FDAIs for TLI

vittguy

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I’ve been running some Apollo 8 TLIs and have noticed a few things that I don’t quite understand.

First, I noticed when I aligned the GDC to the IMU prior to the burn that the thumbwheels required a setting of 257 degrees to null the pitch error needle while FDAI 1 was reading about 180, 357, 0. I thought the two FDAIs would need to read the same at the conclusion of this procedure.

The TLI attitude on FDAI is not what I’d expect. The TLI pad indicates the attitude should be R 179, P 45, Y 0. FDAI 1 appears to be showing an ORDEAL driven attitude of 180, 357, 0. FDAI 2 appears to be in inertial attitude at shows 180, 225, 0 at TLI ignition. Any ideas why the discrepancy between the TLI pad and the FDAI?

Is the vehicle attitude inertial for the burn? I’d expect it to go from LVLH to inertial when the stack goes to the burn attitude but FDAI 2 shows a pitch rate.

Given switch settings of FDAI SEL – 1, FDAI SRC – CMC, and ATT SET – IMU, does FDAI 1 actually display the IMU angles or are they relative to the REFSMMAT (which I’d expect).

Should ORDEAL be turned off for TLI? Seems to me like it should if you want to monitor the TLI burn with two independent systems, and it would be biasing one of the FDAIs if left on.

Thanks!
 

rcflyinghokie

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Just saw this thread! Sorry about no response! Get it all sorted?

Also, the ORDEAL is used to monitor TLI since the SIVB doesn't burn inertially, it actually moves its burn vector at an expected rate which matches what the ORDEAL can generate, so one ball allows the pitch to remain more or less "center" while the other shows inertial attitude as it changes through the burn.
 

vittguy

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Yes, for the most part I got it figured out. I wasn't centering the error needles in the right place. Had to do a Noun 20 to see what the IMU angles really were and then I was able to figure out what I was doing wrong.

Thanks for the explanation of FDAI 1 for TLI. I wondered if that were the case since all it did during the burn was show a yaw attitude change.
 

rcflyinghokie

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Yes, for the most part I got it figured out. I wasn't centering the error needles in the right place. Had to do a Noun 20 to see what the IMU angles really were and then I was able to figure out what I was doing wrong.

Thanks for the explanation of FDAI 1 for TLI. I wondered if that were the case since all it did during the burn was show a yaw attitude change.
There is a chart in the launch/TLI checklists with expected yaw values for this, allowed a SIVB takeover flying the ORDEAL ball essentially.

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