Problem TransX Troubles

[Bloodworth]

I'm having a couple of issues with TransX. I've been using the tutorial by Capt. Crybaby and following it to the letter. I've done this trip 5 times thus far and have the same problems.

1) MJD: I claculate my launch window and write it down, then set Pe, save the scenario and advance the date. starting 24 hours from launch, I do my alignment, which I can get down to an Rinc of 0.000### at which point the number starts fluctuating and I can't get any lower (I figure that an alignment within the 10,000th of a degree range is good enough). The problem is that according to the tutorial, I should reach the MJD that I wrote down 30 seconds from my Pe. Every time I have done this (and as I said, I'm following the tutorial to the letter) the MJD and my Pe have been off by several MINUTES! This last flight, the MJD was off by HALF AN ORBIT (and no, I didn't accidentally flip my Pe and my Ap)!!

2) Cl APP: While calculating my launch window I tend to calculate it to impact (I figure that this will be easier to correct than only getting my cl app within 20k km). By the time I actually LAUNCH, the cl app has drifted out to between 5 and 8 MILLION km. WTF?

 

[vonneuman]

Correction burns. That will solve most of your problems. Launching a few minutes to an a couple of hours off will not hurt you that much as long as you do correction burns. 8 million km is not that bad. I've launched with worse and still gotten to my target. You need to do correction burns. Use the one half rule, do a correction burn every time you get to half the distance to your target from your last burn.

save the scenario and advance the date

Do you mean with the scenario editor under the date setting? Because I had the same problem when I did that. Change the scenario file, that's what I do now and I don't have that problem any more.

 

[vonneuman]

How did it go? Is it working now?

 

[Bloodworth]

Change the scenario file.

Not entirely certain what you mean here. If you mean to go into the directory and edit the date there then that exactly what I have been doing, as this is waht the tutorial says to do...

 

[vonneuman]

I mean did you change the date by hitting Ctrl F4, then scenario editor, then click "date". Or did you save your scenario and edit the file? Because I have had some problems when doing the first one.
But I still think your main problem is not enough correction burns. How often are you preforming correction burns?

 

[Bloodworth]

I mean did you change the date by hitting Ctrl F4, then scenario editor, then click "date". Or did you save your scenario and edit the file? Because I have had some problems when doing the first one.
But I still think your main problem is not enough correction burns. How often are you preforming correction burns?

I've been changing the date by saving the scenario and then editing the scenario file in the orbiter directory. As for my correction burns, the answer is often... The ship i've been using is the James Cook and it takes me 20 to 30 (at least) correction burns to get my Rinc down as low as I do. I do one really long burn at the start which brings my Rinc down to the vicinity of 6 degrees. then a whole crapload of small rcs burns (about every 1 to 5 minutes) to refine it lower than that. I do this because once I get down to about 6 degrees, I simply can't make any long main engine burns because after about 1 second of burn the Rinc starts going the other way...FAST!

 

[Tommy]

When referencing a specific tutorial, it's a good idea to provide a link. I don't have the time to hunt it down (and don't know if he did more than one). From the numbers involved, I suspect this is an interplanetary trip, not lunar?

Unless you calculate the time it takes to get into orbit, and very carefully assure that your orbital period is correct, you will almost never have a PeT that matches the MJD that closely. For an interplanetary flight, a difference of a few hours is minor - ejecting in the right direction is much more important.

As for issue #2, TransX plots courses using conic sections, which isn't perfectly accurate.

Without more information, I suspect that a part of your problem is making too many course corrections, and at the wrong times. 20 corrections is a lot! It can lead to "overcorrecting", and end up costing a lot of fuel. The problem in getting a low RInc could be caused by trying to make plane alignments when you aren't at a node. AlignPlanesMFD (or better yet, IMFD's Map) can help you pick the best time for any MCC containing a plane change.

 

[vonneuman]

6 degrees is to far off to begin your launch. What you need to do is align planes before you launch. As Tommy said trying to make plane alignments when you aren't at a node will waste a lot of fuel.

 

[Tommy]

I'll add that, unless you are doing an off-plane transfer, the node will likely be someplace between your source and your target. Don't make changes to your plane anywhere else, but you can adjust prograde and (to a lesser degree) outward velocities early on.

You also don't really tell what the reference is when you say 6 degrees of RInc. If you mean 6 degrees referenced to the source body, it's high, but survivable. If you mean 6 degrees referenced to the ecliptic you are pretty much screwed.

Remember: plane changes are best made when velocity is low. It's usually best to wait until outside any planet's SOI before making a plane change - so try to arrange your nodes appropriately.