Keith D. Goodfellow

Results of an On-Going Long Duration Ground Test of the DS1 Flight Spare Engine

Ground testing of the DS1 night spare thruster (FT2) is presently being conducted. To date, the thruster has accumulated over 4500 hours of operation. Comparison of FT2 with the performance of the engineering model thruster 2 (EMT2) during the 8.2 khr test shows a transient, lasting for about 3000 hours, during which the discharge chamber efficiency decreases for both thrusters. The flow rates are 2% lower for FT2 than for EMT2 and the discharge chamber performance is 4.5% lower for FT2 during the transient. Sensitivity data obtained during the test show that the lower flow rate accounts for about half of the observed difference. After the initial transients decay, the performance of both thrusters is comparable with the exception of the electron backstreaming margin--which is 6 V lower for FT2.

Methods for Cryopumping Xenon

Design chamclcris(ics ofacryopun~p o~)ti\~]i~,d forxc]]on arc])rcscl)tcd. Sin.glc-stagc Gifford-McMahon hcliun~ cryoprrnrps that])rovidc acryc)rcfrigcratiol) capability of105Wat 50Ktochill coldplatcs arcuscd witioulbafflcsto cnablclargcpurnp spedsonxcn on. Shrouds arcusti torducc ticthcrt]lal loadtodlc cold ]~latcto cnablc. gr~tcrxcl~onpun)p spcrds, but it is notrcquircd tooWratc thcshrouds at1,N2tct))pcra( urcs,Ton ]axin]i~.cp ul])l)s lwtiorlx erlongascss uc.hasnitrogcn, neon and helium can not be cryotrappcd with these pumps. q’t)ccryo1)u~~)psarc52ct~] in length, 16cn~in diarmtc.randwcigh 15.4 Kg, however the cold head assembly is only 29 cm long and 8,3 cm in diameter. The cryorcfrigcralors can be mourr(cd using a 1 S-cm-dia ftangc, and a 10-cm-dia through-hole in the vacuum c}]amber. Alternatively, the cnlirc. cryopump can be mounted inside the vacuum chamber, although preliminary data indic.am that pump efficiency dccrcascs when the pumps arc operalcd in vacuum. I’cst

Segmented ion engine operation and performance

indicate that the full lhcorctical pump speed on xenon is achic.vcd, A pumping systcm designed for the 1,000 hr NSTAR validation test was opaatcd without shrouds or baffles. A poor] y controlled thermal radia[ion load 10 the cryopancls and uncxjxx%.xlly low cryopancl cocfficicnl of thermal conductivity rcsrrllcd in pressure spikes and a drop in the xenon pump speed. A cryopurnping syslcm u[ilixing three cryorcfrigcrators wilh a chilled shroud but without baffles is being used on (hc 8,000-hr NSTAR wear lest to bcticr control the thermal cnvironmcmt Each of [IICSC xenon cryopumps provides a pump speed on xenon of approximately 15,000 1,/s. The, small si~.c of the pumps, coupled with their abil ity to he opcramd in vacuum, provide the user with a grca[ deal of vcrsalili(y as far as locating ihe pumps with rcspca to thrus[crs, tank walls, shrouds, CIC. The COS1 and comp]cxity of installing and operating these cryopumps is a fraclion of the. cost and complexity to install and opera[c diffusion pumps or cryopumps thal deliver sitnilar pump spct.ds on xenon.

Field emission array cathodes for electric propulsion systems

The continuing trend toward smaller and smaller planetary spacecraft to enable the use of smaller, less expensive launch vehicles has nmtivated an examination of new approaches to reduce the sire. and mass of xenon ion propulsion systems for these new spacecraft. A system is proposed which is based on the use of a single 4x15-cm segnxmted ion thruster operated horn a single internally redundant power processing unit. An engine input power throttling range of 380 to 4640 W is projected. The design and operation over a power range of 600 to 2400 W of a laboratory nwdel segrmmted ion engine equipped with carbon-carbon grids is presented. There appear to be no problems a..sociated with interactions with multiple ion sources operating fkom a single set of high voltage power supplies, neutralization of multiple ion sources tkom a single centratly-located neutralizer, or operation of flat, thin carbon-carbon grids.