Aerocapture experiences

[bsalis]

Aerocapture is where you aerobrake into orbit, which I think everyone here should be familiar with.

I understood Kerbin well. 36km was a nice altitude to set the Pe for capture from Mun and Minmus return. Anything under 35km was a full re-entry. So far I have found that 15km is good for Duna. That was a proper intercept near Ap on an eccentric orbit.

So what have peoples experiences been of aerocapture at the new planets? I'd like a heads up, since I prefer to avoid quickload if I can.

EDIT: details so far...

Duna

10.5km Pe, 1900 m/s = 1333 km Ap

11.5 km Pe, 1680 m/s = 1155 km Ap (me again)

12.6km Pe, 1300 m/s - fail, too slow/low (me again)

14.5 km Pe, 1675 m/s - fail too fast/high (me again)

15 km Pe - mixed results... too little or too much depending upon entry m/s

25 km Pe - Fail - too high

Eve

60km Pe, 4000 m/s - capture success

70km Pe (m/s unknown) = success 300km Ap

Jool

90km Pe - epic fail (~120km minimum suggested)

119.5km Pe (m/s unknown) = success (and Laythe capture)

115km (m/s unknown) = 22,000km (~Laythe orbit height)

121km Pe (m/s unknown) = 46,000 km Ap

127km Pe (m/s unknown) = success

Laythe

9km Pe, 7600 m/s - fail, too low

21.88km Pe (m/s unknown) = 235km Ap

Edited October 2, 2012 by bsalis

[elmarko]

If you don't mind using mods, MechJeb's landing module is essential for aerocapture/braking as it gives you a predicted AP depending on where you put your PE in the planet's atmosphere. You may need more than one orbit for greater precision. Also, if we eventually get G-loading/heat sensors for crew survivability, it may be better to do it over a series of orbits instead of one big aerocapture really low down in the atmosphere to get it done in one orbit.

[RoboRay]

The height will depend on your speed. Come from a moon, you'll be moving (probably) relatively slowly and won't need to dip down so low. Zipping in on an interplanetary trajectory you'll have a large velocity difference and will need more aerobraking.

Upon arrival from Kerbin, I aerocaptured at 10.5km over Duna and still had an apodun of 1333km. This was entering the atmosphere at 1900m/sec and departing with 1160m/sec. At 15km, I barely slowed down.

I also aerocaptured at Eve with a 60km periev.

In both cases, subsequent shallower dips into the atmosphere were made to bring apoapsis down where I wanted it before raising periapsis up to circularize. Note that I was aiming for polar orbits in both cases, to map the planets.

Edited September 25, 2012 by RoboRay

[mossman]

My first encounter with Duna I tried aerobraking with a periapsis of about 25 km; That didn't even get me out of my hyperbolic orbit so I did some retrograde burns to get to about a 5000km apoapsis. Then I tried aerobraking at a 10km periapsis, and I had to turn the engines back on to get out of the atmosphere again safely.

Good thing I had plenty of fuel.

[Tauge]

My first Eve encounter ended up with an accidental aerobrake at 70km (didn't take into consideration the thicker Eve atmosphere). Some mild panic before I realized that at worst I'd have an apoapsis at 300km. Adjusted the periapsis up to 80 km and two orbits later I had a nice circular orbit of 120km. I've not tried aerobraking Duna or Jool yet, just retro burn.

[RoboRay]

Oh, since you're summarizing numbers, I believe on my Eve aerocapture I was a little over 4km/sec on entry. I didn't pay attention to the speed on exit, as I was watching my apoev plummet so fast I didn't think I was even going to make it back out of the atmosphere. I cut it pretty close. My initial apoev was about 600km.

As to Duna, I believe I could have safely gone to 10km or maybe even 9.5km to get a more rounded initial orbit. As it was, I had to make two additional braking passes at 18km to get apodun down to 600km and then 200km. Images of the whole process are here: http://imgur.com/a/2ZGCe#0

Edited September 25, 2012 by RoboRay

[RoboRay]

If you don't mind using mods, MechJeb's landing module is essential for aerocapture/braking as it gives you a predicted AP depending on where you put your PE in the planet's atmosphere.

Ah, I did not notice that. I don't use the landing autopilot, so I never checked out it's capabilities. Yeah, apoapsis predictions for aerobraking will be extremely useful to my future attempts, as my current method is "try and reload" a couple of times until I find the right periapsis.

[Koooooj]

Doesn't Aerocapture/Aerobrake altitude depend heavily on the craft's ballistic coefficient? Or do these altitudes work well for pretty much any size craft that could reasonably enter the planets' orbit? I know that the aerodynamics are not yet fleshed out in the game, so I wouldn't be surprised if this is an area where you can't apply real physics yet.

[RoboRay]

Doesn't Aerocapture/Aerobrake altitude depend heavily on the craft's ballistic coefficient? Or do these altitudes work well for pretty much any size craft that could reasonably enter the planets' orbit? I know that the aerodynamics are not yet fleshed out in the game, so I wouldn't be surprised if this is an area where you can't apply real physics yet.

I'm sure the craft's mass and drag values will affect the outcome of the orbit, but when you're aiming +/- 1km for periapsis, I think the variation of the craft's design will probably have less impact than course-deviation errors.

While we will not be able to put together a definitive guide to aerocapture due to variations of craft design and orbital trajectories, we can at least come up with a range of possibilities to get people into the ballpark.

Edited September 25, 2012 by RoboRay

[dikkjo]

When it comes to Jool i suggest not going under 120'000m, my first attempt at 90'000 was pretty catastrophic, no quicksave there

[bsalis]

Thanks guys.

Seems to me so far that 10km is the sweet spot for Duna. My first attempt at 8km was a big fail, so was thinking 15km was the place to be. So little that 2km makes... however velocity matters too obviously. Would be good to get some more info on Jool.

Also with the MechJeb. I use it but did not know that it can help here. Since I only really use it for Ascent, orbital ops (usually), and more recently the Smart ASAS.

[martscht]

I did it for Jool with a periapsis at 121.150 m and ended up with an apoapsis of roughly 46.000 km (http://kerbalspaceprogram.com/forum/showthread.php/21522-Replicate-the-Jupiter-aerobreak-from-2010-the-year-we-made-contact%21). So I'd guess 119.5 km would be the way to go if you wanted to transfer straight to Laythe (which I will try). I transfered to Vall after corrections. BTW, I also failed miserably with 110 - pretty frustrating after 250 days of interplanetary travel...

Edited September 26, 2012 by martscht

[phoenix_ca]

Sad thing is, most of this is likely to go right out the window when re-entry heat gets added. >.<

[elmarko]

Sad thing is, most of this is likely to go right out the window when re-entry heat gets added. >.<

Indeed. So we're going to have to either: 1) Use more fuel for retrograde burning, 2) make a number of passes to slowly reduce apoapsis (takes aaaages), or 3) Some combination of the two.

[Ghery88]

Indeed. So we're going to have to either: 1) Use more fuel for retrograde burning, 2) make a number of passes to slowly reduce apoapsis (takes aaaages), or 3) Some combination of the two.

And we have to take entry angles into account as well. I watched so many fireballs in Orbiter...

Not to advertise my thread, but in it you can find my aerocapture experiences with Jool. http://kerbalspaceprogram.com/forum/showthread.php/21613-0-17-A-Joovan-excursion

Edited September 26, 2012 by Ghery88

[RoboRay]

Well, for aerocapture, your entry angle is always going to pretty shallow. Otherwise, it would either be an impact or a clean miss.

[gosnold]

I managed to aerocapture at Laythe. I was coming through the Jool moon system from the wrong side (ie doing the orbit in the opposite way), so the speed relative to Laythe was 7km/s. The braking was so strong I had to burn upwards while in the atmosphere to get into orbit again.

[phoenix_ca]

Well, for aerocapture, your entry angle is always going to pretty shallow. Otherwise, it would either be an impact or a clean miss.

In the real world, yes. But right now, re-entry heat hasn't been modeled yet, so it's entirely possible to send an object zipping around with a periapsis of 3km in a dense atmosphere without it turning into molten slag.

[martscht]

So I'd guess 119.5 km would be the way to go if you wanted to transfer straight to Laythe (which I will try). I transfered to Vall after corrections

With 119.5 km I got a straight transfer to Laythe: http://kerbalspaceprogram.com/forum/showthread.php/21522-Replicate-the-Jupiter-aerobreak-from-2010-the-year-we-made-contact%21?p=278467#post278467

And with a PE of 21.875 at Laythe I got aerocaptured there.

[Bekiekutmoar]

My speed relative to Laythe was around 7600 m/s and tried areobreaking with lowest point at around 9 km. The kerbonaut is now training his swimming skills in the ocean ...

[bsalis]

I updated my original post again ... also removed some accidental copy-paste from last edit.

So Laythe! I had forgotten that there are moons with atmosphere. Might be while before I get there myself. At least I have an idea of what to expect.

[RoboRay]

In the real world, yes. But right now, re-entry heat hasn't been modeled yet, so it's entirely possible to send an object zipping around with a periapsis of 3km in a dense atmosphere without it turning into molten slag.

Well, yes, but a PE of 3km vice a PE of 10km doesn't change the actual entry angle much, at all. The important thing is the PE itself, as it's defined by the entry angle (and depth of the atmosphere). They are directly related, so if you're talking about changing one, you're changing both. It's just a different way of saying the same thing. We can discuss angles, or we can discuss altitudes, but we're still talking about the same thing in the end.

[antbin]

As said above, MechJeb's landing autopilot has a nice post-aerobraking orbit predictor. But be warned it usually underestimates the braking until your ship enters the upper atmosphere and it can calibrate its estimate of your ship's drag, etc.

So, better to aim high than end up in an unplanned landing/diassembly. Once MechJeb gets dialed in by the upper atmosphere drag, the final orbit estimate becomes much more reliable and you can burn radially up/down to adjust.

I just finished aerobraking through Jool (115km), to set up in an elliptical orbit with apoapsis just higher than Laythe orbit (22,000km), and after 5-6 orbits got a capture. Then, I aerobraked into Laythe orbit (24km), and circularized at a 75km orbit. Soon: landing and return to Kerbin!

Mechjeb (and save-scumming) FTW.

Edited September 29, 2012 by antbin

[bsalis]

Nice double capture antbin. I did one myself recently... however I did a few passes over Jool to luck-hit the Laythe SOI.

Dare I say that a double aerocapture using Jool and Laythe to get to Laythe is The Cool Thing to do at the moment in KSP.

[Trebuchet-Launch]

I performed a Jool aerocapture with a perapsis of 127000m.

Incidentally, it's difficult to build a completely stock, there-and-back-again vehicle which can land on any of the Joolian moons and then return to Kerbin.