The major weakness of the solid-fuel rocket is the fact that, once lit, it burns to completion, and the only thing that can be done is to divert the thrust when it is no longer needed. The lack of burn control for solid-fuel rockets has led to the development of “hybrid” rockets that use a solid-fuel core along with a liquid oxidizer. The solid fuel component in a hybrid rocket is not impregnated with large quantities of an oxidizer material, which makes the rocket much safer to handle and store since it cannot burn efficiently on its own. Lockheed Martin has static-tested a hybrid motor with a butadiene – type solid fuel and liquid oxygen oxidizer. Lockheed Martin has also investigated the use of paraffins as propellants; “paraffins” in this case of course refers to the American usage of the term, meaning candle waxes and related solid hydrocarbons, and not the British usage of the term, which is what Americans call kerosene.
Burt Rutan’s famous commercial suborbital manned spacecraft, “ Spaceship One”, uses a hybrid propulsion system, with a butadiene-type solid fuel and nitrous oxide oxidizer. In this case, the propulsion system is designed for low cost and ease of handling instead of optimal thrust levels. Spaceship One is probably the first thing resembling a operational space vehicle to use hybrid propulsion, and after many years of tinkering the technology seems to be coming of age.
Experiments have also been performed on another approach to the same problem, in the form of “propellant gels”. The idea is to take storable propellants and turn them into gels: hydrazine can be gelled by adding cellulose, and nitric acid can be gelled by adding silicon dioxide (sand, more or less). The results have the consistency of toothpaste. Aluminium powder can be added to provide more “kick”. Since hydrazine and nitric acid are hypergolic, if the two gels come in contact with each other they burn spontaneously — but not for long, since a crust builds up between them that inhibits further combustion. This makes them much safer to handle than their liquid forms.
To get them to burn in a combustion chamber, they are fed under pressure through an orifice that turns them into an aerosol, allowing them to mix properly. The potential advantages of this approach are high energy density, throttle-able operation, and relative safety in handling. Experiments have been performed in determining the suitability of gelled propellants for
military missiles. At the time of writing, the status of research into gelled fuels remains unclear.
