Aerobraking prediction

[MaLuS]

I would love to see in the game something along the lines of what mechjeb does with aerobraking nodes, although i would like to see it shown in the trajectory as well as the nodes similar to how it shows encounters with celestial bodies in the stock game. I think this is a much needed addition as many beginners find it difficult to do aerobraking capture maneuvers and as the games doesn't show what happens with encounters with atmosphere many get caught out on this when starting out.

[Kermunist]

I think this has been considered and discounted for technical reasons.

The current atmospheric model is very simple. A cylinder experiences the same drag going end on or sideways, there is no concept of streamlined or anything like that. This makes it easy to predict how the orbit of a craft changes as it flies through the atmosphere (unless it has wings). So as is, it's not too hard to do what you want, and the hard working folks behind mechjeb have done it.

But the plan is (or was) to change the atmosphere model to a more accurate one. Squad never made it particularly clear how much more accurate (I'm hoping for something like NEAR), but better than the current one anyway. The side effect of this is that it will become much harder to predict the effects of aerobraking, because you'll have to consider the shape of the ship, and it will be different if it enters the atmosphere broadside on. Squad didn't want to write the code to do the prediction for the current model if they would have to rip it all out and start over when they improved the aerodynamics.

Still, it's been a long time since I heard anything about improved aero. It may be that Squad have decided the current aero model, complimented by some excellent mods for those who want more, is enough. In which case it'd be nice to see a feature like this (with an easy API to turn it off for people using NEAR/FAR)

[Andrew Hansen]

Yeah, I agree with Kermunist - I'd much rather have better aerodynamics put into the game than having an aerobraking prediction built into Map View. Plus, with NEAR, you can alter your ship's orientation to somewhat change your orbit by descending lower or ascending out of the atmosphere.

[tntristan12]

Even with realistic aerodynamics, there are models that can be (and are) used to predict aerobraking delta-v. I'm sure someone with influence will discover this eventually.

[Kermunist]

I think you've missed my point a bit tntristan12. It's not that the predictions would be very difficult in an improved aerodynamic model (though the outcome would depend a lot on how the ship's attitude changed during the maneuver, which might be hard to predict), it's that they would be different from the predictions for the current model. So there's no point adding this until the improved aero model is in. Otherwise the code, and the time and effort in writing it, would go to waste when the new model comes in.

[Tex_NL]

Even with the current stock aerodynamics MechJeb is far from accurate. As soon as you start adding wings and control surfaces into the mix the final orbit can be significantly off.

No matter what aerodynamics model you use it will be impossible to predict the exact outcome as there is absolutely no way to know the crafts orientation in advance. In fact; the more accurate the aerodynamics model gets, the more difficult it will become to predict.

As long as people are aware it is only a best guess prediction and not a guarantee this would be a really nice feature to add to the game.

[thereaverofdarkness]

But the plan is (or was) to change the atmosphere model to a more accurate one. Squad never made it particularly clear how much more accurate (I'm hoping for something like NEAR), but better than the current one anyway. The side effect of this is that it will become much harder to predict the effects of aerobraking, because you'll have to consider the shape of the ship, and it will be different if it enters the atmosphere broadside on. Squad didn't want to write the code to do the prediction for the current model if they would have to rip it all out and start over when they improved the aerodynamics.

Simple solution: they just need to write one that predicts how much slowing they will experience if they maintain the same orientation relative to their heading at all times. If that changes with the updated aerodynamic model, it may become more complicated to write. But it would have been a cinch to have written one a long time ago.

[tntristan12]

I think you've missed my point a bit tntristan12. It's not that the predictions would be very difficult in an improved aerodynamic model (though the outcome would depend a lot on how the ship's attitude changed during the maneuver, which might be hard to predict), it's that they would be different from the predictions for the current model. So there's no point adding this until the improved aero model is in. Otherwise the code, and the time and effort in writing it, would go to waste when the new model comes in.

Oh. Indeed I have. I see what you're saying now. Cheers!

[Tery215]

It sounds like a good idea to make it so that the aerobrake is predicted by selecting an orientation, rather than maintaining or assuming an/no orientation.

[MaLuS]

I was in no way expecting the prediction to be 100% accurate but even with an error margin of 10-20% would give you a rough idea on whether you will be captured or not by your current heading and orientation (or planned node). Besides if you have an error margin then it will provide something to help new players learn to use the basic prediction as a guide then experience will help you further along the line. I was mostly thinking of this as a means of not only making sure you get a capture (and roughly how your orbit will sit after) but also as a tool to lower the learning curve of aerobraking for those with less experience with such things.

[arkie87]

I was in no way expecting the prediction to be 100% accurate but even with an error margin of 10-20% would give you a rough idea on whether you will be captured or not by your current heading and orientation (or planned node). Besides if you have an error margin then it will provide something to help new players learn to use the basic prediction as a guide then experience will help you further along the line. I was mostly thinking of this as a means of not only making sure you get a capture (and roughly how your orbit will sit after) but also as a tool to lower the learning curve of aerobraking for those with less experience with such things.

I dont see any reason why the prediction cannot be within this accuracy. The only difference between stock aerodynamics, and something like NEAR/FAR is AoA, and since player can change AoA while aerobreaking, the ultimate prediction might be unrealistic. However, some prediction is better than none, and the aerobreaking predictor can easily assume a reasonable AoA, such as 0° or 180° depending on whether it is a splaceplane or pod i.e. choose whichever orientation is stable (i would argue for 0° default rather than a constant one relative to heading, since the initial AoA might be unstable once thicker atmosphere is reached; nevertheless, the player might want to know how much dV he would lose if he hit Kerbin's atmosphere at 36 km at 90° AoA, so i would still support advanced tools). The only catch would be is if 0° requires too much deceleration and/or heating (if aerodynamic disassembly or deadly reentry is enabled) for high speed/interplanetary kerbin captures, though i suppose the aerobreaking predictor could warn of that as well. To fix that problem, one could try to adjust AoA on the fly until it Apoapsis is not interplanetary. Thus, i think this predictor should be integrated into the patched-conics trajectories which are drawn on the map screen.

Finally, as for the argument that Squad hasnt developed it since they would have to redo it once more complicated aerodynamics is implemented, i think that is bubcus: all they would have to do is make the predictor as general as possible (i.e. more complicated than would be required for stock aerodynamics) such that once new aerodynamics is added, it would still work. For example, even though in stock ksp, lift/drag is not a function of AoA, rather than using this fact when writing the prediction, it should call a function that gets the lift and drag coefficient as a function of AoA (and maybe even Mach). When stock aero is used, it would return a constant value, independent of AoA and when more complex aerodynamics is used, it would return those values. Thus, i dont think the predictor would have to be redone after stock aerodynamics improves...