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Author Topic: Apollo 8 TLI using LTMFD  (Read 4065 times)
BennGunn
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« on: October 31, 2009, 08:01:51 AM »

Dear All

First of all, many thanks for the great work you all do with this project.

I am having trouble with translunar injection on Apollo 8 using the LTMFD. I can calculate what I think is the correct burn data, and I am following the excel checklist for Apollo 8 but doing V37E47E does not recall the dv figures to be gained.

At burn time the SIVB ignites, but the IU does not orient the spacecraft to the correct attitude.

Should I be using the word checklists? These seem to have more steps to take than the excel version.

Any help would be greatly appreciated
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pattersoncr
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« Reply #1 on: October 31, 2009, 08:26:25 AM »

The TLI burn was performed by the IU not the AGC.  P47 allows the AGC to monitor the accelleration imparted by the S-IVB and keep track of the spacecraft's velocity.  Project Apollo MFD is used to input the burn data from IMFD or LTMFD to the IU.
The Launch word checklist goes thru this in detail.
The latest version is available here:
http://www.ibiblio.org/mscorbit/mscforum/index.php?topic=438.210
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BennGunn
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« Reply #2 on: October 31, 2009, 03:23:22 PM »

Thanks for your quick response.

I understand that the IU performs the burn, but I misunderstood the function of P47 (I thought it was supposed to display target dv, whereas it shows actuals)

I managed to complete a TLI burn using LTMFD, but the SIVB shutdown prematurely. I think it ran out of fuel (oxidiser actually)!

I think I setup all the burn parameters correctly. The only paramater that I didn't understand was the TOA (Time of orbit alignment with target), I wasn't sure where I should be getting this figure from.

I'm going to try to redo the TLI using IMFD to calculate the burn, as it has a map so I can monitor the projected trajectory.

Any ideas as to why I ran out of fuel in the SIVB would be appreciated.

Thanks

Rob
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pattersoncr
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« Reply #3 on: October 31, 2009, 04:10:49 PM »

TOA refers to Time of Orbital Alignment.  It's the time at which the planned orbital plane around aligns with the specified lat/long.  For other missions you would set TOA to the time of landing.  For Apollo 8 it probably doesn't matter.
What was the planned dV from LTMFD?
What were the fuel levels when the S-IVB shut down?
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BennGunn
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« Reply #4 on: November 01, 2009, 05:52:54 AM »

The planned dv was around 3,300 m/s, there was no LOX left in the SIVB, a very tiny amount of fuel was left. I had an apoasis altitude of 53.84M

My orbit insertion took about 1m longer than the ckeclist, would this have somthing to do with the problem, that the SIVB was starting with less fuel than it needed for TLI.

I'm going to restart my scenario and try again

Thanks for your help

Rob
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jarmonik
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« Reply #5 on: November 01, 2009, 09:37:15 AM »

Are you sure you are not trying to fly Apollo 8 mission with Apollo 7 configuration ?  There is not enough fuel in Apollo 7 to complete TLI since it was not supposed to do that. 3300m/s should be within a limits for Apollo 8.
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BennGunn
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« Reply #6 on: November 01, 2009, 10:25:42 AM »

I'm using the scenario in Broken Scenarios\Virtual AGC\Apollo 8 - Launch

My insertion cut off time is about 1m later than the checklist, i wonder if the launch auto pilot is leaving me with less fuel than it should be. I get some strange attitude changes during launch, particularly after SII jettison, when the SIVB does a large pitch up movement.

I've redone my launch with similar results, is there a different scenario file i should be using?

Thanks

Rob
« Last Edit: November 01, 2009, 10:29:48 AM by BennGunn » Logged
BennGunn
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« Reply #7 on: November 03, 2009, 01:24:39 PM »

Could someone help me, I am still having trouble with Apollo 8.

My problem is definitely related to Launch and Insertion, my SECO time is about a minute later than the checklist, and the pitch program is much more erratic than that described in the mission report for Apollo 8. It is most strange after SII jettison, where a large pitch up occurs.

I therefore do not have enough fuel to complete TLI successfully.

I am using the scenario in Broken Scenarios/Virtual AGC/Apollo 8 - Launch, is this the least 'broken' scenario or is there one which is more appropriate to use?

Thanks for your help

Rob


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The Aviator
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« Reply #8 on: November 30, 2009, 04:01:40 PM »

Same problem here. Strange behaviour after SII jettison, pitch up and SECO one minute after planned shutdown time. Result: less fuel than I need to perform the TLI.
Anybody have an idea why this occurs? I remember some months ago I completed successfully the whole Apollo 8 mission in vAGC, but I don't remember if I cheated adding some fuel to the SIV.
Could this be an issue came along with the new modules or have anybody experienced the same issue with the old ones?
Thanks for your help.
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BennGunn
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« Reply #9 on: December 01, 2009, 04:55:08 AM »

I am still having this problem with Apollo 8 also, but it does not happen with Apollo 11.

I end up with just over 50% fuel, to be able to complete TLI successfully i had to add fuel via the scenario editor so i have about 72.5%.

I had a look at the scenario config options and some of the project code, but i couldn't see any reason why Apollo 8 in particular shouldn't work.

Thanks

Rob
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bubba
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« Reply #10 on: December 14, 2009, 11:29:56 AM »

i have the same problem with te apollo 8 vAGC Scenario, the TLI burn time calculated by IMFD is 30 seconds longer than the historical correct value of 317 sec? So i run out of gas here?

Best regards

* Apollo 8 MET 001 50 00 0001.scn (69.64 KB - downloaded 87 times.)
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BennGunn
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« Reply #11 on: December 14, 2009, 12:51:03 PM »

there is a known bug in the mass of the apollo 8 SIVB. The fix for this is in CVS, but you would need to recomplie the modules to use this.

Alternativley add the following line to your scenario file (must be before launch) :  S4EMPTYMASS 14880.000
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bubba
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« Reply #12 on: December 14, 2009, 03:42:18 PM »

Thanks, now it should work
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