1. Reaction (2) is exothermic with a very high equilibrium constant driving it in the desired direction. Taking advantage of this, in 1993-94, Zubrin, Price, Mason, and Clark, working at Martin Marietta Astronautics integrated such a Sabatier reactor with a water electrolysis unit to build the first end-to-end Mars ISPP system4'5.
2. For these reasons, it is strongly desired to reduce the total amount of hydrogen that needs to be transported to Mars. One way to do this is to manufacture fuels that have a lower H/C ratio than methane. For example, ethylene, C2H4, has an H/C molar ratio of 2, so if ethylene could be manufactured in place of methane, it would cut the required hydrogen importation by a factor of 2. Even better would be acetylene, C2H2, with an H/C ratio of 1, and such synthesis was proposed by G. Landis circa 1992. However acetylene production is very energy intensive, and acetylene itself is a dangerous monopropellant that can spontaneously explode. However, Benzene, C6H6also has a H/C ratio of 1, and in contrast to acetylene, is safe. Benzene also has the advantage of being a relatively dense storable liquid, and while its manufacture requires some energy, there is a countervailing saving by avoiding the need for cryogenic liquefaction. In 1998, Brian Frankie, then working at Pioneer Astronautics proposed making benzene from methane via:
3. Gashoppers can either be ballistic or winged. Winged systems offer greater performance at the cost of somewhat increased complexity. Pioneer Astronautics has already done a considerable amount of work towards demonstrating the feasibility of the gashopper. In 2000, we developed thermal rocket systems capable of employing CO2propellants and built a prototype ballistic gashopper system forJet Propulsion Lab under a Phase I SBIR contract.7In the gashopper, a bed of material (such as graphite rods of copper-coated steel pellets) was heated electrically to 700 C, after which a supply of CO2stored in liquid form in a propellant tank was allowed to blow down autogenously through the bed. Upon contact with the hot bed, the liquid CO2would flash to vapor, and then be vented out a convergent/divergent rocket nozzle to produce thrust. Specific impulse for this system was measured at about 80 s Isp under Denver ambient conditions, with calculated specific impulses for a more optimized system on Mars being the in the range of 125 s. Several test engines were built and fired on the test stand. Then a protoflight radiocontrolled vehicle with a mass of about 50 Ibs and producing about 45 Ibs thrust was built. To enable flight, this vehicle was attached first to a 20-lb pull bungee cord (for indoor captive testing), and then, for outdoor flight, to a helium balloon with about 20 Ibs of lift. (The bungee cord and balloon auxiliary lift systems stabilized the unit while only partially compensating for the lower gravity of Mars.) In a test flight at Pioneer Astronautics, this vehicle was flown to an altitude of about 50 meters, and then allowed to descend by cutting the engine. The vehicle then descended, and the engine was restarted during flight by remote control to send it back up again. It was then allowed to descend again, and as it approached the ground, the engine was restarted a third time to decelerate the system to a semi-soft landing somewhat in the manner of the DC-X. Photographs of the Gashopper during lab captive tests and in outdoor test flight are shown in Figs 8 and 9.