Establishing orbit around Eve

[milosh]

Hello all,

I can reach Eve without problems at a Periapsis of 1400km but once there I cannot established orbit, no matter what manouver I try I need around 4-5k of Dv/s to get any Ap at all and I only have around 2k left. My orbital speed is quite high at ~5600m/s so I wonder, is this always the case when you try to orbit Eve, do you need an insanly high amount of Dv/s left to get into orbit or am I doing something wrong? I use porkchop selection to rendezvous with Eve and some manual fine tuning on Kerbol (sun) Pe to get my rendezvous Pe down from 60'000km. I didn't choose the optimal launch window which would've cost me only 900 Dv/s to escape Kerbin because I didn't want to wait a year, instead I chose a window that cost me 1900 Dv/s but was only 7 days away. I tried aerobreaking through the athmosphere but the engine on my satellite is only useful in space with a TW/R of 0.19

Is this where my mistake lies, does the difference in Dv/s add up to that high orbital speed I seem to have to work against? I'm not that good at orbital math so I hope someone can help me out or clarify what's going on.

Edited October 12, 2015 by milosh

[FancyMouse]

Yup if you pay more to eject, you add more energy, then you have to pay more to slow down. I usually control my ejection dV within 1.2km/s so that my entry periapsis speed is about 5.2~5.5km/s upon touching atmosphere. Compare to your number, I don't see anything wrong.

Aerobraking is useless if you don't design for it (problem is not engine at all, it's heat), but if you do, you can still pay zero fuel for capturing.

[milosh]

Well the thing is, when I venture off to Duna I only need something over 1.5K of dV to orbit and shift orbit to polar, if anything I would've guessed that trying to orbit a planet on a lower orbit would use less dV. So in other words, there's no way to reach Eve and need only something around 1-2k dV to orbit?

[FancyMouse]

Probably you underestimated the gravity of Eve. In fact it's more about Eve being on an inner orbit around Kerbol. Moho demonstrates this property.

And as I said, you can still do aerocapture (and I do a lot), but you need a careful craft design. Random ship is not going to do it.

Edited October 11, 2015 by FancyMouse

[Snark]

There are various ways to save on dV when arriving at Eve. In order of most-important-first, they are:

Use a good transfer window. This is overwhelmingly the biggest savings you can get. Arrive at Eve along a "good" trajectory, and you don't have all that much dV to kill. Come screaming into the system when you're already >5000 m/s at the edge of Eve's SoI, and you're gonna have a tough row to hoe. So the best thing you can do is set yourself up for success by using a transfer window planner (there are various ones out there, this one is my favorite). "How to set up a good, low-dV encounter" is a whole separate topic, so I won't go into detail here unless you're interested. I'm sure there are plenty of tutorials out there. How to tell if an interplanetary encounter is a "good" one from looking at the map view: If your orbit and Eve's are crossing each other like an X, that's bad. if they're nearly parallel, that's good.

Aerobrake. It's "free" dV. This may or may not be an option for you. First, if you're hitting atmosphere at high interplanetary speeds, you're going to get fried regardless of your ship design. Second, even if you're at a reasonably manageable speed, the amount of aerobraking you can do will be limited by your ship design. Ships that are specifically designed to be aerobrake-sturdy will do a lot better than a ship that was designed without aerobraking in mind. Heat shields are good. Airbrakes are also good. Bear in mind that small, light craft do a lot better at aerobraking than big heavy ones. If you're coming in with a juggernaut, your aerobraking options may be limited.

Make maximum use of Oberth effect. Even if you're not aerobraking, do as much of your retro-burn at as low an altitude as possible. i.e. set up your Eve approach so that your periapsis is as low as possible-- just above the atmosphere, if you can't aerobrake. Wait until you're down at periapsis and then do your burn there. It's pretty easy to arrange for a low periapsis, as long as you do your correction burn a few days before arriving at Eve. If you do it several days in advance (i.e. before you even enter Eve's SoI), you can adjust your periapis altitude for a tiny amount of dV.

So let's say you have a worst-case scenario: you have an existing ship that wasn't well-designed for aerobraking, and you already have a high-velocity encounter set up for Eve, and you have a limited amount of dV left and are trying to figure out how you can make best use of what you have to try to achieve orbit. Here's what you do:

1. A few days before encountering Eve, make a small correction burn to adjust your Eve periapsis to as low as possible, right at the upper fringes of the atmosphere.

2. As soon as you enter Eve's SoI, do a quicksave. You're likely going to have to try this several times to get it right, and you'll probably get fried a few times.

3. We'll try for some aerobraking. Lower your periapsis a bit into the atmosphere. "How far" is going to involve a lot of trial-and-error, that's why you quicksaved in step 2.

4. Just before you hit atmosphere, do a retro-burn to slow yourself as much as you can. (Save a little bit of fuel to adjust your orbit post-aerobraking.) The idea is to get slowed down enough so that the aerobraking doesn't fry you. You want to wait as late as possible to do the burn, because the lower you are when you do it, the more Oberth effect you get. Doing the retro-burn right before you hit atmosphere not only slows you down to a speed that is (hopefully) survivable, but it also lightens your ship by a lot, which means it will aerobrake better. The lighter it is, the more aerobraking will slow it and the less likely it is to fry.

5. Make your aerobraking pass. If there's too much aerobraking, you fry, or end up going down to the surface. Too little, and you go shooting off into space. Hopefully you've hit the sweet spot and end up climbing back out of the atmosphere with a captured trajectory. In that case, just coast up to apoapsis, then do enough of a prograde burn to lift periapsis out of atmosphere. Congratulations, you're in orbit.

Edited October 12, 2015 by Snark

[FancyMouse]

Aerocapture is a completely different business. No multiple passes - you only have one chance. And with the Eve entry speed, for a random ship, the margin between fry and not capturing is a wide negative - something like not exploding >80km, while capturing <73km.

Airbrake is completely useless for Eve capture. It's one of the parts that explode the earliest if not the first, since they're exposed to air, and convective flux is just way way much bigger than that on Kerbin.

Heat shield is not "good", it's a must. Everything must be shielded behind it, otherwise it's just aerobreak to fireworks.

The only thing I haven't tried is to burn slightly to slow down before aerocapturing. I don't know how much fuel you sacrifice can give you a more flexible ship design. But since I can do a zero-fuel capture, I didn't bother this approach at all.

But really. It's fun to play with the heat shields and you'll save 1~2km/s fuel - I suggest take that challenge.

[Tatonf]

I would add that being captured on Eve is quite easy, only 80 m/s with a good transfer window, according to the chart.

What's more costly are the 1330 m/s for circularization, but then it's possible to aerobreak in multiple passes.

But without a heatshield, it's gonna requiring a lot of passes.

[milosh]

Thank you all for your answes, a lot of usefull stuff here.

Aerobreaking is not really available for me as I am trying to position a satellite with a survey scanner in a polar orbit, hence no heat shields or anything that a satellite usually doesn't need. Funny thing though, when I tried it once I felt right through the athmosphere and crashed on the surface, the satellite didn't burn up. Anyway, I think this is my problem:

I'm sure there are plenty of tutorials out there. How to tell if an interplanetary encounter is a "good" one from looking at the map view: If your orbit and Eve's are crossing each other like an X, that's bad. if they're nearly parallel, that's good.

As I said I use MJ's porkchop advanced planet transfer which does all the calculations for me. When I clicked on "asap" it didn't change the launch window at all, it was the same as "lowest fuel", but I was impatient and chose via the chart a window that was only several days away instead of a year and still cost me about as much dV as my second last stage offered. What I didn't pay attention to is my orbit on arrival though and it kinda was intersecting like a flat X. Now that I read it it makes sense and probably explains why "asap" didn't change. I'll try it again today and use the optimal transfer window, hopefully this will get me better results

[technion]

I was impatient

The trick here is.. Kerbal Alarm Clock. Just set an alarm for that window, and time warp until it happens. Eve is one thing, but you'll be in for a very bad time attempting to hit Moho without waiting until it's just right.

[milosh]

When I execute the next node it does already warp to the launch window at max speed (orbit at 640km) but waiting 1.5 years still takes several minutes I didn't want to spend starring at the screen. It's silly and in the end I've spend more time waiting than if I'd initially took the optimal launch window. Or did I misunderstood you, does the alarm clock instantly warp to the set time?

[Warzouz]

As I said I use MJ's porkchop advanced planet transfer which does all the calculations for me. When I clicked on "asap" it didn't change the launch window at all, it was the same as "lowest fuel", but I was impatient and chose via the chart a window that was only several days away instead of a year and still cost me about as much dV as my second last stage offered. What I didn't pay attention to is my orbit on arrival though and it kinda was intersecting like a flat X. Now that I read it it makes sense and probably explains why "asap" didn't change. I'll try it again today and use the optimal transfer window, hopefully this will get me better results

Beware that MJ doesn't optimise course calculation for the lowest dV on arrival. For example, I never had a successful Moho calculation with MJ. Sure I got an intercept but an unusable one (5 or 6k m/s capture...).

Use http://alexmoon.github.io/ksp instead (there is a mod that integrate this tool into the gate wand work nicely with KAC).

[milosh]

The problem was indeed the suboptimal launch window - I tried it again just now and let time warp for half a year to the optimal launch window. I needed some adjustments to make after the burn because contrary to the calculated path I didn't get into capture range, a small 100dV burn corrected this and I ended up at Eve with a Pe of 60'000km. The difference this time was, that my satellite's orbit followed Eve's for a good time, no "X" to speak of. It cost me only 1000 dV to bring Pe down to 700km and into orbit around Eve aligned polar, another 1000 brought Ap down too. Pitty only that thanks to the thick clouds I can barely make out an ore-rich landing side but that's another story. Thank you everyone again for helping me out

@Warzouz: MJ for me is usually quite reliable if I don't choose to ignore it's suggestions. Only issue I have is that it does not calculate the amount of trust my engines produce 100% correctly and thus the predicted path does not always correspond with the path I end up after the burn. Thanks anyway for the mod suggestion but I stick to MJ because it get's always updated right away when a new KSP version is out, it's the only mod I use at all, plus astronomers visual pack.

Edited October 12, 2015 by milosh

[Tatonf]

What you got is not a "window". It's a possibility to do a "fly by".

A window correspond to a possibility of doing a Hohmann transfer, which is the most efficient way of doing an interplanetary transfer (given 2 coplanar bodies).