Need Help Aerobreaking

[mbrittb00]

I've just recently started to fly probes interplanetary and was wanting to try aero-breaking or more to the point aero-capture to capture the probe into an orbit around my target planet. I had found the following calculator on the web, but unfortunately haven't been able to get a successful aero-capture. While I am flying through the atmosphere my Periapsis starts falling, and in the end my Apoapsis ends up in the planet rather than at my desired altitude. I'm sure I'm doing something wrong. I've been trying this on an entry to Duna. My problem may be my relative inclination. It is rather high as I had forgotten to adjust it mid-course.

http://alterbaron.github.io/ksp_aerocalc/?inputDir=prograde

Any suggestions / info on how to use this calculator would be greatly appreciated.

Thanks,

[Skorpychan]

First of all, 'aerobraking' is the word you're looking for. Braking as in slowing down. 'Aerobreaking' is hitting the atmosphere so hard bits fall off, or so deep you smack into the planet.

Secondly, you're probably aiming too deep. You need to use the atmosphere to skim velocity off your craft, not lose all of it entirely.

[Rodyle]

What I usually do: quicksave when I enter the SOI, take a few tries until I get it right. It just depends too much on the particular craft, the exact entry trajectory and a few other factors. I think though, that it's often around 13 to 17 km around Duna, but I could be misremembering.

[Jimbimbibble]

13-17km sounds right for stock. If you're using FAR it's much more difficult to predict and the trial and error method is your best option.

[MircoMars]

If you want, you can use mechjeb in landing mode to calculate a maneuvernode that represents your braking maneuver. after some time you'll be able to estimate a proper periapsis yourself.

otherwise use the F5-F9-F9... method.

[LethalDose]

I've just recently started to fly probes interplanetary and was wanting to try aero-breaking or more to the point aero-capture to capture the probe into an orbit around my target planet. I had found the following calculator on the web, but unfortunately haven't been able to get a successful aero-capture. While I am flying through the atmosphere my Periapsis starts falling, and in the end my Apoapsis ends up in the planet rather than at my desired altitude. I'm sure I'm doing something wrong. I've been trying this on an entry to Duna. My problem may be my relative inclination. It is rather high as I had forgotten to adjust it mid-course.

http://alterbaron.github.io/ksp_aerocalc/?inputDir=prograde

Any suggestions / info on how to use this calculator would be greatly appreciated.

Thanks,

As Skorpychan noted above, you're probably aiming too deep. Aerobraking is more of an art, than a science, and even with the calculator, empirical data from experience is probably more valuable than the calculations. Saving and reloading until you get your maneuver tweaked is probably your best bet. You probably aren't doing anything wrong, you just need a bit of practice before you get it right. A few other notes:

Your relative inclination to the planet shouldn't matter much, if at all.

Your periapsis is falling when you enter the atmosphere because your orbit is slowing, which lowers both your periapsis and apoapsis (slowing only lowers your Ap when you slow down right at your Pe).

When you say your apoapsis is in the planet, do you mean it's in the planet's atmosphere? Or is the periapsis in the planet? I've never seen an orbit with it's apoapsis in the planet.

Also, realize you'll never get a perfect orbit from just aerobraking. You'll need to burn at your new Ap to lift your Pe out of the atmosphere.

[mbrittb00]

When you say your apoapsis is in the planet, do you mean it's in the planet's atmosphere? Or is the periapsis in the planet? I've never seen an orbit with it's apoapsis in the planet.

Sorry, yes, periapsis, got those backwards.

Also, realize you'll never get a perfect orbit from just aerobraking. You'll need to burn at your new Ap to lift your Pe out of the atmosphere.

Understood. Aerobreaking obvously won't rais the apoapsis.

I can play the trial and error game. I've just seen comments from people that the calculator that I linked to has been very successful, and I was more trying to figure out if I was doing something wrong with the results it was giving me.

Also, I don't think the MechJeb I have has a landing mode as suggested by MicroMars. I may need to update.

[technicalfool]

Also, I don't think the MechJeb I have has a landing mode as suggested by MicroMars. I may need to update.

Hi.

I can't see if you've indicated whether you're playing a sandbox or career game-save. In career mode, a lot of Mechjeb's advanced features are disabled, and you'll find more functions become available as you research more unmanned technology.

Of course I could be completely wrong, but I hope that helps anyway.

[mbrittb00]

Hi.

I can't see if you've indicated whether you're playing a sandbox or career game-save. In career mode, a lot of Mechjeb's advanced features are disabled, and you'll find more functions become available as you research more unmanned technology.

Of course I could be completely wrong, but I hope that helps anyway.

Career mode.

[Frank_G]

I recently did a successful aerobraking in Jools atmosphere. I sent a ship on a course with a periapsis of 115 km around Jool, loosing hundreds of dV and escaping unharmed. I dont use FAR and i dont use any calculator... i think, sending in some probes to test it, is the best (and most fun) way to do it.

[mbrittb00]

Ok I've got a slightly different question but on the same subject. I've got a different probe headed to Eve, and I can't seem to figure out which way to burn to get my Periapsis down to the point where I can try aerobreaking. How do you know which way to burn?

[Kryxal]

It's probably going to be one of the radial (blue) directions on the maneuver node, assuming you're a fair distance off. Try the blue handle on the same side of your track as the planet, see what happens.

[xcorps]

Your periapsis is falling when you enter the atmosphere because your orbit is slowing, which lowers both your periapsis and apoapsis (slowing only lowers your Ap when you slow down right at your Pe).

Could you elaborate on that please? How can you set up an aerobreak without being at PE?

[technicalfool]

Sorry for taking a while to get back. Yes, if you're running in Career mode, then Mechjeb is basically crippled until you unlock a whole bunch of unmanned-related technology.

As for how to get your PE down, that depends on where you are, and really a full tutorial would probably suit you better there. If you have Smart ASS on mechjeb, and you're just entering Eve's SOI, then the best direction would probably be anti-radial (or the RAD- button) for about a 70km or so periapsis. That should aerobrake you into an orbit without stranding you on the surface of the purple giant. Don't forget the quicksave button first though.

[LethalDose]

Could you elaborate on that please? How can you set up an aerobreak without being at PE?

Aerobreaking occurs before, during, and after periapsis. It starts as soon as you hit the atmosphere, at which point you should still be going down, meaning you haven't yet reached periapsis. For example, if you're aerobreaking at Kerbin with a periapsis of 30 km, you start aerobreaking/slowing down at 70 km. This slows your velocity and will reduce both your Pe & Ap, because you're not at Pe. Similarly, after Pe, you have to ascend back out of the atmosphere, but you're still being slowed by said atmosphere.

If you were only in the atmosphere at the instant of your periapsis, you'd only aerobreaking in that one instant, which probably wouldn't be very noticeable because of how thin the atmosphere is and how briefly the drag was applied to your vessel.