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Hybrid propulsion systems offer many attractive characteristics such as their inherent simplicity, safe and inexpensive operations, environmentally friendly exhaust, and the capability for throttling. However, the slow burning nature of the classical hybrid fuels requires complex grain geometries and relatively large initial port volumes to achieve the required surface area for producing sufficient fuel mass flow rates, and consequently, thrust levels. Substantial increases in hybrid regression rates could eliminate the need for these complex geometries and would enable more efficient volumetric loading of the fuel through a reduction in both initial port volume requirements and grain sliver mass at burnout. Orbital Technologies Corporation has addressed these issues by the development and testing of a new class of cryogenic hybrid rocket engines that regress 20 to 40 times faster than conventional HTPB-based fuels. Many different hybrid propellant combinations have been tested. This paper focuses on subscale SCHVGOX and SCHt-Al/GOX hybrid development that was conducted under a Phase II NASA/LeRC SBIR. The objectives of the program were to: (1) research the liquid to solid fuel grain formation process, (2) research the gas to solid fuel grain formation process, (3) conduct parametric testing of SCHt/GOX and SCEU-Al/GOX hybrids, and (4) investigate the effects of different types and amounts of aluminum loading. The major findings were: (1) the SCHVGOX and SCHt-AyGOX hybrid firings exhibit smooth, relatively flat and predictable chamber pressure traces, (2) the SCEL^/GOX and SCHt-Al/GOX hybrids obey the classical hybrid regression law and have a similar dependence on mass flux as HTPB-based fuels, but regress 10-20 times faster, (3) as the percent of Al in the grain was increased, the hybrid regression rate decreased, and (4) reducing the initial temperature of SCH4 grains reduces the fuel regression rate. In addition to exhibiting extremely high regression rates, these propellant combinations offer substantial increases in * Member AIAA f Associate Fellow AIAA * Senior Member AIAA * Member AIAA Isp compared to classical hybrid fuels, and are competitive alternatives to bi-propellant and solid propulsion systems. Potential applications include: upper stages, orbit transfer, planetary ascent/decent, and launch vehicles. TEST ENGINES AND SUPPORTING HARDWARE Two hybrid engines were developed and tested during the program. A Low-Cost Hybrid Engine/Freezer was used to research the liquid to solid grain formation process and investigate the effects of different Al loadings. ORBITEC's Mark II engine was used to research the gas to solid fuel formation process and to conduct SCHt/GOX and SCH»Al/GOX firings. The concept behind the Low-Cost Cryogenic Hybrid Engine was to keep it simple, inexpensive, and versatile. This was accomplished by making the main engine components from off-the-shelf pipe fittings. A schematic and photograph of the hardware are shown in Figures 1 and 2, respectively. The engine chamber consisted of a 5-cm ID pipe surrounded by a coolant bath open to the atmosphere. A 5-cm pipe coupling welded to the center of the base plate allowed the main chamber to be screwed into the top and the aft chamber to be screwed into the bottom. A pipe cap with threaded ports for the ignitor, main oxygen, and pressure transducer, screwed onto the top of the main chamber and a copper heat sink nozzle screwed onto the aft chamber. An insulation ring surrounded the coolant bath. The main oxygen, ignitor oxygen, and ignitor methane solenoid valves along with the ignitor glow plug were all computer controlled with ORBrrEC's flexible computer control system. The computer recorded event timing and chamber pressure. A schematic of the Mark II Engine is shown in Figure 3. The engine is encased in a vacuum chamber to Copyright © 1998 by Orbital Technologies Corporation (ORBITECTM) Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 1 American Institute of Aeronautics and Astronautics allow the radiation shield (not shown) to function. A coolant (LHe or LN2) fills the outer engine dewar and chills the wall of the center tube. The methane gas is admitted into the chamber which is maintained at a pressure below the methane triple point, causing the methane gas to freeze directly onto the inner wall of the center tube. When the engine is ready to fire, the inner chamber is exposed to an atmospheric GHe purge and then an ignitor flame. Gaseous oxygen is then injected at the top of the grain. The firing begins and the grain is depleted over time, producing a hot gas emission/thrust. Figure 4 shows the side view of the Mark-II system located in ORBITEC's test facility. All event timing and data acquisition were computer controlled using ORBITEC's cryogenic touchscreen control system. |
