NASA Technical Memorandum 105636 AIAA-92-3516 Corrected Copy Development and Flight History of SERT II Spacecraft r William R. Kerslake Sverdrup Technology, Inc. Lewis Research Center Group Brook Park, Ohio and Louis R. Ignaczak National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio Prepared for the 28th Joint Propulsion Conference and Exhibit cosponsored by AIAA, SAE, ASME, and ASEE Nashville, Tennessee, July 6-8, 1992 NASA - A-11 Trade names or manufacturers' names are used in this report for identification only. This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. F CONTENTS Abstract ............ Introduction ......... Symbols and Acronyms Spacecraft.......................................................... ..............................3 Orbit............................................................ ..............................3 SolarArray ....................................................... ..............................6 Attitude Control ................................................... ..............................7 Gravitygradient ................................................. ..............................7 Ion thruster gimbals ............................................... ..............................7 Backup attitude control system ...................................... ..............................7 Horizonscanners ................................................. ..............................8 ThermalControl ................................................... ..............................8 Auxiliary Experiments .............................................. ..............................8 Spacepotential probe ............................................. ..............................8 Ionbeam potential probe ........................................... ..............................8 Surface contamination experiment ................................... ..............................8 Miniature electrostatic accelerometer ................................. ..............................9 Radiofrequency interference experiment ...........................................................10 Reflector erosion experiment ....................................... .............................10 Data, Command, and Telemetry ..................................... ............................... 10 Ion Thruster System .................................................. .............................11 Propellant Feed System .............................................. .............................11 IonThruster ....................................................... .............................II GimbalSystem .................................................... .............................12 Power-Conditioning Subsystem ....................................... .............................13 Development Program ................................................ .............................14 Thruster Efficiency Optimization ...................................... .............................15 Critical-Life Thruster Components ................................... ............................... 15 Cathodes....................................................... .............................15 Grids.......................................................... .............................16 Neutralizer........................................................ .............................16 Propellant Feed System .............................................. .............................17 Control of liquid mercury .......................................... .............................17 Propellant electrical isolator ...................................... ............................... 17 Thruster Power Conditioner .......................................... .............................18 Development.................................................... .............................18 Power conditioner heaters .......................................... .............................18 Thruster Performance Acceptance Tests ................................. .............................19 Thruster System Ground Life Tests .................................... .............................20 TestM ......................................................... .............................20 Test ......................................................... .............................20 Spacecraft Development ............................................. .............................21 Experimental mass dummy spacecraft ................................ .............................21 Experimental operational spacecraft .................................. .............................21 Prototype spacecraft .............................................. .............................21 Flight Spacecraft Qualification ........................................ .............................22 Flight Spacecraft Launch Integration ..................................... .............................22 Sert11 Flight Results .................................................. .............................23 Initial Ion Thruster Systems Tests, 1970 ................................. .............................23 Thruster operating characteristics .................................... .............................24 Space thrust measurements ......................................... .............................24 Spacecraft potential bias experiment .................................. .............................25 Contamination panel test results ..................................... .............................26 Accelerator grid shorts ............................................ .............................26 ................................ Radiofrequency Interference Tests, 197.0. ................................. .............................27 Thruster Cycling Tests, 197.3. t.o. 1.9.8.1. .................................... .............................27 Grid Short Cleared, 1.9.7.4. ............................................. .............................28 1975 to 1978 Tests ....................................... .............................28 Cross-Neutralization Test, 1979 ..................................... .............................28 Final Thruster Tests, 1979 .to. .1.9.8.1. ...................................... .............................29 Beam endurance test.s. .............................................. .............................29 Plasmathrust tests ........................................... .............................30 Cathode endurance tests ...................................... .............................30 Vaporizer and cathode heater.s. ....................................... .............................31 Insulator resistive changes .......................................... .............................32 Propellant feed tank tests ..................................... .............................32 Power conditioner performance ............................. .............................33 Reflector Erosion Experiment, 1970 to. .1.9.9.1. .............................. .............................33 Spacecraft Subsy.s.te.m.. P..er.f.o.rm..a.n.c.e. ..................................... .............................33 SolarArray ................................................. .............................33 Thermal surfaces ...................................... .............................37 Backup attitude c.o.n.t.ro.l. s.y.s.t.em.. ....................................... .............................38 Taperecorders ................................................. .............................38 Horizon scanners ........................................ .............................38 Control moment gyroscope.s. ......................................... .............................38 Telemetry and command ....................................... .............................38 Housekeeping backup battery .................................. .............................39 Inactive Spacecraft Floating Pot.e.n.ti.a.l. ................................... .............................39 Thruster-Spacecraft Interactions ......................................... .............................42 N onpropellantParticle Efflu.x. .......................................... .............................43 Neutral Pro.p.e.ll.a.n.t .E.f.fl.u.x. ............................................. .............................43 IonBeam ........................................... .............................44 Low-Energy Plasma Efflux ........................................ .............................45 Electromagnetic Fie.ld. .E.ff.l.u.x.e.s. ........................................ .............................45 Concluding R..em..a.r.k.s. .................................................. .............................46 References ................................................ .............................47 IonThruster System ..................................... .............................47 Auxiliary Spacecraft Experime.n.t.s. ...................................... .............................47 Spacecraft and Componen.t.s. ........................................... .............................48 Supporting References .............................48 ii DEVELOPMENT AND FLIGHT HISTORY OF SERT If SPACECRAFT William R. Kerslake and Louis R. Ignaczak National Aeronautics and Space Administration Lewis Research Center Cleveland, OH Abstract age) was operating. Second, an ion thruster beam was neu- tralized in space from a neutralizer cathode located at a A 25-year historical review of the Space Electric Rocket distance of 1 m. Finally, from 1989 to 1991 the spacecraft Test 11 (SERT II) mission is presented. The Agena launch was turned on to obtain long-duration data on the space vehicle; the SERT II spacecraft; and mission-peculiar erosion of an aluminum mirror surface and on solar array spacecraft hardware, including two ion thruster systems, are degradation. described. The 3 1/2-year development period, from 1966 to 1970, that was needed to design, fabricate, and qualify The SERT 11 mission was a NASA Lewis Research Cen- the ion thruster system and the supporting spacecraft com- ter in-house project. The Thorad-Agena launch vehicle was ponents, is documented. Major testing of two ion thruster managed by NASA Lewis personnel. The mission control systems and related auxiliary experiments that were con- center was at NASA Lewis, and was staffed by NASA ducted in space after the February 3, 1970, launch are Lewis personnel. The primary experiment managers were reviewed. Extended ion thruster restarts from 1973 to 1981 the NASA Lewis engineers who had invented and devel- are reported, in addition to cross-neutralization tests. Tests oped the ion thruster system. Over 120 either full or part- of a reflector erosion experiment were continued in 1989 to time NASA Lewis employees worked during the peak years 1991. The continuing performance of spacecraft sub- of the project (1968 to 1970). After launch and for the first systems, including the solar arrays, over the 1970-1991 year 15 people staffed the control room and analyzed data. period is summarized. Finally, the knowledge of thruster- During the extended mission years (1971 to 1991) two spacecraft interactions learned from SERT 11 is listed. engineers and several technicians supported spacecraft operations on a part-time basis. Introduction The reference list of this paper contains all papers (to the authors' knowledge) ever written on SERT II and is divided The SERT If mission was approved in 1966, and after a 3 1/2-year development and qualifying period the solar- chronologically into four major groups: ion thruster sys- powered satellite was launched into a 1000-km-high polar tem, references 1 to 21; auxiliary spacecraft experiments, orbit in 1970. SERT 1, which had been launched in 1964, references 22 to 30; spacecraft and components, references proved that broad-beam ion thrusters would operate and 31 to 51; and supporting references, references 52 to 74. produce thrust in space. The main objective of SERT 11 Table 1 gives a time line of the SERT 11 mission milestones from August 1966, when NASA Administrator James was to demonstrate that an ion thruster system could oper- ate for long periods (6 months) in space. Other objectives Webb approved the mission, until the end of spacecraft were to directly measure ion thruster thrust in space and to operations in November 1991. Table 11 gives the propul- demonstrate the lack of harmful interactions between the sion system performance, and Table 111 gives the system masses. ion thruster system and the spacecraft. Two identical SERT II ion thruster systems (each with 28-mN thrust, 4200-s specific impulse, and 850-W input power) were This paper describes the SERT 11 ion thruster and space- tested in 1970 for 5 months and 3 months, respectively, in craft systems, the ground development program of these space. Operation was stopped on each ion thruster when a systems before launch, the results of the primary mission to web erosion fragment of the accelerator grid caused an space test an ion thruster system (1970), the secondary mis- sion results of six auxiliary experiments conducted in 1970, electrical short. the results of the unplanned extended mission tests of 1971 Because the SERT II spacecraft was still functional, to 1991, and the performance of the spacecraft subsystems additional tests were performed from 1973 to 1981 to dem- during their 21-year (to date) operation. onstrate ion thruster restart capability. The web fragment was cleared from ion thruster 2 in 1973, and testing was Symbols and Acronyms resumed until both ion thruster propellant supplies were exhausted in 1981. Hundreds of restarts were demonstrated AGC automatic gain control before propellant supplies ran out. Two new modes of ion BACS backup attitude control system thruster operation were evaluated. First, thrust was meas- ured when only the main discharge (no high extraction volt- BOL beginning of life TABLE I.—CHRONOLOGICAL LIST OF SERT II MISSION MILESTONES Year 1966 SERT 11 approved by NASA Administrator Discharge chamber optimized 1967 Critical ion thruster components life tested TRW ion thruster life tests begun Ion thruster design frozen Ion thruster operated with breadboard power conditioner Experimental spacecraft assembled Ion thruster operated with experimental power conditioner 1968 In-house ion thruster life tests conducted Prototype power conditioner delivered Prototype spacecraft assembled McDonnell Douglas thruster system life tests started Prototype spacecraft testing Flight spacecraft assembled without thruster systems 1969 Prototype power conditioner problems solved Flight power conditioner delivered Flight thruster systems mounted on Flight spacecraft 1970 Flight spacecraft shipped to launch pad SERT II launched Thruster system I operated 3781 hr at full power Spacecraft potential control demonstrated Ion thrusts measured three separate ways are in agreement Thruster system 2 operated 2011 hr at full power TRW life test stopped at 6612 hr (resource limit) McDonnell Douglas life test ended at 6742 hr (propellant tank empty) Restart attempts to clear grid shorts 1971 Spacecraft activated for STADAN tests 1972 Spacecraft inactive 1973 Cathode restart tests Spacecraft spin stabilized 1974 Spacecraft inactive Thruster system 2 grid short cleared and thruster beam tests again conducted Spacecraft inactive 1975 Spacecraft operated to check attitude Thruster system 2 beam tests, thruster 1 cathode restarts 1976 Both thruster cathodes restart tests 1977 Spacecraft oriented to face Sun and respun 1978 Month-long cathode restart tests and periods of inactivity 1979 Continuous Sun period begins again Thruster 2 operated at 35 percent power for 606 hr Cross neutralization demonstrated Thrust produced by discharge chamber plasma 1980 Neutralizer 2 tank empty after 3878 hr Thruster system 2 operated using neutralizer 1 for 110 hr Neutralizer I tank empty after 4991 hr Thruster 1 tank empty after 7928 hr 1981 Thruster 2 tank empty after 10 096 hr 1982 Spacecraft inactive from 1982 to early 1989 1989 Spacecraft reactivated for third time; continuous Sun period begins REX data obtained over 21-year period 1990 Solar array degradation data measured after 21 years in space 1991 End of data taken, November 1991 1992 2 TABLE 11.—SERT II PROPULSION SYSTEM PERFORMANCE SERT I Space Electric Rocket Test 1, launched in 1964 Total power (BOL), W .................................. 1425 SERT II Space Electric Rocket Test 11, launched in Power for propulsion (BOL), W ........................... 1 195 1970 Number of ion thrusters .....................................2 Thrust per thruster (100 percent beam), mN ....................28 SSU spacecraft support unit Specific impulse (overall), s ..............................4200 Total propellant now (with neutralizer), kg/s .............6.9X10—' STADAN Satellite Tracking and Data Acquisition Total propellant utilization, r(e, percent ........................75 Network Thruster power efficiency, rlp, percent ........................89 Thruster efficiency, 11.X71r, percent ..........................67 OS angle of Sun line to orbit plane normal, deg Power conditioner power efficiency, percent ...................8^ OSA angle of Sun line to solar array plane normal, deg (OS = OSA±2°) when spacecraft is not TABLE 111.—SERT 11 SYSTEM MASSES spin stabilized) Overall mass, Spacecraft kg Agena launch vehicle (dry) 740 The SERT II spacecraft was launched by a Thorad- SERT 11 spacecraft portion 282 Agena, which is shown in Figs. I(a) and (b) on the launch SERT 11 spacecraft support unit 220 pad and at lift-off. Figure 1(c) shows the maneuvers by the Solar array (complete) 193 Thorad-Agena that were required to place the spacecraft in Thruster systems (dry) with PCU, two 59 the 1000-km-high polar orbit. The main bulk of the final Total mass in orbit (BOL, wet) 1524 spacecraft was an empty Agena rocket that was 1.53 m in Ion thruster diameter (Fig. 1(d)). On the Agena aft end (up in Fig. 1(d)) system mass, the two wings of the main solar array each extended out- kg ward for 6 m and were 1.5 m wide. Attached to the for- Ion thruster 3.0 ward (bottom) end of the Agena was the cylindrical Propellant (including neutralizer) 15.0 spacecraft support unit.32 The spacecraft support unit con- Thruster tankage (dry) 4.3 Thruster gimbals 7.7 tained the spacecraft's power conditioning and switching (with support structure) components, telemetry and command system, and attitude Power-conditioning unite 14.5 control components. Attached to the bottom of the space- Power-conditioning thermal 2.4 craft support unit was another cylindrical section called the control spacecraft. The spacecraft section housed two ion thruster Specific systems and all auxiliary experiments. The overall length mass, kg/kW from Agena nozzle to ion thruster mounting deck was 7.9 m, and the total mass was 1435 kg. 35 A block diagram Solar array (1425 W BOL) 135 of the major subsystems is shown in Fig. 1(e). Thruster (843-W input) 3.6 Tankage 5.1 Gimbals 9.1 Orbit Power-conditioning unit 17.7 (952-W input with thermal The SERT Il spacecraft orbit was 1000 km high and control mass) nearly polar (99.10 inclination) with a period of 106.2 min. "Includes 2.0 kg of nonpropulmon hardware. A nonscale representation of the spacecraft and its orbit is shown in Fig. 2. The choice of orbit was influenced by CMG control-moment gyroscopes (two pairs of two several factors. First, the orbit had to be achieved by using the Thorad-Agena launch vehicle. Second, the solar array each) had to be in continuous sunlight for at least 6 months. EMI electromagnetic interference Third, the minimum altitude had to be sufficiently high to avoid excessive aerodynamic drag and torque. Fourth, the g's one g equals Earth's gravity, 9.807 m/s2 maximum altitude had to be sufficiently low that adequate G SFC Goddard Space Flight Center of NASA gravity-gradient torques would be obtained for spacecraft stabilization48 and that excessive solar array degradation MESA miniature electrostatic accelerometer from high-energy protons and electrons would be avoided. PERT program evaluation and rating technique Periods of continuous sunlight were achieved from launch (February 1970) to November 1970; from February 1971 to PCU power conditioner unit November 1971; from January 1979 to April 1979; from August 1979 to May 1980; from July 1980 to May 1981; REX reflector erosion experiment from April 1989 to October 1989; from January 1990 to RFI radiofrequency interference December 1990; and from January 1991 to November (a)Thorad-Agena launch vehicle on pad. 1 C-70-1721 (b)Thorad-Agena launch vehicle at lift-off. Figure 1.--Space Electric Rocket Test II. Ascent phase — — — Orbital phase Ascent; solid- Final orbital propellant altitude _ _ rocket-motor- (1000 km) \ Y case jettison y IX \ V^llel X X, Thorad- Agena 1 separation r lo' Agena engine first start and shroud jettison ;h Age a second powered phase (c) Planned ascent and initial orbit of SERT II. (d) Artist's drawing of SERT II spacecraft in orbit. Ion beam Ion thruster r-- 1. 1^ Neutralizer Propellant tanks I Gimbal I PCU Digitizer Telemetry Spacecraft Commands CMG tj CMG ] Spacecraft support unit Horizon { scanner Agena `^I( Solar Soiar array array (e) Block diagram of major subsystems. Figure 1.—Concluded. 5 1991. Continuous-sunlight orbits will not occur again until the period July 1998 to July 2001. /-- Roll axis Solar Array The SERT II solar array, which was built by Lockheed 31,72 Fitch axis' .\ Missiles and Space Company, was derived from a -Yaw axis design that had flown successfully on several military mis- sions. The configuration of 90 array panels, consisting of x ^ i --Ground- 33 000 2- by 2-cm N-on-P silicon cells with 0.5-mm-thick Equatorial tracking fused silica cover slides, is shown in Fig. 3(a). The main or i p lane ^ ^ stations t thruster array of 76 panels furnished a maximum power of 1195 W at beginning of life (BOL) at 55 V DC and 21.6 A W _ >_ I with a solar flux of 1292 W/m2 and an array temperature of I Earth 327 K. Individual panel output was measured on the t ground and summed to obtain the total array power before launch. Each thruster array panel had 30 of its 370 cells ^ s intentionally shorted before launch. This was done to reduce the open-circuit voltage so that it would fall within Near-polar the specified upper limit of input voltage for the thruster circular orbit power conditioner.45 The remaining 14 panels (called the housekeeping array) furnished 230 W (BOL) at 30 V DC to CD-9017 power the spacecraft support unit and auxiliary experi- ments. The solar arrays were folded at launch and Figure 2.—SERT II vehicle coordinate system in orbit viewed from Sun for spring launch and sunset orbital injection. + Panels with positive potential, negative ground Panels with negative potential, positive ground —19.1ft(5.8m)—^ 21.2ft(6.5m)^ 4.8 ft Location of (1.5 m) test solar cells 45 panels per wing arranged in 15-by-3 configuration. (a) Panel arrangement of solar array. (Fourteen gray-shaded panels furnished housekeeping power; 76 other panels powered ion thruster system.) Upper half of ion thruster K1 — — —— — solar array; 12 920 two- by-two cells _— — (190 parallel, 68 series) O= telemetry monitor y A—__. Center tap House- Housekeeping 5 A Lower half of Thruster Thruster Thruster keeping V array: 5180 ion thruster system 1 system 2 power loads two-by-two I solar array; conditioner cells (74 series, 0 A 12 920 two- heaters for 70 parallel) by-two cells systems 1 (190 parallel, and 2 68 series) Spacecraft structure (b) Block diagram of unregulated power system. Figure 3.—SERT 11 spacecraft power system.