The Tyranny of the Rocket Equation (2012) • Hacker News Board

Re: The Tyranny of the Rocket Equation (2012)

#111

Post by keenerd » Fri, Oct 31, 2014, 3:24 PM UTC

snowwrestler wrote: post #90

rquantz wrote: post #86

Earlier quoted context omitted.

I never have understood why the space elevator gets so much attention, and the launch loop is almost unknown. The elevator obviously had some high-profile proponents (like Arthur C. Clarke), but given how realistic the launch loop seems (only 8 billion dollars??), I would think it would be the thing everyone talks about. The fact that it is not makes me think that it is less realistic or more constrained than it is m…

The launch loop must be perpetually active once established. That's a lot of continuous power needed for something we do pretty rarely--launch stuff into space. And if the power turns off, it falls back to Earth. A heavy, high-speed belt falling 50 miles down along a length of 1,200 miles...not an easy problem to solve. Even if it drops into uninhabited territory, it's not going to be in great shape. A space elevator…

If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall. This is also why most pictures show it operating over the ocean. We have a lot of ocean at the equator.

A space elevator is not passive. It needs active dampening systems to avoid resonances that would tear it apart. The failure modes for a space elevator are also pretty terrifying. The lower the break the less harmful it is, but higher breaks will cause huge amounts of damage. Worst case scenario, the whole cable comes down. That is 75000 km. (Assuming the traditional double-spool deployment.) The Earth's circumference is only 40000 km. It will wrap around the earth twice! 'Blue Mars' has more about what such a disaster might look like.

The loop is not more well known because it was only invented in 1981 and requires non-intuitive physics to explain. Space elevators are "simpler" (if you ignore the active control problems) and have been around since 1895, though the modern design dates to 1959.

Re: The Tyranny of the Rocket Equation (2012)

#112

Post by Retric » Fri, Oct 31, 2014, 3:26 PM UTC

derekp7 wrote: post #89

bcoates wrote: post #84

Earlier quoted context omitted.

Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.

I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam ove…

Your off by ~61,545 times. As, you don't want the energy to boil water, you want the energy you get from burning hydrogen.

1 liter of water is 11.19% hydrogen or ~0.1119kg. Hydrogen is 143 MJ/kg so 1 liter of water ~= 143,000,000J * 0.1119 = 16,001,700J.

So, you want a 1,661,715 megawatt laser. Good luck with that.

PS: You might be able to do laser assisted rocketry where you hit the combustion chamber with energy to increase exhaust velocity. But even that would take insane accuracy and a 100+ terawatt laser to be useful.

Re: The Tyranny of the Rocket Equation (2012)

#113

Post by nyrath » Fri, Oct 31, 2014, 3:31 PM UTC

chii wrote: post #95

dalke wrote: post #53

Earlier quoted context omitted.

The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject. Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.

is there a form of transport that is _not_ limited by the rocket equation?

Solar Sail, Laser Sail, Electric Sail, Magnetic Sail, Mini-magnetospheric plasma sail, MagBeam, Plasma Magnet Sail, Photon Drive, Bussard Ramjet, Ram-Augmented Interstellar Rocket.

Basically any spacecraft that does not carry its own propellant. Most of these have either pathetic thrust or are way beyond our current state-of-the-art.

Re: The Tyranny of the Rocket Equation (2012)

#114

Post by gizmo686 » Fri, Oct 31, 2014, 3:35 PM UTC

FranOntanaya wrote: post #57

Animats wrote: post #32

Right. This has been known for a long time. It's why rockets aren't much better than they were 40 years ago. Chemical fuels are as good as they can get. Space travel with chemical fuels is just barely feasible. In the 1960s, it was assumed that nuclear power would be necessary for space flight. Everybody involved knew the rocket equation. The original plan for Apollo included a nuclear upper stage. The engine (NERVA)…

> Space travel with chemical fuels is just barely feasible. To certain extent, that only applies while assuming rocket launches are hard to iterate. Otherwise it would be a matter of launching enough of them.

By iterate, I assume you mean a multi-stage rocket, which is, in theory, a simple solution to the problem; as well as the on only way we have actually gotten anything to space.

Another potential solution is asparagus staging, where you have multiple fuel tanks that are jettisoned when empty. Assuming you can construct a fuel cross-feed system so that you empty your outermost tanks while you inner tanks are still full, this gets you (mostly) the same benefits as an iterated rocket design. The outertanks also may have engines on them that are jettisoned when they are.

I do not believe that any rocket to date has used this system, However the SpaceX Falcon Heavy (planned to launch in 2015) includes this method. [0] As I understand it, they added a single asparagus stage (with two tanks/engines) to their first stage. Their staging sequence goes like: Fire all 3 lower engines. Jettison outer 2 lower rockets, leaving a single engine with a full tank. Jettison lower engine and fire the upper stage engine.

As a side note, Googling "asparagus staging" returns, with the exception of a single reference to the Falcon Heavy (as the 9th link), exclusively references to the game Kerbal Space Program for the first 3 pages. Tested through a Tor browser session that has not been tainted with my Kerbal related search history. However, the earliest use of the term I found (courtesy of [1]) was from 1997 [2] (Copyright page viewable from [3]). The Kerbal wiki [4] also claims that this is the first usage.

[0] http://en.wikipedia.org/wiki/Falcon_Heavy#cite_ref-nss201111... [1] http://www.reddit.com/r/KerbalSpaceProgram/comments/1b7hml/o... [2] http://books.google.com/books?id=C70gQI5ayEAC&pg=PA143&lpg=P... [3] http://www.amazon.com/gp/product/0486600610/ref=pd_lpo_sbs_d... [4]http://wiki.kerbalspaceprogram.com/wiki/Asparagus_staging

Re: The Tyranny of the Rocket Equation (2012)

#115

Post by nyrath » Fri, Oct 31, 2014, 3:35 PM UTC

dalke wrote: post #53

jordanthoms wrote: post #49

Earlier quoted context omitted.

How about http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi... ?

The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject. Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.

Agreed, but to be pedantic, it applies to all momentum machines which carry its own "propellant", not "fuel".

For instance, in a NERVA rocket, the "fuel" is the uranium rods in the reactor, the "propellant" is the liquid hydrogen that is heated and shot out the exhaust bell.

Re: The Tyranny of the Rocket Equation (2012)

#116

Post by rquantz » Fri, Oct 31, 2014, 3:52 PM UTC

snowwrestler wrote: post #90

rquantz wrote: post #86

Earlier quoted context omitted.

I never have understood why the space elevator gets so much attention, and the launch loop is almost unknown. The elevator obviously had some high-profile proponents (like Arthur C. Clarke), but given how realistic the launch loop seems (only 8 billion dollars??), I would think it would be the thing everyone talks about. The fact that it is not makes me think that it is less realistic or more constrained than it is m…

The launch loop must be perpetually active once established. That's a lot of continuous power needed for something we do pretty rarely--launch stuff into space. And if the power turns off, it falls back to Earth. A heavy, high-speed belt falling 50 miles down along a length of 1,200 miles...not an easy problem to solve. Even if it drops into uninhabited territory, it's not going to be in great shape. A space elevator…

The linked Wikipedia page addresses turning the loop on and off. In fact, it can't be run continuously because it would overheat.

Edit: actually, I may be mistaken. Here is one of the relevant passages:

"When at rest, the loop is at ground level. The rotor is then accelerated up to speed. As the rotor speed increases, it curves to form an arc. The sheath forces it to follow a curve steeper than the rotor's natural ballistic curve, which [clarification needed] , in turn, exerts a centrifugal force on the sheath, holding it aloft. The loop would be anchored to the ground to remain at a fixed height.

"Once raised, the structure requires continuous power to overcome the energy dissipated. Additional energy would be needed to power any vehicles that are launched."

So is this talking about raising it only the first time? And then it has to have continuous power for the remainder of it's operational life? It seems like having power off capability shouldn't be an insurmountable task.

Re: The Tyranny of the Rocket Equation (2012)

#117

Post by marcosdumay » Fri, Oct 31, 2014, 3:54 PM UTC

keenerd wrote: post #111

snowwrestler wrote: post #90

Earlier quoted context omitted.

The launch loop must be perpetually active once established. That's a lot of continuous power needed for something we do pretty rarely--launch stuff into space. And if the power turns off, it falls back to Earth. A heavy, high-speed belt falling 50 miles down along a length of 1,200 miles...not an easy problem to solve. Even if it drops into uninhabited territory, it's not going to be in great shape. A space elevator…

If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall. This is also why most pictures show it operating over the ocean. We have a lot of ocean at the equator. A space elevator is not passive. It needs active dampening systems to avoid resonances that would tear it apart. The failure modes for a space elevator are also pretty terrifying. The lower…

> If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall.

That's for a loop constructed only of superconductors and magnets. If you use active electromagnets instead of the superconductors for saving money (or if your superconductors get too hot), the failure mode is way more harmful.

Anyway, I really don't know why the launch loop is so overlooked. Specifically, I don't know why nobody's trying to build an intercontinental bridge. It may be because of our anti-nuclear culture, and that those things only make any sense when coupled with nuclear power, but that's just speculation.

Re: The Tyranny of the Rocket Equation (2012)

#118

Post by marcosdumay » Fri, Oct 31, 2014, 4:00 PM UTC

mikeash wrote: post #102

Animats wrote: post #32

Right. This has been known for a long time. It's why rockets aren't much better than they were 40 years ago. Chemical fuels are as good as they can get. Space travel with chemical fuels is just barely feasible. In the 1960s, it was assumed that nuclear power would be necessary for space flight. Everybody involved knew the rocket equation. The original plan for Apollo included a nuclear upper stage. The engine (NERVA)…

Everyone Knows™ that putting stuff into space is expensive. Then Everyone Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is what's expensive. If you look at the costs involved in putting something into orbit, the cost of fuel is a trivial detail, on the order of 1% of the total costs. Compare that to an airliner, where fuel is around 1/3rd of the total costs, or a car, where fuel can easi…

People have been trying to make reusable rockets since the beginning. The problem is that a reusable rocket is more complex, what means it's heavier, and that it needs much more fuel to launch, thus a bigger rocket, and the rocket equation makes everything astronomical.

Like everybody else, I cheering for SpaceX to solve this problem, but the challenge is not making a reusable rocket - it's making a light enough reusable rocket.

Re: The Tyranny of the Rocket Equation (2012)

#119

Post by derekp7 » Fri, Oct 31, 2014, 4:02 PM UTC

Retric wrote: post #112

derekp7 wrote: post #89

Earlier quoted context omitted.

I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam ove…

Your off by ~61,545 times. As, you don't want the energy to boil water, you want the energy you get from burning hydrogen. 1 liter of water is 11.19% hydrogen or ~0.1119kg. Hydrogen is 143 MJ/kg so 1 liter of water ~= 143,000,000J * 0.1119 = 16,001,700J. So, you want a 1,661,715 megawatt laser. Good luck with that. PS: You might be able to do laser assisted rocketry where you hit the combustion chamber with energy to…

Ah, I didn't do the calculations myself, just did a couple random Google searches for how many joules it takes to boil off one liter of water (not how much it takes to bring a gallon to boiling point, which is of course much much less). Here's one site I used: https://www.physicsforums.com/threads/energy-required-to-boi..., but I didn't see the "k" in front of joules. Thanks.

Re: The Tyranny of the Rocket Equation (2012)

#120

Post by mcguire » Fri, Oct 31, 2014, 4:05 PM UTC

"If a vehicle is less than 10% propellant, [c]hanges to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say."

This is why you don't let cabinet makers build ships.