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Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 2:10 am UTC
by nowhereman
KrytenKoro wrote:
Well, yeah. That's an orbital elevator.


Well, my idea is more like a mass driver... only more human friendly. As for charges of infeasablility regarding g-forces. As mentioned before, the vehicle can be inclined gradually and the track itself can be largely underground. CERN is good proof that this is technologically feasible, though i doubt it would be economically feasible.

Edited because my phone thinks it knows better than me. Dawn robot revolution!

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 2:59 am UTC
by KrytenKoro
nowhereman wrote:Well, my idea is more like a mass driver... only more human friendly.

I'm picturing it as an orbital elevator with a mass driver on the end. Is this right?

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 5:28 am UTC
by nowhereman
KrytenKoro wrote:
nowhereman wrote:Well, my idea is more like a mass driver... only more human friendly.

I'm picturing it as an orbital elevator with a mass driver on the end. Is this right?


An orbital elevator (space elevator) is a long cable and counter-weight attached to the earth at the equator. A climber (mechanical or electromagnetic) lifts a payload up past geostationary orbit. This process is slow, but energy efficient in comparison to heavy-lift rockets. For more info, read http://en.wikipedia.org/wiki/Orbital_elevator

The system I suggested (augmented by debate on this forum as my original design I thought of when I was 13 would obviously be lethal the humans), is effectively an elevated mass-driver. This driver launches a large rocket payload, infusing it with the power to leave the troposphere and stratosphere reducing the weight of fuel needed to resist the atmospheric drag, and about 15% of the velocity needed to escape earth's gravity. A rocket is still needed to finish ascension. However the amount of fuel needed is reduced and hence cheaper and lighter rocket boosters can be used. Generally this system is more expensive than a space elevator system per launch, however it is both feasible with today's technology and is much faster to use as the initial impulse would take up a span of a few minutes in the launch tube, whereas a space elevator will take hours or days for one use.

An augmentation that could be used (but might not be feasible) is that the shuttle could also have extensible wings so that forward momentum could be converted into vertical lift. If a vessel is launched westward and near the equator, this effect would be magnified tremendously. My aerodynamics however is lacking and so I don't know if this could be done without a) tearing the vessel apart or b) increasing g-forces beyond tolerable limits. Only reason I suggest this is that I hate wasted energy but am not naive enough to think that we could build the ramp steep enough to hit 60 or 85 degrees so that more of the momentum is vertical and not horizontal. If not though, then I know trig well enough to know that the loss in momentum is small enough so as not to matter anyway.

EDIT: I decided to do some math. Simplified to exclude the effects of drag (which are great) the weight of the vehicle should be about 1.005 * 10^6 kg in weight and mass (assuming similar specs to NASA's orbiter). The initial acceleration should require 1.7 gigajoules of energy. Doing a quick calculation using the quadratic drag equation (http://en.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_velocity) the first 10-30 seconds of travel should have 7.2156 * 10^7 newtons of drag force (I used .7 for Cd, sea level air pressure, and the shuttles 8.7 m diameter for cross sectional area). I have no experience with this though so check my math please. After the first 10-30 seconds of travel though, the shuttle will be above most of the atmosphere and drag will drop greatly. There! I need to sleep now. Gotta get up in 5 1/2 hours.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 7:12 am UTC
by keithl
m1el wrote:I don't know why, but there is a very effective space launch method which is not mentioned in this "what-if". And, IMO, it's easier than nuclear propulsion.
http://en.wikipedia.org/wiki/Launch_loop


I've been found out. Yeah, the guy eating the dessicant packs also wrote articles and papers about launch loop in the late 70s and early 80s. For way too much information, see http://launchloop.com or look at John Knapman's http://spacecable.org

A few of us are pushing this forward, though I spend most my time on http://server-sky.com

John and I will be at the Space Elevator Conference in Seattle in a couple of weeks - that community is starting to look at launch loop style "dynamic structures" (Bob Forward's neologism) for a "first stage" of the space elevator, raising the bottom end and the lift lasers above most of the atmosphere and a dozen show-stopping problems. The space elevator still needs unobtanium, subtracting 0.1% of the length doesn't change the tensile forces much, but the tether and climbers no longer need to be ice/wind/ground-lightning resistant, and clouds won't stop the laser beams.

I will be in Maryland three weeks from now, if anyone wants to see my canned lectures about launch loop or server sky.

Energy? The sun wastes 380 trillion terawatts on empty space. If that was converted to terrestrial electric power with 5% efficiency, and then to launch kinetic energy with 25% efficiency (advanced loop designs approach 60%), and we all brought along 10 metric tonnes of space capsule and baggage, then we could launch everyone to 12km/s Mars transfer orbit (5E21 joules kinetic energy after equatorial rotation boost) with about 1 millisecond of solar output. Of course, at a decent 2.5 gees, we would need to stretch that out to a 3000km launch path and 500 seconds or so. That's OK, loops store energy efficiently. If the waste heat dumped in the atmosphere from power conversion and launch inefficiencies is ten times the kinetic energy, the 5E22 joules will heat the atmosphere by about 9 microdegrees Kelvin. But don't forget to turn off the lights when you leave, or the Earth will soon be vaporized.

The actual implementation will be, as Bob Forward once said, "a mere engineering detail" - trivial compared to a mole of moles, or pitching a baseball at 0.9c .

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 7:41 am UTC
by VectorZero
Needs more eezo. Hopefully Curiosity discovers the archives soon.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 8:17 am UTC
by Max™
keithl wrote:
m1el wrote:I don't know why, but there is a very effective space launch method which is not mentioned in this "what-if". And, IMO, it's easier than nuclear propulsion.
http://en.wikipedia.org/wiki/Launch_loop


I've been found out. Yeah, the guy eating the dessicant packs also wrote articles and papers about launch loop in the late 70s and early 80s. For way too much information, see http://launchloop.com or look at John Knapman's http://spacecable.org

A few of us are pushing this forward, though I spend most my time on http://server-sky.com

John and I will be at the Space Elevator Conference in Seattle in a couple of weeks - that community is starting to look at launch loop style "dynamic structures" (Bob Forward's neologism) for a "first stage" of the space elevator, raising the bottom end and the lift lasers above most of the atmosphere and a dozen show-stopping problems. The space elevator still needs unobtanium, subtracting 0.1% of the length doesn't change the tensile forces much, but the tether and climbers no longer need to be ice/wind/ground-lightning resistant, and clouds won't stop the laser beams.

I will be in Maryland three weeks from now, if anyone wants to see my canned lectures about launch loop or server sky.

Energy? The sun wastes 380 trillion terawatts on empty space. If that was converted to terrestrial electric power with 5% efficiency, and then to launch kinetic energy with 25% efficiency (advanced loop designs approach 60%), and we all brought along 10 metric tonnes of space capsule and baggage, then we could launch everyone to 12km/s Mars transfer orbit (5E21 joules kinetic energy after equatorial rotation boost) with about 1 millisecond of solar output. Of course, at a decent 2.5 gees, we would need to stretch that out to a 3000km launch path and 500 seconds or so. That's OK, loops store energy efficiently. If the waste heat dumped in the atmosphere from power conversion and launch inefficiencies is ten times the kinetic energy, the 5E22 joules will heat the atmosphere by about 9 microdegrees Kelvin. But don't forget to turn off the lights when you leave, or the Earth will soon be vaporized.

The actual implementation will be, as Bob Forward once said, "a mere engineering detail" - trivial compared to a mole of moles, or pitching a baseball at 0.9c .

No shit, you're Lofstrom?

Nice to meet ya, love the idea.

I favored the loop idea as soon as I read the feasibility/cost considerations.

I like the fountain idea too, but a loop seems so much cooler.


I was about to mention the launch loop before I read the second page, btw.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 8:26 am UTC
by Clerlic
I'm curious about what would happen to the object on the other end of the space elevator as humans are climbing it. I'm thinking it would deorbit slowly and require a constant boost depending on how many people are climbing it at once. Also wouldn't all these people getting constantly getting launched off the object affect the orbit too? And in case of settling on the object, could the tether handle the object getting extra 400 billion tons of people on it?

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 8:49 am UTC
by Max™
Compared to the mass of the elevator tether itself, lifts traveling along it are fairly insignificant.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 8:56 am UTC
by Dave Rogers
I may be missing the point here, but why is there any net energy needed at all to lift an object from the Earth's surface to escape velocity using an orbital elevator, other than to lift the first load half way and to overcome any losses in the system? I thought the whole point was that the space elevator went from the surface to the point beyond geostationary orbit where the apparent outward force is 1G, so that the climber can generate energy once it gets past the geostationary point, where the net apparent force is outwards, and that this can replace the energy used in the lower half of the climb. The ultimate source of energy is therefore the rotational energy of the Earth, because a part of its angular momentum is lost when the object is released from the top end of the elevator, where it's at escape velocity.

Of course, this either requires the space elevator to be unrealistically rigid or for the orbit of the counterweight to be perfectly stable, which are no less intractable than the materials requirements. But I thought the zero energy requirement for getting to escape velocity was the whole point of the space elevator concept.

Dave

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 10:02 am UTC
by Sandor
Clerlic wrote:I'm curious about what would happen to the object on the other end of the space elevator as humans are climbing it. I'm thinking it would deorbit slowly and require a constant boost depending on how many people are climbing it at once.

In principle, yes. You need to add both potential energy (height above ground), and kinetic energy (orbital velocity) to something climbing the cable. The potential energy comes from whatever powers the climber up the cable. The kinetic energy come from the cable pushing sideways on the the climber, and is stolen from the space elevator's orbital velocity.

However, that causes the elevator to lean slightly as it's now orbiting slower than the Earth is spinning, and the Earth then drags it back up to speed. The net effect is the kinetic energy for the climber comes from the the rotational energy of the Earth (and the Earth's rotation slows slightly as a consequence). In practice the effect is small, as the climber's mass would be tiny in comparison to the elevator cable.

Clerlic wrote:Also wouldn't all these people getting constantly getting launched off the object affect the orbit too? And in case of settling on the object, could the tether handle the object getting extra 400 billion tons of people on it?

Once you reach the geostationary point on the cable, you are in orbit. You can just float away from the cable at that point. You don't exert any force on the cable, so whether it is 1 person or 6 billion makes no difference (as long as they don't all climb the cable at the same time!).

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 10:47 am UTC
by Bloopy
MTGradwell wrote:"No matter how we do it, whether we use rockets or a cannon or a space elevator, moving a 65-kilogram person—or 65 kilograms of anything—out of the Earth’s gravity well requires at least this much energy.
Gravitational potential energy=1/2*65kg*(Earth escape velocity)^2"

Not quite. First of all, suppose we need to accelerate to Earth's escape velocity. We do indeed require that much energy. However, if we start at the equator then we already have a velocity of 464 m/sec, thanks to the Earth's axial rotation. Escape velocity is 11,186 m/sec, but we only need to increase our velocity by that minus 464 i.e. 10,722 m/sec. That's why spacecraft are launched close to the equator.


With a bit of dieting and exercise, and everyone remembering to go to the toilet before they launch, the 65kg would be reduced. That's the best idea I've got!

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 3:04 pm UTC
by leeharveyosmond
put our entire nuclear arsenal in a box


Yes. I own a copy of that book, and I've read it. Lots. The guys that came up with the idea (notably Stan Ulam) were Bomb designers by day and space travel freaks in their spare time. I've come to the conclusion that they put a lot of thought into the propulsion bomb they'd need and discovered things that were interesting. Project Orion got shut down in 1965 and has long since ceased to be a secret; the bombs remain secret, but so does the sort of bomb they had in mind.

As I recall, George Dyson doesn't discuss it and I believe this is by choice. They're just atom bombs. A sketch diagram in the book has a region labelled "radiation case" which is one clue. Freeman Dyson discussed the requirement a bit in a TV interview where he expressed concern about the statistics of how many fatal cancers the fallout from a launch could be expected to cause. [There were other concerns such as bomb debris being trapped by the Van Allen belts and being returned to the atmosphere and so on.]

The requirement is for about a thousand identical bombs, selectable yield to allow for lots of smaller pulses when getting off the ground but minimum fallout (ideally none) at all yields. I think the Project Orion crew came up with a special flavour of H-bomb that remains deeply, deeply secret because of possible other applications.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 3:10 pm UTC
by Patrik3
keithl wrote:Energy? The sun wastes 380 trillion terawatts on empty space. If that was converted to terrestrial electric power with 5% efficiency,


If I recall correctly, what you're describing there is a Dyson shell?

Yeah, having one of those would be mega handy in anything we do... although, I think any biologists might cringe at the idea of blocking out our sun. Maybe if it was a Dyson hemisphere that only ever blocked the half of the sun that we never see? Although, having read Wikipedia last night, that's now sounding like a different megastructure, a Stellar Engine... and might have the unintended effect of dragging the sun away...

(Hehe, 6 posts into this topic and I'm already sounding like an expert in my own head ^^ )

I have another idea about the space elevator, though. Maybe in the future I would do more calculations to check the 'feasibility' of this, but for now I'll just tell you in description.

So, the whole reason space elevators are kind of impossible is because they need to have their centre of mass (henceforth referenced as COM) at 36 megameters above the Earth, right? And the reason for the height is because they have to be in Geosynchronous orbit.

But do they? Why can't they just be in a normal orbit?

So my idea is to build a Ring Rail inside the stratosphere, i.e. where the atmosphere is sparse enough not to cause too many 'air resistance' issues, but not too high for it to be completely impossible to build. Also, if you build outside the stratosphere apparently you have a chance of being hit by meteor debris...

This Ring Rail would sit at about... 30km above the Equator, and follow it around the entire Earth. There would be parallel magnetic tracks. Upon these tracks would sit the carts that would also form the foundations of the elevator. The COM of the elevator would be at about as high as the ISS - 380km (This data collected from http://freemars.org/jeff/speed/index.htm this site, which has a really helpful pictogram and corresponding table!) so the speed of the COM would have to be about 7.68 km/s.

The COM would orbit in the same direction as the Earth's rotation. Also, since the foundations are closer to the centre of the Earth, they would be moving much slower compared to the Earth. I'll do this calculation:

1) The velocity of the Ring Rail: v = 2(pi)r/T v = 2(pi)Radius of Earth + 30km /24 hours = 2(pi)(6378+30)*10^3/86.4*10^3 = 6.3*6408/86.4 = 466 m/s - which checks out, given Wiki's data of 465.1 m/s on the surface.

According to the website, the period of the ISS is approx. 92 mins. Whilst the COM has a tangential speed of 7.68 km/s, the foundations would not share this speed, as mentioned earlier, since they are closer to the centre. So:

2) The velocity of the foundations: v = 2(pi)(6378+30)*10^3 / 92 mins = 6.3*6408 / 5.520 = 7294 m/s

Difference in velocity between Ring Rail and foundations: 7294 - 466 = 6828 m/s or 6.8 km/s

So... still way, way faster than any other terrestrial object, but only about half of escape velocity, and almost 1km/s slower than the tangential speed of the COM. According to Wiki, the effect of sonic booms actually decreases as you push further past the sound barrier? And there is far less air to cause resistance 30km up, anyway.

The construction of the Ring Rail itself seems pretty implausible - it would have to be about 40 megameters long (Circumference of Earth) and probably need huge supports at regular intervals - although possibly the ring structure would hold itself up, and it would only need 'spokes' to tether it in place? Also, since the elevator is flying round the tracks at 6.8km/s, there'd need to be huge slip-roads like on motorways, to get the cargo up to speed before docking with the base of the elevator. The planning permission would also probably be pretty hard to get! :P

However, once in place, if this idea was feasible, we could put almost infinite elevators onto the Ring Rail - the base of each would be less than 1km, so thousands could be installed on 1 Ring Rail, and with more elevators pushing the air around the Earth, the individual air resistance for each elevator would decrease...

---

I was half way though writing this long post when, I hate to say, I thought up another idea. (aside - I'm new to these forums. I write a lot when I get going. Please inform me if I'm verging on spamming. I'm really enthusiastic but if it's filling up the topic too much I'll tone it down a little bit!)

Anyway, really briefly, here's my other idea: Magnet on the moon?

Just place a big magnet on the moon, and use it to pull objects out of the Earth. Simple.

Further, why don't we use magnets for the space elevator? The tensile strength of a string of electromagnets depends on the strength of each electromagnet... so why not intersperse steel cables with magnets? The 'gaps' between the magnets would be inefficient at achieving tensile strength, because the effect of the magnet would decrease sharply (inverse square law) as the gap becomes longer, but the space in each 'gap' would weigh nothing...

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 3:38 pm UTC
by eran_rathan
Patrik3 wrote:Magnet on the moon?

Just place a big magnet on the moon, and use it to pull objects out of the Earth. Simple.

because you said it yourself: (inverse square law)

also, steel has nowhere near the tensile strength required for a space elevator, not do silk or human hair (both of which have higher tensile strength than steel). Carbon nanotubes are the only things (thus far) that are within an order of magnitude of what we need.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 4:29 pm UTC
by Manabu
More on space elevators and other similar ideas:
http://www.orbitalvector.com/Orbital%20 ... vators.htm
http://www.orbitalvector.com/Orbital%20 ... ntains.htm
http://www.orbitalvector.com/Orbital%20 ... ethers.htm

I quite like the idea of space fountains, but some people just think of it as a gigantic penis joke... The best thing about space fountains is that it doesn't needs to go all the way to the space. It can be constructed to be "only" 2 or 10 km high. I'm not sure about the materials needed for the base, as it needs to endure the "impact" of heavy balls changing their speed vectors 180º...

nowhereman wrote:
KrytenKoro wrote:Well, my idea is more like a mass driver... only more human friendly. As for charges of infeasablility regarding g-forces. As mentioned before, the vehicle can be inclined gradually and the track itself can be largely underground. CERN is good proof that this is technologically feasible, though i doubt it would be economically feasible.

Are you considering the deceleration G-forces when the capsule leaves the vacuum chamber of the accelerator and hits the atmosphere? Or it is supposed to be 10+km high and only provide the initial boost to orbit?

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 4:33 pm UTC
by Sandor
Patrik3 wrote:The construction of the Ring Rail itself seems pretty implausible - it would have to be about 40 megameters long (Circumference of Earth) and probably need huge supports at regular intervals - although possibly the ring structure would hold itself up, and it would only need 'spokes' to tether it in place?

If it was strong enough, the ring could support itself, but it would have to be pretty strong. The ring can be thought of as two semi-circular arches, joined base to base. For a normal sized arch, it's easy to see that each base has to withstand a compression force equal to half the weight of the arch. I think this would scale fairly well up to ring rail size, so it would have to be able withstand a compression force equal to a quarter of the weight of the whole ring. That sounds infeasible to me.

Of course, you could have the ring partly supporting its own weight, and partly supported by towers. But I think the launch loop mentioned up thread is a much more practical (and neat) idea. Kudos to keithl :)

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 4:59 pm UTC
by Angry Rooster
Vroomfundel wrote: Project Orion. There is a film about it (http://www.imdb.com/title/tt1039992/) that I have been frantically looking for but failed to find - so any leads are welcome

It would appear to exist on YouTube: http://www.youtube.com/watch?v=v4k_YZAXSEI

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 5:07 pm UTC
by KrytenKoro
Patrik3 wrote:So, the whole reason space elevators are kind of impossible is because they need to have their centre of mass (henceforth referenced as COM) at 36 megameters above the Earth, right? And the reason for the height is because they have to be in Geosynchronous orbit.

But do they? Why can't they just be in a normal orbit?

Because you need certain speeds to maintain orbit and not be torn apart by gravitational torsion. If it's lower than geosynchronous, it needs to be going faster than the Earth's rotation to stay up. If it's higher, it can go slower. My immediate impression is that building a circular ring rail around the Earth that was lower than geosynchronous, AND rotated slower than the Earth itself, would tear the ring apart pretty quickly. Not to mention, I believe, the center of gravity issue, that the ring wouldn't be held up off the surface of the Earth (similar to why Dyson shells would need constant artificial correction). You might be able to get away with incomplete rings that rotate faster than the Earth, though you'd basically have to fly to catch up to the constantly moving elevators attached to it.

Using a space elevator whose center of gravity is at the geosynchronous level keeps simple orbital mechanics from ripping it apart, and allows it to remain stationary with respect to the Earth. There would be a problem with air resistance, but that would be negligible compared to the gravitational stress inflicted by other configurations.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 5:58 pm UTC
by @dmin
You have no idea how much I loved this What-if today. It has restored my faith in all that is xkcd.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 8:29 pm UTC
by keithl
KrytenKoro wrote: My immediate impression is that building a circular ring rail around the Earth that was lower than geosynchronous, AND rotated slower than the Earth itself, would tear the ring apart pretty quickly. Not to mention, I believe, the center of gravity issue, that the ring wouldn't be held up off the surface of the Earth (similar to why Dyson shells would need constant artificial correction). You might be able to get away with incomplete rings that rotate faster than the Earth, though you'd basically have to fly to catch up to the constantly moving elevators attached to it.

Using a space elevator whose center of gravity is at the geosynchronous level keeps simple orbital mechanics from ripping it apart, and allows it to remain stationary with respect to the Earth. There would be a problem with air resistance, but that would be negligible compared to the gravitational stress inflicted by other configurations.


The granddaddy of alternative launch systems was Konstantin Tsiolkovskii's tower - also going out to GEO, but using compression on a Very Big stack of unobtainium. Princeton professor Edwin Fitch Northrup, the inventor of RF induction heating, worked on coil guns for space launch in the 1930s ( and wrote a strange book, "Zero to Eighty" about it, under the pseudonym Akkad Pseudoman, now public domain and available as a PDF for 2 bucks on Lulu ). Strangely, another Princeton professor, Gerard K. O'Neill, also worked on coil guns (and called them mass drivers) in the 1970's, apparently unaware of Northrup's more extensive and better analyzed work. I added information to the Wikipedia Mass Driver article, and the Edwin Fitch Northrup article, but The Powers That Be deleted all reference to it. Sigh.

Arthur C. Clarke read Jerome Pearson's 1975 Acta Astronautica "Orbital Tower..." (space elevator) paper, and was inspired to write his 1978 novel "Fountains of Paradise". A lot of my generation read that, said "Great idea! But it will NEVER WORK" - for many more reasons besides material strength. And more than a dozen of us got busy on other ideas. Roger Arnold and Don Kingsbury were first in print with a two part Analog article about the Spaceport - an orbiting "inverse mass driver" for capturing payloads. I did some similar work, but the dynamics of tidal forces make horizontal orbiting segments unstable. Vertical, and rotating, are a different story - see Han's Moravec's rotovator, and my recent http://launchloop.com/CaptureRail .

Paul Birch wrote about orbital rings in the Journal of the British Interplanetary Society. Ken Brakke also wrote about orbital rings for the L5 News. Benoit Lebon wrote about a ring of orbiting diamagnetic "particles", and another Russian fellow (name forgotten) also wrote a ring article. Again, all this work occurred in the late 70's and early 80's, and we've found each other and kept in touch.

Rod Hyde's "Space Tower" is a vertical mass driver, launching superconducting rings and using ring deceleration forces to hold up a tower. I published a short note on the launch loop in the American Astronautical Society newsletter a little before the much more extensive Space Tower paper, and we both developed our ideas in other publications. Again, all these ideas were indirectly inspired by Jerome Pearson's article. We all later learned of Yuri Artsutanov's 1960 "V Kosmos na Electrovoze" space elevator article in the children's magazine Komsomolskaya Pravda, though that was not technical.

Ideas like this keep popping up. The major issue is stability - almost all these schemes need active control or they literally go nonlinear really fast. Earth tethered horizontal systems are (IMHO) a bit easier to stabilize with cables to the ground, while vertical systems can only go so high before the stabilization tethers become more difficult than the system they attempt to stabilize. At these scales, everything is jello and rubberbands. Precision distance measurement, realtime simulation, and active control (perhaps with solenoid actuators) are required. Fortunately, the speeds involved are only a few microns per nanosecond.

The latest entry in a long saga is the Startram out of the Johns Hopkins Applied Physics Laboratory. This is yet another "magic superconductors" idea. It is surprising that these people don't understand applied magnetics or superconductors. This post is already too long - see my writeup at http://server-sky.com/Brin2012Mar17 and the writeups it points to.

Space launch is Really Hard. Rockets benefited from 8 trillion dollars worth of military optimization, so the US and the Soviets could swap H bombs over the pole. Structure launchers won't fit in a hardened silo, and are ridiculous military systems, so teradollars will not be available for their design and deployment. Existing paying markets and fiendishly clever business plans to exploit them will be necessary before any of these alternative launchers gets built. The technical details are relatively easy, and the innumerate still get those wrong. Little boys have been drawing rocket ships in their school notebooks for a century. Only a few intensely practical organizational geniuses (like von Braun and Korolev) ever manage to pull it off, at terrible human cost.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 8:55 pm UTC
by nowhereman
Manabu wrote:
nowhereman wrote:
KrytenKoro wrote:Well, my idea is more like a mass driver... only more human friendly. As for charges of infeasablility regarding g-forces. As mentioned before, the vehicle can be inclined gradually and the track itself can be largely underground. CERN is good proof that this is technologically feasible, though i doubt it would be economically feasible.

Are you considering the deceleration G-forces when the capsule leaves the vacuum chamber of the accelerator and hits the atmosphere? Or it is supposed to be 10+km high and only provide the initial boost to orbit?


I assume that the launch platform could not exceed 1km (the height of a tall skyscraper). I originally calculated Df ~ 7.2156*10^7. This corresponds to a deceleration rate of about 7m/s^2. This is slightly less than one g. Finally a good reason for heavy rockets. Also the boosters could be fired upon exit which will decrease deceleration. The power requirement is the big problem (1.7 gigawatts).

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 10:56 pm UTC
by wbeaty
Vroomfundel wrote:Anyway, I was short of wonder-struck by the scientific revelations of this one - I could have more arrived more or less the same conclusions myself - but my fascination with atomic spaceflight was once again rekindled by the mention of Project Orion. There is a film about it (http://www.imdb.com/title/tt1039992/) that I have been frantically looking for but failed to find


BBC tv-show version on youtube?

Project Orion: To Mars by A-bomb (1hr)
http://www.youtube.com/watch?v=v4k_YZAXSEI


The best part was the bit about the rear plate having zero wear. Someone stuck a tungsten plate near a Bikini Atoll h-bomb test, found the plate a couple km away, and discovered that fingerprint grease had protected parts of the surface. So, Orion ships must include an oil-sprayer on the back, and the explosions only vaporize the top of the oil layer, not the metal. Much like a gasoline engine: you don't want the burning fuel to etch the piston surface, otherwise you'll be constantly replacing that part.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 11:16 pm UTC
by wbeaty
KrytenKoro wrote:
Patrik3 wrote: A continuous form of reaction would be very nice, yes. However, we have not been able to develop it (cold fusion) yet.


Continuous reaction is called "nuclear reactor?"

Continuous reaction with propellant is called NERVA
http://en.wikipedia.org/wiki/NERVA

Then there's the extremely nasty ground-based ramjet weapon, flies for months wo/refuel:
http://en.wikipedia.org/wiki/Project_Pluto

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 11:26 pm UTC
by Manabu
nowhereman wrote:I assume that the launch platform could not exceed 1km (the height of a tall skyscraper). I originally calculated Df ~ 7.2156*10^7. This corresponds to a deceleration rate of about 7m/s^2. This is slightly less than one g. Finally a good reason for heavy rockets. Also the boosters could be fired upon exit which will decrease deceleration. The power requirement is the big problem (1.7 gigawatts).

But this is the average deceleration rate for the first seconds, right? What would be the maximum one, right after it leaves the launch plataform? Well, if the rocket is acelerating itself when leaving the plataform, it would be a transition from high positive Gs to not so high negative Gs... I don't know... I only suspect the deceleration would be very high, as the re-entry deaceleration G-forces can easily reach 6G, and this starting in a less dense part of atmosphere.

Re: What-if 0007: Everybody Out

Posted: Wed Aug 15, 2012 11:39 pm UTC
by Wnderer
There are a bunch of science fiction movies where, because of pollution, overpopulation, or nuclear war, humanity abandons Earth.
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So the answer is that while sending one person into space is easy, getting all of us there would tax our resources to the limit and possibly destroy the planet. It’s a small step for a man, but a giant leap for mankind.


Um? The Earth is about to be destroyed so we have to abandon the Earth, but if we leave we'll use up all our resources and destroy the Earth. Is that really a problem? I mean it might put a damper on building the 'B' ship and sending away all the telephone sanitizers, but if the Sun is about to expand into a red giant and swallow the Earth, I don't think we'd be too worried about our carbon footprint.

Then again it could be a criticism of my theory, that NASA's main goal should be to depopulate the world and turn the Earth into a nature preserve by doubling the number people living in space every 25 years.

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 4:10 am UTC
by gmalivuk
Patrik3 wrote:My idea was basically to build it on the peak of an enormous artificial mountain of arches, to reduce the necessary length, and thus tension, on the cable, until it could be made out of a 'real' substance. Unfortunately, as per my edit, I've since realized that this mountain of arches is in itself much less feasible than I had originally calculated.
Even if it were totally feasible, it's not helpful, since a space elevator has to go out to geosynchronous orbit. So even a few km or even a few hundred km wouldn't significantly reduce the total length needed, or the tensile strength.

(And in response to your other post about a ring lower down than GEO: space elevators have their center of mass so high because that way they don't have to move relative to Earth's surface. At any other height, you're either wasting energy keeping it synched with Earth's rotation, or it's moving at quite a clip relative to Earth's surface and you've got to catch up to it first, which means you need exactly as much energy as getting into orbit in the first place.)

MTGradwell wrote:Of course we also need to climb up to geostationary orbit *height*. I was going to calculate the energy required for that climb, but the maths looks complicated and I'm fundamentally lazy.
The math isn't that complicated, and it would have been useful for you to have done it. Because then you would have found that it actually takes a *lot* of energy to do that. "Orbit is halfway to anywhere" is a real thing, because the energy of an orbit at a particular height is half of the energy required to escape from that height. Trading altitude for speed doesn't change this fundamental fact.

If orbit at GEO is 3066m/s, then escape from GEO is 4336m/s, which means the energy to get there from Earth's surface (where escape is at 11186m/s) is the difference in escape velocities, or 6850m/s. This is about 23.5 MJ/kg or about 1.5 GJ per person. Not counting any energy required to lift anything else along with them (such as fuel for continued ascent).

A slow trip (e.g. up a space elevator) to GEO doesn't actually require imparting any less energy on the payload, it just allows us to do it more slowly (thus using more efficient energy sources) and without having to bring any propellant along (thus obviating the need for wasteful mass ratios and multistage rockets).

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 5:11 am UTC
by nowhereman
Manabu wrote:
nowhereman wrote:I assume that the launch platform could not exceed 1km (the height of a tall skyscraper). I originally calculated Df ~ 7.2156*10^7. This corresponds to a deceleration rate of about 7m/s^2. This is slightly less than one g. Finally a good reason for heavy rockets. Also the boosters could be fired upon exit which will decrease deceleration. The power requirement is the big problem (1.7 gigawatts).

But this is the average deceleration rate for the first seconds, right? What would be the maximum one, right after it leaves the launch plataform? Well, if the rocket is acelerating itself when leaving the plataform, it would be a transition from high positive Gs to not so high negative Gs... I don't know... I only suspect the deceleration would be very high, as the re-entry deaceleration G-forces can easily reach 6G, and this starting in a less dense part of atmosphere.


From NASA's website (http://www.grc.nasa.gov/WWW/BGH/hihyper.html) re-entry is 7.82 km/s. Mach five is 1.7km/s. Once again using the quadratic drag-force equation, we can easily see why this still creates a major difference in drag on the surface vs. re-entry. The drag force of a moving object is proportional to the square of the velocity, however the drag is unit proportional to the atmospheric density. The re-entry speed is 4.6 times as fast as mach 5, which makes the square 21.2. I do not know where the shock wave of re-entry begins (from atmospheric drag as opposed to frictional heating), but it would seem from the equation that drag force is affected far more by velocity than temperature.

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 5:48 am UTC
by gmalivuk
What does that post have to do with what you quoted? Did Manabu say anything about temperature or the primary cause of drag?

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 4:26 pm UTC
by nowhereman
gmalivuk wrote:What does that post have to do with what you quoted? Did Manabu say anything about temperature or the primary cause of drag?


His point was observational. Specifically that the atmosphere is much thinner up high yet re-entry hits 6 Gs, whereas I calculated a -.8 G force from deceleration upon exiting the tube. The implication is obvious, that I had somehow messed up creating an underestimation of the g-forces involved. This is why I pointed out that velocity is a much larger determinant of drag. I could also of pointed out that cross-sectional area is just as important as velocity. So to answer you, he never directly mentioned temperature or the primary cause of drag, but did ask why I have faith in my figure. I thought my answer was adequate in defending my argument. Though I really was miffed that I forgot cross-sectional area. I mean, come on that was obvious. Yet another reason why I am not an engineer.

EDIT: Before I get slapped for not having noticed, that last line of my post was supposed to read "velocity than pressure" not "Velocity than temperature". Given that mistype, your entire post make complete sense.

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 7:04 pm UTC
by HopDavid
As mentioned, mass goes up exponentially with delta V. If hydrogen and oxygen is the propellant, each 3 km/s added to the delta V budget is a square on this chess board:

Image

However, if you could refuel along the way, you get to start over on each square on the chess board where there's a propellant depot:

Image

Planetary Resources hopes to park a propellant rich asteroid in high lunar orbit or at EML2. Having orbital propellant would break the exponent in the rocket equation. Given propellant in LEO and EML1, the delta V budgets are much less. You'd still have an ~9 km/s delta V budget to reach LEO.

The Keck Institute For Space Studies Asteroid Retrieval Feasibility Report points out many asteroids are quite close to high lunar orbit in terms of delta V. The proposed retrieval vehicle ion rocket has an exhaust velocity of 30 km/s.

The lunar cold traps are likely to have volatiles that could be made into propellant. And these are 2.5 km/s from EML2.

In summary, there's two big flaws with Randall's article. 1) There's possible extra-terrestrial propellant sources, so we're not necessarily stuck with 13 km/s (or greater) delta V budgets. 2) There are ion rockets with much higher exhaust velocity than 4.5 km/s.

Given some infrastructure, chemical is more than adequate to move about the solar system. Space elevators or Orions aren't needed.

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 7:56 pm UTC
by ekolis
Patrik3 wrote:2) (Silly) The Earth's already unsuitable to live upon, right? (according to the "What If"). So why not use nukes to blast a chunk of the moon out of orbit, and then use the landed chunk (which will form a gigantic 1000 mile high mountain) to climb into space from there?


Or just use the nuke ships as described in the article... get everyone in the ships, then so long and thanks for all the fish!

Come to think of it, this could also make for a scorched-earth strategy for interplanetary conquest: raid a planet of its valuable resources, load them into nuclear pulse powered ships, and set off for the next planet - the nukes will ensure that nobody else can use the planet for a darn long while!

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 7:58 pm UTC
by HopDavid
gmalivuk wrote:[(And in response to your other post about a ring lower down than GEO: space elevators have their center of mass so high because that way they don't have to move relative to Earth's surface. At any other height, you're either wasting energy keeping it synched with Earth's rotation, or it's moving at quite a clip relative to Earth's surface and you've got to catch up to it first, which means you need exactly as much energy as getting into orbit in the first place.)


Incorrect.

The higher a tether's orbit, the slower it moves.

For example:

Image

This tether is centered on an orbit 800 km altitude. It's angular velocity is .00104 radians/second. The tether is pictured descending to a 300 km altitude. This tether foot is moving about 7 km/s. Orbital velocity is about 8 km/s at 300 km altitude.

(7/8)^2 = ~.76

Catching up to this tether foot would take about 3/4 the energy as achieving orbit at that altitude.

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 9:41 pm UTC
by KrytenKoro
HopDavid wrote:
gmalivuk wrote:[(And in response to your other post about a ring lower down than GEO: space elevators have their center of mass so high because that way they don't have to move relative to Earth's surface. At any other height, you're either wasting energy keeping it synched with Earth's rotation, or it's moving at quite a clip relative to Earth's surface and you've got to catch up to it first, which means you need exactly as much energy as getting into orbit in the first place.)


Incorrect.

The higher a tether's orbit, the slower it moves.

For example:

Image

This tether is centered on an orbit 800 km altitude. It's angular velocity is .00104 radians/second. The tether is pictured descending to a 300 km altitude. This tether foot is moving about 7 km/s. Orbital velocity is about 8 km/s at 300 km altitude.

(7/8)^2 = ~.76

Catching up to this tether foot would take about 3/4 the energy as achieving orbit at that altitude.

Which means he is completely correct, actually. In order for the ring to stay in orbit below the geosynchronous line, it would have to be moving faster than the Earth's rotation.

And I believe he was comparing "energy to get in orbit with geosynchronous elevator" with "energy to get in orbit with otherkind elevator + energy to catch up to elevator", not just the energy to catch up to the elevator.

Re: What-if 0007: Everybody Out

Posted: Thu Aug 16, 2012 10:22 pm UTC
by HopDavid
KrytenKoro wrote:Which means he is completely correct, actually. In order for the ring to stay in orbit below the geosynchronous line, it would have to be moving faster than the Earth's rotation.


That's the first part of what he said. Of course orbits below geosynchronous move faster than earth's rotation.

But read more carefully. His statement goes on to say "or it's moving at quite a clip relative to Earth's surface and you've got to catch up to it first, which means you need exactly as much energy as getting into orbit in the first place."

Which is completely incorrect, actually.

Re: What-if 0007: Everybody Out

Posted: Fri Aug 17, 2012 12:00 am UTC
by Xaphnir
Math error in this comic. 4 gigajoules times 7 billion people is 2.8x10^19 joules, not 2.8x10^18 joules.

Re: What-if 0007: Everybody Out

Posted: Fri Aug 17, 2012 1:12 am UTC
by gmalivuk
HopDavid wrote:
KrytenKoro wrote:Which means he is completely correct, actually. In order for the ring to stay in orbit below the geosynchronous line, it would have to be moving faster than the Earth's rotation.


That's the first part of what he said. Of course orbits below geosynchronous move faster than earth's rotation.

But read more carefully. His statement goes on to say "or it's moving at quite a clip relative to Earth's surface and you've got to catch up to it first, which means you need exactly as much energy as getting into orbit in the first place."

Which is completely incorrect, actually.
What's wrong with that? Are you claiming it takes a different amount of energy to get to the base of a tether and ascend to a point in orbit than to get to that point otherwise, given that I'm only talking about the specific energy per kg of payload, rather than adding in extra for rocket propellant?

I think you missed the second part of KrytenKoro's post, which correctly interpreted my meaning, about reaching the base of the elevator *and then* going up to the point moving at orbital velocity for its height.

Re: What-if 0007: Everybody Out

Posted: Fri Aug 17, 2012 6:59 am UTC
by HopDavid
gmalivuk wrote:
HopDavid wrote:His statement goes on to say "or it's moving at quite a clip relative to Earth's surface and you've got to catch up to it first, which means you need exactly as much energy as getting into orbit in the first place."

Which is completely incorrect, actually.
What's wrong with that? Are you claiming it takes a different amount of energy to get to the base of a tether and ascend to a point in orbit than to get to that point otherwise,


No.

I am claiming it would take less energy to catch up to a tether foot than it would getting into orbit.

gmalivuk wrote:given that I'm only talking about the specific energy per kg of payload, rather than adding in extra for rocket propellant?


Propellant mass is central to the article that spawned this thread. Recall:

"What’s important is the ratio between Δv and vexhaust — the speed we want to be going compared to the speed that the propellant exits our rocket. The kilograms of fuel needed per kilogram of ship is e to the power of this number, which gets big very fast. For leaving Earth, we need a Δv of upwards of 13 km/s, and vexhaust isn’t much higher than 4.5 km/s, which gives a fuel-to-ship ratio of at least e13/4.5 ≈ 20."

If Δv can be achieved by other methods than firing reaction mass from a rocket, the number above is changed.

Re: What-if 0007: Everybody Out

Posted: Fri Aug 17, 2012 11:56 am UTC
by Exodies
Youse brutes are being held back by your obsession with a brute force departure. Why not simply leave the Higgs bosons behind, take off with the wind and pick up some new mass once in interstellar space?

Re: What-if 0007: Everybody Out

Posted: Fri Aug 17, 2012 12:07 pm UTC
by AvatarIII
Exodies wrote:Youse brutes are being held back by your obsession with a brute force departure. Why not simply leave the Higgs bosons behind, take off with the wind and pick up some new mass once in interstellar space?


you might aswell have just said "Why not open a Einstein-Rosen Bridge somewhere and simply walk off planet?"

Re: What-if 0007: Everybody Out

Posted: Fri Aug 17, 2012 2:14 pm UTC
by Sandor
gmalivuk wrote:...space elevators have their center of mass so high because that way they don't have to move relative to Earth's surface. At any other height, you're either wasting energy keeping it synched with Earth's rotation, or it's moving at quite a clip relative to Earth's surface...

For lower (and shorter) elevators, one idea is to spin the cable so it kind of "rolls" along the atmosphere, with higher the end of the cable moving at twice the orbital velocity, and the lower end almost stationary (all with respect to the Earth's surface). This is called a sky hook. The lower end dips into the atmosphere, where you can grab onto it. Doing this will steal momentum from the sky hook, so you need some mechanism to boost it so it maintains it orbit.