Flight Question Sling Planning in IMFD?

TMac3000

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I know this is one of those things you aren't really supposed to be able to do...but I think I might be close to cracking it.


I started by using a combination of Transfer MFD and the scenario editor to find the Jupiter-Saturn window. Then I entered that information into IMFD on the right screen, and set the current date back to the present.


Then I open a separate IMFD on the left, and set up my Earth-Jupiter leg. I set the arrival date to the same as the departure date on the Jupiter-Saturn leg, and adjusted the Earth depart date to get the lowest oV possible.



THEN I change the Saturn arrival date to get the oV on that leg to match the iV on the Earth-Jupiter leg, and go.


So I'm monitoring my outbound trajectory on IMFD Map as I head out to Jupiter, and I see it is predicting that I will shoot past Jupiter and out to Saturn's orbit! Success?:please: My heart was pounding. I've always wanted to do a slingshot.


Nope :( When I entered Jupiter's SOI and started setting up Planet Approach, Plan on the Map screen said I was going to wind up about 2 AUs BEHIND Saturn pretty much no matter what I did.


Rats. Was my approach angle at Jupiter wrong? Did I leave at the wrong time?


I was amazed that I could even get out to Saturn orbit! I feel like I am SO close to nailing this once and for all? Can anyone give me a boost over the top?


PS: How you guys doin'?
 

Marijn

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I've only tried IMFD for slingshots on an earth-Venus-Mars trip. I am using data from NASA's trajectory browser for the planning. The UI doesn't show the Venus arrival MJD, but it's possible to extract that information from the page source. (it's down currently)

Then I set the Course Program up for a trajectory to Venus arriving on the MJD found. On arrival, I setup the Planet Approach program in Slingshot mode if I remember correctly which burns a small amount of fuel to proceed to Mars.

So I think the trick is to know the correct arrival MJD at Jupiter. Not sure if and how Transfer MFD would give you that.
 

TMac3000

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Not sure if and how Transfer MFD would give you that.

It gives it to you in seconds, under the TIn number. If you were using TransX, you would convert for the notation, divide this number by 86,400 and add the result to your current MJD to get your arrival date.


In IMFD, you enter the TIn number from Transfer MFD directly into the TIn field in IMFD, and it not only converts it for you, but recalculates based on that date.


Anyway, my current slingshot method is not quite working. Although, like I said, it did get me out to Saturn orbit, which surprised the hell out of me:)
 

Marijn

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It gives it to you in seconds, under the TIn number. If you were using TransX, you would convert for the notation, divide this number by 86,400 and add the result to your current MJD to get your arrival date.
If I am not mistaken, you also need to make sure you have the correct periapsis altitude. The slingshot will be different depending on how close you pass Jupiter is it not? And you are not mentioning it. What does TransX say about this and did you follow up upon it?
 

TMac3000

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If I am not mistaken, you also need to make sure you have the correct periapsis altitude. The slingshot will be different depending on how close you pass Jupiter is it not? And you are not mentioning it. What does TransX say about this and did you follow up upon it?

You can set the periapsis altitude in IMFD in the initial planning before you ever take off, by selecting MOD, then Offset Disabled, and use the + button to change this to Vel. Frame. Then you select Rad and set it to your desired periapsis.


I set mine to 70M (70,000 km), remembering that the Pl/Pe ratio had to be 1 in TransX, which I assume means you have to set it to the radius of the planet you are slinging from? (Jupiter in this case.)


I did set up the sling plan in both TransX and IMFD, but the one on TransX (which I assume is right) looked quite different then the IMFD. So I set up IMFD using the dates in TransX, but I couldn't find anything in those results that gave me a clue as to what I had done wrong the first time--like the EIn being zero (it wasn't) or the PeA being really close to one of the two planets (it was off by the usual amounts in both cases). I'm trying to figure out if there is some clue in there that would tell me how to plan this without TransX.


Now, I did notice one thing: in the revised IMFD plan based on the TransX plan, the phase angle between my Jupiter intercept and my Saturn intercept looked a lot like the phase angle between Jupiter and Saturn that I found in Transfer MFD at the very beginning of the planning stage. That interests me very much, and I intend to follow up on it when I get a chance. I'll let you know how it goes :)
 
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