Thiokol Rubber Fuel
In 1942, John Parsons created a rocket motor that employed asphalt as a binder and fuel component, combined with potassium perchlorate as an oxidiser. The binding agent was used to maintain the optimum shape of the combustion chamber. Ammonium perchlorate replaced potassium perchlorate as the standard oxidiser in the late 1940s. This reduced smoke production and increased specific impulse.
Charles Bartley inproved on this idea by replacing the asphalt with thiokol LP-2, a polysulfide rubber. The polysulphide rubber binder gave improved performance over asphalt, particulary in terms of storage temperature range and hardness. He also developed a star shaped combustion channel running throughout the motor. The earlier asphalt motors had burnt propellant from end to end - the same mechanism as used by gunpowder burning rockets and fireworks. However, composite solid propellants tend to burn slower than blackpowder, generating the need to expose a greater surface area of propellant at any moment in time. In addition, the rubber binder allows the propellant to bond to the motor case, enabling a channel to exist in the middle of the rubber based fuel. However, a cyclindrical channel proved inefficient. The longer the motor burns, the greater the surface area of exposed rubber bound propellant. To keep burn rate consistent throughout the flight, a star shaped cross-section is employed. This was initially referred to as a burning star configuration. This core burning design also allowed for lighter motor cases, as the rubber propellant acted as an insulator to protect the casing
Further research into polysulfide rubber-perchlorate propellants resulted in the "Thunderbird" rocket. This vehicle demonstrated a polysulphide rubber composite-propellant, internal-burning star-grain motor in 1947. The vehicle had an acceleration of 100G.
This idea evolved into the motor for the Sergeant missile (MGM-29A). Sergeant was the first large missile to employ a thiokol rubber bound composite propellant. Sergeant was a single stage, surface to surface missile. It was first deployed in 1962. The complete missile was 10.5 m long, it had a range of 139 km, and could carry a 200 KT nuclear warhead. The Sergeant was powered by a Thiokol XM100 polysulphide rubber composite motor. This could produce 200 kN of thrust for 34 seconds. A total of 473 Sergeants were built.
In addition to the Sergeant, several other motors employed polysulfide rubber binders. The sustainer motor of the Nike-Hercules surface to air missile used thiokol rubber, as did numerous small sounding rockets, and the retro-rockets of the Mercury and Gemini manned spacecraft.
Two engineers, Keith Rumbel and Charles Henderson, then found that adding aluminium significantly increased the specific impulse of the solid propellant. They used 21% aluminium, 59% ammonium perchlorate and 20% plasticised polyvinyl chloride. Thiokol have added aluminium to their rubber binders ever since.
In the early 1950s Thiokol chemists set out to reduce the sulphur content of the polysulfide rubber used in their propellent. They experimented with several polymers and copolymers. Polybutadiene rubber polymers were found to be the best candidates. Eventually, PBAA (polybutadiene-acrylic acid) was discovered and used in the first stage of the Minuteman 1 ICBM. PBAA was an improvement on the original thiokol rubber binder, but lacked tear strength. PBAA is a copolymer of butadiene and acrylic acid.
However, the search for ever better rubber binders continued. This led to a new synthetic rubber binder: PBAN ( acrylic acid, acrylonitrile and butadiene terpolymer). Thiokol introduced PBAN in 1954. This had superior physical properties to PBAA, due to the addition of 10% acrylonitrile. Thiokol then developed CTPB (carboxyl-terminated polybutadiene). This was even better, but too expensive for widescale use. Thiokol binder evolution ended with HTPB (hydroxyl-terminated polybutadiene). This low-cost, low-viscosity propellant has become the new industry standard.