Going to Gilly. Do I capture near Eve or far away from it?

[juanml82]

I've been thinking, now that aerocapture isn't really a thing, at least on Eve (well, I could give it a shot, but I'm not counting on it)

I'm going to refuel most of my Eve Fleet on Gilly, so here's the question:

If set my Pe very close to Eve and do my capture burn there, I take advantage of the Oberth effect and save fuel to capture. But I end up in a highly elliptical orbit, so my Gilly capture burn (once I have an intercept) is going to cost plenty of dV

If I set my Pe farther from Eve, the capture burn ought to be more expensive, as there is no Oberth effect. However, I will end in a near circular orbit which is probably close to Gilly's orbit (save for inclination, of course), so the capture burn at Gilly should be a lot cheaper.

Does anyone know which one is the best option?

Edited October 7, 2015 by juanml82

[Phil deCube]

I did a similar thing when arriving in the Jool system, with a base to land on Pol. I set the Pe to the same distance from Jool as Pol is and circularised there. It seemed to work very well. I didn't do a comparison with the other option though.

[Tatonf]

How about you try by yourself ?

Set your encounter with Eve, and on the planned orbit set an other maneuvre node, and write down the delta-v requirement for both option. Then choose the better.

But due to the high eccentricity of Gilly, the efficiency of your burn will depend of where they are.

The better option would be to place your periapsis where Gilly's one is, I think.

[FancyMouse]

Why aerocapture is not really a thing? I aerocapture on Eve a lot. You need to a careful design, though. A rocket-sized cargo bay from some mods helps a lot, but it's still doable with stock - you would need to point absolutely steady during entry and let the shock cone from your heat shield to protect your craft. Eve capture is still possible, unlike Jool where your heat shield itself would explode and that's what really not a thing.

[GoSlash27]

I've been thinking, now that aerocapture isn't really a thing, at least on Eve (well, I could give it a shot, but I'm not counting on it)

I'm going to refuel most of my Eve Fleet on Gilly, so here's the question:

If set my Pe very close to Eve and do my capture burn there, I take advantage of the Oberth effect and save fuel to capture. But I end up in a highly elliptical orbit, so my Gilly capture burn (once I have an intercept) is going to cost plenty of dV

If I set my Pe farther from Eve, the capture burn ought to be more expensive, as there is no Oberth effect. However, I will end in a near circular orbit which is probably close to Gilly's orbit (save for inclination, of course), so the capture burn at Gilly should be a lot cheaper.

Plus you've got to match Gilly's goofy inclination. It's a lot easier and cheaper to do that on a highly- elliptical orbit where your velocity is lower.

Does anyone know which one is the best option?

juanml,

I don't know for a fact that the low altitude retroburn would be "best", but it's definitely what I would do. If you're already in an elliptical orbit to intercept Gilly, the circularization burn at that end is cheap. Coming into Eve at interplanetary velocity, OTOH... it's expensive if you don't do it efficiently.

And FWIW aerocapture at Eve is still a thing. You've just got to be a little more careful.

Best,

-Slashy

Edited October 7, 2015 by GoSlash27

[Tatonf]

A little more careful ?

It's damn hard and quite random. The periapsis window between slowing enough and blowing up is quite small. I wouldn't recommand it.

Aerobreaking have become stupidly hard since 1.0.4. Kerbin have a real atmosphere so it's ok, but Duna apart, the other atmospheres are way too dense too quickly.

And the devs don't even plan to change that on 1.1.

[Phil deCube]

And the devs don't even plan to change that on 1.1.

Hmm... I think I read somewhere that they were making the atmospheres more linear as far as density changes go. I can't remember where I read that, and I could have dreamed it, but I'm reasonably sure that's the case.

[Kryxal]

If you capture with a low PE, your speed at AP is next to nothing ... but Gilly's speed isn't really all that high that far out anyway (274 m/s to 945 m/s depending on the point in the orbit). I'd say capture low, possibly with some minor aerobraking as well.

Also, this will make any needed plane-change maneuver fairly easy with a capture crossing the plane very low.

[FancyMouse]

A little more careful ?

It's damn hard and quite random. The periapsis window between slowing enough and blowing up is quite small.

Nope it's really not that hard. Design goal is simple - heat shield should be the only part exposed to air flow for a time period longer than a couple of seconds, and there are various design techniques to achieve this goal.

If this goal is not met, then sure it's going to blow up. But if it is met, then it can handle entry speed about up to 5.5km/s without a single dip of fuel to capture. It may even handle faster speed but I didn't need that and haven't tested faster speeds.

As a reference, periapsis is about 62km~70km based on the ship shape and mass and entry speed. It's certainly not something that can be achieved by a random ship.

[juanml82]

juanml,

I don't know for a fact that the low altitude retroburn would be "best", but it's definitely what I would do. If you're already in an elliptical orbit to intercept Gilly, the circularization burn at that end is cheap. Coming into Eve at interplanetary velocity, OTOH... it's expensive if you don't do it efficiently.

And FWIW aerocapture at Eve is still a thing. You've just got to be a little more careful.

Best,

-Slashy

If you capture with a low PE, your speed at AP is next to nothing ... but Gilly's speed isn't really all that high that far out anyway (274 m/s to 945 m/s depending on the point in the orbit). I'd say capture low, possibly with some minor aerobraking as well.

Also, this will make any needed plane-change maneuver fairly easy with a capture crossing the plane very low.

Nope it's really not that hard. Design goal is simple - heat shield should be the only part exposed to air flow for a time period longer than a couple of seconds, and there are various design techniques to achieve this goal.

If this goal is not met, then sure it's going to blow up. But if it is met, then it can handle entry speed about up to 5.5km/s without a single dip of fuel to capture. It may even handle faster speed but I didn't need that and haven't tested faster speeds.

As a reference, periapsis is about 62km~70km based on the ship shape and mass and entry speed. It's certainly not something that can be achieved by a random ship.

Alright, I'll go with a low Pe

As for aerocapture, I think it's likely on smallish ships. But once you make big things, you can't reliably keep everything behind a heat shield for the entire dip in the atmosphere, specially because the ship will turn. In a smaller probe, say a short 1.25m probe, you can put a 2.5m and you'll surely be covered by it - the risk is having the shield's skin overheat faster than ablator gets burnt

[FancyMouse]

OK since there's some interest in aerocapture, it's time to reveal some of my design tricks to keep heat shield front (and I'd be glad to hear if anyone has some more!).

Yes totally agree that the main challenge is to prevent it turn during the most dense part of the atmosphere. But it is solvable. The below list comes with priority based on my experience.

1. Make sure CoM is near the heat shield. Place heavy parts like fuel tanks as near as possible. Move all your fuel there as close as possible.

- there's even a small trick about choosing the tanks. If the same stage is doing transfer burn as well, then even if a single tank is sufficient, you would want to split to two tanks. Explanation: suppose heat shield is at bottom, and I have a fuel tank on top of it with half fuel. Using one fuel tank - CoM is at 1/2 tank height (ignoring shield mass); if we use two identical tanks, and move fuel down upon entry, then CoM is at 1/4 tank height, much closer to the shield! Small trick like that actually makes a huge difference from my testing.

2. Spam SAS. You really need them. My last resort would be plenty of RCS at the tail but so far SAS is enough for me.

3. Use rocket-size cargo bay (modded parts) or fairing (this is stock) to give a larger margin of attack of angle. As long as most of the area is covered by heat shield, fairing can handle a small radius increase without problem, giving you some flexibility of designing payload.

So far the biggest thing I sent there was a ~40t base (2.5m-sized mostly, with legs, panels etc.) covered by 3.5m heat shield placed at the bottom. It isn't huge but still not that smallish, I think.

[Lukaszenko]

Hmm... I think I read somewhere that they were making the atmospheres more linear as far as density changes go. I can't remember where I read that, and I could have dreamed it, but I'm reasonably sure that's the case.

http://forum.kerbalspaceprogram.com/threads/133786-Devnote-Tuesday-Turning-Up-The-Heat%21?p=2186139&highlight=Lukaszenko#post2186139