Apollo 8 Lunar Orbit GDC Align Question

[Wedge313]

Hello. So I'm in a 59.9 x 60.5 lunar orbit. After every P52 the checklist asks for a GDC alignment to the IMU. What do I want to look at? The ORDEAL is in ORB RATE, do I want to switch to INRTL before setting the Attitude Set thumbwheels to the FDAI attitude? Thanks.

(I appreciate the help. I DO look for answers first before asking on this forum. I know you answer these same questions many times. But I must stink at searching.)

 

[Thespacer]

You probably have noticed that each time you perform a P52, small torquing angles are generated. While generally imperceptible, you may notice, when you hit “torque” in the P52 procedure, your FDAI #1 gimbal angles adjust ever so slightly. That is the visible effect of the IMU realigning itself.

Assuming FDAI #1 (generally IMU-driven) and #2 (generally GDC-driven) were in sync before the P52, they are no longer in sync after this realignment. While hardly noticeable, it is likely that such small difference would still be significantly risky if you’re about to perform a burn. Should you lose the IMU for any reason, the backup GDC-driven attitude indicator is your primary reference for burn attitudes. This is why you perform the GDC alignment, to ensure the backup (generally FDAI #2) remains an accurate source of attitude information.

You should switch to Inertial, if in Orb Rate. Technically you are setting the Att Set thumbwheels to the IMU values, and then aligning the GDC using those same values. It gets tricky when pitching in Orb rate but it is doable. If easier, you can either reduce the sim rate to 0.5, or, pause spacecraft rotation, to mitigate the effects of the slow pitch of orb rate.

Keep asking questions

 

[Wedge313]

That helps me understand what's going on. Good explanation, thanks.

 

[MrFickles]

If it helps you, you can use FDAI 2 for your ORDEAL instead of FDAI 1. I personally prefer this as the error needles are on FDAI 1.