iMFD and Hohmann Transfers

gder

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I picked up Orbiter last week and dug in with zeal. I've gone through the Go Play in Space book by Bruce Irving. A wonderful piece of work that brings most of the important concepts in Orbiter right home, and you don't even need to be a rocket scientist to read it (my only comment would be to update the Mars chapter to reference the newer version of iMFD).

I have some questions for the more experienced Orbiter users, particularly the ones that use iMFD for interplanetary flight. Before posting here I have gone through the iMFD manual and also looked over several tutorials I found (not just iMFD but TransX as well). The tutorials were very helpful but I'm still left with some puzzling questions.

I've been able to reach mars orbit successfully a few times now using the "Target Intercept" program to calculate a base trajectory and eject time. My base scenario for practice has been the included "DGIV in Orbit" modified so that my rINC to Mars is 0 and the DGIV is fully refueled. Essentially leaving from Earth orbit and arriving in Mars orbit.

Once I have that set up I sync it to the "Orbit Eject" program and fine tune the ejection time until i find the lowest DV required to git 'er done so to speak. As soon as I leave Earth's gravitational influence I'm able to change my course reference to the Sun and source body as the DGIV. I then use the burn calculations in concert with the map to make a manual burn and adjust my PeA to Mars to where I want it (This seems to eliminate the need for a more expensive planetary approach burn later in the orbit). Once I get close to Mars PeA I've been able to enter a stable orbit using two methods; one is a simple retro-burn initiated at roughly PeT 30-60, and the other is using iMFDs orbit insert program.

So this is all well and good for Mars with the DGIV but poses a problem for bodies further out. Jupiter is my next objective with a possible refueling stop on Europa Base (to get a hang for scenario editing) and continuing on to one of the outer planets.

I have tried several times to use the "Tangential Transfer" program to determine a launch window for a Hohmann transfer orbit. I've noticed that once I get the orbit set up so that my PeA distance is as low as I can get it (I try to shoot for within 1M meters) the number doesn't stop changing. I have time warp set to 1x and even when I'm not pressing any of the MFD buttons the DST value continues to rise.

Okay, so I figure that maybe the calculation isn't perfect since I'm doing it from orbit and not on the ground. I tried doing the same thing with a DGIV sitting on a landing pad on Earth and get the same results...odd but hrmmm, okay whatever I'll run with it.

Once I have my eject time figured pretty well I use the scenario editor to move the current date closer to my launch date (which usually ends up being a couple years in the future). As I adjust the simulation date I notice that my orbital calculations get further and further out of wack, I'm no longer calculating an eject time to meet up with Mars, apparently iMFD just wants to pass through and wave hello on its way to becoming the first man made kupier belt object...

Okay, no biggie, I can readjust the parameters when I get closer to the eject date and perform a burn in-transit to correct any major errors. So as launch (man I wish it was lunch) time approaches I sync up to the "Orbit Eject" MFD and start figuring out which tEJ offers the lowest DV for a successful transfer. In "Orbit Eject" I first set tEJ to equal the tEJ provided by the course program and then try to tweak it from there.


I suppose the main problem I'm having is confusion as to why the DST number in the course MFD is constantly changing, the affect seems to get worse the further out you want to go (Jupiter for instance seems nearly impossible to lock into for me).

The other main question I have is how do you relate the tEJ provided by the course program into something usable for the Orbit Eject program? Since Orbit Eject still has to be configured manually.

On a side note if someone could please explain the difference between the "off-axis" and "realtime" settings I would appreciate it. The two settings seem to produce very different results but off-axis seems to provide a lower DV. Most of the tutorials I've read seem to suggest using the off-axis setting but none of the explain why.

Anyway, if you've made it through this post congratulations, I realize it is somewhat of a ramble. I welcome comments from any of you Ol' Flyers out there.

Regards
Gder

Edit note: Underlined my actual questions, the rest of just background.
 

gder

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I think I may have answered some of my own questions just through playing around with the MFD. I found it much easier to configure iMFD to use MJD rather than GET times for displaying launch and intercept dates.

I also played around with Tangential Transfer program some more. It seems that as you get closer to launch date the output starts to become more stable, I need to play with it more though.
 
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