Solid Rocket Motor for Ultralow Temperature Operation During the Mars Sample Return Mission

Our primary technology insertion opportunity within NASA is the Mars Sample Return (MSR) mission. The Mars climate enforces a uniquely stressing environment that every propulsion system must deal with. However, we are also aware that NASA is focusing upon spacecraft and lander missions to asteroids and other near-earth bodies that will require operating at extreme low temperatures approaching -270�C—a scant 3�C above absolute zero—that will test the limits of every known material. From a propulsion system perspective, such missions demand, to an even greater extent than MSR, the kind of performance siloxane binders appear to offer. We are not aware of any polymer with a glass transition temperature near the liquid temperature of helium, so these missions will require electric heaters for the motor. Siloxane binders would reduce the power requirements for such spacecraft. Other potential NASA uses will be in the Space Launch System (SLS) program, perhaps used as ullage settling motors (USM), tower jettison motors, or even crew escape system motors. We are aware, of course, that motor designs currently exist (and have been tested) for these uses, but there may come a time when the additional structural margin at low launch temperatures is needed. Siloxane binders can fill that need.

Tactical missiles and rockets are generally required to operate between -65�F and +160�F, a temperature range often referred to as the tactical temperature limits. However, there are several systems in the field today that are restricted to minimum operable temperatures well above the lower tactical limit because grain structural margins are not adequate at -65�F. This is a direct result of a mismatch in coefficient of thermal expansion between propellant and case, as well as from the high glass transition temperature (Tg) of HTPB. The propellant stiffness goes up dramatically as Tg is approached, which has an undesirable effect on the allowable stresses and strains in the grain. A siloxane-based composite formulation, with its significantly lower Tg, could ease those low temperature firing limitations. Additionally, the hydrophobic nature of siloxane polymers may also have a positive benefit to motor service life, since hydrolytic scission of the binder—a major component of composite propellant aging—will no longer be possible. Ballistic parachute deployment is a commercial market in which a motor that uses the proposed binder would serve well. It is also a market with which we already have an association. In the spring of 2007, ASI was approached by Ballistic Recovery Systems for help in scaling their current deployment motor. We will re-engage that company during execution of this Phase I to determine whether the new binder system would fit their needs. More »