Advanced Question Visiting multiple planets in one go. TransX Slingshot Question

[blixel]

This issue is for all the TransX gurus among you.

For the last several days, I've been having a great time exploring the Solar System. For the longest time, I've been stuck playing around in LEO and/or going to the moon and back, but I recently learned enough about TransX to make a trip to Mars, Venus, Jupiter, and finally, Mercury.

My first successful interplanetary trip. I was extremely happy when I made it!

Wow - I even made it to one of the outer planets. It's almost like I know what I'm doing.

I had to "cheat" big time with Mercury. (Using the scenario editor to refuel more than once. No wonder it took NASA over 6 years to get MESSENGER in orbit around this feather weight.)

It was my trip to Mercury that most piqued my interest in doing the whole slingshot thing. (Well, that, and the fact that as soon as you figure something out, you immediately want to raise the bar.)

When I was arriving at Mercury, it became painfully clear to me how hard it is to realistically enter an orbit around this tiny planet. (I did it with brute force ... using an unrealistic amount of fuel.)

So after that trip was in the history books, I immediately started dabbling with TransX to try and figure out how to set up an Earth -> Escape -> Venus -> Escape -> Mercury plan using TransX.

I spent a couple hours going forward and backward through the stages, and I just can't figure out what to do. I thought that plan might be too advanced, so I tried setting up an Earth -> Escape -> Venus -> Escape -> Earth plan ... as well as trying some combinations with Mars, but I think I must be missing something "simple" (or perhaps not so simple) ... because I just can't get the alignment to work out for 2 planets in a row.

I do understand that there is a delicate balance with the timing ... when you sling past one planet, the next planet has to be at the right place and at the right time, otherwise you're just going to orbit the sun indefinitely. I worked with the date forward and backward, but couldn't find a solution.

So for now, I'm done guessing. There has to be an easier way to figure this out.

Here is a scenario I'm working on. It uses the basic Delta-glider and there are no other vessels in the scenario (for maximum compatibility). If any of you TransX gurus would be so kind as to load this up in Orbiter and have a look, and offer your thoughts, I would be quite appreciative.

BEGIN_DESC
Orbiter saved state at T = 5982
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 55618.6213859022
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 30.00
END_CAMERA

BEGIN_HUD
  TYPE Surface
END_HUD

BEGIN_MFD Left
  TYPE User
  MODE TransX
  Ship  GL-01
  FNumber 5
  Int 1
  Orbit True
  Vector  2683221.2204 4748259.4861 -3293046.75069
  Vector  358.046599588 -78.0899423542 179.143197475
  Double  3.98600439969e+014
  Double  55618.621385
  Handle Earth
  Handle NULL
  Handle NULL
Select Target
 0 Escape
Autoplan
0 0
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0 0
Plan
0 1
Plan
0 0
Plan
0 0
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0 0
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0 0
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0 3
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Advanced
0 0
Pe Distance
 2  6570331.06978
Ej Orientation
 0  0
Equatorial view
0 0
Finvars
  Finish BaseFunction
  Int 2
  Orbit False
  Handle Sun
  Handle Earth
  Handle Venus
Select Target
 0 Venus
Autoplan
0 0
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0 2
Plan
0 0
Plan
0 0
Plan
0 1
Select Minor
 0 None
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0 0
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0 1
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 0  0
Man. date
 0  55618.6213859
Outward vel.
 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
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0 0
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Scale to view
0 0
Advanced
0 0
Prograde vel.
 5  -7897.9674183
Eject date
 6  55618.6024544
Outward vel.
 6  0
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 3  655.397
Finvars
  Finish BaseFunction
  Int 4
  Orbit True
  Vector  -588093189.106 763889762.519 -6092760556.67
  Vector  1550.89419933 -2018.2041416 16082.4336187
  Double  3.2485863e+014
  Double  55759.233013
  Handle Venus
  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
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 0  0
Man. date
 0  55618.6208895
Outward vel.
 0  0
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 0  0
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0 0
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0 0
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0 0
Advanced
0 0
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0 0
Finvars
  Finish BaseFunction
  Int 3
  Orbit True
  Vector  -20726788557 2650447806.08 105529362800
  Vector  -25414.588616 14654.5841179 -11415.4814005
  Double  1.32712764814e+020
  Double  55763.611669
  Handle Sun
  Handle Venus
  Handle Mercury
Select Target
 0 Mercury
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  55618.6208858
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 0  0
Ch. plane vel.
 0  0
Intercept with
0 0
Orbits to Icept
0 0
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0 0
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0 0
Advanced
0 0
Velocity.
 0  0
Outward angle
 6  0
Inc. angle
 6  0
Inherit Vel.
0 0
Eject date
 0  55763.611669
Finvars
  Finish BaseFunction
  Int 5
  Orbit False
  Handle Mercury
  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
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0 0
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0 1
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 0  0
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 0  55618.604211
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 0  0
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0 0
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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 1869201.57 -6313460.46 1419679.23
  RVEL -7252.442 -1728.811 1893.204
  AROT 30.00 0.00 50.00
  AFCMODE 7
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
GL-01:DeltaGlider
  STATUS Landed Earth
  POS -80.6825940 28.5969240
  HEADING 329.94
  RCSMODE 0
  AFCMODE 7
  PRPLEVEL 0:1.000000 1:1.000000
  NAVFREQ 402 94 0 0
  XPDR 0
  GEAR 1 1.0000
  AAP 0:0 0:0 0:0
END
END_SHIPS

BEGIN_ExtMFD
END

BEGIN_VistaBoost
END

 

[Grover]

well, if you entered mercury orbit from a sling from jupiter, you will have had trouble anyway. escape velocity from jupiter is high enough, but you then gained K inetic energy as you lost gravitational potential energy relative to the sun, if you came straight from earth it would have been easier to insert

there is another trick that i use tomake direct moon-earth re-entry a little easier

there's a "Sweet spot" to burn at, where you will slow down, but not adjust PeA of a parabolic approach, so you can make your insertion easier (though PROBABLY less fuel efficient) by slowing down once youre at the edge of the SOI, making the insertion burn shorter, and the window of opportunity wider

the rules go that at Pe, this sweet spot is retrograde, so turning to about 120* (and 0* plane change) is usually about right for the moon-earth burn, you can find the Mercury one bu giving it just a little throttle, then yawing to perfect your attitude before opening the throttle entirely

or it may be possible to mathematically calculate it and put that in an MFD function

there are ways to make it easier, you just need to be a little bit creative (and have some background knowledge of basic orbital mechanics)

good luck with your second, more realistic attempt!

 

[agentgonzo]

Yes, getting the alignment of two slings is not an easy thing to be doing.

One thing that may help is the 'orbits to intercept' parameter in TransX. It allows you to say "I don't want to intercept the planet on this orbit of the sun, but I want to perform one (or two, three...) complete orbit of the sun and then intercept the planet the next time I cross it's orbit". This allows for more flexibility with slings and intercepts if it's not possible in the current orbit.

Slinging to mercury from Venus isn't easy. It requires a lot of DeltaV (without a sling from memory it's of the order of 10km/s from LEO) and Venus isn't that great at slinging due to its low mass (but this is compensated for by its high speed)

 

[blixel]

Yes, getting the alignment of two slings is not an easy thing to be doing.

One thing that may help is the 'orbits to intercept' parameter in TransX. It allows you to say "I don't want to intercept the planet on this orbit of the sun, but I want to perform one (or two, three...) complete orbit of the sun and then intercept the planet the next time I cross it's orbit". This allows for more flexibility with slings and intercepts if it's not possible in the current orbit.

Interesting. I've seen that variable but I didn't know what it was for. Going around the sun a couple of extra times doesn't sound very practical, but I'll have to play around with that just to see what I can come up with.

Slinging to mercury from Venus isn't easy. It requires a lot of DeltaV (without a sling from memory it's of the order of 10km/s from LEO) and Venus isn't that great at slinging due to its low mass (but this is compensated for by its high speed)

At this point I'm mainly interested in learning how to set up sling maneuvers. If Venus -> Mercury is impractical (even in simulator terms), then I'll focus on some other sling maneuver that makes more sense. Perhaps a simple sling around Venus back to Earth would be more reasonable? I'm not sure how all the physics play out on this kind of thing.

Hmm.... I just thought of that awesome video Tex posted yesterday.

He seems to take the Arrow Freighter off from the moon, sling around Earth, and head out to Jupiter. Maybe Moon -> Earth -> other planet is a good starting point for learning how to set up multiple stages.

 

[agentgonzo]

At this point I'm mainly interested in learning how to set up sling maneuvers. If Venus -> Mercury is impractical (even in simulator terms), then I'll focus on some other sling maneuver that makes more sense.

An easier multiple-sling target may be the Voyager 2 trajectory that did Jupiter-Saturn-Uranus-Neptune. Their greater mass make the slings a lot easier to target (though you will have to set the sim date back to the 70s. There are a reason that probes launched to Mercury do multiple slings (MESSENGER did one of earth and two of Venus before getting to Mercury).

 

[Grover]

basically, choose a good date to start at, they need to be in the right places. the Voyagers got this far because they were launched at a good time, and all 4 planets were in the right place at the right time, this is both why they were launched at first, and why their mission was extended once they got past Saturn

 

[blixel]

basically, choose a good date to start at, they need to be in the right places.

I do understand that there is a delicate balance with the timing ... when you sling past one planet, the next planet has to be at the right place and at the right time, otherwise you're just going to orbit the sun indefinitely.

So I guess the question becomes ... is there a utility to calculate the launch date? If I want to keep it relatively basic, and just do 1 sling past Jupiter on my way to Saturn, is there some spreadsheet, MFD, or other application that will let me know when Saturn is in the right alignment with Jupiter to make such a sling possible?

 

[Grover]

you could always just set the date to Voyager's launch date: 5th September '77, at least we know that that date works

 

[blixel]

you could always just set the date to Voyager's launch date: 5th September '77, at least we know that that date works

True enough.

 

[agentgonzo]

So I guess the question becomes ... is there a utility to calculate the launch date? If I want to keep it relatively basic, and just do 1 sling past Jupiter on my way to Saturn, is there some spreadsheet, MFD, or other application that will let me know when Saturn is in the right alignment with Jupiter to make such a sling possible?

What I've done in the past is to load up Celestia and get a 'downwards' view on the solar system and use that to see where the planets are at a certain time. You can 'get a feel' of how fast your vessel would move whilst slinging and imagine it flying on each leg. It's not exactly accurate and does require 'having a feel' of orbital speeds etc, but it's a quick and dirty way of scanning for future launch windows.

 

[blixel]

Very good call on using the Voyager date!

I created a Jupiter to Saturn slingshot and was able to figure out the new TransX screens that the slingshot stage opens up. Unfortunately I didn't get to finish the journey due to a crash. (And I didn't save. Grr...) But I got to Jupiter, making several small course corrections along the way to keep everything lined up, and I made the slingshot around Jupiter and headed off toward Saturn.

The key, from what I could tell, was to watch the slingshot trajectory TransX screen as you are warping ahead. As you see the green line (the actual path) start to drift off the yellow dashed line (the desired path), then you need to come out of time warp and do a maneuver to line things up again. I did that several times on the way to Jupiter.

I'm sure I would have made it all the way to Saturn because once the slingshot stage was complete, the next leg of the journey is just like a regular trip to any planet. (Earth to Mars for example.) I was well on my way to Saturn, having made a couple of mid-course corrections, when suddenly I got a CTD.

One thing I noticed was that I had to make a rather huge plane change after pulling out of Jupiter's gravity well. (Huge as in 1.3k+ dV.) I don't know if that's normal, or if I messed up my approach/departure inclination. Hmmm.... can you get a free plane change? I'm not sure how the physics work in that regard.

For the benefit of others, I'd like to post a few details about what I did ... in case someone runs into this thread in the future, but before I can do that, I'll need to start the scenario over and take some notes.

Really cool stuff!

 

[garyw]

What I've done in the past is to load up Celestia and get a 'downwards' view on the solar system and use that to see where the planets are at a certain time. You can 'get a feel' of how fast your vessel would move whilst slinging and imagine it flying on each leg. It's not exactly accurate and does require 'having a feel' of orbital speeds etc, but it's a quick and dirty way of scanning for future launch windows.

This is exactly what I do as well. nothing quite like that view of the solar system to work out positioning of planets and to get your head around launch windows.

 

[Tex]

So I guess the question becomes ... is there a utility to calculate the launch date?

Thanks to Artlav's [ame="http://www.orbithangar.com/searchid.php?ID=4864"]Videnie addon[/ame] you can view all the planets positions and orbits inside Orbiter very easily. In addition, there are some web sources to help you determine the position of planets and launch windows. Try these in conjunction with Videnie which I highly recommend anyway:

http://www.fourmilab.ch/cgi-bin/Solar/action?sys=-Sf
http://clowder.net/hop/railroad/sched.html

 

[blixel]

Try these in conjunction with Videnie which I highly recommend anyway:

Thanks, I'll check out those links you provided.

I have the Videnie module, but for some reason it doesn't work. The text file says to press ALT+I to enable it, but nothing happens when I do that. (And, of course, I have ensured that it is enabled in the Modules Orbiter Launchpad.) Maybe it's not compatible with the D3D9Client?

 

[Tex]

The key, from what I could tell, was to watch the slingshot trajectory TransX screen as you are warping ahead. As you see the green line (the actual path) start to drift off the yellow dashed line (the desired path), then you need to come out of time warp and do a maneuver to line things up again. I did that several times on the way to Jupiter.

You actually want to watch the 'Slingshot' view in stage 3 because the other view in stage 3 is only showing you a top down view of your sling. That means your course may appear to overlap the dotted line however since you can only see the top down view you are not getting all the information you need. Hitting view to get to the 'slingshot' view will display 'R Inc' and 'Pe Ratio'. This allows you to setup a real tight sling. The goal is to setup the sling so that R Inc is on 0 or as closely as possible and Pe Ratio is on 1.000.

Additionally, when setting up a sling, one of the most important numbers you want to monitor is 'Pe/Pl Rad'. That determines the distance you will pass the previous planet in the sling. So for example, your first target is Jupiter for a sling out to Saturn. After you get your plan setup to Jupiter and you click forward to setup the sling out to Saturn in stage 3, you will see the figure 'Pe/Pl Rad'. You want to ensure that stays above 1.0 or your sling will cause you to hit Jupiter.

Also, I think you saw it, but in case you didn't flytandem walks you through all of this here:

EDIT - One more thing.. You should only do a mid course correction when you are about half-way there. As I understand it, you cycle through all your variables when setting up an mcc to keep the entire sling tight.

 

[blixel]

You actually want to watch the 'Slingshot' view in stage 3...

Ah, I see. Thanks for all the great information. I definitely understand what you're saying about the top-down view only giving you part of the picture. As I was making the journey, I knew I didn't have everything worked out exactly right. I'm learning as I go. A week ago I still couldn't get beyond the moon, so I'm picking up a lot of new information here. I'm finally starting to feel comfortable with TransX.

Also, I think you saw it, but in case you didn't flytandem walks you through all of this here:

Great video. Thanks for posting that. I now have enough experience with TransX to be able to follow what he's doing. I'm definitely going to save that link.