Current chemical rockets use the hydrogen-oxygen reaction (bipropellant), there are many other reactions that provide a larger specific energy release but this release can not be taken advantage of in a rocket engine, due to the product being non-gaseous. For the rocket to function the product of the reaction must take a gaseous state due to the fact that the exhaust velocity of this gas produces the thrust, clearly any other state would be a non-starter!

It is perhaps natural to think that a tripropellant would produce an improved specific energy release. This was exactly the idea of many rocket scientists, however the experimental results have been extremely disappointing, the best results showing only marginal improvements.

In what seems like the final straw for chemical rockets there is now increased research into the area of high energy density matter (HEDM) propulsion. The concepts are relatively new and little is known of the true potential. There are, however, several easily identifiable problems.

This branch of research seeks to achieve very impressive increases in specific impulse over standard chemical propellants, while maintaining high thrust levels, by introducing either free radicals or metastable atoms. The most researched possibilities are atomic hydrogen (a free radical) and metastable helium, the estimated specific impulses are 2100secs and 3150secs respectively (contrast with 400-500secs of 'standard' chemical rockets).

HEDM Possibilities

The problems begin with the production and storage of atomic hydrogen, which is extremely difficult, substantial improvements over current methods are required before it can be considered for rocket propulsion.
Unfortunately even with development it is expected large magnets and cryogenic systems will be required for storage and this will add a great deal of weight to the spacecraft, considerably reducing the advantages of the weight saving high-density propellants.

Metastable helium is easier to produce than free radicals, both lasers and particle bombardment can excite an atom to its metastable state. Storing them however is extremely difficult as the lifetime is very short, indeed without a fundamental breakthrough it is probably impossible.

There is also talk of using metallic hydrogen. Here a great deal of energy is realised by allowing the compressed hydrogen to relax, specific impulse is thought to be in the region of 1700secs.
Production is again very difficult, requiring a pressure of 1.4 Mbar and a shock heating to 3000 K. There is potential storage problems as well, it is thought that upon the release of the pressure at the formation the hydrogen may be a metastable and as we have already discussed if this is the case it will be probably be impossible to store.

In general the problem with HEDM is the need to store it as a cryogenic solid, which is difficult to achieve and harder to maintain. The pressures involved will also require the tanks to be made of materials much stronger than anything we have today (nanotechnology will be able to help here).
These problems further imply a great deal of the tank mass saved by using such dense propellant could be lost in equipment to store the propellant, if storage is even possible.

Given this and the specific impulse of HEDM propellant is not as good as other future concepts, the outlook is not too promising here. It must though be remembered that HEDM does provide a rocket with very high thrust levels, so could be a major improvement on current chemical propulsion for use in the near Earth environment.

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