Question about aerobraking

[xcorps]

So I did an Eve flyby and used a bit of aerobraking to grab some science. I made an elliptical orbit that allowed my probe to pass through the atmosphere at about 85k, then left. I dropped a stage in that orbit that was out of fuel, and then I decided to watch it descend to get a real idea of what aerobrakeing does.

Here's my confusion:

The velocity of my debris is always the same at peri. The ap is falling, and velocity at AP is dropping each time. So the distance and time from AP to PE drops with each arrival at PE, so why doesn't velocity drop as well?

EDIT:

Blah, I should have waited to post this. It is dropping, but not noticeable until AP is below a million meters.

Edited May 13, 2014 by xcorps
Not enough science done

[SSSPutnik]

Is there a probe core or better on it? If not, the debris is basically on rails.

[Northstar1989]

Your debris is not slowing down (much) while it is at periapsis because it is actively falling lower while in the atmosphere, making up for any losses due to drag. However, it should be moving slower at each periapsis, as it is falling from a lower apoapsis each time...

85 km also isn't a terribly low periapsis, even on Eve. It's going to take a while at that rate to see measurable declines in velocity at periapsis at each pass...

[xcorps]

Is there a probe core or better on it? If not, the debris is basically on rails.

Right, but the rails went through atmosphere, so there was some deceleration. Apoapsis dropped every orbit, I guess I was just expecting velocity at Peri to drop faster than it did.

It's going to take a while at that rate to see measurable declines in velocity at periapsis at each pass...

Roger that, exactly what happened. I didn't see any velocity drops worth noting until Apoapsis was under a million meters, then it started dropping by 1-2 m/s per pass. Under 500k it was dropping about 10-15 m/s per pass. Then as periapsis finally started to drop, the debris spent more time under 90k, so it fell even faster.

My expectations were too high.

[Kasuha]

Is there a probe core or better on it? If not, the debris is basically on rails.

Debris or not debris - does not really matter. Either you follow it, then it aerobrakes, or you don't follow it and then it's on rails and eventually disappears when under 23 km.

[Red Iron Crown]

I don't think objects on rails experience aerobraking, physics is not simulated for them. So if you leave debris in an orbit that enters the atmosphere but doesn't go below the 23km delete limit, it will stay in that orbit perpetually. Unless you can get an active craft within 2.5km of it while it's in the atmosphere, that will make physics kick in and it will decelerate as expected.

[Kasuha]

I don't think objects on rails experience aerobraking, physics is not simulated for them. So if you leave debris in an orbit that enters the atmosphere but doesn't go below the 23km delete limit, it will stay in that orbit perpetually. Unless you can get an active craft within 2.5km of it while it's in the atmosphere, that will make physics kick in and it will decelerate as expected.

Yes, you can even leave a satellite in orbit well inside the atmosphere which will stay there as long as you don't switch to it.

[BudgetHedgehog]

It's not the altitude that determines if something is on rails, it's the pressure. 0.01 atm to be precise, and 0.01 atm of pressure on Kerbin is at about 23km.

[Red Iron Crown]

It's not the altitude that determines if something is on rails, it's the pressure. 0.01 atm to be precise, and 0.01 atm of pressure on Kerbin is at about 23km.

Interesting, I didn't know that. Seems like more calculation would be required than for a hard limit. So an object would be deleted at much higher altitude on Eve and much lower on Duna, yes?

[BudgetHedgehog]

So an object would be deleted at much higher altitude on Eve and much lower on Duna, yes?

Yup. Eve is about 43km, Duna is about 9, Jool is 73 and Laythe is about 17.

[magnemoe]

Interesting, I didn't know that. Seems like more calculation would be required than for a hard limit. So an object would be deleted at much higher altitude on Eve and much lower on Duna, yes?

Its probably an set attitude on each body equal to 0.01 atmosphere, at this attitude something in orbit will deorbit anyway.

And yes this allows you to cheat and do an rescue.

[MarvinKitFox]

Debris or not debris - does not really matter. Either you follow it, then it aerobrakes, or you don't follow it and then it's on rails and eventually disappears when under 23 km.

Exactly.

Atmosphere only matters if physics is loaded, i.e. if your point-of-view is within ~2.5km of the object.

On Kerbin, it does another check at ~23km. Anything below this alt, rails or no, gets unloaded if its not in physics. (Anyone know what this alt is for the other atmospherics?)

Yup. Eve is about 43km, Duna is about 9, Jool is 73 and Laythe is about 17.

Ah, Should read more before I ask.

In one of my worlds I have a spacestation "orbiting" in a 28x35km very low Kerbin orbit.

I'm just not allowed to visit it

Edited May 13, 2014 by MarvinKitFox
alleviating foot-in-mouth disease.

[Duna]

Just a tip if your aerobraking on Duna don't go lower than 13,000 m because lower than that you could hit the highest mountain on Duna (8264 m) and below 13,000 you could end up having an unplanned landing on Duna

[Claw]

Its probably an set attitude on each body equal to 0.01 atmosphere, at this attitude something in orbit will deorbit anyway.

And yes this allows you to cheat and do an rescue.

It's not fixed to the altitude, but to atmospheric pressure. You can confirm that by watching the debug screen when an on-rails object gets deleted.