IMFD How to launch on a free return trajectory? (followed tutorial but i don't understand)

[spacekid1998]

Hi

I followed the Moon free return tutorial on the IMFD website but I don't understand why I'm inputting the values that I use. Could someone tell me how to launch on a free return trajectory (to say, Mars) ? I know how to get TO mars just fine.. And I can set up an orbit once I get there and land. But I would like to set up a free return. Thanks

 

[Tommy]

Those variables are based on the known time of a lunar free return trajectory and the moons gravitational pull. Essentially, you are setting TEj and TIn to create a trajectory with enough velocity (a free return is much faster than a Hohman) and "offsetting" so that you ascend past the moons orbit the correct amount ahead of the moon for it's gravity to be able to alter your course sufficiently. With some patience, you can alternately vary the trip time and offset to find a free return trajectory for some Planet - Moon combinations. The moon needs to have sufficient mass to alter your trajectory, and no other moons can be near enough to affect your trajectory in a significant way. A Mars - Demios free return, for instance, is pretty much a Hohman transfer since Demios doesn't have much gravity. Trying to do a free return to one of Saturn's outer moons (from a Saturn orbit, of course) could also be problematic if one of the inner moons gets in the way.

Setting up an interplanetary free return is quite a lot different from setting up a lunar free return, and isn't possible for all planets. A lunar free return is far simpler, since the major body will always be there when you return to Pe.

For an interplanetary free return, launch windows aren't as common. Also, the period of the trajectory must be such that the originating body is in the correct place when you get there. For instance, if you tried a Mars free return, and the trip lasted 5 1/2 years, when you got back to the Earth's orbit, the Earth would be on the opposite side of the sun!

I haven't tried, but it may be possible to set up an Earth-Mars-Earth slingshot in IMFD or TransX. January 14, 2014 should be a valid launch window for a Free return that uses Mars gravity as well as the sun's (about two months faster than a solar-only free return). This pdf may also help find a window:
http://techreports.jpl.nasa.gov/1997/97-1121.pdf

I suspect you'll have to use Mars as the reference in IMFD's Map program if you want to see the classic "figure 8" trajectory, from a Sun reference it may appear as a pointy ovoid! And since IMFD can't calculate a slingshot before launch, you'd need to have a TEj and TIn for the Earth - Mars trip that is suitable.

 

[JamesG]

In TRANSX book another leg going back to Earth. It will then adjust your insertion velocity to sling you around and escape back to Sunward. At Mars perigee cancel (or just ignore) TRANSX and burn to circularize your orbit.

Oh! Beat me to the punch...

It would be a good flight profile if you wanted a fail-safe, "come back home" if anything catastrophic occured, vs. sailing off into the outer solar system or risking an unstable orbit around Mars. As Tommy said, its not perfect and might take years to complete...

 

[flytandem]

Going to Mars with fail-safe (free return) back to earth is fairly easy in TransX.

Start with an Earth eject with target Earth. Add prograde that raises the Ap so you are doing one orbit around the sun when the Earth does 2 orbits. It doesn't have to be a precise close pass back at earth. No need to add plane change or outward. The purpose is only to get an approximate amount of prograde for a 2 year journey back to Earth.

Then change to targeting Mars instead of Earth, and advance the date until you are arriving at Mars on either the outbound or inbound part of the journey. You will need to add some plane change in the transfer to hit Mars. Now you have a plan that hits Mars and as long as Mars doesn't affect you trajectory too much you also have a 2 year return to Earth.

Then do a normal sling off Mars to go back to Earth. Note the sling radius at Mars. If it's several Mars radii you can always tweak it by going back to the original transfer from Earth and try adding some prograde (like maybe 50 m/s worth), adjust the date to hit Mars again with this changed prograde amount and then go forward to redo the sling to see what the pass at Mars becomes. If it's further from Mars (worse) then go back and subtract some prograde. Just 3 or 4 tries should get you a nice close pass at Mars for the sling back to Earth. Now you have a plan that both goes from Earth to Mars and a close pass at Mars to arrive back at Earth 2 years later give or take a couple months.

OK maybe it's not simple if you are just starting out doing slings, but if you have a fair bit of experience doing slings then it is quite straightforward.

---------- Post added at 06:41 PM ---------- Previous post was at 11:43 AM ----------

I went ahead and tweaked a TransX plan for a very close pass for the free return of Mars. It's the ~2 year trip mentioned above. The pass at Mars is 88 km agl at the Pe. I think it's about as close as one can get without suffering noticeable drag losses that could mess up the sling.

It's made with the latest TransX 3.13.1.3 at
the [ame="http://www.orbithangar.com/searchid.php?ID=3039"]transx orbit hangar page[/ame].

Ship is on the runway at the Cape and time to launch is at start of the scenario and heading is something like 89.5 degrees. There are about 8 orbits (parking) while waiting on the planned eject date to arrive.

BEGIN_DESC
TransX plan set up for a flyby of Mars just a few km above the atmosphere and free return to Earth.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 55900.6336687269
END_ENVIRONMENT

BEGIN_FOCUS
  Ship 3
END_FOCUS

BEGIN_CAMERA
  TARGET 3
  MODE Cockpit
  FOV 60.00
END_CAMERA

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  3
  FNumber 5
  Int 1
  Orbit True
  Vector  -4505435.77461 3776365.51967 -2455588.13928
  Vector  273.860767988 130.742626709 -301.406470435
  Double  3.98600439969e+014
  Double  55900.6336683
  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 0
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  55900.6336683
Outward vel.
 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
Orbits to Icept
0 0
Graph projection
0 3
Scale to view
0 0
Advanced
0 0
Pe Distance
 1  6577235.51455
Ej Orientation
 2  -0.720192662543
Equatorial view
0 0
Finvars
  Finish BaseFunction
  Int 2
  Orbit False
  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 1
Prograde vel.
 0  0
Man. date
 0  55900.6336683
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  5448.63229417
Eject date
 0  55901.1489432
Outward vel.
 0  0
Ch. plane vel.
 0  -853.307
Finvars
  Finish BaseFunction
  Int 4
  Orbit True
  Vector  6203147946.13 -318476153.917 209480965.151
  Vector  -10894.8028433 565.377767447 -371.099015299
  Double  4.28282991638e+013
  Double  56035.4397537
  Handle Mars
  Handle NULL
  Handle NULL
Select Target
 0 Escape
Autoplan
0 0
Plan type
0 1
Plan
0 0
Plan
0 1
Plan
0 0
Select Minor
 0 None
Manoeuvre mode
0 0
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  55900.6258295
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
View Orbit
0 0
Finvars
  Finish BaseFunction
  Int 3
  Orbit True
  Vector  -244073107559 5416240085.06 -28012202179.9
  Vector  -7163.06304392 -1769.17126476 -21432.2740952
  Double  1.32712482783e+020
  Double  56042.0270893
  Handle Sun
  Handle Mars
  Handle Earth
Select Target
 0 Earth
Autoplan
0 0
Plan type
0 2
Plan
0 0
Plan
0 0
Plan
0 2
Select Minor
 0 None
Manoeuvre mode
0 0
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  55900.6258291
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
Velocity.
 0  0
Outward angle
 0  1.78616018264
Inc. angle
 0  0.14032447186
Inherit Vel.
0 0
Eject date
 0  56042.0270893
Finvars
  Finish BaseFunction
  Int 5
  Orbit True
  Vector  8380356418.3 -3380604986.1 -1170066461.26
  Vector  -8391.25722965 3392.65645164 1174.05398149
  Double  3.98600439969e+014
  Double  56566.0325179
  Handle Earth
  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  55900.6258195
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
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  RPOS 3796031.91 -3722164.29 4108395.18
  RVEL -4932.550 -5871.786 -757.526
  AROT 110.00 -10.00 80.00
  PRPLEVEL 0:1.000
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
3:DeltaGlider
  STATUS Landed Earth
  POS -82.4257483 22.9796405
  HEADING 60.02
  RCSMODE 0
  PRPLEVEL 0:1.000 1:1.000
  NAVFREQ 484 94 0 0
  XPDR 8
  GEAR 1 1.0000
  TRIM 1.000
END
END_SHIPS

BEGIN_ExtMFD
END

BEGIN_VistaBoost
END

---------- Post added 02-12-10 at 03:52 AM ---------- Previous post was 02-11-10 at 06:41 PM ----------

Here's the scenario approaching Mars... :thumbup:
On the way back to Earth I found a MCC of about 45 m/s was needed. Pretty typical.

BEGIN_DESC
Orbiter saved state at T = 12237690
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 56042.2735969132
END_ENVIRONMENT

BEGIN_FOCUS
  Ship 3
END_FOCUS

BEGIN_CAMERA
  TARGET 3
  MODE Cockpit
  FOV 10.00
END_CAMERA

BEGIN_HUD
  TYPE Orbit
  REF Mars
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  3
  FNumber 3
  Int 4
  Orbit True
  Vector  52424272.7434 720535.964691 111292.417965
  Vector  -10921.3488451 565.746664042 -363.293066008
  Double  4.28282991638e+013
  Double  56042.2735967
  Handle Mars
  Handle NULL
  Handle NULL
Select Target
 0 Escape
Autoplan
0 0
Plan type
0 1
Plan
0 0
Plan
0 1
Plan
0 0
Select Minor
 0 None
Manoeuvre mode
0 0
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  56042.2735967
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
View Orbit
0 0
Finvars
  Finish BaseFunction
  Int 3
  Orbit True
  Vector  -243976677388 5402429816.7 -28591604403.6
  Vector  -7051.9557949 -1840.52654917 -21478.4642444
  Double  1.32712482783e+020
  Double  56042.3283946
  Handle Sun
  Handle Mars
  Handle Earth
Select Target
 0 Earth
Autoplan
0 0
Plan type
0 2
Plan
0 0
Plan
0 0
Plan
0 2
Select Minor
 0 None
Manoeuvre mode
0 0
Base Orbit
0 0
Prograde vel.
 0  0
Man. date
 0  56032.7399006
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
Velocity.
 0  0
Outward angle
 2  1.78427522705
Inc. angle
 2  0.144164196214
Inherit Vel.
0 0
Eject date
 0  56042.3283946
Finvars
  Finish BaseFunction
  Int 5
  Orbit True
  Vector  8707311508.05 -3172890070.85 -919426804.987
  Vector  -8277.35704644 3426.66819221 1041.4271504
  Double  3.98600439969e+014
  Double  56566.8823728
  Handle Earth
  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  55900.6258195
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 Orbit
  PROJ Ship
  FRAME Ecliptic
  ALT
  REF Mars
END_MFD


BEGIN_SHIPS
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  RPOS -4755116.06 2349563.73 -4151473.69
  RVEL 3697.356 6728.137 -425.562
  AROT 110.00 -10.00 80.00
  PRPLEVEL 0:1.000
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
3:DeltaGlider
  STATUS Orbiting Mars
  RPOS 52424098.00 720545.02 111286.61
  RVEL -10921.349 565.747 -363.293
  AROT 122.96 86.43 97.95
  RCSMODE 2
  PRPLEVEL 0:0.441 1:0.985
  NAVFREQ 484 94 0 0
  XPDR 8
  TRIM -1.000
END
END_SHIPS

BEGIN_ExtMFD
END

BEGIN_VistaBoost
END