NASA NTRS1972
The thruster development effort of the biowaste resistojet program is reported. Under this program, several variations of the concentric tube resistojet were built and tested. Because of the corrosive nature of the biowaste gases available for propellants, thruster effort was concentrated on the use of noble metals (platinum and its alloys) for the critical center tube heating element. This tube must operate at high temperatures. Initially the biowaste gases were projected to be CO2, CH4, H2O or H2 with only very small amounts of O2 or N2. However, during 1971, results from experimental life support tests indicated that the oxygen content would probably be in the range from 0.5 to 1.0%. To achieve the desired high specific impulses, metal temperatures as high as 1700 K (3060 R) were considered desirable goals. The first thruster, designated Mark 1, employed the alloy platinum-rhodium for the center tube. Although this material was oxidation resistant and had high strength relative to pure platinum, experimental materials tests showed that this alloy suffered from carbonyl reactions when tested on pure CO2. These reactions resulted in a depletion of rhodium and significantly weakened the tubes. The next series of thrusters were designated Mark 2 and employed platinum-iridium for the center tubes.