NASA Technical Memorandum 1023 10 

AIAA-89-2840 

. Investigation of a Liquid-Fed 

Water Resistojet Plume 

D.H. Manzella 

Sverdrup Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 

and i 

L.M. Carney 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

25th Joint Propulsion Conference 

cosponsored by the AIAA, ASME, SAE, and ASEE 

Monterey , California, July 10- 12, 1989 1 

r 

T! 

'\ #89;27706 [ NASA -TH- 10 23 10) Ibl VESTIGB TIOa f)P A - 

LIQUID-PED UATER RESISTWET PLURE {HAS&. 

Lewis Research Center) 10 p CSCL 21H 

Unclas 

G3/20 0225969 
INVESTIGATION OF A LIQUID-FED WATER RESISTOjET PLUME 

D.H. Manzella 

Sverdrup Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 44135 

and 

L.M. Carney 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

ABSTRACT 

Measurements of mass flux and flow angle were taken 

throughout the forward flow region of the exhaust of a 

liquid-fed water resistojet using a quartz crystal 

microbalance (QCM). The resistojet operated at a mass 

flow rate of 0.1 g/s with a power input of 330 Watts. 

Measured values were compared to theoretical predictions 

obtained by employing a source flow approximation. 

Excellent agrement between predicted and measured mass 

flux values was attained; however, this agreement was 

highly dependent on knowledge of nozzle flow conditions. 

Measurements of the temperature at which the exhaust 

condensed on the QCM were obtained as a function of 

incident mass flux. 

INTRODUCTION 

Recently, thqe has been renewed interest in resistojets 

capable of utilizing water as a propellant. The 

multipropellant resistojet baselined for Space Station 

Freedom will potentially use water as a propellant.1 Also. 

the commercially-sponsored, man-tended space platform 

recently considered for development baselined water 

resistojets as the primary on-orbit propulsion.2 Resistojet 

thrusters have been successfully integrated into spacecraft 

dating back to 1%5,3 and although water was a candidate 

ropellant for the biowaste resistojets of the Manned Orbital i esearch Laboratory (MORL) proposed in the early 

1970k.4 a liquid-fed water resistojet has not been flight 

tested. 

Successful integration of the water resistojet requires an 

accurate assessment of the potential impact that the water 

vapor plume may have on the spacecraft operation and 

missions. Since water vapor is readily condensible. one 

impact of primary concem is the possible contamination of 

spacecraft surfaces due to mass deposition. Sensitive 

optical surfaces, solar arrays, and thermal control surfaces 

can be adversely affected by the deposition of even a few 

angstroms of water. Field of view interferences caused by 

environmental contamination are also a concern for 

spacecraft users. Other potential impacts which must be 

addressed include thrust losses. disturbance torques. or 

theamal loading due to plume impingement The magnitude 

of these impacts are highly dependent on the spacecraft 

architecture. 

To predict the potential contamination effects of a water 

resistojet propulsion system. the plume must be modeled 

analytically, and, where possible. verified experimentally. 

A complete analysis of the exhaust flow field requires a 

solution to the Boltzmann equation. With the advent of 

supercomputers. great progress has been made in obtaining 

approximate numerical solutions of exhaust plumes using 

methods such as the Method of Characteristics (MOC)>a6 

Direct Simulation Monte Carlo (DSMC)7-8 and kinetic 

theory models.9.10 G.A. Simons presented a widely used 

source flow method for the prediction of rocket exhausts.11 

This method accounts for the expansion of the supersonic 

portion of the boundary layer of the nozzle, along with the 

inviscid core. Plume properties are expressed in terms of 

conditions at the nozzle exit and in terms of the boundary 

layer thickness. As this method was primarily devoted to 

the prediction of high thrust rocket nozzles with very thin 

boundary layers, there was no provision for subsonic 

boundary layer expansion. This method has been 

successfully utilized in the past by Genovesel2 to predict 

spacecraftlplume interactions. Agreement between values 

calculated for a hydrazine monopropellant thruster plume 

interacting with a solar array and actual flight data was 

good. alia and Brook13 also found good agreement with 

mass flux measurements made at large angles from the 

plume centerline of a small-scale reaction control engine. 

Experimental verification of any of the above-mentioned 

techniques in a ground-based facility is a difficult task as it 

is not possible to accurately simulate the space environment 

while the thruster is operational. However, meaningful 

plume data can be obtained in the forward region of the 

plume (downstream of the nozzle exit plane) at facility 

pressures less than Torr for condensable propellants 

with the use of cryogens such as gaseous Helium at 16 K 

(GHe) or liquid Nitrogen at 77 K (LN2). A number of 

diagnostic techniques for the measurement of plume 

properties are well established. Experimental studie~l~,1~ 

of pure gas, conical nozzle expansions into hard vacuum 

have been made to investigate the effects of nozzle area ratio 

and lip geometry. facility pressure, propellant condensation, 

and inadequate cryopumping in the back flux region. 

Preliminary investigations of the resistojet thruster 

plume have been reported by Zana. et They made a 

comparison between mass flux measurements taken with a 

QCM and predicted values using a modified version of 

Simons' method. Because the resistojet thruster is 

characterized by highly viscous flow in very small nozzle 

geometries. the method of Simons' was modified by 

Hofhan17 to account for the thick boundary layers present 

in the nozzle. The mass flux data were obtained in the 

exhaust of a laboratory model resistojet operating on 

unheated CO2 using a cryogenicallycooled quartz crystal microbalance ( QCM ). In order to avoid limitations in the 

dynamic range of the QCM. a small amount of H20 in the 

C02 was used as a tracer of the actual flow field. 

Qualitative agreement between theoretical predictions and 

experiment was found in the forward flux region. Breyley. 

et al.18 conducted a similar investigation addressing he 

effect of nozzle geometry on the exhaust plume. Again, 

agreement between the modified Simons' method and 

experiment proved to be qualitative. Both studies indicated 

that the suitability of the modified Simons' technique for 

predicting the exhaust flow field of viscous, low-thrust 

nozzles was promising but further experimental data were 

needed. 
The objective of this work was to experimentally map 

the exhaust flow field of a liquid-fed. water resistojet and 

identify potential plume effects on a spacecraft with the 

primary focus on contamination. A cryogenically-cooled 

quartz crystal microbalance was used to obtain a mapping of 

mass flux in the forward flux region of the plume. Also, 

local deposition rates as a function of surface temperature 

were obtained at various locations in the plume. In 

addition, an analytical study using the modified Simons' 

method for predicting the forward flow plume was 

conducted and compared to experimental measurements. 

This paper briefly describes the modified Simons' 

method and provides details of the measurement technique 

and experimental hardware. The QCM response during 

data collection as a function of time and crystal temperature 

is described. Profiles of measured mass flux as a function 

of QCM angular orientation with the flow streamlines are 

presented. The measured mass flux data are then compared 

with the plume density calculations based on the modified 

Simons' method. The sensitivity of the analysis to the 

selection of major plume parameters used in the calculations 

is also discussed. 

SYMBOLS 

plume normalization constant 

angular density variation 

QCM constant. g.Hz 

mass. g 

nozzle throat radius, cm 

spherical radius, cm 

nozzle exit radius, cm 

nozzle exit plane velocity, m/s 

limiting velocity, m/s 

limiting velocity in the boundary layer, mls 

distance along nozzle wall to exit, cm 

ratio between and tTlim 

plume parameta 

specific heat ratio 

boudary layer thickness, cm 

angle from plume centerline, degrees 

nozzle half angle, degrees 

angle to edge of boundary layer, degrees 

limiting turning angle, degrees 

simpification for p determination, Q.(8) 

viscosity. Ndm2 

Randtl-Meyer limiting angle, degrees 

maximum Randtl-Meyer limiting angle, degrees 

density, g/cm3 

nozzle throat density, g/cm3 

natural resonant fresuency of sensor crystal, Hz 

ANALYSES 

The modified Simons' method was chosen as the 

method of analysis because of its simplicity and ease of 

implementation. It is a semi-empirical, far-field 

approximation based on the more rigorous numerical 

solutions. This section summarizes the equations used to 

describe the plume flow field. Detailed discussions of the 

rationale and methodology of Simons' method and later 

modifications of Hoffman may be found in Refs. 11 and 

17, respectively. 

In the far field, rocket nozzle flow may be treated as 

though it originates from a point source. The plume 

structure, however, may be divided into two distinct 

regions: the region which originates in the inviscid core of 

the nozzle. and the region which originates in the 

supersonic boundary layer. Using continuity, the density at 

any point in the plume can be calculated in closed form 

based on conditions in the nozzle according to the following 

equatiOlX 

p/p* = A(R'/r)2 f(8) (1) 

The density varies inversely as the square of the distance 

from the thruster,and is related to the centerline value by a 

functionfle). This angular variation is described by two 

different expressions. The first expression is applicable for 

angles less than eo, the limiting turning angle for the 

plume's inviscid core. In this part of the plume the density 

1s modeled by a cosine power function: 

The second expression for the angular density variation is 

applicable in the outer region of the plume resulting from 

the expansion of the supersonic portion of the boundary 

layer; the subsonic portion of the boundary layer is 

neglected in this analysis. This corresponds to angles 

between eo and e,. In this region the model predicts that 

the density drops off exponentially: 

There is no provision for flow beyond the calculated 

limiting turning angle, 8,. The plume normalization 

constant in equation (I) is calculated using the following 

expression which is a simplification based on inviscid 

theory: 

2 1 ytl If2 A=-[-] [z] 8 7-1 e, (4) 

The limiting turning angle, e, may be calculated with the 

following expression: 

e, = V- V+ e,,0z (5) 

where v- is the limiting expansion angle for a flow with 

an infiite Mach number, vis the limiting expansion angle 

for a flow with the nozzle exit plane Mach number, and e,, 

is the geometric nozzle half angle. The remaining 

parameters 0, and /3 are functions of the nozzle exit 

conditions and the boundary layer thickness: 

1 

-sin K2 4 6,+rrt.os6,-l (7) 

where IC is: 

and Ulim = aUlirn an6 0.5 $a 5 1 .O. The boundary layer 

thickness, a,, is calculated using a classical flat plate 

analysis for laminar flow from ScNichting.19 

(9) 

2 
. 

APPARATUS 

Test Facility 

The experimental investigation was conducted in Tank 5 

of the Electric Power Laboratory at the NASA Lewis 

Research Center. The cylindrical vacuum chamber is 18 m long and 5 m in diameter. The pumping system consists of 

twenty, 0.8 m diameter oil diffusion pumps with four rotary 

lobe-type blowers and four rotating piston roughing pumps 

installed in parallel. The facility is also cryogenically 

pumped with two large cryopanels installed at one end of 

the chamber (Fig. 1). The panels were LN2-cooled to 

temperatures less than 90 K prior to testing. This portion of 

the tank was separated from the remainder of the tank by an 

auxiliary baffle and movable louvers. These louvers were 

in the open position during operation. 

The thruster was located 0.2 m from the tank wall and 

was oriented such that it exhausted along an axis 

perpendicular to the tank's major axis. Tank pressures 

were monitored at three different locations during operation 

using hot-cathode ionization gauges. During thruster 

operation the tank pressure ranged from 1x10-5 Torr near 

the cryopanels to 7x10-5 Torr directly across from the 

resistojet. 

,Resistojet 

A h 

Plume &yopanels 

Figure 1. - Tank 5 including resistojet and cryopanels 

Thruster 

The water resistojet employed for this study was a first 

generation thruster developed during an on-going research 

program at the NASA Lewis Research Center. A complete 

performance characterization of this device was conducted 

by Morren and Stone.20 Fig. 2 displays a cross sectional 

schematic of the resistojet. The major components of this 

thruster were fabricated from stainless steel, while the 

helical heating coil was constructed from an insulated 

nichrome wire. The converging-diverging nozzle had a 

twenty degree half angle, a throat diameter of 0.012 cm, 

and an expansion ratio of 1OO:l. 

0 

Figure 2. - Fit Generation Liquid-Fed Water Resistojet 

Quartz Crystal Microbalance 

Mass flux measurements were taken in various parts of 

the plume using a cryogenically-cooled quartz crystal 

microbalance (QCM). QCMs have been used in similar 

investigations of exhaust flow fields.1~16~1*~* The QCM 

is a simple intrusive diagnostic tool consisting of a matched 

pair of precision crystals which oscillate at 10 MHz. The 

two crystals are LN2-cooled to temperatures sufficient to 

condense mass on one of the crystals (sensor crystal). The 

other crystal serves as a reference and is not exposed to the 

incident mass flux. As mass condenses on the exposed 

surface of a sensor crystal the frequency of the oscillation 

changes according to the following relationship: 

do= Am (a?/k) (10) 

where o is the natural resonant frequency of the crystal, k 

is a constant dependent on the crystal cut, and dm is the 

change in mass. The difference between the two 

frequencies, the beat frequency, which is directly 

proportional to the incident mass flux, was monitored using 

a digital meter. Pertinent operating characteristics and 

dimensions of the QCM may be found in Table I. 

Table I - characteristics of QCM 

Crystal Frequency. MHz 10 

Operating Ternpaamre. K 

Beat Frequency Output, kHz 0-100 

Mass Sensitivity, glcm2.s 4.5~ 10-9 

10 - 360 

crystal Collection kea, cm2 0.3167 

Outer Dimensions, cm 0.318 dia x 3.18 

The QCM was mounted on an actuator system capable 

of being translated linearly in directions both parallel and 

perpendicular to the thrust axis as shown in Fig. 3. The 

QCM could also be rotated about its centerline with a rotary 

actuator. Axially, the QCM could be positioned between 20 

and 120 cm from the thruster exit plane. Radially, the QCM 

could be positioned rt42 cm relative to the thrust axis. The 

QCM could also be rotatedk90 degrees with respect to the 

thrust axis. A rotary angle of zero degrees corresponds to a 

parallel alignment of the normal to the QCM crystal surface 

with the thrust axis. The linear positional accuracy was 

estimated to be within fo.2 cm and the rotational accuracy 

was estimated to be within fo.2 degrees. 

Figure 3. - Probe Actuator System 

PROCEDURE 

The experiment was designed to map the plume 

produced by a liquid-fed water resistojet. The thruster was 

operated at a nominal mass flow rate of 0.10 g/s and a 

power input of 330 Watts. Additional performance data are 

given in Table II. The reported value of thrust is from 

measurements taken by Morren and Stone.18 Performance 

of this thruster was found to be stable and repeatable. In 

the current investigation. the mass flow rate, heater voltage, 

heater current and five internal temperatures were 

monitored routinely during operation. 
Table II - Water Resistojet Performance Data 

Feed pressure, KPa 

Voltage. volts 

Current, amps 

Power, Watts 

Mass Flow Rate, mg/s 

Thrust, mN 

Specific Impulse, s 

Heater Temperature, K 

inlet 

middle 

exit 

667 

43.9 

7.5 

330 

102 

170 

170 

1020 

lo05 

1018 

In all tests the resistojet was run until stable operation 

was achieved. At this point, the QCM was positioned at the 

location of interest and LN2 flow through the cooling lines 

was initiated. At a particular temperature mass began to 

collect on the sensor crystal. This initial condensation 

temperature was recorded. The amount of mass collected, 

the mass rate of change and the QCM temperature were 

recorded digitally using a computer once every second. 

Once the rate of mass collection stabilized the QCM was 

rotated. The rate of mass collection was monitored until 

constant and the QCM was again rotated. This procedure 

was repeated until the sensor crystal was saturated. It was 

assumed that once mass started condensing on the crystal at 

a constant rate, there was no further temperature 

dependance. The QCM response at six different angular 

positions and the QCM temperature are plotted as a function 

of time in Fig. 4. Each cluster of data points was taken at a 

different angular orientation. After saturating the sensor 

crystal. the LN2 flow to the cooling line was discontinued, 

the surface of the crystal was heated until clean, and the 

entire procedure was repeated. The reproducibility of the 

measured mass rate of change at a point was within f5 

percent. 

40.0 

"b 

3 7.5 

B 

3 

a 8 5.0 

2 2.5 

0.0 

'\, -CP Mass 

, 

, 

, 

, 

\ 

, P , _-- Temperature 

/ 

0 30 60 90 1 

Time, sec 

. '. I 

0 30 60 90 1 

Time, sec 

172 

171 * 

$ 

E 

170 

169 t.. 

168 

1 67 

6 

Figure 4. - QCM Response 

To determine the mas flux at any location in the plume 

from the measured mass collection rate a capture coefficient 

must be considered. The capture coefficient is defined as 

the fraction of impinging molecules which adhere to the 

surface of the crystal. For this investigation a capture 

coefficient of 1.0 (independent of the rate of incidence on 

the surface) was assumed for simplicity. Values as low as 

0.92 have been observed for H20 condensing on a 77 K 

surface.22 Mass flux values were determined at 

approximately sixty locations in the resistojet flow field. 

Rotary variation in mass flux as a function of the QCM 

angular orientation was determined at forty of these 

locations. 

In addition to rotary surveys, measurements were taken 

with the QCM at a fixed angular orientation. These 

measurements were taken along three radial lines originating 

from the center of the nozzle exit plane at angles of 0. 10, 

and 20 degrees from centerline. The QCM was moved to a 

position along one of the lines and oriented at an angle of 70 

degrees with respect to the line (see Fig. 5). When the 

QCM was in position, LN2 flow was initiated and the 

sensor cooled. When the rate of collection stabilized, the 

mass flux was recorded. If the sensor crystal was not 

safurated after obtaining a stable mas flux reading the QCM 

was moved to a new position along the radial line (still 

oriented at a 70 degree angle with respect to the line) and 

additional mass flux measurements were taken. If the 

crystal was saturated, the heating/cooling cycle had to be 

repeated. 

Nozzle 

Figure 5. - QCM Orientation for Fixed Angle 

M&WlWtlents 

The mass flux measured by a rotary QCM has been 

found to exhibit a cosine variation about the s~reamline.~~~~~ 

By utilizing this relationship and only measuring a small 

component of the total mass flux, the limit in the dynamic 

range of the QCM could be avoided. Mass flux 

measurements taken at a known angle with respect to a 

particular streamline could be corrected to give the mass 

flux that would have been measured along the seeamline at 

that location in the plume. Using this procedure, 

measurements were obtained as close as 35 cm on centerline 

from the thruster. The existence of this cosine variation 

centered about the direction of the radial lines was 

subsequently verified at a minimum of one position along 

each of the three lines by conducting a complete rotary 

survey. The uncertainty in any of the mass flux 

measurements was estimated to be within f 30 percent 

primarily due to the temperature dependence of the capture 

coefficient. 

RESULTS AND DISCUSSION 

The QCM was used to experimentally map the plume 

produced by a liquid-fed water resistojet. The data were 

compared to theoretical predictions obtained using the 

modified Simons' method, and the sensitivity of the 

modified Simons' source flow approximation to selection of 

two critical plume parameters was considered. 

QCM Measurements 

As expected, the condensation temperatures were found 

to depend on the magnitude of the incident mass flux. Fig. 

6 is a plot of condensation temperature versus the steady 

state mass flux measured at many different QCM positions 

and orientations. The variation is uniform and the mlts 

were repeatable. This result indicates that the amount of 

condensation of water vapor from a resistojet on sensitive 

spacecraft surfaces can be successfully addressed 

experimentally. By placing a QCM at the location of 

interest relative to the resistojet. surface temperatures below 

which condensation would occur can be determined. 

4 
The variation in mass flux with angle is shown in Fig. 

7@) for a location in which the magnitude of the mass flux 

resulting from the directed molecules of the plume was 

approaching background values. The interaction between 

the directed molecules of the plume and the randomly 

directed molecules that have undergone collisions resulted 

in a cosine variation that approached the uncertainty of the 

measurements. The characteristic cosine variation was still 

evident, but the maximum measured mass flux was only 40 

percent higher than the mass flux values measured 

perpendicular to the measured streamline of 45 degrees. 

From the rotary surveys, the local streamlines were 

determined at each location by curve-fiuing a cosine to the 

measured mass flux values as shown in Fig 7. The relative 

magnitude of the on-streamline mass flux and the direction 

of the streamline are plotted in Fig. 8. The magnitude of the 

mass flux and the flow angle on both sides of the thrust 

centerline were approximately equal indicating that the 

plume was symmetric. However, there was a variation in 

the background mass flux measured at 90 degrees to the 

streamlines on either side of the thrust axis. Measurements 

taken on the side of the plume nearest the LN2 cooled 

cryopanels consistently resulted in a lower measured 

background mass flux. The difference in the measured 

background mass flux as a function of location in the tank 

was indicative of facility interaction. During prolonged 

runs it became evident that a significant portion of the total 

amount of water exhausted into the vacuum chamber had 

been collected on the LN2 cooled cryopanels. Therefore, 

the difference in measured background mass fluxes 

reflected the existence of a density gradient along the length 

of the tank caused by an asymmetry in the pumping system 

with respect to the resistojet exhaust. Three ionization 

gauges placed at different locations within the vacuum 

chamber also verified the existence of a density gradient. The ionization gauge closest to the cryopanels consistently 

indicated a lower tank pressure (1x10-5 Torr) while the 

resistojet was operating. 

VI 

fl 

53- 

32- K 

r' 

B 

GI' 

s 

... 175 ' .... 185 ' .. Condensation Temperature, K 

Figure 6. - Mass flux vs. Condensation Temperature 

As previously stated, mass flux measurements were 

taken as a function of QCM angle at over forty different 

locations. These rotary surveys demonstrated a cosine 

variation centered on the streamline. A typical profile is 

shown in Fig. 7(a) where an angle of zero degrees denotes 

parallel alignment of the QCM surface normal with the 

thrust axis. The existence of this cosine distribution is easily 

understood by considering the geometry of the fluid-QCM 

interaction. At any particular orientation. the mass 

condensing on the sensor crystal was due to both the 

directed molecules of the exhaust and randomly directed 

molecules resulting from collisions. When the surface of 

the sensor crystal is aligned perpendicular to the flow 

streamline the maximum number of directed molecules 

impinges on the surface. As the orientation of the QCM 

changes, the molecules moving in the direction of the 

streamline have a smaller projected area of the sensor crystal 

to condense on since the projected area varies with the 

cosine of the angle. The mass flux resulting from the 

randomly directed molecules was sufficiently low in 

comparison to the mass flux resulting from the exhaust 

throughout the forward flow region of the plume. For the 

measured profile in Fig. 7(a) the measured local flow angle 

occurred at 40 degrees with respect to the thrust axis. 

axial pos. 15.0 cm 

radial pos. 33.0 cm 

1 1 

v) 

c\ 

E 

Y 

rD 

2 

K 

0 20 40 60 80 100 120 

Axial Position. cm 

Figure 8. - Mass Flux Vectors from Rotary Surveys 

The magnitude of the mass flux vectcrs determined 

from the rotary surveys reached a low of 1.3x10-6 @cm% at 

a location 10 cm downstream of the nozzle exit and 33 cm 

off the centerline. The maximum mass flux measured at 

angles greater than 45 degrees from the thrust axis was 

2.7~10-6 g/cm%. A background mass flux of 4.6x1P6 

g/cm% was calculated based on the assumption of randomly 

directed ambient temperature molecules at a pressure of 

7x10-5 Torr. This pressure was measured with an 

ionization gauge on the tank wall directly opposite the 

resistojet. The differences between the calculated 

background mass flux and the measured mass fluxes also 

indicated that a significant density gradient existed in the 

vacuum chamber. .. 

A consideration of these facility effects has been 

included because measurements of plume properties taken 

-90 45 0 45 90 

Angle of Rotation, degrees 

in vacuum facilities are strongly dependent bn he ability of 

Figure 7@). - Mass Flux Variation Near Background Values a facility to accurately simulate a space environment It was 

5 
recognized that meaningful measurements could not be 

obtained in all regions of the plume in the facility used for 

these tests. Those measurements taken in lower density 

regions of the plume have a higher degree of uncertainty 

than those taken in higher density regions; however, no 

measurements were taken where facility effects were 

thought to predominate over the expansion process. 

Consequently, no data were obtained at high angles from 

the centerline (near 90 degrees). 

No rotary surveys were taken near the plume centerline 

because the water molecules from the exhaust condensed on 

the sensing crystal at such a rapid rate that the dynamic 

range of the device was quickly exceeded. In many cases 

the condensed layer of water molecules was so thick it 

prevented the crystal from oscillating at all. As previously 

mentioned, to avoid these limitations, the cosine 

relationship with rotary angle was employed. The QCM 

was oriented at a particular angle to a radial line and the 

measured value was corrected assuming the radial line was 

a streamline. The radial lines were experimentally verified 

to be streamlines by taking rotary surveys at a minium of 

one point along each line. The resulting measured 

streamline angles determined along radial lines 0, 10. and 

20 degrees relative to the thrust axis are included in Fig. 8. 

Fixed angle mass flux measurements were taken along 

the plume centerline. The QCM was oriented at an angle of 

70 degrees to the plume centerline. The centerline mass 

flux was then calculated assuming a cosine distribution 

centered at zero degrees. These values are presented in Fig. 

9. A I/r2 relation was fit to the experimental data. The 

small scatter in the data relative to the l/r2 curve 

demonstrates the reliability of the QCM measurements. 

4 I 

\ 

\ 

\ 

\ 

? 

o Experimental data 

----_ 1/9 curve fit 

6' - O- -a- - 

-.-.-.-.-.-.-. 

20 40 60 80 100 120 140 

Distance to Thruster. cm 

Figure 9. - Centerline Mass Flux 

Parametric Analyses 

The modified version of Simons' method was used to 

obtain the number density distribution in the resistojet 

plume. As discussed previously, the density distribution in 

the plume is functionally dependent on several critical 

parameters which require a knowledge of conditions at the 

nozzle throat and exit plane. An accurate value of the 

normalizing throat density is required for an absolute 

magnitude of number density in the plume. For this 

analysis, the throat density was based on a onedimensional, isentropic analysis of the nozzle using the 

measured thrust and stagnation pressure to calculate a 

stagnation temperature at the throat. A throat density was 

then calculated from ideal gas dynamic relations. To 

account for real gas effects such as viscous dissipation in 

the nozzle. a 92 percent nozzle efficiency was assumed and 

the ideal throat density was increased by 8 percent for the 

analysis. 

Fig. 10 displays the parametric sensitivity of the 

modified Simons' method to &,a, and a for a given set of 

conditions. The thruster nozzle exit plane is located at the 

origin. In the figures. the plotted contour represents a 

number density of 1012molecules/rm3. Marked differences 

in the calculated number density were observed for varied 

parameters holding all else constant. For example, the 

selection of the limiting turning angle is calculated from a 

knowledge of specific heat ratio and the Mach number at the 

exit plane ( Eq. (S) ). For the resistojet operating on H20 

with a yof 1.33. an exit plane Mach number of 6.1 was 

calculated from isentropic flow relations. This gives a 

calculated limiting turning angle of 74 degrees. However, 

previous experimental data indicates that the flow extends 

beyond the calculated limiting turning angle.16 . This is 

characteristic of low Reynolds Number nozzle flows which 

have large boundary layers. Again, this analysis neglected 

the expansion of the subsonic portion of boundary layer. 

Fig. 10(a) presents number density contours for limiting 

angles of 74', 105'. and 135'. 

Axial Position, cm 

Figure 10(a). - Variation in 1.0~10'~ mol/cm3number 

density contour with the limiting turning angle, e, 

0 20 40 60 80 100 120 

Axial Position, cm 

Figure lo@). - Variation in 1.0~10~~ mol/cm3 number 

density contour with the plume parameter, a 

" 7-- T 7 

0 20 40 60 80 100 120 

Axial Position, cm 

Figure lqc). - Variation in 1.0x10~~mol/cm3 number 

density contour with the boundary layer thickness, 4 

Similarly, the variations in density contours with the 

selection of a are shown in Fig. lo@). Varying aaffects 

the solution only in that portion of the plume resulting from 

the expansion of the boundary layer (angles greater than 31 

degrees) since a affects the rate of exponential decay of the 

plume flow originating from the boundary layer through the 

parameter p ( Eq. (7) ). The calculations displayed in Fig. 

100) are for a limiting turning angle of 105 degrees. The 

model predicts that more flow will go into the backflow 

region with increasing a 

The effect of boundary layer thickness was also 

investigated. This parameter determines the location of the 

6 
transition from core to boundary layer flow ( Eq. (6) ). It 

also controls the rate of exponential decay in the plume flow 

originating in the boundary layer through the parameter 0. 

In this analysis, the boundary layer thickness was 

determined using a flat plate analysis and was calculated to 

be on the order of 25% of the exit area. Number density 

contours were calculated for boundary layer thicknesses 

twice the calculated value and one half the calculated value. 

The variation in the calculated contours is shown in Fig. 

lqc) for a = 1 .O and 8, = 105 degrees. These calculations 

show that the model is not sensitive to the value of S, for the 

given flow conditions. 

The above discussion implies that the selection of a and 

8, is critical in comparing theory and experimental values. 

These functional dependencies also indicate that measured 

values in the plume can provide insight into conditions at 

the nozzle exit plane. 

Comparison of Model with Experiment 

A comparison of the experimental values of mass flux 

and those from the analyses is made in Fig. 1 1. The dashed 

lines in the figure were generated from a linear interpolation 

scheme of the measured mass flux. The solid lines in the 

figure were determined based on number densities 

calculated using the modified Simons' method and values of 

the critical plume parameters which gave the best agreement 

with the experimental values. Conversion of the calculated 

number density values to mass flux requires the particle 

velocity. The calculated limiting velocity was chosen. This 

velocity was calculated assuming the nozzle flow expanded 

to zero static pressure. The limiting velocity used to convert 

the calculated density to mass flux was 1660 m/s. The 

difference between the gas velocity based on measured 

specific impulse and the calculated limiting velocity was less 

than one percent. The values of all variables used in the 

analysis are listed in Table JII. The agreement between the 

measured and calculated values is excellent. 

--- Measured 

- Calculated 

0 20 40 66 80 l& 120 

Axial Position. cm 

Figure 1 1. - Measured and Calculated Mass Flux Contours 

Table Ill - Input Parameters 

Plume normalization constant, A 9.76~10-' 

Nozzle throat radius, R* 

Nozzle exit radius, re 

Limiting velocity in the boundary layer,alh 1660 m/s 

1 .o 

Plume parameter, p 5.69 

Specific heat ratio, y 1.33 

Boundary layer thickness, Se 9.95~10-~ cm 

44' 

Limiting tuning angle, e, 105' 

Nozzle throat density, p* 3.85~10~ &n3 

3.8~10-~ cm 

3.8~10~ cm 

Limiting velocity. Ulh 1660 m/s 

Ratio between Ulh andalh, a 

Angle to edge of boundary layer. eo 

The excellent agreement between the calculated values 

and the measured values indicate that the Simons' source 

flow approximation is a viable tool for predicting the 

exhaust from a liquid-fed water resistojet. The 105 degree 

limiting turning angle indicated that the plume expanded 

more than anticipated. This was the result of either 

backscatter from the forward portion of the plume. or a 

significant subsonic boundary layer that was neglected in 

the analysis, or a much lower exit plane Mach number than 

calculated. It must be stressed that experimental data are 

required to define nozzle conditions before accurate plume 

calculations can be made. In particular, the value of the exit 

Mach number is critical as it defines the limiting turning 

angle. 

CONCLUDING REMARKS 

This paper has presented an experimental and analytical 

investigation of the exhaust from a liquid-fed water 

resistojet. The resistojet was operated at a mass flow rate of 

0.1 g/s with a heater input of 330 W. Mass flux 

measurements were taken throughout the foward flow field 

using a cryogenically-cooled quartz crystal microbalance 

(QCM). In consideration of the potential impact to sensitive 

spacecraft surfaces, the surface temperature at which water 

began to condense was determined as a function of the 

incident mass flux. Condensation temperatures between 

168 and 186 K were measured at various locations in the 

plume. 

Complete rotary surveys were also obtained with the 

QCM. As has been demonstrated with pure gas expansions 

into vacuum, a cosine variation was exhibited about the 

flow streamline. The relationship between mass flux and 

flow angle was employed to obtain data at higher density 

regions of the plume where the dynamic range of the QCM 

was exceeded. Data were taken at a known angle to an 

assumed streamline and corrected to obtain the appropriate 

value of mass flux. In total, QCM measurements were 

taken at over sixty locations in the plume on both sides of 

the thrust centerline. The resistojet plume was axisymmetric; however, the rotary surveys with the QCM 

indicated differences in the background levels of mass flux 

on either side of the plume centerline due to non-uniform 

pumping in the vacuum tank. Evidence of pressure 

gradients in the tank were documented with the QCM 

measurements and also with pressure gauges. 

Finally. the mass flux data were compared to theoretical 

predictions based on a modified version of Simons' 

method. Excellent agreement between theory and 

experiment was attained; however, a careful selection of 

several critical plume parameters was required. The 

sensitivity of the calculated density contours on three plume 

parameters was discussed. 

1. 

2. 

3. 

4. 

REFERENCES 

Jones, R.E., Morren. W.E.. Sovey. J.S. and Tacina, 

R.R.. "Space Station Propulsion," NASA TM100216. Dec. 1987. 

Louviere, A.J., Jones, R.E. Morren, W.E.. and 

Sovey. J.S.. "Water-Propellant Resistojets for ManTended Platforms," IAF 87-259. Oct. 1987. (NASA 

TM-100110). 

Stone, J.R., "NASA Electrothermal Auxiliary 

Propulsion Technology," NASA TM-87271, June 

1986. 

Greco, R.V. and Bliss, J.R., "Resistojet Systems 

Studies Directed to the Space Station/Space Base, Vol. 

I - Statioaase Resistojet System Design," (MDC7 
G2125, McDonnell Douglas Astronautics Co.; NASA 

Contract NASl-10127) NASA CR-111879. April 

1971. 

5. Suebold, J.G.. "Use of Viscous Nozzle Flow Program 

and the Method of Characteristics Plume Program to 

Predict Plume Expansion in the Back Flow Region for 

Small Thrusters." JANN AF 9th Plume Technology 

Meeting, Chemical Propulsion Information Agency, 

Laurel, MD. CPIA-PUBL-277, 1976, pp.1-14. 

6. Cooper, B.P.. "A Computational Scheme Usable for 

Calculating the Plume Backflow Region," 10th SDace 

Simulation Conferenc e, New York, AIAA. 1978. pp. 

150-1 52. 

7. Bird, G.A.. "Breakdown of Continuum Flow in 

Resistojets and Rocket Plumes." 32th International 

SvmDosium on Rarefied Gas Dynamics, S.S. Fisher, 

ed., aAA. New York, 1980. pp. 681-694. 

8. Boyd, I.D. and Stark, J.P.W., "Modelling of Small 

Hydrazine Thruster Plumes Using Discrete Particle and 

Continuum Methods," AIAA-88-2631, June 1988. 

9. Riley, B.R. and Scheller, K.W.,"Kinetic Theory 

Model for the Flow of a Simple Gas from a TwoDimensional Nozzle." 16th International Symposium 

on Rarefied Gas Dynamics Proceedings. December, 

1988. 

10. Chug. C. and De Witt, K.J., "Rarefied Gas Flow 

Through Two-Dimensional Nozzles," AIAA-89-2893. 

July 1989. 

11. Simons. G.A., "Effect of Nozzle Boundary Layers on 

Rocket Exhaust Plumes." AIAA Journal, Vol. 10. No. 

11, Sept. 1972, pp. 1534-1535. 

12. Genovese, J.E., "Rapid Estimation of Hydrazine 

Exhaust Plume Interaction." AIAA-78-1091, July 

1978. 

13. Calia, V.S. and Brook, J.W.. "Measurements of a 

Simulated Rocket Exhaust Plume Near the PrandtlMeyer Limiting Angle," J Spacecraft and Rockets, 

14. Bailey, A.B., Price, L.L., McGregor. W.K. and Mau, 

R.J.. "Flow Field Mapping of Gas/Particle Nozzle 

Expansion Into Vacuum ," AEDC-TR-84 -3 8, July 

1985. 

15. Bailey. A.B.and Price. L.L. "Flow Field Mapping of 

Carbon Dioxide Nozzle Expansion Into Vacuum." 

16. Zana, L.M.. Hoffman, D.J., Breyley. L.R., and 

Serafini J.S., "An Analytical and Experimental 

Investigation of Resistojet Plumes," NASA TM88852. Jan. 1987. 

17. Hoffman, D.J., "Resistojet Plume and Induced 

Environment Analysis," NASA TM-88957. May 1987. 

18. Breyley. L.R.. Hoffman, D.J.. Zana, L.M., and 

Serafini, J.S.. "Effect of Nozzle Geometry on the 

Resistojet Exhaust Plume,'' AIAA-87-2121, June 

1987. 

19. Schlichting. H.. Boundarv J.aver Theow, McGraw 

V01.12, NO.4, April 1975, pp.205-208. 

AEDC-TR-85-26. July 1985. 

Hill, New York, 1979, p. 140. 

20. Morren. W.E. and Stone J.R.. "Development of a 

Liquid-Fed Water Resistojet," AIAA-88-3288, July 

21. Chirevella, J.E.,"Hydrazine Engine Plume 

Contamination Mapping." AFRPL-TR-75-16. Oct. 

1975. 

22. Brown, R.F. and Wang, E.S., "Capture Coefficients 

of Gases at 77' K," Advances in Crvoeenic 

beinwring, Vol. 10, K.D. Timmerhaus, Plenum, 

New York, 1964. pp.283-291. 

1988. (NASA TM-1W27). 

8 
nv\sn National Aeronautics and Report Documentation Page 

Space Administration 

2. Government Accession No. NASA TM-102310 

AIAA-89-2840 

1. Report No. 

I 

4. Title and Subtitle 

Investigation of a Liquid-Fed Water Resistojet Plume 

7. Author(s) 

D.H. Manzella and L.M. Carney 

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 

-. 

3. Recipient’s Catalog No. 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-5005 

10. Work Unit No. 

506-42-3 1 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Prepared for the 25th Joint Propulsion Conference cosponsored by the AIAA, ASME, SAE, and ASEE, 

Monterey, California, July 10-12, 1989. D.H. Manzella, Sverdrup Technology, Inc., NASA Lewis Research 

Center Group, Cleveland, Ohio 44135; L.M. Carney, NASA Lewis Research Center. 

16. Abstract 

Measurements of mass flux and flow angle were taken throughout the forward flow region of the exhaust of a 

liquid-fed water resistojet using a quartz crystal microbalance (QCM). The resistojet operated at a mass flow rate 

of 0.1 g/s with a power input of 330 Watts. Measured values were compared to theoretical predictions obtained 

by employing a source flow approximation. Excellent agreement between predicted and measured mass flux 

values was attained; however, this agreement was highly dependent on knowledge of nozzle flow conditions. 

Measurements of the temperature at which the exhaust condensed on the QCM were obtained as a function of 

i--ident mass flux. 

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

Resistojets 

Electrothermal thruster 

Plumes 

Contamination steam 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 20 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No of pages 22. Price’ 

10 1 A03 

. 

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