# What are the practical limits of a rocket that receives propellant from the surface?

The Orion spacecraft was supposed to be propelled by igniting nuclear bombs behind it. Its proponents successfully demonstrated the concept using conventional explosives, on a small scale, for a few seconds.

My questions:

Instead of carrying propellant and energy by itself (whether in the form of a nuclear bomb or otherwise):

• can a rocket be launched from the Earth's surface by shooting propellant mass at it (either solid in the form of bombs, or fluid in the form of gas or plasma), thereby benefiting from nearly unlimited specific impulse?

• what is the maximum distance a bomb or gas jet can reach before it stops accelerating the rocket?

• I don't quite follow the question - to shoot propellant at a rocket, you need another rocket, which has the same limitations as the first one. Launching from a ground-based railgun that requires no on-board propellant is very inefficient in an atmosphere, so a rocket's the only practical option. Not sure what you're asking with the "maximum distance a bomb or gas jet could reach" - a rocket can achieve escape velocity and go infinitely far from Earth, if properly configured. Oct 23, 2020 at 13:57
• Allow me to clarify - I only think this works as a booster stage, because beyond a certain velocity you won't be able to shoot propellant at it fast enough as you say. Imagine throwing a live grenade up into a rocket nozzle - provided the nozzle is strong enough, how many grenades can we throw before the rocket has gone too high/fast? Oct 23, 2020 at 14:02
• There is no limit - if the rocket is going so fast that a grenade can't catch up with it, just throw a grenade at the grenade. You're asking if you can make a rocket that so fast that it's impossible for another rocket to catch up with it. But clearly, you have the ability to make a rocket go that fast in the first place. Oct 23, 2020 at 14:07

The showstopper for Earth launch is the atmosphere. Drag grows proportional to velocity squared, so materials accelerated up to several km/s are going to slow down very quickly. Each doubling of velocity is only going to give you the same amount of extra jet range as the previous doubling. This is a very efficient ceiling for how fast things can go inside the atmosphere.

But looking more abstractly at it beyond Earth launch, there's nothing wrong with this idea, the only limitation being relative velocity.

If the craft is travelling faster than the jet hitting it, there's no acceleration. And even before that, this has diminishing returns due to the relative velocity between the jet stream and the craft approaching zero.

So how fast can we make those jet streams?

• Conventional chemical reactions limits you to 4-5 km/s. Note how this isn't even enough to reach orbital velocity.
• Devices based on the heating of gas have velocities limited by how high a temperature they can stand. Around 15 km/s at most, and reactors probably limited to about 10 km/s.
• Electric and magnetic ways of accelerating a jet could reach a 100 km/s and more, but can only accelerate a very small mass flow.
• Even light carries momentum, so shining a torch at the craft theoretically has no acceleration limit, even if the thrust is absolutely minuscule. (Efficiency is still lost to red shift as the relative velocity gets very large).

There's no "maximum distance" as such, there's no problem waiting years for a jet stream to catch up to you in space.

The idea of accelerating a bomb up to you has a more fundamental flaw: If we have a way to make the bomb reach such a velocity, why aren't we using that for our craft instead? If the answer is to use bombs to accelerate the bombs, that's what the rocket equation is about in the first place. You save no mass by doing this, but introduce the complication of fragmenting your "rocket" into many different parts.

• This is basically the beamrider concept, which appears in many interstellar proposals. A large infrastructure in the solar system you are leaving sends you momentum in some form, which you use to accelerate. This could be light as in the Flight of the DragonFly, a particle beam, or even a series of small, very robust guided missiles, accelerated to relativistic velocity by high G magnetic launcher. Decellerating again is a problem. Oct 23, 2020 at 15:18
• Pournelle wrote sf novels featuring a launcher with an ablative-lined nozzle. A ground-based laser zapped the nozzle. Oct 23, 2020 at 18:04
• @SteveLinton "sends you momentum in some form, which you use to accelerate" Is there any way to avoid accelerating if you have received momentum? :-)
– uhoh
Oct 24, 2020 at 3:34
• @uhoh Fair point Oct 24, 2020 at 7:01
• If energy is sent, there may be an opportunity to reradiate it. I guess the only "momentum radiators there are besides reaction mass ejectors (thrusters etc.) would also be things which radiate light. I like that word; "momentum radiator"; Iwish I could think of a question and a place to ask it.
– uhoh
Oct 24, 2020 at 7:55