I did not come around here for a long time and how I was surprised to see people still use EOCA, something I built a few years ago!
And the version I put online is a very early beta compare to the latest I have on my disk.
May be I should give you my point of view on all of this:
- I initially began EOCA due to the lack of MFD to compute 2 things:
1/ The position of a spacecraft in some time from present, in a stable way according to the gravity perturbations. Orbit MFD, for example, tell you where you are but not where you will be within one hour, 2 orbits or 2 days later. And situation has not really changed since almost no MFD can do that.
2/ The stars in sight in an attitude burn in order to perform the sextant star check in P40/P41.
- As I ran into the math for EOCA, Gauss equations and so on I realised that, if I could do the non spherical gravity field and perturbations calculations, I should be able to add a force on the spacecraft in almost the same way: I decided to enhance EOCA and to make it compatible with the computation for the delta V's required for any maneuver in a simple way for the user: for example, put the major axis orientation you want into EOCA and it will tell you which burn you have to do.
- As time passed it became obviously that the goal of EOCA overtook the excel spreadsheet capability. I always considered that this tool was more a demonstration of what we need and in which way, here in the NASSP community, in charge for the talented developers to further build the final tool.
This happened partially since IMFD5.5 is almost perfect for any basic maneuver .
Iím happy to see that Jarmo came on this thread and ask some technical questions on the coelliptic RDV cause my experience says that it just means weíll probably have a new feature on IMFD in a couple of time
. Just letís be patient.
- As far as the accuracy of EOCA is concerned: I donít remember exactly which version you use around here but I know that Iíve improved greatly the accuracy in the latest version I was working on. Unfortunately, all that files are less than beta and since Iím the worst developer of the world I wrote no commentary anywhere and today, I donít understand exactly my own work!!!
I know that orbiter changed the state vector propagation and it probably does not match anymore the way EOCA did.
But may be I should see if I can put online one of the latest version I get if one is stable enough.
- About the purpose of this thread, the Apollo 7 RDV: Great question!
At the end, before I left the work on EOCA due to lack of time and a bit of discouragement I was trying to figure out the problem of coelliptic RDV in a general way to incorporate it in EOCA and especially the Apollo 7 and above all the Apollo 9 RDV that is quite the most beautiful amongst all.
I saw some members as well as Tschachim put here the links for some useful docs about RDV. I have many on my hard disk and since Iím back with orbiter and Apollo Iíll have a look on it again.
AFAIR, the coelliptic RDV itself is not so complicated, although CSI and TPI are not as simple as they look due to some operational constraints the NASA wanted to satisfy.
The biggest problem, I think, is the phasing.
I explain myself:
To consider how and when the different maneuvers occur in the whole RDV sequence, the best way is to depart from the end with the most important question:
Where do you want to dock?
Answering this question, you know the position of the target, and the time when final RDV should occur. Thatís why the capability of position and time prediction for a vessel in orbiter is so important.
Then you know where and when the TPI should occur according one input: the delta H, that is the difference of height between the target orbit and the chaser orbit. No other input is required since the constraint for the TPI is that the chaser attitude during the burn is such as the tgt is on the line of sight.
Normally, if you set a 15 nm delta H (if I remember correctly), the CENTANG is around 130į.
An opposite way is to depart from a 130į of CENTANG and then you compute a maneuver and the attitude burn is more or less with the tgt along the line of sight.
Then you know where and when the TPI should occur.
In the same manner you may find time and position for CDH and CSI and optionally a plane change between them, according to some operational constraint such as a min time of 10 minutes between 2 maneuvers. I know there is some other constraints I donít remember exactly.
As soon as I get the CSI time and position I ran into the biggest problem for me:
Assuming the position of my vessel, which maneuver for which orbit should I do to get on the CSI burn at the right time?
In other words, I was in front of the problem of the phasing.
For a LM landed on the moon, about to lift off to rendezvous with the CSM: No problem: the orbit insertion is almost always the same and the phasing is only to wait on the surface the proper time for launch.
For a vessel already in orbit (like the Apollo 7 CSM, the Apollo 9 LM or even the Apollo 10 LM) this problem is known as the Lambert problem and I found that I was on the edge of my knowledge as well as the limit of excel. Sure I know some methods to solve the Lambert problem but I was not really able to do it in a satisfactorily way with Excel.
I hope Jarmo will take time to incorporate tha coelliptic RDV feature in a further IMFD version.
Meanwhile, may be should I build a simplified spreadsheet to calculate and display a whole RDV sequence?
No need for maneuver calculation anymore. We can do that with delta velocity prog of IMFD.
The spreadsheet should give only the GETI and the orbit parameter.
May be you have some comments and request?
And happy to be back again around here!