TransX Two questions on interplanetary planning

[laukejas]

Hello,

I'm using Orbiter 2010 P1 with TransX 3.13.

As I tried more and more of interplanetary flight, I noticed strange thing.

When I create a simple TransX plan Earth-Mars, I have to add much of plane change velocity (about 1.1km/s). Later, when I launch, zero RInc, and prepare manoeuvre with the same settings, trimming as needed, I find that my burn vector is somewhere about 20 degrees above my orbit plane. This, of course, is logical, since manoeuvre includes plane change, but that raises the question: if I need to burn off plane, why not launch in that inclination, that already is aligned, and I can burn completely prograde? How can I make a plan while sitting on Earth that doesn't need to add plane change velocity, but instead, launch in the correct plane? If I make myself unclear, please ask, I'll put in more detail.

And the second question: when I am in parking orbit before escape burn (150km), and I have to make a long burn, TransX gives me a vector that is very left of the prograde vector. That means, that when I start my burn, I'm burning towards Earth a bit, and lower my PeA. Sometimes that ends up burning in the atmosphere. Isn't there some precaution, for example, make a manoeuvre that allows to burn me prograde for a while, until escape velocity is achieved, and then finish it in any direction required?

 

[flytandem]

If you post a scenario or scenarios I would be happy to take a look at what you have done.

 

[C3PO]

The large "downward" LVLH is caused by the time it takes to accelerate, and TransX's calculations are based on instantaneous DV.(Zero burn time)

You can improve accuracy by making the parking orbit highly eliptical, but make sure Pe is in the correct point. As you get close to ejection (ejection minus one orbit), adjust the ApD so that you arrive at Pe at the correct time. This way the remaining DV is smaller, and burn time shorter.

 

[laukejas]

Flytandem - here's what I do:

Scenario Test1 - DGIV on Earth, with TransX plan for Mars, fine tuned, just before launch to 150km parking orbit.

Scenario Test2 - in orbit, in the correct circular orbit, with near-zero Rinc. Manoeuvre is not yet set.

Scenario Test3 - I set up manoeuvre in first stage, using values from second stage escape plan. Question - why even after setting the exact values, if I delete escape plan (reset all values) in stage 2, to see what trajectory manoeuvre will provide, I end up so short of Mars? (open stage 2 in Test3 scenario, and you will see. Manoeuvre values are exactly what was there in stage 2 escape plan).

Scenario Test4 - I re-planned the manoeuvre so that I can intercept Mars. I decided not to change eject date - it is 60583.9563, but instead, add minus off-plane velocity. Maybe that's what causing the burn-towards-earth problem? Scenario is timed 620 seconds before eject burn. As it approaches, you will see that burn vector is severely off plane (orbit hud shows it's about -22 degrees off plane, and about -18 degrees to the left of orbit vector).

Scenario Test5 - after eject burn. As I predicted, burn lowered PeA to a mininum of 129.3km. Not dangerously low, but I guess a trip to Jupiter would made me end up in atmosphere. Note that even though my burn was very accurate, full thrust, small corrections applied quickly to leave DV at 0.003, as I turned off manoeuvre mode, stage 2 now shows Cl. App. of 2.8G, while it should have been about 37M. Why? Burn was precise and quick, why such a difference in final Cl. App.? Did the low PeA (atmospheric braking) have to do something with it?

I would appreciate your comment at my problems.

 

[flytandem]

Got it. thanks.
In Test3 you have the maneuver all set up. You entered the prograde correctly but you should not have manually entered the date from the plan. By doing that you are somewhere at random in the parking orbit when you do the maneuver. Try warping to the last full orbit prior to the date of the plan and then set up a maneuver with prograde as you did but then for the date of the maneuver, advance it slowly to lay the yellow dashed trajectory on top of the plan. For more explanation see

and about 1:15 into the vid it shows what I mean.

If you don't want to touch the atmosphere during the maneuver you might also want to make the parking orbit a bit higher. Not super critical for the burn to Mars but going to Jupiter you might want to have the orbit at 200 Km agl for example.

 

[laukejas]

Thank you very much for your reply. I think I understand it. But what about that off-plane velocity? Can you explain me, why can't I make a plan so that I launch in the inclination that gives me the right plane for interplanetary flight, so in eject, I would only need prograde velocity, but no off-plane?

 

[flytandem]

Thank you very much for your reply. I think I understand it. But what about that off-plane velocity? Can you explain me, why can't I make a plan so that I launch in the inclination that gives me the right plane for interplanetary flight, so in eject, I would only need prograde velocity, but no off-plane?

Actually you have done just that in your scenarios. I have taken your test2 scenario and set up a maneuver with the prograde as shown you need. (you already had a maneuver started so I turned it off first then back on). I set the date to a bit before the planned eject date. Then I advanced date to overlay the maneuver to match the planned trajectory. This is as shown in the explanation in my post above.

Then I tweaked the date and prograde amount to fine tune the arrival at Mars. The burn is just prograde. So I am not really sure of your question.
Here is the modified test2 scenario with the maneuver all set to go.

BEGIN_DESC
test2 modified to be maneuver only.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 60583.9197032549
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 40.00
END_CAMERA

BEGIN_HUD
  TYPE Orbit
  REF AUTO
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  GL-01
  FNumber 3
  Int 1
  Orbit True
  Vector  867064.878815 -3731246.20241 5277171.91417
  Vector  -6758.29881156 2599.62765338 2948.51679322
  Double  3.98600439969e+014
  Double  60583.9178435
  Handle Earth
  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  3832.33065806
Man. date
 0  60583.9571435
Outward vel.
 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
Orbits to Icept
0 0
Graph projection
0 1
Scale to view
0 0
Advanced
0 0
Pe Distance
 0  7645212
Ej Orientation
 0  0
Equatorial view
0 0
Finvars
  Finish BaseFunction
  Int 2
  Orbit True
  Vector  148384955740 -103964003.883 20784065452.7
  Vector  -5123.95041231 -1614.27795933 32648.0715107
  Double  1.32712838556e+020
  Double  60583.9571435
  Handle Sun
  Handle Earth
  Handle Mars
Select Target
 0 Mars
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 0
Prograde vel.
 0  0
Man. date
 0  60583.9192194
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  0
Eject date
 0  60583.9187344
Outward vel.
 0  0
Ch. plane vel.
 0  0
Finvars
  Finish BaseFunction
  Int 5
  Orbit True
  Vector  2669418577.74 -3372084650.26 -3729777205.14
  Vector  -1284.5842734 1627.53228684 1800.47128517
  Double  4.28282991638e+013
  Double  60864.6410066
  Handle Mars
  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  60583.9192156
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

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ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
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  VROT 0.01 0.01 0.10
  AFCMODE 7
  PRPLEVEL 0:0.657364
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
Mir:Mir
  STATUS Orbiting Earth
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  AROT 5.69 -2.90 94.19
  VROT -0.08 -0.01 -0.00
  AFCMODE 7
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  XPDR 482
END
Luna-OB1:Wheel
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  AROT -0.01 -0.03 134.72
  VROT 0.00 0.00 10.00
  AFCMODE 7
  IDS 0:560 100 1:564 100
  XPDR 494
END
GL-01:DeltaGliderIV
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  ;-------------------------- Skin parameters must contain directory name of skin (without space) 
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  ;-------------------------- Cargo payload must contain name of config file or "none".(see doc) 
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  GEAR 0 0.00
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  N2tankALevel 99.9831
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  N2tankBLevel 100.0000
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  ;------------------Crew parameters  UMMUCREW Function-Name-Age-CardiacPulse-WeightKg (fonction of Pilot must be: Capt) 
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  UMMUCREW Capt-Mr_Captain-37-66-78
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  FailRightTurbo 0
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END
END_SHIPS

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