Why aren't catapults used for rockets?

I guess the efficiency of a rocket is very low at slow speeds, so why don't they build some sort catapult such as we use on aircraft carriers to accelerate the rocket? A shuttle launch cost almost $1 billion, so maybe spending $1 billion on a huge catapult to increase payload by 10% would pay back quickly?

(I'm sure there is a good reason they don't build a catapult; I'm just trying to understand what it is.)

Originally posted by cloudyday
I guess the efficiency of a rocket is very low at slow speeds, so why don't they build some sort catapult such as we use on aircraft carriers to accelerate the rocket? A shuttle launch cost almost $1 billion, so maybe spending $1 billion on a huge catapult to increase payload by 10% would pay back quickly?

(I'm sure there is a good reason they don't build a catapult; I'm just trying to understand what it is.)

They cannot produce escape velocity.

reply to post by cloudyday


Q: Why aren't catapults used for rockets?

A: Look what happened to Wile E. Coyote.


Thwack.

Originally posted by Violater1

Originally posted by cloudyday
I guess the efficiency of a rocket is very low at slow speeds, so why don't they build some sort catapult such as we use on aircraft carriers to accelerate the rocket? A shuttle launch cost almost $1 billion, so maybe spending $1 billion on a huge catapult to increase payload by 10% would pay back quickly?

(I'm sure there is a good reason they don't build a catapult; I'm just trying to understand what it is.)

They cannot produce escape velocity.

What I'm talking about is using a catapult to accelerate the rocket in the early phase where it is moving so slowly that the rocket is inefficient. Then use the rocket in the phase where it is more efficient.

There seems to be a slight problem with humans surviving the G forces which would be produced at launch.

A typical shuttle launch produced about five g's. Which was only sustained for about three minutes. This, in it's self, is not comfortable, but survivable. A catapult could produce ten to twelve g's, which would crush the bones in a human body.

There was talk, at one time, of using a "mass-driver" system for satellites but there they are too easily damaged for it to be used. They could be in a slow speed-up mode, but would require a large area for their construction and a lot of power for each launch.

I realize a "mass-driver" is not the same as a catapult. It would be a high tech equivilant.

you can read all about it here. Not with a catapult though, but with a "Space gun", which is more powerful than your catapult. You can also read about the problems with it.

en.wikipedia.org...

Of course, there is some research being done in using less acceleration over a longer time, by using linear accelerators.

Because it is cheaper and simpler and proven technology to just make the propellant tanks a little bigger. Thus catapults dont make much sense unless the speed from catapult launch is at least several kilometers per second, as in StarTram concept or Launch Loop.

Originally posted by Maslo
Because it is cheaper and simpler and proven technology to just make the propellant tanks a little bigger. Thus catapults dont make much sense unless the speed from catapult launch is at least several kilometers per second, as in StarTram concept or Launch Loop.

edit on 24/3/12 by Maslo because: (no reason given)

Can you explain the reason the speed must be several km/sec before it is cost effective? I'm sure the rocket efficiency doesn't improve much until the speed is several km/sec, but even if the catapult could save 5 seconds of fuel it might be quite a bit of extra payload mass. (I think about this whenever I watch a rocket creeping slowly off the launch pad.)

The other possibility I've wondered about is a giant balloon. Float the rocket as high as the balloon will go and then turn on the rocket for the remainder.

Again, I'm sure rocket launches are optimal design already. I'm just trying to understand why these ideas don't work.

Oh, right. I forgot. No school today.

Originally posted by Aliensun
Oh, right. I forgot. No school today.

"mood: misanthropic" hmmmm

Originally posted by cloudyday

Can you explain the reason the speed must be several km/sec before it is cost effective? I'm sure the rocket efficiency doesn't improve much until the speed is several km/sec, but even if the catapult could save 5 seconds of fuel it might be quite a bit of extra payload mass. (I think about this whenever I watch a rocket creeping slowly off the launch pad.)

The other possibility I've wondered about is a giant balloon. Float the rocket as high as the balloon will go and then turn on the rocket for the remainder.

Again, I'm sure rocket launches are optimal design already. I'm just trying to understand why these ideas don't work.

edit on 24-3-2012 by cloudyday because: (no reason given)

edit on 24-3-2012 by cloudyday because: (no reason given)

At several km/s I can imagine that the payload mass increase will be so great that it may be cost effective. But maybe not. Absolute cost per launch is also important, not just cost per kg, and thats where simpler system is cheaper overall.

As for the giant baloon, I also doubt the whole contraption will be worth it just to avoid air drag losses in lower atmosphere.

EDIT: I think reusable rockets and high launch rates are the most plausible way to lower launch costs. Skylon also seems promising.

reply to post by cloudyday


Alot of missle systems use an initial kinetic force to propell it into the air before deploying its main thrusters. Of course the biggest and most epic fail for the initial concept of this was the PIAT use in ww2.

Originally posted by Wertdagf
reply to post by cloudyday

 

Alot of missle systems use an initial kinetic force to propell it into the air before deploying its main thrusters. Of course the biggest and most epic fail for the initial concept of this was the PIAT use in ww2.

So I wonder why it makes sense for these missiles and not for space launches? I suppose a missile warhead can be designed to handle 100 G's but a satellite needs to be as light as possible?