For several years an intensive program has been in progress at the Uni
versity of Stuttgart to investigate and develop thermal arcjets for pr
opellants including ammonia, nitrogen-hydrogen mixtures simulating hyd
razine, and hydrogen. Since hydrogen yields the highest specific impul
se I-sp and best efficiencies eta, special emphasis was placed on this
propellant. Arcjet power levels between 0.7-150 kW have been studied,
including water- and radiation-cooled laboratory models and flight ha
rdware. Results yielded a maximal attainable I-sp as a function of the
design and power level and showed that increasing power increased I-s
p. Radiation-cooled arcjets show better eta and I-sp than water-cooled
devices, but raise technical problems because of the high temperature
s of the thrusters, which require the use of special refractory materi
als. Proper arcjet optimization was done with a thorough thermal analy
sis, including the propellant flow. A further improvement of these thr
usters was reached by regenerative cooling and by optimizing the const
rictor contour. The constrictor now is modeled by a three-channel mode
l, the results of which are compared with experimental data. A new two
-dimensional computational fluid dynamics (CFD) approach for hydrogen
arcjet thrusters is presented. In 1996 a 0.7-kW ammonia arcjet is sche
duled for a night on the P3-D AMSAT satellite.