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 Author Topic: Apollo 7: Calculating the Corrective Combination Burn  (Read 922 times)
almostfm
Full Member

Posts: 39

 « on: February 20, 2010, 07:54:07 AM »

This is going to seem a bit rambling, so I apologize in advance

I've been playing around with STK (taking advantage of the 30 day trial of the Astrogator module), and after a week, finally got a Corrective Combination burn.  I'm going to try to do the same thing with GMAT, which has the advantage of being free (I'm afraid to ask what Astrogator costs ) but the disadvantage of being largely undocumented, AFAIK.  I'm hoping my knowledge of STK can transfer over...

A couple of things I've noticed--when inputting the state vector from Orbiter, I had to transpose the Y and Z values for both position and velocity to get the proper orbit.  This may be tied in to something I found when STK actually calculated the burn, which I'll go into in a moment.

Also, I found if I use the historic value of -5.7 fps for the phasing maneuver, I'm way too far ahead of the SIVB, which would require a significantly bigger burn to put the CSM in the correct position for CSI.  One of the wiki's mentioned that a -2.5 fps burn puts you in the correct position, so I used that, and it worked very well.

After running Astrogator, I got delta V's close to the historic values--about 6 fps off in X, 2 fps in Z, and within .5 fps in Y, but the signs are reversed for the Y and Z values.  My guess is that it has something to do with transposing the values from the state vector.  It's almost as if the two coordinate systems are mirror images of each other about the X axis.  Over the weekend, I'm going to try to do the actual rendezvous sequence, and see how it turns out.

The issue of the phasing burn value raises a question in my mind--as much as we want to fly "by the numbers", I suspect that there are going to be times when the "real world" numbers don't quite work in the Orbiter world.  Do we acknowledge that, and if so, how?
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kneecaps
Project Team Member
Full Member

Posts: 161

 « Reply #1 on: February 21, 2010, 09:45:32 AM »

Hi,
Have a look into educational licenses for STK if you are a student, even better many faculties are actually able to give students a copy. Luckily my employer provides a system for me.

What co-ordinate system are you using in STK?

There are numerous reasons why the numbers won't match. Differences in orbital perturbations for one, but most importantly relative motion. In rendezvous situations relative motions calculated onboard are usually better than ground/tracking derived solutions. However i've seen this 3 - 4 fps difference a lot and don't recall why. I haven't had the time to look at the Apollo/STK stuff for a while, but at one point in time I had BCS defined, and axis' for the COAS and scope (as sesnsors, scope with pointing angles and COAS boresighted to S/C axis with an offset). I was able to calculate burns and provide 'check stars' that would be visible in the COAS and scope .

That's where my knowledge of math and astrodynamics because too little so I've been studying more over the past 18 months in order to return stronger to NASSP .

Let me know the specifics of your STK issues and I'll see what I can do.
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