TransX Problem with Phobos

[garanixx]

Hello everyone!

I am experiencing an odd problem with TransX. I was trying to play through one of Flytandem's tutorial scenarios, the one that simply has you rendezvous with Phobos from Mars orbit. http://flytandem.com/orbiter/tutorials/transfer to Phobos/index.htm

The problem is when I turn on maneuver mode and set up the variables, the closest approach distance will slowly rise and will continue to do so. This occurs both before I have done the burn and even after the burn is finished and I've turned off maneuver mode. I set the variables to arrive within 2 kilometers of Phobos, and by the time I get to the orbit intersection where I was supposed to arrive at Phobos, the closest approach has climbed all the way to 20 kilometers or more and I am very far away from my target. I have tried getting to Phobos with the stock transfer MFD and I arrive spot on, no problems. This makes me believe the problem is TransX specific.

This is really strange, is anybody else experiencing this or perhaps has a clue on what could be causing this? I am using the latest version of Orbiter and TransX.

Thank you!

 

[Goth]

You can do midcourse corrections.
1-You basically do the plan again using transx
OR
2-You select Phobos in the orbitMFD and reading the value of PEA (perigee) you try to keep it low thrusting the engines in the correct direction, which if you've understood orbital mechanics will be pretty obvious. Generally prograde/retrogade should do well if the error in the trajectory is not too great.

For what I remember transx hardly puts you to a target without big margin errors, so midcourse corrections are always needed.
Generally IMFD has a bit more precision but less flexibility (possible things to do).

 

[garanixx]

Yes, of course I know about mid-course corrections. But I would literally have to be doing them all the time at the rate the closest approach is changing. This also does not happen with Earth's moon for example, and Flytandem doesn't mention it in his beginner-oriented tutorial either, which makes me assume it's not supposed to be happening.

 

[Goth]

In this case you may have done something wrong. I just ran the scenario and I reached Phobos with an error of very few K, I've basically seen it big as 1/3 of the screen, without midcourse corrections.
The only hypothesis I still have are:
-you've burned too early / too late (you did not respected the "man. date")
-you did the burn wrong, because:
* you didn't placed the nose very carefully in the green X;
or
* you started to burn too early / late;
or
* you didn't keep the nose aligned while thrusting (using linear translation);
Even if you think it's nothing like that, just try again the flight keeping in mind all those things; so maybe you'll do more precise manouvres and the margin error will be lower. The closest approach will always increment, but doing better manouvres you will be able to make it increment very very slowly, so you'll reach phobos with very few km of error.

If all goes wrong again, just upload the scenario of your plan and I'll try to see what corrections could be done.

 

[garanixx]

I know how to use transX and I do follow that green X very precisely. I am starting to suspect that somekind of add-on I installed might be causing interference. I think I'll try to do a clean install of Orbiter and if that doesn't help, I'll try again and upload the scenario file.

Thank you!

 

[flytandem]

Sounds like you are working TransX correctly. Watching the closest approach to either Phobos or Deimos drift in value is normal. If you are not very far away (let's say a few hundred Km at most) and are taking most of an orbit to reach it with a slow encounter speed (let's say under 50 or 100 m/s), the drift is most noticeable and frustrating. It is similar to chasing the moons around Jupiter. If you are arriving faster from many thousands of Km with a several hundred m/s encounter speed the drift seems less noticeable.
It is a bit of an art to decide when to do the MCCs. It's very easy to do too many.

I think a very early version of Orbiter had much more stable values when chasing Phobos or Deimos. I could be wrong. But I recall it was as stable as if the moons (Phobos and Deimos) were ships that you were rendezvousing with. At some point they became less stable as far as the slow arrivals went. I don't know why though. Ephemeris?

Wish I could be of more help.

 

[garanixx]

Thank you for the response Flytandem!

I can confirm that this is NOT an add-on related problem. I tried to do a clean install of Orbiter with only TransX installed, and the problem is still there. It must be simply what Flytandem said. A lot of drift due to low encounter speed. Transfer MFD does not suffer from the problem however, which leads me to believe it handles the low speed situations with more accuracy than TransX.

Thanks once more for the advice!