Light weight Structures Filament-wound tanks are used to store high pressure liquids or gases. This type of tank can reduce the mass of propellant and pressurant tankage by 10 to 50 percent. The greatest savings are only enabled at very high pressures of several thousand psi. For simpler pressure-fed propulsion systems, filament wound tankage may provide a lower weight alternative to all-metal tankage made of aluminum or titanium alloys. 8.6.3 High Energy Density Propellants These propellants include metastable molecules and free radical atoms. A very high energy can be released from these materials. This high energy can be translated into a very high specific impulse: up to three times higher than current O2/H2 propulsion. For example, the Space Shuttle Main Engine delivers approximately 455 s (4463.6 N-s/kg). Atomic hydrogen may ultimately deliver 1500 s (14,715 N-s/kg). Studies of HEDP have shown that the mass of launch vehicles can be reduced by 50- 80 percent [Palaszewski, 1990]. One such free radical propellant is atomic hydrogen [Palaszewski, 1990]. Whereas molecular hydrogen (H2) is used in current space propulsion systems, atomic hydrogen (H) is a single atom. It can be stored in a matrix with solid molecular hydrogen. Atomic hydrogen would be used as a mono propellant. Because only one propellant is used, it may simplify the design of future launchers. Additional detailed understanding of the physics of high energy density propellants is needed. The main challenge for the vehicle designer will be to operate the propulsion system at 2 to 4 K temperatures. Also, atomic hydrogen releases its energy through recombination of the atoms. This recombination occurs very rapidly if not checked and controlled. Thermal control of the propellant is therefore extremely important. Designs for cryogenic solid particle (or two-phase flow) feed systems to take the solid matrix from the storage tank to the rocket engine will be required. The major challenges that must be overcome are the production, storage, lifetime and utilization of these fuels. A matrix with up to 2 percent atomic hydrogen in H2 has been made. The total mass of atomic hydrogen stored is several nanograms. Very large amounts that would be needed for launch vehicles (100's of tons) have not yet been created. 8.6.4 Aerobrake/Aerocapture Aerobraking uses the atmosphere of a planet to slow down and go into a low altitude orbit after returning from higher altitudes. This is in contrast to aerocapture which is used to brake into orbit around a planet after traveling on an interplanetary trajectory. Aerobraking can reduce the mass of chemical propellant transfer vehicles by 50 percent. This technology can reduce the transportation cost and mass for lunar and GEO payload delivery missions. The aerobrake is a large aerodynamic structure that provides a protective thermal barrier against the heat of atmospheric entry. It can be very large in diameter: 20 to 30 m for a GEO or lunar transfer vehicle. In the selection process for the aerobrake, the total vehicle mass with and without the aerobrake must be determined. There are applications where the mass of the aerobrake may exceed the propellant required for the return from high orbit. Careful selection of the type and configuration of the brake is also required. The thermal heating during atmospheric entry may require a design that completely surrounds the vehicle. 8.6.5 Air Breathing Propulsion An alternative to pure rocket propulsion is air breathing propulsion. With this technology, the air from the atmosphere can be used as the oxidizer in the vehicle engine. For Earth to Orbit vehicles, the total mass of propellant can be reduced very significantly if air is used in lieu of liquid oxygen. With a vehicle using oxygen and hydrocarbon fuel, the typical mixture ratio is 2.6. A mixture ratio is the ratio of the oxidizer mass to the fuel mass. This means that (2.673.6) or 72.2 percent of the propellant mass would no longer be carried on board the rocket. For a rocket using oxygen and hydrogen, the mixture ratio is 6.0 and total savings would be (6/7) or 85.7 percent of the propellant mass. This is a critical part of the development of the National Aerospace Plane (NASP) that is planned in the United States. Another potential application is the Two Stage to Orbit (TSTO) vehicle.
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