Aerobraking lifts Ap? Bug?!?

[kurja]

So I went to Eve and aerobraked to get captured, then raised my Pe a bit higher so I could do another aerobraking pass without ending up on Eve and maybe go to Gilly... But, when I touch the atmosphere my Ap is rising??? What's going on, has anyone else seen this happen?!?

And moments later....

[Murph]

With a 77Mm apoapsis and a 93km periapsis, you'll be getting near enough zero aerobraking, as you're not deep enough into the atmosphere to get much effect, and will be travelling so fast (due to the extreme apo) that you'll be in and out of the atmosphere in a flash. So, it's more likely just rounding errors or something like that. Movement of the ship (changing attitude) can cause the numbers to jitter a little as well, it could just be that (it's a very small % change).

You need a periapsis of about 80km or less to get much aerobraking, if memory serves. At a rough guess, you'll need a periapsis of maybe 70-75km on your next pass to pull your apoapsis down about half way towards Gilly, but it's been a week or 2 since I last skimmed through Eve's upper atmosphere. Don't go below 70km unless you're certain of the required altitude, as it quite soon becomes a landing below that.

Edit: You can see the near zero aerobraking in your engineer numbers there, less than 1kN of drag at periapsis, likely not enough to make a measurable difference to your >4km/s velocity, and certainly orders of magnitude away from the force you'll require to pull that apoapsis down towards Gilly quickly.

Edited November 30, 2013 by Murph

[kurja]

With a 77Mm apoapsis and a 93km periapsis, you'll be getting near enough zero aerobraking, as you're not deep enough into the atmosphere to get much effect, and will be travelling so fast (due to the extreme apo) that you'll be in and out of the atmosphere in a flash. So, it's more likely just rounding errors or something like that. Movement of the ship (changing attitude) can cause the numbers to jitter a little as well, it could just be that (it's a very small % change).

You need a periapsis of about 80km or less to get much aerobraking, if memory serves. At a rough guess, you'll need a periapsis of maybe 70-75km on your next pass to pull your apoapsis down about half way towards Gilly, but it's been a week or 2 since I last skimmed through Eve's upper atmosphere. Don't go below 70km unless you're certain of the required altitude, as it quite soon becomes a landing below that.

I did spend more than a few seconds in the atmosphere, and the Ap was rising steadily, not flickering about in a random rounding error fashion.

I know I don't get very much breaking at that altitude, I thought better to do it little by little than too deep...

[UmbralRaptor]

I'm less concerned by the apoapsis rising (I could write that off as aerodynamics actually working) than by the semi-major axis.

[kurja]

I'm less concerned by the apoapsis rising (I could write that off as aerodynamics actually working) than by the semi-major axis.

Why, what's the problem with semi major?

Hm by hitting the atmosphere both Ap and Pe rise, I'm not supposed to gain energy with friction, am I?

[UmbralRaptor]

Why, what's the problem with semi major?

Violation of conservation of energy for one.

Hm by hitting the atmosphere both Ap and Pe rise, I'm not supposed to gain energy with friction, am I?

Nope.

[kurja]

Violation of conservation of energy for one.

You'll have to elaborate if you want me to understand.

[UmbralRaptor]

You'll have to elaborate if you want me to understand.

Your second line answered it; an object's semi-major axis is directly related to its overall (potential + kinetic) orbital energy. Atmospheric friction* is supposed to reduce your orbital energy (and by extension semi-major axis), but in those pictures isn't.

What does your craft look like, anyway?

*In the real world drag at this speeds involves almost no friction, but given the limitations of KSP stock aerodynamics...

[kurja]

tomatos, tomatoes...

It's a ~30 part probe ship, abt 3 tons

[Daddy Cecil]

I'd reckon you're getting a gravity assist from Eve, since a. you're just skimming through Eve's atmosphere and b. Eve has a huge gravity well. Having an elliptical Jool orbit has sent all my probes flying off from gravity assists from the moons.

[mhoram]

I'd reckon you're getting a gravity assist from Eve

Gravity assists are only in effect, when SOI changes are involved, And in the OP this is not the case.

Otherwise your orbit would always increase while near PA.

[kurja]

Gravity assist from orbiting a single body, hm that's a new one... Who wrote ksp physics, someone from the flat earth society?

[sgt_flyer]

by any chance, do you have control surfaces on your ship ? - in this case, especially if you have your SAS turned on, i would guess some form of the infiniglide bug.

[kurja]

by any chance, do you have control surfaces on your ship ? - in this case, especially if you have your SAS turned on, i would guess some form of the infiniglide bug.

Hm no, there are no wings or fins or anything.

[allmhuran]

It is odd to see Ap go up and I don't have any explanation for it other than some minor bug in the orbit calculation part of the engine, but at 93 km your periapsis is not low enough to get any noticeable effect from aerobraking. If you dropped it to about 80 it would still take at least one more pass to get your Ap down to Gilly, and many more passes (like, dozens) before you risked deorbiting.

[kurja]

Yes, I know that I get more aerobraking at lower altitudes. It has also been brought up in this thread a couple times now. That is not the point of this thread.

[allmhuran]

Yes, I know that I get more aerobraking at lower altitudes. It has also been brought up in this thread a couple times now. That is not the point of this thread.

I already responded to your question in the first sentence. But if you're going to be a jerk about it I'll be sure not answer any more of your questions.

Edited December 1, 2013 by allmhuran

[kurja]

I already responded to your question in the first sentence. But if you're going to be a jerk about it I'll be sure not answer any more of your questions.

When substance of your response consists of "I don't know" is it really worth your while to make that post?

Sorry about being a jerk.

[Vanamonde]

Please keep the tone of discussion civil, ladies and/or gentlemen. The purpose of this sub-forum is to try to help each other.

[ddenis]

Your Pe it way too high, your craft isn't aerobraking. Ap changes because of turning rocket by SAS. Because the Ap is so higher than Pe and far from Eve, so even such small actions have big influence.

[kurja]

Your Pe it way too high, your craft isn't aerobraking. Ap changes because of turning rocket by SAS. Because the Ap is so higher than Pe and far from Eve, so even such small actions have big influence.

Turning a craft can cause some wiggling of the orbit, especially if it's a big ship but this is a small probe and we're looking at both Ap & Pe rising, Ap rising over a kilometer per second spent in the upper atmosphere. That's just not supposed to happen, ship is gaining energy by slowing down in the atmosphere... Either that, or even worse, it's accelerating by hitting the upper atmosphere.

[numerobis]

I've noticed this same effect, quite consistently, in high-altitude aerobraking. Your Pe drops by a few meters, your Ap goes up, rather than, as you'd expect, the opposite. After a very large number of passes, your Pe goes low enough that you start losing significant orbital energy and finally you get back to more normal behaviour. I also have no explanation.

[Nao]

From the images OP posted it looks like atmosphere actually do slow you down as the specific angular momentum is going down. (The 26m increase in Ap is much less energy gained than the loss of 3m at Pe).

I think the Ap going up is just a rounding error. And as numerobis points out the orbit should decay after many passes.

Edited December 2, 2013 by Nao

[numerobis]

The semi-major axis is increasing, so definitely the energy is increasing, which is nonsensical. But the Pe is falling, so it's not a game-breaking violation of conservation laws.

[Nao]

The semi-major axis is increasing, so definitely the energy is increasing, which is nonsensical. But the Pe is falling, so it's not a game-breaking violation of conservation laws.

Is this a fact actually?

I mean i did check the values in screenshots and even thou SMA is increasing both total specific angular momentum and Areal Velocity are going down actually.

I don't think SMA have much to do with energy, especially for highly elliptical orbits where the increase in Ap cost's *much* less than increasing Pe by the same altitude value.

edit: enlightened myself a little, it seems that Total Specific Energy is constant for constant SMA ... welp i forget these things constantly, sry and carry on

Edited December 2, 2013 by Nao