oxidizer, may reduce the risk of catastrophic failures associated with cracks in propellant grains or debonds at the case. The ability to control the flow of liquid oxidizer permits throttling and engine shutdown.

An investment should be made in demonstrating the technology necessary to validate the engineering practicality of the hybrid rocket motor for large, high-thrust, strap-on applications. As envisioned, the fuel grain would be a hydrocarbon type and the oxidizer would be liquid oxygen. Technology efforts need to be directed to demonstrating satisfactory combustion characteristics along the length of the fuel grain and minimizing the residual fuel at the completion of burn. This includes tailoring the internal geometry of the motor to achieve the best combination of these characteristics. The combustion process needs to be free of oscillations that may introduce unacceptable stage vibrations or detrimental internal conditions. These investigations must be accomplished at a scale that is large enough to be representative of a full-scale booster motor, but sufficiently small to keep costs affordable. This development activity might include tests at a variety of thrust levels to permit the establishment and evaluation of scaling criteria. Hybrid rocket motor development should be advanced to the point that it can be quantitatively evaluated in competition with solid and liquid bipropellant systems designed to directly comparable criteria. This hybrid motor technology should be targeted initially for thrust-augmentation booster applications such as the 135,000-pound payload class vehicle, NLS-1.

The modular plug engine, as conceived by Aerojet General Corporation, consists of 12 identical modules arranged circumferentially to form a central, truncated surface, as shown in Figure 4. In addition, each of the modules contains 16 very small rectangular thrusters that burn the propellants and internally expand the exhaust gas, directing it along the central surface for additional, external expansion to the surrounding environment. One may think of the modular plug engine as having a nozzle that self-adjusts its expansion to the external pressure that exists at the vehicle's flight altitude. Thus, in spite of some loss caused by the truncation of the central surface, the modular plug engine is especially well suited for single-stage-to-orbit (SSTO) vehicles because it promises to operate relatively efficiently at low and high altitudes without mechanical nozzle extensions. Its fully modular design may greatly simplify the logistic and maintenance operations, thus reducing turnaround time. Also, a novel platelet² construction promises practical and cheaper fabrication of small thrusters. Although the actual performance