Orbital coilgun accelerator

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stoppedcaring
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Orbital coilgun accelerator

Postby stoppedcaring » Wed Apr 30, 2014 5:07 pm UTC

Okay, got an idea for an orbital launch system that might actually be feasible with current technology and materials (20th-century technology, TBH) and would almost definitely result in significantly lower launch costs despite the high initial construction cost.

As What-If #58 references, it's quite popular to try and come up with ways of getting into space differently than the way we do it now. Tons of the "why don't we send a rocket up to space and THEN go sideways", which of course misses the point of how carefully launch trajectories are designed to be as efficient as possible. Threads like this one, for example.

Existing challenges:
Spoiler:
Chemical rocket engines have a low specific impulse, driving up mass fraction and thus driving up vehicle size and cost while making reusability a huge challenge, but we need them for their high thrust-to-weight ratio in order to get off the ground in the first place. As many have pointed out, if there was a way to get up to space and stay there without falling, you could use a high-specific-impulse engine, like a solar ion engine, to get up to orbital speed at your leisure. It's the staying-up-there part that's so tricky.

Space guns are a great idea -- it's way cheaper if most of the launch energy comes from the ground rather than being carried with you, and you could use a high-efficiency high-reusability electromagnetic launcher -- but they are the worst in terms of drag. Having maximum velocity at the start means you lose most of it to drag.

Of course, there's the combination -- why not lift a space gun into space and then fire a payload up to orbital velocity? But the cost of lifting a heavy space gun and all its fuel is prohibitive and wasteful. If only there was a way to have the space gun already up there....
So here's the idea.

Orbiter Design

First, launch 400 robust low-resistance metal rings into orbit. Each ring will need to be about 8 meters across. Bolt them together with a robust, lightweight frame, around 10 meters apart to form a tube around 4 km long. Yes, that is a very large object to have in orbit. Yes, this will be a very high initial cost. Not much we can do about that. Of course, the materials and design elements themselves will be nowhere close to the cost of the Large Hadron Collider (which is a loop seven times longer than the tube we're talking about); the highest cost is going to be getting everything into orbit and putting it together once it's up there.

Add solar panels, high-efficiency solar ion engines, and a central control center with a habitable module.

Wire this tube such that a high burst of current can be sent through each of the rings in sequence, turning the whole thing into a gigantic solenoid with a uniform magnetic field through the center.

Now that the orbital accelerator is all set up, all we need to do is set up a payload!

Launch System

Design and build a fully-reusable mid-range rocket capable of suborbital space flight and re-entry with a peak speed of around 4 km/s. This level of performance was achieved in the 60s with the Jupiter class of medium-range ballistic missiles; modern improvements should make it quite easy and inexpensive. Mass fraction would be significantly lower than orbital launch; consider a mass fraction of 5:1 using SpaceX's Merlin-class engine vs a mass fraction of 28:1 for orbital launch with the same engine. Using a peak-efficiency liquid rocket could reduce mass fraction as low as 3:1. It may be that this is low enough for air-launch systems to be cheaper and more efficient, but that's another question entirely.

Incorporate a lightweight, robust electromagnet design into the rocket's outer skin. Also, give it some reusable retro-rockets for ultra-fine flight control.

The launch trajectory will be designed to intersect the orbital path of the space-station-tube. High-speed computers will determine the exact trajectory of both spacecraft and line them up such that the tube catches up to the rocket and passes around it from back to front. At this point, both the tube solenoid and the rocket's electromagnet are activated, exerting tremendous force on the rocket and thus transferring momentum from the tube to the rocket. Acceleration on the rocket will be on the order of 200 gees, boosting it from 4 km/s well up to orbital velocity.

Because the mass of the orbiting coilgun is so much greater, it will lose only a small amount of speed in the exchange and can make up the difference with a very short station-keeping burn of its high-specific-impulse ion engines.

The returning spacecraft can execute the exact same maneuver in reverse when it wants to de-orbit, dropping from orbital velocity down to 4 km/s. This should be enough to greatly reduce heating on re-entry and enable a parachute-assisted descent with pinpoint vertical landing.

Hypothetical specifications
Spoiler:
SpaceX's Falcon 9 is a little over 500 tonnes and can place 13 tonnes of payload into LEO. If we want a similar payload requirement and assume payload is half the mass of our otherwise-empty craft, then our mass fraction of ~3:1 would give us a 100-tonne launch mass and a 25-tonne orbital entry mass. Falcon 9 is 3.7 meters wide and 70 meters long; our launch vehicle could carry the same payload with a length of only 15 meters, barely larger than Falcon 9's second stage.

In this case, we'd need the coilgun to exert a force of 50 MN. Note that this would produce an acceleration on the 2800-tonne spacecraft (see below) of a mere 0.14 gees, reducing the station's speed by only 106 m/s. This is still around 4 tonnes of propellant, but that beats the heck of out the ~250 tonnes of extra propellant that our rocket would require to achieve orbital velocity on its own.
Challenges

Cost. If we assume that the conductive rings will each be about 10 cm thick, then they'll each weigh on the order of 3.5 tonnes. SpaceX claims a target launch cost of $500 per pound, which means that if the rings compose half the mass of the spacecraft (the rest being frame, engines, modules, and so forth), we're looking at a total mass of 2800 tonnes and a launch cost of $3.1 billion. On the plus side, the cost of the materials is going to be negligible in comparison, and this is less than half the cost of the Large Hadron Collider and nowhere near the pricetag of the ISS.

Power. Achieving rapid enough energy discharge into both the rocket's electromagnet and the orbital coil will be tricky. For this purpose, a flywheel may actually be the best way to store and discharge electrical energy from the solar panels on the space station. Powering the electromagnet in the rocket is trickier, though a flywheel could certainly do the trick here as well.

Thoughts? Feasibility?

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Re: Orbital coilgun accelerator

Postby p1t1o » Thu May 01, 2014 8:31 am UTC

Whilst this idea may have some merits, here are the first thoughts that spring to mind:

stoppedcaring wrote:...can make up the difference with a very short station-keeping burn of its high-specific-impulse ion engines.


This should say "...an *extremely long* station-keeping burn...". Ion engines tend to have very low thrust:weight ratio.
Problems here with keeping the blasted thing in orbit - as soon as a rocket is "launched" through it, it will immediately start to fall out of orbit and will have to be re-boosted to its previous altitude - there is a question of whether ion engines will be able to provide enough thrust to keep the coilgun in orbit after launching a rocket.

stoppedcaring wrote:Acceleration on the rocket will be on the order of 200 gees


That is quite extreme, the extra weight needed to construct a rocket capable of surviving this may well push the idea to infeasibility - not to mention the payload itself would have to survive it, and thus be heavier.

This is further exacerbated by the weight of your electromagnetic components on the rocket itself.


I suppose the idea behnd this is that you are exchanging low-efficiency liquid rocket thrust on your rocket for the high-efficiency thrust of ion engines mounted on your coilgun - you take fuel off your rocket and in exchange use a lighter weight of fuel stored aboard the coilgun.

Whether this exchange can be made worth constructing a giant orbital coilgun is the question.

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elliptic
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Re: Orbital coilgun accelerator

Postby elliptic » Thu May 01, 2014 9:52 am UTC

stoppedcaring wrote:The launch trajectory will be designed to intersect the orbital path of the space-station-tube. High-speed computers will determine the exact trajectory of both spacecraft and line them up such that the tube catches up to the rocket and passes around it from back to front.


This part also seems, errr... fairly ambitious? The coilgun is in LEO so the closing speed is ~8km/s and any error in alignment leads to total destruction.

[edit] re-reading slightly more carefully you have the launch vehicle at 4km/s at intersection, which halves the closing speed. But since the objective is to minimise the amount of fuel required by the rocket, I'm not sure why you want it to have any residual speed at all?

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Re: Orbital coilgun accelerator

Postby peregrine_crow » Thu May 01, 2014 12:31 pm UTC

p1t1o wrote:Problems here with keeping the blasted thing in orbit - as soon as a rocket is "launched" through it, it will immediately start to fall out of orbit and will have to be re-boosted to its previous altitude - there is a question of whether ion engines will be able to provide enough thrust to keep the coilgun in orbit after launching a rocket.


But you perform the same trick in reverse for reentry, which pushes the coilgun into a higher orbit. Provided you start at a high enough altitude and the difference between the weight of your coilgun and the weight of the craft is high enough you could theoretically pingpong between the initial higher orbit for launching stuff into space and the lower orbit for catching stuff coming back from space.

Still leaves the problem of getting back tot the coilgun and the question of why you'd want the craft to return back to earth in the first place (as there would obviously not be any living people inside the craft when it accelerates at 200g).
Ignorance killed the cat, curiosity was framed.

stoppedcaring
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Re: Orbital coilgun accelerator

Postby stoppedcaring » Thu May 01, 2014 1:02 pm UTC

peregrine_crow wrote:
p1t1o wrote:Problems here with keeping the blasted thing in orbit - as soon as a rocket is "launched" through it, it will immediately start to fall out of orbit and will have to be re-boosted to its previous altitude - there is a question of whether ion engines will be able to provide enough thrust to keep the coilgun in orbit after launching a rocket.


But you perform the same trick in reverse for reentry, which pushes the coilgun into a higher orbit. Provided you start at a high enough altitude and the difference between the weight of your coilgun and the weight of the craft is high enough you could theoretically pingpong between the initial higher orbit for launching stuff into space and the lower orbit for catching stuff coming back from space.

Still leaves the problem of getting back tot the coilgun and the question of why you'd want the craft to return back to earth in the first place (as there would obviously not be any living people inside the craft when it accelerates at 200g).

It's true, the need for launching stuff out of orbit and into re-entry is significantly lower than the need for launching stuff into orbit in the first place, so the amount of incoming mass would be much lower than the amount of outgoing mass. Most re-entry is human cargo.

But the coilgun won't immediately fall out of orbit whenever it launches a rocket. It's not going to be parked at the lowest possible altitude, after all; there'll be a safety margin. If it's clipping along at 8 km/s, losing 106 m/s is only going to decrease the size of its orbit slightly, not take it out of orbit altogether.

elliptic wrote:
stoppedcaring wrote:The launch trajectory will be designed to intersect the orbital path of the space-station-tube. High-speed computers will determine the exact trajectory of both spacecraft and line them up such that the tube catches up to the rocket and passes around it from back to front.


This part also seems, errr... fairly ambitious? The coilgun is in LEO so the closing speed is ~8km/s and any error in alignment leads to total destruction.

[edit] re-reading slightly more carefully you have the launch vehicle at 4km/s at intersection, which halves the closing speed. But since the objective is to minimise the amount of fuel required by the rocket, I'm not sure why you want it to have any residual speed at all?

One way to add a safety margin would be to use a parabolic section rather than a ring. Especially because the rocket is going to need to be pushed upward slightly over the first 3/4 of the launch path. This way, the rocket wouldn't have to pass through the track; it would just have to pass over the track, which gives a whole upward degree of freedom in case of emergency.

Doing a launch at half of orbital speed is already a huge savings on propellant and staging, and the speed reduces closing velocity while also reducing the necessary size of the coilgun and/or the g-forces on the rocket. We could, of course, reduce the rocket's launch speed to something like 1.5 km/s and extend the length of the coilgun.

p1t1o wrote:
stoppedcaring wrote:Acceleration on the rocket will be on the order of 200 gees


That is quite extreme, the extra weight needed to construct a rocket capable of surviving this may well push the idea to infeasibility - not to mention the payload itself would have to survive it, and thus be heavier.

We could always make it an 80-km coilgun and reduce the acceleration to a human-survivable level. But that would push launch costs alone upward of $62 billion, which is starting to get pricey even for a project of this magnitude. Still way cheaper than building a space hook or space elevator, of course.

stoppedcaring wrote:...can make up the difference with a very short station-keeping burn of its high-specific-impulse ion engines.

This should say "...an *extremely long* station-keeping burn...". Ion engines tend to have very low thrust:weight ratio.
Problems here with keeping the blasted thing in orbit - as soon as a rocket is "launched" through it, it will immediately start to fall out of orbit and will have to be re-boosted to its previous altitude - there is a question of whether ion engines will be able to provide enough thrust to keep the coilgun in orbit after launching a rocket.

I suppose the idea behnd this is that you are exchanging low-efficiency liquid rocket thrust on your rocket for the high-efficiency thrust of ion engines mounted on your coilgun - you take fuel off your rocket and in exchange use a lighter weight of fuel stored aboard the coilgun.

Whether this exchange can be made worth constructing a giant orbital coilgun is the question.

Incidentally, the nature of the momentum exchange is such that even using the same bipropellant liquid rocket fuel on the coilgun station, you'd still only be using a fraction of the fuel. Around 70 tonnes (as opposed to the ~300 tonnes the rocket would need to use for itself).

But this admittedly might be the point where current technology fails us. We might need to use several beefed-up electrostatic dual-stage 4-grid electrostatic thrusters or a series of VASIMR thrusters. Both can be scaled-up more easily than a Hall Effect ion thruster and both output thrust based on the available power. One nice thing about having a multiple-kilometer space station is that there's plenty of room for solar panels!

It's been a while since I took E&M. Anyone care to take a stab at how much power would be required to generate 50 MN of conservative magnetic force using this sort of arrangement?

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Re: Orbital coilgun accelerator

Postby p1t1o » Thu May 01, 2014 1:19 pm UTC

Well "higher" or "lower" orbit and "falling out of orbit" are slight misnomers here, which I bear some guilt for.

What will happen to the coilgun on accelerating a rocket is its orbit will become elliptical, with the apoapsis at the same initial height and the periapsis at some lower height. Using the coilgun to decelerate a rocket could occur at the apoapsis, restoring the original circular orbit of the coilgun and putting the rocket into an elliptical, atmosphere-contacting one. Transitions will always occur at the same altitude (accel or decel) otherwise the orbit of the gun will be really screwed up.

So with extreme precision, yes I think you could remove the need for lots of thrusting on the orbiting coilgun, but only assuming you have roughly equal masses of stuff going up and down.

By the time we have the ability to build and succesfully operate this, we won't need to.
For example, the projected Skylon spaceplane, which could be operational within mere decades, is already projected to bring launch costs down to roughly £650 per kg (~$1000/kg).

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Thu May 01, 2014 1:33 pm UTC

p1t1o wrote:By the time we have the ability to build and succesfully operate this, we won't need to.
For example, the projected Skylon spaceplane, which could be operational within mere decades, is already projected to bring launch costs down to roughly £650 per kg (~$1000/kg).

SpaceX is already promising launch costs on the order of $1000/kg from simply using fully-reusable VTOL boosters. That, in fact, is what I used as the cost basis for the launch of the coilgun.

The orbital coilgun would make launch costs drastically lower than that.

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Re: Orbital coilgun accelerator

Postby p1t1o » Thu May 01, 2014 1:49 pm UTC

I dunno... your quoted price of 62 billion for an 80km orbiting coilgun seems waaaay too low. The raw cost of launching the mass of the gun to space is only a fraction of the total cost. I'd say 62 billion is a reasonable amount to expect to pay to build an 80km coilgun on the ground.

As a comparison the total project cost for the B2 spirit bomber was 44billion and they are only like 50 metres across and dont go anywhere near orbit :D


And if you're only going to save a few hundred bucks per kilogram, will that justify a multi-muti-billion dollar mega-project?

Lets say the coilgun halves the $1000/kg cost to $500/kg, so you save $500 per kilogram.
If we assume the gun costs $100billion (for ease of calculation), you will have to launch 200,000tons (444 ISSs) into orbit to break even.
Whilst this isn't a completely unimaginable amount, the cost isn't taking into account the maintenance and operational (and insurance) costs of an extremely precise orbiting megamachine bigger than anything we have ever built (except maybe the LHC, but that doesn't count as its not free-standing)

And bring back 200,000tons down from orbit if we are using the "ping-pong" style launch-recover scheme.

I'm sorry I'm on a bit of a downer but it just doesn't feel like apractical solution to me :(

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Thu May 01, 2014 2:18 pm UTC

p1t1o wrote:I dunno... your quoted price of 62 billion for an 80km orbiting coilgun seems waaaay too low. The raw cost of launching the mass of the gun to space is only a fraction of the total cost. I'd say 62 billion is a reasonable amount to expect to pay to build an 80km coilgun on the ground.

Oh, certainly. I was just trying to get an idea of launch cost alone. A project like this is scaled on an order-of-magnitude basis; I was wanting to make sure the launch cost wouldn't be prohibitively expensive.

Development would be a huge cost. The solar panels and engines will be pricey. And then it's upkeep, which...honestly, I don't really have any idea how much the operational upkeep costs would be.

And if you're only going to save a few hundred bucks per kilogram, will that justify a multi-muti-billion dollar mega-project?

Lets say the coilgun halves the $1000/kg cost to $500/kg, so you save $500 per kilogram.
If we assume the gun costs $100billion (for ease of calculation), you will have to launch 200,000tons (444 ISSs) into orbit to break even.

Oh, I'm thinking we can get launch costs down close to space elevator levels.

Whilst this isn't a completely unimaginable amount, the cost isn't taking into account the maintenance and operational (and insurance) costs of an extremely precise orbiting megamachine bigger than anything we have ever built (except maybe the LHC, but that doesn't count as its not free-standing)

Well, neither is this; it's free-falling. ;)

Remember that the 80 km one is only if you want human-survivable launch. For a cargo-only launch system, it's just 4-8 km, with a launch cost of under $10 billion.

All this is predicated on the idea that a contra-electromagnetized coilgun design is the best way to massively decelerate an object quickly and non-destructively in a weightless vacuum. If anyone else can think of a better way, I'm all ears. :)

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Re: Orbital coilgun accelerator

Postby p1t1o » Thu May 01, 2014 2:33 pm UTC

stoppedcaring wrote:Remember that the 80 km one is only if you want human-survivable launch. For a cargo-only launch system, it's just 4-8 km, with a launch cost of under $10 billion.


I've stuck to the 80km figure not for human-survivable purposes but because of the weight penalty associated with building a rocket and cargo that can survive 200gs may well single-handedly negate any other efficiency gains.

I do like coilguns but the only way we are going to get this off the ground is by using it as an enourmous space-based weapon. That way we can spend as much as we want! (oh snap, political!)

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Thu May 01, 2014 4:48 pm UTC

p1t1o wrote:
stoppedcaring wrote:Remember that the 80 km one is only if you want human-survivable launch. For a cargo-only launch system, it's just 4-8 km, with a launch cost of under $10 billion.


I've stuck to the 80km figure not for human-survivable purposes but because of the weight penalty associated with building a rocket and cargo that can survive 200gs may well single-handedly negate any other efficiency gains.

Well, the proposal by Quicklaunch would involve around 1800g, so I was thinking 200g was conservative. To my knowledge, the Sprint anti-ICBM missile didn't have significant problems withstanding the g-forces...though of course it wasn't exactly reusable.

But we certainly don't need to go all the way out to the 80 km. One thing to keep in mind is that the amount of impulse provided would be 100% adjustable. There's no requirement that every launch be exactly the same; the launch vehicle could have any initial velocity we wanted to give it. If we used a 35 km coilgun (launch cost approximately $24 billion), we could handle a wide range of launch profiles:

possible launch specifications for 35 km orbital coilgun
(initial launch with liquid-fueled rocket, Isp 450 seconds)
(minimum delta-V to altitude = 1.53 km/s)

Image

So having this coilgun in orbit could make every launch SSTO-capable, even passenger launch. In the case of hardened cargo, the launch vehicle could release the payload and return like a sounding rocket without ever reaching high velocity or needing hardening at all; a fully-reusable non-hardened VTOL sounding rocket is super cheap to build and operate.

Right now, spacelaunch has moderate overhead with huge mission costs. This would be a much bigger overhead, but with minimal mission costs. Once multiple sensitive-cargo deliveries had been made, engines and tanks could be disassembled and loaded into a single sensitive-cargo shuttle to be shipped back to Earth. Nothing gets wasted; everything can be re-used just like with normal aircraft flight.

EDIT: Does anyone know off the bat where Randall might have gotten this graph?
Image
I mean, obviously he drew it. And the concept is also obvious. But how do you come up with a launch profile? Is there any good rule-of-thumb for determining the optimal acceleration to use so that both gravity drag and aerodynamic drag stay as low as possible?

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Re: Orbital coilgun accelerator

Postby p1t1o » Thu May 01, 2014 6:30 pm UTC

Well the sprint missile (one of my favorites) had a burn-out velocity of around ~4km/s, but a ceiling altitude of only some 30km. A sprint missile modified to reach LEO could be built, but it would be huge - and therefore expensive.

My problem with the acceleration is not whether or not we could build a rocket to withstand it, as you point out, we already can. But if cost saving is your aim - building a rocket that strong, and then launching the extra weight that it requires, will eat up your projected profits.

Im not so sure about quicklaunch, his projected numbers are nice but the inventor doesn't seem to have had much success moving on from there - didn't he work on the Iraqi supergun?
1800g is fine if you are delivering fuel, but not so much with the sattelites and people.


Goddamn I love the Sprint though - did you know that because its mission profile was low in the atmosphere and its speed and acceleration so great, that if you were to play an oxy-acetylene torch over it in flight, it would cool it down :D


**edit**
Ejecting the payload and sending the rocket itself back unaccelerated is a good idea though. I still think it'd be easier/cheaper to use a small disposable rocket stage instead of a kilometres long coilgun :D
I have heard somewhere that a launch profile that sticks around 1G (upwards, so 2G) is optimal for aero/gravity drag balance, at least until after MaxQ. Can't remember where I heard it so may not be reliable.

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Thu May 01, 2014 6:52 pm UTC

p1t1o wrote:Well the sprint missile (one of my favorites) had a burn-out velocity of around ~4km/s, but a ceiling altitude of only some 30km. A sprint missile modified to reach LEO could be built, but it would be huge - and therefore expensive.

My problem with the acceleration is not whether or not we could build a rocket to withstand it, as you point out, we already can. But if cost saving is your aim - building a rocket that strong, and then launching the extra weight that it requires, will eat up your projected profits.

There are plenty of buildable SSTO launch systems that fall just shy of the 7-8 km/s you need for orbital insertion. So the advantage of the orbital coil launcher would be that it could provide an extra boost for almost any system. The lower your initial mass ratio is, the easier it is to achieve reusability and the lower your overall costs are.

The acceleration is obviously much lower if you have a slightly larger coilgun and use some initial velocity.

1800g is fine if you are delivering fuel, but not so much with the sattelites and people.

Which is why I modified the design so that g-forces range from 6 to 100.

Goddamn I love the Sprint though - did you know that because its mission profile was low in the atmosphere and its speed and acceleration so great, that if you were to play an oxy-acetylene torch over it in flight, it would cool it down :D

Damn.

**edit**
Ejecting the payload and sending the rocket itself back unaccelerated is a good idea though. I still think it'd be easier/cheaper to use a small disposable rocket stage instead of a kilometres long coilgun :D

Which is basically a two-stage rocket. See SpaceX. ;)

I have heard somewhere that a launch profile that sticks around 1G (upwards, so 2G) is optimal for aero/gravity drag balance, at least until after MaxQ. Can't remember where I heard it so may not be reliable.

Good enough for government work. So just estimate where MaxQ will be using thrusters at 2G and then open up afterward?

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Re: Orbital coilgun accelerator

Postby mfb » Thu May 01, 2014 7:06 pm UTC

stoppedcaring wrote:Incidentally, the nature of the momentum exchange is such that even using the same bipropellant liquid rocket fuel on the coilgun station, you'd still only be using a fraction of the fuel. Around 70 tonnes (as opposed to the ~300 tonnes the rocket would need to use for itself).
You would still have to carry that fuel to orbit, and then you don't save anything (you could simply burn the fuel while it is still in the rocket to get the same momentum change - actually even a bit more).

Did you take the ion drive fuel into account in your calculations? This directly reduces the payload.

The general acceleration mechanism looks problematic. You want to produce a uniform magnetic field - but how does the acceleration work then? Your rocket does not decelerate in a uniform field. You would need an alternating field - the rocket won't be able to handle that, so your rings have to switch their orientation. Good luck switching a meter-sized magnetic field of the order of 1 Tesla in a millisecond - as comparison, the LHC magnets need 20 minutes to ramp up or down. In addition, the rings would generate huge and time-dependent forces on each other, so you need a massive support structure. Parabolic parts don't work, you need a current return.

You could use an eddy current brake. It is way easier to set up, if you dump the heat into the orbital structure it should be able to handle that. Deorbiting is not possible that way, of course.

Speaking of deorbiting... if you slow something to 4km/s, it will basically start to fall down with ~9m/s^2. Starting (optimistic) from LEO at 400km height, after 4-5 minutes it reaches the upper atmosphere with a vertical velocity in excess of 1km/s. That means you have about 20-30 seconds between "starting to decelerate somewhat" and "Oh me yarm I'm in very thick air". Your peak power at the outer surface will be higher than for conventional deorbits.

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Re: Orbital coilgun accelerator

Postby p1t1o » Thu May 01, 2014 9:43 pm UTC

stoppedcaring wrote:
**edit**
Ejecting the payload and sending the rocket itself back unaccelerated is a good idea though. I still think it'd be easier/cheaper to use a small disposable rocket stage instead of a kilometres long coilgun :D

Which is basically a two-stage rocket. See SpaceX. ;)



Thats basically what I was getting at :lol:

stoppedcaring wrote:Good enough for government work. So just estimate where MaxQ will be using thrusters at 2G and then open up afterward?


Roughly speaking, works in KSP anyhoo :mrgreen:

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Re: Orbital coilgun accelerator

Postby stianhat » Fri May 02, 2014 1:26 pm UTC

I'd say that a huge benefit would be the incredibly much better control you can have over the rocket trajectory after the coil gun. Rockets, being rockets, deliver their thrust over time and many kilometers, and even though the guys at the different space centers are quite skilled at planning the burns - reducing this to impulse-like acceleration would increase the precision of most movements, which is less important for sending a probe the way of saturn but more important for scoping out an asteroid. For instance.

I do have a problem with arranging the LEO coil gun and a freshly de-atmospherized rocket to meet at d4000 m/s though, anything misaligned by the fraction of a second or anything and you have destroyed a large swath of common exit trajectories AND LEO with debris.

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Fri May 02, 2014 7:56 pm UTC

mfb wrote:
stoppedcaring wrote:Incidentally, the nature of the momentum exchange is such that even using the same bipropellant liquid rocket fuel on the coilgun station, you'd still only be using a fraction of the fuel. Around 70 tonnes (as opposed to the ~300 tonnes the rocket would need to use for itself).
You would still have to carry that fuel to orbit, and then you don't save anything (you could simply burn the fuel while it is still in the rocket to get the same momentum change - actually even a bit more).

This is true, but only if we're assuming a two-stage design. The momentum transfer enables the coilgun to act as the second stage; since it never leaves orbit, the weight cost associated with a second stage is zero even if the fuel is not. The rocket would need substantially more fuel to achieve the same velocity with an SSTO design, which is part of what I'm trying to achieve.

What SpaceX has done is pretty cool...instead of trying to achieve final-stage reusability (e.g. Space Shuttle) or true SSTO with reusability (Skylon), they've opted for full reusability in the first stage with an expendable payload stage. Since the first stage has the biggest engine group and the most complex systems, that's a substantial savings; the payload stage is really not nearly as expensive. And their boosters are designed such that they can be easily strapped together for super-heavy lift, so cost per pound actually goes down.

I estimated the required fuel for the coilgun to be around 4 tonnes of xenon, which would admittedly reduce the effective payload. It would be substantially less if they used argon instead with a VASIMR engine. I actually designed a VASIMR engine in highschool before I knew what it was. But that's with the older estimated deltaV and so forth, not the modified/longer coilgun design and adjusted payloads.

I had been using a specific impulse of 4400 m/s, which I now realize came from the Space Shuttle Main Engines. But those had half the thrust-to-weight ratio of something like the Merlin family, which is more what I'd want to use. I should probably re-run the numbers using the project specific impulse from the Raptor-class engines SpaceX is building and assume a slightly more modest thrust-to-weight ratio.

I must have accidentally had it in my head that higher orbits have higher speeds, which is obviously false. The higher we place our launch station, the less delta-v it would need to impart. Of course, we don't want to put it so high that we end up losing more than we save in gravity drag. The ISS is out at over 400 km but has an average speed of 7.6 km/s, only a few hundred m/s slower than speed at the minimum altitude, so it's probably better to stick with a low-altitude insertion.

Here's the table used to come up with the required delta-V at launch insertion for a range of configurations (note that the lowest gee-level from 25 km up to 45 km is 0.00, corresponding to a sounding-rocket launch):
Spoiler:
Image
And here's a table showing the requisite mass fraction if we're using an SSTL design with specific impulse comparable to the projected Raptor engines:
Spoiler:
Image
Of course this mass fraction doesn't take the mass of the ion drive fuel into account, but if we go with a VASIMR thruster group then it shouldn't be much; we can take it off the top of payload.

So it's clear that even with the smallest possible launcher and the lowest possible gee-levels, we've still brought mass fraction down to a range where a single-engine-cluster, single-stage rocket is possible. That's pretty much the holy grail of launching, so that's encouraging.

This sort of launch system could accommodate a really wide range of launches; quite a few payload configurations would be possible. But since the goal is standardization and reusability, the optimal standard approach would be to use the same engine cluster for everything and simply switch out payload and fuel tank size according to need. If we go with the 30-km launcher (so that a sounding-rocket launch is possible at only ~100 gees for partially hardened cargo), then for a crewed payload comparable to the DragonRider (and conservatively tripling payload from the Dragon's ~7500 kg to account for the total launch vehicle weight, including VASIMR fuel), the total launch mass for a 4-gee crewed launch would be around 270 tonnes, half the standard launch mass for the Dragon 9v1.1 currently in use by SpaceX. Lifting such a launch vehicle at a nominal acceleration of 2g (1g plus gravity drag) would take only four Merlin-D-sized rocket engines.

Using this as our standard launch cluster and working backward provides the following payload capacities for each launch class:

Image

One fully-reusable engine cluster. One orbital launcher. All enabling many different launch configurations, all essentially single-stage-to-launch. The action mass needed for the VASIMR engines is modest; a single hardened-cargo launch can carry enough VASIMR fuel for its own launch plus 20+ sensitive-cargo launches. I've asterisk'd the crewed launches, as they will almost always be two-way, meaning the space station can make up the lost momentum on the return trip. The fuel for a hardened-cargo launch would actually be slightly lower, as the launch vehicle returns to Earth and doesn't need to be accelerated, but it's only a 5% difference in the end.

Speaking of deorbiting... if you slow something to 4km/s, it will basically start to fall down with ~9m/s^2. Starting (optimistic) from LEO at 400km height, after 4-5 minutes it reaches the upper atmosphere with a vertical velocity in excess of 1km/s. That means you have about 20-30 seconds between "starting to decelerate somewhat" and "Oh me yarm I'm in very thick air". Your peak power at the outer surface will be higher than for conventional deorbits.

SpaceX considers its launch profile to be proprietary, so I wasn't able to get a specific value for how much velocity their rocket has at first-stage separation.

But, based on the vacuum specific impulse and sea level specific impulse of their engines, coupled with the known burn time and thrust of each stage, I can pretty reasonably predict that first-stage separation takes place at 3.5-4.5 km/s, well within my deorbit entry speed for sensitive cargo. I'm not sure how they decelerate their first stage booster from that speed. They plan on using a disposable ablative shield with hypergolic engines to land DragonRider, but I don't think their booster has an ablative shield. In any case, I'm sure it wouldn't be too much of a challenge to use a short burn (fuel is cheap; rockets are not; we have an incredibly good mass ratio already and our hardened-launch capacity will make it feasible to store large amounts of fuel in orbit) and parachutes to pull off a controlled landing using the same basic profile as they use.

The general acceleration mechanism looks problematic. You want to produce a uniform magnetic field - but how does the acceleration work then? Your rocket does not decelerate in a uniform field.

It's been a while since I took E&M, so I'm a little rusty...but if the rocket is set up as a giant bar magnet and is run through the center of a solenoid with a uniform field running in the opposite direction, won't that do the trick?

In addition, the rings would generate huge and time-dependent forces on each other, so you need a massive support structure. Parabolic parts don't work, you need a current return.

I'm sure we could come up with current multipliers or something so that it would loop underneath with minimal field, rather than over the top. And the support structure is definitely going to be an issue...but hey, this IS a giant space station, so we're already thinking along those lines.

You could use an eddy current brake. It is way easier to set up, if you dump the heat into the orbital structure it should be able to handle that. Deorbiting is not possible that way, of course.

Hmm. What about a regenerative braking system that can be used as a launcher in the reverse case? Collect the kinetic energy of the system during orbital insertion and discharge it during deorbital launch. A linear inductive motor is another possibility.

I do have a problem with arranging the LEO coil gun and a freshly de-atmospherized rocket to meet at d4000 m/s though, anything misaligned by the fraction of a second or anything and you have destroyed a large swath of common exit trajectories AND LEO with debris.

Well, on the one hand, once we've got it up and functioning we will be able to put another one in orbit a decade later for a much lower cost.

And using linear inductive motors, a differentially-shaped ring, or a regenerative brake system can allow for the launch path to be one-sided, allowing for an escape vector in case of catastrophic failure. You really want the "landing" part to be smooth, though. :)

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Re: Orbital coilgun accelerator

Postby p1t1o » Fri May 02, 2014 8:00 pm UTC

Forget putting one up a decade later, the destruction of a multi-kilometre structure by a multi-km/s impact will push us straight into full blown Kessler syndrome at this stage. We'd be lucky to be going back to space at all within a decade :D

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Fri May 02, 2014 8:46 pm UTC

p1t1o wrote:Forget putting one up a decade later, the destruction of a multi-kilometre structure by a multi-km/s impact will push us straight into full blown Kessler syndrome at this stage. We'd be lucky to be going back to space at all within a decade :D

My point being that using a flat or curved launch system rather than a tube would decrease the risk of catastrophic failure, and having a secondary one would make repairs after a minor accident less costly.

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Re: Orbital coilgun accelerator

Postby p1t1o » Fri May 02, 2014 9:08 pm UTC

Do you get minor accidents at 4km/s?

How close does the vessel would have to pass the gun?

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Fri May 02, 2014 9:27 pm UTC

p1t1o wrote:Do you get minor accidents at 4km/s?

How close does the vessel would have to pass the gun?

Well, there would be a whole range of entry speeds. Catastrophic failure would only be likely if you lined up your approach improperly...basically the same as a fighter jet dipping too low on entry for a carrier landing and smashing into the back of the carrier deck. KABOOM. It might be possible to construct a one-time-use reactive shield on the front end of the launch track for exactly that eventuality.

Once you've made the approach properly, the force is going to lift you up and push you back, so only the slightest of accidents would be possible. Nothing catastrophic. I'm guessing the launch track would be wider at the beginning and smaller toward the end, so as to have the largest margin for error at the highest speeds and taper to a smaller margin of error as speeds equalized.

That's basically what we're talking about, here. Where an aircraft carrier launches and receives aircraft, this is a spacecraft carrier. But with an electromagnetic launch/receive mechanism, rather than a catapault and tailhook. Tailhooks wouldn't work so well at 3-8 km/s.

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Re: Orbital coilgun accelerator

Postby peregrine_crow » Mon May 05, 2014 8:26 am UTC

stoppedcaring wrote:
p1t1o wrote:Do you get minor accidents at 4km/s?

How close does the vessel would have to pass the gun?

Well, there would be a whole range of entry speeds. Catastrophic failure would only be likely if you lined up your approach improperly...basically the same as a fighter jet dipping too low on entry for a carrier landing and smashing into the back of the carrier deck. KABOOM. It might be possible to construct a one-time-use reactive shield on the front end of the launch track for exactly that eventuality.


p1t1o does have a good point though. You're putting a massive construction in space, it is going to accelerate kessler syndrome. There don't even have to be any critical accidents with the coil gun itself, if space debris from another satellite hits the coil gun it will cause debris which itself can hit other satellites, etc. Being a huge construction just makes it that much more likely to get hit (and fall apart into more pieces in case of a catastrophic breakdown).

I'm not sure to what extend we are able to shield satellites from this sort of things, but it might be more feasible to shield this thing than it is to shield our regular satellites, just due to its size (shield it from regular debris, of course, not from payloads being smacked into it at 4km/s).

I wonder how existing space debris will respond to activating the gun, will it noticeably attract/repel nearby stuff?
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Re: Orbital coilgun accelerator

Postby speising » Mon May 05, 2014 8:34 am UTC

don't forget the political dimension. whoever controls a doomsday weapon like this rules the world.

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Re: Orbital coilgun accelerator

Postby Neil_Boekend » Mon May 05, 2014 11:33 am UTC

p1t1o wrote:Do you get minor accidents at 4km/s?

Theoretically yes, as the mass of the object decides the energy of the collision. I do not know how low the mass must be for a normal satellite part to survive it, but I would hazard a guess that it is between the mass of a single atom and a gram.
p1t1o wrote:How close does the vessel would have to pass the gun?

Quite close, as the efficiency of the power transfer decreases drastically with distance.
Maybe a magnetic net of a couple of km could be used. The shape of the magnetic field lines in a simple electromagnet seem to do a nice job of guiding an object to the middle of the electromagnet. Theoretically you could just set the current in one direction for all coils so they form one massive electromagnet. All ferromagnetic objects are automatically guided to the center of the coil, where the coilgun uses the same coils to accelerate the object in the standard coilgun way.
Assuming the field is strong enough and the payload has sufficient ferromagnetic parts even a 4 km/sec payload would enter the coilgun in the correct way in a semi foolproof manner (Never assume something is fully foolproof. There are really advanced fools out there).

As for acceleration to keep the thing in place, why not use the earth's magnetic field as a stator? Can't find the details now but I seem to remember an experiment where a induction coil is supplied with a current to generate a force to drag an object in a specific direction. This thing would be made of coils so it might be suitable. Dunno about the direction constraints though.
If it turns out to be feasible the coil doesn't even need fuel for ion engines, because how would you refill that fuel? Electricity is quite available if you cover even only a part of the structure with solar panels (which you need to do anyway to power the coilgun itself).
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Re: Orbital coilgun accelerator

Postby thoughtfully » Mon May 05, 2014 2:07 pm UTC

It seems to me that balance is going to matter a lot. If the mass of ferromagnetic material is not centered fairly well on the center of mass, erratic trajectories could result. And how much of the ferromagnetic mass has to be mechanically bonded to the superstructure?

Coil guns are great for accelerating a uniform cylindrically symmrtrical mass of ferrous metal, but what about a large heterogenous body?
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Re: Orbital coilgun accelerator

Postby Whizbang » Mon May 05, 2014 2:09 pm UTC

speising wrote:don't forget the political dimension. whoever controls a doomsday weapon like this rules the world.


One ring to rule them all...

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Re: Orbital coilgun accelerator

Postby Zamfir » Mon May 05, 2014 3:56 pm UTC

We can't worry simultaneously about the object's vulnerability to impacts and its awesome power as a weapon...

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Re: Orbital coilgun accelerator

Postby speising » Mon May 05, 2014 4:11 pm UTC

Zamfir wrote:We can't worry simultaneously about the object's vulnerability to impacts and its awesome power as a weapon...


not simultaneously, but alternatively. if we solve one problem, the other becomes active.

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Mon May 05, 2014 6:12 pm UTC

Whizbang wrote:
speising wrote:don't forget the political dimension. whoever controls a doomsday weapon like this rules the world.


One ring to rule them all...

Hehe.

Yes, this could double as a kinetic bombardment system. Then again, any country capable of putting a satellite in orbit can put up a kinetic bombardment system any time they want and no one will be any the wiser...in comparison, anything launched via something like this would be highly visible.

The potential for kinetic bombardment using tungsten rods deorbited from LEO is pretty scary. Especially with respect to something like assassination potential. A hostile regime like North Korea could (if they ever manage to keep a satellite in orbit for more than 15 minutes) land an impossible-to-defend-against impactor pretty much anywhere in the world with zero warning. Because of the incredibly short launch-to-impact time, they could do so at the beginning of a speech or other public appearance and it would be virtually impossible to anticipate.

peregrine_crow wrote:p1t1o does have a good point though. You're putting a massive construction in space, it is going to accelerate kessler syndrome. There don't even have to be any critical accidents with the coil gun itself, if space debris from another satellite hits the coil gun it will cause debris which itself can hit other satellites, etc. Being a huge construction just makes it that much more likely to get hit (and fall apart into more pieces in case of a catastrophic breakdown).

I'm not sure to what extend we are able to shield satellites from this sort of things, but it might be more feasible to shield this thing than it is to shield our regular satellites, just due to its size (shield it from regular debris, of course, not from payloads being smacked into it at 4km/s).

Putting light shielding on it would be fairly cheap in comparison to its overall cost, unlike with most satellites where they are trying to keep weight as low as possible. Unfortunately, debris impacts could have practically any speed from 2 km/s up to as high as 15 km/s; orbits can run in any great circle around the Earth's center of gravity and so you can get impactors coming in at orbital speed from any direction.

Fortunately, most space debris is not very dense, and this works to our advantage. Another thing that works to our advantage even better is orbital height. LEO is most crowded with debris above 500-600 km; at 150-200 km the thermosphere is still thick enough that satellites must perform regular station-keeping. That's where our space launch carrier would be placed. Its very high cross-sectional density would make it much less susceptible to atmospheric drag. Uncontrolled space debris in this area loses momentum quickly and deorbits, making it the safest place for such a high-impact space station. As long as we're using the system regularly, it won't be hard to keep it supplied with small amounts of VASIMR fuel for station-keeping.

Neil_Boekend wrote:As for acceleration to keep the thing in place, why not use the earth's magnetic field as a stator? Can't find the details now but I seem to remember an experiment where a induction coil is supplied with a current to generate a force to drag an object in a specific direction. This thing would be made of coils so it might be suitable. Dunno about the direction constraints though. If it turns out to be feasible the coil doesn't even need fuel for ion engines, because how would you refill that fuel? Electricity is quite available if you cover even only a part of the structure with solar panels (which you need to do anyway to power the coilgun itself).

I doubt that the geomagnetic gradient at 100-200 km is going to be significant enough to use for any appreciable changes to the orbital velocity of a 21,000 tonne launcher, but I could be wrong. Thankfully, lifting the ion fuel shouldn't be that significant of an issue. I calculated that the most extreme launch profiles -- 0 km/s to orbit with hardened cargo -- could launch 125 tonnes of raw materials (fuel, food, water, air, construction equipment) while carrying 25 tonnes of VASIMR fuel to correct the station's orbit (above the 100 tonnes of rocket fuel used by the booster rocket to lift the payload to altitude). 125 tonnes of fuel to launch 125 tonnes of payload. And this is without using a launch window designed to regain momentum on payload deorbit. In comparison, the best launch systems presently in existence would require over 3,000 tonnes of fuel to achieve the same payload in an SSTO configuration. This system's 1:1 mass fraction is a dozen times better than the best we have now.

Maybe a magnetic net of a couple of km could be used. The shape of the magnetic field lines in a simple electromagnet seem to do a nice job of guiding an object to the middle of the electromagnet. Theoretically you could just set the current in one direction for all coils so they form one massive electromagnet. All ferromagnetic objects are automatically guided to the center of the coil, where the coilgun uses the same coils to accelerate the object in the standard coilgun way.

I would definitely want to go with a tapering design, so that whatever acceleration system I was using (eddy current braking, induction motor, regenerative brake, solenoid coil) would have the widest opening at the entry end and narrow down. Widest margin of error at the beginning.

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Re: Orbital coilgun accelerator

Postby mfb » Mon May 05, 2014 7:48 pm UTC

stoppedcaring wrote:
mfb wrote:
stoppedcaring wrote:Incidentally, the nature of the momentum exchange is such that even using the same bipropellant liquid rocket fuel on the coilgun station, you'd still only be using a fraction of the fuel. Around 70 tonnes (as opposed to the ~300 tonnes the rocket would need to use for itself).
You would still have to carry that fuel to orbit, and then you don't save anything (you could simply burn the fuel while it is still in the rocket to get the same momentum change - actually even a bit more).

This is true, but only if we're assuming a two-stage design. The momentum transfer enables the coilgun to act as the second stage; since it never leaves orbit, the weight cost associated with a second stage is zero even if the fuel is not. The rocket would need substantially more fuel to achieve the same velocity with an SSTO design, which is part of what I'm trying to achieve.
You still need a fuel tank, so all you save is the rocket engine and a bit of other infrastructure.

So it's clear that even with the smallest possible launcher and the lowest possible gee-levels, we've still brought mass fraction down to a range where a single-engine-cluster, single-stage rocket is possible. That's pretty much the holy grail of launching, so that's encouraging.
I don't see this. You still need 7.31km/s with 4km track and 3g. That is nearly the full speed, especially if you add 1.5km/s for atmosphere and gravity. Sure the mass ratio is exponential in this, but still...
If your calculations hold, then a weak (~1km/s), reusable first stage would do the same. Without all those other problems of the design.

Speaking of deorbiting... if you slow something to 4km/s, it will basically start to fall down with ~9m/s^2. Starting (optimistic) from LEO at 400km height, after 4-5 minutes it reaches the upper atmosphere with a vertical velocity in excess of 1km/s. That means you have about 20-30 seconds between "starting to decelerate somewhat" and "Oh me yarm I'm in very thick air". Your peak power at the outer surface will be higher than for conventional deorbits.

SpaceX considers its launch profile to be proprietary, so I wasn't able to get a specific value for how much velocity their rocket has at first-stage separation.

But, based on the vacuum specific impulse and sea level specific impulse of their engines, coupled with the known burn time and thrust of each stage, I can pretty reasonably predict that first-stage separation takes place at 3.5-4.5 km/s, well within my deorbit entry speed for sensitive cargo. I'm not sure how they decelerate their first stage booster from that speed. They plan on using a disposable ablative shield with hypergolic engines to land DragonRider, but I don't think their booster has an ablative shield. In any case, I'm sure it wouldn't be too much of a challenge to use a short burn (fuel is cheap; rockets are not; we have an incredibly good mass ratio already and our hardened-launch capacity will make it feasible to store large amounts of fuel in orbit) and parachutes to pull off a controlled landing using the same basic profile as they use.
The point is not the horizontal speed - we know that deorbiting works. The point is the combination of horizontal and vertical speed, making the deceleration much quicker and harder (several g, probably with a peak beyond 10, even more if you include vibrations). Even without a crew this is a serious issue. Sure, you can get rid of this 1km/s with a rocket to get a flatter landing profile, but then you lose payload mass again.

It's been a while since I took E&M, so I'm a little rusty...but if the rocket is set up as a giant bar magnet and is run through the center of a solenoid with a uniform field running in the opposite direction, won't that do the trick?
No. A magnet in a uniform magnetic field does not feel an acceleration (simple question: why should in be in one direction and not the other?).

I'm sure we could come up with current multipliers or something so that it would loop underneath with minimal field, rather than over the top. And the support structure is definitely going to be an issue...but hey, this IS a giant space station, so we're already thinking along those lines.
I don't know what you mean with current multipliers. Yes it is a giant space station, but making it a factor 2-10 heavier is an issue. Especially as mass is your dominant cost factor.

Hmm. What about a regenerative braking system that can be used as a launcher in the reverse case? Collect the kinetic energy of the system during orbital insertion and discharge it during deorbital launch. A linear inductive motor is another possibility.
A linear induction motor is basically what I proposed with the active rings.

Its very high cross-sectional density would make it much less susceptible to atmospheric drag.
Only for the timescale. Its high overall cross-section would mean significant stationkeeping effort.

peregrine_crow wrote:I wonder how existing space debris will respond to activating the gun, will it noticeably attract/repel nearby stuff?
Not in a distance of more than ~100m, so basically "no".

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Re: Orbital coilgun accelerator

Postby PM 2Ring » Tue May 06, 2014 12:47 pm UTC

stoppedcaring wrote:I doubt that the geomagnetic gradient at 100-200 km is going to be significant enough to use for any appreciable changes to the orbital velocity of a 21,000 tonne launcher, but I could be wrong.


Maybe...

http://en.wikipedia.org/wiki/Electrodynamic_tether
Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy.[1] Electric potential is generated across a conductive tether by its motion through the Earth's magnetic field.

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Re: Orbital coilgun accelerator

Postby stoppedcaring » Tue May 06, 2014 1:46 pm UTC

mfb wrote:
stoppedcaring wrote:The momentum transfer enables the coilgun to act as the second stage; since it never leaves orbit, the weight cost associated with a second stage is zero even if the fuel is not. The rocket would need substantially more fuel to achieve the same velocity with an SSTO design, which is part of what I'm trying to achieve.
You still need a fuel tank, so all you save is the rocket engine and a bit of other infrastructure.

True; if the launch platform were using the same low-specific-impulse fuel as the rocket engines, you'd only be saving the weight of the rocket engine and the weight of the added body and fuel system. Which is helpful, but not that helpful. Of course that's not all we're saving; the incredibly high specific impulse of a VASIMR thruster (which has already been successfully vacuum-tested in its launch configuration) means its fuel cost is exponentially lower.

So it's clear that even with the smallest possible launcher and the lowest possible gee-levels, we've still brought mass fraction down to a range where a single-engine-cluster, single-stage rocket is possible. That's pretty much the holy grail of launching, so that's encouraging.
I don't see this. You still need 7.31km/s with 4km track and 3g. That is nearly the full speed, especially if you add 1.5km/s for atmosphere and gravity. Sure the mass ratio is exponential in this, but still...

If your calculations hold, then a weak (~1km/s), reusable first stage would do the same. Without all those other problems of the design.

Noting the lowest-gee, shortest-launcher configuration was more proof of concept than actual proposal; the idea would be make it fully-functional as soon as possible and build larger and larger, using its launch capacity as part of the construction process.

The thing about SSTO is that we are capable of doing it...but only barely. Earth's atmosphere and gravity combine to make SSTO possible, but prohibitively expensive. The Atlas family of rockets used a 1.5-stage design where the small booster engines of Stage 0 were jettisoned (in the same way as the SLS) even though they weighed less than the total payload and could have been carried along for the ride. Of course, the Atlas only achieved this by using balloon-supported fuel tanks (their outer skin wasn't strong enough to support their weight, so they relied on the positive pressure of the fuel to hold the thing up).

The point being that we only need to shave a tiny bit off of LEO velocity in order to make reusable SSTO feasible. Sure, we could build a weak reusable first stage to do the same trick, but only at that particular level. The launch system would be primarily for launching hardened payloads, and it would get much bigger than just 4 km long. The point is to enable 3g launch with SSTO boost at very early in construction.

The point is not the horizontal speed - we know that deorbiting works. The point is the combination of horizontal and vertical speed, making the deceleration much quicker and harder (several g, probably with a peak beyond 10, even more if you include vibrations).

What I should have pointed out is that 400 km is not the release point. We'd be looking at something closer to 100-150 km, which means 2.3 km/s less than dropping from 400 km. Not so much of a long-drop-and-sudden-stop scenario. Again, I don't know the horizontal velocity or altitude of Falcon 9's booster separation, but clearly they figured out a way to deal with it.

Its very high cross-sectional density would make it much less susceptible to atmospheric drag.
Only for the timescale. Its high overall cross-section would mean significant stationkeeping effort.

Let's see...the ISS has a cross-sectional area of roughly 2,000 square meters (this is an approximation, but bear with me) and a mass of 450 tonnes, making its maximum cross-sectional density around 225 kg/m2. In comparison, the coilgun launcher would have a cross-sectional area probably closer to 3,750 square meters (if the front is 150x25) but a mass of 21,000 tonnes, making its cross-sectional density a whopping 5.6 tonnes/m2. According to a table here, nominal density in the thermosphere at ~425 km is three orders of magnitude lower than at ~130 km, meaning the drag deceleration on our launcher would be roughly 80 times greater than on the space station. The ISS loses about 2 m/s per month to drag deceleration, so we can expect orbital decay on our launcher of around 160 m/s per month.

PM 2Ring wrote:
stoppedcaring wrote:I doubt that the geomagnetic gradient at 100-200 km is going to be significant enough to use for any appreciable changes to the orbital velocity of a 21,000 tonne launcher, but I could be wrong.

Maybe...

http://en.wikipedia.org/wiki/Electrodynamic_tether

Just off the top of my head, I'm guessing that in order for an electrodynamic tether to appreciably move a 21,000 tonne object, it would need to be so long that you'd be better off going with a space hook from the get-go.

EDIT: I found this thread discussing the first-stage separation speed/altitude of Falcon 9's first stage. As I had suspected, the exact launch profile is tailored to the mission, so these values vary; however, separation seems to typically occur between 80 and 100 km with a downrange velocity of around 2-2.5 km/s.

The velocity change of a deorbital insertion launch would be limited by acceleration, but it would otherwise be completely controllable. Launch could take place with any exit vector needed; the craft could exit along the path of orbit, or at a depressed attitude, or at a higher attitude, all the better to control re-entry. I'm not saying that thermal re-entry would always be eliminated -- it might still be the best choice for some launch profiles -- but I'm sure there would be options. Plus, it might actually be economical to store some high-gee launched fuel onboard the station for this exact purpose.

p1t1o
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Re: Orbital coilgun accelerator

Postby p1t1o » Tue May 06, 2014 2:41 pm UTC

You seem to have done plenty of maths to back up each of your statements, and you seem to have been fairly dilligent in approaching the problem in terms of forseen difficulties (depending on how much the reader sees your solutions translating into practical realities.)

So colour me impressed.

But.

We are still talking about a very (very!) expensive, very (very!) large project. (And thats ignoring any issues related to weaponisation).

Compared to various other low-cost/low-risk projects that are maturing *today* I am extremely sceptical of the cost-benefit here.

I think you have convinced us that, technically, from an engineering perspective - this project is possible, using known physics and contemporary materials and fabrication techniques.

But then so is building a replica of the Eiffel tower on Mars.

With price-to-orbit dropping to mere hundreds of dollars per kg *today*, and further drops in price as technology matures and the markets develop (which could be significant), is this project going to have any benefit whatsoever over far simpler solutions? For this to be the next paradigm-shift in ground-to-orbit transport you are going to have to knock that down at least another order of magnitude (around the $10/kg mark) and I don't see this technique having that capability.

If you would like to tackle this problem, I would suggest that you standardise the sort of cargo you are talking about - are we talking about 10kg at a time? Or 10 tons? Or 1000tons? How big is the rocket? How much fuel? Are we going with cargo only, or passengers too? Is the station going to de-orbit assets too, or just orbital insertion? How many launches are possible per day? And probably more details too. For example, its entirely possible that it may simply be out-businessed by independant companies doing it the old way because *they don't have to book a slot to use your gun*.

At the moment this seems like the sort of project that will only be likely in a sort of "Post-Scarcity" era in which we have routine access to unlimited energy and resources. I think if you are going to lift 20,000tons of materiel into space, you'd make more money building a chain of space hotels :D

stoppedcaring
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Re: Orbital coilgun accelerator

Postby stoppedcaring » Tue May 06, 2014 5:19 pm UTC

p1t1o wrote:You seem to have done plenty of maths to back up each of your statements, and you seem to have been fairly dilligent in approaching the problem in terms of forseen difficulties (depending on how much the reader sees your solutions translating into practical realities.)

So colour me impressed.

I think you have convinced us that, technically, from an engineering perspective - this project is possible, using known physics and contemporary materials and fabrication techniques.

Well, that's the first challenge! To my knowledge, there's no other non-rocket orbital launch proposal that's immediately feasible. A short-length rotating spacehook is a possibility, but it still has way more challenges and unanswered questions to deal with. So even if it's totally unrealistic, that's still something worth coming up with.

With price-to-orbit dropping to mere hundreds of dollars per kg *today*, and further drops in price as technology matures and the markets develop (which could be significant), is this project going to have any benefit whatsoever over far simpler solutions? For this to be the next paradigm-shift in ground-to-orbit transport you are going to have to knock that down at least another order of magnitude (around the $10/kg mark) and I don't see this technique having that capability.

Hmm...mere hundreds/kg? SpaceX is promising $1100/kg once Falcon 9R is fully-functional, but the current pricetag is closer to $4200/kg. I wasn't able to find any other launches under $8k per kg. This would be really promising if I could get to under $350/kg, I think.

If you would like to tackle this problem, I would suggest that you standardise the sort of cargo you are talking about - are we talking about 10kg at a time? Or 10 tons? Or 1000tons? How big is the rocket? How much fuel? Are we going with cargo only, or passengers too?

This is a mass driver, no question about it. However, it's a mass driver that can also act as a launch-assist for other types of launches. And it's also a massive space station with awesome research opportunities.

Standardizing cargo is absolutely a worthy goal. What I talked about earlier was going with a single standard launch mass and engine cluster, but varying tank size and payload in accordance with the launch profile. A set of fully-reusable modular components fit together depending on what kind of launch you need. My target launch weight was about half the size of a Falcon 9, or 270 tonnes; payload mass would depend on maximum allowable gees.

But for the sake of getting an estimate of lower-bound launch costs, I'll run the numbers for a Falcon 9-sized launch mass so that I can use their engine specs. The Falcon 9v1.1 has a launch mass of 505 tonnes (I'm assuming that includes payload to be conservative) and puts 13.2 tonnes of payload into LEO for a cool $57 million. They claim that 9R will drop launch costs to $1100/kg, which would mean a launch pricetag of only $14.5 million. Pretty damn cheap. I'll use this number; obviously we won't be using nearly as much fuel, but fuel is cheap anyway.

Using specs for a max-gee hardened-cargo launch...the Merlin 1D engines are just under half a tonne each and it takes nine of them to lift the 505-tonne vehicle. Conservatively tripling this to account for staging and body gives us a dry vehicle mass of about 12 tonnes (liquid-fuel tanks for anything other than liquid hydrogen are typically on the order of 1-2% of their content mass, which is negligible for the purposes of this estimate). If we go with the liquid-methane fuel of the Raptor engines rather than the lower-specific-impulse kerosene of the Merlin-class engines, our ve is 4.4 km/s. This booster only needs to lift them into orbit, not get them up to speed, so aerodynamic drag will probably be lower...I'll estimate 1.4 km/s in gravity and aerodynamic drag to get up there, plus 0.8 km/s to land softly, for a required delta-v of 2.2 km/s. This gives us a mass ratio of 0.65:1, meaning that the launch mass will be only 40% fuel. Subtract this and the dry vehicle weight and you end up launching 291 tonnes.

There's also the compressed argon for the VASIMR engines on the launcher. Of course, some configurations allow for alternate momentum recovery on the launcher, but we'll assume that's not in use for the purposes of this estimate. I won't spell out the math here, but it works out to about 50 tonnes of fuel at presently-tested specific impulse on a VASIMR engine. So subtract that from our payload, and we end up with 241 tonnes of hardened payload to orbit.

For $14.5 million. That's $60 bucks per kilogram.

On the lower end of launch size, suborbital spaceplanes and reusable sounding rockets can currently carry payloads under 1 tonne to an altitude of 100-200 km for $120-160 per kilogram. Economies of scale can be expected to halve that within the next five years if a large market enough market existed, placing us pretty close to the same cost as a super-heavy rocket lift. Since we don't need great speed, other launch systems like air-breathing engines or rockoons could become cost-effective, but I won't dive into that here.

Of course this doesn't include the rather-significant costs of manning the launcher itself. But even if the price for a launch slot is three times the cost of the booster, that's still just $240/kg. More than an order of magnitude improvement over current costs, and a quarter the cost of the Falcon 9R's projections.

At the same time, the space platform can be providing launch assist to crewed and sensitive-payload lost at minimal cost, making reusable SSTO an achievable standard for ALL launches.

Is the station going to de-orbit assets too, or just orbital insertion? How many launches are possible per day?

De-orbit won't be necessary for the mass driver aspect, but de-orbiting crews and sensitive payloads at perigee will serve to help with station-keeping and thus lower fuel costs. Energy for launch is not a problem, as a regenerative brake that charges flywheels should be more than enough to power the VASIMRs. If we're using super-heavy launch as our standard, the limiting factor will probably be how long it takes the station to get back up to speed. The station can have as many megawatt-class VASIMRs as we want, in theory, but would probably need at least two or three orbits to stabilize speed. We're looking at 3-4 hours minimum between launches, or about a half-dozen launches per day at maximum. If we're putting much smaller payloads into orbit from suborbital spaceplanes or sounding rockets, momentum recovery becomes negligible and we could do a launch each orbit, up to 16 per day.

If we say that 25% uptime is maximum capacity (12 hour days, every-other-day off for maintenance) and there is a steady supply of heavy-launch rockets, we'd be launching 11,000 tonnes into space each month, netting $23 billion annually in launch slot fees. Should be more than enough for upkeep. Of course that assumes an unrealistically high demand for launch...but if you build it, they will come. :mrgreen: Next stop: Mars.

I think if you are going to lift 20,000tons of materiel into space, you'd make more money building a chain of space hotels

Hey, help build this first, then I'll give you a bulk discount on launching the parts for your space hotel chain. ;)

stoppedcaring
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Re: Orbital coilgun accelerator

Postby stoppedcaring » Wed May 07, 2014 1:39 pm UTC

A couple more challenges (and some dangerously nifty solutions) I thought of last night....

I see two main problems arising with the current configuration. The first is an issue with propulsion. It's going to take a lot of force to accelerate a 21,000 tonne ship in any reasonable timeframe. We can give it a couple of super-heavy VASIMR thruster banks, but that adds another issue: concentrating all the thrust in one area can lead to differential stresses. You'd almost want to cover the whole ship with dozens of evenly-spaced thrusters to best distribute stress and eliminate torque on the airframe.

The second has to do with where the payload ends up. Usually, a rocket will take a payload into orbit and have just enough fuel left over to park it in the desired orbit. The launcher, on the other hand, is only able to bring the payload up to orbital speed; it can't even capture the payload, let alone put it into a particular orbit. Obviously, that won't be an issue for launch assist provided to crewed flights (both low-gee and mid-gee); in those cases, the launch vehicle will retain its original engines and use them to take the crew wherever they need to go. But it's absolutely an issue for hardened payloads that are purely ballistic and have no guidance engines. It's also an issue for sensitive cargo; the launch vehicle will be able to dock with the launcher, but there won't be any way to get the cargo contents out to where they're supposed to go.

However, I think these two problems can be solved at the same time. By space tugs. The space tugs wouldn't have to be built or operated or maintained by whoever owned/ran the launcher; they would probably be third-party owned and operated. Corporations would build compact, robust shuttles with large VASIMR (or even chemical, if they want extra power) engines and some kind of tug mechanism. Each corporation would pay to launch them into orbit via the launcher and then dock them along the sides.

If you wanted to launch a satellite into a particular orbit, you'd build the satellite, then buy a slot on a scheduled launch. You'd also contract with one of the various space-tug companies to deliver your payload to its desired orbit; the fee would depend on what orbit you wanted and how fast you needed it to get there.

When the payload containing your satellite was launched through the launcher, a space tug would go out and retrieve the cargo pod and pull it into a bay. One of the tug operators would open it up, pull out your satellite, and then the tugs would drag it out to whatever orbit you wanted and return.

The space tug companies would have to pay to have fuel sent up periodically via the hardened launcher. In exchange for being allowed to dock on the sides of the launcher, they'd be expected to let their engines be used to augment the main thrusters, so as to help accelerate it after launches and reduce overall stress.

Because an individual tug won't be terribly complicated or expensive, there would be lots of competition, hopefully driving down costs and spurring on innovation. It would only take a moderate initial investment to build a couple of tugs and send them up. Plus, the tugs could be used as escape pods in case of extreme emergency. Sufficiently advanced tugs could even be used to put together larger structures.

Another issue we've already touched on is meteor and debris collision. The cross-section presented by the station is rather high. While it would have a standard Whipple shield over most of its body to protect from micrometeoroids, larger debris or larger meteors would be a threat. A particularly scifi solution: radar and lasers. Mount a couple of radar dishes to scan for any debris larger than a penny for 200 miles in every direction and mount a single high-kilowatt-class solid-state laser on the opposite side. Once a piece of debris was spotted that could conceivably threaten the integrity of the launcher, the laser would be rapidly brought to bear and either use ablation to deflect it or simply vaporize it entirely. We'd need to come up with a cooling solution, of course, but the vacuum of space would help the laser hit its target with minimal losses.

Pew pew, my friends. Pew pew.

Would a visible-light 100-kW solid-state laser be visible at 125 km? The density of the thermosphere there is 28 micrograms per cubic meter.

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Re: Orbital coilgun accelerator

Postby mfb » Wed May 07, 2014 9:15 pm UTC

stoppedcaring wrote:
mfb wrote:
stoppedcaring wrote:The momentum transfer enables the coilgun to act as the second stage; since it never leaves orbit, the weight cost associated with a second stage is zero even if the fuel is not. The rocket would need substantially more fuel to achieve the same velocity with an SSTO design, which is part of what I'm trying to achieve.
You still need a fuel tank, so all you save is the rocket engine and a bit of other infrastructure.

True; if the launch platform were using the same low-specific-impulse fuel as the rocket engines, you'd only be saving the weight of the rocket engine and the weight of the added body and fuel system. Which is helpful, but not that helpful. Of course that's not all we're saving; the incredibly high specific impulse of a VASIMR thruster (which has already been successfully vacuum-tested in its launch configuration) means its fuel cost is exponentially lower.
This discussion started with "we could even use the same rockets" I think. At least this was the point I was objecting to.

What I should have pointed out is that 400 km is not the release point. We'd be looking at something closer to 100-150 km, which means 2.3 km/s less than dropping from 400 km. Not so much of a long-drop-and-sudden-stop scenario. Again, I don't know the horizontal velocity or altitude of Falcon 9's booster separation, but clearly they figured out a way to deal with it.
How can you reduce ~1km/s by 2.3km/s? Dropping from 100-150km would make the whole thing way more manageable, but then orbital decay really gets an issue:

Let's see...the ISS has a cross-sectional area of roughly 2,000 square meters (this is an approximation, but bear with me) and a mass of 450 tonnes, making its maximum cross-sectional density around 225 kg/m2. In comparison, the coilgun launcher would have a cross-sectional area probably closer to 3,750 square meters (if the front is 150x25) but a mass of 21,000 tonnes, making its cross-sectional density a whopping 5.6 tonnes/m2. According to a table here, nominal density in the thermosphere at ~425 km is three orders of magnitude lower than at ~130 km, meaning the drag deceleration on our launcher would be roughly 80 times greater than on the space station. The ISS loses about 2 m/s per month to drag deceleration, so we can expect orbital decay on our launcher of around 160 m/s per month.
160m/s but for a mass of 21000 tonnes. Your ion drives with an exhaust velocity of 50km/s would need 0.3% of the station mass per month, or 60 tonnes. You get the same result if you save the intermediate step of dividing the value by the station mass The station mass cancels out in the effort to keep it up. I don't see how you get your area, however. Your rings have a diameter of 8 meters and are (guessed) ~50cm thick (outer radius minus inner radius)? At the typical speeds and mean free pathlengths of atoms there, this should mean drafting is a small effect. Your 400 rings all have their cross-section of ~12m^2, giving ~5000 m^2 in total, not counting support structures and solar panels.

Solar panels for energy are an issue. Using your values, to support 1m^2 of front area, you need 160m/s / 3750 * 21000 tons = 800kNs per month or 0.3 N. How much solar power do we need? 0.3N at 50km/s exhaust velocity costs 7.5kW. With ~300W/m^2 for nice solar cells, this needs 25m^2.
Your total area of solar panels oriented towards the sun has to be 25 times the total area towards the flight direction. This will be a very thin, long structure.

stoppedcaring
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Re: Orbital coilgun accelerator

Postby stoppedcaring » Wed May 07, 2014 10:02 pm UTC

mfb wrote:This discussion started with "we could even use the same rockets" I think. At least this was the point I was objecting to.

Oh, my point was merely that even if the same fuel was used, it still offers an advantage over a multistage approach because you save the weight of the stages, the weight of the extra engines, and you eliminate most of gravity drag. But of course the station wouldn't want to use the same fuel.

How can you reduce ~1km/s by 2.3km/s?

Well, I'm not sure how you got your ~1 km/s, but dropping from 400 km to 125 km will get you up to 2.3 km/s (v = sqrt(2ax)), so that at least is something you don't have to deal with.

Solar panels for energy are an issue.

Your total area of solar panels oriented towards the sun has to be 25 times the total area towards the flight direction. This will be a very thin, long structure.

Well, it was going to be a pretty long, pretty thin structure from the very beginning. Originally I was thinking of nothing but the coils themselves for the majority of the distance. Rather than coils, I'm looking at more of a flat or slightly curved launch platform without gaps. I'm targeting a width no more than 150 meters, a height no more than 25 meters, and a length of 30 km. So....yeah, long and thin. Very long and thin. Extremely long and thin. But with more than ample space for solar panels.

It's too bad that a ramscoop-to-VASIMR design isn't possible in the thermosphere. That would be pretty awesome.

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Re: Orbital coilgun accelerator

Postby Neil_Boekend » Thu May 08, 2014 6:58 am UTC

mfb wrote:Solar panels for energy are an issue. Using your values, to support 1m^2 of front area, you need 160m/s / 3750 * 21000 tons = 800kNs per month or 0.3 N. How much solar power do we need? 0.3N at 50km/s exhaust velocity costs 7.5kW. With ~300W/m^2 for nice solar cells, this needs 25m^2.
Your total area of solar panels oriented towards the sun has to be 25 times the total area towards the flight direction. This will be a very thin, long structure.

Of course it would be a long, thin structure. 8 m cross section rings in an array 4 or 8 km long! The coilgun HAS to point exactly in the direction of the orbit it sends satellites. It also HAS to move exactly in that direction, else the force would tear it apart. Thus the coilgun has to point exactly aligned with it's orbit. It's frontal cross section would be only a couple of square meters. A factor 25 is no problem.
The drag would be a reason to use a higher orbit. As it usually is.
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Re: Orbital coilgun accelerator

Postby stoppedcaring » Thu May 08, 2014 1:10 pm UTC

Neil_Boekend wrote:The coilgun HAS to point exactly in the direction of the orbit it sends satellites. It also HAS to move exactly in that direction, else the force would tear it apart. Thus the coilgun has to point exactly aligned with it's orbit. It's frontal cross section would be only a couple of square meters. A factor 25 is no problem.
The drag would be a reason to use a higher orbit. As it usually is.

A lower orbit is more attractive for two reasons: first, it lessens the risk of debris impact (because most space junk at 125 km drops out of the sky pretty quickly), and second, it makes deorbit less delta-v expensive.

Not sure if that would be enough, though. I'm guessing there would be some sort of cost/benefit analysis to find the ideal parking spot.


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