Six docking attempts were required to achieve initial latch engagement during the Apollo 14 translunar docking event. Although subsequent performance of the docking hardware was normal, the docking probe was retained for a thorough postflight investigation. Pertinent design details of the docking system, the mission events related to the anomaly, and a discussion of the postflight investigation of the cause of the anomaly are presented

Langley, R. D.

English

NASA Technical Reports Server

THE APOLLO 14 DOCKING ANOMALY
By Robert D. Langley*
ABSTRACT
Six docking attempts were required to achieve initial latch engagement during the
Apollo 14 translunar docking event. Although subsequent performance of the docking
hardware was normal, the docking probe was retained for a thorough postflight investi-
gation. Pertinent design details of the docking system, the mission events related to
the anomaly, and a discussion of the postflight investigation of the cause of the anomaly
are presented in this report.
INTRODUCTION
The Apollo lunar-landing missions require that two docking maneuvers be per-
formed: the first is called transposition or translunar docking and the second is re-
ferred to as lunar-orbit docking. Because the required dockings had been performed
successfully on five previous Apollo missions, docking had become a routine procedure
until the Apollo 14 docking anomaly. The Apollo 14 translunar docking required six at-
tempts to achieve capture-latch engagement. Although subsequent mission operations
were normal, the docking probe was returned to earth with the spacecraft so that a de-
tailed investigation could be conducted. The purpose of this paper is to present a
limited description of the docking mechanism, the associated mission events, and a
discussion of the investigation and findings.
The material presented in this paper was derived from the Apollo 14 Mission
Anomaly Report prepared by the NASA Manned Spacecraft Center Mission Evaluation
Team.
DOCKING SYSTEM DESCRIPTION
The Apollo docking system consists primarily of the probe, mounted on the for-
ward end of the command module (CM), and the drogue, mounted within the lunar mod-
ule (LM) tunnel (figs. 1 and 2). The docking probe consists of numerous subassemblies;
however, a description of only the capture-latch assembly is Presented because the
Apollo 14 docking anomaly is associated with that area.
*NASA Manned Spacecraft Center, Houston, Tex.
191
https://ntrs.nasa.gov/search.jsp?R=19730010157 2020-03-17T08:20:36+00:00Z
The probe capture-latch assembly (fig. 3) is contained within the self-centering,
gimbal-mounted probe head and serves as the method for achieving initial coupling be-
tween the CM and LM. The assembly consists of three hooks that are pin mounted to
the probe head and that are spring loaded so that the hook protrudes beyond the surface
of the probe head. Opposite each of the hook pivot points is a two-piece toggle link that
connects the hook to a fixed point on the probe head. When the hook is extended, as
shown in figure 3, the toggle link pins are almost in line, providing a means of locking
the hook.
Latch locking and release are determined by the axial position of a single, sym-
metrical spider that is spring loaded to the full-forward, locked position. In this po-
sition, a roller on the spider rests beneath each of the hook toggle links so that the
hooks cannot be depressed. To unlock the latches, the spider must be moved aft and
retained until subsequent latch lock is required.
Spider retention and release (fig. 4) are achieved by means of triggers located
within each of the latch hooks. When the spider is moved aft of the triggers and re-
leased, pins located on the outer tip of the spider bear against the back face of the
trigger and prevent forward travel of the spider. To release the spider, all three
triggers must be depressed because one or more triggers will retain the spider in the
aft (unlocked) position. The spider can be moved from the forward (locked) to the aft
(unlocked) position by manually depressing the plunger in the probe head or by rotating
the torque shaft. When the torque shaft is rotated, either by manually actuating the
capture-latch release handle or by powering the torque motors in the actuator assem-
bly (fig. 5), the rollers turn in the cam slots and force the cam and the spider aft.
When power is removed from the torque motors, the torsion spring on the torque shaft
rotates the shaft back and allows the spider to move forward until cocked; that is, the
spider pins ride against the back of the triggers.
APOLLO 14 MISSION SEQUENCE
Functional checkout of the Apollo 14 docking probe was completed in August 1970,
and the retracted docking probe was installed in the CM docking ring on November 8,
1970. At that time, a tension-tie mechanism was installed between the probe head and
the launch escape system (LES) boost protective cover to provide a system compatible
with LES abort requirements (fig. 6). As shown in figure 6, the tension-tie installa-
tion places the capture latches in a mechanically cocked launch configuration.
On January 31, 1971, after a rain shower, the Apollo 14 spacecraft was launched
from the NASA John F. Kennedy Space Center; as part of the normal boost phase of the
flight, the LES was jettisoned. This event sheared the pins that attached the tension
tie to the probe head, allowing the tension tie to remain with the jettisoned LES and
leave an exposed, intact, docking probe. As part of the earth-orbit spacecraft-
checkout activity, the crewmen extended the docking probe by operating a switch on
the CM main-display panel. This extension sequence also applied power to the probe
direct current torque motors to rotate the torque shaft. The crewmen verified proper
extension by observing a talkback indicator on the same panel. The spacecraft was
then injected into a translunar flight trajectory by firing the S-IVB booster engine.
192
The transposition and docking events (fig. 7) were initiated by separation of the
command and service module (CSM) from the adapter and ejection of the adapter panels
that house the LM. Then, the crew translated the CSM approximately 30 meters
(100 feet) away from the LM and turned'the CSM 180° with a pitch maneuver to aline
the CSM and LM in preparation for the docking. Before initiating a CSM/LM closure
rate, the crew placed one of the docking switches to the retract position and certified
that the probe capture latches were still in the cocked configuration. The CM pilot
then experienced five unsuccessful docking attempts.
After docking, the crew examined the probe and drogue and no abnormal opera-
tions could be observed; however, there were marks on the drogue, as is shown in fig-
ure 8. Although subsequent docking functions were completely normal, the docking
probe was returned with the CM.
ANOMALY INVESTIGATION
The initial phase of the anomaly investigation consisted of activity to reduce the
list of possible failure modes based on flight data and the crew debriefing. A summary
of the docking attempts is presented in table I. The first attempt consisted of four
distinct probe/drogue contacts near the drogue apex, whereas each succeeding attempt
consisted of withdrawal of the CM and subsequent initiation of a new closure rate.
These data support the conclusion that the contact conditions were normal for all dock-
ing attempts and capture-latch engagement should have been achieved for the five un-
successful attempts.
The second conclusion, which is that the capture latches were not in the locked
configuration during the unsuccessful attempts, is based on the following reasons.
1. The probe-status talkback displays functioned properly before and after the
unsuccessful attempts, indicating proper switch operation and power to the talkback
circuits. The talkback displays always indicated that the capture latches were in the
cocked position during the unsuccessful attempts.
2. Electrical power to the motors is not required because the system is cocked
before flight and the initial capture operation is strictly mechanical and must be trig-
gered by the drogue. Conversely, a review of flight electrical data showed that there
was no unexplained current usage during the docking maneuvers because power to the
probe torque motors would have retained the latches in the cocked position.
3. Each of the six marks on the drogue resulted from separate contacts by the
probe head (fig. 8). A docking impact with locked capture latches should result in
three double marks (to match the latch hooks) within 2.54 centimeters (1 inch) of the
drogue socket.
Because the latches were not locked, apparently the anomaly was caused by fail-
ure of the capture-latch spider to reach the locked position. Therefore, the investiga-
tive activity was focused on the items that could influence the position of the spider.
The results are summarized as follows.
193
Analysis
As a result of rain on the launch day, it was possible that water could have en-
tered the probe head and frozen during the boost phase of flight. A maximum of
30 grams of ice could have formed; however, this amount would sublime within 15 min-
utes, well before the docking event that occurred approximately 3-1/2 hours after
launch.
A trajectory analysis, based on Apollo 14 flight-acceleration data, was conducted
to determine if the tension-tie shear-pin remnants could have reentered the capture-
latch area. This analysis, which was verified by tests, showed that, if the remnants
separated from the tension tie, impact would be well out of the capture-latch area.
The mating parts and surfaces of the capture-latch assembly were analyzed to
determine if a worst-case tolerance accumulation combined with a 11° K (20° F)
thermal gradient would produce interference. In all cases there were no metal-to-
metal interferences.
Ancillary Testing
Because improper shearing of the tension-tie shear pins could prevent movement
of the capture-latch plunger, various design combinations of pins were shear tested.
In each test, the shear was clear, regardless of the allowable pin-to-hole dimensional
combination. In addition, the shock load associated with pin shearing had no effect on
performance of the latch mechanism.
As part of the acceptance testing of each probe, a capture-latch timing test is
performed to record spider travel as a function of time. To determine if shelf life de-
grades the latch mechanism, a timing test was performed on a probe that had been idle
for approximately 5 months (similar to the Apollo 14probe). The results were com-
pared with previous time traces, and there was no evidence of degradation.
Coincidental with the Apollo 14 investigation, acceptance.testing was being per-
formed on the Apollo 16 docking probe when the capture latches failed in a manner
symptomatic of the Apollo 14 anomaly. It was determined that, for a given torque-
shaft alinement and the lateral load resulting from the torsion spring (fig. 9), the ball
end of the shaft would fall into the cam slot. Then, a friction lock would occur between
the ball and pin, preventing the ball from riding out of the cam slot. This, in turn,
would prevent subsequent movement of the cam and spider train. Upon disassembly,
the torque-shaft ball was found to be galled severely.
Apollo 14Docking-Probe Inspection and Tests
A visual inspection of the probe head revealed several small scratches and burrs
on the inner wall of the tension-tie bushing (fig. 10). The scratches were caused by an
object moving in the aft direction; the burrs were the material buildup. This burring
could have been caused by a foreign particle lodged between the plunger and the wall of
the bushing because the initial motion of the plunger is in the aft direction when the
194
capture hooks are depressed. Therefore, foreign particles in the area of the capture-
latch plunger were considered a potential failure mode.
During disassembly of the probe, 12 contaminant particles were found. Three of
the particles, which were foreign to the probe, were iron oxide, double-back tape, and
c_tdmium particles. The largest of the 12 particles was 0.15 centimeter (0.060 inch)
long, and, of those particles large enough to cause mechanical interference, none were
hard enough to prevent operation of the mechanism.
After the Apollo 16 probe capture-latch problem, attempts were made to dupli-
cate the problem on the Apollo 14probe. With the latch assembly in a horizontal posi-
tion so that the weight of the shaft added to torsion-spring lateral load, one malfunction
was experienced during 60 latch cycles. This also remained a potential failure mode.
Tests were conducted to measure capture-latch timing, torque motor output,
friction drag, and spring forces. The results were compared with preflight measure-
ments and no significant degradation was noted.
A detailed inspection of each latch-assembly component did not reveal any wear,
damage, or out-of-tolerance parts that could be related to the problem.
CONCLUDING REMARKS
The cause of the Apollo 14 docking anomaly was either (1) foreign material (that
was subsequently lost) restricting mechanical operation or (2) jamming of the cam. To
ensure proper performance for future systems, cleanliness requirements were im-
proved and a design change was implemented to prevent jamming of the cam. Since
then, the Apollo 15 and 16 missions have been completed successfully without recur-
rence of the docking problem.
DISCUSSION
W. W. Weber:
How were the supporting tests correlated with the actual failure environment (for
example, zero gravity)? By vertical or horizontal positioning?
Langley:
During postflight testing of the Apollo 14 docking probe, the failure could not be
reproduced with the probe in the vertical position (simulating zero gravity). The
failure could only be produced with the probe horizontal, so that the weight of the torque
shaft produced additional side loading (one failure in 60 activations).
195
TABLE I. - RELATED DATA AND FILM INVESTIGATION RESULTS
Docking
attempt
1A
1B
1C
1D
Estimated
velocity,
m/see
(ft/sec)
(a)
O. 03
(O.i)
0. 043
(0.14 max. )
0. 043
(0.14 max. )
O. 088
(0.29 max. )
0.122 to 0. 152
(0.4 to 0.5)
O. 122
(0.4)
0. 122 to 0. 152
(0.4 to 0.5)
0. 076
(0.25)
0.061
(0.2)
Socket
contact
time,
see
(b)
1.55
1.65
1.4
2.35
1.7
2.45
6.5
2.9
In and hard
docked
+X
thrusting
after contact,
sec
None
None
0.55
1.95
None
None
6.2
None
14.3
Comments
No thruster activity
Contact moderately close to apex
Contact close to apex
Contact close to apex
Contact close to apex
Contact close to apex
Contact close to apex
Contact close to apex
Contact close to apex
Contact moderately close to apex
Retract cycle began 6.9 seconds
after contact
Initial latch triggered 11.8 sec-
onds after contact
aThe system is designed to capture with closing velocities between 0. 030 and
0.305 m/sec (0.1 and 1.0 ft/sec) and with initial probe contact within 0. 305 meter
(12 inches) of the center of the drogue, These criteria were met on all docking
attempts.
bThe maximum capture-latch response time is 80 milliseconds.
196
LAUNCH ESCAPE SYS
BOOST PROTECTIVE CC
COMMAND MODULE
SERVICE MODULE
DOCKING DROGUE
ADAPTER
LUNAR
MODULE
INSTRUMENT UNIT
S-lVB
)CKING
PROBE
*l
ml
!
m
ml
S-]_" B
!
, S-ll
bq
,S-IC
i
Figure 1.- The Apollo spacecraft.
- FORWARD
TUNNEL
DROGUE PROBE "DOCKING HATCH
DROGUE ASSEMBLY "-7 RING(C 7 t _ _CM
ASS
TUNNEL ASSEMBLIES TUNNEL
RING _'FORWARD
MOUNTS'--_
_.LMADUvMP \ A_ ;EMBLY- 7
UPPER -LM RiNG
HATCH TUNNEL BULKHEAD
Figure 2.- Major assemblies of the docking system.
SPRING---._ /--" .._I"IUI:K / \,,' * ',. \...1"_'J
HOOK '_ / /RETURN _ _, ,._ ,;_"
SPRINGS--_ \_ / _ --_; _\ /';_
.,,_ L/ /_ ._.._'-::3f_i_, _ ---',..i _," •
TRIGGER - -.... -: -,:, .
,o,s,o,
TOGGLE ._.._ _V"-I_iJEZII SPRING __--.A_N_::.: .___ _.':_-:_._._.L,NKS_ n Y_ I 7 _\,_.," _:::-:,!_, ""_-_.-_,_.
_"_2_ D-.'-=_.............._,,_--'-__-,_"= _.' ' '--""_'- .... ""- _'_ ._, ,__,,I ',, V
/ _-7_ == __ I "-._
SPI DE RZ7 _ J"* _' TORQF_ EA
BALL JOINT
Figure 3. - Probe capture-latch assembly shown in locked position.
_ _..PIDER ROLLER
_PLUNGER
_HTO?GKER
.SPIDERPIN'
_ cAM--.
TORQUE
SHAFT
ROLLER
LOCKED POSITION RELEASEDPOSITION
RETURN
SPRING
SPRING
RETURN
SPRING
_UE
SHAFT
COCKED POSITION
Figure 4. - Relationship of probe latch and cam mechanism.
198
Figure 5. - Cutaway of probe assembly in extended and cocked position.
MILD DETONATING FUSE ATTACHES TO BOOST
SEPARATES DOCKING RING PROTECTIVE COVER
FROM COMMAND MODULE 4t
(RING AND PROBE REMAIN LAUNCH ESCAPE J !1
EATcT:pCEHEsDSTTEOMLAUNCH) _ SYSTEM ABORT _ SHEAR PINS
_ LATCH HOOK
PROBE CAP
LAUNCH ESCAPE
SYSTEM--_ __TENSION TIE
SEPARATES
NORMAL ASCENT
TOWER JETTISON
Figure 6.- Tension-tie operation.
199
ADAPTER FREE
SEPARATION FLY-AROUND DOCKED
Figure 7. - Transposition and docking.
LM
REMOVAL
A
7 cm
/_ (2-3/4 IN.)
3 cm
(1-1/8 IN.)
6 ¢m
F__DROGUE APEX_
5 cm i2-1/8 IN.)_ /lcm
5 cm _1-3m (5/8 ,N.)_%.(1/4 IN.)
':y/
• ALL MARKS ARE SINGLE
• E AND F ARE SHINY MARKS
IN DRY LUBRICANT
• A, B, C, AND D ARE WIDE
SINGLE MARKS HAVING SLIGHT
DEPRESSIONS WITH SCRATCHES
THROUGH DRY LUBRICANT
IN CENTER
Figure 8.- Drogue assembly and location of radial marks.
200
POSSIBLE SIDE LOAD
_BALL END OF TORQUE SHAFT
JAMMED INTO CAM SLOT
__T TQRSION SPRING
ORQUE SHAFT
Figure 9.- Side-load reaction on torque-shaft operation.
Figure 10. - Scratches and burrs adjacent to capture-latch plunger.
201



The Apollo 14 docking anomaly

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