IMFD Jupiter to Europa IMFD transfer question

[Grover]

as i have been trying for a while now, i just made another attmept to get a UGCO arrow to Europa using IMFD, and ive had some sucess!

i managed to get into stable orbit around jupiter, and get a very accurate transfer to europa (wait for it)

as i got into europa's soi, i activated the "planet approach" guide and set a PEA of just under 200k, and an inc of 80* (i was aiming for a very northerly target, the UGCO base there) i noticed that my previous inc was the wrong way, and being picky, i decided to swap my orbital direction back to CCW (normal, inc between -90 and 90) it the autoburn began to work, but before i knew what had happened (not looking where i was going) id hit europa at 8Km/s.

now, im 100% confident that i set the PeA to 200k, so why did it aim for the centre of the planet? i also turned on the autoburn as the gravitational influence of europa got to 0.47 according to the stock orbit MFD.

as far as i know, its bad practice to tune an approach outside the SOI of a body, but should i bend this rule for jupiter's moons due to the high gravity of jupiter? or did i just do something wrong?

thanks guys
-=Grover=-

 

[jambooger]

I always attempt Planet Approach burn at 1.5X the planets SOI. I also ck and burn several times during approach. I hear its ok up to 2x the planets SOI.

 

[Grover]

ahh, so i WAS wrong... big suprise there eh :rofl:

ill give it a go later on, hopefully ill finally manage to land on europa!

thanks
-=Grover=-

 

[Tommy]

i noticed that my previous inc was the wrong way, and being picky, i decided to swap my orbital direction back to CCW (normal, inc between -90 and 90)

I'm not positive that I understand this, I'm assuming you mean you were initially approaching Europa for a retrograde orbit? (this would be "clockwise" in IMFD's map).

For most moons, a retrograde orbit is fine. They are "tide-locked", so have a rather long rotational period. This means there is little advantage to a prograde orbit over a retrograde orbit. This is even more the case with high inclinations, so I would have just kept the retrograde insert, aiming for Inc of 100 degrees.

Did you hit Europa while you were still in the approach burn? If so, then it's clear that you hit Europa because your trajectory hadn't changed all the way to the prograde approach. Remember, when you switch from a retrograde approach to a prograde approach, your trajectory will have to switch from one "side" of the target to the other - meaning that it will pass through the target during the change.

Planet Approach can be used while still outside a target's SOI, typically at up to 3 times the SOI. However, Jupiter's immense gravity means it's a bit better to wait until closer to the target's SOI - it can be rather inaccurate if performed too soon. It's also quite common to make more than one approach burn. The first would be made well outside the target's SOI, because the angle of deflection get's higher the closer you are to the target. Second, even third, burns are made as you reach the target's SOI - and these will be more accurate.

As you've noticed, Jupiter's immense gravity complicates things when approaching one of it's moons because the moon's SOI is so small. Often, by the time you are close enough for Planet Approach to work, you are too close - and your PeT may be less than the burn time required!

I recommend using Target Offsetting to tune your approach to Europa right from the start. Use IMFD's map, with the CNT set to "p-europa", and have Int, Disp, and Plan enabled. Use a Lon of 180 if you want to approach for a prograde insert, or a Lon of zero for a retrograde insert. Then increase the Rad (you can use the target's radius for a starting point, but expect the final Rad to be quite different) until Map shows your planned trajectory to have the PeA you want. You can even try increasing or decreasing the Lat to adjust your inclination. Get the Rad set first, then alternate between adjusting the Lat (to adjust the inclination, but will also change your PeA, and adjusting the Rad (to correct the PeA, but will also change the Inc.

If you set the target base as the "Landing Target" on IMFD's main config page, you can even target an approach that will take you directly over the target base (you may have to hit the <MOD> button on Map a couple times until the target base is shown, and an Ang value is shown. The lower the Ang, the closer your trajectory will be to passing directly over the target. You won't always be able to get the Ang to zero this way, but it can hep lower the base alignment burn.

I'm getting closer to finishing my part of the update to IMFD Full Manual, which explains Offsetting much more thoroughly, and includes an example of a lunar transfer that targets an insertion that passes directly over a target base. Think I can have it ready by Christmas.

 

[jambooger]

Start the trip at a very high Jupiter orbit, that helps due to Jupiters high gravitational influence. Make sure you are setting the Ref to Europa when using Planet Approach and Orbit Insert. I made that mistake in the past.

 

[Tommy]

Start the trip at a very high Jupiter orbit, that helps due to Jupiters high gravitational influence

This will also give you a rather high RVel at the rendezvous, which can make the orbit insert problematic (insert burn may be much longer that the time you'll be in Europa's SOI.)

If you start with a Jupiter orbit that is close to Europa's altitude, that will make the RVel low and leave you with a much easier approach and insert - but will likely make the sync take quite a bit longer.

The best way is to target Europa directly, and never bother with a Jupiter insertion - but that's a tricky manuever I don't think I can teach in a forum thread. It will be covered in the update (actually a direct Titan intercept - but the same principle applies). With a Europa direct intercept, you'll need to "fudge" the initial trajectory (set it slightly outside Europa's alt, and have your arrival a bit "early" to account for the fact that you'll begin the Europa inset burn well before you actually reach Europa. Unless you are a math wiz, that will take some trial and error!

 

[Grover]

well i had started at an orbit alt of 70-100M below europa, and the speed wasnt too huge, i guess i just didnt leave enough time to calibrate my approach for a normal inclination

thanks guys
-=Grover=-