Compton Gamma Ray Observatory - Lessons learned in propulsion

G.A. Dressier*, G.W. Joseph*, H.W. b "hrens* r"I'R_V, Redondo Beach, CAbD.I. Asato* (,\.ISA GSFC, Greenbelt, MD) and i¢ _. Carlson (Rancho Palos Verdes. C.t;ABSTRACTThe Compton Gamma Ray Observatory was thesecond of NASAs Great Observatories. At [7_,Z tons. itwas the heaviest astrophysical payload ever flown at thetime of its launch on April 5, 1991 aboard the SpaceShuttle. During initial, on-orbit priming of thespacecraft's monopropellant hydrazine propulsion system,a severe v,aterhammer transient was experienced. At thattime. anomalous telemetry readings were received fromon-board propulsion system instrumentation. This led toground analyses and laboratory investigations as to theroot cause of the waterhammer, potential damage tosystem integrity and functionality, and risks for switchingfrom the primary (A-side) propulsion system to theredundant (B-side) system. The switchover to B-side wasultimately performed successfully and the spacecraftcompleted its basic and extended missions in thisconfiguration. Nine years later, following a criticalcontrol gyroscope failure, Compton was safely deorbitedand re-entered the Earth's atmosphere on June 4. 2000.Additional risk assessments concerning viability of A-and B-sides were necessary, to provide confidence inattitude and delta-V authority and reliability to managethe precisely controlled reentry. This paper summarizesthe design and operation of the propulsion system used onthe spacecraft and provides "'lessons learned" from thesystem engineering, investigations into the propellantloading procedures, the initial priming anomaly, missionoperations, and the commanded re-entry following theg',ro failure.INTRODUCTIONThe Compton Gamma Ray Observato_ (CGRO} wasa large scientific spacecraft designed for celestialobservations from low Earth orbit. It was the secondelement launched in NASA's deployment of four "'GreatObservatories" (HST. CGRO. Chandra-AXAF. SIRTF)and carried instruments dedicated to the highest part ofthe electromagnetic spectrum. The objective of the CGROmission was to obtain gamma-ray measurements over theentire celestial sphere v, ith unprecedented sensitivity.spectral range and resolution.CGRO was launched aboard the Space ShuttleAtlantis (STS-37} on April 5. 1991, and was deployedApril 7 into a 45 _ km circular orbit at 28.5 degreesinclination IFigure 1 h At the time of its deplo.,,ment it sett_o rect_rds for non-m_l_tary spacecraft: tt ,,,,as the largestspacecraft launched by STS and it had the largestm_,nt>prapellant propuls_t>n s,,stem ever tlo'an." Member of A[A.\Figure 1. CGRO Deployment from STS-37After nine years of exciting scientific disco,,eries _: ofvery energetic celestial phenomena (far exceeding itsminimum mission life of 27 months). CGRO '*as safelyde-orbited _ith controlled re-entry into the Earth'satmosphere on June 4. 2000. Pieces of the spacecraftsurvived the re-entry, landing in a remote part of thePacific Ocean near the equator, approximately' 3.862 km(2.-100 miles} southeast of Hawaii.The CGRO mission was a NASA cooperativeprogram managed by the NASA Goddard Space FlightCenter IGSFC} and included co-investigators from theUnited States, Federal Republic of German_. Netherlands.ESA and United Kingdom. The Observatory carried fourhighly sophisticated instruments capable of makingsimultaneous measurements over six decades of energy120 keV-30 GeV? These instruments ,*ere: the Burstand Transient Source Experiment (BATSE t. the OrientedScintillation Spectrometer Experiment (OSSE_. theImaging Compton Telescope (COMPTELh and theEnergetic Gamma Ray Experiment Telescope ,EGRET}The CGRO spacecraft _,,as designed and de_elopedb_ TRW in Redondo Beach. CA. Table i presents asummary of the spacecraft subs._ stems. Themonopropetlant hxdrazine propulsam subs'.stemconsisted of "'A-side'" and full,:' redundant "'B-s_de" sets ofthrusters, feed system c_)mp,ments and pr_)pe]]ant tanks.Although norma ly inacti',e during science gathering, thepropulsion system ,._as :o be used e;er,, _--, '.ears toreb, n_st CGRO to ,_ttset decay in orbital altitude due toatmo.,pherw dragThe ',pacccr:tt't'_ propui>l_m sub,;2,stem .had :'._, majoron-,,rbtt an_wnalie,, durine :he mls_,i_m [he ;-_r.: .m,,malv,c,:'.,rred durt ny' che.:k_ut -nd actt', atu m. ,t :he .{xwecr:fftarter being released tn,m 'he Shuttle [mmed_,::c'.', up,,n_pcntng,mc,_tthe pn_pe!lant:ank >_lati,,n,.._['...:.. r_:hc('_p_,rIght ')2001 bv TRW [nc Published b', Amcrwan Institute ,t \c>'f_at_t:,- _,,_rz_i \,rr,,r:a::_:_,. [*;c. '._th _c::_:..,, _