Polyurethane Rubber Rockets
Although Thiokol won the competition for the Minuteman missile contract, they lost out on the equally lucrative Polaris missile contract. The reason was simply one of energy.
The Polaris missile was subject to even greater size and weight constraints than the Minuteman ICBM. One variant of the Minuteman family, the LGM-30C was intended to be a land mobile missile. However, the physical constraints of submarine technology at the time required any submarine launched missile to be even smaller. Thiokol fuel/binders of the time lacked sufficient chemical energy to allow such a missile to achieve the required range.
An alternative fuel/binder was presented by Aerojet: Polyurethane rubber.
Polyurethane rubber offered a significantly higher specific impulse by weight than either thiokol rubber, or the new polybutadiene-acrylic acid (PBAA) rubber fuel/binder developed by Thiokol.
So why was the Poseidon missile, the Polaris replacement, powered by a polybutadiene rubber? Unfortunately the higher specific impulse of polyurethane rubber fuel is offset by its low viscosity. This creates problems in forming and maintaining the star shaped combustion chamber within the solid propellant.
Various fuel binders were developed over the years. The Polaris A1 missile employed a polyether-polyester-polyurethane composition with 15% aluminium and ammonium perchlorate.
Polyurethanes were the second elastomer fuel binder. The group of polymers known as polyurethanes were made by combining polyols with isocianates. The versatility in polymer chemistry was such that a large number of starting materials having varying molecular weights were available.
Compared with the polysulfides, the average molecular weight of the polyurethanes' exhaust gases was lower. This was because the polyurethanes contained only carbon, hydrogen, oxygen, and nitrogen atoms (not sulphur). An additional benefit was claimed in the processing: the backbone polymer contained substantial amounts of oxygen. It was not necessary therefore to use as great a percentage of oxidiser in the formulation of the propellant to achieve comparable energies. The increased proportion of binder to oxidiser provides added elongation and other good mechanical properties to the propellant, permitting the addition of other energetic fuels (for example aluminium).
From the logistics standpoint, the starting ingredients for manufacturing polyurethane were available from a large number of chemical suppliers, whereas the liquid polysulfide rubbers were manufactured almost exclusively by a single company. This had its effect on cost, quality, and delivery time.
One of the advantages of polyurethane was that a high concentration of nitrate ester could be incorporated in the binder to give increased energy. A commonly used polyurethane binder material was ESTANE, a product of B.F. Goodrich Chemical Company.