PAD (Pre-Advisory Data)

PAD (Pre-Advisory Data)
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Apollo TEI PAD as generated by Chuck Deiterich for Apollo 8.
Apollo TEI PAD as generated by Chuck Deiterich for Apollo 8.

Throughout the mission, large lists of numbers, called PADs, will be read up to the crew which give them the information necessary to carry out a particular maneuver. PAD stands for Pre-Advisory Data. Some of these "block data" are for planned maneuvers such as the TLI (Translunar Injection) or LOI (Lunar Orbit Insertion) burns. Other PADs, such as the "TLI plus 90" and "Lift-off plus 8" mentioned here are the first of 27 abort options which will be read up to the crew at scheduled times throughout the early and middle portions of the mission. Note that the TLI+90 PAD has nothing to do with TLI itself but would occur 90 minutes after a successful TLI burn in the event of an abort. However, mission planners have decided that at no time beyond Earth orbit will the crew be without a get-us-home PAD. Then, any time they might lose communication with Earth, they will have the information to hand to get themselves back manually.

To simplify the voice transmission of these huge lists of numbers and reduce the likelihood of errors, each type of PAD was precisely formatted both in Mission Control's and the crew's paperwork - all the crew needed to do was fill in the blanks.

Scott, from 1998 correspondence - "Another example of precision planning (a major key to Apollo success)."

The abort PADs are the responsibility of RETRO, one of the flight controllers in the front row of the MOCR (Mission Operations Control Room). Chuck Deiterich was one of those who occupied the RETRO console throughout Apollo.

Chuck Deiterich, from 2003 correspondence - "There is quite a bit of protocol in the PAD process. Empty PADs were in tablets of no carbon required (NCR) paper. We would make about 6 copies and use a red ballpoint on the top (original) so the CapCom would be sure what was part of the printed form and what was data."

Chuck supplied a sample PAD form from the Apollo 8 mission.

Deiterich, from 2003 correspondence - "You can see where the capcom checked each item during the crew readback. A printed X on the pad said no data goes here, a printed 0 indicated the computer would always have a zero here (ignition time for example), a heavy box on a square indicated a + or - sign should go here (sometimes a letter like D.) Only the red data was read up, thus saving comm time."

Page 2-15 of the Flight Plan includes a schedule of all the "Return to Earth Block Data" PADs. Three basic types of abort PAD are shown; "Complete P30" which defines all the parameters for a return to Earth, "P37" short versions which give data for the computer's P37 Return to Earth program, and the "Abbreviated P30" which assumes some data already has already been given in a full P30 abort PAD. The intention of these PADs is to ensure that at all stages of the flight, the crew have the information to hand with which they can get themselves on a homeward trajectory, in the event that they permanently lose communication with Earth. (ap15fj)

[edit] PAD for S-IVB restart sequence

The timings for events relating to the launch vehicle are defined relative to a number of time bases, each of which start with a particular event. This allows controllers to move complete sequences of events relative to the overall mission time. The restart sequence for the S-IVB's single J-2 engine is tied to time base 6. When TB-6 begins, all subsequent events to restart the engine such as tank repressurisation, engine chilldown, ullage, etc., follow on, leading to the engine start command 9 minutes, 30 seconds later, and ignition 8 seconds after that.

The crew also have tasks to perform in the minutes leading up to the TLI burn and they use their event timer to help them. Around 002:40:23, TB-6 begins and this is shown by both the 'Uplink Activity' and 'S-II Sep' lamps coming on. The former is illuminated for ten seconds, the latter for 38 seconds. At 9 minutes to ignition, the point at which the 'S-II Sep' lamp is extinguished, Dave will start the event timer counting up, having previously set it to 51:00. This will give a visual count-up to and beyond ignition to aid the crew in sequencing their final tasks before and during TLI. Items in the checklist are therefore shown with times from 51:00, through (1:)00:00 and upwards. (ap15fj)

The PAD is interpreted as follows.

S-IVB restart sequence PAD (ap15fj)
TB-6 predict light This comes on at 002:40:23, which implies that TB-6 begins at 002:40:31 and that Tig (time of ignition) will be at 002:50:01.
Attitude for TLI 180°, 45°, 001° in roll, pitch and yaw respectively. This attitude is with respect to the local vertical/local horizontal. Duration of burn: 5 minutes, 55 seconds.
Delta-Vc' 10,401.1 fps (3,170.3 m/s) will be entered into the EMS to allow the crew to monitor the remaining velocity to be added.
VI Indicated velocity at engine cut-off is 35,599 fps (10,850.6 m/s).
Separation attitude The correct attitude for separation of the CSM from the launch vehicle is 359°, 77°, 320° in roll, pitch and yaw respectively with respect to the local vertical/local horizontal. Among the criteria for adopting this attitude is solar illumination of the LM to assist the docking procedure.
Extraction attitude The correct attitude for extraction of the LM from the S-IVB is 301°, 257°, 40° in roll, pitch and yaw respectively.
R2 align 045.0.
R2 ignition 038.0.
ORDEAL Start Relative to the count-up on the event timer, the ordeal should be started at 56:45. [see: Orb-rate explanation] The ORDEAL (Orbital Rate Display - Earth And Lunar) drives the FDAI (Flight Director Attitude Indicator, or "8-ball") to make it display the spacecraft's attitude relative to the ground below.
Yaw 001
LM extraction The LM will be extracted from the top of the S-IVB stage by the docked CSM at 4 hours, 16 minutes GET.

    This article is based on material extracted, with permission, from the [Apollo Flight Journal] by David Woods and Frank O'Brien. The NASA Apollo Flight Journal files are copyright © 1998 - 2004. W. David Woods and Frank O'Brien.(more...)