1. Copyright© 1998byOrbitalTechnologiesCorporation (ORBITEC™). Published byAmerican Instituteof gaseous oxygen enters the combustion chamber it is forced toward the head-end by the favorable axial pressure gradient and forced outward by centrifugal acceleration resulting from the tangential direction of injection. The oxidizer spirals upward along the fuel surface, mixing and burning with vaporized fuel. At the top of the chamber, this outer vortex spills over into adownward spiralinginnervortexthateventually passes through the nozzle. The vortex hybrid uses a singlecylindrical centerport. Approximately 80% of the vortex hybrid grain volume is fuel. In addition, high solid fuel regression rates, on theorder of 800% higher than those in classic hybrids operating at the same conditions, have been demonstrated experimentally in a lab-scale engine1"2. During these studies, both PMM and HTPB were burned with GOX.
2. Pump-fed hybrids optimize at a modestly higher chamber pressure than comparable pressure-fed hybrids, because of the weight savings in the LOX tank and pressurization system. The higher chamber pressure improves hybrid engine performance as a booster because theengine can be fitted with a larger expansion area ratio skirt. A further benefit is possible by using a topping cycle feed system in which the LOX is gasified in a preburner or in combustion chamber coolant passages and used to drive the turbopump. In topping cycles the turbine exhaust flow becomes theoxidizersupply to the main chamber. The topping cycle offers a performance gain of 2% or more in specific impulse over cycles that bleed the turbine drive gas overboard. In the topping cycle, the oxygen is delivered to the fuel grain as a warm gas instead of cold liquid. Combustion processes in the grain port are also improved. The large volumetric flow of the gas allows more uniform oxidizer distribution within the grain port and the delay time associated with vaporization of LOX is eliminated. Further, the use of gaseous oxygen permits its injection with a large swirl componenttotheinjection velocity withoutexcessive viscous losses. Early design concepts for large hybrids (circa 1987), envisioned the tangential injection of the pump-fed gaseous oxidizer via multiple ports positioned along the length of the grain. Figure 1 illustrates such a concept as a replacement forthe Space Shuttle SRB's. The result of anearly attempt to test a model of such an engine using injection ports along the full length of the port is illustrated in Figure 2. Unexpected flame outflow from the injection ports nearthenozzleentrance gave the first indication of anunusualflow field within the vortex-fired combustionchamber.
3. 1 Definition ofEngine Configuration.
4. Value 2.684xlO6pa [300psia] 3603.0°K [6485.40°R] 1.590kg/mJP^SxlO^lbm/ft3] 1.2306 1926.0J/kg-°K [0.46010Btu/lbm-°R) 7.6649X10"4m2[1.1881in2] 23.034 1215.5m/s [3987.8ft/s] 9.5840xlQ-5kg/m-s [2.00xlO-6lbf-s/ft2] 1795.0m/s [5889.0ft/s] The axial length of the GOX injector port was determined as follows: The port is assumed to extend circumferentially around the entire grain port diameter. The axial length of the injector port is determined from the computed GOX flow rate and the desired injection Mach number of 0.8. First, the speed of sound of the GOX is calculated from the equation:
5. Value 2.684x10" pa [300psia] 2.243kg/mj[1.2637X10'1Ibm/ft3] 1.166kg/mJ[7.2793xlO'2Ibm/ft3] 3603.0K [6485.40R] 3039.7K [5471.5R] 4915.2K [8847.4R] 2830.8K [5095.4R] 109.33m/s [358.70ft/s] 890.43m/s [2921.3ft/s] 7.1830m/s [23.566ft/s] 1.740xlO-2m [6.850X10'1in] 1121.4m/s [3679.0ft/s] 360.967N-m/kg-K [67.075ft-lbf/lbm-R] 1.2306 23.034 9.5840xlO'5kg/m-s [2.00xW6Ibf-s/ ft2] 1795.0m/s [5889.0ft/s]