[TransX] Earth - Mercury with Venus slingshot with aerobrake

[laukejas]

Hello,

I'm trying to put Chapman probe into Mercury orbit, launched from Earth with Saturn 5. I've got more than enough fuel to get to Venus, but not enough for Mercury, and this probe has only 2500m/s2 dV, so orbit insertion at Mercury can be very difficult. I'm thinking of Earth - to Venus, slingshot, then another slingshot around Venus, and then to Mercury. However, I'm missing Mercury by 8G. Not much, but I can't get the number any lower because I'd have to go through Venus surface. So it would seem impossible. But I have an idea. What if I made aerobraking at second slingshot at Venus, reduce Sun relative orbit speed, and then intercept Mercury? I could then plan several slingshots around Mercury to reduce relative speed, and finally capture.

Problem is this aerobraking. Aerobrake MFD has very useful feature, it shows how much dV I would lose if I enter atmosphere at specific altitude. But I do not know how much DeltaV I would need to lose. How to determine that with TransX? Is it possible to somehow add hypothetical dV change at slingshot periapsis, to see what would happen? I tried turning off "inherit velocity", but then it doesn't take Venus gravity assist into account.

I watched Flytandem's aerobrake with TransX tutorial, but it was performed with Delta Glider, and my probe has no aerodynamic surfaces, so I can't "fly inverted" or control aerobrake at all. I just need to plan it before, determine how much dV I need to lose, and then put probe in good approach with Aerobrake MFD.

Can anyone help? How to plan with TransX what would happen if I lose specific dV while doing slingshot?

By the way, it would also be good to plan a manouevre while doing slingshot, because it is most effective at periapsis.


Edit: I tried setting up simple manouevre in periapsis of Venus (determined by encounter date), but once I add retrograde velocity, graph starts jumping over and never stops. This doesn't happen if I add prograde velocity.

Edit2: I found the name: it is called "powered slingshot". If I could plan such with TransX, I could just as easily see how much dV I need to gain/lose with aerobraking. How to plan powered slingshot?

Edit3: Okay, by practice I saw that aerobrake was a bad idea. It didn't help to get to Mercury. So only powered slingshot remains here. How to plan it? I tried setting up manouevre in Venus SOI, but it didn't affect the following section of TransX plan. Why?

[flytandem]

This from memory... It's possible to have maneuvers in latter stages of TransX but not mixed in with plans. Example, first set up the plan to get to Venus and get into you parking orbit on Earth before the trans Venus burn. The trans venus burn can be a maneuver and set this maneuver up and kill the plan by making the prograde, outward and plane change all become zero values. After the maneuver is set up to hit venus, then forward to stage 2 where you can add a maneuver if you like to fine tune your arrival at venus, and then forward to stage 3 (encounter at venus) where you can set up another maneuver. Note that maneuver in latter stages is not going to show up as a view choice. To get "maneuver" go back to stage 1 and show the maneuver view the and just click forward to next stages and it will give you the maneuver view in these stages. I don't think transx was designed to be doing these multiple stage maneuvers since it is not a latter stage option. But I have found it to be possible and even useful in certain cases like what you are trying to do.

[laukejas]

But if I do not plan everything in advance, how can I determine if I'll have enough dV for executing it?

Edit: and another question: when planning slingshot, how to determine what inclination relative to the ecliptic will be after slingshot? For example, I now reached Mercury (after 6 slingshots via Venus), and have to perform a number of Mercury slingshots to equalize orbits. First I want to equalize inclination. I can choose "Inclination Angle" when planning slingshot, but I can't see anywhere in what inclination will it result relative to ecliptic.

[laukejas]

Can anybody answer my last question, please? I really need that information...

[dgatsoulis]

Could you please post the scenario with the TransX plan of the journey you are trying to make?

[laukejas]

Yes, I can. I'm attaching a scenario in which I have a spacecraft (DG here, for compatibility, but in my scenario, it is Chapman probe. The dV budget here is real one, about 2.670k dV).

The goal of this mission is Mercury orbit insertion. Since capture burn in Mercury periapsis would take a lot of dV, I plan on executing series of slingshots around Mercury to alter my orbit to match Mercurian, and when orbits are enough similar, perform cheap capture burn.

After 6 orbits around the Sun I'll come into close proximity with Mercury. Now I need to make a slingshot that would make my orbit more like that of Mercury (reduce inclination, change apoapsis longtitude), be able to return to Mercury after some 3-7 orbits for another slingshot, and not go through Mercury's surface to make that slingshot.

Problem is, I don't exactly know what my orbit will be like after this slingshot. I can visualize it horizontally because of how TransX is drawing, so I see that after slingshot my orbit will be closer to Mercurian. But with inclination, I have no idea. I can't find any number in TransX that will tell me what my inclination will be like after this slingshot. Maybe my plan is all wrong, and I'm actually increasing R.inclination with Mercury?

Please advise. How can I find out what my inclination will be like after this nearest slingshot?

[dgatsoulis]

I haven't had much time to find a solution, but from a quick test run I did, I'd say that it's going to be very difficult to get to Mercury from that starting trajectory.

Even with a "perfect" approach to Mercury, you need ~1.2 km/s of dV for (circular) orbit insertion, leaving you with 1.4 km/s for manoeuvres.
The main problem is that your perihelion is set substantially bellow Mercury's orbit.
The second problem is that Mercury is not massive enough to make great changes to your trajectory, meaning that it will require several passes before you make an approach with low enough velocity.
I'd start with a 0.7 km/s manoeuvre that can get you for a first pass within the trajectory you are now, instead of 6 orbits later.
After that you should be able to make passes every ~2.5 to 3 orbits each time lowering your aphelion.

You don't have a way to see what your inclination will be after the slingshot with TransX but you can guess it if you see the line of nodes changing.
Like I said before, Mercury is too small to change your trajectory by much, your best bet is to go for an off-plane approach, each time lowering your aphelion, matching planes would take a lot more slings.

Hope this helps

[flytandem]

Sorry I hadn't seen the progression of this post until now.
It struck me that you had probably done a slingshot at Venus at a location not especially ideal to go to Mercury.
I have done some Earth Venus Mercury trips and in trying to minimize dv I usually started by finding out the location of the node of Venus and Mercury orbits. To do this I placed a ship on Mercury and made a prograde transx plan to show me the nodes (there is probably other ways to find this out). I then started my Earth to Venus planning so that I would end up arriving Venus at that Venus Mercury node. This gave me the best chance of doing a sling that had me heading to Mercury in plane with Mercury. I have no idea what the overall dv requirements are however. The stock deltaglider had plenty to make it and not only get a low orbit, it could land on Mercury with plenty of fuel to spare.

Also, having just 2.6 km/s dv while enroute from Venus to Mercury isn't going to insert to low orbit. Doing it in reverse with going from Mercury low orbit to eject to anywhere near Venus height is about 4.5 km/s dv for the eject maneuver. So I think after leaving Earth, a ship would need probably about 5 km/s dv to comfortably make corrections and sling Venus then arrive Mercury and insert to a low orbit. Have no idea what the chapman probe has for dv budget after leaving LEO.

---------- Post added at 02:15 PM ---------- Previous post was at 09:11 AM ----------

Here's the kind of Earth Venus Venus Mercury slings I think might be helpful in getting limited dv ships to Mercury. This has a fully loaded DG so on arrival mercury there should be plenty of fuel left over.

BEGIN_DESC
Ready to depart heading 090 for a 5 year 9 month trip to Mercury.
 Eject to Venus with sling to Venus then Mercury. 
 Stage 6 slips the "Orbits to Icept" on startup and should be set to "5". 
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 57231.6471036720
END_ENVIRONMENT

BEGIN_FOCUS
  Ship 3
END_FOCUS

BEGIN_CAMERA
  TARGET 3
  MODE Cockpit
  FOV 60.00
END_CAMERA

BEGIN_HUD
  TYPE Surface
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  3
  FNumber 7
  Int 1
  Orbit True
  Vector  2554620.18478 1435108.60957 5657223.71463
  Vector  -336.862904179 -73.476756653 170.755824645
  Double  3.98600439969e+014
  Double  57231.6471034
  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  57231.6471034
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
 0  6571242.46973
Ej Orientation
 0  -0.0494277244165
Equatorial view
0 0
Finvars
  Finish BaseFunction
  Int 2
  Orbit False
  Handle Sun
  Handle Earth
  Handle Venus
Select Target
 0 Venus
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  57231.6463916
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
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 0  -3964.90370426
Eject date
 0  57232.0129932
Outward vel.
 0  -59.5
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 0  1187.47814134
Finvars
  Finish BaseFunction
  Int 4
  Orbit True
  Vector  5243283389.7 -1385883552.13 3009596859.86
  Vector  -7481.57150094 1990.53959953 -4312.59488102
  Double  3.2485863e+014
  Double  57392.8972036
  Handle Venus
  Handle NULL
  Handle NULL
Select Target
 0 Escape
Autoplan
0 0
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0 1
Plan
0 0
Plan
0 1
Plan
0 0
Select Minor
 0 None
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0 0
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0 0
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 0  0
Man. date
 0  57231.6464016
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 0  0
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Finvars
  Finish BaseFunction
  Int 3
  Orbit True
  Vector  -97325228905.9 4998575889.71 -46656558476.5
  Vector  6106.1732539 -2382.40271682 -32016.3514852
  Double  1.32712764814e+020
  Double  57400.9718906
  Handle Sun
  Handle Venus
  Handle Venus
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 0 Venus
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0 0
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0 2
Plan
0 0
Plan
0 0
Plan
0 2
Select Minor
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0 0
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 0  0
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 0  57231.6464103
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 0  0
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0 0
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0 11
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0 0
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0 0
Advanced
0 0
Velocity.
 0  0
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 0  1.97035769405
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Inherit Vel.
0 0
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 0  57400.9718906
Finvars
  Finish BaseFunction
  Int 4
  Orbit True
  Vector  6148291508.86 788569755.615 218767944.644
  Vector  -8790.0205878 -1086.60766775 -332.80296648
  Double  3.2485863e+014
  Double  58516.3593118
  Handle Venus
  Handle NULL
  Handle NULL
Select Target
 0 Escape
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0 0
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0 0
Plan
0 1
Plan
0 0
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 0  57231.6464194
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  Int 3
  Orbit True
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  Vector  6563.99383736 -4368.23810888 -31072.2524239
  Double  1.32712764814e+020
  Double  58524.4373748
  Handle Sun
  Handle Venus
  Handle Mercury
Select Target
 0 Mercury
Autoplan
0 0
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0 2
Plan
0 0
Plan
0 0
Plan
0 2
Select Minor
 0 None
Manoeuvre mode
0 0
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0 0
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Man. date
 0  57231.6471034
Outward vel.
 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
Orbits to Icept
0 11
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0 0
Scale to view
0 0
Advanced
0 0
Velocity.
 0  0
Outward angle
 3  2.57778170146
Inc. angle
 2  0.241310618644
Inherit Vel.
0 0
Eject date
 0  58524.4373748
Finvars
  Finish BaseFunction
  Int 5
  Orbit True
  Vector  275480793.285 -54039418.2398 -965974874.698
  Vector  -1333.23962269 625.982117009 5543.96713289
  Double  2.20320914692e+013
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  Handle Mercury
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Finvars
  Finish BaseFunction
END_MFD

BEGIN_MFD Right
  TYPE User
  MODE TransX
END_MFD

BEGIN_SHIPS
3:DeltaGlider
  STATUS Landed Earth
  POS -117.2601388 34.0891835
  HEADING 90.00
  RCSMODE 0
  AFCMODE 7
  PRPLEVEL 0:1.000000 1:1.000000
  NAVFREQ 0 524 84 114
  XPDR 6
  GEAR 1 1.0000
  TRIM 1.000000
  AAP 0:0 0:0 0:0
END
END_SHIPS

BEGIN_VistaBoost
END

[flytandem]

The plan here flew predictably. Arrival at Mercury required a bit under 3 km/s dv to be captured and an additional 1.2 km/s to create a nice low 50 by 50 km agl polar orbit. Have no idea what the dv of the chapman probe is but perhaps this is a workable solution to fly it to Mercury. [edit: I see in the first post that the chapman probe has 2500 m/s dv limit. My gut feel is that it would be impossible to insert the probe at Mercury. Any arrival at Mercury that comes from Venus will need more than that based on the flight done here. Or maybe I'm missing something?]

[laukejas]

Thank you all for your input, it really helped. And sorry for such a late reply, I was out of orbit for a long time