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!
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!