LOX gel fuel

The two main types of rocket propellant are solid and two-liquid fuels. (There are also hybrids using a solid fuel and liquid oxidiser, and monopropellants.)

Solid fuels are simple to use and need only simple rockets - basically a tube with a nozzle at the end. They have the disadvantages of being non-controllable once lit, and being quite costly. Worse yet, their specific impulse is not that great.

Two-liquid fuels are typically liquid oxygen combined with either liquid hydrogen (best for impulse per unit weight) or kerosene (best for impulse per unit volume). The liquids are pumped into a combustion chamber, mixed and burned. This has the advantage of using cheap fuels and giving a very good specific impulse. It's also throttlable. The drawback is the complexity (=cost, weight) of the engine.

I suggest a sort of hybrid fuel which would have the high impulse of two-liquid systems, but the simplicity of a solid rocket. (It would not have the throttlability of a liquid system, though).

Liquid oxygen and liquid hydrogen can be mixed, and are fairly stable (at least, they aren't hypergolic - they don't ignite spontaneously). Now add a few percent of a liquid which will gel at very low temperatures (might be kerosene) and mix (gently). With luck, you will get a semi- solid gel which is >90% LOX plus liquid hydrogen. With a bit of luck, this gel will behave much like a solid propellant (ie, it will simply burn once lit), but with the high impulse afforded by the intimate mixing of LOX and H2.

There are some issues with this. First, the gel is going to have to maintain its temperature by venting, just as LOX or liquid H2 alone do. This might lead to the formation of gas bubbles in the gel. On the other hand, provided the gel is surrounded by a porous mesh, boil-off from the surface should be favoured, and might prevent the insides from bubbling.

(You might even be able to use a porous, combustible solid in place of the gelling agent, soaking the liquids up like a sponge; I'm thinking of maybe open-cell porous aluminium or magnesium, or an open-cell plastic foam.)

Second, the mix may be sensitive to premature detonation. However, agents could be added to the gelling agent to stabilize the mixture (for example, free- radical scavengers) and make it semi- stable.

Finally, handling this is going to be a nightmare - it's going to be a semi- solid that wants to turn into a gas. However, there may be some way to prepare the gel in-situ, by filling the tank with the three components and allowing them to mix and gel in situ.