IINASA Technical Memorandum 1031.9_0............

AIAA-90-2579 ..................

Arcjet Load characteristics _ _

John A 5Hamley .............................

Lewis Research Center

Cleveland, Ohio _

, T

(NA_A-TM-103190) ARC JET LOAD

CHAPACTFRISTIC S (NASA) 18 p

CSCL 2iH

N90-25181

G3/eo

Prepared for the

........... _21stlnternational Electric_ Propulsion Confer eence ___:-_ _

, _ . _cosponsored by the AIAA, DGLR, and JSASS

Orlando, Florida, July 18-20, 19901_IIL _ 7 -

ARC JET LOAD CHARACTERISTICS

John A. Hamley

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

Experiments were conducted to define the

interface characteristics and constraints of 1 kW class

arcjets run on simulated decomposition products of

hydrazine and power processors. The impacts of

power supply output current ripple on arc jet

performance were assessed by variation of the ripple

frequency from 100 Hz to 100 kHz with 10% peakto-peak ripple amplitude at 1.2 kW. Ripple had no

significant effects on thrust, specific impulse or

efficiency. The impact of output ripple on thruster

lifetime was not assessed.

The static and dynamic impedances of the arcjet

were quantified with two thrusters of nearly identical

configuration. Superposition of an AC component

on the DC arc current was used to characterize the

dynamic impedance as a function of flow rate and

DC current level. A mathematical model was

formulated from these data. Both the static and

dynamic impedance magnitude were found to be

dependent on mass flow rate. The amplitude of the

AC component was found to have little effect on the

dynamic impedance. Reducing the DC level from 10

to 8 amps led to a large change in the magnitude of

the dynamic impedance with no observable phase

change. The impedance data compared favorably

between the two thrusters.

Ap_p

AC

C

I3(:

f

iac

iarc

J

Ki

m

n

Pi

Prms

qi

NOMENCLATURE

Amperes peak-to-peak, A

Alternating current

Arc jet dynamic impedance gain

constant

Direct current

Frequency, Hz

AC arc current, A

Arc current (total), A

DC arc current, A

(:iGain constant for a specific

pole-zero pair

Total number of zeros

Total number of poles

Dynamic impedance pole

RMS power, W

Dynamic impedance zero

s Complex frequency

variable, j2_xf

T Thrust,g

vac AC arc voltage, V

Varc Arc voltage (total),V

Vdc DC arc voltage, V

Zd Dynamic impedance,

0 Impedance phase angle,

degrees

x Transportation lag, seconds

INTRODUCTION

Arcjets were first considered as advanced

propulsion devices in the 1950's, and this early

research program carried into the 1960's. At this

time, NASA's interest was primarily in planetary

missions and research centered mainly on high

powered arcjets with hydrogen propellant. 1-4

Ammonia was also considered as an alternate

propellant. Arcjets in the 1 to 2 kW class were

considered for an auxiliary propulsion role, and these

devices were run with some success on hydrogen

propellant. 5,6 Limited efforts to run the devices at

these lower power levels on storable propellants met

with little success. 7 In 1965, a review of these

programs was presented by Wallner and Csika. 8

Interest in arcjets renewed in the last decade,

especially low power devices run on storable

propellants. A 1-2 kW device was selected for northsouth stationkeeping missions (NSSK) on

geosynchronous spacecraft. Current programs have

demonstrated stable operation of these devices over a

wide range of power levels and mass flow rates.

Specific impulse values in the range of 450 to 550

seconds have been demonstrated at power levels and

propellant pressures expected in a NSSK mission. 9

14

In response to the need for system hardware, pulse

width modulated (PWM) switching power supplies

with integral starting circuits were designed to

provide reliable start-up and steady-state operation

with vortex flow stabilized arcjets 15. These power

processing units produced 13 kHz ripple of

approximately 1.5 ampere peak-to-peak amplitude,

however, the effect of ripple on arc jet performance

was unknown. Preliminary testing indicated a

relatively fast response in the arc column, leading to

a step response specification on the order of 500

Copyright © 1990 by the American Institute of Aeronautics

and Astronautics, Inc. No copyright is asserted in the

United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under

the copyright claimed herein for Governmental purposes.

All other rights are reserved by the copyright owner.
l.ts.15-18No detailedwork to characterizethe

dynamicimpedanceofthearcjetwasdoneduringthe

development of thepowerprocessors, however,a

preliminaryinvestigationtocharacterize thedynamic

impedancewasconductedafterward, whichdeveloped

themethodsusedhere.19

Thispaperpresentsheresultsof initialeffortsto

evaluateandcharacterizethearcjet/power processor

interface.First,thestaticimpedanceof thearcjet

wasquantifiedasa functionof DCinputcurrent

overarangeof 8to 11.5A.Theeffectof flowrate

changeswerealsoinvestigatedoverthisrangeof

inputcurrents.Thedynamicimpedanceof thearc

wasquantifiedbyinjectinganACcomponent onto

theDCarccurrentoverthefrequencyrangeof 100

Hzto 100kHz.Effectsof variationsin flowrate,

DC currentlevel,andrippleamplitudeon the

dynamicimpedancewerealsoquantified.The

impactsof rippleon thrusterperformancewere

assessedwith10%peak-to-peak rippleamplitudeby

simultaneous measurement ofhrustanddissipated

powerwhilevaryingtheripplefrequencyoverthe

rangeof 100Hz to 100kHz. Thrust,specific

impulseandefficiencywerethenplottedasfunctions

of ripplefrequency.Theissuesof lifetimeand

electromagnetic compatibilityasafunctionofripple

amplitudeandfrequencywerenotaddressed here.

Thiseffortwill provideinputsfor criteriato

determinetherippleacceptablein futurepower

processor designs,whichcouldofferreductionsin

massinoutputfilters.Thedynamicimpedancedata

arealsoapplicableto powerprocessorcontrols

designintermsof stepresponseandcompensation.

Thiswillprovidemoredetailedinformationastothe

characteristics ofthearcjetandpowerprocessor asa

systemtothepowerprocessor designer.

APPARATUS

Arciet Thruster

Two modular thrusters of nearly identical

configuration were used. A cutaway schematic of the

thruster is shown in Figure 1. The thruster has been

described in detail elsewhere. 20 The constrictor was

0.51 mm in diameter and 0.25 mm in length. A

3.18 mm 2% thoriated tungsten welding rod with a

conical tip and 30 ° half angle was used for the

cathode. The anode also was made from the same

material and had a 30° half angle convergent section

and a 20 ° half angle divergent section. The arc gap

was set to 0.58 mm.

Propellant Managcm¢nl

A 2:1 stoichiometric ratio of hydrogen and

nitrogen gas was used as a propellant throughout the

testing which simulated the decomposition products

of hydrazine monopropellant. The flow rates were

maintained using commercially available mass flow

controllers.

A block diagram of the power supply used is

presented in Figure 2a. A 150V 15A laboratory

supply with a series pass regulator to maintain

constant current was used to supply power for the

experiment instead of a standard arcjet power

processor. A diode in series with the regulator

protected it from the high voltage pulse used to start

the arcjet` A shunt modulator injected an AC signal

of variable frequency and amplitude onto the the

regulated DC supply current. Figure 2b details the

operation of the modulator. The output of the

current regulator was represented as ireg and the

current through the modulator imod, was controlled

by the function generator. The arc current iarc was

expressed as:

i_= i_g- imod (1)

where imod depended on the input from the function

generator. The modulator required a 2A DC bias

current to operate in a linear fashion, thus, the DC

component of the arc current, idc was lower than the

output of the current regulator. This bias current

remained constant while the device is in operation. If

a sinusoid was input to the modulator the arc current

was represented as:

i_ = idc + iacsin (2rift) (2)

where idc represents the difference between the

regulator current and the DC bias current of the

modulator. The amplitude and frequency of iac were

dependent upon the input from the function

generator. This approach was used because the

frequency response of the current regulator was

insufficient to modulate its output directly. A

vacuum relay protected the modulator from the high

voltage ignition pulse. After the arcjet was started,

the vacuum relay was closed and the DC bias on the

modulator was applied slowly, avoiding any abrupt

changes in arc current.

The arcjet was started with a pulse transformer

derived from the 1 kW arcjet power electronics. Its

operation is described in detail elsewhere 15,18. The

pulse was initiated using a pushbutton on the front

panel of the equipment rack. An interlock inhibited

the operation of the starting circuit if the vacuum

relay in the modulator were not open. Coaxial cable

was used to supply the current to the arcjet.

_saungamtmn

Arc current was measured using a non-intrusive

current probe and a current shunt. The current probe

was a commercially available probe with a

bandwidth of DC to 5 MHz which measured both
DCandAC currentcomponents, whiletheshunt

wasusedto measureDCcurrentonly.Arcvoltage

wasmeasuredusinganisolatedoscilloscopeprobe,

andstandardigitalmultimeters(DMMs).These

deviceswerecapableof measuringtheACandDC

componentsofthevoltageandcurrent.Thecurrent

probeandshuntwerelocatedatthefeedthroughto

the vacuumchamberandthearc voltagewas

measuredusingpotentialeadsmountedto the

thrustertominimizetheeffectsof thepowercable

andconnectionimpedances.

A gain/phasemeterwasusedto measurethe

relativemagnitudesandphasesoftheACarccurrent

andvoltage.Therearetwo inputson themeter.

Channel Aisusedasareferenceandtheratioof the

ACamplitudesofthetwochannelsaredisplayedas

20log(B/A)alongwiththephaseanglebetweenthe

twochannelsindegrees. Anoscilloscopewasused

to verify the magnitudesof the arc current

components andthefrequencyoftheACcomponent

wasdisplayedonafrequencyounter.

Thrustmeasurements weremadeusinga flexure

displacement thruststandwhichwascalibratedin

situ. This thruststandis describedin detail

elsewhere. 17

Facilities

The experiments were carried out in two

facilities 20. All impedance measurements were done

with the thruster mounted in a small bell jar 0.64 m

in length and 0.64 m in diameter. A single

mechanical roughing pump provided 21,000 lpm

pumping capacity. Pressures on the order of 100 Pa

were maintained at the maximum propellant flow.

All performance measurements were carried out in a

vacuum tank 5 m in length and 1.5 m in diameter.

Four 0.81 m diameter oil diffusion pumps backed by

a lobe type blower and two mechanical roughing

pumps maintained the background pressure at

approximately 0.05 Pa at maximum propellant

flow.

PROCEDURE

To quantify the effect of power processor ripple

amplitude and frequency on thruster performance it

was first necessary to establish a baseline

performance level with a DC input current. The

thrust level, specific impulse, and efficiency were

dependent on the power input to the device, which

was determined by the static impedance only for the

DC case. As ripple was introduced, the dynamic or

AC impedance of the device was included in the

equation used to determine the dissipated power.

This relation is discussed in depth later. Because of

this, the static and dynamic impedances of the arcjet

were quantified prior to performance testing.

Performance data were taken on only the first of the

two arcjets, hereafter referred to as thruster 1. The

second arcjet was referred to as thruster 2. Dynamic

impedance data were taken for both arc jets.

All impedance data were taken in the smaller of

the two facilities described previously. Static

impedance data were taken using thruster 1 only.

The static impedance was measured using a shunt

resistor and a DMM to monitor the arc current and

another DMM to monitor the arc voltage. Potential

taps were added to the thruster and brought out

through feedthroughs. The shunt resistor was

mounted at the power feedthrough. Both DMMs

were battery operated to avoid ground loops.

Flow rates of 4.3x10 -5, 4.5x10 -5 and 4.8x10 -5

kg/sec were used to investigate the dependence of the

dynamic impedance on flow rate. These flow rates

are representative of those expected on a NSSK

mission. The flow rate to the thruster was set and

maintained constant throughout a test. The arc jet

was started and allowed to stabilize thermally at 10A

DC operating current and 4.3x10 -5 kg/sec flow for

30 minutes. The current was then reduced to 8A and

a 10 minute equilibration period followed. Static

voltage and current data were then taken. This

process was repeated increasing the current 0.5A DC

increments for each data point. Ten minutes were

allowed at each setpoint for equilibration.The

process continued to the 11.5A DC level and was

repeated at each flow rate.

To quantify the dynamic impedance it was

necessary to inject an AC component onto the DC

arc current. This AC arc current caused an AC

component to appear on the arc voltage. The

dynamic impedance of the arcjet was defined as the

ratio of this AC arc voltage to the AC arc current. A

magnitude and phase were associated with this

impedance, and in these data the arc current was used

as the reference for the phase angle. The magnitude

of the impedance in dB was _given by:

l zal = 201ogt0[v'c/ [dB] (3)

and the phase angle 0 was expressed as:

These data can then be presented in the form of

Bode plots and a mathematical description of the

following form:

11

I'l( s + qi)

i=l

Za(s ) = K i (5)

m

FI (s+Pi)

i=l

Can be obtained empirically. The zeros of the

characteristic equation, qi, are roots of the

numerator, and the poles, Pi are the roots of the

denominator. It has been shown that the presence of
apoleresultsinadecreaseinthemagnitudeof the

characteristicequationof 20dBperdecadeat the

comerfrequencyassociatedwiththepole,andthe

phaseanglecontributionis-90°.A zerowill cause

themagnitudetoincreaseata rateof 20dBper

decadeatthecomerfrequencywitha90° phaseangle

contribution. Thecomerfrequenciesaredetermined

bythenumerical valueof thepolesandzeros.The

theoryof systemfunctionsandtheirpolesandzeros

isexplainedindetailelsewhere. 21Theintentof the

of theexperimentswastomapthepolesandzerosif

present inthefrequencyrangetested.

Thruster1wasinstalledinthebelljarasdescribed

forthestatictests.Acurrentprobewasaddedatthe

feedthroughtomonitortheinjectedACarccurrent

component, andanisolatedoscilloscopeprobewas

usedtomonitorthecorrespondingACarcvoltage.

Themassflowratewasadjustedtothedesiredlevel

andthearcjetstartedwiththeDCarccurrentlevel

setto 10A.ThisDCcurrentlevelwasmaintained

throughout thetestingof thruster1.A 30minute

equilibrationperiodfollowed.Theoutputof the

currentregulatorwasincreasedto 12ADCandthe

modulatorstarted.ThemodulatorDCbiascurrent

wasadjustedto2A,reducingthearccurrentto10A.

Thefunctiongenerator wasadjustedtoafrequencyof

100Hzandtheamplitudesetsothatthemodulator

injecteda 1Ap_psinusoidal currentcomponent, an

AC currentlevel was that was maintained

throughout eachtest.

Theoutputofthecurrentprobeamplifierwasset

forACcouplingandinputto thegainphasemeter

channelA, theoscilloscope, andthefrequency

counter.Theoutputof thevoltageprobeamplifier

wasinputtochannelBof thegain/phasemeterand

theoscilloscope. Thegain/phasemeterwassetto

displaytheratiooftheamplitudesB/Aandthephase

angleof channelB withrespecttoA.Thegainsof

theinstrumentation amplifiersweresetsuchthatthe

ratiodisplayedwas20 lOgl0(Vac/iac). Withthis

configurationthemagnitudeandphaseof the

dynamicmpedancewerereaddirectly.Thefrequency

wasthenvariedinstepsof oneoctavewithineach

decadefrom100Hzto 100kHz,verifyingtheripple

frequencywiththefrequencycounter.A five minute

equilibration period was allowed following each

change in frequency in early tests but was found to

be unnecessary.

The procedure was repeated at all flow rates. To

quantify the repeatability of the data this test matrix

was repeated three times. Attempts to obtain data at

higher frequencies were made but the data were found

to be unreliable due to the inductance of the power

leads in the bell jar.

This procedure was repeated for thruster 2 with the

following additions. The DC current level was

reduced to 8 ADC and data were taken at 10% ripple

amplitude for the three flow rates to determine the

effect of DC current level on the dynamic

impedance. Three ripple amplitudes were also used at

the 10 ADC level to investigate the linearity of the

impedance. The ripple was varied in discrete steps

from 8 to 12%.

For performance measurements thruster 1 was

installed on a flexure thrust stand in the larger

vacuum facility. Calibration and operation of the

thrust stand are outlined in detail elsewhere. 17 To

measure the arc current, the current probe was

installed near the vacuum feedthrough along with a

standard 100mV/10A shunt. The arc voltage in this

case was measured at the vacuum feedthrough using

a DMM and the isolated oscilloscope probe. The DC

current level was maintained at 10 ADC throughout

the performance tests, and the ripple amplitude was

set at 10%. The arcjet was started at the lower flow

rate and allowed to run for 30 minutes at the 10

ADC current level. Thrust measurements were taken

at the DC operating point and used as a baseline for

comparison to the measurements with ripple. The

procedure from this point is identical to that of the

dynamic impedance measurements, and these data

were taken in tandem with the thrust measurements.

To ensure the accuracy of the impedance data, the

instrumentation was calibrated using a 50£1 RF load

resistor prior to the tests. The gain of each of the

instrumentation amplifiers was set so that the

gain/phase meter read the impedance magnitude

directly in dB. The impedance of the resistor was

measured to be 50.5_. It was assumed that the

imaginary component of the load impedance was

negligible in the frequency range of interest,

therefore, the dynamic impedance of the device

should have a magnitude of 34.1 dB (201og(50.5))

and a phase angle of 0 degrees from 100Hz to

100kHz. Since the device is purely resistive, the

dynamic and static impedances are identical. Any

deviations from the 50.5 ohm impedance were

assumed to result from gain errors and phase lags in

the amplifiers. The resulting errors were small, but

all data taken using the thruster were corrected

accordingly.

RESULTS AND DISCUSSION

For a given configuration the performance of the

arc jet was dependent on the mass flow rate and the

power dissipated in the device. To properly compute

dissipated power, it was necessary to quantify the

arcjet impedance. For this reason, the impedance data

are presented in'st, followed by the performance data.

, ,tiOmmaaa 

The static impedance data for thruster 1 are

presented in the form of volt-ampere curves in

Figure 3. The arc voltage decreases with increasing

current, and this behavior is typical for high pressure

4
arcs as described in detail elsewhere. 22 An increase

in the mass flow rate resulted in an increase in the

static impedance. This was due to the associated

increase in arc chamber pressure and a reduction in

the mean free path between propellant molecules.

The shortened free path length resulted in an increase

in the collision rate between particles, and more

energy was needed to maintain the necessary

ionization levels at a given current. Thus, a larger

electric field was required, increasing the arc voltage.

This phenomenon is described in detail elsewhere. 23

Dvni_nic Im_texlanceMagnitude

The magnitude of the dynamic impedance for

thruster 1 is presented in Figure 4a, and the phase

data in Figure 4b. The DC arc current was 10A and

the AC component had an amplitude of 1Ap_p. For

a given flow rate it can be seen that the magnitude

of the impedance remains relatively constant from

100Hz to 10kHz, with a 2 dB decrease in magnitude

from 10kHz to 100kHz. There was some indication

of a possible minima in the impedance magnitude at

80 kHz. In general, as the flow rate increased the

magnitude of the dynamic impedance increased. This

increase was not linear over the frequency range

tested, but varied from approximately 0.2 to 0.5 dB

from the 4.3x10 5 to the 4.8x10 -5 kg/sec mass flow

rate.

Dynamic Im_t_dance Phase Angle

The phase angle of the impedance was 190° at

100 Hz, and remained constant, within 10°, to 10

kHz, where the phase angle began to decrease. The

phase angle remained greater than 90° for the range

of frequencies tested, therefore, the dynamic

impedance had a negative real component for this

frequency range. This indicated that the V/I

characteristic for this frequency range maintained a

negative slope. From these data it appears that the

flow rate had little effect on the phase angle of the

impedance. A discontinuity in the magnitude and

phase data occurred at 1 kHz. This was an

experimental artifact traceable to the input filters on

the gain phase meter. A 1 kHz low pass filter was

used for frequencies below 1 kHz. For higher

frequencies a 100 kHz filter was used. This filter was

not responsible for the minima in the impedance

magnitude at 80 kI-Iz, as the tests on the RF load

verified flat response in this region.

Data Re_oeatability

As shown in Figures 5a and 5b, the dynamic

impedance tests on tests on thruster 2 yielded results

similar to those obtained with thruster 1. The

magnitude of the dynamic impedance of the two

thrusters agreed to within 1 dB and the phase angle

to within 5 degrees.

Mathematical Model

From these data, no poles or zeros can be mapped

because the magnitude of the impedance remained

nearly constant. However, the phase angle of the

dynamic impedance decreased as the ripple frequency

increased. This phenomenon can be characterized by

a transportation lag, which is mathematically

described as follows:

-|'[

zd(s) = e (6)

where s = j2rff, and x is the transportation lag in

seconds. This function has a magnitude of 1

because"

I cos ( 2nfx ) -j sin ( 2nfx ) I = 1 (7)

The phase angle is 0 for low values of f and

becomes more negative as f increases 23. The

magnitude of the dynamic impedance was greater

than 1 however, and this can be accounted for by

introducing a gain constant C. The absolute value of

the gain constant can be obtained from the dynamic

impedance data for a given flow rate. The magnitude

of the dynamic impedance was found using equation

3, thus, the value of the gain constant is:

Izdl [dg ]

C = 10 • (8)

Where I Zd I was taken from the data of Figure 4a.

The phase angle of the arc impedance was 180 ° for

low frequencies and this can be accounted for by

multiplying equation 6 by -1, which introduces a

180 degree phase shift for low frequencies. The

phase angle will decrease as frequency increases

depending on the constant x in equation 6 can be

estimated from the phase data of Figure 4b and was

found to be on the order of 1.5x10 "6 seconds. Since

the phase data showed no significant dependence on

flow rate, this value of x can be used for all flow

rates tested. The gain constant C, however, is

dependent on flow rate. Typical values were on the

order of 3.76 for the 4.8x10 -5 kg/sec flow. The

dynamic impedance can therefore be approximately

expressed as

-(15 xl06) s

Zd(S) =-C e (9)

for the frequency range of 100 Hz to 100kHz.

Figures 6a and 6b compare the measured dynamic

impedance magnitude and phase with those predicted

by equation 9. The data compares favorably with the

calculated values. The maximum error of 2 dB

occurs at 100 kHz. These values for the dynamic

impedance magnitude were higher than those

predicted by the static V/I curves, which were on the

order of 6dB. This value was obtained in the

following manner. When a 1A peak-to-peak AC

current component was superimposed on the DC arc

current, the arc current had a maximum value of

10.5A and a minimum value of 9.5A. The difference

between the corresponding arc voltages for these
current values quantified the dynamic impedance

since Z=V/I. The magnitudes obtained in the

frequency range tested were greater than this expected

value. This may indicate the presence of a pole-zero

pair at very low frequencies. This is further

supported by the 190 ° phase shift at 100 Hz. A

phase angle of only 180 ° is expected from the static

curves. Further work is necessary to investigate this

phenomenon. From these data, there is no indication

that the regulator step response need be changed.

Thruster 2 was used to investigate the linearity of

the dynamic impedance. If the dynamic impedance

was linear, the impedance magnitude should remain

constant regardless of the magnitude of the AC

component. Figure 7a presents the magnitude data

and Figure 7b the phase data for a DC current of

10A and a fixed flow rate. The magnitude of the AC

component was varied from 8, 10 and 12% of the

DC level. The magnitude of the dynamic impedance

appears to decrease as the magnitude of the AC

component increases. The total deviation over this

range of tipple amplitudes is on the order of .25 dB.

The results of the phase angle measurements were

similar.

The effect of the DC current level on the dynamic

impedance was investigated with arc currents of 8

and 10 amperes respectively. The magnitude of the

AC component was maintained at 10% for the

investigation. The static impedance data in Figure 3

show that the slope of the static volt-ampere curve

was more negative at 8 than at 10A. Thus, it was

expected that the magnitude of the dynamic

impedance at 8A would be greater than at the 10A

point. This indicates that the gain constant C of

equation 9 is also dependent on the DC current level.

The impedance magnitude data are presented in

Figure 8a. At 8 amperes the magnitude of the

dynamic impedance was in fact an average of 4 dB

greater than at the 10 ampere level. The impedance

phase data are presented in Figure 8b. The phase

angle for the 8A case was virtually identical to the

10A case.

Thruster Performance

The effects of power processor ripple on arc jet

performance had not been determined. To quantify

these effects, the thrust produced by the arc jet was

measured while varying the tipple frequency. For

these measurements, the ripple peak-to-peak

amplitude was maintained at 10% of the I)(2 value.

One possible scenario for a degradation in

performance was a change in the dissipated power in

the arcjet with tipple present when compared to the

DC case. This was supported by previous work

which demonstrated that the pressure in the arc

chamber of the thruster was proportional to the

input power, and therefore, thrust and specific

impulse were also related to the input

power24.When calculating the power dissipated in

the DC case, only the static impedance

characteristics were of interest, thus the power

dissipated was simply the product of the DC arc

voltage and current. However, when the AC tipple

component was superimposed, the dynamic

impedance was also considered. If the current input

to the arcjet was:

iarc= iac + iacsin (2rift) (i0)

where iac was the amplitude of the ripple, or one

half of the peak-to-peak amplitude, then the

resulting arc voltage was:

Var c = Vdc+ vacsin (2rift + 0) (11)

where 0 was the phase angle between the AC arc

voltage current, using the current as the reference.

The amplitude of the AC arc voltage and the phase

angle 0 were determined from the dynamic

impedance data presented in Figures 4a and b. The

dynamic impedance magnitude data of Figure. 4a

were measured by the gain/phase meter as the ratio

of the amplitudes of the AC arc voltage over the AC

arc current, represented by equation 3. Since the

amplitude of the AC arc current was 1A peak-topeak, the AC voltage peak-to-peak amplitude was

exactly equal to the dynamic impedance in ohms,

which could be calculated using equation 8. This

peak-to-peak amplitude was then divided by 2 to

determine the actual amplitude of the AC arc

voltage. The phase angle 0 was taken directly from

the data of Figure 4b, and the RMS power

dissipation was then calculated with the relation:

Vaclac ._x

Pnns = Vdcidc+ _ COS_O) (12)

Since the dynamic impedance had a magnitude

and phase, a real and complex or imaginary

component were associated with the impedance. If

the value of the real component of the impedance

was negative, the value of cos0 was also negative.

This was the case for the entire frequency range of

this experiment since 900<0<270 ° as shown in

Figure 4b. The tight hand term of equation 12 then

constituted a loss when compared to the pure DC

case, since the DC current level and DC arc voltage

remained constant. The magnitude of the loss

depended on the magnitude of the AC V-I product

and the value of the phase angle.

The experiment apparatus allowed the

simultaneous measurement of the thrust, AC and

DC are voltages and currents, and the flow rate.

Thus, the thrust and power dissipated could be

continuously monitored while the tipple frequency
wasvaried.Theimpedancedatatakenwhileon the

thrust stand correlated with the impedance data

shown in Figures 4a and 4b, taken in a separate

facility. In the frequency range tested, the value for 0

remained greater than 90 ° , thus the value of the

right hand term of equation 12 remained negative

throughout. However, the magnitude of the V*I

product for the AC component was small, in this

case less than 5 watts, which is less than 1% of the

total power.

The thruster was operated at a fixed flow rate with

a DC current level of 10A with an AC component

of 1Ap_p. Figure 9 presents the thrust data for three

flow rates. The DC thrust levels for each flow rate

are represented by a horizontal line. The thrust level

remained constant throughout the frequency range

tested, therefore, the specific impulse also remained

constant. The overall efficiency is presented in

Figure 10. There are minor deviations in the

efficiency, but these are more likely due to the

propagation of measurement errors in the

calculations. The changes in performance were

insignificant at all frequencies tested, as was

expected from the power calculations.

CONCLUSIONS

The interface between a 1 kW class arc jet and

future power processors was characterized by

determining the impacts of power supply output

characteristics on arcjet performance and by

characterizing the arcjet as a static and dynamic

electrical load. The impacts of power supply ripple

on arcjet performance were assessed with a ripple

amplitude of 10% peak-to-peak of the DC current

level.

Characterization of the arc jet as an electrical load

required the measurement of the static and dynamic

impedances of the arc. The static impedance data

show that the DC operating voltage of the arc jet is a

function of the input current and flow rate. The arc

voltage decreases with increasing current and

increases with flow rate. A curve fit of the V/I data

results in an equation that fairly accurately describes

the static impedance for a given flow rate, provided

that the cathode has been run for a sufficient time to

render the effect of further erosion negligible.

Variations in the static impedance from thruster to

thruster were not characterized.

The dynamic impedance also exhibits similar

trends with flow rate. Initially it was intended to

assign poles and zeros to the impedance using

frequency response methods. However, from these

data it was difficult to assign poles and zeros to the

dynamic impedance because the magnitude remained

constant to within 2 dB across the 100 Hz to 100

kHz range tested. A phase shift of approximately

180 ° was noted at low frequencies, but decreased at

frequencies above 10 kHz. This is representative of a

transportation lag in the arc column on the order of

1.5 _. The transportation lag was multiplied by a

negative gain constant to account for the 180 ° phase

shift at low frequencies. The value of the gain

constant was found to be dependent on flow rate and

the DC current level. Thus, for the frequency range

in tested the dynamic impedance of the arc can be

represented as

.(15x16_)i

zd(s) =-C e

A typical value for C was found to be on the order

of 3.76 for a 10A DC current level. This value

increased as the DC current level decreased for a

given ripple amplitude of 10% peak-to-peak.

This magnitude, however, is greater than that

expected from the static curves. This may indicate

presence of a pole-zero pair at a very low frequency.

Further work is necessary to investigate this

phenomenon. From these data, there was no

indication that the step response of the power

processor regulator should be changed. These data

were taken with two thrusters of nearly identical

configuration and the data compared favorably,

indicating that these data were transportable between

thrusters.

The effects of ripple on arc jet performance were

assessed by measuring DC performance levels at

various flow rates to provide a baseline for

comparison to thruster performance with power

supply ripple present. An AC component of

constant amplitude and variable frequency was then

injected to simulate power supply ripple. Thrust and

input power were measured simultaneously while

the ripple frequency was varied from 100 Hz to 100

kHz. The dynamic impedance of the arc jet caused the

dissipated power in the arcjet to decrease slightly due

to the phase angle associated with the dynamic

impedance which was greater than 90 ° . The total

reduction in power was less than 1%, thus, the AC

component was found to have no measurable effect

on thrust, specific impulse or overall efficiency over

the frequency range tested. This indicates that present

power supply designs, with output ripple currents

on the order of 10% peak-to-peak, are not

compromising overall system performance. The

impact of ripple on thruster lifetime was not

assessed in this work, however, past experience

indicates that this is not an issue for mission

lifetimes of 1000 hours.

REFERENCES

1. John, R.R., Connors, J.F., and Bennet, S.,

"Thirty Day Endurance Test of a 30 kW Arcjet

Engine", AIAA Paper 63-274, June 1963.

7
2. John, R.R., "Thirty Kilowatt Plasmajet RocketEngine Development," RAD-TR-64-6, Avco Corp.,

Wilmington, MA, NASA CR-54044, 1964

3. Todd J.P., and Sheets, R.E., "Development of a

Regeneratively Cooled 30-kW Arcjet Engine,"

AIAA Journal, Vol. 3, No. 1, Jan. 1965, pp. 122-

126.

4. Todd, J.P., "30 kW Arcjet Thruster Research,"

APL-TDR-64-58, Gianni Scientific Corp., Santa

Ana, CA, Mar. 1964. (Avail. NTIS, AD-601534.)

5. Ducati, A.C., Humpal, H., Metzler, J.,

Muehlberger, E., Todd, J.P., and Waltzer, H., "lkW Arcjet-Engine System-Performance Test,"

Journal of Spacecraft and Rockets, Vol. 1, No. 3,

May-June 1964, pp. 327-332.

6. Mc Caughey, O.J., Geideman, W.A., Jr., amd

Mueller, K., "Research and Development of a 2 kW

Arc-Jet Thruster," GRC- 1646, Plasmadyne Corp.,

Santa Ana, CA, NASA CR-54035, 1963.

7. Shepard, C.E., and Watson, V.R., "Performance

of a Constricted-Arc Discharge in a Supersonic

Nozzle," Physico-Chemical Diagnostics of Plasmas,

T.P. Anderson, R.W. Springer, and R.C. Warder,

Jr., eds., Northwestern University Press, 1964, pp.

261-272. (AIAA Paper 63-380).

8. Wallner, L.E., and Csika, J. Jr., "Arc-Jet

Thrustor for Space Propulsion," NASA TN D-2868,

1965.

9. Nakanishi, S., "Experimental performance of a 1

Kilowatt Arcjet Thruster," AIAA Paper 85-2033,

Oct. 1985. (NASA TM-87131.)

10. Curran, F.M., and Nakanishi, S., ',Low Power

dc Arcject Operation with Hydrogen/Nitrogen

Propellant Mixtures," AIAA Paper 86-1505, June

1986. (NASA TM-87279)

11. Hardy, T.L., and Curran, F.M., "Low Power dc

Arcjet Operation with Hydrogen/Nitrogen/

Ammonia Mixtures," AIAA Paper 87-1948, June

1987. (NASA TM-89876)

12. Knowles, S.C., Smith, W.W., Curran, F.M.,

and Haag, T.W., "Performance Characterization of a

Low Power Hydrazine Arcjet," AIAA Paper 87-

1057, May 1987.

13. Knowles, S.C., "Arcjet Thruster Research and

Technology, Phase I, Final Report," 87-R-1175,

Rocket Research Co., Redmond, WA, Sept. 1987.

14. Simon, M.A., Knowles, S.C., Curran F.M.,

and Hardy, T.L., "Low Power Arcjet Life Issues,"

AIAA Paper 87-1059, May 1987.

15. Gruber, R.P., "Power Electronics for a l-kW

Arcjet Thruster," AIAA Paper 86-1507, June 1986.

(NASA TM-87340)

16. Curran, F.M., and Haag T.W., "Arcjet

Component Conditions Through a Multistart Test,"

AIAA Paper 87-1060, May 1987. (NASA TM89857)

17. Haag, T.W., and Curran, F.M., "Arcjet Starting

Reliability: A Multistart Test on Hydrogen/

Nitrogen Mixtures," AIAA Paper 87-1061, May

1987. (NASA TM-89867)

18. Sarmiento, C.J., and Gruber, R.P., "Low Power

Arc jet Thruster Pulse Ignition," AIAA Paper 87-

1951, July 1987. (NASA TM-100123)

19. Knowles, S.C.,Arc jet Thruster Research and

Technology Phase II., NASA CR 182276, 1990,

(To be published)

20. Curran, F.M., and Haag, T.W., "An Extended

Life and Performance Test of a Low Power Arcjet,"

AIAA Paper 88-3106, July 1988. (NASA TM100942)

21. Gupta, S.C., and Hasdorff, L., Fundamentals of

Automatic Control, Robert E. Krieger Publishing

Co., Florida, 1983

22. Cobine, J.D., Gaseous Cond_l_tors, Dover

Publications Inc., New York, 1941

23. Ogata, K., Modem Control Engineering.

Prentice-Hall Inc., Englewood Cliffs, New Jersey,

1970

24. Curran, F.M., "An Experimental Study of

Energy Loss Mechanisms and Efficiency

Considerations in the Low Power dc Arcjet," AIAA

Paper 85-2017, September 1985. (NASA TM87123)

8
INJECTION

FRONT DISK "_

REAR INSULATOR _

INLET CATHODE

Figure 1. - Cutaway view of arcjet thruster.

POWER

SUPPLY

150V

ISA

DIODE

REGULATOR CURRENT ___ MODULATOR

/

I GENERATOR FUNCTION

CURRENT COAXI AL

SHUNT PROBE

CABLE RESISTOR

VACUUM

RELAY I PROBE

I PULSE I AMPLIFIERP

[TRANSFORMER I

REF, l

I GAIN/PHASE 1"

T?

OSCILLOSCOPE/I I

COUNTER

VACUUM

FACILITY

ARC JET

I AMPLIFIER ISOLATION

Figure 2a. - Apparatus block diagram.

9
POWER

SUPPLY

ISOV

1SA

ireg iarc

CURRENT I +

REGUL

-- _ imod

FUNCTION I

GENERATOR S VACUUMRELAY

Figure 2b. - Regulator/Modulator operation.

o"

t_

o

125

120

115

110

8.0 910 10.0 il.0

Arc Current,A

Figure 3. - Static Volt/Ampere characteristics for various flow rates, Thruster 1.

Mass Flow kg/sec

O 4.8x10 o5

[] 4.5x10 -5

A 4.3x10 -5

10
m

qD

e"

'10

t.-

o=

31

12

11

10

[] 0 [] 0 0

0 0 O0

A A A AA

A

• • . • • w,.| • • • =.i|

10 2 10 3 10 4

Frequency, Hz

A

A

[]

&

| • • w •=

0 5

Mass Flow [kg/sec]

O 4.8x10-5

[] 4.5x10 -5

A 4.3x10-5

Figure 4a. - Dynamic impedance magnitude vs. frequency at various flow rates, thruster 1.

es

_e

t'

=g

ets

20O

180

160

140

120

100

B B []

[] []

B

[]

= • • • . _.=| . • . . = .m,| = • • • • i i

0 2 10 3 10 4 10 5

Frequency,Hz

Mass Flow,kg/sec

O 4.8x10 -5

[]4.5x10 -5

A 4.3X10 -5

Figure 4b. - Dynamic impedance phase angle vs. frequency at various flow rates, thruster 1.

11
g0

"o

@

"o

¢-

14

13

12

11

10

9

0 0 0

0 0 O0 0

[] [] n[] [] [] n n

0

r-l

0

[]

OoC

0

0

Dr

i , • i , ,,w| , • , roll| w • w , , , ,

10 2 10 3 10 4 10 5

Frequency, Hz

0 Thruster2

[] Thruster 1

Figure 5a. - Comparison of dynamic impedance magnitude, thrusters 1 and 2, flow rate and DC current fixed.

220

2OO

180

@

160

,<

i/)

" 140 0,.

120

100

0

0 19 8 813 [] []

P'B

• i , , , , m| i , , ,l| |

10 3 10 4

Frequeney, i_Z

0

[]

B

0

%

, i i i I,

O 5

0 Thruster2

[] Thruster 1

Figure 5b. - Comparison of dynamic impedance phase angle, thrusters 1 and 2, flow rate and DC currrent fixed.
Ill

"O

Qt_

12.0

11.5

11.0

10.5'

10.0'

9.5'

9,0'

8.5'

8.0

FI FI I-I r"l I"1 - I

[((((l[ [[((((( tt[([[ [[(I.t(|/[(((( tin I[([ (([f[(([¢[£[C( ((] I[(( [[[([[ [[(({[[ [[(([[[ [[[[[ i

/

121

= i • , • ,, ,| , • , | i i, w| • • i m i

0 2 10 3 10 4 10 5

Frequency,Hz

O Model

[] Thruster

Figure 6a. - Comparison of dynamic impedance magnitude predicted by model and thruster data, .00048 kg/sec flow rate.

e-

<

t_

eD.

2OO

150

100

50

0 0 0 0 0 0

I IIIilli IIIIhq IIIIII III1111 II II_'l _lllh I II1_

• ' J''l " ' ' "vTI ' ' ' ' . .'

0 2 10 3 10 4 10 5

Frequency, Hz

0 Thruster

13 Model

Figure 6b. - Comparison of impedance phase angle predicted by model and thruster data, .00tN8 kg/sec flow rate.

13
m

0"

"O

t_

12

11

10

- 0

0 0

0 A

0 [] n

0 0 A []

[]

0

[] 0

[]

A

0

[]

A

0

A

[]

' • ° ' °'I ' ° ° °'|I ° " ' ''

0 2 10 3 10 4 10 5

Frequency, Hz

Ripple Amplitude

O 8o/0

[] 10%

A 12%

Figure 7a. - Dynamic impedance magnitude vs. frequency for various peak-to-peak ripple amplitudes, thruster 2.

6_

Q

O')

.<

rat

2OO

180

160

140

120

100

0 2 10 3 10 4 10 5

Frequency,Hz

Ripple Amplitude

0 80/0

[] 10%

A 12%

Figure 7b. - Dynamic impedance phase angle vs. frequency for various ripple amplitudes, thruster 2.

14
"0

6

==

t,-

18

16

14

12

10

8

0 0 O0

0

0 0

0

[] [] on [] [] DO

0

[]

0

[]

0

[]

• , , ,,,| , , , , J, | , , , | ,r

0 2 10 3 10 4 10 5

Frequency, Hz

DC Current

0 8A

[] 10A

Figure 8a. - Dynamic impedance magnitude vs. frequency for different DC current levels, thruster 2.

O

,<

t_

f

2O0

180

160

140

120

[]

fl [] Oo

0

0

8fl

100 ..... , ...... ,

10 2 10 3 10 4

fl

8

O

[3

DC Current

O 8A

[] 10A

0 5

Frequency, Hz

Figure 8b. - Dynamic impedance phase angle vs. frequency for different DC current levels, thruster 2.

15
Z

E

t==

J_

p22O I

215 _ 0 0 04 )

210 t

[] [] 13 1

205

200

195

190 ..... , ...... ,

10 2 10 3 10 4 10 5

Mass Flow, kg/sec

O 4.8x10 -5

[] 4.5x10 -5

A 4.3x10 -5

Frequency, Hz

Figure 9. - Thrust vs. ripple frequency for 10% peak-to-peak tipple amplitude. Horizontal lines denote DC performance levels.

0.41

0.40'

o

c =

e

I.U

0.39 -

0.38

O 2

[] [] []

0

A

• , ,,,| ..... ,!

10 3 10 4

Frequency, Hz

A A

• , , • , , ,,

O5

Mass Flow, kg/sec

O 4.8x10 -5

[] 4.5x10 -5

A 4.3x10 -5

Figure 10. - Efficiency vs. tipple frequency for 10% peak-to-peak tipple amplitude, thruster 1.

16
Report Documentation Page National Aeronautics and

SpaceAdministration

1. Report No. NASA TM-103190 2. Government Accession No, 3. Recipient's Catalog No.

AIAA-90-2579

4. Title and Subtitle 5. Report Date

Arcjet Load Characteristics

7. Author(s)

John A. Hamley

9, Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

6. Performing Organization Code

8. Performing Organization Report No.

E-5578

10. Work Unit No,

506-42-31

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15 Supplementary Notes

Prepared for the 21st International Electric Propulsion Conference

JSASS, Orlando, Florida, July 18-20, 1990.

cosponsored by the AIAA, DGLR, and

16 Abstract

Experiments were conducted to define the interface characteristics and constraints of 1 kW class arcjets run on

simulated decomposition products of hydrazine and power 9rocessors. The impacts of power supply output

current ripple on arcjet performance were assessed by variation of thc ripple frequency from 100 Hz to 100 kHz

with 10 % peak-to-peak ripple amplitude at 1.2 kW. Ripple had no significant effects on thrust, specific impulse

or efficiency. The impact of output ripple on thruster lifetime was not assessed. The static and dynamic

impedances of the arcjet were quantified with two thrusters of nearly identical configuration. Superposition of an

AC cornponent on the DC arc current was used to characterize the dynamic impedance as a function of flow rate

and DC current level. A mathematical model was formulated from these data. Both the static and dynamic

impedance magnitude were found to be dependent on mass flow rate. The amplitude of the AC component was

found to have little effect on the dynamic impedance. Reducing the DC level from 10 to 8 amps led to a large

change in the magnitude of the dynamic impedance with no observable phase change. The impedance data

compared favorably between the two thrusters.

17. Key Words (Suggested by Author(s))

Space propulsion

Electric propulsion

Arciet thrusters

Power processing

18. Distribution Statement

Unclassified - Unlimited

Subject Category 20

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No, of pages

Unclassified Unclassified 18

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