What-if 0007: "Everybody Out"

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Re: What-if 0007: Everybody Out

Postby gmalivuk » Fri Aug 17, 2012 2:30 pm UTC

HopDavid wrote:
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.
Then there's no disagreement, since I never said anything different.
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Re: What-if 0007: Everybody Out

Postby HopDavid » Fri Aug 17, 2012 2:52 pm UTC

gmalivuk wrote:
HopDavid wrote:
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.
Then there's no disagreement, since I never said anything different.


I can see the catching up part in your quote (bolded above). I guess I need special glasses to see the part about ascending the tether after catching up.

Edit: After rereading your post I see:
gmalivuk wrote: 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).


So you did mention that elevators can save propellant and multi-staging, if not energy. My bad.

Tethers have the potential to do the same. But tethers are far more plausible than a full blown space elevator. Your earlier post seems to dismiss anything that's orbiting lower and faster than geosynch.
Last edited by HopDavid on Fri Aug 17, 2012 3:37 pm UTC, edited 1 time in total.

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Re: What-if 0007: Everybody Out

Postby HopDavid » Fri Aug 17, 2012 3:18 pm UTC

Sandor wrote: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.


Correct, making a suborbital catch will steal momentum, lowering it's orbit.

There's proposed ways to restore that momentum. The electro dynamic tether runs a current along its length as it moves through earth's magnetic field. Unlike the rolling tether you mention, this is a gravity gradient stabilized tether that tends to align itself with the local vertical.

Also a tether can restore momentum lost to suborbital catches by making catches from higher orbits. For example, if Planetary Resources succeeds in snagging a water rich asteroid to high lunar orbit, it may be sending propellant to low earth orbit. The EML1 transfer ellipse in this illustration could also be a transfer orbit from EML1 to LEO:

Image

Each catch not only saves 3 km/s from propellant delivery but also restores momentum the tether's lost to suborbital catches.

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Re: What-if 0007: Everybody Out

Postby gmalivuk » Fri Aug 17, 2012 5:05 pm UTC

HopDavid wrote:Tethers have the potential to do the same. But tethers are far more plausible than a full blown space elevator. Your earlier post seems to dismiss anything that's orbiting lower and faster than geosynch.
I don't dismiss anything. I just pointed out that no alternative skyhook/tether system gets you out of the total amount of energy each unit of payload mass needs to get to a particular position and velocity. Lower-orbiting solutions may be more practical for engineering reasons, but they don't magically allow you to escape Earth's gravity with less energy than a full-on geosynchronous space elevator.
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Re: What-if 0007: Everybody Out

Postby Wnderer » Fri Aug 17, 2012 6:58 pm UTC

Here's a dumb question. Why do we need to achieve escape velocity to leave the planet? Sure you need to achieve escape velocity if you want to achieve orbit. But why do you need it to just combat the force of gravity? It's possible to build a rocket vehicle that just hovers a 100 ft above the ground. It's not in orbit but it is fighting against gravity. Well if 100ft is okay why not a 1000ft? 10000ft? A mile? 10 miles? Why can't we just build a rocket that just keeps going higher and higher?

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Re: What-if 0007: Everybody Out

Postby gmalivuk » Fri Aug 17, 2012 7:17 pm UTC

The most energy efficient way to do it is to just give something escape velocity all at once. Hovering at 1000ft requires a constant output of energy for exactly zero gain in height. Actual rockets are somewhere in between (they have to fight gravity and atmospheric drag to get up in the first place, but they try to do this as quickly as possible so they can achieve orbital speeds without wasting too much additional energy).

Basically, think of it like this: near the surface, any actual acceleration needs to be in addition to the 9.8m/s^2 needed just to stay in one place. If your actual acceleration is 1m/s^2, you have to put out enough thrust for 10.8m/s^2, and are thus wasting about 91% of your power output. If your actual acceleration is 3g, on the other hand, then you're putting out thrust for 4g and only wasting 25%. If 100g was possible, you'd only have to waste about 1% of your power output to fight Earth's gravity, and you'd only even be doing that for the 10 or so seconds it took to reach escape velocity.
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Re: What-if 0007: Everybody Out

Postby Max™ » Fri Aug 17, 2012 9:03 pm UTC

Wnderer wrote:Here's a dumb question. Why do we need to achieve escape velocity to leave the planet? Sure you need to achieve escape velocity if you want to achieve orbit. But why do you need it to just combat the force of gravity? It's possible to build a rocket vehicle that just hovers a 100 ft above the ground. It's not in orbit but it is fighting against gravity. Well if 100ft is okay why not a 1000ft? 10000ft? A mile? 10 miles? Why can't we just build a rocket that just keeps going higher and higher?

Just felt the need to add, that's actually not a dumb question, as shown by the answer it received above this post.
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Re: What-if 0007: Everybody Out

Postby HopDavid » Fri Aug 17, 2012 10:06 pm UTC

There is a math error in Randall's article:

Δv=vexhaustln(mstart/mend)
mstart and mend are the total mass of the ship+fuel before and after the burn, and vexhaust is the “exhaust velocity” of the fuel, a number that’s between 2.5-4.5 km/s for rocket fuels.
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.


mend is the dry mass of the ship after it's burned all it's propellant. So mstart = propellant mass + mend

So with substituting,
Δv=vexhaustln(mstart/mend)
becomes
Δv = vexhaustln((propellant mass + mend )/mend)
Δv/vexhaust = ln((propellant mass + mend )/mend)
eΔv/vexhaust= (propellant mass + mend )/mend
eΔv/vexhaust= (propellant mass/mend) + (mend/mend)
eΔv/vexhaust= (propellant mass/mend) + 1
eΔv/vexhaust - 1 = propellant mass/mend

so the last phase in that paragraph should be
... which gives a fuel-to-ship ratio of at least e13/4.5 - 1...

The minus 1 part doesn't make a big difference when delta V budget is high.

But with lower delta V budgets that minus 1 makes a huge difference. Say the delta V budget is 3 km/s.

Randall's equation would say propellant mass to ship mass ratio is e3/4.5 or about 2. Actually it would e3/4.5 - 1 which would be about 1.

It can also be shown his equation is wrong with this absurdity: Let's say delta V budget is zero. Then fuel mass/ship mass would be e0/4.5.
Of course e0/4.5 = e0 = 1. So to achieve a delta V of zero kilometers per second, we'd need the ship's mass in propellant!

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Re: What-if 0007: Everybody Out

Postby HopDavid » Fri Aug 17, 2012 10:13 pm UTC

gmalivuk wrote:
HopDavid wrote:Tethers have the potential to do the same. But tethers are far more plausible than a full blown space elevator. Your earlier post seems to dismiss anything that's orbiting lower and faster than geosynch.
I don't dismiss anything. I just pointed out that no alternative skyhook/tether system gets you out of the total amount of energy each unit of payload mass needs to get to a particular position and velocity. Lower-orbiting solutions may be more practical for engineering reasons, but they don't magically allow you to escape Earth's gravity with less energy than a full-on geosynchronous space elevator.


Nor have I said tethers have magical powers.

I do say tethers confer many of the same advantages of the their much larger cousin.

The big difference between tethers and a geosynchronous bean stalk: One is doable, the other only remotely plausible science fiction.

Randall's article gives the impression we'd need something like Orion or a Clarke tower to get off planet. There are much more plausible devices. We don't have to resort to outdated 1970's science fiction to imagine humans getting beyond LEO.

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Re: What-if 0007: Everybody Out

Postby KrytenKoro » Fri Aug 17, 2012 10:35 pm UTC

HopDavid wrote:
gmalivuk wrote:
HopDavid wrote:Tethers have the potential to do the same. But tethers are far more plausible than a full blown space elevator. Your earlier post seems to dismiss anything that's orbiting lower and faster than geosynch.
I don't dismiss anything. I just pointed out that no alternative skyhook/tether system gets you out of the total amount of energy each unit of payload mass needs to get to a particular position and velocity. Lower-orbiting solutions may be more practical for engineering reasons, but they don't magically allow you to escape Earth's gravity with less energy than a full-on geosynchronous space elevator.


Nor have I said tethers have magical powers.

I do say tethers confer many of the same advantages of the their much larger cousin.

The big difference between tethers and a geosynchronous bean stalk: One is doable, the other only remotely plausible science fiction.

Randall's article gives the impression we'd need something like Orion or a Clarke tower to get off planet. There are much more plausible devices. We don't have to resort to outdated 1970's science fiction to imagine humans getting beyond LEO.

That is a huge mischaracterization of the article, which is discussing the energy cost of humanity (note, not humans) leaving the planet all at once. Space elevators are presented as a lower bound, and if they can be built, they would be pretty ideal.

Noting that it would be (economically) cheaper to build tethers for small-scale use and then denigrating the article for that is...well, I don't want to say dishonest, but definitely misguided.

There is a math error in Randall's article:

This is true. However, it doesn't greatly impact Randall's point about the immensity of the energy needed.
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Re: What-if 0007: Everybody Out

Postby Max™ » Fri Aug 17, 2012 11:15 pm UTC

I'm not sure how you'd compare the energy costs of a rocket to an elevator, as we don't know what the manufacturing of elevator materials would cost, nor what it would take to move a counterweight into place, though if it is already made, then yes it wins handily.

Wouldn't it make more sense to compare rocket vs launch loops or fountains or even skytrams?

Once you get a loop established, correct me if I'm wrong keithl, you just need to counteract the losses incurred due to holding it up/launching payloads, but that is still quite a bit less than what is required for moving the same amount of mass using rockets isn't it?
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Re: What-if 0007: Everybody Out

Postby HopDavid » Fri Aug 17, 2012 11:47 pm UTC

KrytenKoro wrote:That is a huge mischaracterization of the article, which is discussing the energy cost of humanity (note, not humans) leaving the planet all at once.


Whether we're sending 4 billion humans or 1000 is immaterial. There are more plausible ways to get beyond LEO than this outdated 1970s science fiction.

KrytenKoro wrote:Noting that it would be (economically) cheaper to build tethers for small-scale use


Why would tethers be restricted to small scale use?

Tethers haven't been the only thing I've been discussing. Also propellant depots and propellant sources other than earth's surface.

KrytenKoro wrote:and then denigrating the article for that is...well, I don't want to say dishonest, but definitely misguided.


There are much more doable options for beyond LEO transportation than Dyson's Orion or a Clarke tower.

That Randall and the readers of this forum are unaware of them is deeply disappointing. It is a symptom of how deeply we've sunk into the Britney Spears zeitgeist. As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.



KrytenKoro wrote:
There is a math error in Randall's article:

This is true. However, it doesn't greatly impact Randall's point about the immensity of the energy needed.


Randall gives 13 km/s as the minimum delta V budget. As mentioned, the minus 1 doesn't affect a 13 km/s delta V budget much.

But, given orbital propellant, delta V budgets are broken into smaller legs:

Image

With a .7 or a 1.2 km/s delta V budget, Randall's error is huge.

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Re: What-if 0007: Everybody Out

Postby Wnderer » Fri Aug 17, 2012 11:56 pm UTC

gmalivuk wrote:The most energy efficient way to do it is to just give something escape velocity all at once. Hovering at 1000ft requires a constant output of energy for exactly zero gain in height. Actual rockets are somewhere in between (they have to fight gravity and atmospheric drag to get up in the first place, but they try to do this as quickly as possible so they can achieve orbital speeds without wasting too much additional energy).

Basically, think of it like this: near the surface, any actual acceleration needs to be in addition to the 9.8m/s^2 needed just to stay in one place. If your actual acceleration is 1m/s^2, you have to put out enough thrust for 10.8m/s^2, and are thus wasting about 91% of your power output. If your actual acceleration is 3g, on the other hand, then you're putting out thrust for 4g and only wasting 25%. If 100g was possible, you'd only have to waste about 1% of your power output to fight Earth's gravity, and you'd only even be doing that for the 10 or so seconds it took to reach escape velocity.


Great explanation. Thanks

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Re: What-if 0007: Everybody Out

Postby keithl » Sat Aug 18, 2012 12:14 am UTC

Max™ wrote:Wouldn't it make more sense to compare rocket vs launch loops or fountains or even skytrams?

Once you get a loop established, correct me if I'm wrong keithl, you just need to counteract the losses incurred due to holding it up/launching payloads, but that is still quite a bit less than what is required for moving the same amount of mass using rockets isn't it?


In the perfect ideal case, you can get pretty close to 1/2 mv^2 - for 12 km/s to Mars transfer, that would be 72 MJ/kg . At $0.12/kw⋅h that's $2.40 per kilogram.

In real life, most of the energy ends up heating the rotor, so multiply the above by about 2.5 ... 180 MJ/kg and $6.00 per kilogram. LEO and MEO will be cheaper.

You need to power elevators (a "mere" 50km) and run vacuum pumps, and push against drag from residual gas. Measurement, communication, control, and actuators will require power, as will the big magnets (you don't want to use superconductors, if you change the field rapidly, they quench). All these associated energy costs add up to about 300MW continuous (whether you are launching or not).

And you need delta V at the arrival end. If you are aiming for GEO, you need to add an additional 1500 meters per second at apogee to circularize the orbit. You will either need rockets for that, or some trick like http://launchloop.com/CaptureRail . Even the capture rail will need makeup delta V - either by hauling up liquid argon for a VASIMR ion engine, or sending mass down from the moon, or launching stuff back at the earth. You also need some delta V and expense for thrusters, plane changes, etc. This will probably be the most expensive, since you have to add a lot of expensive hardware per payload. Still, you might be able to get by with rubber ablation panels and lasers, both for accurate position measurement and vaporizing the ablation panels for small amounts of thrust. It's fun to imagine a vehicle that is a wooden pallet, with a magnet rail strapped to the bottom and ablation panels nailed to the sides.

So launch loops will still cost energy, just not nearly as much as a rocket. In the long run, they can launch the space power collection systems that power the loops and the robot factory systems that build both. Over time, the energy just gets cheaper and cheaper.

Rockets could be a lot cheaper, too. The fuel cost for a rocket is a small fraction of the cost of a launch. The biggest cost is logistics, which is where I expect SpaceX automation to excel. Elon Musk automated banking, and to date the black hats haven't taken all his money away - rockets should be easier. The key to all of this is relentless pursuit of quality and automated logistics. I have a vision of a fully automated SpaceX, with a mechanic to fix the automated factory, a range safety officer who is also the janitor between launches, and (of course) the sales weenie.

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Re: What-if 0007: Everybody Out

Postby gmalivuk » Sat Aug 18, 2012 1:28 am UTC

HopDavid wrote:That Randall and the readers of this forum are unaware of them is deeply disappointing. It is a symptom of how deeply we've sunk into the Britney Spears zeitgeist. As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.
Oh give us a fucking break.
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Re: What-if 0007: Everybody Out

Postby Wes Janson » Sat Aug 18, 2012 4:38 am UTC

leeharveyosmond wrote:
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.


This was essentially my analysis of George Dyson's book as well. There's a fair amount of widely-disseminated information available on fission/fusion bomb design out there, which makes me deeply suspicious of what we aren't told about. Chief amongst them would be things like Dyson's shaped-charge warheads. I don't think that's why NASA and the USAF abandoned Orion, but it would partially account for the lack of impetus towards resurrecting his ideas.

The point missed by the majority of posters in this thread and in most space-based threads is this:

When it comes to the future of space exploration and humanity, the single most important obstacle to boot-stepping our way into space is getting mass out of our gravity well.

The only currently-known technology capable of launching very large masses into orbit efficiently using plausible technology and materials is a bomb-based Orion derivative.

Thus, if we want to get serious about space exploration...we need Orion. It's not a matter of choice. It's about destiny, and whether we're going to spend the next hundred years sending glorified RC cars to our next-door neighbor, or if we're going to actually get out there and colonise the fucking universe. We can do it. The technology exists, the money is available, the only true obstacle that remains is public opinion. The rest is just getting out of the way of the engineers and letting them build a few miracles.

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Re: What-if 0007: Everybody Out

Postby HopDavid » Sat Aug 18, 2012 4:11 pm UTC

gmalivuk wrote:
HopDavid wrote:That Randall and the readers of this forum are unaware of them is deeply disappointing. It is a symptom of how deeply we've sunk into the Britney Spears zeitgeist. As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.
Oh give us a fucking break.


Routes to space range from the doable to wildly implausible. Orions and bean stalks belong to the latter category.

The plausible proposals are a cry in the wilderness. They are drowned out by the din of wildly unlikely science fiction. And so we will remain below LEO.

Until you educate yourselves, you remain part of the problem. You don't deserve a fucking break.

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Re: What-if 0007: Everybody Out

Postby Max™ » Sat Aug 18, 2012 4:26 pm UTC

HopDavid wrote:Routes to space range from the doable to wildly implausible. Orions and bean stalks belong to the latter category.

The plausible proposals are a cry in the wilderness. They are drowned out by the din of wildly unlikely science fiction. And so we will remain below LEO.

Until you educate yourselves, you remain part of the problem. You don't deserve a fucking break.


Is this one, in your esteemed opinion:
Image

Cause there is quite a bit of thought put into ideas like that and others done by the guy who posted literally three posts before the one I quoted... perhaps when you toss around calls to "educate yourselves" and use "you" broadly, you should check the rest of the thread, Mr. Smartypants.
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Re: What-if 0007: Everybody Out

Postby gmalivuk » Sat Aug 18, 2012 6:48 pm UTC

HopDavid wrote:
gmalivuk wrote:
HopDavid wrote:That Randall and the readers of this forum are unaware of them is deeply disappointing. It is a symptom of how deeply we've sunk into the Britney Spears zeitgeist. As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.
Oh give us a fucking break.


Routes to space range from the doable to wildly implausible. Orions and bean stalks belong to the latter category.

The plausible proposals are a cry in the wilderness. They are drowned out by the din of wildly unlikely science fiction. And so we will remain below LEO.

Until you educate yourselves, you remain part of the problem. You don't deserve a fucking break.
I meant give us a break in your overly broad generalizations about everyone else on this forum, plus Randall. Instead of acting like you're the only intelligent person in a world of sheep, then, give us a fucking break, and accept that you are not the smartest or most knowledgeable person on this forum, or even in this thread.
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Re: What-if 0007: Everybody Out

Postby KrytenKoro » Sat Aug 18, 2012 7:03 pm UTC

HopDavid wrote:
gmalivuk wrote:
HopDavid wrote:That Randall and the readers of this forum are unaware of them is deeply disappointing. It is a symptom of how deeply we've sunk into the Britney Spears zeitgeist. As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.
Oh give us a fucking break.


Routes to space range from the doable to wildly implausible. Orions and bean stalks belong to the latter category.

The plausible proposals are a cry in the wilderness. They are drowned out by the din of wildly unlikely science fiction. And so we will remain below LEO.

Until you educate yourselves, you remain part of the problem. You don't deserve a fucking break.

No, seriously, you're whining that the article about the energy cost of moving humanity, when depicting the lower bound, doesn't talk about your higher-cost system that would be somewhat easier to build if you're using it on low numbers of people. That's completely irrelevant to the discussion, which is lowest bound. I'm pretty sure everyone here has at least heard about tethers, even if they're not familiar with them, but most of us aren't complaining about it because we actually understood the topic of the article.

For analogy:

Q: What would be the least amount of only vegetables someone would need to survive for a year?
Randall: Well, the lowest bound would be:...
You: Randall is so stupid, because he didn't talk about how if you add some meat you would have a more well-rounded diet!

Also:
Routes to space range from the doable to wildly implausible. Orions and bean stalks belong to the latter category.

You might have missed this:
The second approach is nuclear pulse propulsion, a surprisingly plausible method for getting huge amounts of material moving really fast.

Emphasis mine. Orion is not about just getting off Earth. It's about going fast enough to get anywhere else in the universe before you die. Unless stuff like at-will wormholes are invented, it simply isn't possible to get to Alpha Centauri without using nuclear if not anti-matter propulsion, within a lifetime.
From the elegant yelling of this compelling dispute comes the ghastly suspicion my opposition's a fruit.

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Re: What-if 0007: Everybody Out

Postby keithl » Sun Aug 19, 2012 12:47 am UTC

Arguments about Randall, plausibility, etc.

Hey folks, we are pretty much on the same team here. We want to go right now. We can't pay for it. Let's not take out our frustrations on each other.

Option 1: Get very rich, earn lots of discretionary dollars, create space systems a la Elon Musk or just buy a trip like Mark Shuttleworth. Requires discipline, market focus, decades of long hours, luck.

Option 2: Learn lots. Do the math. Do small experiments. Write papers, contribute to public wikis, solve problems. Reduce the barrier for whoever succeeds with option 1. Also requires discipline and long hours, somewhat less luck.

Option 3: Read SF. Go to movies and watch TV. Live in your parent's basement. Complain that whoever seems to have lots of money isn't spending it on space. Make zero progress. Get old, and watch your dreams slip away.

No guarantees, but trying for option 1 (and probably failing) or option 2 (getting something small but useful done) seem more useful than option 3. If you want something, work for it. There is glory in success, but there is honor in making progress.

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Re: What-if 0007: Everybody Out

Postby HopDavid » Sun Aug 19, 2012 1:24 am UTC

KrytenKoro wrote:For analogy:

Q: What would be the least amount of only vegetables someone would need to survive for a year?
Randall: Well, the lowest bound would be:...
You: Randall is so stupid, because he didn't talk about how if you add some meat you would have a more well-rounded diet!


A number of topics were covered in his article.

Paragraphs 1 through 6 talk about energy requirements.

At paragraph 7 he starts talking about ways to get people off planet. Paragraphs 7 through 12 he talks about rockets and the Tsiolkovsky rocket equation. Some of this discussion is wrong.

Paragraphs 13-17 he talks about other options than rockets. It is paragraphs 13 through 17 where he extols the virtues of the Celebrity Pure Foods Diet, to borrow your awkward analogy.

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Re: What-if 0007: Everybody Out

Postby KrytenKoro » Sun Aug 19, 2012 1:34 am UTC

HopDavid wrote:
KrytenKoro wrote:For analogy:

Q: What would be the least amount of only vegetables someone would need to survive for a year?
Randall: Well, the lowest bound would be:...
You: Randall is so stupid, because he didn't talk about how if you add some meat you would have a more well-rounded diet!


A number of topics were covered in his article.

Paragraphs 1 through 6 talk about energy requirements.

At paragraph 7 he starts talking about ways to get people off planet. Paragraphs 7 through 12 he talks about rockets and the Tsiolkovsky rocket equation. Some of this discussion is wrong.

Paragraphs 13-17 he talks about other options than rockets. It is paragraphs 13 through 17 where he extols the virtues of the Celebrity Pure Foods Diet, to borrow your awkward analogy.

...see, when the article is an answer to the question:
Is there enough energy to move the entire current human population off-planet?


And Randall follows up "Rockets would require a shit-ton of energy" with "these methods would be more efficient", and one of them is an idealized lower bound...and then he summarizes with "yes, there is enough energy, but it would require almost all we have".

...I kind of think that, yeah, his discussion was about how much energy it would take, and that that's why he brought up the space elevator.

Again, given that you're woefully wrong in asserting that the rest of us are trash-tv-loving troglodytes who've never even heard of tethers, I think maybe you need to step back and just, you know, reread the article. You are totally misrepresenting the point of the article and its contents, and you've been doing it enough that it almost seems you're doing it on purpose to pick a fight.
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Re: What-if 0007: Everybody Out

Postby Max™ » Sun Aug 19, 2012 1:37 am UTC

Picking a fight on the xkcd forums, no less... that's like walking into a high school AP Calculus Class and shouting "LET EPSILON BE LESS THAN ZERO!" isn't it?
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Re: What-if 0007: Everybody Out

Postby HopDavid » Sun Aug 19, 2012 1:41 am UTC

Max™ wrote:
HopDavid wrote:Routes to space range from the doable to wildly implausible. Orions and bean stalks belong to the latter category.

The plausible proposals are a cry in the wilderness. They are drowned out by the din of wildly unlikely science fiction. And so we will remain below LEO.

Until you educate yourselves, you remain part of the problem. You don't deserve a fucking break.


Is this one, in your esteemed opinion:
Image

Cause there is quite a bit of thought put into ideas like that and others done by the guy who posted literally three posts before the one I quoted... perhaps when you toss around calls to "educate yourselves" and use "you" broadly, you should check the rest of the thread, Mr. Smartypants.


Well, it's only 1/4 the length of the Great Wall of China. But it's about 100 times taller than Burj Khalifa, the world's tallest building. Sorry, not plausible.

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Re: What-if 0007: Everybody Out

Postby HopDavid » Sun Aug 19, 2012 2:58 am UTC

Max™ wrote:I'm not sure how you'd compare the energy costs of a rocket to an elevator, as we don't know what the manufacturing of elevator materials would cost, nor what it would take to move a counterweight into place, though if it is already made, then yes it wins handily.


Not necessarily

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Re: What-if 0007: Everybody Out

Postby HopDavid » Sun Aug 19, 2012 3:07 am UTC

KrytenKoro wrote:
eran_rathan wrote:Earth's escape velocity is close to 11 km/sec (depending on location and direction of launch - the closer to the Equator you are, the lower the velocity, plus you can pick up some speed by pointing east, essentially adding the speed of the rotation of the Earth to your velocity).

Yeah, I uh, googled it instead of rerunning the calculations.

This.

So, lesson learned. Google doesn't know escape velocities.


Google does know escape velocities. Following link: 6,951 miles/s (11,190 km/s)

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Re: What-if 0007: Everybody Out

Postby KrytenKoro » Sun Aug 19, 2012 4:09 am UTC

HopDavid wrote:
KrytenKoro wrote:
eran_rathan wrote:Earth's escape velocity is close to 11 km/sec (depending on location and direction of launch - the closer to the Equator you are, the lower the velocity, plus you can pick up some speed by pointing east, essentially adding the speed of the rotation of the Earth to your velocity).

Yeah, I uh, googled it instead of rerunning the calculations.

This.

So, lesson learned. Google doesn't know escape velocities.


Google does know escape velocities. Following link: 6,951 miles/s (11,190 km/s)


Ah, well screw me then.
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Re: What-if 0007: Everybody Out

Postby Max™ » Sun Aug 19, 2012 4:49 am UTC

HopDavid wrote:Well, it's only 1/4 the length of the Great Wall of China. But it's about 100 times taller than Burj Khalifa, the world's tallest building. Sorry, not plausible.

So you dismissed it without knowing anything about it at all?

It is actually built as a floating structure, is it a megastructure project? Yes, but it's closer to building a bunch of high tech oil rigs pipelines and LHC tunnel segments than the Burj. After activating it, the structure lifts itself using the same principles that would be involved for getting payloads to orbit.
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Re: What-if 0007: Everybody Out

Postby keithl » Sun Aug 19, 2012 5:43 am UTC

HopDavid wrote:
Well, it's only 1/4 the length of the Great Wall of China. But it's about 100 times taller than Burj Khalifa, the world's tallest building. Sorry, not plausible.


Look at the website (http://launchloop.com), read the paper, do the math. If it still isn't plausible to you, I would love to hear an informed opinion why. But if your reason is "it doesn't resemble stuff I already know about", then it would help to learn about the history of engineering, and be amazed at what technological imagination can make possible.

Before it was built, the Great Wall of China was implausible. The 828 meter Burj Khalifa, with construction started in 2004, would have been implausible in 1904, when the world's tallest building was the Philadelphia City Hall at 167 meters. All are compression structures.

But compression is not the only way to hold stuff up - consider the airplane, which is a structure held up by deflected mass. A stream of airplanes, nose to tail, takeoff to landing, could be made into a very strange, very tall structure. One of the taller "structures" of recent times was the helicopter and 1 km tether that Ben Shelef arranged for the 2009 space elevator games, which my friend Jordin Kare won with his multi-kilowatt laser.

The launch loop is a "dynamic structure". One example of a dynamic structure is a firehose stiffened by moving water. Fast moving water can hold the hose up many meters, while the same water, uncontained and unmoving, forms a shallow puddle. The launch loop contains a stream of iron moving very fast, supporting a narrow (10 cm diameter) vacuum enclosure with actively controlled magnets. Seems crazy until you do the math. But then, the New York Times couldn't do the math in 1920 when it claimed Goddard's rockets needed something to push against.

Here's an amusing small scale demo of a dynamic structure by a student in Virginia. http://launchloop.com/EugeneKovalenko . His loop rose only about a meter, but at 1:80000 scale, it is closer to a practical launch loop than Ben's kilometer tether is to a space elevator with a 100,000km counterweight. Eugene got all the pieces at Home Depot (and rented the motor tool). Somewhat cheaper than Ben's multimillion dollar setup.

The Curiosity landing was one of the most implausible things I've ever witnessed. But those amazing engineers at JPL did the math. The machine did, on Mars, what the calculations did in the computer years before. Launch loop will certainly be more of a challenge than Curiosity, and will rest on an equally long string of incremental accomplishments by many people over a long time. But Curiosity's story began in that field in Auburn Massachusetts in 1925, in von Neumann's Maniac computer, in Noyce's planar integrated circuit, and in many other observations, speculations, experiments, and inventions. You might not find any of those plausible before they happened, but all resulted from hard work motivated by knowledge and imagination.

Russian schoolteacher Konstantin Tsiolkovsky dreamed of many things like Curiosity in his log cabin in Kaluga - we can only hope we can be as creative and forward looking. It is OK to dream, to be unreasonable, to use you mind to see things that others haven't noticed yet. Then start analyzing and building. The way to predict the future is to invent it.

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Re: What-if 0007: Everybody Out

Postby Max™ » Sun Aug 19, 2012 6:05 am UTC

[sarcasm]Nah, heck with that, the only way to do things is my way or the highway, if you don't like it, go back to back to watching MTV![/sarcasm]
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Re: What-if 0007: Everybody Out

Postby gmalivuk » Sun Aug 19, 2012 5:49 pm UTC

HopDavid wrote:
KrytenKoro wrote:
eran_rathan wrote:Earth's escape velocity is close to 11 km/sec (depending on location and direction of launch - the closer to the Equator you are, the lower the velocity, plus you can pick up some speed by pointing east, essentially adding the speed of the rotation of the Earth to your velocity).

Yeah, I uh, googled it instead of rerunning the calculations.

This.

So, lesson learned. Google doesn't know escape velocities.


Google does know escape velocities. Following link: 6,951 miles/s (11,190 km/s)
Um, notice the comma, while Google normally uses the American system where decimal points are periods. So it has the digits right, but is off by a factor of 1000. That's even more wrong than it looked like at first.
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Re: What-if 0007: Everybody Out

Postby Faramir » Mon Aug 20, 2012 1:29 am UTC

As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.


Not to get all philosophical, but I'm more concerned not with the arrogant tone but with the idea that Earth is merely a "cradle" which humanity will presumably outgrow. I'm perfectly happy living on "cradle" Earth and hope we never have to leave. Whatever your problems are that make you wish you could get off of Earth, I'm pretty sure that they're not going to be magically solved on a space station/the Moon/Mars/Alpha Centauri.

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Re: What-if 0007: Everybody Out

Postby KrytenKoro » Mon Aug 20, 2012 2:44 am UTC

Faramir wrote:
As we get dumber and dumber, our chances for breaking free of cradle earth grow dimmer and dimmer.


Not to get all philosophical, but I'm more concerned not with the arrogant tone but with the idea that Earth is merely a "cradle" which humanity will presumably outgrow. I'm perfectly happy living on "cradle" Earth and hope we never have to leave. Whatever your problems are that make you wish you could get off of Earth, I'm pretty sure that they're not going to be magically solved on a space station/the Moon/Mars/Alpha Centauri.


We do have rather pressing deadlines, unless we somehow learn how to switch the Sun with another star, and being able to colonize other planets has pretty obvious benefits that would make humanity as a species exponentially more long-living and well-equipped.
From the elegant yelling of this compelling dispute comes the ghastly suspicion my opposition's a fruit.

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Re: What-if 0007: Everybody Out

Postby Max™ » Mon Aug 20, 2012 8:26 am UTC

Yeah, the death of the sun isn't just "a problem", but we still have the light speed barrier and the eventual heat death rather certainly dooming us.

Maybe we'll become the Downstreamers, instead?
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Re: What-if 0007: Everybody Out

Postby eran_rathan » Mon Aug 20, 2012 5:42 pm UTC

Wes Janson wrote:The only currently-known technology capable of launching very large masses into orbit efficiently using plausible technology and materials is a bomb-based Orion derivative.


Piffle, the NERVA engines are a far better design, and far more likely to actually get built.
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Re: What-if 0007: Everybody Out

Postby Max™ » Mon Aug 20, 2012 6:29 pm UTC

eran_rathan wrote:
Wes Janson wrote:The only currently-known technology capable of launching very large masses into orbit efficiently using plausible technology and materials is a bomb-based Orion derivative.


Piffle, the NERVA engines are a far better design, and far more likely to actually get built.

Image

They're called S2 engines.
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Re: What-if 0007: Everybody Out

Postby Yakk » Mon Aug 20, 2012 6:47 pm UTC

eran_rathan wrote:
Wes Janson wrote:The only currently-known technology capable of launching very large masses into orbit efficiently using plausible technology and materials is a bomb-based Orion derivative.
Piffle, the NERVA engines are a far better design, and far more likely to actually get built.

I wasn't aware that anyone actually planned on using NERVA engines to "launch things into orbit" -- are you talking about the upper-stage Saturn rocket plan?
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Re: What-if 0007: Everybody Out

Postby eran_rathan » Mon Aug 20, 2012 6:54 pm UTC

Yakk wrote:
eran_rathan wrote:
Wes Janson wrote:The only currently-known technology capable of launching very large masses into orbit efficiently using plausible technology and materials is a bomb-based Orion derivative.
Piffle, the NERVA engines are a far better design, and far more likely to actually get built.

I wasn't aware that anyone actually planned on using NERVA engines to "launch things into orbit" -- are you talking about the upper-stage Saturn rocket plan?


They were designed to replace the J-2s, yes, but there were designs to use a similar style engine for SSTO launches - Wehner Van Braun drew up one of the earliest designs for it.


Additionally, this website has a ton of good information re: nuclear rockets.
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Re: What-if 0007: Everybody Out

Postby DennisP » Wed Aug 29, 2012 2:31 pm UTC

All you really need to do is get to low-earth orbit. Once you're there, you can use high-isp, low-thrust rockets to take you the rest of the way with much better efficiency. For bonus points, you can get your reaction mass from near-earth asteroids...those billionaire asteroid miners are hoping to start by mining water, for exactly that purpose. Or you could use a solar sail and forget about reaction mass.

The "launch loop" and various other non-rocket ideas could get us to LEO very efficiently (though not the space elevator, which wouldn't impart the sideways velocity). Failing that, we could at least go with higher-ISP rockets like the "nuclear lightbulb" or solid-core fission. Multistage rockets help, too. If one of the boron fusion projects works out (eg., polywell or focus fusion), we'll do even better.


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