To Gilly!! Maybe!!!!

[capi3101]

Hey guys.

Well it took some doing, but I was able to resupply Jeb's extended mission to Eve after a botched shot at Gilly. I'd still like to try to hit Gilly if I could, but at this point I'm not sure if it's even possible.

Here's the data I've got right now on the mission as it sits:

The mission "Eris 2" is currently in a circular polar orbit around Eve at an altitude of approximate 350,000 m. And by circular, I mean the apoapsis is at 359,000 and the periapsis is a little over 350,000. Descending node is -81.4 degrees.

The current orientation of the orbit is almost exactly perpendicular.

Interplanetary tug's got five LV-Ns for propulsion. Lander unit's got one Aerospike

Current mass is 98.89 tonnes. Liquid Fuel is at 4968/7920. Oxidizer is at 6073/9680. MonoProp is at 2916/3750.

So, based on this data, can I reach/orbit/land on Gilly and get back to Kerbin, or should I forget the notion of a Gilly landing? If I can hit Gilly, what are my phase and ejection angles going to look like?

[Colonel_Panic]

Hey guys.

Well it took some doing, but I was able to resupply Jeb's extended mission to Eve after a botched shot at Gilly. I'd still like to try to hit Gilly if I could, but at this point I'm not sure if it's even possible.

Here's the data I've got right now on the mission as it sits:

The mission "Eris 2" is currently in a circular polar orbit around Eve at an altitude of approximate 350,000 m. And by circular, I mean the apoapsis is at 359,000 and the periapsis is a little over 350,000. Descending node is -81.4 degrees.

The current orientation of the orbit is almost exactly perpendicular.

Interplanetary tug's got five LV-Ns for propulsion. Lander unit's got one Aerospike

Current mass is 98.89 tonnes. Liquid Fuel is at 4968/7920. Oxidizer is at 6073/9680. MonoProp is at 2916/3750.

So, based on this data, can I reach/orbit/land on Gilly and get back to Kerbin, or should I forget the notion of a Gilly landing? If I can hit Gilly, what are my phase and ejection angles going to look like?

Hi, I actually did something very similar to this just this morning in sending a map satellite from eve to gilly. I can't tell you much about your capacity given your supplied fuel stats, but I can tell you that you need less than 1000 or so delta v of fuel to reach gilly, and almost no thrust to weight ratio to land. Furthermore from gilly, it takes less than 500 delta v to return to kerbin using an eve gravity assist, so in theory, it will take only a tiny bit more fuel (between 400 and 700dv worth) to land on and return from gilly en-route to kerbin than it will take to go to kerbin directly.

Here's the method:

1) wait until gilly is in the right position around eve for a hohmann intercept from your current orbit. This will require a lot of guesswork and maneuver node experimentation. Set gilly as a target and pop maneuver nodes to prograde to get an orbital intercept and look to see where gilly will be when you get there... and if it's too far behind in orbit, wait another revolution around eve and try again. If you're close, fine-tune it by tinkering with your prograde and radial burns to bring your arc out and get an intercept with the right timing. expect around 800dv if you do it close to the middle of gilly's orbit.

2) fine tune your approach because gilly has a very small SOI, so you want to come in real close, but avoid hitting any mountains. Don't worry too much about inclination until this point. Inclination burns cost fuel, though you can adjust it almost for free on capture if you do it right as soon as you enter gilly's SOI. It doesn't need to be perfect, but the closer you can get it to 0 inclination around gilly, the better.

3) circularizing burn. Here's where you need some TWR since you have a very small burn window and will shoot past fast. I couldn't do it with my ion powered mapsat, but you can with your LVN most likely. it takes less than 200 dv IIRC.

4) landing and takeoff is straightforward. gilly has almost no gravity. Make sure your landing craft has a wide enough base to not float off the surface and tip over.

5) return is the hard part... because of gilly's eccentric orbit, you'll need to find a kerbin return window that is compatible with your position in gilly's orbit. You want to do it when gilly is near it's apoapsis and eve is near opposite from kerbol while kerbin is at the right phase angle all at the same time. You might need to skip a window or two waiting for this to happen. When it does, burn to gilly's retrograde to drop down to an eve orbit with a periapsis just above eve's atmosphere (~250dv). Hopefully you get lucky with inclination (might want to quicksave first). Then wait until you reach the periapsis before doing your ejection burn from eve to maximize the oberth effect and put you on an intercept with kerbin (~150dv) and the rest is just nailing your kerbin approach.

[capi3101]

Hmm...if I'm doing the math right, assuming I don't use any RCS fuel, I've got about 6400 m/s of delta-V remaining.

That's assuming one unit of liquid fuel plus one unit of oxidizer has a mass of 1/200 of a tonne and one unit of monoprop has a mass of 1/250 of a tonne, and 800 Isp from the LV-Ns.

(Thus I'm carrying 55.205 tonnes of fuel for the thrusters and 11.664 tonnes of RCS fuel - yes, I know I've got waaaaaay to much RCS, but it's saved my bacon more times than I can count)

I'm in Eve orbit. 1650 delta-V to get to Gilly, 210 to orbit, 35 to land, 35 to launch, 210 to get back to orbit, another 1650 to get back to Eve, 1310 to orbit Eve, and then 1030 to get back to Kerbin. 6130 total. Even with the fudge-up factors in, it looks like I've got enough delta-V to make an attempt. So that's out of the way.

Okay...so "wait until Gilly is in the right position around Eve for a hohmann intercept from your current orbit". Uh......the Interactive Illustrated Interplanetary Guide and Calculator won't do a transfer calculation from Eve to Gilly. So I'm left with the second part of my original question - what's the proper phase and ejection angles in this case?

126,123.27 meter sphere of influence...no wonder I screwed it up so badly the first time.

[Jod]

Umm... I could be wrong, but if you are around Gilly then it is probably easier to just leave Eve SOI and transfer while you are in Kerbol orbit, not to burn your way to the planet for an assist.

another 1650 to get back to Eve, 1310 to orbit Eve, and then 1030 to get back to Kerbin

Seem like counter-productive maneuvers to me.

Edited June 12, 2013 by Jod

[capi3101]

I mentioned "fudge-up factors"; the trip back to Eve and establishing the orbit is what I meant. That 1030 to get back to Kerbin, though, is probably not.

[Spaceisbeautifulul]

I think you have two options if your concerned for your Kerbals. (I did no math at all, so don't trust me)

Send them to Gilly, you should have enough fuel to get there. After that take another ship and send it to Gilly for them to dock together and one take the fuel, and go back home.

You could also abandon the mission to go to Gilly and send 'em straight back to Kerbin at the first window.

[Colonel_Panic]

Hmm...if I'm doing the math right, assuming I don't use any RCS fuel, I've got about 6400 m/s of delta-V remaining.

That's assuming one unit of liquid fuel plus one unit of oxidizer has a mass of 1/200 of a tonne and one unit of monoprop has a mass of 1/250 of a tonne, and 800 Isp from the LV-Ns.

(Thus I'm carrying 55.205 tonnes of fuel for the thrusters and 11.664 tonnes of RCS fuel - yes, I know I've got waaaaaay to much RCS, but it's saved my bacon more times than I can count)

I'm in Eve orbit. 1650 delta-V to get to Gilly, 210 to orbit, 35 to land, 35 to launch, 210 to get back to orbit, another 1650 to get back to Eve, 1310 to orbit Eve, and then 1030 to get back to Kerbin. 6130 total. Even with the fudge-up factors in, it looks like I've got enough delta-V to make an attempt. So that's out of the way.

Okay...so "wait until Gilly is in the right position around Eve for a hohmann intercept from your current orbit". Uh......the Interactive Illustrated Interplanetary Guide and Calculator won't do a transfer calculation from Eve to Gilly. So I'm left with the second part of my original question - what's the proper phase and ejection angles in this case?

126,123.27 meter sphere of influence...no wonder I screwed it up so badly the first time.

Do you have a picture of the vessel? your figures sound high unless you have almost no dry mass right now. That said if you have anywhere near that much delta v you can very easily do it.

You do NOT need to expend more than 250 delta v to get back to eve, and you are not circularizing at eve but rather just using it as a gravity assist to reach kerbin, thus dropping your *total required delta v* to reach kerbin from the surface of gilly to less than 400. It really is *that low*, so give or take a little on fudge factor you should be perfectly fine doing it that way. Furthermore reaching gilly shouldn't take that much delta v either unless you're intercepting at the absolute worst point in its orbit (and if that's an option, then intercepting at the absolute best point also is).

Another poster mentioned just burning to escape from gilly prograde, and then doing a hohmann transfer from kerbol orbit. If you have as much fuel as you say you do, you can do that too. I mentioned using the slingshot maneuver because it dramatically reduces the fuel necessary to bring your kerbol apoapsis up to kerbin intercept, so the difference in required fuel between landing on gilly and not landing on gilly is minimized.

Because you're in a polar orbit (and gilly is highly eccentric), the phase angle calculators do not apply, and it won't be a proper hohmann transfer. What you need to do is guesstimate when gilly and your ship will cross planes. Set it as a target, and you'll want to intercept at the ascending or descending node. Timing is critical because you're not in the same plane as your target. To time it, create a maneuver node opposite from the planned point of intercept to burn to prograde and bring your apoapsis up until it intersects with gilly's orbital line, and the intercept markers appear on the map. There will be a marker for your ship and a marker for gilly. If they're far apart (gilly's marker is still way back behind your intercept on its orbital path) you'll need to cancel the node and rotate around eve some more to let gilly get closer to your intercept point to try again. If the markers are close together, but you don't get capture, try adding some radial thrust from eve into the burn. This will push your orbital line out away from eve, thereby adding a tad more time before your intercept and allowing gilly to catch up. You just have to adjust your node's prograde/retrograde to keep your orbits intersecting so you can tinker with the burn until your intersect markers get close enough to get an encounter. Once you have an encounter, just do your burn at the node, and then set up a second node to fine-tune your approach and make sure you're still on target.

EDIT: I take back what I said about inclination change in my previous post. Because you're entering gilly SOI perpendicular to its plane, it is not possible to improve your inclination on capture. Instead, to minimize delta v of the inclination change before your ejection burn, try to land near gilly's equator and then takeoff into a 0 inclination orbit to the east.

Edited June 12, 2013 by Colonel_Panic

[foamyesque]

Eh, at Gilly's orbital altitude he could do an inclination change pretty cheap while he's circularizing.

[Colonel_Panic]

Eh, at Gilly's orbital altitude he could do an inclination change pretty cheap while he's circularizing.

that has nothing to do with gilly's orbital altitude but rather gilly's low gravity. It wouldn't be expensive per-se to change inclination at gilly's equatorial AN/DN, but because of the fact that you're injecting from perpendicular to gilly's pane, there's no possible way to save fuel by doing it earlier on capture. It's a simple case of "you can't get there from here." as any inclination change you make coming at gilly from above/below will still leave you inclined 90 degrees, just in a different direction relative to gilly's orbital path. The only way to shave inclination change cost off the basic change around gilly, is to do your landing at the equator and then take off at your desired inclination.

[capi3101]

Do you have a picture of the vessel? your figures sound high unless you have almost no dry mass right now.

Just got done with a resupply mission; that's why the numbers are as high as they are.

Don't have time to respond to the rest of everybody's posts right this second; will do when I get an opportunity.

Edited June 12, 2013 by capi3101

[Mr Shifty]

The only way to shave inclination change cost off the basic change around gilly, is to do your landing at the equator and then take off at your desired inclination.

How does this work? Don't you have to match Gilly's rotation to land?

[Colonel_Panic]

How does this work? Don't you have to match Gilly's rotation to land?

Simply put the act of changing inclination is removing velocity in one direction and adding it in another. Comparatively, landing is removing it, and taking off is adding it. If you accomplish both int he same maneuver, you don't have to do it twice.

gilly's rotation isn't a huge factor as far as I'm aware.

[Mr Shifty]

Simply put the act of changing inclination is removing velocity in one direction and adding it in another. Comparatively, landing is removing it, and taking off is adding it. If you accomplish both int he same maneuver, you don't have to do it twice.

gilly's rotation isn't a huge factor as far as I'm aware.

But here's a hypothetical. Suppose you're in a polar orbit with an orbital velocity of 1000 m/s around a moon that is not rotating at all:

- In order to change your inclination to 1000 m/s eastward, you need to burn 1,414m/s directly south-eastward while ascending past the equator.

- In order to land you need to burn 1000 m/s south to kill your orbital velocity. Then to return to a 1000 m/s eastward orbit, you need to burn 1,000 east, for a total of 2,000 m/s.

I don't see how you can save delta-V for an inclination change by landing.

[capi3101]

Okay...so overall it sounds like a pretty standard planet-moon transfer aside from the difficulties associated with Gilly's inclination to Eve, the fact I'm in a polar orbit, and its tiny SOI. Clarifying: I need to make sure the AN/DN is at the intercept point?

Landing the whole ship at the equator is going to be problematic - there's two pieces to it, a lander and an interplanetary booster (a "tug" if you prefer). The lander is supposed to undock, do its business and come back. I could have the lander take off on an equatorial orbit around Gilly, but I'd probably have to change the tug's inclination anyway. I might be able to pull that off with RCS only - Lord knows I've got enough monoprop to at least try and the lander's got enough RCS for the return trip if push comes to shove.

I'll make the attempt at Gilly either tonight or tomorrow night; I'll let y'all know what happens. Thanks for the pointers.

[capi3101]

Success!!! Took a couple of tries to get the intercept; that was really the tough part. Quicksave rules...

Wow. When they say Gilly's gravity is low, they mean it's low. Like practically non-existant, that low. Tug's in a 10k orbit only going 20 m/s. Once I had established an orbit, I was able to handle the rest of the maneuvers and the entire landing on RCS and torque thrusters only. Hell, I used RCS to increase my rate of descent once it was vertical. Gave the aerospike a quick tap at the end to slow down to a safe landing velocity. Easiest landing I've done so far.

Moving around has proved interesting. You move like half a kilometer on each step. Jetpacking is essentially the same as it is in space. I might be able to get the "land navigation arrow" simply by walking around a bit. I've got time; it'll be a while before the next Kerbin return window comes up.

Next challenge: fix the damn ladder on the lander...

I suppose getting back to Kerbin might be the next challenge. Fortunately, I still have most of my fuel reserve for that. Here's my last status screenshot:

Math says I've got 4,967.8 m/s of delta-V remaining not accounting for available RCS.

Edited June 13, 2013 by capi3101

[Colonel_Panic]

But here's a hypothetical. Suppose you're in a polar orbit with an orbital velocity of 1000 m/s around a moon that is not rotating at all:

- In order to change your inclination to 1000 m/s eastward, you need to burn 1,414m/s directly south-eastward while ascending past the equator.

- In order to land you need to burn 1000 m/s south to kill your orbital velocity. Then to return to a 1000 m/s eastward orbit, you need to burn 1,000 east, for a total of 2,000 m/s.

I don't see how you can save delta-V for an inclination change by landing.

Simple, because if you need to land anyway, you're going to be burning that 2000 m/s either way, so any inclination change you do beforehand is just wasted fuel that you need to add on to it.

Success!!! Took a couple of tries to get the intercept; that was really the tough part. Quicksave rules...

Wow. When they say Gilly's gravity is low, they mean it's low. Like practically non-existant, that low. Tug's in a 10k orbit only going 20 m/s. Once I had established an orbit, I was able to handle the rest of the maneuvers and the entire landing on RCS and torque thrusters only. Hell, I used RCS to increase my rate of descent once it was vertical. Gave the aerospike a quick tap at the end to slow down to a safe landing velocity. Easiest landing I've done so far.

Moving around has proved interesting. You move like half a kilometer on each step. Jetpacking is essentially the same as it is in space. I might be able to get the "land navigation arrow" simply by walking around a bit. I've got time; it'll be a while before the next Kerbin return window comes up.

Next challenge: fix the damn ladder on the lander...

I suppose getting back to Kerbin might be the next challenge. Fortunately, I still have most of my fuel reserve for that. Here's my last status screenshot:

Math says I've got 4,967.8 m/s of delta-V remaining not accounting for available RCS.

I forgot just how slow the gilly orbit was, seems an inclination change there is free anyway.

You'll get home no problem.

easiest method: just burn to Eve escape from gilly's prograde, then wait for a hohmann transfer window to get home.

free method: wait for the right moment to do that eve slingshot maneuver I talked about, and use less than 1/10 your remaining fuel.

Okay...so overall it sounds like a pretty standard planet-moon transfer aside from the difficulties associated with Gilly's inclination to Eve, the fact I'm in a polar orbit, and its tiny SOI. Clarifying: I need to make sure the AN/DN is at the intercept point?

you probably already found this out, but for anyone else's benefit reading this thread, if you're in a near-perpendicular plane to your target and want to intercept, the AN and DN are the only places your planes intersect, and therefore your only option for a successful transfer without burning a ton of fuel is to burn at one intersect and then intercept at the other.

Edited June 13, 2013 by Colonel_Panic

[Mr Shifty]

Simple, because if you need to land anyway, you're going to be burning that 2000 m/s either way, so any inclination change you do beforehand is just wasted fuel that you need to add on to it.

Good point. I must not have had my coffee yet yesterday.

[Valentina Tereshkova]

I hadn't realized just how difficult it would be to make it to Gilly orbit. I didn't look up the extreme inclination and eccentricity first. Well, heck, it's harder if you don't know what to expect.

The mission was to survey both Eve and Gilly in one mission. I had originally planned two separate sats, one for each, but it wound up being more fuel efficient to keep everything attached all the way to Gilly. Fortunately I brought a lot of fuel with me, and the payload wasn't that large.

Anyone who thinks that it's trivial to rendezvous and land using MechJeb should try to orbit Gilly. Hint: MechJeb doesn't help all that much.

Now if I can just map out the geography and kethane deposits on Gilly, I'll have a refueling point in the inner system. Eventually.

[Kenobi McCormick]

Anyone who thinks that it's trivial to rendezvous and land using MechJeb should try to orbit Gilly. Hint: MechJeb doesn't help all that much.

If you give Jeb the correct orders Jeb makes a Gilly rendezvous look like a simple Mun landing.