1. EP missionsprior to 1975 mostly involvedthrusters orpropellantsthatannotcurrentlybeiginvestigated(e.g. memuy ion engks, teilon pulsed plasma thrusters). Activities since 1975haveemphasizedfourEPdevicesthat arethemostlikelycandidatesfornear-termmisrions:
2. Since 1980morethan 140hydrazineresistojetshave been used for north-south stationkeepingon commercial satellites, andthey arejustifiably viewed as "off-the-shelf' hardware [17.46,47,49,109]. Currentperformancefigures (350-500 mN thtust, 0.6-0.8 kW input power, 300 s specific impulse, 500,000 N s total impulse [IS]) make hydrazine resistojets a viable alternative to bipropellant chemical systems for NSSK The demonmated total impulse shows that the problem of non-volatile residue deposition (leadingtoflowblockage in someearlierunits) hasbeensuccessfullyresolved[17,110].
3. During the 1% high-power resistojets with hydrogen propellant achieved impressive performance in groundtests(850sspecificimpulse,85%thruste 5ciency, 1kWto 30kWinpt-1) (1131. Thesethrusterscould pmvesuitableforcertainorbitaltrausfermissions[55], but the lackofhydrogendstojet developmentprogramssince 1970impliesthat 2-3yearsofneweffottwouldberequired to verify the early performance data and to establish an adequate operating lifetime [47,111]. Using hydrogen propellant for low-thrust orbit transfer missions with resistojw orarcjetsrequiresacorrespondingdevelopment of cryogenic storage and feed systems, but the relevant technicalissuesappeartobewellunderstood [114.1151 FlG. 7. Rocket R-h Compmfs 1.8-kW hydradne arcjet and power pnvessorhavebeenspsccqualified for 12 yeam of north-southstationkeeping(fromRef.52). 6. Arcjets
4. A 1.8-kWhydrazinearcjetand power processorhave been qualified by Rocket Research for the 12-47 NSSK mission on MMAS Series 7000 satellites (see Fig. 7) [Sl,52]. Given this level of design maturity,low-power arcjwwillbereadyforexpandeduse in the mid-199Os in applications such as NSSK at GEO and dragmakeup at LEO [116]. Ongoing ground tests and modeling [9,53,117-1191 to understand the impact of EMI,plasma interactions, heat loads, and surface modification will broaden the base of potential users beyond commercial satelliteslike Series 7000. Low-powerarcjetsfrom Eumpe andJapan[65,120-1231areatalevelofreadiness 2-3 years behind theRocketResearchthruster.
5. Arcjwformedium-power(3-10kw) andhigh-power (10-30 kw) applications are being developed through USAF and NASA technology programs [8,106,124,125]. Flightversions ofthe USAF ammoniathrusterand power proQssor are undergoing qualification for the ESEX demonstration in 1995 [126,121. Medium-power and high-powerarcjwh m Germany and Italy [120,121,128] existasadvancedlaboratorymodelsbut have not reached thefightquali 6cationstage.