Circularizing Orbit With the Apollo CSM SPS Burn Calculator

[TonyTheFlyer]

With Gemini, I was always able to very precisely circularize my orbit within 2 KM and often less every time (one time, I even got it down to a few tenths of a KM). And once I learned how to do a burn with the SPS in the Apollo CSM, I started practicing by maneuvering to a higher orbit, but no matter how close I get the numbers in the SPS Burn Calculator, the Ap and Pe of the resulting orbit after the second burn is always 10 to 20 NM off from the calculated orbit. Only once was I able to get the difference down to around 2 NM with a third burn but that seemed like a fluke.

So my questions are:

1. Why is the actual orbit never the same as the orbit calculated by the Apollo CSM SPS Burn Calculator?

2. Is there any trick to better circularizing the orbit with the Apollo CSM SPS?

[Hector Melo]

Thank you for playing our wonderful Sim, the Apollo CSM SPS Burn Planner is a Work In Progress tool up t o be modified/upgraded or replaced in the future. With this tool you are able to calculate burns for different space maneuvers, right now we don't have an official checklist for Circularizing the orbit with the CSM by using that planner. In a near future (we hope) we will be able to plan more maneuvers in i realistic and accurate way.
Remember that you are also free of joining us at our Discord channel: http://discord.gg/reentrygame or directly to me here: https://discord.com/channels/767750390602399764/784063201125597254
there you can ask me anything, keep in orbit Reentrynaut!

[wilhelmsenstudios]

The CSM Burn Planner is still in an experimental state and I'm working putting it all together.
However, the burn planner requires the IMU to be aligned with the preferred burn direction, so it can maintain the tangent as best as possible during the entire burn.

So the ORDEAL needs to be in INRTL then:
Use the Burn Planner to calculate the burn you wish.
Use the P30 to set up for the burn
Use P52, option 1 (V37E 52E wait until the program is loaded, takes a few seconds then key V22E +00001E PRO PRO and notice the FDAI realign.
Then use P40 where you set automaneuver to +00000, +00000, +00000 as the IMU has been set so prograde is the direction of the burn.

DAP configurations might help a but too.

During the burn, pay attention to the Orbital parameters on the map view, and IF the SPS is still burning when you reach the desired orbit, hit PRO to cut it. Working on the cutoff timing and logic.

With the TSS implementation, everything is now in a much more realistic scale. In the initial versions of Reentry, the spacecrafts were visually huge to overcome floating point precision issues. This made it easier to rendezvous as the objects that would meet up were so large (miles big). A lot of visual tricks were used. TSS removed all this, and the position and scale is 1:1 with the physics layer. This makes everything much harder, and equally on my side when it comes to precision in how the tools work.

I would be happy to join you through MOCR one day to monitor the spacecraft during the burn live, might be some good input for me there. MOCR is the new Mission Control module for Apollo, with the primary intension on making it easier for me to improve things, add systems, and test them at a larger scale. I will soon post more information on this.

[TonyTheFlyer]

Originally posted by wilhelmsenstudios:

The CSM Burn Planner is still in an experimental state and I'm working putting it all together.
However, the burn planner requires the IMU to be aligned with the preferred burn direction, so it can maintain the tangent as best as possible during the entire burn.

So the ORDEAL needs to be in INRTL then:
Use the Burn Planner to calculate the burn you wish.
Use the P30 to set up for the burn
Use P52, option 1 (V37E 52E wait until the program is loaded, takes a few seconds then key V22E +00001E PRO PRO and notice the FDAI realign.
Then use P40 where you set automaneuver to +00000, +00000, +00000 as the IMU has been set so prograde is the direction of the burn.

DAP configurations might help a but too.

During the burn, pay attention to the Orbital parameters on the map view, and IF the SPS is still burning when you reach the desired orbit, hit PRO to cut it. Working on the cutoff timing and logic.

With the TSS implementation, everything is now in a much more realistic scale. In the initial versions of Reentry, the spacecrafts were visually huge to overcome floating point precision issues. This made it easier to rendezvous as the objects that would meet up were so large (miles big). A lot of visual tricks were used. TSS removed all this, and the position and scale is 1:1 with the physics layer. This makes everything much harder, and equally on my side when it comes to precision in how the tools work.

I would be happy to join you through MOCR one day to monitor the spacecraft during the burn live, might be some good input for me there. MOCR is the new Mission Control module for Apollo, with the primary intension on making it easier for me to improve things, add systems, and test them at a larger scale. I will soon post more information on this.

Thank you for the replies from Hector and Wilhelmsenstudios. I always have the ORDEAL set to earth/rate (haven’t tried it in lunar orbit yet) and very closely monitor the FDAI during the burn, manually keeping it at zero pitch and yaw as the DAP doesn’t seem to hold it very precisely (as you said, some better DAP configurations might help also). I will try setting it to INRTL and running P52 option 1 tonight and see how that works. And let me know when is good time for you to join up through MOCR. I’ve been meaning to get on discord also, just been dragging my feet ;-)

[TonyTheFlyer]

Okay, I followed your instructions exactly, planned and executed two burns to raise my orbit to 300 NM and it worked! The resulting orbit was 299.7 x 299.4 NM. Realigning the IMU appears to be the key to circularizing your orbit. I don’t think the burn planning tutorial ever mentioned realigning the IMU, in fact, it actually says to set the ORDEAL to ORB RATE.

Thanks for the help!

[wilhelmsenstudios]

Excellent, thanks for the update.

Yes you are correct about this, P52 and REFSMMATs are a new feature I made, and the Academy lags behind. You can still perform burns in ORB RATE, but the longer they are, the more offset you will get.

The reason is that the burn energy is calculated from the impulse point and tangent, and that should be fixed. The Orb Rate mode maintains Local Vertical and thus, makes the burn direction rotate along with the local vertical instead of staying fixed.