Aerobraking Woes

[Choctofliatrio2.0]

So I sent my first probe to Duna today. The first time I did it, I set my periapsis to about 20,000 meters, and quicksaved (so if something went wrong I could reload that)

Then, when aerobraking time came, the atmosphere slowed me down some, but it wasn't enough, and I ran out of fuel, so I reloaded the quicksave. The reload was... odd. When I looked at my periapsis, I saw it was rapidly going up for no reason. I could burn to reduce it, but the instant I stopped burning it started increasing again. I did manage to get it down to 15,000 and keep it there with timewarp. When it was time, the atmosphere slowed me down too much, and I crashed.

It took several more tries, and probably luck, to get it into orbit, and the aerobrake didn't really help.

So what I'm wondering is, what is the optimal aerobraking height for Duna? And, in the optimal aerobrake, should I have to use my engines? Is there any explanation for my rapidly increasing periapsis? Help would be appreciated.

 

Edit: I apologize if this has been asked before. I'm sort of an idiot

Edited December 31, 2015 by Choctofliatrio2.0

[technion]

In 1.05, you won't be able to aerobrake effectively without also using some engine burn. Heat damage kicks in and becomes a problem at a much higher altitude than you need for the atmosphere to slow you down.

[Sovek]

Heatshield. Use a transfer stage to get there and let it burn up in the atmosphere of the target body. Stage with a heatsield protecting your lander/return craft.

[Geschosskopf]

19 hours ago, Choctofliatrio2.0 said:

So what I'm wondering is, what is the optimal aerobraking height for Duna? And, in the optimal aerobrake, should I have to use my engines? Is there any explanation for my rapidly increasing periapsis? Help would be appreciated.

Duna is about the only place where aerocapturing is still possible in 1 pass without burning.  Unfortunately, there is no "optimal aerobraking altitude" at any planet, due to all the variables involved which will differ for every ship.  The variables include:

1. The desired altitude you want for your Ap after the pass, which depends on the ship's mission and fuel state.  Is the ship going to low Duna orbit, high Duna oribt, or heading for Ike?  With all other variables the same, the higher your desired post-aero Ap, the higher you need to set your Pe.
2. The orbital speed of the ship relative to Duna as it reaches Pe.  This depends on how you did  your transfer burn back at Kerbin, which determines how fast the ship crosses the distance and the angle at which it crosses Duna's orbital path.  The smoother your ship's transfer orbit merges with Duna's orbit, the slower your ship will be going relative to Duna, so it won't need to dig as deep in the atmosphere to achieve the desired post-aero Ap.  But even with a flotilla of several ships in tight formation, they will still arrive days to weeks apart at Duna, meaning all will have different speeds.  Thus, with all other variables the same, each will still need a different Pe altitude.
3. The aerodynamic shape of the ship.  Aerobraking is all about using aerodynamic drag so its effectiveness is directly related to how sleek or draggy the ship is.  IOW, if you've got 2 ships with the same speed going to the same Ap, the sleeker one will have to dig deeper.

Because of all these variables, and because the stock game map view won't show you the effects of aerobraking on your future path, aerobraking and aerocapturing are always matters of trial-and-error F5/F9.  There is a mod called Trajectories that attempts to show you in advance what aerobraking will do for you, but I find it more of a good estimate than an exact prediction.  And at present it doesn't work with FAR if that matters to you (it probably will soon, however).

As to why your Pe kept rising, I sometimes run into that bug myself.  It's usually a sign of memory approaching the limit and means you should exit the game and restart.

There are some other causes of weird future path movement, however.  Wobbly ships have wobbly trajectories, apparently caused by the CoM oscillating as if the wobbles are producing a net thrust.  The wobbles can be stopped by a brief application of warp, but will start again when you next reorient the ship or do a burn.

But the big and highly annoying cause of wobbly trajectories for me is RCS.  Just having RCS turned on, not even actively using it, will make your future path wobble around a fair amount.  And if you really are using RCS to orient your ship, your path will go nuts.  You can fix this by turning RCS off, but then you can't use RCS to fine-tune your Pe.  Which is a real bummer because I used to do precisely that.  The path wobbles from RCS were introduced in the same update that gave us asteroids, which was the last time the map view got any attention from the developers AFAIK.  Hopefully this will be fixed in 1.1