During the Apollo 15 mission, a boom with an attached mass spectrometer was required to retract periodically so that the instrument would not be in the field of view of other experiments. The boom did not fully retract on five of 12 occasions. Data analysis indicated that the boom probably retracted to within approximately 2.54 centimeters (1 inch) of full retraction. The pertinent boom-design details, the events in the mission related to the anomaly, a discussion of the inflight and postflight investigation of the problem, a discussion of the design changes to the boom mechanism as a result of the investigation, and subsequent flight performance are presented

White, R. D.

English

NASA Technical Reports Server

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THE APOLLO 15 DEPLOYABLE BOOM ANOMALY
By Robert D. White*
ABSTRACT
During the Apollo 15 mission, a boom with an attached mass spectrometer was
required to retract periodically so that the instrument would not be in the field of view
of other experiments. The boom did not fully retract on five of 12 occasions. Data
analysis indicated that the boom probably retracted to within approximately 2.54 centi-
meters (1 inch) of full retraction. The pertinent boom-design details, the events in the
mission related to the anomaly, a discussion of the inflight and postflight investigation
of the problem, a discussion of the design changes to the boom mechanism as a result
of the investigation, and subsequent flight performance are presented in this report.
INT RODUC TION
While the Apollo 15 commander and lunar module pilot were on the lunar surface,
the command module pilot completed 34 lunar orbits, operating scientific instrument
module (SIM) experiments to obtain data concerning the lunar surface and the lunar
environment. One of these experiments involved the use of a mass spectrometer that
was deployed from and retracted into the SIM bay by means of a deployable boom mech-
anism. A boom was necessary because the experiment was used to detect released
gases from the lunar surface and, therefore, had to be placed away from the immedi-
ate vicinity of the command/service module to minimize any contamination of the'in-
strument by offgassing products of the vehicle. The boom was required to retract
periodically so that the mass spectrometer would not be in the field of view of other
experiments and also to allow firing of the service propulsion system engine. How-
ever, during the Apollo 15 flight, the boom did not fully retract on five of 12 occasions.
The problem that occurred in flight, the postflight investigations, and the subsequent
design changes and results are discussed in this report.
DEPLOYMENT MECHANISM
Two experiments, a gamma-ray spectrometer and a mass spectrometer, were
mounted<on similar extendable/retractable boom mechanisms in the SIM bay of the
Apollo service module (fig. 1). When deployed, the mass-spectrometer boom ex-
tended 762 centimeters (25 feet) and the gamma-ray boom extended 817.8 centimeters
*NASA Manned Spacecraft Center, Houston, Tex.
15
https://ntrs.nasa.gov/search.jsp?R=19730010141 2020-03-17T08:18:33+00:00Z
(26 feet 10 inches). The mass-spectrometer unit that was attached to the end of the
boom weighed approximately 11 kilograms (25 pounds). The deployment mechanism
(including the jettison mechanism) had dimensions of 33by 43 by 84 centimeters (13 by
17 by 33 inches) and weighed 41 kilograms (90 pounds).
The boom is formed of two tempered-steel tapes that are 14.2 centimeters
(5.6 inches) wide and 0. 03 centimeter (0. 012 inch) thick. Before deployment, the
tapes are stored flat on two motor-driven reels. As the tape is unreeled during de-
ploym6nt, it assumes its natural C-shaped cross section and joins the other tape to
form a circular cross section boom 5.08 centimeters (2 inches) in diameter (fig. 2).
The deployment cycle is terminated by allowing a roller follower on the extend limit
switch (fig. 2) to drop through a slot cut in one of the steel tapes, opening the circuit
to the extend winding of the reel-drive motors and a talkback indicator that is monitored
by the command module pilot. The retraction cycle is terminated when the experiment
mounting flange engages the retract limit-switch actuation rod and opens the circuit to
the retract winding of the reel-drive motors and to the talkback indicator.
Cycling time of the mass-spectrometer boom assembly is a function of several
factors (for example, temperature and available bus voltage). At a nominal 28 V dc,
the boom assembly would take approximately 140 seconds for extension and 173 sec-
onds for retraction. Boom-position-monitoring capability is provided by means of
talkbacks. However, these boom-status monitors indicate full deployment, full retrac-
tion, boom in transit, and boom jettison only; intermediate boom positioning necessi-
tates timing by the crewman.
The experiment is connected electrically to the SIM bay by means of a cable that
is coiled around the boom at deployment (fig. 3) and that is stored within the boom-
actuating mechanism housing (fig. 2) when the boom is retracted. The experiment
power cable is a bundle of 20 electrical conductors and one coil spring wire having a
total cross section of 1.3 centimeters (0.5 inch). The coil spring is included to control
the location of the power cable and the diametrical size of the coils during extension
and retraction. The pitch of the coiled cable reverses direction at the midpoint in the
cable length so that there is no torsional effect from the spring on the experiment dur-
ing extension and retraction or while extended and operating. The length of the power
cable is approximately 1920 centimeters (63 feet), but, in the relaxed coiled configura-
tion, the length is approximately 260 centimeters (8.5 feet). A force of approximately
4.5 kilograms (10 pounds) is required in order to compress or extend the coils from
the relaxed position; however, the drive motors will not stall until an excess of
200 kilograms (440 pounds) is applied to the boom.
To guide the power cable into its storage volume during retraction, the mechan-
ism housing is flared at the end and the experiment mounting flange has six fingers
attached that act like scoops during retraction (figs. 2 and 3). The flare on the housing
has cutouts so that the fingers can push the cable bundle past the housing lip.
Because the retracted experiment and its boom mechanism must be supported in
the SIM bay for launch forces, the experiment and the boom are mounted on two rails
that are supported by trusses from the bottom side of a SIM-bay shelf (fig. 4). These
guide rails also allow the experiment to extend and then retract for support during
space-flight acceleration forces with the service propulsion system engine. Linear
bearings attached to the experiment pick up the tapered ends of the guide rails during
retraction.
16
Because of the limited mechanical strength of the boom, it must be retracted
before any service propulsion system engine firings to avert buckling of the boom,
which could damage the service module. If the boom cannot be retracted because of a
malfunction, the boom-actuating mechanism, the boom, and the experiment could be
jettisoned. The spring-powered jettison mechanism is shown in figure 4. On proper
circuit closure, the fusable wire actuator would release the restraining link, and the
boom-actuating mechanism, the boom, and the experiment all could be jettisoned at a
rate of approximately 213 cm/sec (7 ft/sec).
The mass-spectrometer installation in the SIM includes a thermal and contamina-
tion cover (figs. 4 and 5) that closes over the outboard face of the experiment. This
protective cover opens with the initial motion of the boom mechanism during extension
and closes with the final motion during retraction. The cover prevents heating and
contaminant damage to the experiment that would result from reaction control system
(RCS) jet firings and effluent dumps.
FLIGHT ANOMALY
During the Apollo 15 flight, the mass-spectrometer boom mechanism was re-
quired to retract 12 times. On five of these occasions, the boom did not retract fully.
Normally, the deploy/retract talkback indicator is gray when the boom motors are off
and the boom is retracted or extended fully. The indicator is "barberpole" when the
boom is eithe_ extending or retracting. However, because the indicator is wired in
series with the drive motors, a half barberpole will usually show if the drive motors
stall. When the motors stall, the current increases and causes a voltage drop. The
voltage drop then causes the indicator to activate only partially to the barberpole posi-
tion. The command module pilot noted this half-barberpole condition that indicated
stalled motors and, therefore, incomplete retractions.
Telemetry data from the spacecraft electrical buses plotted with a time base in-
dicated that the current driving the retract motors was normal on the five stall oc-
casions until the boom was within 2 to 5 centimeters (0.79 to 1.97 inches) of full
retract. At that point, the current increased from a nominal 3 to 4 amperes to the
typical stall level of approximately 9 amperes.
A space cold soaking of the fully deployed boom and power cable preceded each
anomalous retraction. In every case, after a warmup period in the sun, the boom
could be retracted fully after first again extending it approximately a meter. Also, a
special inflight test to investigate the boom anomaly was accomplished during the
spacecraft coast back to earth. This test supported the theory that the malfunction
was related to thermal conditions; that is, after a cold soak, the boom did not retract
completely, but, after a subsequent hot soak, retraction was completed.
An inflight photograph of the extended boom (fig. 6) revealed that the power-
cable coils remained circular but were not concentric to the boom and appeared to
touch the boom on most of its length. The assumption had been made that in zero-g this
sagging would not occur significantly because of the coiled spring in the power cable.
17
The extravehicular activity by the command module pilot to the SIM bay to re-
trieve film cassettes confirmed the assumption that the boom was within approximately
3 to 4 centimeters (1.18 to 1.57 inches) of full retraction. The boom had stalled before
the extravehicular activity and was left in the stalled position for visual inspection by
the command module pilot. He reported that the mass-spectrometer thermal cover was
approximately 30 to 40 degrees open and that he could see only the tapered tip of the
guide rail protruding from the experiment support bearing, as simulated in figure 5.
This corresponds to a boom position of approximately 3.5 centimeters (1.38 inches)
from full retract, as shown by the data in figure 7.
Although retraction problems occurred, the boom was always manipulated (sun
exposure and short recycles) so that the experiment did not have to be jettisoned to
allow SPS engine firings. Valuable mass-spectrometer data would have been lost if the
stalled boom had been jettisoned. However, only a small percentage of the experiment
data-gathering time was lost. The similar boom-deployment mechanism for the
gamma-ray spectrometer worked normally throughout the Apollo 15 mission.
PROBLEM INVESTIGATION
No postflight hardware analysis or testing could be conducted with the Apollo 15
boom mechanism because the service module that contained the SIM bay was jettisoned
before earth entry. However, because gamma-ray-spectrometer and mass-
spectrometer boom mechanisms were to be used on the Apollo 16 mission, an exten-
sive anomaly investigation was conducted in an attempt to correct the problem. The
investigation was based on the following list of most probable causes of the anomaly.
.
2.
bearing
3.
4.
5.
6.
Cable bunching and jamming at the mechanism housing annulus opening
Cable sagging and bunching between flared housing and fingers or experiment
Cable service-loop snagging on housing or support structure
Interference between guide rails and spectrometer bearing
Low-temperature stiffness of the power cable
Marginal adjustment or malfunction of the retract limit switch or the actuator
rod to the switch
7. Interference between experiment and cover
The investigation started with the following analysis and testing.
1. A documentation review of all pertinent drawings, specifications, analyses,
and discrepancy records was conducted to identify any irregularity that may have
caused the retract anomaly.
18
2. A visual inspection of the Apollo 16 mass spectrometer and gamma-ray spec-
trometer was accomplished at the launch pad while the instruments were installed in
the flight configuration. The inspection included a short extension and retraction of
each assembly.
3. After return of the Apollo 16 mass-spectrometer boom assembly to the con-
tractor, additional visual inspection and dimension verification were accomplished. In
addition, short extensions and retractions were performed at room-ambient conditions
under various orientations with respect to gravity to simulate worst-case cable-sagging
conditions.
The major problem discovered was that of sagging in the power cable. On a few
occasions, the sagging would allow the cable to bunch up and jam or snag between the
flared housing and a guide finger or between the flared housing and an experiment sup-
port bearing. An abnormality of this kind is shown in figure 8.
This same cable-sagging problem had been observed during the original flight-
qualification testing of the deployment mechanism, but most of the sag was attributed
to the one-g environment, and the assumption was made that the sag would be confined
withir, Lheflared housing and the guide fingers during zero-g conditions. Therefore,
for the majority of the qualification tests, the cable was counterbalanced when the
boom was cycled.
Also, in power-cable developmental tests that were conducted before the quali-
fication testing, no stiffness change was detected in the cable coils when the coils were
exercised at room temperature and at 188.15 ° K (-85 ° C). This fact was verified
during the anomaly investigation.
INVESTIGATION CONCLUSIONSAND CORRECTIVE ACTIONS
After all plausible causes for the Apollo 15 flight anomaly were examined, and
based on the analyses and testing discussed in this report, the conclusion was reached
that the problem occurred because of one of the following reasons.
1. Improper stacking of the power-cable coils into the annulus of the mechanism
housing during retraction
2. Jamming of the power cable either between the experiment support bearing
and the mechanism housing or between the guide fingers and the housing
The first possible cause cited could not be corrected in time to support the
Apollo Program schedule. The housing annulus opening would have had to be redesigned
and, possibly, the cable-coil diameter would have had to be decreased. These changes
could not have been accomplished and requalified for flight in time for the Apollo 16
19
launch. However, for the second possible cause, several simple changes could be made
to the existing design and could be tested within the schedule. The following changes
were implemented.
1. Tab extensions were added to the flare on the mechanism housing (fig. 9).
2. A second cable clamp was added to retain the cable service loop better at the
experiment interface (fig. 9).
3. The inboard experiment-support bearings were modified by adding a ramp to
serve as a fairing during cable retraction (fig. 10).
4. The guide fingers on the experiment-mounting flange were extended where
possible to improve gathering of a sagging cable (fig. 9). A similar change was made
to the Apollo 16 gamma-ray-spectrometer deployment mechanism.
After these modifications were added, the Apollo 16 mass-spectrometer mech-
anism (with a prototype spectrometer) was tested at room-ambient and low-temperature
conditions and at the low-temperature gradients in a vacuum simulating the worst-case
low-temperature-gradient environment of lunar orbit. No test anomalies were ex-
perienced, and the mechanism performance was nominal.
The mass spectrometer deployment mechanism assembly was completely re-
cycled through standard acceptance-test procedures before return to the launch pad.
However, in the event the changes did not solve the problem for the next flight (such as
improper stacking of the retracting cable), a proximity switch was also added to each
of the spectrometer experiment mechanisms (fig. 10). This switch would indicate when
the boom was within 30 centimeters (11° 81 inches) of full retract even if the motors did
stall. The boom is safe for an SPSengine firing within this 30centimeters (11.81 inches),
and this knowledge could prevent either experiment from being jettisoned if the boom
did stall.
CONCLUDING REMARKS
The Apollo 15 mass-spectrometer-deployment mechanism could not be returned
for an evaluation to determine the cause of the flight anomaly. A postflight investiga-
tion revealed two probable causes; however, the flight schedule for Apollo 16 prevented
any extensive redesign of the mechanisms. Only simple modifications could be added
to the mechanisms in an attempt to avert the problem from occurring again and also to
add a proximity switch to indicate when the boom was in a safe position for service
propulsion system engine firing. These modifications were thought to be sufficient be-
cause Apollo 16 was the last spacecraft to use these spectrometer mechanisms.
The Apollo 16 mission proved that the changes were not adequate because both
mass-spectrometer and gamma-ray boom mechanisms would not always retract fully
when commanded. However, the proximity switches on both experiments always in-
dicated they were retracted within 30 centimeters (11.81 inches), saving the experi-
ments from being jettisoned while gathering data in lunar orbit. _ It is the opinion of this
author that the basic problem was improper stacking of the cable into the mechanism
2O
housing annulus during retraction; this should be recognized as a significant design
problem. If similar concepts of long retractable booms that have an external cable are
needed in future space efforts, an ample stowage area should be provided for the cable.
DISCUSSION
D. D. Laine:
Are internal electrical cables being considered by the MSC for future booms of
this basic cross- sectional configuration ?
White:
There will be no booms with electrical cables used on any other Apollo flight, and
I would say that it is too early to know this type of detail for the space-shuttle program.
A. B° Hunter, Jr."
On the double C-spring boom mast, was there any resistance to twisting and was
this a problem? Also, did the outer coiled cable impart any twist to the boom?
White:
The boom tapes did have restraints or clamps located at each end of the boom;
however, it was known from ground-based testing that the boom would have 40° to 60°
thermal twist. This twist was not a problem with the gamma-ray experiment but the
mass spectrometer inlet scoop had to be redesigned. The external power cable did
not cause a torsional problem while extended because the pitch of the coiled cable re-
versed direction at the cable-length midpoint to cancel the torsion from the coil spring.
21
Figure i.- Apollo SIM-bay experiments.
EXTEND LIMIT SWITCH
ELECTRICAL
CONNECTOR
RETRACT
LIMIT
SWITCH
SWITCH-ACTUATION ROD
PUNT BEARINGS
FINGER
(6 TOTAL)
SWITCH RC
FOLLOWER
EXPERIMENT
CONNECTOR
IMENT-
MOUNTING
FLANGE
MOTOR-DRIVEN
STEEL TAPE REELS
COILED EXPERIMENT
POWER CABLE
RAIL-MOUNT BEARINGS
_AECHANISM
HOUSING
VIEW ROTATED 180 °
Figure 2.-Mass-spectrometer boom-actuating mechanism.
22
Figure 3.- Mass-spectrometer boom partially extended.
.°_..
ELF
L'_,_ I _--"'J F-TRUSS SUPPORT
_,SSEMBLY
THERMAL/CONTAMINATION ___
COVER {OPEN)
£MASS JETTISON
MECHANISM
GUIDE
-.'s_>'_"_ "_. RAILS
__ CABLE COIL-PITCH REVERSAL
_ W I _ EXPERIMENT BEARINGS
"" _! -"
Figure 4.- Deployment mechanism and support structure.
23
Figure 5.- Simulation of command module pilot observation.
Figure 6.- Boom deployed in lunar orbit.
24
54
GUIDE-RAIL TIP
OUTBOA'RD
EXPERIMENT-SUPPORT
BEARING AT
FULL-RETRACT POSITION
DISTANCE BEHIND RAIL
TIP TO BEARING, crn
3
TAPERED
PORTION
2 - OF RAIL
10
KINEMATIC RELATIONSHIP
DOOR
FULLY
OPEN
PILOT ESTIMATION
RAIL TIP l S. L Z
0 20 30 40 50 60 70 80 90 100
COVER OPEN ANGLE, DEG
Figure '/. -Relationship of mass-spectrometer cover to bearing position.
Figure 8.- Boom stall caused by jammed cable.
25
Figure 9.- Design changes to the deployment mechanism.
Figure 10.- Additions to the deployment mechanism.
26


The Apollo 15 deployable boom anomaly

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