TransX Manouevre doesn't work in "Slingshot" stage

[laukejas]

Hello,

I've been trying out Voyager addon, and it has default scenarios with TransX plans for Grand Tour. I decided to try it. I noticed that in multi slingshot plan, TransX allows to set up manouvre, and see what trajectory will be not only with the first slingshot planet, but second one also. This allows to set up very precise and fuel efficient burns. For example, as I was approaching Jupiter, I could already see how MCCs will affect my approach to Uranus.

But then a problem arose. When I was about 2/5 way from Earth to Jupiter, I made MCC burn to put me on correct Pea ratio and inclination on "slingshot" stage for Jupiter (the one with only one sphere, and two green lines, dashed and solid), bringing it less to 1.000 Pea ratio and 0.00 degrees of relative inclination. Everything looked good. But then suddenly first stage ("Sling direct") was deleted, and I was left with "slingshot" stage. Only to find out that setting up manouevre doesn't affect that Pea ratio and r.inclination. In fact, it doesn't affect any further stages as well. Meaning that I'm left with guessing in which direction and how much should I burn. And since Voyager has 281m/s2 of dV, and extremely low impulse, it was no surprise that after Jupiter encounter I was 300M off Saturn, with no way to get back on course.

So, two questions: first, why setting up manouevre in "slingshot" stage doesn't affect estimations on Pea ratio and r.inclination? How can I fix it?
Second, why were trajectory estimations so inaccurate? I was right on course just before "sling direct" was deleted (1.000 Pea ratio and 0.00 degrees of relative inclination), but when I approached Jupiter, it was like 0.9600 of Pea ratio, and few degrees of r.inclination).

I also noticed that this "Slingshot" estimations seem to be stable when halfway to the planet, but as I enter it's SOI, it begins to drastically change. It is not clear when it is time to make a MCC, and when it's better to wait (but the more you wait, the more fuel expensive MCC will be).

TransX version 3.13, non-spherical gravity sources is OFF.

Please help.

 

[dgatsoulis]

first, why setting up manouevre in "slingshot" stage doesn't affect estimations on Pea ratio and r.inclination? How can I fix it?

Please post a scenario with the transX plan after the sling direct stage has been deleted.

Second, why were trajectory estimations so inaccurate?

2. A differrence of 0.04 of the Pe/Pl ratio and a few degrees of Rel. Inc. is normal and expected with transX. That's why you need MCCs.

 

[laukejas]

ok. I attached a scenario just before auto-deletion of first stage. "Slingshot" stage is perfectly aligned, as you can see (stage 2). The next slingshot stage in Saturn (stage 4) is also pretty close.

Now just accelerate time 1 day further. 1st stage gets deleted, and suddently, accuracy of "slingshot" stage drops dramatically - from Pea ratio of 1 to 0.7, inclination - from 0 to 0.7. Why?

And now try to set up manouevre in stage 1 (Jupiter slingshot). You will see that no matter what numbers you input in prograde, outward or change plane velocity, slingshot view (with Pea ratio, that sphere with two lines) doesn't get updated. And further stages as well. So basically, until you exit Jupiter's slingshot, you can't set up any manouevres.

If no MCC is made now (because it's too difficult without setting up manouevre), in my attempt, I reached Jupiter Pea with Pea ratio of 0.9976 and inclination of 0.004067, meaning that in arrival at Saturn, Pea ratio is 1.7 (you can see that in further stages). Not horrible, but since I can't fix that with MCC until I exit Jupiter SOI, I'll have very expensive burn). And in practice, it is said that planar burns in perigee are most efficient, so this is missed opportunity also (for example, I couldn't make powered slingshot if my original plan would include that).

So what am I doing wrong here? Why accuracy drops so much after auto-delete of first stage (at least it gets back after a while), and most importantly, why manouevre doesn't work?

 

[laukejas]

And low look here. Another scenario file, just after the Jupiter slingshot stage was deleted. As I was exiting Jupiter SOI, slingshot stage for Saturn was getting off and off, and when the Jupiter slingshot stage was deleted, Pe ratio for Saturn is 36 (and rapidly increasing for some reason). No MCC made, nothing. When I was in Jupiter slingshot, as I said, Pe ratio was only 0.9976 and inclination 0.004067. So that's pretty close, but now for some reason I'm passing Saturn 36 times farther than slingshot should have taken me.
Isn't that a little too much for a normal accuracy error? What has happened here? To correct that, I would need several hundred m/s2 of dV.

 

[dgatsoulis]

As you have seen for yourself, as you approach Jupiter, there is actually a small difference between the initial plan and your encounter.
There are two ways to deal with this in TransX. The first one is what I usually use for multislings. Both of these are for after the first stage has been deleted.

First way:
Press the VW button to see the approach view of the first stage. Open OrbitMFD set Jupiter as the reference planet and press the HUD button. Orient your vessel in a prograde (relative to Jupiter) direction (you'll have to do this manually, do NOT press the prograde AP because it will orient you relative to the sun). Apply linear RCS thrust to get the Pe/Pl ratio and Rel.Inc. to the desired values. (Usually takes a little bit of "left/right" and "up/down" thrust).
Repeat the process as you get closer to Jupiter. (I usually make 3 or 4 of these approach corrections, two outside the SOI and another one or two inside).

Second way. (Manoeuvre)
Wait untill the Pe/Pl ratio reaches a maximum and the R.Inc a minimum and turn the autoplan off and the advanced on, on the first stage. Select "Plan type": Cruise Plan and "Plan": Sling Direct. Then press the VW button to get to the manoeuvre view. Now you can set up a manoeuvre and the results of the changes will be visible on the other stages of the plan.

Personally I prefer the first way. It's easier and uses less dV.

Now to answer your question as to why this happens. Here are some quotes from agentgonzo on replies to some similar transX questions.

The model that TransX uses for the solar system (perturbed patched conics) is not super-accurate. As such, the predictions will change as you travel between spheres of influence of different bodies. The behaviour that you are experiencing is totally normal. The 'standard' operation of TransX is to set up a 'fairly' accurate plan (PeD seems to change more than the inclination) and then do a coure-correction burn as soon as you move to the next stage in TransX, and then one every time your time to the encounter halves (or as you deem necessary)

Nasa's guidance programs are a lot more accurate than TransX. TransX makes many assumptions that mean that it's not perfectly accurate (but is very accurate). These innaccuracies mean that you need to do a course correction burn after encounters and your figures are fairly normal.

Imfd's predictions are better than transx, but transx can do much more cOmplicated sling trajectories. Both have plusses and they complement eachether very well

Hope this helps

 

[laukejas]

Thanks. But you see, I can't do first way, because it's low impulse, low dV spacecraft, so making even 1m/s2 MCC can take about an hour. So it's pretty difficult to eyeball such manouevre.

I tried second option, but then "slingshot" view is not available. And I have to rely only on further stages. I ended up using all fuel, and still missing. Too inaccurate this way.

How can I set up manouevre and still preserve "slingshot" stage?



On these quotes, I know that TransX is not perfect, but come on, how can I miss a planet by 36 Pea ratios after almost perfect slingshot? Nothing like that happened on first journey leg (predictions for Earth-Jupiter were very accurate, except for date).

 

[dgatsoulis]

I can see your point, it's difficult when you are flying such a low impulse craft.

If you want increased accuracy, you'll need a combination of TransX and IMFD.

First setup your initial multisling plan on transX as close to a perfect plan as you can. In the first scenario you posted, your passes at Jupiter and Saturn are ok, but your passes at Uranus and Neptune are sloppy.

Here is an example of a correct multisling plan:

BEGIN_DESC
Contains the latest simulation state.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 43993.9508474037
END_ENVIRONMENT

BEGIN_FOCUS
  Ship Voyager\Voyager-2
END_FOCUS

BEGIN_CAMERA
  TARGET Voyager\Voyager-2
  MODE Cockpit
  FOV 50.00
END_CAMERA

BEGIN_HUD
  TYPE Orbit
  REF AUTO
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  Voyager\Voyager-2
  FNumber 8
  Int 0
  Orbit True
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  Vector  -10462.635706 -543.082195293 -161.976447842
  Double  1.32712439955e+020
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  Handle Sun
  Handle Earth
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0 0
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0 2
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0 0
Plan
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Plan
0 0
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Outward vel.
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Ch. plane vel.
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Finvars
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  Int 4
  Orbit True
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  Vector  -532.861067313 -803.287057082 7963.36226245
  Double  1.26686534397e+017
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  Handle Jupiter
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0 1
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Plan
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Plan
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Select Minor
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Outward vel.
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Ch. plane vel.
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Orbits to Icept
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Scale to view
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Advanced
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Finvars
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  Int 3
  Orbit True
  Vector  -597799951101 11229847588.7 543838752944
  Vector  -16521.4538249 874.939438773 -10602.6795207
  Double  1.32839126489e+020
  Double  44060.7176815
  Handle Sun
  Handle Jupiter
  Handle Saturn
Select Target
 0 Saturn
Autoplan
0 0
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0 2
Plan
0 0
Plan
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Plan
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Select Minor
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 0  43993.9506969
Outward vel.
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Inherit Vel.
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  Orbit True
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  Vector  -10349.5543372 516.174340595 -3186.16002891
  Double  3.79311866084e+016
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  Handle Saturn
  Handle NULL
  Handle NULL
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 0 Escape
Autoplan
0 0
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0 1
Plan
0 0
Plan
0 1
Plan
0 0
Select Minor
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Outward vel.
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  Vector  3557.87413806 -327.733047437 -20101.3562533
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  Handle Sun
  Handle Saturn
  Handle Uranus
Select Target
 0 Uranus
Autoplan
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0 2
Plan
0 0
Plan
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Plan
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Select Minor
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Manoeuvre mode
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Man. date
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  Handle Uranus
  Handle NULL
  Handle NULL
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Plan
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Plan
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Select Minor
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Man. date
 0  43993.9507573
Outward vel.
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Ch. plane vel.
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  Int 3
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  Handle Sun
  Handle Uranus
  Handle Neptune
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Plan
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Plan
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Plan
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Select Minor
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Advanced
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Velocity.
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Finvars
  Finish BaseFunction
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Plan
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Select Minor
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END_MFD

BEGIN_MFD Right
  TYPE User
  MODE TransX
END_MFD

BEGIN_SHIPS
Voyager\Voyager-2:Voyager\Voyager-2
  STATUS Orbiting Sun
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  RVEL -10462.636 -543.082 -161.976
  AROT -84.35 -74.97 -74.67
  RCSMODE 0
  AFCMODE 7
  PRPLEVEL 0:0.892921 
  NAVFREQ 0 0 0 0
  XPDR 0
  AAP 0:0 0:0 0:0
END
END_SHIPS

BEGIN_ExtMFD
END

Then make a note of the transX predicted approaches of each planet. You are interested in PeD and Inc.

You can then use IMFD's much more accurate map program to see how close to the transX plan your trajectory will actually be.

This should help you make your adjustments earlier and with less dV.

For practise, try first a scenario with a DG that you'll set to have the same dV as Voyager and only when you are confident that you can do it, try it with Voyager itself.

 

[laukejas]

thanks, I'll try that. I just heard that IMFD Map program is buggy, it crashes with addon solar systems - is that true? (since I use Orbiter Galaxy sometimes).

And one more question, how come IMFD is so much more accurate? Does it use some different prediction method? If so, why TransX doesn't?

 

[dgatsoulis]

thanks, I'll try that. I just heard that IMFD Map program is buggy, it crashes with addon solar systems - is that true? (since I use Orbiter Galaxy sometimes).

I don't know about that. I've never had a crash with IMFD5.5, but I rarely use other solar systems. I guess the way to find out is to try it for yourself.

And one more question, how come IMFD is so much more accurate? Does it use some different prediction method? If so, why TransX doesn't?

Different MFDs, written by different people, who wanted to achieve different goals. If you learn how to use both, you'll find that they complement each other extremely well.
AFAIK TransX is open source, so anyone can have a crack at it, if they think they can improve it.