Copyright ©1996, American Institute of Aeronautics and Astronautics, Inc.

AIAA Meeting Papers on Disc, July 1996A9637135, AIAA Paper 96-2961

Power electronics for a miniaturized arcjet

Luis R. Pinero

NASA, Lewis Research Center, Cleveland, OH

Glen E. Bowers

Gilcrest Electric, Brook Park, OH

AIAA, ASME, SAE, and ASEE, Joint Propulsion Conference and Exhibit, 32nd, Lake

Buena Vista, FL, July 1-3, 1996

A 0.3 kW Power Processing Unit (PPU) was designed, tested on resistive loads, and then integrated with aminiaturized arcjet. The main goal of the design was to minimize size and mass while maintaining reasonableefficiency. In order to obtain the desired reductions in mass, simple topologies and control methods wereconsidered. The PPU design incorporates a 50 kHz, current-mode-control, pulse-width-modulated (PWM),push-pull topology. An input voltage of 28 +/- 4 V was chosen for compatibility with typical unregulatedlow-voltage busses anticipated for smallsats. An efficiency of 0.90 under nominal operating conditions wasobtained. The component mass of the PPU was 0.475 kg, and could be improved by optimization of the outputfilter design. The estimated mass for a flight PPU based on this design is less than 1 kg. (Author)

Page 1

POWER ELECTRONICS FOR A MINIATURIZED ARCJET

Luis R. Pifiero *National Aeronautics and Space Administration

Lewis Research CenterCleveland, Ohio 44135

Glen E. Bowers tGilcrest Electric

3000 Aerospace ParkwayBrook Park, Ohio 44142

Abstract

A 0.3 kW Power Processing Unit (PPU) was designed, tested on resistive loads, and thenintegrated with a miniaturized arcjet. The main goal of the design was to minimize size andmass while maintaining reasonable efficiency. In order to obtain the desired reductions inmass, simple topologies and control methods were considered. The PPU design incorporates aSO kHz, current-mode-control, pulse-width-modulated (PWM), push-pull topology. An inputvoltage of 28 ± 4 V was chosen for compatibility with typical unregulated low-voltagebusses anticipated for smallsats. An efficiency of 0.90 under nominal operating conditionswas obtained. The component mass of the PPU was 0.475 kg and could be improved byoptimization of the output filter design. The estimated mass for a flight PPU based on thisdesign is less than a kilogram.

Introduction

Since the 1980's, various arcjet systems have beendeveloped for power levels from 1 to 30 kW>2 A 1.8kW hydrazine arcjet system with a specific impulse of500 s is currently operational on Lockheed Martin 7000Series spacecraft.! A 2.2 kW, 600 second specificimpulse system is currently baselined for North-SouthStationkeeping on a new GEO comsat series.3 A 0.5kW arcjet system is being developed and tested under ajoint NASA / industry program for both primary andauxiliary propulsion.4-5 Potential applications for thissystem are Stationkeeping, orbit insertion, and dragmake-up for communication satellites and primarypropulsion for near-Earth science spacecraft

NASA's current plans for Earth-space and planetarymissions require the miniaturization of spacecraft.

Spacecraft subsystems must be small, light-weight, andefficient due to the limited power and thermal controlcapacity inherent in small spacecraft design. A highperformance low power arcjet system may benefitmultiple missions by reducing the on-board propellantrequirements compared to resistojet, chemical, or coldgas systems while retaining a relatively simplepropellant system architecture.

To support NASA's initiative to reduce spacecraft size,a miniaturized arcjet system is being evaluated. Theminiature arcjet was designed to operate using eitherammonia or hydrazine propellant at a nominal powerlevel of 0.3 kW. This thruster has demonstratedthrottleability to 0.2 kW.6 Recent studies haveconcentrated on improving miniature arcjetperformance.7

* Electrical Engineer, On-Board Propulsion Brancht Electronics Systems Mechanic

Copyright © 1996 by die American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in theUnited States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights underthe copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner.

As part of the low-power arcjet development effort, a0.3 kW breadboard power processing unit (PPU) wasdesigned with the goal of minimizing size and masswhile maintaining reasonable efficiency. Powerelectronics for low-power arcjets have been developed inthe past based on both a full-bridge topology and current-mode pulse-width-modulated (PWM) control.8 Thisbreadboard was optimized for efficiency but not forminimum mass and complexity. In order to obtain thedesired reductions in mass and size in the miniaturizedarcjet effort, simple topologies and control methodswere considered. An input voltage range of 24 - 32 Vwas selected for compatibility with the unregulated lowvoltage power busses anticipated for smallsats.

This paper documents the design process andperformance characteristics of the PPU, and presents theresults of resistive load tests and integration tests with aminiature arcjet.

Design Considerations

Spacecraft / PPU Interface

Typical smallsats are anticipated to have unregulatedlow voltage power busses. To maintain compatibility,a nominal input voltage of 28 ± 4 V was chosen forthis design. Also, input-output isolation was desired toconform with single point grounding schemes. Eventhough this is a low power application, it is importantto obtain the highest possible efficiency to minimizeimpact on the spacecraft's power and thermal controlsystems. The efficiency goal for this design wasapproximately 0.90 which corresponds to 0.030 kW oflosses at a typical 0.3 kW output level. An EMI filterwas not designed because it was not within the scope ofthis program.

Thruster / PPU Interface

Typical voltage-current characteristics of theminiaturized arcjet for both simulated hydrazine andammonia propellant have been reported. 6 Ammoniaperformance was substantially higher than hydrazine andslightly higher arc voltages for the same arc current andflowrate setpoints were observed. Previous data showthat typical arc voltages for the flowrate range ofinterest and a power level of approximately 0.3 kW arewithin 130 to 170 V. The PPU was designed with anominal open circuit voltage of 200 V so that it could

operate the thruster using either propellant and couldprovide some contingency for higher voltage operation.A reliable pulsed ignition technique for hydrazine arcjetshas been demonstrated in previous work and was alsoused in this design.8-12 Initial values for the ignitionpulse of approximately 1.9 kV and 12 (is were chosenfor this design based on past experience. This may needoptimization but modifications are not expected toimpact the PPU design greatly. A maximum steadystate current ripple requirement of 15 to 20 percent wasused based on previous work.8-!!

Topology

The design goal was to minimize PPU mass andcomplexity while maintaining a reasonable efficiency.An isolated topology was also required to simplifyintegration of an arcjet system to a spacecraft by beingcompatible with single point grounding schemes.Previous arcjet PPU designs have used a phase shiftedfull-bridge topology.8 This topology was not chosenfor this effort because it requires a complex power stageincluding four power switches, isolation for the drivercircuits, and additional circuitry for gate drive phaseshift. A flyback topology with an additional winding toreset the power transformer (also known as forwardtopology) was also considered. This topology requiresonly one power switch without driver isolation. Themain disadvantages are that it is unstable during opencircuit operation and the power transformer core has avery low utilization factor because it is only excited inone direction. Finally, push-pull topology wasevaluated and chosen for this design. The push-pull hasbeen successfully used with higher power arcjets inprevious work.9 It requires two switches and a bifilarprimary winding as shown in the PPU schematic inFigure I. One issue associated with this particulardesign was the fabrication of the power transformer. Theexpected high primary current requires a considerableamount of wire to minimize conduction losses in thetransformer. As a result, special winding techniques toreduce leakage inductance and improve magneticcoupling are difficult to implement in the small coreselected to reduce the mass of the PPU. Current-mode-control pulse-width-modulation was used on this designto avoid staircase saturation in the power transformerand for primary current limit.

Transformer

The power transformer was wound on a toroidal 3F3ferrite core. Ferrite materials yield a lower mass for thesame cross sectional area compared to tape wound cores.Also, ferrite cores have lower core losses compared tomany metal cores. A switching frequency of 50 kHzand a maximum magnetic flux density of 0.27 T werechosen to reduce transformer size. Both windings weremade of multiple strands of 24 AWG magnet wire toreduce losses due to skin effect. The maximumcalculated core and conduction losses for thistransformer at nominal conditions were 3.1 W and 0.6W, respectively.

Output Filter

A maximum output current ripple of 15 to 20 percentwas used as in previous PPU designs.8-11 Because 2.0A of output current was specified for this design, a largeinductor is required to meet the ripple specification.Using a switching frequency of 50 kHz on the PPUreduces the tune the output inductor has to providecurrent to the arcjet while running at a certain dutycycle. This, in turn, reduces the inductance needed tomeet the ripple specification. An amorphous metallicalloy C-core was chosen because its high saturation fluxallowed both number of turns required and core size tobe reduced. The output inductor also has an integralstart winding which is part of the arcjet ignition circuit.Details on this ignition technique are presentedelsewhere.12 An advantage of this technique is that thepulse characteristics can be easily tailored by minorchanges in the ignition circuit to meet systemrequirements.

Physical Characteristics

Minimizing the mass and size of the PPU was the mostimportant requirement of this design. The totalcomponent mass of the breadboard PPU as seen hi thephotograph shown hi Figure 2, is 0.475 kg. The totalweight is 0.95 kg. The heaviest component in the PPUis the output inductor (0.175 kg). The inductor corewas oversized due to limited core availability, but itsweight and size could be considerably reduced by using asmaller core. Another heavy component is the powertransformer which weights approximately 0.100 kg.This transformer design compares favorably with sizesof other designs and its window utilization is very good.

Based on the breadboard weight and the anticipatedimprovements, it is estimated that a flight PPU couldweigh less than a kilogram.

Performance

Output Characteristics

The PPU output characteristics were evaluated viaoperations on a resistive load. The voltage-currentcharacteristics of the PPU, at an output current of 2.0 Aand for input voltages of 24, 28, and 32 V, are shownin Figure 3. The open circuit voltage is between 185and 245 V over the input voltage range. Table 1 showsperformance data for various input and load conditionson the PPU. Both load and line regulation are betterthan 1% over the range of load conditions and inputvoltages.

The arcjet ignition circuit is also powered by the inputbus. A family of ignition pulses from the PPU isshown in Figure 4. The magnitude and width of theignitions pulse is a function of the input voltage. Forinput voltages of 24, 28, and 32 V, the pulsemagnitudes were 1.6,1.8. and 2.1 kV,respectively, witha duration of approximately 12 {is. These result inpulse energies of 14.1,18.3, and 25.3 ml, respectively.

Efficiency

The efficiency of the power supply was measured usingdigital multimeters to measure the input and outputvoltages and currents while operating the power supplyon a resistive load. Table 1 shows efficiency data forvarious conditions. The efficiency varied between 0.88and 0.92 for typical load values over the input voltagerange of interest. The nominal operating condition wasan input of 28 V, an output current of 2.0 A, and a load60 to 70 Q. The measured efficiency at this nominalpoint was 0.90. Notice that all efficiency numbersquoted herein include housekeeping power. It wasobserved that the efficiency was higher when the PPUwas running at lower input voltage or when it washeavily loaded. This was anticipated because at theseconditions the power stage operates at higher dutycycles which reduces the required energy storage in theoutput filter. Calculated power losses for variouscomponents at nominal conditions are shown in Table2. The major contributions are due to switching andconduction losses in the power stage, core losses hi the

power transformer and the output inductor, conductionlosses in the output rectifiers, and losses in thesnubbers due to the leakage inductance of the powertransformer.

Efficiency could be unproved by using larger cores forthe magnetic components to reduce core magnetic fluxdensities. Also, the switching losses could be reducedby reducing the switching frequency . But, since themost important goal of this design was to minimizemass and volume, neither of these options wereimplemented. The power transformer windings werenot optimized but it is anticipated that better windingtechniques could help reduce the leakage inductancewhich would result in reduce power losses in thesnubbers.

Thruster Integration

The PPU was integrated with a prototype miniaturizedarcjet which is described in detail elsewhere.6"7 Thearcjet consisted of a single piece anode (W/2%ThO2),brazed to the rear-half of the thruster. The nozzle haddivergent and convergent half-angles of 15° and 30°,respectively. The constrictor had a diameter of 0.25mm and a length of 0.13 mm. The cathode was alsomade of W/2%ThO2 with a diameter of 1.6 mm and a30° half-angle conical tip. The arc gap was set to 0.41mm. The total weight of this thruster was 0.18 kg.Testing was done in a facility described elsewhere.13 Astoichiometric mixture with a 3:1 ratio of hydrogen andnitrogen was used to simulate ammonia. Thepropellant flowrate for the test was 15.0 mg/s.

Thruster Ignition

The arcjet testing was started at 28 V input voltage tothe PPU. At this input voltage, the ignition pulses hada magnitude of 1.9 kV, a duration of approximately 12{is, and an energy of approximately 18.3 ml. Theminiaturized thruster was reliably started throughout thewhole input voltage range. A typical breakdown of theminiaturized arcjet is shown in Figure 5. Prior to theignition pulse, the PPU open circuit voltage wasapproximately 215 V. As can be seen from the figure,the arcjet broke down close to the peak of the ignitionpulse. The current overshot to approximately 2.7 Aafter ignition and then ramped down to the nominal 2.0A setpoint in less than 2.0 ms.

Steady State Operation

During the integration test, the arcjet never stabilized.Large voltage variation and oscillations in the plumewere observed. It was presumed that this was caused bya cathode/anode alignment problem. For a 2.0 Acurrent, the arc voltage fluctuated around 140 ± 10 Vwhich corresponds to a power level of approximately0.280 kW. While it was not possible to resolve thisproblem in time to impact this report, the fact that thePPU was able to maintain this operating mode was veryencouraging as it represents a much harder operatingcondition than the steady state

Arc voltage and current waveforms for a nominaloperating point are shown in Figure 6. The currentripple is approximately 200 mA which corresponds to10.0 percent ripple at a 2.0 A output current. The arcjetwas operated for approximately 2.0 hours withoutincident

Conclusions

A 50 kHz, push-pull, current-mode PWM powerprocessor for miniaturized arcjets was successfullydeveloped. It was tested on resistive loads at powerlevels between 0.150 to 0.350 kW at input voltages of24 to 32 V. Line and load regulation was better than1 percent and efficiency ranged from 0.88 to 0.92 fortypical operating points. This design was based onprevious 1-kW class PPU designs and also included anintegral start winding on the output inductor for arcjetignition. The component mass of the PPU was 0.475kg and it could be further unproved by optimization ofthe output inductor design. It is presumed that a flightPPU using this design could weigh less than akilogram.

The PPU successfully operated a miniaturized arcjetdespite the fact that the device was running poorly dueto assembly issues. It was tested for approximately 2.0hours at a flowrate of 15.0 mg/s and a power level of0.280 kW. Multiple starts were successful at variousinput voltages to the PPU.

References

1. Smith, R.D., et al., "Qualification of a 1.8kW Hydrazine Arcjet System," Proceedings of the 23rd

International Electric Propulsion Conference, Sept. NASA TM 106204).1993, pp. 93-107.

2. Wong, S.P. and Britt, E.J., "Non-Isolated 30kW Class Arcjet PCU," NASA CR NAS3-25609,March 1994.

3. McLean, C.H., et al., "Life Demonstration ofa 600-Second Mission Average Arcjet," AIAA Paper 94-2866, June 1994.

4. Curran, P.M. and Sarmiento, C.J., "LowPower Arcjet Performance," AIAA Paper 90-2578, July1990 (also NASA TM 103280).

5. Curran, P.M. and Haag, T.W., "Extended Lifeand Performance Test of a Low-Power Arcjet, " J. ofSpacecraft and Rockets, Vol. 29, No. 4, July-Aug.1992, pp. 444-452.

6. Sankovic, J.M. and Jacobson, D.T.,"Performance of a Miniaturized Arcjet," AIAA Paper 95-2822, July 1995.

7. Sankovic, J.M. and Hopkins, J.B.,"Miniaturized Arcjet Performance Improvement," AIAAPaper 96-2962, July 1996.

8. Hamley, J.A. and Hill, G.M., "PowerElectronics for a Low Power Arcjets," AIAA Paper 91-191, June 1991 (also NASA TM 104459).

9. Gruber, R.P., "Power Electronics for a 1-Kilowatt Arcjet Thruster," AIAA Paper 86-1507, June1986 (also NASA TM 87340).

10. Hamley, J.A., et al., "10 kW PowerElectronics for Hydrogen Arcjets," NASA TM 105614,February 1992.

11. Gruber, R.P., et al., "5-KW Arcjet PowerElectronics," AIAA Paper 89-2725, July 1989 (alsoNASA TM 102108).

12. Sarmiento, C.J. and Gruber, R.P., "LowPower Arcjet Thruster Pulse Ignition," AIAA Paper 87-1951, June 1987 (also NASA TM 100123).

13. Sankovic, J.M. and Beras, D.H., "Performanceof a Low-Power Subsonic-Arc-Attachment ArcjetThruster," AIAA Paper 93-1898, June 1993 (also

Vin(V)

32.0032.00

28.00

28.0024.0032.0032.00

28.0028.0024.0024.00

Iin(A)

9.4011.90

10.6113.4412.267.279.25

8.2510.509.4812.03

Pin(W)

300.8380.8297.08376.32294.24232.64296.00

231.00294.00227.52288.72

Vout(V)

133.9173.8

133.70173.80133.70117.20152.60

117.30152.80117.00152.20

lout (A)

2.002.00

2.002.002.001.751.75

1.751.751.751.75

Pout(W)

267.8347.6267.40347.60267.40205.10267.05

205.28267.40204.75266.35

Eff(%)

89.0391.2890.0192.3790.8888.1690.22

88.8690.9589.9992.25

Table 1. Miniaturized arcjet PPU efficiency data for various input and load conditions.

Conduction Losses:

Transformer:Output Inductor:MOSFETs:Rectifiers:

Core Losses:

TransformerOutput Inductor:

Switching Losses:

Housekeeping:

Snubbers:

Other

Total at 267.4 W output:

0.6 W0.4 W2.2 W4.0 W

3.1 W2.7 W

4.5 W

2.5 W

5.5 W

4.2 W

29.7 W

Table 2. Measured and calculated power losses at nominal conditions.

Figure 1. Miniaturized arcjet PPU schematic.

Figure 2. Minitaturized arcjet PPU breadboard.

250-

< >200-

iso-

100-

50'

0 o coo oo o0 o o ooo— - °

+ 24 Vin

O 28 Vin

O 32 Vin

"5o

lout (Amps)

Figure 3. Miniaturized arcjet PPU output characteristics at 2.0 A output current and various input voltages.

IgnitionPulses500V/div

5jis/div

Figure 4. Family of ignition pulses at various input voltages.

Arc Voltage500V/div

Arc CurrentlA/div

0-

5(is/div

Figure 5. Voltage and current waveforms for arcjet ignition.

Arc Voltage20V/div

Arc CurrentO.SA/div

Figure 6. Output voltage and current waveforms for steady state operation of a miniaturized arcjet.