Lunar Rocket-sled to Orbit
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- Using a lunar rocket-sled to orbit (LRSTO) provides the chance to recapture nearly all rocket exhaust and recycle the precious hydrogen while providing man ratable transportation to lunar escape velocity (about 2400 meters per second). Since a rocket-sled moves nearly horizontally, it is possible to encase the track on which the vehicle rides in a long pressure vessel set upon the lunar surface for that portion of the track the rocket-sled uses while burning. Open doors at the ends of the pressure vessel allow the sled to enter and escape. A short section of track is moved out of the way and a door is closed after the rocket passes as it enters and as it exits the pressure vessel. This traps the exhaust gas, which is mainly water. The water is then electrolyzed to recover the hydrogen and oxygen. With a rocket-sled to orbit, the electrical power can be stored in cryogenic hydrogen and oxygen until launch. So, large capacitor banks, large storage batteries, or large generating capacity is not needed. Power storage requirements and high power mass-driver coil requirements typically limit the mass accelerated by an all electric mass-driver to 10 kilograms or less in early stage lunar export transportation proposals.
- The rocket sled concept was demonstrated on Earth to more than lunar escape velocity. On the 30th of April, 2003 the United States Air Force set the absolute land speed record with a rocket sled traveling at over 2868 meters per second. Various changes would be necessary to adapt this concept to lunar conditions and the requirement to carry passengers.
- The lunar version rocket-sled would ride on a magnetic levitating track. In the following example likely values are specified by estimation to give a general idea of the type of performance to be expected. If a real rocket-sled is built, specifications are likely to be different. An electric tractor would push the sled up to 850 meters per second in 18 seconds with a maximum acceleration of 50 meters per second squared using about 7.7kilometers (4.8 miles) of track. The sled would coast for 1.2 seconds and enter the pressure vessel while the tractor would be diverted on its separate track going outside the pressure vessel. The electric tractor would be on a narrower gage track so the tractor and its track could drop beneath the rocket sled track. The remaining 1550 meters per second to escape velocity would be provided by a liquid hydrogen-liquid oxygen rocket with an exhaust velocity of 4300 meters per second. So 70% of the mass of the vehicle at rocket ignition would reach escape velocity. Rocket acceleration would start at 34.9 meters per second squared and reach a maximum of 50 meters per second squared before burnout. The trip through the long pressure vessel would last about 37 seconds and cover about 61 kilometers (38 miles). If the track starts level and levitates the rocket-sled until circular orbit velocity is reached and then merely provides a means of correcting the position of the rocket-sled, keeping it in the center of the long pressure vessel, then the pressure vessel would follow the shape of the orbital path and end up near 300 meters higher than the starting elevation. If the track starts a downhill slope somewhat before circular orbit velocity is reached, the track exits the pressure vessel closer to the starting level with a slightly upward slant. After exiting the pressure vessel, the rocket sled would separate into one portion consisting of the main rocket engine, empty fuel tanks, and the runners held magnetically at a fixed distance from the rails; and a second portion consisting of the spacecraft carrying cargo or carrying passengers, environmental equipment, maneuvering thrusters, radio navigation and communications. The portion remaining on the rails would be slowed and moved to maintenance facilities for reuse. The whole facility would require 128 kilometers (80 miles) of magnetically levitating track, 61 kilometers (38 miles) of which would be encased in a pressure vessel with doors at both ends.
- A rocket-sled to orbit would provide reasonably low cost transportation to orbit only if the high capital cost could be spread over a large number of tons. So plans must be made to use a large number of tons of cargo in cis-lunar space profitably before the rocket-sled is built. Commonly suggested uses are space based solar power and space habitats.
- While Earth bound tests did not run a rocket-sled through a pressure vessel, they did run a rocket sled through an 11,000 foot long 184 inch diameter tube full of helium. The pressure vessel on Luna could be constructed from local materials, perhaps using Sintered Brick Construction methods. The electrical portion of the acceleration helps avoid damage to the pressure vessel because the rocked sled is already moving when the rocket engine starts. So the otherwise erosive effects of rocket exhaust are dispersed along the length of the pressure vessel.
- An LRSTO would be used for personnel transportation for a well developed lunar base. It might also be easier to build than an all electric cargo launching system. If so, it could serve for cargo launch until an all electric system is built. The example configuration of a LRSTO would require much more electric power during the electric portion of the launch than a minimal all electric cargo launching system and might use temporarily diverted electrical power. So the example configuration would not be easier to build than some minimal all electric cargo launching systems.