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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)

#91

Post by chton »

baq wrote: post #80
iliis wrote: post #72

Earlier quoted context omitted.

You might be interested in Copenhagen Suborbitals (copsub.com): A danish group trying to reach space in a DIY fashion financed by donations. It's quite impressive what they've already achieved with not that much more than duct tape!

as the article says, space != orbit. i don't see how you can get 8 km/s delta-v in a garage. edit: doesn't mean it's not interesting, but orbit is so much more challenging than just space it's not funny.

It's true that orbit is more challenging, but you have no hope of getting there if you can't do a suborbital jump first. Aiming for suborbital first might see them get a lot more funding if they succeed. You need to learn to crawl before you can run.

Re: The Tyranny of the Rocket Equation (2012)

#92

Post by Symmetry »

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…

More information than you require about different nuclear (and other) rocket designs.

http://www.projectrho.com/public_html/rocket/enginelist.php

Re: The Tyranny of the Rocket Equation (2012)

#93

Post by im2w1l »

What about firing the rocket from Jules Verne's space gun[0]? Now, I don't mean an actual gun what with the high g-forces and so, but only some machinery that gives the rocket high initial speed. Maybe a super sonic evacuated tube maglev that ends with a ramp? Put it on Mt. Everest for less air resistance.

[0] http://en.wikipedia.org/wiki/Space_gun

Re: The Tyranny of the Rocket Equation (2012)

#94

Post by Symmetry »

nraynaud wrote: post #61
MPSimmons wrote: post #58

Earlier quoted context omitted.

If you were in a vacuum, that could be a wonderfully efficient way of converting mechanical movement into kinetic energy. We have to deal with atmospheric friction, though, and the places where the cannon would launch from (the surface) has the thickest part of the atmosphere, so the most friction.

what about vacuuming the cannon? like those ping pong ball cannons?

You would also have to vacuum all the space between the cannon and space, and at that point you have a space elevator you can just climb up.

Re: The Tyranny of the Rocket Equation (2012)

#95

Post by chii »

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.

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

Re: The Tyranny of the Rocket Equation (2012)

#96

Post by Symmetry »

MPSimmons wrote: post #58
nraynaud wrote: post #45

just a noob question: why transport all the fuel, and not start from a cannon/catapult, something that make you start with some speed? edit: it's not clear, I was talking about partially using a cannon, not about a complete ballistic launch.

If you were in a vacuum, that could be a wonderfully efficient way of converting mechanical movement into kinetic energy. We have to deal with atmospheric friction, though, and the places where the cannon would launch from (the surface) has the thickest part of the atmosphere, so the most friction.

That's why some people have thought about sending a fuel factory to the Moon, refining the fuel there, and using mass drivers to send it to Earth orbit for serious interplanetary missions.

Re: The Tyranny of the Rocket Equation (2012)

#97

Post by snowwrestler »

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…

Pretty much any large rocket today has an explosive self-destruct mechanism to cover failure modes where the rocket goes in an unintended direction. So that's one reason a nuclear rocket might explode at low altitude.

Re: The Tyranny of the Rocket Equation (2012)

#98

Post by moron4hire »

yoha wrote: post #17
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…

Starting from a ground propulsion is a priori a bad idea due to friction forces in the atmosphere. An almost plausible design though, would be to have a first stage taking advantage of the atmosphere (i.e. using jet engines with oxygen from the environment). To build one's intuition about space exploration, Kerbal Space Program is an excellent start.

It's amazing how quickly and clearly KSP makes this line from the article quite evident:

>> The giant leap for mankind is not the first step on the Moon, but in attaining Earth orbit.

Re: The Tyranny of the Rocket Equation (2012)

#99

Post by Symmetry »

jordanthoms wrote: post #49
Sharlin wrote: post #40

Earlier quoted context omitted.

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 ele…

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

The rocket equation still applies, but the propellent velocity is so high that you don't a large mass fraction.

Re: The Tyranny of the Rocket Equation (2012)

#100

Post by sehugg »

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)…

NERVA was fine if you didn't start it up until you were in LEO and took care to avoid any trajectories that might involve Earth reentry. But even for manned flight outside the atmosphere, shielding was an issue and servicing it was pretty much a suicide mission. Not insurmountable problems, but more than we needed for our modest goals at the time which didn't involve going beyond the Moon.

http://www.wired.com/2012/09/nuclear-flight-system-definitio...

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