KSP 23.5 (and earlier?) -- Aerobraking only works on focused craft

[LitaAlto]

[Edited to clarify I wasn't calling this a bug to start with--this is just an observation. Thanks!]

It occurred to me that it might be more efficient to send up one rocket with multiple probes than one probe at a time.

I created a rig with nine probes on it--one main probe to serve as a communications relay and to operate a rocket-powered orbiter, and eight others clustered in groups of four around the scaffolding connecting the orbiter to the fuel and engine. The cluster probes each have a QBE for heat resistance, with a modicum of solar panels and antennae and instruments, topped with a M16 chute. Perfect for seeding Eve or Laythe with probes! (They might even survive Duna but I have doubts one M16 is enough.) While there is a chance of failure for one or two probes, I could probably get enough probes to survive to make it worthwhile. (And in my test below I found that was the case with a 75% survival rate on Kerbal.)

Once in orbit around their final destination, the main probe would adjust the orbit to allow for aerobraking. Then, it starts releasing probes, a minute or two at a time to allow for some separation, before readjusting its orbit for stability.

Before sending these probes half-way across the Kerbol system, I thought I should do a little test closer to home first.

So I sent up my rocket, put it in an orbital with the periapsis around 50km, deployed the probes, moved the orbiter back to a stable orbit, and waited.

And waited. And waited. Nothing was aerobraking.

I switched focus to one of the probes to see what was going in. Then, and only then, did aerobraking happen, with chute and touchdown coming at the end of the second orbit after focus-switch.

The rest of the probes kept orbiting happily, totally ignoring that, technically, they were passing through atmosphere that should have degraded their orbit.

I had to switch focus to each of them individually and keep the focus on them to get them to aerobrake and land.

If I let aerobraking happen on the first orbit, then switched to another probe, the first probe would continue in the degraded orbit, again as if it weren't passing through atmosphere each time, until I switched back again.

This isn't entirely a bad thing, since it allows for better control of when (and by extension where) each probe lands. If the destination isn't tidally locked, it's easy to scatter the probes across the world's surface. Even if it is tidally locked, like in the case of Laythe, I presume that if I'm orbiting it and not Jool then I should be able to pick moments in its orbit around Jool which would be good for keeping the probes separated.

But it's not realistic. In this test I released all the probes only moments apart, hoping at best for a few kilometers of scattering, not having them more or less evenly spaced all around Kerbal. To scatter them better, I anticipated waiting until my orbiter was climbing out of the atmosphere during its sole aerobraking run, then releasing the probes several minutes apart as I traveled the rest of the orbit, with only minutes to readjust the orbit to prevent further aerobraking of the orbiter. But it's obvious now that it makes no difference--the only difference is in how long I go in-between switching focus between probes.

Again, I'm sure Squad wasn't even thinking of scenarios like this when they designed the game. And I'm sure there may be good gameplay reasons not to allow aerobraking for anything except the focused craft.

But it's something to think about.

Edited May 25, 2014 by LitaAlto
Clarifying comment added.

[Cirocco]

It's not gameplay reasons, but technical ones. Physics calculations are hard on your computer, they take a ton of processing power. Especially for large craft because the game has to calculate physics for each part and then add it all up. So the game takes shortcuts to run more smoothly. For example: anything in timewarp is not subjected to physics, just the orbital mechanics. (exception being physical timewarp, but that only goes up to 4x) Another shortcut is that anythin that is not in focus view or a few kilometers near the point of focus is also not subjected to physics. That is the reason you cannot aerobrake with stuff that isn't in focus. The game also assumes that anything that is in an atmosphere and below a threshold (I believe 20 km?) has/will crash and will delete it.

[undercoveryankee]

The game also assumes that anything that is in an atmosphere and below a threshold (I believe 20 km?) has/will crash and will delete it.

The threshold for vessels that are unloaded in atmo to despawn is a pressure of 0.01 atm. This is about 23 km above sea level on Kerbin, but it will be different on other planets.

[LitaAlto]

Fair enough. This is mainly an observation on how it'll impact my gameplay, and I thought I'd share it here. Thanks for the explanation!

[RenevB]

As said before, it's not a bug, it's a feature. Imagine KSP trying to calculate physics for all parts on all your ships everywhere. That wouldn't be very practical and it's the reason for the 2.5 km (give or take a couple 100 meters) physics bubble.

Edited May 25, 2014 by RenevB
typo

[LitaAlto]

No, now that I've taken off my space geek hat and put on my CompSci one--of course I'd need a supercluster at minimum to adequately model aerobraking for multiple objects at once.

It's just... it feels like CHEATING. I shouldn't be able to put a bunch of probes in an aerobraking orbit, then choose which probe to aerobrake first. They should all deorbit the moment they start to hit atmosphere. At least, that's what would happen in reality.

And no, I'm not really complaining. I bet NASA wishes they could dump a bunch of probes into an aerobraking orbit then decide when and where each probe will land.

[Guest]

Wait for Principia mod for KSP. It overhauls physics engine with symplectic integrators, making a lot of impossible stuff work (including N-body simulation!). Squad doesn't seem to know enough about maths to implement that themselves, but Eggrobin does. Really, this can be done, it just requires the kind of mathematical knownledge Squad doesn't have (being game devs not astrophysicists might have something to do with it, but they could at least try to hire an expert...).

And no, it doesn't melt your computer, the algorithm is actually quite efficient.

[LitaAlto]

Color me intrigued! I'm looking up the Principia mod now to see if there are any status updates. Thanks for the tip!

[technicalfool]

As said before, it's not a bug, it's a feature. Imagine KSP trying to calculate physics for all parts on all your ships everywhere. That wouldn't be very practical and it's the reason for the 2.5 km (give or take a couple 100 meters) physics bubble.

I would still call that a bug. Just a rather difficult to deal with bug. I'd like to see some kind of limited outside-the-bubble physics that would enable aerobraking (and get rid of orbiting debris that only skims the atmosphere), but I can understand that not being a huge priority right now.

[nickexists]

Couldn't they implement a system so that the game calculates a single drag coefficient for each inactive ship and then calculates aerobraking forces on them in real time as though each ship were one part?