FD12/Spacecraft 101 ASP Mission EngineerOCT 24 196767-FM-T-96FM/Deputy ChiefImmediate restart of the SPS engine under PGNCS control
1. This memo is in response to your question as to whether there was anything in the spacecraft computer that would prevent attempt- ing to restart the SPS engine immediately if it shut down prematurely for some reason. I passed this question on to the Guidance and Per- formance Branch (MPAD). The following paragraphs are quoted almost verbatim from Rick Nobles' response.
2. During an SPS burn, if the delta V monitor program of the CMC detects that the accumulated velocity falls below 0.64 meters per second for at least two computer cycles (i.e., 2 to 4 seconds), it does the following:
a) terminated the Engine On command
b) maintains Vg computations
c) displays 59-59 on the DSKY (R1)
d) turns off the TVC DAP
e) turns on the RCS DAP
f) recycles to time-of-ignition minus 5 seconds (TFI-4) in the thrust program, and starts counting down
g) asks for Engine On enable which is given with an “enter”
h) commands the SPS on at TFI-0 (and no sooner!)
i) switches from RCS DAP to TVC DAP
At this point the delta V monitor checks acceleration again and if the thrust is on it will go right on through the program: if the thrust is still too low to pass the 0.64 meters per second test it recycles again as previously described.
3. The program actually cycles back to TFI-5 in exactly the same pro- gram used for a nominal SPS burn. Accordingly, manual ullage must be provided if it is needed and the engine cannot be started earlier than stated above. This all means that the engine will be off for a minimum of 7 to 9 seconds in this guidance control mode.