NASSP Apollo 11 P30 DOI burns too long. Why?

[mnurick]

Hello! I’m performing a DOI burn from ~60 nm orbit following the values on the MCC P30 pad changing my velocity ~70 FPS. Which all seems accurate. And yet after the burn and nulling all residuals my periapsis is 6.7 nm. Far too low for PDI. What do you think is happening?

Thanks for your help!

 

[indy91]

Where does is it say 6.7NM? The Project Apollo MFD? That altitude is relative to the mean radius of the Moon. The Apollo 11 landing site is about 1.6NM below the mean radius, which means that you periapsis height would be 8.3NM instead of 6.7NM, which would be quite accurate. If the 6.7NM appears on the DSKY with V82 or so then that would be another story.

 

[mnurick]

Thanks for your reply. I'm seeing that 6.7 nm stat in the stock Orbiter MFD (which also states my pre-burn orbit at roughly circular 60 nm). I never use this view, but since I find myself crashing on the moon during PDI before I even reach pitchover, I'm digging for answers!

 

[indy91]

Thanks for your reply. I'm seeing that 6.7 nm stat in the stock Orbiter MFD (which also states my pre-burn orbit at roughly circular 60 nm). I never use this view, but since I find myself crashing on the moon during PDI before I even reach pitchover, I'm digging for answers!

The Orbit MFD also shows altitude (although in kilometers) relative to the mean radius of the Moon. So I don't think that is an issue, your number sounds reasonable for the Apollo 11 landing site.

Weird that you are crashing during the descent. Is your landing radar doing work? That would usually prevent you from crashing, even if some other issue is going on that causes you to be low in altitude (bad state vector for example).

 

[mnurick]

Landing radar is definitely powered. But let me run the procedure again and check to see if its actually doing anything. Will report back after I get home from work tonight eastern time. But I do recall the Range Alt indicating an altitude of ~45,000 ft when I begin PDI.

But I'm starting to suspect a bug with my save state.

 

[indy91]

But I do recall the Range Alt indicating an altitude of ~45,000 ft when I begin PDI.

That's a bit low, but shouldn't be a big issue.

But I'm starting to suspect a bug with my save state.

I can check your scenario for any issues if you want. The scenario files are too large to be attached to a post here in the forum, so you would need to upload it externally somewhere.

---------- Post added at 21:58 ---------- Previous post was at 16:40 ----------

Ok, I did an initial check of the scenario you sent me. I'm already seeing multiple things going on. I can't load the scenario without deleting the S-IVB from it. Did you delete most of the S-IVB scenario lines manually? And which NASSP and Orbiter versions are you using?

The biggest issue I am quickly seeing is that the AGC clock is off by nearly 2 minutes (check V16 N65). That can quickly lead to the state vector deteriorating and in consequence the LM crashing.

 

[mnurick]

Ok, I did an initial check of the scenario you sent me. I'm already seeing multiple things going on. I can't load the scenario without deleting the S-IVB from it. Did you delete most of the S-IVB scenario lines manually? And which NASSP and Orbiter versions are you using?

Strange. I didn't touch the save file code. I'm running Orbiter 2016 and NASSP 8, whatever were the latest builds as of a month ago.

The biggest issue I am quickly seeing is that the AGC clock is off by nearly 2 minutes (check V16 N65). That can quickly lead to the state vector deteriorating and in consequence the LM crashing.

Huh! Off relative to what? Is that human error on my part? Or is the virtualaAGC pegged to Orbiter time?

And huge thanks again for helping me troubleshoot. Loving this community!

---------- Post added 02-28-20 at 06:08 AM ---------- Previous post was 02-27-20 at 09:07 PM ----------

So I synced the AGC time to the Apollo MFD, retried my DOI and PDI, and boom! Soft touchdown on the lunar surface. Glorious. Thank you so much for your help.

Now lets see if I can bring it home.

 

[indy91]

Huh! Off relative to what? Is that human error on my part? Or is the virtualaAGC pegged to Orbiter time?

Might be a human error. Which scenario did you start the mission with? On the launchpad or later on? If you went through the LM activation then you will have initialized the LGC clock as part of that procedure. So maybe you did an error during that.

So I synced the AGC time to the Apollo MFD, retried my DOI and PDI, and boom! Soft touchdown on the lunar surface. Glorious. Thank you so much for your help.

Great that you got it working!

 

[mnurick]

Might be a human error. Which scenario did you start the mission with? On the launchpad or later on? If you went through the LM activation then you will have initialized the LGC clock as part of that procedure. So maybe you did an error during that.

Likely human error then. I started at T-4hrs to launch, and have used time acceleration up to 20x. I really thought I nailed the mark on the LGC clock initialization in the LM, but possible my mouse pointer keystrokes were a bit off at 1am. Serves me right for skipping rest periods.

Thanks again!