What would make a rocket sled launch feasible?

Question

Wikipedia introduces the idea of a Rocket Sled Launch (also catapult launch, ramped launch, etc) as follows:

With this concept the launch vehicle is supported by an eastward pointing rail or maglev track that goes up the side of a mountain while an externally applied force is used to accelerate the launch vehicle to a given velocity. Using an externally applied force for the initial acceleration reduces the propellant the launch vehicle needs to carry to reach orbit. This allows the launch vehicle to carry a larger payload and reduces the cost of getting to orbit. When the amount of velocity added to the launch vehicle by the ground accelerator becomes great enough, single-stage-to-orbit flight with a reusable launch vehicle becomes possible.

The idea has been used in fiction several times, but so far as I know, no one is even discussing building one. In these days of SpaceX and competitors, I'd expect that someone would be talking about it if it were feasible, so I assume that it's not currently.

Even if a direct launch to orbit from the sled isn't feasible, I'd think that this would be a efficient way to get a regular rocket into the upper atmosphere at significant velocity, instead of starting from a standing start on the ground. The rocket would then be able to push the payload the rest of the way into orbit with far less fuel than a traditional one would need.

What obstacles would a system like this have to overcome to be considered? Is it a matter of location? Materials engineering? Simply financial? Have there even been feasibility studies?

Answer 1

The main barrier to making a cost effective rocket sled launch is the atmosphere. It's a huge barrier to making a cost effective rocket sled launch for a few reasons:

1. Atmospheric drag at terrestrial altitudes reduce the effectiveness of building up speed on a terrestrial track.
2. Stresses of moving a rocket through the atmosphere at higher speeds requires stronger rocket structures that are necessarily heavier.
3. Reaching altitudes above mountain tops is a huge engineering and materials science challenge

An example is if you built a track up Mt Denali, you could easily make it up to 700 m/s at just 3 gees if you started horizontally before ascending from the base of the mountain. The problem is that that mountain is only at an altitude of 6.2km (20,300ft). Even if you got to 700 m/s at the top of mt Everest, you'd only be at 8.84km (29,000ft). To compare to a shuttle launch, at 8.84km the shuttle is moving at about 230 m/s (~750 ft/s), so 700 m/s would be about 3 times as fast and have air drag on it of 4.5 times as much (for why see this answer for a very helpful graph showing various atmospheric variables in a space shuttle launch: What barriers are there to creating a reusable rocket sled lanchpad? ). That extra drag would suck away a lot of the velocity gained via the sled, and would require a heavier rocket structured to withstand that speed at that altitude. I don't know how to estimate the extra fuel cost and rocket construction cost of a heavier rocket, so I'll have to end that thought there.

Similarly for reaching higher altitudes where the atmosphere would pose less of an issue, I don't know how to estimate the feasibility of building the structure it would take to get much higher than earth's mountains.

But lets say you didn't want a stronger, heavier rocket, and just wanted to get up to the speed the shuttle usually got to at mountain-altitude. I've been told the amount of fuel it takes to get up to orbital height is only a small fraction of the amount of fuel it takes to get up to orbital speed (8 km/s), which would mean you can't save much fuel from a mountain-altitude rocket sled. It would certainly save some, but possibly not much.

And there might be easier avenues at the moment for cost savings. Designing fully reusable vehicles might be able to save more money in launches than the minimal fuel and structure reductions a rocket sled launch could make. Also, a working scramjet engine would be able to use aircraft to get rockets up beyond the atmosphere and to orbital speed, which would replace almost all the rocket fuel needed to get to LEO with much cheaper aircraft fuel.

So in summary, the barriers to economically feasible rocket sleds are problems that have yet to be overcome in the design of rocket sled launches and more promising alternatives.

All the above said, since the atmosphere is the main barrier, on bodies like the moon with no atmosphere, a rocket sled is a much more attractive option.

Answer 2

There are two limitations here, both of which are serious:

1) As LocalFluff says, atmosphere. Any velocity component from any sort of launch track is going to be basically horizontal. If you're in vacuum and the launch velocity is a good portion of orbital velocity this would be a good thing.

In atmosphere, though, there are three cases:

1a) A fairly low launch velocity, say Mach 1. The launch impulse is basically bled away in drag. You gained nothing but lost a lot of energy while your rocket turned from horizontal to vertical. (Something that rockets don't do well.)

1b) A medium launch velocity, say Mach 10. Things get even worse--you need to beef up your rocket to handle this and you'll still bleed most of the velocity to drag.

1c) A very high launch velocity, say Mach 40 (Yes, I realize this is above orbital velocity. You'll need to be well above orbital velocity to make up for drag losses.) This could actually be of some use. It's only for payloads that can tolerate a lot of gs, both because of the launcher and the brutal deceleration that will happen when it hits the atmosphere. You'll also need a beefy launch capsule to do this, but you get to orbit for only the cost of a circularization burn. If you have enough such cargo it might make sense.

2) You have a big problem with the strength of a rocket. Rockets are normally built like eggshells--strong against the intended load, very weak against forces they aren't going to encounter. You'll need to beef your rocket up substantially so it can survive sitting there on the launch cradle before someone pushes the button.

Beefing a rocket up isn't actually that expensive construction but it's extra weight--and the rocket equation is brutal indeed. Every extra pound holding your booster together is a pound that comes off your payload. You only want to go to these costs if there is a substantial benefit to be gained.

Answer 3

A special case of a launch sled would be to incline it up the side of a mountain. At which point, the lower end would be in the atmosphere, the top end thinner due to altitude.

You would probably want to enclose it and evacuate the tube so you could accelerate through a vacuum or close to it.

Since you now have a tube, you might want to use magnetic induction to propel it. But if not, you might want to ignite something behind it, and you have a launch gun.