TransX Moon-Earth Return - Need Help Understanding TransX

[LTrotsky]

I'm having enormous difficulty understanding TransX, despite reading the Orbiter, Delta-Glider, TransX Quickstart, and TransX manuals, and watching both David Courtney's and flytandem youtubers.

Situation : In orbit around the moon @ 2.957rad, zero eccentricity, plane aligned to earth with close to zero (.0001) relative inclination. Watched this video to get some idea of planning an earth return : [ame="http://vimeo.com/13319674"]TransX Tutorial - Moon to Earth on Vimeo[/ame] .

I set up the return as best I could, following the instructions as best I could. What is confusing is that you have to enter a negative prograde delta to get an earth return ellipse, and then do a positive prograde burn at burn time, as reported in the escape plan. In any case, in the video, after the prograde burn which shows a green trajectory line matching the dashed yellow planned escape, at some point the transx plan 'switches stages' which allows for an MCC manuver. In my attempt, the transx never switches stages, and I do not appear at that point to be able to use transx for anything useful for a return to earth orbit. So, there are still fundamental things I dont understand about transx at this point. I should note that using transfer and dead reckoning for the return have been far more successful in my efforts (for example, just escaping the Moon ZOI and visually heading for earth and then using the OrbitMFD once the hud switch to earth happens).

So, I am looking for some handholding here. I can post screenies on request of the current situation with shots of the transx reports, and what happens after the burn.

 

[dgatsoulis]

A pre-burn and a post-burn scenario would help in understanding what you are doing exactly.

In my attempt, the transx never switches stages, and I do not appear at that point to be able to use transx for anything useful for a return to earth orbit.

TransX should automatically delete the first stage of the plan when you are outside of the Moon's Sphere Of Influence (SOI).
If you are looking at OrbitMFD with reference to Earth, that should happen at about "G 0.40" (Down-Center of OrbitMFD).

----------------------------------------------------------------------------------

Instead of clicking buttons and hopping that you are doing the right thing, I believe it's more important to first understand and visualize what exactly is going on during the Moon-Earth flight-plan in TransX.

First let's take a look at the initial orbit and the retrograde burn, without taking the moon into consideration.

If your spacecraft was in an orbit around the Earth exactly the same as the Moon is, and you wanted to get your periapsis inside Earth's atmosphere, all you'd have to do is to point the spacecraft retrograde and apply enough ΔV to lower your periapsis.

This is exactly what you are setting up on Stage2 of the TransX plan. Assuming a near circular orbit, it takes ~850 m/s to lower your periapsis from the Moon's orbital altitude all the way down to Earth's atmosphere.

(Drawings NOT to scale)
Blue circle: Earth
Gray circle: initial orbit
Green ellipse: transfer orbit
Big red arrow: Direction of the retrograde burn

Now let's include the moon, a spacecraft in orbit around it and zoom in.
This is the first Stage of the TransX plan. You can see that eventhough you are making a prograde relative to the Moon burn, you are actually exiting the Moon's SOI in a retrograde relative to Earth direction.
Orange circle: spacecraft's orbit around the moon
Blue line: post-burn trajectory
thin red arrow: Direction of the burn

In order to get into the green transfer orbit, the exit point* direction of the post-burn trajectory, must be parallel to the direction of the retrograde burn in pic 1.

*The "exit point" is at the edge of the sphere of influence

P.S. Replace the Earth with the Sun and the Moon with Earth an you understand the basics of interplanetary journeys.

 

[LTrotsky]

Flight Profile/Telemetry

Here is a composite screenie of the relevant data. I can do the burn and post screens of what happens after the burn, when I should be out of the moon SOI, there was no stage change, and hence, I got stuck.

Following the point of the screen capture, I waited to the burn time, and did the prograde burn specified. After that burn, a green line ellipsis appears on the right hand side of transx, but one which is somewhat out of place from the predicted ellipsis described by the yellow dashed line. I waited until a point at which I thought I was or should have been out of the moon's SOI for transx to switch stages, but nothing happened. I can post screenies of this progression.

 

[LTrotsky]

Here are the screenies of the pre-burn and post-burn data as reported in transx.

Note that in the post-burn screen, the green ellipsis is not where it was predicted.

The prograde burn was NOT done using the manuver view (the target display with the little green x) as I could find no way to bring that up.

I believe I understand that because the escape burn is prograde, the ellipsis immediately after the burn will not be in the predicted place, until after negative DV is applied when out of moon SOI. At that point, it would seem to me, the green line would be close to it's predicted location, and/or, the transx stage would switch, allowing some kind of precise manuver to earth orbit. Ideally, it would seem possible to do a manuver in Earth SOI which aligns partially or very closely with the desired landing base (KSC), but this kind of thing is well beyond my skill or familiarity with transx at this point.

 

[BruceJohnJennerLawso]

Pretty much what :hesaid:

I'm busy this afternoon, so I cant help with this right now. If I have some time this evening I'll try to help you out with this.

But just in general, do try to be patient. I know exactly the sort of hair-pulling madness that you're experiencing right now; exactly the same thing that I was doing about a year and a half ago. Just try to be patient, think about what your maneuvers are doing, experiment with different things, and before you know it, you'll wake up one day knowing exactly how to use it & never have a problem navigating again. Thats just the TransX way of things I suppose.

Oh and dont forget to

 

[dgatsoulis]

Please post the scenario a couple of minutes before the burn.

 

[LTrotsky]

Here is the mission progression to the point at which the hud switches to the Sun (i.e. now orbiting the sun).

Here is where I should probably display more of my confusion and ignorance. The relative position of the ship on the earth return ellipsis seems to me fairly inefficient in terms of time. I would have liked to slingshot out of moon orbit for a more direct return ellipse, with some mid-course manuver to align the orbit prior to re-entry as close as possible to an orbit transversing the desired landing area, even if that means waiting a few orbits. Not averse to doing an orbit align once in Earth orbit, and no matter what you do in some cases this may be necessary. The method I used here doesn't seem close to this ideal, and I am virtually certain it is due to not understanding how to use transx.

 

[LTrotsky]

Pretty much what :hesaid:

I'm busy this afternoon, so I cant help with this right now. If I have some time this evening I'll try to help you out with this.

But just in general, do try to be patient. I know exactly the sort of hair-pulling madness that you're experiencing right now; exactly the same thing that I was doing about a year and a half ago. Just try to be patient, think about what your maneuvers are doing, experiment with different things, and before you know it, you'll wake up one day knowing exactly how to use it & never have a problem navigating again. Thats just the TransX way of things I suppose.

Oh and dont forget to

Thanks for letting me know that hair-pulling and other forms of self-loathing are normal for people just getting familiar with transx. :tiphat:

 

[BruceJohnJennerLawso]

Thanks for letting me know that hair-pulling and other forms of self-loathing are normal for people just getting familiar with transx. :tiphat:

Well not self-loathing, just tirades at the MFD for being so frustrating :lol:.

For a while I thought that I was going to have problems with TransX forever. I still dont find it perfect, but if you can remember to think in SOI to SOI terms, using the MFD will go very easily.

 

[LTrotsky]

Please post the scenario a couple of minutes before the burn.

Here it is, should be 120 seconds pre-burn. I should note that this is derived from a custom scenario which loads with the current datetime, and has no other ships (including the ISS).

View attachment BB_3_CDT_DG1_TrainingMission01_TransXPlan_120secPreBurn.scn

Okay I hope that is the correct file.

 

[dgatsoulis]

Ok got it.
I'll have a look and get back to you in a few mins.


A note on posting scenarios.
Instead of posting them as an attachment, do this:

Open the scenario with an editor and press Ctrl A to select everything and then Ctrl C.

In your post press the code button from the icons bar (right next to the quote icon). You should see this:

[CODE][/CODE]

Paste what you copied between the

 tags.

It should look like this:

[CODE]BEGIN_DESC
Orbiter saved state at T = 271
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 56567.7292802025
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 60.00
END_CAMERA

BEGIN_HUD
  TYPE Orbit
  REF AUTO
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  GL-01
  FNumber 2
  Int 1
  Orbit True
  Vector  872913.724762 251778.891371 -2814214.28542
  Vector  1228.16045237 40.9059139083 384.535088894
  Double  4.90279493298e+012
  Double  56567.7253418
  Handle Moon
  Handle NULL
  Handle NULL
Select Target
 0 Escape
Autoplan
0 0
Plan type
0 0
Plan
0 1
Plan
0 0
Plan
0 0
Select Minor
 0 None
Manoeuvre mode
0 1
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  56567.7253418
Outward vel.
 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
Orbits to Icept
0 0
Graph projection
0 2
Scale to view
0 0
Advanced
0 0
Pe Distance
 0  2956501.70866
Ej Orientation
 0  0
Equatorial view
0 0
Finvars
  Finish BaseFunction
  Int 2
  Orbit False
  Handle Earth
  Handle Moon
  Handle NULL
Select Target
 0 Escape
Autoplan
0 0
Plan type
0 2
Plan
0 0
Plan
0 0
Plan
0 1
Select Minor
 0 None
Manoeuvre mode
0 0
Base Orbit
0 1
Prograde vel.
 0  0
Man. date
 0  56567.7292797
Outward vel.
 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
Orbits to Icept
0 0
Graph projection
0 0
Scale to view
0 0
Advanced
0 0
Prograde vel.
 0  -830.04
Eject date
 0  56567.723569
Outward vel.
 0  0
Ch. plane vel.
 0  0
Finvars
  Finish BaseFunction
END_MFD

BEGIN_MFD Right
  TYPE User
  MODE TransX
END_MFD

BEGIN_PANEL
END_PANEL

BEGIN_SHIPS
GL-01:DeltaGlider
  STATUS Orbiting Moon
  RPOS 1279741.58 262889.12 -2653011.96
  RVEL 1158.610 24.275 561.204
  AROT 2.51 -64.23 -7.36
  RCSMODE 2
  AFCMODE 7
  PRPLEVEL 0:0.858895 1:0.952101
  NAVFREQ 94 484 486 114
  XPDR 166
  RCOVER 1 1.0000
  TRIM 0.025077
  AAP 0:0 0:0 0:0
END
END_SHIPS

BEGIN_ExtMFD
END

 

[LTrotsky]

Hope this discussion helps other as well

As this post title says, I hope this discussion will help others who may be struggling or encourage people learn all the details of transx.

The problem for me is that I need to understand the whys, which mean I need to understand what is behind virtually every interaction with an MFD, down to the simplest ones. The tutorials I've looked at go a bit fast, and tend not to explain the smaller details - this is okay for most folks, but not me. David Courtney's tutorials are the most detailed, and consequently are the longest videos, but even those gave me only a very general conceptual understanding of transx (what is possible and what isn't) but not enough specific detail for someone at my very low level of understanding and familiarity. With respect to most other aspects of Orbiter, his and the other tutorials have indeed been very helpful, and I can successfully use most of the other default MFDs. Once I understand all of them, I will begin to add some other MFDs, and would of course like to progress to the more advanced ships and mission types.

 

[dgatsoulis]

Ok, I had a look at the scenario. My main concern was if you had the date setup up wrong in stage2, something that I could not see in your preburn screenshots, but everything seems ok.

I also looked at the postburn pics on post#4 and it looks like you performed the burn correctly.

Now let's address the problems you mentioned.

What is confusing is that you have to enter a negative prograde delta to get an earth return ellipse, and then do a positive prograde burn at burn time, as reported in the escape plan.

I hope the pics and explanation on post #2 covered that.

In my attempt, the transx never switches stages, and I do not appear at that point to be able to use transx for anything useful for a return to earth orbit.

I think that you are expecting the TransX stage to delete itself earlier than when it actually does. I mentioned when you are supposed to see only one stage on the same post.

If you are looking at OrbitMFD with reference to Earth, that should happen at about "G 0.40" (Down-Center of OrbitMFD).

Let me make it a bit more clear. After you have completed the burn, open OrbitMFD and choose Earth as your reference. Time-warp (not more than x10000) and watch the G number at the bottom center of the MFD. TransX should delete the stage when you are at around "G 0.40". (In your scenario it happens slightly earlier at 0.34, but the exact number varies with each flight. 0.40 is a nice round number to remember).

The relative position of the ship on the earth return ellipsis seems to me fairly inefficient in terms of time. I would have liked to slingshot out of moon orbit for a more direct return ellipse, with some mid-course manuver to align the orbit prior to re-entry as close as possible to an orbit transversing the desired landing area, even if that means waiting a few orbits.

Have a look here to see how to setup a faster journey to Earth with TransX.
But first I would advise to get the "normal" method right and then move on to faster transits.

 

[blixel]

Here it is, should be 120 seconds pre-burn.

There is nothing wrong with your scenario. (Though I don't think it's necessary to get into such a high orbit around the moon.) I executed your plan without changing anything. When I warp time forward far enough, TransX updates the stages just fine. (Note: This does NOT happen as soon as the HUD changes to Orbit Sun, rather, the stages update when the moon's Gravitation influence drops to about 0.02 according to Orbit MFD.)

The PeA at Earth is about -1.7 thousand kilometers, so you just need an outward burn for your MCC. I did the burn when the Gravitational influence at Earth was 0.82, so it cost me about 90 m/s of dV. (If you do the burn sooner, it won't cost as much, but Orbit MFD won't give you a reliable PeA until you're closer to Earth.)

EDIT: I think seeing a video is often easier than reading text. So in this 5 minute video, I'm completing the trip back to Earth using your scenario:

 

[LTrotsky]

Thanks to blixel and dgatsoulis. Dgat, your explanations are very clear and advance my understanding of both the physics and transx. Blix, awesome that you made a video which progressed from my scenario state, with the helpful mouse hovers to highlight what is going on.

Let me absorb all of this great help and hopefully progess myself.

 

[LTrotsky]

Dgatsoulis :

1. The illustration in pics 1 and 2 were very helpful.

2. I ran the scenario further on, and indeed tranx did "switch stages"; due to the plan type all that meant was one stage dropped.

3. I used OrbitMFD as you suggested and returned to KSC (it wasn't a pretty landing, but I was on the base and undamaged.) I've used OrbitMFD before for the same thing, but I guess I was just expecting to be able to use tranx for an earth encounter from the moon.

4. I will look at your link.

Thanks for all the help guys.

 

[BruceJohnJennerLawso]

3. I used OrbitMFD as you suggested and returned to KSC (it wasn't a pretty landing, but I was on the base and undamaged.) I've used OrbitMFD before for the same thing, but I guess I was just expecting to be able to use tranx for an earth encounter from the moon.

Well its not really possible to break a DG, but Congrats!!! :10sign::11sign:

I remember my first flight around the moon well:

http://www.orbiter-forum.com/blog.php?b=1261

 

[LTrotsky]

Blix : Even though you cant damage the DG, I still pretend you can. I was forced to use hover engines at KSC to get down safely. :tiphat:

 

[Shifty]

I have a sort of related question, and I'm sorry if I'm thread-jacking here, but why not use IMFD for this? I'm an Orbiter newbie, but was able to set up a direct re-entry to Earth from lunar orbit and land deadstick at CSSC with an XR2 using IMFD pretty simply. What advantages does TransX have?

 

[wingnut]

The reason to use TransX over IMFD is, in my opinion, that you have to do more things manually to plan and execute your trajectory thus giving the user a better understanding of the orbital mechanics and how certain variables affect your trajectory.

TransX is quite tedious for me in certain aspects, like finetuning slingshots for example. I've spent long times tuning the slingshot variables to encounter my next target which might just be caused by my incompetence in the finer understanding of the mechanics. You tune one variable and changing the next totally misalignes the encounter etc.

TransX really could need a (half-) automated burn mechanism as it is quite hard to be precise with it manually and provisions to decide manually when stages are deleted would be beneficial, too.