Dr. Steve Koontz
The objective of Detailed Test Objective 829 (DTO-829) was to
provide QCM measurements both transient and persistent surface
contamination produced by: 1) Shuttle Primary Reaction Control
System (PRCS) engines, 2) Russian 13 kg attitude control and
reboost thrusters on Mir, and 3) the Shuttle environment during
nominal operations. The last measurement, (3), provides
additional sensor background data in the on-orbit Shuttle cargo
bay environment to support data analysis. The DTO-829 data
objectives were achieved during STS-74. Final data analysis will
be completed following post-flight sensor thermal calibration at
QCM Research, the sensor vendor. QCM research is preparing a cost
estimate for the thermal calibration at this time and final data
analysis and reporting is expected by September 30, 1996. The
post flight thermal calibration is needed because QCM sensors are
very sensitive to the thermal environment. Sensor output changes
as a result of varying radiant heat transfer environments (deep
space viewing, sun viewing and earth viewing), even when
thermostated. Different sensor-output thermal effects are
obtained for different external and internal heating functions so
that the post flight thermal calibration had to be designed
around the as- flown thermal environments of STS-74 which could
only be determined post-flight. Preliminary results using
pre-flight sensor thermal correction functions are, however,
reported below.
The DTO-829 measurement objective was determined by the need
to assess the impact of hypergolic engine plume impingement
contamination on the functional life of sensitive International
Space Station (ISS) surfaces. These assessments are important
cost drivers for the ISS program because the outcome of the
assessments are used to: 1) predict functional life, 2) set
hardware replacement schedules, and 3) choose proximity
operations scenarios. Assessments produced during the Space
Station Freedom program showed that the then current engine plume
contamination estimates, derived from ground- based, small-engine
laboratory data (laboratory vacuum systems have vacuum pumping
speeds that are only a small fraction of the equivalent vacuum
pumping speed of the space environment) or Apollo-Skylab data,
predicted significant degradation of both photovoltaic panels,
and thermal control surfaces. Particle impact damage to the
Space- Shuttle-Orbiter body flap tiles caused by vernier engine
plume impingement seemed to qualitatively confirm the assumptions
about engine plume contamination used in the early SSF
assessments while raising new issues in the area of surface
erosion by particle impact. Program cost growth resulting from
limited subsystem life and/or operational fixes were
possibilities indicated by these early assessments.
Only the relatively large PRCS engines fire at space station
during the terminal phase of proximity operations. The CONTAM
hypergolic engine simulation software package maintained by the
Chemical Propulsion Information Agency at Johns Hopkins
University predicts that Shuttle PRCS engines burn propellant
more efficiently and produce far fewer damaging particles than
either Shuttle vernier engines or the small laboratory scale
engines used to establish the basis for the early SSF program
engine plume contamination assessments. CONTAM predictions were
also more consistent with Johnson Space Center space flight
experience, for example with the Induced Environment
Contamination Monitor (IECM) contamination monitor subjected to
PRCS engine plumes during STS-2 to STS-5, showed no visible
evidence of plume exposure. In addition the Soviet Program has
reported some anecdotal information on the subject of hypergolic
engine plume contamination or surface damage, though except for
view port transparency reduction, no important system impacts
were established. The Soviet and later Russian experience is
important in light of the fact that the Russian 13 kg thrusters
are similar to the Shuttle verniers in overall size and
performance.
Space flight experience and CONTAM predictions both suggested
that the basis for the SSF engine plume effects assessment was
far too conservative and that significant cost avoidance was
possible if a more realistic assessment basis could be
established. Alternately, real proof of the validity of the
earlier conservative basis for the assessment was needed to
justify the implicit increase in program cost and complexity. The
plume sticking fraction, i.e. the mass fraction of the total
plume gas flow which sticks permanently on a sensitive surface is
the most important factor in the ISS contamination effects and
functional life assessments. The plume sticking fraction,
expressed as a percentage of engine plume mass flux, was
estimated at 1.0 percent, on the basis of the data sources
described in the previous paragraph, during early SSF
assessments.
During STS-52, direct QCM measurements of PRCS engine plume
contamination with the Shuttle Plume Impingement Experiment
(SPIE) revealed no detectable persistent or transient
contamination with a QCM sensor temperature range of 305 to 330
degrees Kelvin. SPIE QCM data combined with post flight chemical
analysis of SPIE hardware resulted in program acceptance of a new
plume sticking fraction percentage of 0.02 percent, a
conservative estimate based on sensor and analysis precision and
accuracy limits only, in that no definite deposition was
observed. The Shuttle Plume Impingement Flight Experiment
(SPIFEX) payload which was subjected to numerous PRCS and vernier
engine plume impingement events during STS-64. Post flight
chemical analysis of SPIFEX witness coupons resulted in an even
lower estimate of the persistent sticking fraction of 0.001
percent, the greater apparent sensitivity resulting from the very
large number of engine plume impingement events compared to SPIE.
DTO-829 was designed to verify, in a general way, the results of
SPIE and SPIFEX on Shuttle PRCS engines while making a first set
of measurements on the Russian 13 kg thrusters. By passively
cooling the QCM sensors prior to the plume impingement events,
more transient deposition should be observed. Atomic oxygen (AO)
and solar ultraviolet (UV) were combined with plume impingement
to determine if transient deposition can be fixed permanently by
AO/UV in the low-Earth orbit (LEO) environment, a phenomena
observed in ground based laboratory studies.
The experiment logic defining DTO-829 procedures and data
analysis is somewhat different from that defining SPIFEX. The
SPIFEX sticking fraction estimate was derived from post-flight
analysis of witness coupons subjected to repeated engine plume
impingement. The SPIFEX data is subject to some uncertainty
resulting from prolonged exposure of the witness coupons to
ambient atmosphere prior to analysis (some speculation concerning
the possible evaporation of persistent plume contamination after
exposure to humid air has been voiced from time to time).
Therefore, DTO-829 was planned to provide measurements of engine
plume contaminant deposition and re- evaporation phenomena in the
space environment, with the best possible sensitivity given the
limitations imposed by mission time line, sensor engine plume
heating, and propellant utilization. Pre-flight planning
calculations showed that the DTO-829 sensor set should be able to
detect persistent sticking fractions greater than 0.003 percent
of the net plume mass flow for the PRCS engines and greater than
0.01 percent for the Russian 13 kg thrusters.
DTO-829 Flight Procedures
and Mission Environments.
DTO-829 was successfully carried out during STS-74. Two
nominally identical Quartz Crystal Microbalance (QCM) sensors
were mounted on the Shuttle remote manipulator end effector (RMS
EE) and operated using a payload general services computer (PGSC,
IBM Think Pad) interfaced to the sensor flight electronics unit
(FEU). Both the computer and the FEU were mounted in the Shuttle
aft flight deck area. The National Instrument Corporation's
"LabVIEW for Windows" instrument control and data
acquisition software package was used with the PGSC to provide a
flexible and user friendly crew interface to DTO-829. Floppy
disks containing the flight experiment data were provided to the
DTO-829 Investigator less than two weeks after Shuttle landing.
DTO-829 consisted of three operational periods: 1)
measurements of engine plume contamination produced by the
Russian 13 kg thrusters, 2) a long term observation (about 16
hours) of the Shuttle cargo bay, and 3) measurements of Shuttle
primary reaction control (PRCS) engine plume contamination. The
Shuttle-Mir operations were completed at an altitude of 215 nmi
(398 km) and the Shuttle-PRCS measurements were conducted at an
altitude of 185 nmi (342 km). Atomic oxygen ram flux depends on
both altitude and solar activity, measured as the 10.7 cm radio
flux and the geomagnetic index Ap. The average value of the
daily10.7 cm radio flux between Nov. 13 and Nov. 19, 1996 was 75
1.2 and the average
value of Ap was 4.14 1.4 (Reported by NOAA, Boulder, Colorado). The
atomic oxygen ram flux depends primarily on the magnitude of Ap
and the 10.7 cm radio flux and the spacecraft orbital altitude.
The approximate oxygen ram flux of 3 x 10
O-atoms/cm
sec.
at 398 km altitude and 6 x 10 O-atoms/cmsec. at 342 km (more
accurate calculations of the atomic oxygen flux incident on the
sensors during STS-74 are in progress).
The attached figures 1 and figure 2 show the Shuttle RMS
configuration for each of the plume impingement measurements.
Prior to both Mir and Shuttle engine contamination measurements
the sensors were oriented to deep space to produce passive
radiative cooling to below 300 K, though conflicting pointing
requirements prevented cooling to below 280 K. The Shuttle-MIR
stack attitude during the Mir jet measurements was defined as IO
5.1. The Shuttle attitude during the PRCS measurements -XLV,
-YVV. For the Mir 13 kg thruster measurements the sensors were 40
feet away from the subject thruster and positioned on the engine
plume axis, with the sensor-set normal directed at the thruster.
For the Shuttle PRCS measurements (figure 2)
the sensors were on plume axis, 34.7 feet away from the F3U PRCS
engine. In this case the angle between the sensor normal and the
engine plume was 35 degrees to accommodate the need for
simultaneous exposure to the atomic oxygen ram flux and solar
ultraviolet during the engine firings. The angle between the
sensor normal and the sun vector was 9 degrees and the angle
between the velocity vector and the sensor normal was 35 degrees.
Long term observations of the shuttle cargo bay involved
parking the RMS above the cargo bay with the sensor-set normal
pointed toward the center of the cargo bay (RMS EE coordinates
were X=-575 inches, Y=+5.50 inches and Z=-740 inches in the
Shuttle operations coordinate system). The sensors were not
actively heated during this time so that frequency changes
reflect passive thermal heating and cooling effects resulting
from changes in the radiative thermal environment during each
orbit.
DTO-829 Results and
Discussion
Sensor frequency and temperature data for sensor B is shown in
figure 3 (Mir jet firings), figure 4 (Shuttle cargo bay observation
period), and figure 5 (Shuttle PRCS
measurements). Mass deposition on the sensors produces an
increase in the output frequency of the sensors, however various
thermal effects also change sensor frequency. Throughout the
mission sensor A produced more noise in the frequency data than
sensor B so that the sensor A data is viewed as less reliable
than sensor B data (sensors A and B showed qualitatively similar
responses).
The calculation of engine plume sticking fraction estimates
from QCM sensor frequency data requires an estimate of the total
engine plume mass flux to the sensor during the experiment as
well as the mass deposited on the sensor. Mass deposition is
calculated from change in sensor output frequency,
f (with
frequency output corrected for thermal effects), resulting form
the mass deposition event, and then multiplying
f by the
sensor mass-frequency calibration constant (equation 1).
(f (Hz) - f
(Hz)) x 4.42 x 10
(g/cm Hz) = G(g/cm ),
(1).
where
f = sensor frequency, corrected for thermal effects,
after the mass deposit event, f = sensor frequency, corrected for thermal
effects, before the mass deposit event, G = grams/cm
deposited on the sensor.
The 13 kg Mir engine and the Shuttle PRCS engine both produced
an immediate contamination mass deposit as indicated by the
increase in sensor output frequency coincident with the engine
firing event. Using equation 1, the immediate mass deposit for
each engine can be calculated.
Mir 13 kg engine initial deposit = 0.62
micrograms/cm
Shuttle PRCS engine initial deposit = 15.6
micrograms/cm
The initial deposit from both engine types sublimed or
evaporated rapidly so that the sensor output frequency, corrected
for thermal effects, was near the pre-mass deposit value by the
end of the experiment period indicating that little or no
persistent mass deposition had resulted from the engine firings.
Because small differences in sensor output frequency are involved
in the calculation of the amount of persistent mass deposit and
the final post-flight sensor thermal re-calibration will not be
completed for several months, the estimates of the persistent
mass deposit given below are provisional. Nonetheless, little or
no persistent mass deposit is indicated from either engine type
and no important increase in the estimated mass deposit is
anticipated.
Some comment on the thermal effects visible in the expanded
scale data plots is appropriate. Abrupt step-function changes in
frequency are associated with movement into and out of sunlight
as is shown in figure 4, figure 6, and figure
7. The direct photothermal effect is well known in the
industry and the instrument vendor verifies that the effect is of
the correct order of magnitude in DTO-829 data set. In addition,
sensor assembly temperature changes, measured by a thin-film
resistance thermometer on the sensor crystal. Both effects are
taken into account when calculating f.
Mir 13 kg engine persistent deposit = 0.01 +
0.09 micrograms/cm
Shuttle PRCS engine persistent deposit = 0.1
+ 0.09 micrograms/cm
A preliminary estimate of the engine plume
persistent sticking fraction (the basis of the ISS contamination
performance and functional life estimates as described above) can
be made using simple rocket engine plume models to estimate the
engine plume mass flow past the sensor location during the test.
The PRCS engine plume model is used directly and the Mir 13 kg
engines are modeled using a model of the Shuttle vernier engine
plume, since the Russian 13 kg thruster is nearly identical to
the Shuttle vernier in size, thrust and propellant mass flow rate
(differences in propellant mass flow rate were taken into
account). More detailed calculations using current ISS program
baseline engine plume models are in progress, however, no
important change in the conclusions are anticipated at this time
because DTO-829 measurements were conducted on engine plume axes.
The persistent sticking fraction is calculated by dividing the
measured sensor mass deposition by the total engine plume mass
flow delivered to the sensor location (calculated using the
approximate plume models given in the Shuttle/Payload
Contamination Evaluation Program MCR-81-509, NAS9-15826, Feb.
1981.).
Mir 13 kg engine persistent sticking
fraction (percent) = 0.006 0.05 percent
Shuttle PRCS engine persistent sticking
fraction (percent) = 0.007 0.006 percent
Conclusions
The sensitivity of any technique used to measure the sticking
fraction of rocket engine plume gases in the LEO environment is
ultimately limited by the sensor signal to noise ratio and the
total mass flow of plume gases over the sensor. The results of
DTO-829 confirm the SPIE results for Shuttle PRCS engines and
extent the measurement to greater sensitivity as a result of the
increased number of engine firings and the smaller separation
between the sensor set and the F3U engine. In addition, cooling
the sensors allowed for a greater transient contamination deposit
which did not show detectable permanent fixation as a result of
the combined effects of atomic oxygen and solar UV radiation.
Finally, the first measurements, to which we have access, of
contamination produced by the Russian 13 kg engines have been
made and no persistent contamination has been detected which
alleviates some program concerns about 13 kg engine plume
degradation of the ATCS radiators on ISS. SPIE, SPIFEX, and
DTO-829 together, provide the data needed to set the persistent
plume sticking fraction for hypergolic engines of interest to the
International Space Station Program at 0.001 percent of the net
plume gas flow for contamination assessment and functional life
predictions. The estimate of 0.001 percent is based on SPIFEX
measurements and observations confirmed by the fact that DTO-829
measurements revealed no persistent contamination with a
comparable though somewhat lower sensitivity. An accurate and
realistic basis for performance assessments leads directly to
significant program cost avoidance and reduced program risks.
Acknowledgments
Payload integration, payload development, flight planning and
flight operations support were excellent throughout this DTO
effort. EV/Gary Bourland did an excellent job with developing the
LabVIEW Windows software application to DTO-829 despite the
difficulties involved with the ongoing change from the Grid to
the IBM Thinkpad PGSC platform. MT2/Vanessa Ellerbe developed the
Integration Plan (IP) and Annexes that made the integration
process proceed smoothly and efficiently despite the need to
develop joint operations with the Russian Program during IP
development. DF44/RSO/Alberto Magh Developed RMS procedures for
both the Shuttle and Mir components of the mission. The Paylaod
Officer, DO621/Tim Baum, and the Timeliner, DO451/Gail Schneider,
were especially helpful in negotiating the joint operations
agreement with the Russian Program, which ultimately agreed to
commit 8 kg of valuable propellant and develop custom thruster
firing software to support the DTO-829 measurements. OB/ETS/Buck
Gay provided invaluable assistance in interfacing with the
Russian Program. KSC support (Bob Peterson, Tom Kelley, and the
RMS support team) was excellent and included a custom
modification of an RMS EE thermal blanket to accommodate the DTO
829 QCM sensor set, though some misunderstandings developed in
the area of PGSC support for the in vehicle test (IVT) (probably
as a result of the PGSC platform change process).
Flight operations support was also excellent with controllers
and back-room staff showing a detailed knowledge of the DTO-829
requirements and informing the PI of any deviations.
OB/ETS/Carolynn Conley provided excellent support in the CSR
during the mission.
