TransX TransX accuracy

[laukejas]

Hello,

I'm using Orbiter 2010 P1. For interplanetary flights, I used IMFD, and it worked well. But as I red about TransX ability to plan complex multi-stage flights (e.g. launch, escape planet SOI, sling here, sling there, approach, insert, land in one multi stage plan, able to perfect from beginning to end even before launch), while I didn't find that IMFD could do similar multi-plans in advance, only a few stages. So I thought that switching to TransX would make a huge difference in interplanetary flights.
However, as I tried TransX for Earth-Moon trip, I discovered that it is highly inaccurate, missing target by a large margin, requires multiple correction burns back and forth to make plan work, while IMFD usually worked well with one or no correction burns.

This would mean much higher fuel cost for interplanetary travel, and quite disturbing experience with these numerous corrections. Seems like switching back to IMFD is only choice. Or am I wrong? Can I make TransX more accurate? Or make IMFD able to make multi-stage planning?

Is it possible to make Voyager type plan (Earth->Escape, Sun->Jupiter, Jupiter->Escape, Sun->Saturn, Saturn->Escape, Sun->Uranus, Uranus-
>Escape, Sun->Neptune, Neptune->) with IMFD in advance, e.g. before launch?

 

[Mister Mxyzptlk]

As a long time transx user, I would agree that it isn't as accurate as IMFD, but it is not as bad as you make it sound. Since you obviously are a novice at transx I would ascribe the inaccuracies you experienced to operator error.

The mistake that most people make is performing course corrections too early and too often. If you use transx a lot you will learn that it is more accurate than you think.

I go to the moon with no correction burns or one minor burn routinely. If you ultra fine tune your initial burn with transx and use manuever mode when available for your initial burn, transx is highly accurate, just talk to Flytandem if you want to learn the finer points of transx. Also remember that transx is not mastered overnight.

 

[laukejas]

Maybe you're right... I just followed tutorials, so I thought I do everything ok. And maybe indeed I did corrections too early. When should I do them? Half the trip? How can I calculate when is half?

 

[Tex]

How can I calculate when is half?

After you eject earth and the stage changes, you can see your trajectory in relation to earth and your targets orbit. That could be used as a rough visual reference for a half-way point. You could also figure it by time. Take your eject and encounter MJD, figuring half that way. Unless my closest approach is way off, I do my first course correction just past half-way, then I do a final course correction shortly before arriving to setup a perfect encounter. That only takes a little translation to complete usually, not much fuel waist at all.

 

[Arrowstar]

Of TransX and IMFD, I believe one uses a patched conics approximation, while the other uses another method (direct integration of the equations of motion?) to determine flight plans. Since you're saying TransX appears less accurate, I suspect it's that one using the approximation. This will be the cause of some inaccuracies, though it should not present too much of a problem.

 

[laukejas]

Thank you for advice! I'll dig in deeper into manuals, and try some more. Anyway, I like the whole TransX idea of multi-stages a lot, so I'll try to perfect it

 

[Mister Mxyzptlk]

Laukejas, If you could post a scenario where you used transx and thought it was inaccurate, I could take a look at it and let you know what you might be doing right or wrong. Set up your flight plan and save the scenario about a 1000 seconds before burntime.

Use just stock vessels and scenarios please.

 

[statickid]

yeah transx always puts me right where I want to be. Do you check the FWD button to see your predicted approach to the place you are going?

 

[laukejas]

Ok, Mister Mxyzptlk, here's what I do:

1. Zero relative inclination between my orbital plane and moon's.
2. In transx: first stage - target moon, FWD to create new stage, fire up right MFD with transx, and view that second stage;
3. In first stage, manoeuvre mode on, raise prograde vel. to 3.099k, Man. date to 51982.5594. Right MFD (stage 2) shows Min alt. to moon - 147.1k.
4. (scenario saved, 2.1k seconds to burn)
5. When Begin Burn reaches zero, full thrust to shown vector, until Delta V reads less than 0.01. However, Delta V is constantly increasing every second, (after half a minute, it reads 8m/s, and continues to raise)
6. Wait...

Tried this time as precisely as possible. With no course corrections, I arrive at moon with periapsis of 39M, well away from moon's SOI.

Attaching scenario 2.1k before burn.

Where is my mistake?

 

[Tex]

Why is this bit in your scenario?

BEGIN_OrbiterGalaxy
  CurSys Sol
  CurStar Sun
  CurSysCube 0.000000 -798.000000 2.000000
  CurSysPos -0.000002 -0.000004 0.250000
  CurSysPlane 90.019997 60.180000 -96.400002
END

 

[laukejas]

Well, because I use Orbiter Galaxy addon. Some problems? I can disable it for this scenario, if it causes problems.

 

[dgatsoulis]

On the first stage, you set up the closest approach at 5.616M away from the center of the moon. So even if the second stage says that you'll pass ~150k above the surface, you see that this cannot happen.
Try to bring the first stage, much closer (a few kilometers) from the center.

Here, try this:

BEGIN_DESC
Contains the latest simulation state.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 51982.5351872387
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 50.00
END_CAMERA

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  GL-01
  FNumber 2
  Int 0
  Orbit True
  Vector  4393078.3064 -282394.336561 -5094785.90744
  Vector  5792.98618319 -596.798106292 5028.43132938
  Double  3.98600439969e+014
  Double  51982.5307147
  Handle Earth
  Handle NULL
  Handle Moon
Select Target
 0 Moon
Autoplan
0 0
Plan type
0 0
Plan
0 0
Plan
0 0
Plan
0 0
Select Minor
 0 None
Manoeuvre mode
0 1
Base Orbit
0 0
Prograde vel.
 4  3098.98029315
Man. date
 5  51982.5600793
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
Finvars
  Finish BaseFunction
  Int 5
  Orbit True
  Vector  316760875.476 -31603638.8363 219851558.514
  Vector  -650.896248525 64.9397813877 -451.691019859
  Double  4.90279493298e+012
  Double  51981.6673243
  Handle Moon
  Handle NULL
  Handle NULL
Select Target
 0 None
Autoplan
0 0
Plan type
0 1
Plan
0 0
Plan
0 2
Plan
0 0
Select Minor
 0 None
Manoeuvre mode
0 0
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  51982.5351868
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
Draw Base
0 0
Finvars
  Finish BaseFunction
END_MFD

BEGIN_MFD Right
  TYPE User
  MODE TransX
END_MFD

BEGIN_SHIPS
GL-01:DeltaGlider
  STATUS Orbiting Earth
  RPOS 6137836.82 -477978.06 -2724980.23
  RVEL 3090.157 -399.569 7036.391
  AROT -54.04 -60.01 86.79
  AFCMODE 7
  PRPLEVEL 0:0.553000 1:0.900000
  NAVFREQ 0 0 0 0
  XPDR 0
  AAP 0:0 0:0 0:0
END
END_SHIPS

BEGIN_ExtMFD
END

This plan will get you to the moon with minimal corrections.

 

[Tex]

Also your orbital plane was not perfectly aligned with the Moon. I fixed that for you and setup a new plan, eject date is less than 1000 seconds away.

 

[Tex]

Sort of ninja'd by dgatsoulis there. Nice plan mate! That will surely get him on the surface of the Moon with only one burn! Can't say for sure you'll survive the impact though. :lol:

 

[laukejas]

dgatsoulis, I tried your scenario - with after precise burn, but without corrections, I ended up with PeA of 50M. What went wrong? What PeA do you get without correction burn?

 

[dgatsoulis]

Sort of ninja'd by dgatsoulis there. Nice plan mate! That will surely get him on the surface of the Moon with only one burn! Can't say for sure you'll survive the impact though. :lol:

:rofl:

With that plan, it only takes a ~10m/s correction to get aligned with any base you want on the moon. You did raise an interesting point though.

-Plane alignment

I used the same plane that laukejas was already on, but made sure that i'd arrive on a node. (So my trajectory was a bit off-plane).

Yours was dead-on! Hard to make a mistake with that one!:thumbup:

@laukejas

I think that you'll find artlav's [ame="http://orbithangar.com/searchid.php?ID=4864"]Videnie addon as valuable as i have found it.[/ame]
It's a great tool to help you visualise what's going on when you are in orbit.
A great way to try it is this:
Get your hands on as many playbacks as you can, zoom out and watch!
Try to compare that, to what you are doing, and also use the scenario editor to place ships in completely different orbits around the same planet.
-Try having the same Inclanation but different LAN from the playback and see what goes on there.
-Try having different inclanation but the same LAN, and also see what happens there.

Practice, practice, practice.

---just before posting i saw this---

dgatsoulis, I tried your scenario - with after precise burn, but without corrections, I ended up with PeA of 50M. What went wrong? What PeA do you get without correction burn?

With that burn, you almost smack right in the middle of the moon.
:hmm: How exactly do you perform your burns?
Do you wait for the "Time to burn" in stage1 to go to 0 and burn prograde, untill your Delta-V reaches zero?

 

[Tex]

laukejas: If you setup the manoeuver, then using target view for your burn is the most accurate. When begin burn reaches 0, burn keeping the green X centered in the target until delta v reaches 0.000

What are your realism settings? Are you playing with 'Nonspherical gravity sources' on? With that activated you will see some deviations in your trajectory which require course corrections. While it may be more realistic, I personally find that option annoying, so I have it disabled.

 

[laukejas]

laukejas: If you setup the manoeuver, then using target view for your burn is the most accurate. When begin burn reaches 0, burn keeping the green X centered in the target until delta v reaches 0.000

That's exactly what I do! The thing is, that even if I zero dV, it starts to increase immediately.

I have enabled 'Nonspherical gravity sources', I like things realistic. Doesn't TransX take that into account?

 

[Tex]

If you like to play with that on, then I'm afraid you will not be able to avoid course corrections (just like real spacecraft). As you learn the margins of error with nonspherical gravity sources activated, then you will know what to shoot for with your variables in TransX so that you minimize the number and amount of course corrections.

That will just take trial and error. Practice, practice!

 

[laukejas]

Well, thank you very much.

Just one more question: when it's best to do course corrections in regard to accuracy and fuel efficiency?