extrapolation of the currently existing technologies and studies. In fact, one should remember that 50 years ago, in 1942, chemical propulsion development was only beginning. So, some new technologies may be discovered in the far future and modify the results. Lowering the Cost of Space Transportation There are many alternatives to reduce the cost of space transportation. Combining some of the methods of improved propulsion and different structural concepts may make the idea of cheap space transportation a reality. 8.6.1 Metallized Propellants Metallized propellants are gelled liquid propellants with metal particles suspended in them. By using these propellants, we can increase either the specific impulse or the propellant density, or both. Also the safety of the propulsion system is increased because the propellants are gelled liquids. Gelling the fuel reduces size of any accidental spillage and also reduces the explosion hazard if a high-velocity particle (micro meteoroid, accidentally dropped hammer, etc.) were to hit the vehicle's tankage.Past studies have shown that the density and/or the specific impulse increases of metallized propellants can increase the payload of the Space Shuttle (Palaszewski, 1991) by 14 to 35 percent. Other applications of metallized propellants can significantly increase the payload of upper stages: up to 100 percent increases are possible for planetary missions and 19 percent for LEO-GEO missions. The major technical challenges that occur with metallized propellants are the assurance of high engine efficiency with metal combustion. Particle combustion and the erosion of engine nozzles are very critical factors in using metallized fuels. Work is continuing to improve the performance of these engine with sub scale testing in the USA and Japan. The redesign of the propellant feed system to use non-Newtonian gelled fuels is also required. 8.6.2 Lightweight Upper Stages One powerful way to reduce the cost of current space transportation is to replace existing upper stages with lighter weight systems. By reducing the mass of the upper stage, the launch vehicle does not have to lift so much into orbit. This can reduce the class of the launch vehicle one uses for a flight. Alternatively, the same launch vehicle can take more payload into orbit. This approach can dramatically reduce the size and cost of the required launch vehicle or improve the launch efficiency. The reductions in stage weight are possible with either electric propulsion, high energy chemical propellants or reduced structural masses. Each of these areas will be discussed below. Electric Propulsion A general discussion of electric propulsion is provided in the section 8.2.4. By using electric propulsion, the Isp of the upper stage propulsion system is increased very significantly: up to 5000 s versus the typical values of 300 s for storable chemical propulsion. An example of reducing the launch vehicle size is the use of solar electric ion propulsion for the deployment of Global Positioning Satellites (GPS). By using ion propulsion rather than the current Inertial Upper Stage (IUS), a smaller class of launch vehicle can be used. To launch the GPS/IUS combination, a Titan 4 is needed; only a Delta launcher is needed for the GPS launch with the ion propulsion upper stage. The cost difference between these two options is $250M for the Titan versus $80M for the Delta. Over the life of the GPS system, the total cost savings would be many billions of dollars. Chemical Propulsion High energy chemical propulsion systems can also provide high leverage for reducing costs. Examples of this are the use of high temperature materials to increase the performance of storable propellants. Because the engine does not require film cooling, the propellant is used more efficiently and will deliver higher Isp. High performance cryogenic O2/H2 engines are also planned for advanced missions. These engines can deliver up to 485 s (4757.9 N-s/kg) using high chamber pressures (1000 to 1500 psi) and high expansion ratios (up to 1000).
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