Jool Aerobrake - Bug or what?

[cwalrus]

Hi human particle conglomerations.

After doing much research and checking charts etc, I think something is.... amiss.

I am entering Jools SOI from Kerbin like a normal person, and hitting Jool's atmo. at PE of 197km to try aerobraking. Obrit speed of 2500 i think.

Two odd things happen -

One, as I get close to PE, my orbit vector line on the map seems to dip into the atmosphere of Jool.. sort of bending... Is this normal? Why's it bending? That looks bad!

Secondly, the MOMENT i hit the atmosphere, I explode. No warning, not heating up gauges, etc.

Help! I must aerobrake to save the other monkeys I stranded at Tylo!

thanks,

Cam

[Kryxal]

You'd be better off trying either a gravity assist off Laythe/Tylo or aerobraking at Laythe ... Jool is just insane now.

[Jesrad]

Jool has too much gravity, on an interplanetary transit you'll be hitting the atmosphere at speeds ranging 6-8 km/s, and any speed above ~4 km/s is unsurvivable in current KSP. The heating model used by KSP scales the heat to the cube of velocity, so your ship is getting hundreds, maybe thousands of times more heat than you're expecting.

I tried aerobraking on Eve, and at 4.6 km/s from Kerbin transit I kept burning up as soon as my ship reached under 80 km altitude, until I resigned to brake with the engines first to bring my velocity down to 4 km/s and then did a long, boring series of 82 km altitude aerocapture passes that each brought it on the verge of overheating.

Long story short: aerobraking days are pretty much gone except for Minmus/Mun return.

As above, use gravity assists instead to turn a low-Pe hyperbolic trajectory into a high-Pe circularish orbit, or use your main engines to slow down. Just like Real Space Programs.

[Geschosskopf]

HiI am entering Jools SOI from Kerbin like a normal person, and hitting Jool's atmo. at PE of 197km to try aerobraking. Obrit speed of 2500 i think.

Two odd things happen -

One, as I get close to PE, my orbit vector line on the map seems to dip into the atmosphere of Jool.. sort of bending... Is this normal? Why's it bending? That looks bad!

Secondly, the MOMENT i hit the atmosphere, I explode. No warning, not heating up gauges, etc.

Yes, there are bugs (reported already) with orbits in the near vicinity of Jool. The most annoying one is that if you're in a more-or-less equatorial orbit, you can't warp if you're below 1 million meters altitude.

As to the heat for aerobraking, yes, that's a big problem due at least partially to faulty game mechanics. You might want to read.....

* How Do You Deal with Interplanetary Aerocapture in 1.0.4?, which is a community discussion of these issues.

* The above thread references some testing I did in this thread.

* And I did that testing due to bad experiences in 1.0.2 in this post.

The bottom line, though, is that in 1.0.4, heat at Jool increased by about 4x compared to what it was in 1.0.2, and there's really no good way to aerocapture there now that isn't more trouble than it's worth. So for the time being, I recommend instead that you set up a braking gravity assist off Tylo or one of the other big moons to capture at Jool, then go from there to where you really want to go. The last page or 2 of the "How Do You...?" thread discusses this a lot.

[A_name]

As above, use gravity assists instead to turn a low-Pe hyperbolic trajectory into a high-Pe circularish orbit, or use your main engines to slow down. Just like Real Space Programs.

So real spaceships do not aerobrake, ever?

[Geschosskopf]

So real spaceships do not aerobrake, ever?

Well, aerobraking is different from aerocapturing. With aerocapture, you're coming in on an open, hyperbolic orbit at interplanetary transfer speed and using the atmosphere instead of retro thrust to close your orbit. With aerobraking, you're already in a closed orbit so are at lower speed, and you're just using the atmosphere to lower your Ap.

Real spaceships that land on Earth use aerobraking all the time---it's unavoidably part of the landing process . I don't think there's been but 1 or 2 real examples of aerocapture, however. The main reason for that, I suppose, is that our real solar system is an order of magnitude than KSP's, so you need proportionally more dV to go anywhere, and so arrive proportionally more speed. This makes it way more difficult, if not impossible. But if KSP planets have "realistic" atmospheres, then the speeds involved in KSP shouldn't preclude aerocapturing even at Jool. Unfortunately, Jool's air ain't "realistic" at the moment (and never has been AFAIK).

[xtoro]

I managed to aerobrake a probe on Jool. Small probe that had a 2.5m heat shield on the front, dipped in at 198500m and got an elliptical orbit out of it. Braked a couple more times at 199km's. Ended up losing a solar panel and atmosphere science thingy but I had more so no big deal. This was in 1.0.3 though... I have one en route to hook now in 1.0.4 so we'll see how it goes...

[cwalrus]

Well as a matter of fact, I am only trying to get to Tylo anyway. So is my basic understanding of gravity assist correct: approach on the correct side of the planet according to the wiki, and you will lose DV to help you brake. This loss- it is proportional to your proximity to the moon in question? Does it basically make your subsequent retrograde burn at PE cost much less because of the gravity assist? or is burning retrograde at the PE of a potential gravity assist pointless, and only after the subsequent trajectory can you reap the benefits? i.e. entering into an orbit at Tylo cannot be assisted by its gravity...?

What is the most DV efficient way of getting to Tylo? Gravity assist at Tylo, or Aerobrake at Laythe? I had thought it was aerobrake at Jool.. clearly not.

And finally, WHY!!!! did the programmers change the statistics of Jool from one version to the next, especially so dramatically!!!!?!

[Geschosskopf]

I managed to aerobrake a probe on Jool. Small probe that had a 2.5m heat shield on the front, dipped in at 198500m and got an elliptical orbit out of it. Braked a couple more times at 199km's. Ended up losing a solar panel and atmosphere science thingy but I had more so no big deal. This was in 1.0.3 though... I have one en route to hook now in 1.0.4 so we'll see how it goes...

Oh, it's possible, just barely, in 1.0.4. Especially for something small you can wrap up in fairings and heat shields. But beyond that, it gets problematic. The main issue seems to be that no planet except Kerbin has a thin, wispy upper atmosphere, so even with barely getting into Jool's air you're hitting a brick wall, which ain't realistic.

Well as a matter of fact, I am only trying to get to Tylo anyway. So is my basic understanding of gravity assist correct: approach on the correct side of the planet according to the wiki, and you will lose DV to help you brake. This loss- it is proportional to your proximity to the moon in question? Does it basically make your subsequent retrograde burn at PE cost much less because of the gravity assist? or is burning retrograde at the PE of a potential gravity assist pointless, and only after the subsequent trajectory can you reap the benefits? i.e. entering into an orbit at Tylo cannot be assisted by its gravity...?

What is the most DV efficient way of getting to Tylo? Gravity assist at Tylo, or Aerobrake at Laythe? I had thought it was aerobrake at Jool.. clearly not.

And finally, WHY!!!! did the programmers change the statistics of Jool from one version to the next, especially so dramatically!!!!?!

Read those threads I linked a few posts above. The last page or 2 of the 1st thread has all the answers.

As to why the changes, I think that in the rush to get things out the door, Squad optimized the heat/aero stuff for Kerbin without bothering to check its effects at other planets.

[cwalrus]

DANG BLABBIT! So i tried the Lathe aerocapture, and KABOOM! Instantly. I was going 5400 m/s... is that enough to make the difference?

I'm going to play with those atmos. settings now cuz this ain't funny

[Streetwind]

Make sure you are not in timewarp when hitting the atmosphere. Timewarp is screwing up heat calculations all over the place right now. Sure, the game pulls you out of timewarp the moment you hit the atmosphere, but that one moment may be enough to wreck you.

Also, using physics timewarp during reentry also tends to cook you alive.

[Old Foxboy]

Just FYI,

in the real world, any ship that's returning from just a LEO (LOW Earth orbit!), is travelling at around 7000 m/s and enters Earth's atmosphere at this speed.

Just so that you know atmospheric heating in upper atmosphere in KSP is totally crazy off.

[Streetwind]

That's fully intentional - if KSP used realistic numbers, capsules returning from LKO might not even get reentry effects

But the elevated numbers probably scale up way more than expected (and way more than RL numbers would) at higher speeds.

[Warzouz]

This is very sad because aerocapture is fun to do.

[627]

Well, I tried to brake on kerbin on a return trip from dres. That didn't end well, choices were back to sun orbit, burn up in the atmo or cheat some fuel on the empty tanks to brake below 4km/s before hitting the atmosphere.