8.6.6 Slush Hydrogen Another method of storing hydrogen fuel is called slush hydrogen. This technique allows the density of hydrogen to be marginally increased. The thermal heat capacity of the slush is also significantly higher than liquid hydrogen. This improves the ability of the fuel to cool the external surfaces of the aircraft during ascent and reentry. This is critically important for applications such as the NASP, which has a very large hydrogen tank. By using a higher density fuel, the propellant tank is considerably reduced in size. The drag reduction and the thermal advantages of slush hydrogen make it an enabling technology for the NASP. Extensive testing of slush hydrogen is underway as part of the NASP program in the USA. 8.6.7 In-Situ Propellants In-situ propellants are fuels and oxidizers produced on other planets, moons or other bodies in the solar system. By producing propellants on extraterrestrial bodies, the transportation around the solar systems can be greatly improved and more cost effective. This is because all of the mass for propulsion does not have to be lifted from the Earth. Taking less mass along with you on your vehicle can allow a faster mission, and improve the payload delivered to the final destination. Using in-situ propellants, the mass launched for a Mars Sample Return Mission can be reduced by more than 50 percent. For lunar transportation, the total mass lifted from the Earth can be reduced by 33 to 50 percent. Current research is underway at a modest level in the rocket engine technology for using both 02/Al for the moon and O2/CO propellant for Mars. Additional technology is being developed at a low level to address the production of propellants on both the Moon and Mars. 8.6.8 Mass Drivers A mass driver can electrically accelerate a payload to high speeds. The payload will experience a very high acceleration: from 100's to 1000's of gravity. It is therefore not recommended for human flight. Mass drivers are particularly applicable to launching payloads from the Moon toward a liberation point or perhaps into lunar orbit. Past studies of these devices have shown that they may reduce the cost of launching objects from the Moon for space construction projects. There has been consideration of these devices for Earth launch as well. The size of sub scale experiments with mass drivers has been relatively small: several grams to one kilogram. Very extensive scale increases would be needed to launch the payloads for Space Solar Power Program development 8.6.9 Gun Propulsion An alternative to rockets for Earth launch is gun propulsion. This type of system literally uses the same principle as a gun: expanding gases accelerate a projectile or payload to high speeds. As with the mass driver, high velocities and very high accelerations are produced. As with the mass driver, scale experiments have been conducted. 8.6.10 Laser Propulsion This technology uses a laser to heat a propellant to high temperatures. The resulting gas is then expanded through a rocket nozzle to produce thrust. The performance of such an engine can be 1000 s (9810 N-s/kg) using hydrogen as a fuel. Laser propulsion can be used for Earth to orbit transportation. The payload for an Earth to Orbit (ETO) laser system will have to be fairly small: 1000 kg. This is because the laser on the ground will be limited in size: under 100 MW. Sending large payloads (> 10,000 kg) into orbit would require many 1000's of MW in laser power. Such giant lasers would be potentially be impractical in size. Small manned laser propelled vehicles as well as cargo vehicles have been studied. Very small scale experiments have been conducted. 8.6.11 Nuclear Thermal Propulsion Nuclear thermal rockets use a nuclear reactor to heat a working fluid to very high temperatures. The fluid is expanded through a DeLaval nozzle to produce thrust. These engines have the potential to produce up to 1000s of Isp. The reactor technology for this system is under development in the United States by a joint program between the Air Force, the Department of Energy and NASA. A demonstration of the engine is possible in the next 15 years.
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