Advanced Question Plane Issue w/Slings - TransX - Cassini Mission

[RSWingman]

How to distill this down:

I'm attempting to retrace the trajectory of Cassini. I've even downloaded a scenario saved in well-organized parts. My greatest trouble comes with the most critical sling of all - the final Venus encounter enroute to Earth.

Setup (using TransX): I've just lost the cruise stage from the DSM and the Venus Encounter stage is now Stage 1 ("cruise to Earth" is 2, "Earth Encounter" is 3, "Cruise to Jupiter" is 4... getting ahead of myself now).

On stage 3 (Earth Encounter), I've got the green line going directly through Earth. This obviously means that stage 2 (cruise) is right on. And in stage 1 (Venus Encounter), I've perfectly matched the dashed yellow line at around Pe-6.5M. Rinc is nearly 0. That's how right-on I am. Aren't I?

I've got my quicksave at this point so that I can keep picking-up from there. But every time, after the sling and right after I lose the Encounter stage, I am revealed to be several G off course (an amount that requires several K of dv correction - way unacceptable). But I was RIGHT ON; this doesn't make sense and it's driving me nuts. :compbash2:

I've seen successful stages (performed by others) where the plane was maybe 30 degrees off and was apparently disregarded; but the coming maneuver worked out fine. This is a contradiction I can't wrap my brain around (though I'm beginning to think that it may have to do with keeping on Earth's plane - I'll be experimenting more with that theory). :hmm:

I know my problem is at least plane related as I've begun to notice upon loading the quicksave (just by eyeballing an imaginary line from Earth, through Venus, to myself, that I am off plane. I'm seeing how Venus is throwing me off plane; and I've been using hover to correct this by eye, pre-sling. My result after losing Encounter is better but not enough and this can't be the right way.

Is my Rinc not supposed to be 0 approaching Venus? How do I determine what approach plane will produce the right release angle to Earth? What procedure am I missing in the approach to Venus? How is the appropriate plane determined in any given situation?

 

[atuhalpa]

Flag this question for flytandem. He is the master of transx slings.

 

[RSWingman]

Please forgive me - I'm a noob here. How do you mean flag? Use the "contact us link" at flytandem.com? The main site appears to have nothing to do with Orbiter and there even appears to be no way to navigate from there, to flytandem/orbiter. And there is no "contact us" link at the flytandem/orbiter site. So I want to be sure of what you're telling me to do before contacting someone out of the blue.

Or are you saying that this person has an account here and to contact him there? If there is some kind of "flag" feature, I'm not yet familiar with it.

 

[2552]

Just an idea, but if you have Manouvre mode on, try turning it off. I left it on once while flying the Galileo trajectory to Jupiter and seemed to be way off. Turning off Manouvre mode fixed it.

 

[RSWingman]

Right - good call, but that's happened to me before and I know well now, to turn off Maneuver mode after using it. As long as it's on, what is shown is only hypothetical and not live.

Though, the Venus Encounter view does exhibit similar behavior until it drops and I get a real update on my situation.

I'll have to look into that again. But I think I've checked and the Encounter view doesn't seem to allow you to turn off Maneuver mode on THAT stage.

 

[atuhalpa]

RSWingman, all I meant was to either send him a private message or put his name in the title box like " flytandem, please read this post!" something like that. He is amazing at transx trajectories and he really likes to help other orbiters out. He has a few tutorials that are well worth working through.

 

[RSWingman]

Ok. It's the middle stage where Maneuver can't be turned off (Sling Direct). There's no Maneuver mode to speak of unless the entire stage is one. I've made sure that it is turned off in both Encounter stages. I've got the second Encounter stage saying I'm going right through Earth. But it's after I complete the Venus sling, and the first Encounter stage drops, that I find I'm several G off course. I'm just stumped & overwealmed.

 

[flytandem]

Post your scenario text, I'll see if I can find out what's happening.


I did a cassini mission a few years ago, scenario pack is near the bottom of this page
http://www.flytandem.com/orbiter/scenarios/
and it uses TransX 3.10 I think which there is a copy available at the top of the page.
I remember the final Venus Earth sling did a slalom like a snow skier going around gates.

 

[RSWingman]

Yup, the last pass is to the left, so that it throws out. How do you mean, "scenario text"?

Found out I had TransX 3.0. Downloaded the latest. Played with it.

I must be missing some fundamental understanding of how to do this:

Where both Encounter stages appear to be "live" and show that I'm right on, Sling-Direct seems to be hypothetical. When I make a change there, I'm not sure of the right way to affect that change. But any change I do make (like thrusting in a given direction) throws everything else off. It's impossible to get all three stages to agree with each other, so I don't know what to listen to, what to ignore and when.

 

[tblaxland]

How do you mean, "scenario text"?

Do a quick save of the scenario whilst you have the TransX plan open in one of the MFDs. Use Notepad to open the scenario you just saved (it will be in the Orbiter\Scenarios\Quicksave folder). Select all the text and paste it in a post, preferably putting

  tags around it, so it looks like:
[code];scenario text here

 

[RSWingman]

I begun this playthrough (before re-quicksaving) upon eject from Earth.
Since I installed the latest TransX, I had to reprogram Sling-Direct (Stage 2). While I was at it, I corrected some of my obvious plane error with some hover by eyeballing it.

Now, both stages 2 & 3 are right on. But if I make any change (trying to match the yellow dashed line by burning or ), that will throw it off and I'll have to recalibrate Sling-Direct all over again. It's a vicious circle.

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[flytandem]

When you open your scenario, in stage 1, click VW as needed to get the slingshot view. You'll see a circle with a yellow dashed radius line and a solid green radius line. The dashed line is what you should have had your green line laying on top of with exactly the same length in the final days and weeks coming up to the sling.
Yours is actually off by quite a lot both in direction and length. The relative inclination is off by 7.572 degrees. I usually shoot for within a half a degree at most and often I'm within 3 or 4 thousandths of a degree. And Pe Ratio (the length) is quite critical. Your Pe ratio is 0.8711 or about 13% shorter than the planned Pe. Best to try to keep it within 1% of the proper length. I usually also try to hit it within afew 10ths of a percent in length. This will be your sling distance and very much provides the strength of the sling. So from your planned sling you will have a much stronger sling off Venus.

What's interesting is the date of arrival at Venus and the date of arrival at Earth. As well as the velocity. When I ran the Cassini I could have been off by as much as a day on the inner slings I really can't remember, but you are off by 3 weeks on this Venus sling. You are July 15 instead of June 24. And my velocity at the sling Pe was something like 2K m/s faster than yours. So you are late and slow which is why your flight as it's going right now will not have the energy to sling Earth to Jupiter. At best you will make it to an Ap about half way from Earth to Jupiter.

Did you do the deep space maneuver between the two Venus slings?

I suppose being at the correct dates only really matters if you are trying to replicate an historic mission. To make one's own mission is fun and has huge value by itself. But best to try to nail the sling as it's planned or you'll find as you did, that huge corrections will be needed after the sling.

---------- Post added at 05:49 AM ---------- Previous post was Yesterday at 11:21 PM ----------

Overnight I thought of additional tricks about getting the sling into the right slot.

The most accurate time to know what the sling will be seems to be, for most planets, just before the stage switches. Immediately after the switch it often is quite inaccurate but comes back into place as one approaches the Pe. Especially true of Jupiter.

Earth slings are best checked for being in the slot at exactly one moon orbit away from Pe. Pull up Orbit, ref earth, target moon and check the moon's period then in TransX as you approach the time to Pe as matching this value, slow to real time and do a small maneuver to nail the sling. Actually what I do at this time isn't a proper "maneuver" with setting up velocities and target to aim at etc. I rotate the ship nose prograde using HUD, ref earth, and with a bit of LIN manually slide sideways and up/down (not prograde) to bring the Pe into place. I don't want to change the velocity of the sling. Usually less than .5 m/s is all one needs this far away from the Pe. Do a save and then warp to the Pe to see how it fared. If it's off, look at how much and what direction in the slingshot view and either accept it and continue, or restart from the last save and again with a bit of LIN offset the sling to account for the discrepancy. Perhaps like knowing a gunsight is off so aiming a bit offset to compensate.

SUMMARY, roughly get the sling into place for the sling when at least several months away, 1 year for outerplanets. Then make the sling perfect just before the stage switches, or 1 moon period ahead at Earth. YMMV.

 

[RSWingman]

Yes, my arrival time is quite off in this case and I don't care as much anymore if I get to Jupiter, as much as figuring out my own "missing-link". The tedium of making sure to arrive at proper dates loses it's appeal after so many failed attempts, knowing I'm missing a critical procedure. I figured I would spend some extra fuel this time in the name of expediency. Later, I could really clean up my act with limited fuel. Translation: I'm frustrated & getting sloppy.

Sure I did the DSM; but I'm really not sure what I'm shooting for other than a date & a close pass. Is that all the accuracy I need at that stage? I don't need Earth in my sights until after the DSM (and maybe even most of the cruise), right?

I'm also not sure what I'm shooting for on my first Venus sling other than a date & a close pass. There is no "Target" (place to be) at the planned DSM, so I'm shooting quite blind. My procedure has been to pass as close as I can for maximum sling and work out the right Venus 2 encounter at DSM. The bigger the ellipse, the better, right? Or does it only matter that I select the trajectory that returns me most naturally to Venus at the proper date using the least Dv?

I would also like to develop a sense of how to do some more Freestyle slings. I know NASA's already got this one all figured out, but if they waited for a perfect scenario, it would never have even happened, right? To some degree, they've got to work with what they're given, right? Should it be plausible to disregard precise dates for a sense of adventure? But yeah - getting to Jupiter seems pretty critical on this one. Mars doesn't appear to be anywhere close for an assist.

I tried what you said and I think that did it. I think I was a hundred or so k off. It looked like a really underhanded throw and I was sure it was a fail, but it turned-out sweet. I achieved this alignment quite easily with some Hover & Manual Thrust. That might be the piece I was looking for. I'll have to try it over when I get a chance. Busy day. Until then, thanks so much.

---------- Post added at 04:37 AM ---------- Previous post was Yesterday at 07:55 AM ----------

Yeah, that seems to have fixed it. I played through from the DSM and hit every encounter to the day. For some reason, when I arrived at Venus 2, the prescribed Pe was a little too low; so I had to go a little wide and spend over 1000k dv between correcting that & staying on schedule. But at least I know where it went.

Got to Jupiter. Been to Saturn and also, the encounter velocity looked really tame (I'm sure they planned it that way). Looked around for some place else that would take advantage Jupiter's full potential. Surprisingly, I couldn't seem to find anything deeper in a favorable position.

In case anyone wants to take a crack at it (and I do have a crapload of extra planetoids/comets/asteroids...):

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Thanks for all the patience & help.

 

[atuhalpa]

I have found that by judicious use of the "linear translation mode" I can nail the Rinc AND the Pe ratio perfectly on approach.