How Do You Deal with Inerplanetary Aerocapturing in 1.0.4?

[ShadowZone]

Moons and planets are pretty predictable.

Other than that, the know-how is well documented.

I know the concept, however I would not know how to predict when to make a grav assist using the mun to get to Jool and then use Tylo to get captured. If I did that, I would eyeball it all the way and probably end up burning a lot of fuel to get the necessary encounters, spending more than I would like.

Is there a mod or calculation program/website to get grav assists right, especially when using it for really long range travel?

[regex]

I know the concept, however I would not know how to predict when to make a grav assist using the mun to get to Jool and then use Tylo to get captured. If I did that, I would eyeball it all the way and probably end up burning a lot of fuel to get the necessary encounters, spending more than I would like.

Encounters with Tylo are very easy to get, although you may have to slingshot around Laythe or Vall to get one. Really not that hard to do. The best part is that you can set one up inside the edge of Jool's SOI for around 100m/s. I estimate I saved nearly 2km/s on my current Jool mission by using Tylo to get into a stable orbit and I fixed my inclination while doing it.

AFAIK, gravity assists off the Mun aren't entirely worth it, but they can help. I've never done one but the Mun is much like Ike in that it's in everyone's way.

[Feradose]

I use 120%, and always use a double heatshield for Eve. Returning from anywhere beyond Mun, I prefer slowing down first, or really shallow entries with Pe 65km. My drop pods for kerbals and science use structural parts stacked so they can drop straight down to another planet.

[Violent Jeb]

MULTIPLE passes dipping into the atmosphere between 55-70. Staging to slow myself if needed. I usually have 500 dv on my crafts for getting on the ground.

I have been trying to avoid the heat-shield in lieu of more "re-usable" parts. I try to keep my sensitive equipment behind tanks or within the angle of the command pod.

My most recent find is that a decoupler can be placed on the craft "upsidedown", will still detach fine and can serve as a very functional reusable heat shield. Not sure if it's cheaper than a heat shield, I just know that it works. Who knew there was ablative material on the decoupler lol.

I have seen some of the "huge" shields, and they are not aesthetically pleasing to me, nor would they be fun to get into orbit.

[Wjolcz]

Encounters with Tylo are very easy to get, although you may have to slingshot around Laythe or Vall to get one. Really not that hard to do. The best part is that you can set one up inside the edge of Jool's SOI for around 100m/s. I estimate I saved nearly 2km/s on my current Jool mission by using Tylo to get into a stable orbit and I fixed my inclination while doing it.

http://i.imgur.com/9GTmJUS.png

AFAIK, gravity assists off the Mun aren't entirely worth it, but they can help. I've never done one but the Mun is much like Ike in that it's in everyone's way.

Ooooh, so it's possible to do a Tylo assist for a stable Jool orbit. Cool! Was planning to do that with Laythe aerocapture, but wasn't sure of the results. Time to change the plans, I guess.

And yes, Mun can help you to get out of the Kerbin SOI, but not necessarily the way you would want. Then you need to make correction burns and all that jazz, so it's not really worth it. Better to make a straight SOI exit burn.

[Geschosskopf]

Encounters with Tylo are very easy to get, although you may have to slingshot around Laythe or Vall to get one. Really not that hard to do. The best part is that you can set one up inside the edge of Jool's SOI for around 100m/s. I estimate I saved nearly 2km/s on my current Jool mission by using Tylo to get into a stable orbit and I fixed my inclination while doing it.

I am a proud descendant of Grandpa Erectus. Just as he chipped his rocks the same way for over 1 million years, so I, left to my own devices, never think that I might be able to do things differently. I had a routine of always aerocapturing off Jool then going to the various moons, so never considered there might be other options. However, like Uncle Neanderthal in the Châtelperronian period, I do recognize and steal good ideas from my new-fangled Homosap neighbors .

So today, while performing some more controlled testing of aerocapture, I decided to give this Tylo thing a try. You can see the full report (and the test results) here. In this thread, suffice to say that I got a pretty non-ideal encounter which, though it did capture me in Jool's SOI, put me into a retrograde orbit relative to Jool. But it was that or nothing, due to the phases of the moons when I got there. Still, I managed to fix the retrograde and end up in a 100km orbit at Laythe (after aerocapture there) for a total of 484m/s after entering Jool's SOI, without touching Jool at all. Thus, this was sort of "MOAR Fuel Lite": somewhat (but not excessively so) more than a Jool aerocapture and rather less than using thrust alone.

The efficiency of using Tylo obviously depends on where it is when you get to Jool. Ideally it would be like in the pic on the KSP wiki, where you meet it pretty much tangent to its orbit to the right of Jool as you go in. I could only hit it at right angles on the back side of Jool. I doubt this is the absolute worst case but I'm thinking it's up there. Other than being totally unable to get a Tylo encounter (say due to having to fly through Jool), what is the worst case? Is it even possible NOT to get a Tylo encounter? If you build your ship to rely on using Tylo can can't hit it, you're screwed.

It's this uncertainty about always being able to get Tylo encounter that makes me hesitant to use this as a standard tactic. While you can, if you want to spend the time, set up a perfect Tylo encounter when you do the burn at Kerbin, this is only really useful if you're just sending the 1 ship instead of a flotilla and can warp directly to the burn. If you change focus away from the ship, you'll hose the accuracy of the burn. And even if you avoid that, any inaccuracies in burn execution, trajectory bumps and SOI boundaries, and trajectory sways when rotating the ship via RCS, can all change your Jool arrival date by days to weeks, throwing your prediction of Tylo's position out the window anyway. So I'm thinking Tylo's position when you get to Jool is all luck of the draw. If you can ALWAYS get an encounter, then just bring fuel for the worst-case scenario and you're fine. But if there's a chance you can't hit Tylo at all.............

-----------------------------

Anyway, back to the actual aerocapture testing. After dancing around Tylo, my ship soon hit Laythe's air doing 4200m/s, which is rather fast, seeing how ships hitting Laythe direct from Kerbin usually arrive with about 4500-5000m/s. I still had reentry heat set to 20%, which proved WAY too low. Nothing got hot at all even going down to a 34km Pe. Wow.

So I learned something important. For this particular ship in 1.0.2, aerocapture at Laythe was even more severe than at Jool, even though at a lower speed than this. I had therefore been expecting Laythe to be a real bear this time, given what Jool was like in 1.0.4. However, it turns out that 1.0.4 treats Laythe differently than 1.0.2 did. Instead of being way worse, in 1.0.4, Laythe is way easier, despite 1.0.4 apparently making about 4x times as much heat (at least at Jool). And that's a serious point in favor of 1.0.4's heat model.

Anyway, I'll have to try this again (now that I have a named save I can play with) using higher heat settings and see what happens.

Hmmmmmmmm............

The reason I started this thread was that I had come to the belief that trying to force things to be dangerous for LKO reentries had made any sort of useful aerobraking, let alone aerocapturing, at other planets pretty much impossible. Here's how I arrived at that conclusion:

* In 1.0.2, things were quite bad at Jool and even worse at Laythe

* In 1.0.4, things are about 4 times worse at Jool, so I assumed Laythe was right out the window given as the game had been making it even harder than Jool. And by extension, Eve and returning to Kerbin would be quite problematic.

This assumption has proved incorrect, however. For some reason, the various aero and heat changes in 1.0.4 appear to make Laythe quite doable, although further testing is required to determine how bad things really are there at higher heat levels. In any case, things aren't so bad as I originally thought, although I think Jool's a bit overblown.

Anyway, I started this thread to get more knowledge and that seems to be working. Thanks to all who've contributed and kept it clean so far.

[DJK]

I use 200% thermal heat settings (tweaked the save file) and do a proper reentry. Multiple passes if speeds are extreme. Works like a charm.

[regex]

It's this uncertainty about always being able to get Tylo encounter that makes me hesitant to use this as a standard tactic.

I don't rely on it either, which is why I pack enough fuel for a proper braking burn, but it is nice when it happens and it's common enough that you can make it work. At the very least, you can use Vall or Laythe to help slow down as well, a point I made earlier.

Anyway, the whole Tylo thing was obviously tangential to the original discussion, I was just answering questions. Sorry to get in the way.

[Mischief]

Jool is tougher, no question. If you're flying aero you'll want to come in high and shy. Nose up to maybe 5 deg angle of attack. While it seems a bit counter intuitive it spreads the heat. You go in ballistic and you're essentially using your nose to eat all the heat.

Nose up a bit and it'll spread out more, also plop a pair of radiators on the top of the plane just back of the cockpit. Helps .... cog forward and they will eat a ton of that heat. Just nose up so it stays in the shadow of the craft and isn't building heat of its own.

Just nose up shallow. Top edge of prograde indicator or it'll rip you apart.

Airbreaking a ssto/aero design on jool is ballsy but it can be done and I can see where the 2k dv you save can be important to make the build fly.

[Anglave]

I had been unaware that 1.0.4 upped reentry heating, until trying to aerobrake at Jool (for a Laythe orbit). My conclusion is that (using stock parts with 100% heating) touching Jool's atmosphere AT ALL at interplanetary transfer speeds is suicide. There's no way to hit high enough nor shallow enough to not burn up instantly. It steps directly from no atmosphere to lethal, which is ridiculous.

Since then I've also tried aerocapture at Eve, and had to make 20+ passes to avoid catastrophe.

In the future I'll be playing with a lower heat setting. I didn't even notice the heating difference in 1.0.4 until trying to do interplanetary transfers, where it's all but instantly fatal. I understand a desire for realism, but the current system (at 100%) makes atmospheric braking somewhere between "infinitely tedious" and "instantly fatal".

[Frank_G]

Remembering that the whole purpose of aerocapturing is just to save some fuel, how much did you end up saving afer deducting the costs of launching, docking, and carrying this structure to Jool? If I read the dV maps aright, the most aerocapture at Jool can save you is less than 2000m/s, rather less than the cost of launching something from Kerbin.

One is for the coolness... Two is, the assembled heatshield weighs about 20 tons, the whole craft easily adds a zero and more. So the fuel for 2.000 delta V is by far more massive than the shield.

[Geschosskopf]

I don't rely on it either, which is why I pack enough fuel for a proper braking burn, but it is nice when it happens and it's common enough that you can make it work. At the very least, you can use Vall or Laythe to help slow down as well, a point I made earlier.

Anyway, the whole Tylo thing was obviously tangential to the original discussion, I was just answering questions. Sorry to get in the way.

I consider this dead on topic. The question was how folks deal with interplanetary aerocaptures these days. One of the answers to the poll is to not do it, and another answer is "other". Tylo qualifies under both.

So, what's the likelihood of NOT being able to get a direct (as in not bouncing off other moons or Jool first) Tylo encounter? It's got to be pretty low just based on the geometry. Jool, atmosphere included, really isn't that wide on the scale of its system of moons, so there's not much Tylo can hide behind. And if you go indirect, with bank shots off Jool itself and/or Laythe and/or Vall if they're handy, you can "shoot around the corner" to at least some extent, and further reduce Tylo's ability to hide.

Now, bouncing off Tylo can definitely put you in a retrograde orbit relative to Jool. This would seem to be the likely result if you hit Tylo when it's ahead of Jool. But that's really not so bad if your Ap is still out near Pol and your orbit is pretty eccentric (as in long and skinny). Then it only costs a couple hundred to flip it over to prograde.

The more I think about this, the safer it seems.

Jool is tougher, no question. If you're flying aero you'll want to come in high and shy. Nose up to maybe 5 deg angle of attack. While it seems a bit counter intuitive it spreads the heat. You go in ballistic and you're essentially using your nose to eat all the heat....Airbreaking a ssto/aero design on jool is ballsy but it can be done and I can see where the 2k dv you save can be important to make the build fly.

Thanks for the info.

I'm in the habit of designing Laythe SSTOs and space-incapable airplanes just to do their specific jobs and shipping them out there docked to transfer tugs/stages, rather than compromise their job performance by making them do the trip under their own power. So adding the fuel for them to not need Jool areocapture just means a bigger transfer tug/stage. which is relatively trivial as things go in KSP. But it's quite annoying to HAVE to do that instead of being able to aerocapture off Jool, because it's a degree of player freedom taken away. Dragging a tug/plane combo through Jool's air these days at 100% heat is pretty much hopeless unless you're willing to build a huge shield, spam radiators (WHICH SHOULD NOT WORK), or some such. This makes me sad.

I had been unaware that 1.0.4 upped reentry heating, until trying to aerobrake at Jool (for a Laythe orbit). My conclusion is that (using stock parts with 100% heating) touching Jool's atmosphere AT ALL at interplanetary transfer speeds is suicide. There's no way to hit high enough nor shallow enough to not burn up instantly. It steps directly from no atmosphere to lethal, which is ridiculous.

This is my main complaint. I'm thinking the root cause is that the curves for the present aero model, if allowed to run out to their asymptotes, would result in much taller atmospheres at places like Jool than allowed by tradition and the arbitrary caps we have. Thus, the thin, wispy, upper areas we'd normally be aerocapturing in have been lopped off at the shoulders by the arbitrary height caps.

[Brotoro]

Setting up a Tylo or Laythe gravity capture into Jool orbit is not as difficult as you might think. The trip out to Jool takes a long time compared to the orbital periods of those moons, so just increasing or decreasing your travel time by altering your trans-Jool trajectory can get you there at the proper time for the encounter you want. You just need to tweak you prograde/retrograde in combination with your radial outward/inward components (while focusing on the Jool system as you tweak the distant maneuver node in the background) to see ho this changes things. I usually do this targeting burn just after leaving Kerbin's SOI. The prograde/retrograde component will shift your arrival time, and the radial component can be used to target the encounter. You want to arrange for the encounter to be tangential to the moon's orbit...and the distance you pass by the outside of the the moon will determine the resulting capture orbit around Jool. I play with it until I get an acceptable result (I like the Precise Node mod for the fine tweaking)

[Geschosskopf]

Ah, the master has spoken Thanks for the tips, Bro.

The disconcerting part to me is that just the time it takes to move from the edge of Jool's SOI to where the big moons life (about 12x 24-hr days) is also a long time compared to the orbital periods of the moons. If you delay getting the intercept until you enter the SOI, you really have no idea which way to shift your orbit, and it can be quite frustrating to fiddle around with if there's any significant inclination difference between your ship and the moons at that point.

It seems to me that the ideal case would be as follows: If Jool is the center of the clock face and your ship enters the SOI at 6 o'clock, then you want the intercept at 3 o'clock relative to Jool, passing just inside the moon. If you intercept near 9 o'clock you'll end up in a retrograde orbit of Jool afterwards. Is that correct?

[Mischief]

Can someone post a pick of a ship that gets insta-gibbed going to eve or jool? Speeds too?

I am having a different issue - I have to get pretty insane on my approach to really risk cooking up. I'd it my design? Speed? Do I just play it super safe compared to everyone else?

I'd love to see an example of a ship that is really not happy with an eve/jool aerobreak and what speeds that involves.

Edited July 1, 2015 by Mischief

[Brotoro]

Ah, the master has spoken Thanks for the tips, Bro.

The disconcerting part to me is that just the time it takes to move from the edge of Jool's SOI to where the big moons life (about 12x 24-hr days) is also a long time compared to the orbital periods of the moons. If you delay getting the intercept until you enter the SOI, you really have no idea which way to shift your orbit, and it can be quite frustrating to fiddle around with if there's any significant inclination difference between your ship and the moons at that point.

It seems to me that the ideal case would be as follows: If Jool is the center of the clock face and your ship enters the SOI at 6 o'clock, then you want the intercept at 3 o'clock relative to Jool, passing just inside the moon. If you intercept near 9 o'clock you'll end up in a retrograde orbit of Jool afterwards. Is that correct?

What determines whether or not a gravity assist will speed up or slow down a ship depends on how the directions of the ship's incoming and outgoing velocity vectors compare to the direction of the velocity vector of the body you are encountering. If the outgoing vector is closer to the direction of the planet's vector than the incoming vector was, then the ship will be sped up by the encounter. If the outgoing vector is further away from the direction of the body's velocity than the incoming vector was, then the ship will be slowed down by the encounter. So if you are coming in tangentially to the body's orbit (the ship's velocity vector is in the same direction as the body's velocity vector), then the ship will be decelerated by the encounter if the ship passes on either side of the body (since the outgoing vector will be bent further away from the planet's velocity vector). By shifting the path to either side, you can try to get as close as you can to a desirable post-encounter orbit.

For an example, see the "Tricky Tylo Transfers" section of Long-term Laythe episode 31. I had sent in a payload for Tylo, intending to use a weak aerobraking at Laythe to slow it into Jool orbit (without stopping at Laythe)...but I found out that a near miss of Laythe (with no aerobraking at all) resulted in a gravitational capture with a periapsis out by Tylo. See the image of the capture trajectory.

The same thing happened in the "But Wait, There's More! Maneuver Mayhem!" section near the end of Long-term Laythe episode 35 when the Pol and Bop payloads arrived. Again, I had originally thought to use aerobrakings at Laythe to slow them into Jool orbit, but instead shifted them into Tylo gravity captures (now that I understood gravity captures better). The capture trajectories are shown.

Edited July 2, 2015 by Brotoro

[NathanKell]

Geschosskopf, thanks for this topic.

Just as a note, I think you're right on the money with the issue being that heating is optimized for Kerbin. However, I don't think it has to be an either-or situation, or a "use funky numbers because ~Kerbal~" situation. In fact, in real life a gas giant's atmosphere produces rather less shock heating than Earth's, and that can (and IMO should) be modeled in KSP. A long time ago in the DRE thread ferram calculated shock temperature vs. velocity curves for various atmosphere types, and I think KSP's heating system would be well served by varying shock temperature (or transfer coefficient) by atmosphere type.

[ferram4]

Oh, you mean these guys?

Temperatures below 3000 m/s:

Temperatures at higher velocities

velocity in m/s, stagnation temperature in K

Thinking back, I'm not sure exactly how accurate these are, especially at the higher energies. The right next step would be to do an Equilibrium Gas calculation and work from there instead, but getting the Gibbs Free Energy values (or something equivalent) for ionization is... difficult.

[Geschosskopf]

Just as a note, I think you're right on the money with the issue being that heating is optimized for Kerbin. However, I don't think it has to be an either-or situation, or a "use funky numbers because ~Kerbal~" situation. In fact, in real life a gas giant's atmosphere produces rather less shock heating than Earth's, and that can (and IMO should) be modeled in KSP. A long time ago in the DRE thread ferram calculated shock temperature vs. velocity curves for various atmosphere types, and I think KSP's heating system would be well served by varying shock temperature (or transfer coefficient) by atmosphere type.

I don't think it has to be an either/or thing, either, and I wish it wasn't. Like with Jool, if its upper atmosphere is supposed to be mostly hydrogen, hydrogen is a wonderful heat-absorber---it has the highest specific heat of any common element. Of course, it's not very dense but even so, it can still do wonders. So just as water in a plastic bottle will keep an external match flame from melting through the bottle by sucking away all the heat, you'd think going through the upper layers of Jool would be pretty benign, as shown on Ferram's graph.

While I've got the attention of both the DRE guy and the air guy, what has to be done differently from stock to make this sort of thing happen? And do your respective mods do this or plan to in the near future? Or do you all know if Squad has any plans to address this?

What I mostly complain of is the lack of wispy upper layers at other planets. You can hit Kerbin at like 3500-4000m/s coming home from Minmus or Duna, yet no flames appear at all until you're below 40km, nearly half-way to the surface. But hitting Laythe at comparable speeds and it's full-on flames just under the surface, and even more-so with Jool. I still haven't gotten around to trying Eve in 1.0.4 but from what I hear, it's the same way. What can be done about that? Would it be better to increase atmosphere heights or revise whatever formulae define them?

[Superfluous J]

I've one foot in the "Aerobraking is just one of my many tools" circle, and another in the "I haven't done it enough - yet - to really know" crowd. Sadly 1.0.4 came out right in the middle of a whole bunch of RL stuff that has caused me to play almost no KSP for weeks. However, I've done a couple aerobrakings (at Eve of all places) and can see the pitfalls and benefits of it.

So far, I like the 100% heat setting and the challenges it provides. Sometimes I miss the "straight down to your landing spot" re-entry methods of 0.90 and before, but more for nostalgia than any actual desire for it to be the norm.

[NathanKell]

Geschosskopf: I can't speak for Squad, but I certainly for myself prefer making similar changes to what you describe, and it's something I suggest we both, err, suggest, for 1.1.

Adding curves like this would be very possible for Deadly Reentry (although that's Starwaster's baby rather than mine), but it wouldn't do what you want about lower-pressure upper atmospheres. For that you'd need to change the celestial body curves, which is a fairly trivial piece of code to load and apply new curves.

Part of what's going on here is that the heat transfer coefficient for convection is rho^0.5, which "matters" more in Jool's very-not-dense upper atmosphere than it does in the comparatively denser Kerbin upper atmosphere (i.e. at rho ~= 1e-7 that's a density coefficient for convection of 3e-4 but drag still uses 1e-7). Not sure about Laythe though.

[Brotoro]

I sent a test payload to Laythe for aerocapture...a new Big Advanced Nuclear Tug with a random payload stack (something made for Duna, actually).

Below is the trajectory (focused on Jool) after the fine-targeting maneuver made at the descending node (which happened to be a moderately short way out of Kerbin's SOI).

Once the ship entered Jool's SOI, I used the RCS to tweak the intercept altitude to a bit over 43 km (MechJeb was indicating a post-aerocapture apoapsis of 3,000 km for that altitude).

The aerocapture had lots of flames, but showed no overheating.

Note that this Laythe intercept was nearly tangential to Laythe's orbit, which is the best case situation because the intercept velocity is minimal that way (this is what I normally shoot for).

The actual apoapsis after aerocapture was 2,500 km instead of the intended 3,000 km.

The heating did indeed seem to show up very suddenly once the ship dipped into the outer edge of the atmosphere (under 50 km), so the atmosphere doesn't seem to fade out gradually, but instead cut off sharply at 50 km.

CONCLUSION: Direct aerocapture at Laythe is possible without special heat-shielding designs if you use a tangential intercept.

Edited July 2, 2015 by Brotoro

[Geschosskopf]

Part of what's going on here is that the heat transfer coefficient for convection is rho^0.5, which "matters" more in Jool's very-not-dense upper atmosphere than it does in the comparatively denser Kerbin upper atmosphere (i.e. at rho ~= 1e-7 that's a density coefficient for convection of 3e-4 but drag still uses 1e-7). Not sure about Laythe though.

I don't understand what this "convection" is all about in the context of a reentering ship. I was an engineer in a prior career but didn't mess with thermodynamics except in college. These days I'm a firefighter by trade so study and deal with heat transfer on a daily basis, but in a less rigorous way than before, so my technical vocabulary has probably slipped a few cogs. But as I deal with heat these days, heat transfers in 3 ways: radiation, conduction, and convection. Radiation is electromagnetic waves/photons, conduction is through direct physical contact, and convection is how heated fluids expand, thereby becoming lighter than their surroundings, and so move upwards. Regardless of method, heat transfer is a separate thing entirely from heat creation. You have some process that creates heat, and then that heat transfers into the environment via one of these methods.

Thus, in the case of a ship hitting an atmosphere, the heat we're concerned with is being generated by the mechanical friction of the air on the ship, mostly on its front end. The rate of heat production thus depends on the speed of the ship, the density of the atmosphere, the sleekness of the ship---everything that makes for more air molecules hitting the ship, and hit it harder, in a given unit of time. Some of this heat is being radiated away in all directions (if you can see the flames, heat can see you). Some of the heat is conducting through the rest of the ship from the point of heat creation. Heat is also being conducted through the air from the source into the surrounding air, but if any pre-heated air hits another part of the ship, that's another, separate source of heat creation by friction. The only convection going on is in the ship's wake, as the air heated by its passage rises in the surrounding atmosphere, but won't cause any heat transfer to the ship, which is long gone. And the fact that this convection is happening means not all the heat created by the friction was absorbed by the ship.

So obviously, we mean different things by "convection". What does "convection" mean in KSP's heat model?

CONCLUSION: Direct aerocapture at Laythe is possible without special heat-shielding designs if you use a tangential intercept.

Looks good. Thanks.

[DoToH]

Thus, in the case of a ship hitting an atmosphere, the heat we're concerned with is being generated by the mechanical friction of the air on the ship, mostly on its front end.

I think that's wrong. IIRC, reentry heat is generated mainly by air compression. Air in front of the object entering atmosphere is compressed due to object's speed, thus raising its temperature proportionally to compression. Heat is then transfered from air to the ship.

Someone please correct me if I'm wrong.

On topic:

Do you guys think Jool's atmosphere values should be tweaked to allow aerocapture?. TBH, I never tried the tylo assist, aerocapture at Jool was the easiest option until 1.0.3 and I never tried another.

Edited July 2, 2015 by DoToH

[Geschosskopf]

I think that's wrong. IIRC, reentry heat is generated mainly by air compression. Air in front of the object entering atmosphere is compressed due to object's speed, thus raising its temperature proportionally to compression. Heat is then transfered from air to the ship.

By what mechanism does the heat from the compressed air get into the ship? It's not convection, certainly--that's just totally the wrong mechanic for this. There would be some radiation, but as the radiation goes in all directions, most of it is heating the surrounding atmosphere, not the ship. And no individual air molecule is in contact with the ship long enough to do much conduction. However, that air molecule is hitting the ship at Mach-whatever, which generates heat all by itself at the point of impact. Thus, I'm pretty sure the main way of heating the ship is by mechanical friction. I suppose any radiant heat it absorbs is indirectly caused by the mechanical friction of compressing the air, but I don't see that in itself as the major heating cause.

Instead, I believe most of the ship's kinetic energy loss does indeed go into compressing the air, which heats it, and then that heat is released and transfers to the surroundings as the air decompresses after the momentary passage of the shockwave. But most of that heat radiates in directions other than towards the ship.

Do you guys think Jool's atmosphere values should be tweaked to allow aerocapture?

Hmm. Jool aerocapture IS possible right now in limited circumstances. I would change it so it's doable under a wider range of circumstances.

You cannot look at a reentry into Jool, Laythe, or Eve, and say it's realistic that flames start the instant you barely touch the top of their atmospheres. Consider how on Kerbin you have to get below 40km, halfway to the ground, before flames start. That's more like what you'd expect if the atmosphere faded out to nothing realistically instead of being chopped off at the shoulders by an arbitrary cap. IOW, the atmospheres of the other planets are too dense at their tops.

So, if you want realistic aerodynamics and atmospheres, the other planets need to have thinner upper parts. Whether this happens by raising the tops of their altitudes to where their existing curves would have them fade away, or change the curves so they fade at their existing upper limits, I really don't care. But one or the other needs to happen, strictly from a "realism" standpoint. Having too much density at the top like they do now, these atmospheres are more like liquid oceans than gases, which is just wrong.

Now, a side effect of fixing these atmospheres will be the creation of thin, upper layers in which interplanetary aerocapture will be rather easier than it is at present. More like at Kerbin coming home from Minmus or Duna. But what's wrong with that? If it's possible, and "realistic" under an internally consistent set of rules within the game, then I certainly have no problem. And I think it's good for gameplay because it gives players more scope for more ambitious projects without saddling them with tedious workarounds. But then, I get my jollies from KSP out of thinking up something cool to do at another planet, designing the hardware for that, and then playing with it at the destination. I see the trip out there as just "fly-over country" and warp through as much of it as possible. You might have other priorities.

I'm sure somebody will say "but nobody does interplanetary aerocaptures in real life so they shouldn't be able to in KSP". But such folks are forgetting that KSP is toy-sized compared to real life. It takes an order of magnitude less dV to get anywhere in KSP, which means ships are moving proportionately slower when they arrive, so don't hit atmospheres nearly as hard as in real life. If you're willing to accept the small size of KSP, then you have to live with things like this. If the atmospheres are "realistic" in KSP and its rules are internally consistent at all planets, then interplanetary aerocapture should naturally be something commonly used. It only makes sense to do it if you can.

Edited July 2, 2015 by Geschosskopf