Apollo lunar mission tutorial: Transposition, Docking and Extraction
This page should be moved to the Apollo Flight Journal. By John Dunn
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This is where the CMP gets to do some of that “Pilot Stuff”. Unlike most of the major maneuvers performed during the course of the mission, TD&E is flown almost entirely by hand. As the name implies, the process consists of three phases:
[edit] Transposition
The maneuver begins with separation of the CSM from the S-IVB (containing the Lunar Module), with the SLA panels tumbling away into space. 15 seconds after separation (distance approximately 5 meters) a pitch up maneuver is initiated, rotating the CSM 180˚ to face the LM/S-IVB. The reason a pitch up rather than pitch down or yaw maneuver is used is that it brings the target into the CMP’s field of view that much sooner. It is interesting to note that while the checklist calls for a 0.5˚ per second pitch rate - which would place the CSM some 60 meters or so from the LM/S-IVB at the completion of the maneuver – in many cases a much higher pitch rate was used. According to the Apollo 15 Flight Journal, CMP Al Worden used a 2˚/sec pitch rate – 4 times the rate specified in the checklist.
[edit] Docking
Following the 180˚ pitch maneuver, the CSM is translated with plus-X thrusters, closing the distance between the CSM and the LM/S-IVB at rate of approximately 1 ft/sec. The CSM is then rolled approximately 45˚ left to align the COAS (Crew Optical Alignment Sight - mounted to the #2 Left Rendezvous Window frame) with the docking target on the LM. Y and Z thrusters are also used to fine tune the approach. The final few feet are covered at a closure rate of approximately 0.1 ft/sec.
   The procedure for executing this maneuver in Orbiter is fairly straight forward. However a soft touch with the thrusters is required to avoid over-correcting. One thing that has caused me some trouble here is occasionally forgetting to shift between Translation mode and Rotation mode as necessary. My advice is to always glance up at the HUD indicator before initiating any maneuver. |
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Typically the docking probe on the nose of the CM will contact the drogue on the LM and slide to the center with the capture latches effecting a “soft dock”. The probe is then slowly retracted, aligning the docking rings on the CM and LM and damping any relative motion between the two vehicles. Once the retraction of the probe brings the two vehicles together, twelve automatic docking latches snap into place around the back surface of the docking flange to achieve a “hard dock”.
Apollo 14 required six such docking attempts before the CSM was successfully docked with the LM. Examination of the docking probe afterward revealed no problems. It was therefore assumed that the capture-latch assembly must not have been in the locked configuration during the first five attempts.
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[edit] Extraction
After docking the cabin pressures of the two spacecraft are equalized and the CSM forward hatch removed to inspect the docking latches. Electrical umbilical cables are connected within the tunnel and the hatch reinstalled. After further systems checks the CSM/LM stack is then ejected from the S-IVB, followed by a 1 ft/sec separation maneuver.
[edit] Trans Lunar Coast
Routine trans-lunar activities included various tasks such as conducting experiments, performing systems checks and photography. The CDR and LMP transfer to the LM for several hours of housekeeping and systems checks. One or two midcourse correction burns are performed, setting up the final lunar orbit insertion trajectory.
At various times during the trans-lunar coast, when a stable attitude is not necessary for platform realignment or other purposes, the spacecraft is placed into a slow continuous roll - typically 0.35° per second, or approximately 17 minutes to complete a full 360° roll. This is called the Passive Thermal Control maneuver, often referred to as "Barbecue'" mode. The purpose of this maneuver is to maintain an even heating and cooling of the CM heat shield, the SM RCS quads, SPS propellant tanks and structure, and the LM propellant and battery systems.