Swiss Army Rockets

Question

Is it possible to build a rocket that uses several different types of fuel/fuel delivery methods at different times during flight?

For example, it may use nuclear fuel to travel to a different planet and then, using the hydrogen from the atmosphere, land successfully on its surface (or being able to harness a variety of liquid propellants in a similar manner).

The Questions

-Could such a rocket possibly be built and function accordingly?

-What methods might the engineers of such rocket use, so that it could use many fuel sources during flight?

Answer 1

A rocket is essentially an engine, payload, and fuel tank. What you're describing is a craft with multiple propulsion systems.

It would certainly be possible, although you'd face big engineering issues, and leaving Earth's atmosphere would be out of the question.

You see, engines are usually designed within a very careful set of parameters, and are quite big/heavy (after all, they're lifting/pushing tons and tons of cargo around). Having multiple engines aboard one craft means that when one is active, it has to carry not only the regular cargo, but the "dead weight" of the inactive engines and their fuel as well!

No sane engineer would waste fuel in such a way! This is why we don't have cargo ships which can crawl out of the ocean and turn into trains. The cargo ship is optimized for carrying things at sea, the train for rolling on tracks.

Now, one might nitpick that even nuclear submarines have diesel engines for emergency power, etc, but that's just it - those are redundant/emergency systems which are not meant to match the main system in efficiency or power.

Answer 2

An example of two different methods of propulsion in one engine is the SABRE. It is a turbojet running on liquid hydrogen fuel up to an altitude of about 28km. Then the inlet shuts, the compressor is turned off, and liquid oxygen is pumped into the combustion chamber, at which point it becomes a liquid fueled rocket.

Answer 3

Yes it's possible. Whether or not it would be in common depends on if it's less efficient than simply using one of the fuel types.

We currently use multiple types of propulsion when entering orbit vs maneuvering space. Spacecraft propulsion

Cars currently take this approach with electricity and gasoline. So it's not unreasonable.

Answer 4

There are several ways to think about this question. You could be talking about adjusting the way a particular rocket engine works or you could be speaking about how to use different types of engines.

Multi mode engines have been investigated, but none have been flown to date AFAIK. A simple examples is the "DUMBO" NTR, which is an ordinary NTR using hydrogen flowing through a nuclear reactor core, but with the option of injecting LOX into the engine nozzle to increase thrust (at the expense of ISP). This is actually the general way many multi mode rockets work. Dusty Fission Fragment rockets have fission fragments from fissioning uranium dust exiting the rocket at @ 1% of c. This is an insanely high ISP but has a minimal thrust. Injecting water into the exhaust stream provides more mass and therefore thrust, but once again at the expense of ISP. Similar tricks can be used for most rockets. The Atomic Rockets engines page provides lots of details of different types of engines.

The VASMIR engine uses a similar technique of varying the amount of plasma in the engine to "change gears" from high thrust/low ISP to low thrust /high ISP

The second means of having different modes is to have two different engines. A nuclear reactor can serve both as the main propulsion unit for a NERVA type rocket, but after the high thrust burn is complete, the rocket nozzle can be closed off and the reactor run as a power generator (the coolest is diverted from the rocket nozzle to a Brayton, Rankine or even Stirling generator to get electrical energy), which powers a low thrust/high ISP electric drive like an ion engine or VASMIR in the "high gear" mode.

Edit to add: Atomic Rockets has a worked example of a bimodal NERVA/ion drive ship.

In general, a singular multi mode rocket engine injecting extra remass into the exhaust stream would seem to be the easiest and most reliable means of doing so.

Answer 5

For a rocket, you need basically two things: reaction mass to propel in the direction opposite to the one you want to go and energy to propel the reaction mass.

A simple example is a water rocket: water is the reaction mass, compressed air stores the potential energy to propel the water out of the rocket.

Now, what can possibly make this better? Fire, of course. Welcome, steam rocket, part of the thermal rocket family. You can even go nuclear with those.

This allows for plenty of fuels and kinds of reaction mass to choose from.