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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)

#41

Post by claudius »

has2k1 wrote: post #22
claudius wrote: post #16

Earlier quoted context omitted.

You are always pointed "up", but as long as you don’t accelerate too much, earth’s gravity keeps you on a circular trajectory. This is especially true if you fix the vehicle to the ground using e.g. rails. It would then be conceivable to accelerate to a high speed on ground-level rails (which is considered “free” as you can get energy delivered through the rails) until your speed is such that gravity alone does not h…

You would have to build up a lot more speed if all you are going to do is decelerate once you "take-off". Also noteworthy, once you get to the escape velocity you would require an additional force just to keep you circular.

You can still have a rocket on your vehicle to accelerate further after take-off, but you certainly need to accelerate less if you already at a certain velocity than if you are stationary. Since the “certain velocity” is free by assumption (i.e. supplied by ground-based electricty generators or something similar), I don’t see how you could be off worse. But I still didn’t do the maths fully.

Re: The Tyranny of the Rocket Equation (2012)

#42

Post by ericd »

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)…

I wouldn't dismiss the space elevator out of hand. It requires carbon nanotubes of a few meters length to achieve the required strength, and you wouldn't need to lift it pre-built - you could build it with a guideline and cable laying cars traveling up and down, adding to the cable, much like they do with suspension bridges. Long term, it seems like far and away the best approach if we can solve the materials science problem.

EDIT: I found this book to be a pretty great primer on the subject: http://www.amazon.com/Space-Elevator-Earth-Space-Transportat...

Re: The Tyranny of the Rocket Equation (2012)

#43

Post by ericd »

phreeza wrote: post #33
has2k1 wrote: post #11

Earlier quoted context omitted.

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 f…

Yep, you just need the ground to fall away from the bullet as quickly as the bullet is falling towards the ground.

Re: The Tyranny of the Rocket Equation (2012)

#44

Post by wolf550e »

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)…

A better example of air launch is Pegasus [0]. An example of an air breather that has passed the physics plausibility test is Skylon [1].

0 - https://en.wikipedia.org/wiki/Pegasus_(rocket)

1 - https://en.wikipedia.org/wiki/Skylon_(spacecraft)

Re: The Tyranny of the Rocket Equation (2012)

#46

Post by RBerenguel »

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…

I wonder if the treaty about nuclear weapons in higher atmosphere/space has something to do with it. A reactor is far from being a bomb, even if it explodes mid-air the explosion would essentially be hydrogen+oxygen (with maybe a high amount of radioactive plume, but not a proper nuclear detonation).

Re: The Tyranny of the Rocket Equation (2012)

#47

Post by knz42 »

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)…

About your last point: if you have a space elevator you can have a mass going down to balance for the mass going up, which reduces the energy needed merely to working against the friction on the cable and the connection point at the top. That's cheap.

Re: The Tyranny of the Rocket Equation (2012)

#48

Post by SamReidHughes »

has2k1 wrote: post #39
SamReidHughes wrote: post #28

Earlier quoted context omitted.

i'll spare everybody else the snark here

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

Okay. In this scenario where you accelerate upward then turn 90 degrees and accelerate horizontally, are you talking about this being powered by a rocket or by a track?

Re: The Tyranny of the Rocket Equation (2012)

#49

Post by jordanthoms »

Sharlin wrote: post #40
shalmanese wrote: post #35

Earlier quoted context omitted.

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 ele…

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

Re: The Tyranny of the Rocket Equation (2012)

#50

Post by [deleted] »

tempestn wrote: post #36
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

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