Flight Question Approaching a planet to get to a moon

[mbertelsen]

Specifically, I'm trying to figure out how to sequence maneuvers to approach Jupiter in order to end up orbiting Europa, in a "realistic" way. I'm using IMFD 5.3b, but would be happy to hear any guidance for other tools too. This is with the UCGO arrow in the provided scenario for Europa.

If I do things one at a time, I can get to where I'm approaching Jupiter at a low EqI and insert into a circular Jovian orbit fine. From there it's just a target intercept to europa, etc.

But can it be done directly, going directly from a hyperbolic Jovian orbit into a target intercept for Europa, and braking into Europan orbit? Or will I just be going too fast for that to work?

Any guidance on the right inclinations, altitudes, eccentricities, etc to make this work. I spent a lot of time burning up in Jupiter's atmosphere when I thought my peA was high enough.

Are there general principles that could apply to any similar approach?

Thanks!

 

[Ark]

What I've been doing is waiting until I've just crossed into Jupiter's SOI, then setting IMFD's reference to Jupiter and target to Europa. Play around with the values and you should be able to make a reasonable burn to intercept Europa without stopping in Jupiter orbit and aligning. Be wary, though, you'll still be carrying all of your interplanetary velocity, so the braking burn can be tricky. Also, with this method you'll have a pretty high inclination relative to Europa's equator.

 

[Adelanthal]

What I tend to do is, similar to Ark, once I hit Jupiter's SOI, switch TransX or IMFD to focus on Jupiter and plan an intercept with Europa. I also try to intercept Europa as it's orbiting "away" from me so I've got a couple less km/s to kill. But, again, remember that you will have been falling towards Jupiter for several months by the time you intercept and will be moving .... rather sprightly, as in 17-20 km/s. You'll end up starting your intercept burns before you enter Europa's SOI, as otherwise you'll whip in and out far too fast for you to slow down in time. Just as an FYI, the UCGO base is towards the northern pole.

 

[mbertelsen]

Yeah, I just can't get the last burn right.

I'm getting the flyby of Jupiter right, and ending up in the neighborhood of Europa at what looks like the right time, but orbit insert never works. Either it won't give me a burn vector at all or it burns me into some (apparently) random orbit around Jupiter.

Any tips? How close do I need to get to Europa? How far in advance do I plan the orbit insert and hit AB? Is IMFD just not going to do this for me?

 

[Ark]

I use the circularize function with my PeA pretty close to the surface. I take the predicted time of the burn, then watch my PeT and start when it's about half the burn time. You can tweak that for what works best, just remember that circularizing before perapsis tends to raise the PeA. It takes some experimentation to get an accurate, low-altitude orbit out of it, but that usually does the job.

 

[Piper]

But can it be done directly, going directly from a hyperbolic Jovian orbit into a target intercept for Europa, and braking into Europan orbit? Or will I just be going too fast for that to work?

Yes you can.

It's not technically the best way, that would be by putting yourself into an elliptical orbit around Jupiter, and then sling-shotting around the 4 Galilean moons to bleed off deltaV. However for Orbiter, it presents a huge problem for planning and performing (regardless of the MFD), and you end up waisting enormous amounts of fuel on course corrections.

Going straight from a hyperbolic orbit to a orbit around Europa is much simpler method, and it's the method I chose for my Io Orbiter mission, but there definitely is a right way, and a wrong way.

The wrong way is just targeting Europa, and intercepting it on any date.

Basically, you want to intercept Europa while it's on the outer edge of its orbit. If you use TransX, you want to select Europa as your target, and set it up so the encounter looks something like this:

The above image is for encountering Io, but it's the same concept. The key point, is to get the Pe Lat/Long as close to 0 as humanly possible. To do this, adjust your Outward (for the date of intercept) and Prograde (for altitude) values to get everything to line up. Also, check the inclination relative to Jupiter. If it is above 90 (ie, retrograde), you are intercepting on the wrong side of the orbit, it must be prograde to Jupiter (it does not matter if it is prograde or retrograde to Europa).

I don't know how to do the intercept in IMFD, but it's the same concept, get the intercept as closest to the outermost edge of Europa's orbit, and prograde relative to Jupiter.

---------- Post added at 01:56 ---------- Previous post was at 01:45 ----------

Yeah, I just can't get the last burn right.

I'm getting the flyby of Jupiter right, and ending up in the neighborhood of Europa at what looks like the right time, but orbit insert never works. Either it won't give me a burn vector at all or it burns me into some (apparently) random orbit around Jupiter.

Any tips? How close do I need to get to Europa? How far in advance do I plan the orbit insert and hit AB? Is IMFD just not going to do this for me?

If you try turning retrograde for Europa, but are too far away (you have to be very close to Europa for it to select that), you will turn retrograde to Jupiter instead, regardless of the target in the Orbit MFD. Instead, use the circularizing feature of IMFD instead, or line-up manually using the Orbit MFD with Europa as its target.

 

[tblaxland]

The key point, is to get the Pe Lat/Long as close to 0 as humanly possible.

What is the reference frame for those values in that screenshot? Juptier centred? Io centred? Ecliptic frame or equatorial frame? Just curious and trying to learn more about TransX...

 

[Piper]

The Major is Jupiter, and the Target is Io. It's also zoomed in to target.

 

[mbertelsen]

Hey thanks everybody, I finally made it!

The keys were making sure I burn to approach Jupiter prograde, not retrograde. Then once I have a good hyperbolic approach, in Target Intercept tell IMFD to delay Tin until Europa is in a good spot on the other side and moving generally in the same direction as my orbit rather than trying to intercept on the way in.

The final burn with orbit circularize worked like a charm once I figured out I needed to do a final approach burn first to get my Pe up off the planet's surface! :rofl: I haven't tried with orbit insert yet.

Anyways, cheers. If I ever have time (not soon), maybe I'll write my flight plan up as a tutorial.

 

[Tommy]

Even with Orbit Insert you will want to make a "final approach burn" using Planet Approach. Adjusting your Io Pe can also be done manually using IMFD's Map and mostly RCS thrusters long before Planet Appoach can be used, and the earlier this is done the more efficient it will be.

This is interesting, and I'm sure I'll be playing around with it a bit. I suspect that a more efficient approach will be to enter the jovian system with a low perijove, and performing a retro burn to attain capture and lower the Ap to Io's altitude. I think this will take advantage of the Oberth effect to make the initial capture much cheaper, leaving a much smaller dV needed to circularize at Io. The trick will be in the timing - Io should be reaching a point opposite your perijove when you reach it's altitude on the way back up. I'll have to figure out a good methodology to test this - probably save a flight a few months before entering Jupiter's SOI, performing the appropriate planet approach burn for each method, refilling the fuel tanks, etc, to eliminate as many extraneous variables as possible.

A "direct" approach would be better for Titan, since it has a sufficient atmoshere to allow aerobraking for both initial capture, and Titan capture.