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The Tyranny of the Rocket Equation (2012)

nasa.gov

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Re: The Tyranny of the Rocket Equation (2012)

#31

Post by lordnacho »

"If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."

I thought this was quite though-provoking from a Drake Equation standpoint. From what I've gleaned, the earth-like planets we know of seem to be a big bigger than Earth. Perhaps if there's civilization out there, they are hampered by the misfortune of being on a planet that's practically impossible to escape from. If they find it hard to put up a Hubble Telescope, perhaps they'll just not be as likely to bother.

My other remark is to the engineering. There's a quip that you have to be an engineer to make something that only just satisfies the requirements. Plenty of people build houses without much in the way of calculation. Even cars can be built by enthusiasts without degrees. This is what makes the space stuff such amazing engineering.

Re: The Tyranny of the Rocket Equation (2012)

#32

Post by Animats »

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) was built and tested. A Nuclear Assembly Building at Canaveral was planned. But, because the goal was so narrow ("man, moon, decade"), the solution chosen was a disposable rocket the size of a 50-story building to send an RV-sized payload to the moon.

The crash of a nuclear rocket would produce a radioactive mess. Not Chernobyl or Fukishima sized, but at least small-town sized. Launching from an isolated island would help.

Various schemes have been tried or proposed to beat the rocket equation. Launching from a balloon was tried early. Launching from an aircraft is still used by Virgin Aerospace. It helps a little.

A space vehicle that's an air-breather while it's in the atmosphere and transitions to rocket mode once out has been proposed many times, but making something that's both a rocket and an airplane is hard and adds a lot of weight. As an airplane, it has to go hypersonic to get up enough speed that it's worth doing this. Building a hypersonic aircraft is very hard; so far, only a few small demo craft have done it. The National Space Plane (hypersonic single-stage-to-orbit) was proposed in the 1980s. Ben Rich, head of the Lockheed Skunk Works and the designer of the SR-71's propulsion system, declined to let Lockheed bid on it. (His comment: "We used titanium (on the SR-71). You know anything stronger?") Remember, it has to be strong at a few thousand degrees.

The same problems apply to launch track systems. Going hypersonic near the ground is possible; the Holloman AFB test track, 50,000 feet of very straight railroad track, has been used to reach Mach 8.6. The required acceleration is about 14g. Far too much for humans.

The "space elevator" requires not only unreasonable strong materials but the ability to put so much mass in space that you wouldn't need a space elevator if you had that kind of launch capacity.

Re: The Tyranny of the Rocket Equation (2012)

#33

Post by phreeza »

has2k1 wrote: post #11
ars wrote: post #6

Lest you despair of ever making space flight routine, a rocket is not the only way to get into orbit. Virtually all of the needed velocity is tangent to the surface, not away from it. So you can accelerate the vehicle along the ground at least part of the way, and only then turn heavenward and burn fuel to get into orbit. With this boost you significantly reduce the amount of fuel needed. There are many way of doing…

Given that earth is a ball, aren't you always pointed heavenward if you are between 0-180 degrees. So you might as well start of by pointing directly heavenward, i.e 90 degrees. Plus, turning requires acceleration you cannot just capitalise on the velocity you have built up otherwise you would violate the 1st law of thermodynamics.

What really helped me understand why you do not want to go up is the following thought experiment:

Imagine you are standing on Mt Everest (and the atmosphere has magically disappeared), and you would like to shoot a bullet once all the way around the earth, how would you have to fire it? It seems pretty intuitive that shooting it up into the air won't do the trick. If you want to shoot around the earth, you need to fire in parallel to the earths surface, with a sufficiently high muzzle velocity. And really an orbit is nothing else than a shot around the earth.

Re: The Tyranny of the Rocket Equation (2012)

#34

Post by ajuc »

has2k1 wrote: post #11
ars wrote: post #6

Lest you despair of ever making space flight routine, a rocket is not the only way to get into orbit. Virtually all of the needed velocity is tangent to the surface, not away from it. So you can accelerate the vehicle along the ground at least part of the way, and only then turn heavenward and burn fuel to get into orbit. With this boost you significantly reduce the amount of fuel needed. There are many way of doing…

Given that earth is a ball, aren't you always pointed heavenward if you are between 0-180 degrees. So you might as well start of by pointing directly heavenward, i.e 90 degrees. Plus, turning requires acceleration you cannot just capitalise on the velocity you have built up otherwise you would violate the 1st law of thermodynamics.

Turning requires acceleration, but you can convert velocity direction with reasonable efficiency by pushing on something (like rails or magnetic field in a tube or just road).

Re: The Tyranny of the Rocket Equation (2012)

#35

Post by shalmanese »

ytturbed wrote: post #2

>Currently, all our human rated rocket engines use chemical reactions (combustion of a fuel and oxidizer) to produce the energy. Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began befo…

Fusion devices aren't rockets and are thus, not subject to the Tyranny of the rocket equation.

Re: The Tyranny of the Rocket Equation (2012)

#36

Post by tempestn »

tempestn wrote: post #3

> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy and hydrogen as the propellant. That was intriguing, but didn't go into detail on why a nuclear thermal rocket hasn't been tried since. The obvious explanation is that there could be serious consequences if such a rocket exploded, spreading radioactive materi…

There's a nice easy to read primer on nuclear rocketry here: http://www.lanl.gov/science/NSS/issue1_2011/story5full.shtml

Re: The Tyranny of the Rocket Equation (2012)

#37

Post by tempestn »

daeken wrote: post #10
tempestn wrote: post #3

> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy and hydrogen as the propellant. That was intriguing, but didn't go into detail on why a nuclear thermal rocket hasn't been tried since. The obvious explanation is that there could be serious consequences if such a rocket exploded, spreading radioactive materi…

> And rockets tend to explode sometimes. Chemical rockets -- the only ones we've ever actually used -- explode because that's what they're intended to do. The only difference between a successful rocket firing and a catastrophic rocket failure is the speed at which the explosion happens. A nuclear rocket engine has basically no risk of explosion; tearing itself apart at speed maybe, if the aerodynamics aren't done pr…

Normally at least the first stage of the rocket will still be chemical, so the risk of explosion is still there. Fortunately the risk of spreading radioactive material appears limited regardless.

Re: The Tyranny of the Rocket Equation (2012)

#38

Post by Sharlin »

lordnacho wrote: post #31

"If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport." I thought this was quite though-provoking from a Drake Equation standpoint. From what I've gleaned, the earth-like planets we know of seem to be a big bigger than Earth. Perhaps if there's civilization out there, they are hampered by the misfortune of being on a planet that's practically impos…

> From what I've gleaned, the earth-like planets we know of seem to be a big bigger than Earth.

This is entirely a selection effect. Big planets are easier to find, that's why we have mostly found big planets.

Re: The Tyranny of the Rocket Equation (2012)

#39

Post by has2k1 »

SamReidHughes wrote: post #28
has2k1 wrote: post #25

Earlier quoted context omitted.

I see it this way. You have to accelerate the mass to the escape velocity. You also have to achieve a net effect of being in orbit (some distance x above surface of earth). The most direct vector to that distance is perpendicular. The two combined should give you the minimal energy requirement. Any engineering (and aerodynamics) creativity cannot give you anything better.

i'll spare everybody else the snark here

Zero feelings involved, just analysis before numerical computation. The concepts need to make sense first.

Re: The Tyranny of the Rocket Equation (2012)

#40

Post by Sharlin »

shalmanese wrote: post #35
ytturbed wrote: post #2

>Currently, all our human rated rocket engines use chemical reactions (combustion of a fuel and oxidizer) to produce the energy. Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began befo…

Fusion devices aren't rockets and are thus, not subject to the Tyranny of the rocket equation.

Anything that pushes stuff out from a small hole in order to accelerate in the opposite direction is a rocket, by definition, and subject to the rocket equation. If a chemical reaction is used to create the energy to push the stuff out, it's a chemical rocket. If a nuclear reactor is used to heat the stuff so it's pushed out, it's a nuclear thermal rocket. If a nuclear reactor or solar panels are used to generate electricity that's used to push the stuff out, it's a nuclear electric or solar electric rocket respectively.
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