Apollo 16 Lunar Descent: P63 and AGS Preparation Questions

[Wedge313]

First the good news, Orion has landed safely at Descartes. The procedural guidance that you all provided for the Apollo 15 flight I did served me well once again, so thanks.

But a few questions: I've been trying to work more on the AGS procedures, but I hit an early snag when I'm asked to load the AGS abort constants. On the actual flight the crew was told to load numbers from a data card: from the AFJ "096 20 23 Irwin: Roger; because when you do load those constants, we want you to load nominal values from the data card." I couldn't find that data. Is there a way to generate any AGS data (besides the 231 RLS) using the RTCC? Or any other way to get these entries?

And during the AGS initialization, I'm not seeing what I would expect when I run V83. I'm guessing this data (range, range rate) is computed using the CSM and LM SVs? If so, I'm not uplinking these correctly because I'm getting zeros. I think I'm uplinking the LM SV ok (the LM seems to know where it is), but could you walk me through uplinking the CSM SV to the LM?

And finally, an important safety tip: don't start the descent with the RDR TEST switch in the LDG position. Of course I would never do that, but I imagine all kinds of wrong stuff would result if you did .

 

[indy91]

The ALSJ has the Apollo 16 Data Card Book: https://www.hq.nasa.gov/alsj/a16/a16DataCardBookChngE.pdf

The RTCC MFD does have a way to calculate the abort constants, but because the rendezvous profile following an abort was changing from mission to mission and because from Apollo 11 to 12 they switched to an entirely different scheme of abort constants I haven't made the calculation compatible with all missions yet. Only Apollo 11 and 12 are supported in this way so far sadly.

How did your Apollo 15 LM find the way back to the CSM without LGC CSM state vector uplinks? You go to the uplink page, then type 21 for "21: LGC CSM Navigation Update" and select the name Casper with the TGT button. Calculate the state vector and uplink.

 

[Wedge313]

How did your Apollo 15 LM find the way back to the CSM without LGC CSM state vector uplinks?

Yeah, I thought I knew how to do this, I didn't have a problem on 15. I think I messed up the TGT, I probably had Orion vs. Casper. But with Casper as the TGT V83 gives me values now that seem reasonable. But the AGS 317R and 440R don't match up, so I'll have to go back and see what I did wrong with that.

Another question: About P76, we run this after the CSM circ burn. So the TIG and dVs we input are from the circ burn PAD, correct? And trying to understand what's going on, we do this to tell the LGC "the CSM was here, then at this time we made a burn of xxx dV", and then the LGC can figure out where the CSM will be? Why do this P76 instead of just uplinking a new CSM SV after the burn?

Thanks.

 

[indy91]

Another question: About P76, we run this after the CSM circ burn. So the TIG and dVs we input are from the circ burn PAD, correct? And trying to understand what's going on, we do this to tell the LGC "the CSM was here, then at this time we made a burn of xxx dV", and then the LGC can figure out where the CSM will be? Why do this P76 instead of just uplinking a new CSM SV after the burn?

Yes, that's what P76 does. It "simulates" the burn as an instant maneuver at the TIG with the given DV. The circ burn happens behind the Moon, so no uplinks until AOS. And as happened on the actual Apollo 16 mission, things don't always go as planned and the circ burn didn't happen on schedule. And it's always good to have valid state vectors onboard, especially when CSM and LM are separated. They might have to perform an unscheduled rendezvous, and nearly half the time in lunar orbit mission control can't help you with that, behind the Moon.

 

[rcflyinghokie]

Yeah, I thought I knew how to do this, I didn't have a problem on 15. I think I messed up the TGT, I probably had Orion vs. Casper. But with Casper as the TGT V83 gives me values now that seem reasonable. But the AGS 317R and 440R don't match up, so I'll have to go back and see what I did wrong with that.

Did you do an AGS update after updating the LGC SV's?

 

[Wedge313]

Did you do an AGS update after updating the LGC SV's?

I did V47/414+10000E, that should update the AGS, correct?

 

[rcflyinghokie]

I did V47/414+10000E, that should update the AGS, correct?

Correct, if your SV is good in the LGC for both vehicles AND your AGS time/bias time are correct that should give you proper 373 and 440 values.

 

[Wedge313]

AND your AGS time/bias time are correct that should give you proper 373 and 440 values.

I did a V47/337+xxxxx to update the time, and that combined with the correct CSM SV (designating the TGT as Casper in the RTCC) did the trick, the V83 and 373/440 values sync up. Not sure what happened to the AGS time, I'm 99% sure I did that correctly when I first initialized the AGS after undocking, but hey it wouldn't be the first time I made a mistake.

Yes, that's what P76 does. It "simulates" the burn as an instant maneuver at the TIG with the given DV. The circ burn happens behind the Moon, so no uplinks until AOS. And as happened on the actual Apollo 16 mission, things don't always go as planned and the circ burn didn't happen on schedule. And it's always good to have valid state vectors onboard, especially when CSM and LM are separated. They might have to perform an unscheduled rendezvous, and nearly half the time in lunar orbit mission control can't help you with that, behind the Moon.

OK P76 makes sense to me now. This is something I haven't really been paying too much attention to, the whole AOS/LOS thing. I've been spoiled because I can generate uplinks from the RTCC anytime, it doesn't seem to care if I'm LOS. I paid more attention going through the descent this last time, and sure enough right at planned AOS the Earth appears on the lunar horizon, and the signal strength on the S-Band jumps up, and I'd be able to accept uplinks in the real world. I'll try not to cheat as much going forward .

So I've landed at Descartes four times, with varying degrees of AGS participation. What I'd like to try next is a few aborts. If I want to learn about the AGS, that's probably where I need to go.

 

[rcflyinghokie]

So I've landed at Descartes four times, with varying degrees of AGS participation. What I'd like to try next is a few aborts. If I want to learn about the AGS, that's probably where I need to go.

Aborts are actually pretty fun! Remember the PGNS also has full abort capability, so a fun analogy is to run a PGNS abort and then reload and run an AGS abort at the same time and note the differences.

Additionally, AGS can be used for ascent and rendezvous, another great way to learn the AGS (more so than the aborts IMO)