Work on large solid-fuel grains for the Minuteman and Polaris programs also led to the development of the first all-solid-fuel space launch vehicle, the LTV “Scout”, which would have a long career putting small payloads into space. In addition, the Minuteman development effort had a direct connection to the development of segmented solid rocket motors, with Aerojet testing the concept in 1961 by the simple measures of cutting a Minuteman first stage in half and then splicing it back together with a lock ring joint. Both Aerojet and United Research, which would later become a component of the modern United Technology Center (UTC), performed further work and static test firings of segmented rocket boosters. In 1962, UTC won a contract from the USAF to build a five-segment SRB for the Titan III space launch vehicle. The resulting SRBs went into service in 1965, with two SRBs straddling the liquid-fuel Titan core.
The initial Titan III SRB motor was 3.05 meters (10 feet) in diameter and 25.8 meters (84 feet 8 inches) long. It led in the 1980s to the 5.5 segment motor for the Titan 34D and the subsequent 7 segment motor for the Titan IV, which produces about 7,565 kN (771,000 kgp / 1.7 million lbf) thrust per SRB. The biggest solid-rocket motor ever to be put into operation is the SRB for the US space shuttle. Each SRBs is 45.5 meters (149 feet 2 inches) long. The precise composition of the shuttle SRB grain by weight is: 69.6% ammonium perchlorate oxidizer 16.0% aluminum booster 12.4% polymer binder 2.0% epoxy curing agent 0.4% iron oxide combustion catalyst (thermite reaction with aluminum) The shuttle SRBs are made up of four segments stacked on top of each other. The hole up the center of the SRBs is cone-shaped at the bottom, leading to an 11-point star that runs to the top. This scheme gives maximum thrust of 11,770 kN (1.2 million kgp / 2.65 million lbf) at liftoff, falling off to a sustained level of thrust after that. The nozzle is steerable.
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, “SpaceshipOne”, 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. SpaceshipOne 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. Aluminum 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, throttleable operation, and relative safety in handling. Experiments have been performed in determining the suitability of gelled propellants for military missiles. The status of research into gelled fuels remains unclear.
