1.0.4 Aerocapture Testing

[Geschosskopf]

Chapter 1: Jool

So this afternoon I sat down to do some testing of aerocapturing at Jool in 1.0.4. I invite others to do similar testing and post their experiences.

For this series of tests, I used a copy of the Dual FOOL (an orange tank nuke tug with 2 small airplanes radially attached).

Using this ship, I set up a scenario to replicate as closely as I could what the original ship experienced in my 1.0.2 game. I used HyperEdit to put it in a Jool orbit with a Pe of 210km and an Ap just barely inside Jool's SOI, hitting Jool at about the same angle relativel to its orbital path. I also tweaked the fuel so that the orange tank was about 1/2 empty and all others full, and left the airbrakes closed. I then warped this around until it was crossing Laythe's orbit on the way in towards Jool, at which point I created a named save from the KSC screen. From that save, I could vary the ship's Pe and run it through the atmosphere repeatedly. Also, I had a file where I could tweak the reentry heat value.

In 1.0.2, I had a ModuleManager file that raised the maxTemps of various parts from the 1.0-1.0.2 stock value of 2000 to 3000. I did not use that in these tests, instead just letting stock values see what they could do.

NOTE: In 1.0.3/1.0.4, most parts had their STOCK temps raised to 2400. So much for Squad saying "realistically, everything should melt at 2000" . And now I don't feel so bad about my MM file .

What I did instead in these tests was vary the amount of reentry heating. This is easily changed in save files by altering the last line in the PARAMETERS/DIFFICULTY section up at the top.

When 1.0.4 first came out (I never played 1.0.3 due to the quick change there), I tried this test with 100% reentry heat. The result was literally instant vaporization of the ship upon 1st contact with the atmosphere. It happened so fast I didn't have time to see what blew up first. So I tried 50%. Same thing. After that, I got results that could be studied, which I now present:

Test 1: 10% Reentry Heat, Jool Pe 195km

Here is the ship at its actual Pe (due to some slowing) of 194.9km, still moving at nearly 9500m/s. Only the roots of the (NON-deployed) airbrakes even got warm.

The speed is within a 100-200m/s of what the real ship experienced in 1.0.2 on an actual interplanetary transfer, so I consider the test scenario valid for speed. I had the ship rolled so the planes were knife-edge to the atmosphere so their wing lift wouldn't screw things up.

This very shallow penetration resulted in a post-aerobraking (not capturing because it began with a closed orbit) Ap way out beyond Pol's orbit, which would require multiple passes to bring down over the course of about a month of game time.

Test 2: 10% Reentry Heat, Jool Pe 190km

Given that the ship had survived no problem and the Ap was unacceptably high, I repeated the test with a Pe of 190km.

In this case, the airbrakes got a bit warmer than before. Also, the nosecones on the planes' de-orbit rockets and the docking port on the front of the whole thing got a little warm.

A difference in 5km of Pe altitude caused the final Ap to be about exactly as far out as Tylo's orbit.

This was totally acceptable for an Ap, but it seemed the heat was so low I might as well just disable it completely. So, for the remaining tests, I kept the Pe at 190km and increased the heat to 20%.

Test 3. 20% Reentry Heat, Jool Pe 190km

Doubling the heat made things rather more interesting. Nearly everything got to the borderline between green and yellow, and several parts got high into the red, but nothing exploded. Interestingly, while the leading docking port and the airbrakes were amongst the hottest parts, the wingtip nosecones were not despite getting warm at 10% heat. Instead, the intakes on the planes came close to blowing off.

This test came very close to reproducing the heating experienced by the real ship in 1.0.2, although the real ship ended up using a rather higher Pe of about 198km IIRC, which wasn't enough to capture.

Changing the heat had no effect on Ap. It was still out at Tylo's orbit, same as 190km with 10% heat.

At first, you might think this is to be expected. However, as I understand things, part of the heat system is (or, at least, was up through 1.0.2) the leading parts heating the air, which then gave extra heat to the trailing parts. When you heat air, it becomes less dense. sp should provide less resistance, which should have resulted in a higher Ap with the higher heat. So either that doesn't happen anymore, or isn't as "realistic" as some would think, I'm just wrong in thinking this. Whatever.

Anyway, all through these tests so far, I had not used the airbrakes on the planes. This was by accident rather than design (you'll note the brakes light is on in the above screenshots), but the original ship hadn't used them either so this actually helped in making comparisons. I made sure to change that for the last test.

Test 4: 20% Reentry Heat, Jool Pe 190km, 4 Airbrakes In Use

Using the airbrakes had no noticeable effect on the heat experienced by the ship. This is rather different from 1.0.2 where airbrakes could, at least in some cases, pretty much eliminate it. The same parts appear to have gotten just as hot with the brakes out as with them in above.

Where the airbrakes had a noticeable effect was on reducing Ap. This time, the ship ended up just inside Vall's orbit.

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CONCLUSIONS

1. Jool Pe Altitude Has Little Effect on Ship Heat

Jool's atmosphere is so dense that only slight penetrations are necessary for aerocaptures, despite the extreme speed of the ship. The deepest penetration here was only 5% of its depth, compared to about 45% routinely used at Kerbin coming back from Mun or Minmus at much lower speeds even in 1.0.4. With such shallow penetrations, you would not expect much difference in heat between 195km and 190km, and that is indeed borne out by comparing Tests 1 and 2. Of course, deeper penetrations do greatly increase heat, but just skimming the surface like this doesn't make much difference.

2. The Amount of Reentry Heat Increase Between 1.0.2 and 1.0.4

When the real ship did this in 1.0.2, it had 100% reentry hit. It nearly lost parts at a higher altitude and with a few parts tweaked to burn at slightly higher temperatures than 1.0.4 stock. In the 20% tests, the test ship nearly lost parts. To me, this means that the change from 1.0.2 to 1.0.4 increased reentry heat by somewhat less than a factor of 5, but probably more than a factor of 3. As indicated above, the difference in heat at different Pe altitude (in the shallow penetrations used here) was not significant, and the parts that had tweaked maxTemps in 1.0.2 weren't all the same ones that got the hottest in 1.0.4, so that doesn't seem significant, either.

Thus, I estimate that 1.0.4 increased reentry heat by about 4x over 1.0.2. Somewhere in that ballpark, anyway. Quite a significant change.

3. Which Parts Get Hottest in 1.0.4

In 1.0.2, the parts that were most affected by heat were the vertical AV-T1 fins of the planes, then the ram intakes on their noses. Nothing else on the ship was in danger of burning up. Not the docking port on the front of the ship, not the antennae sticking off the planes as you might expect.

In 1.0.4, OTOH, at 10% the 1st things to experience heat were the airbrakes even when retracted, followed by the lead docking port and the nosecones on the planes' wingtips. But at 20%, the nosecones cooled off and the ram intakes got hot. Given that all these parts (except the airbrakes, see below) were leading the charge, this makes more sense to me than what was happening in 1.0.2. So 1.0.4 does seem to have a better heat model than 1.0.2. It's just way more heat.

About the aribrakes in 1.0.4. I note that these parts still have a maxTemp of 2000, so naturally they would reach a higher fraction of that before parts with 2400, and hence start showing temperature gauges sooner. Given what seems otherwise to be an across-the-board increase to 2400, I believe the airbrakes probably fell through a crack. They seem a likely candidate for a ModuleManager patch to bring them in line with the 1.0.4 standard.

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OPINION AND FURTHER RESEARCH

Reentry Heat in General

I'm good with reentry heat as a general concept. I have, after all, frequently used DRE in the past. However, I think it should be just something you have to be aware of and take proper but relatively simple precautions, and then go on about your business. It should not totally preclude you from doing what you want, nor should it require massive changes to how you do them. Furthermore, reentry heating should not in any way be based on how things work on Earth because that's not in the least bit realistic for the KSP universe.

But my opinions are really beside the point. What really matters, and can be considered factual observations of the KSP community, is the effect of reentry heat on gameplay. Is there any gameplay benefit, as in making the game more fun and/or challenging, in having such intense reentry heat? I say there is not, for the following reasons.

Let us assume for the moment that you choose to leave 1.0.4 reentry heat at 100%, or even increase it to the max of 120%. Then, if you want to go to Jool, you have 3 basic options

1. Using stock parts, spend a lot of time and effort designing, testing, and ultimately lifting into space some monstrous shield proof against the heat, and use it to aerobrake any way you want to. This seems merely an attention-seeking display of self-torture given that the weight of the shield and the effort to launch it would probably cost more fuel than saved by the aerobraking.

2. Just carry enough fuel out there to slow down without touching the atmosphere. This just means building a bigger rocket, a trivial exercise, and probably what most folks would do.

3. Somebody will no doubt soon release a mod for a huge, inflatable heatshield like in the movie "2010", or something similar. This is even more trivial. Slap the mod part on pretty much your standard Jool rocket and away you go like the 1.0.4 heat never happened.

Options 2 and 3 clearly add nothing of substance to gameplay, making the reentry heat worthless as a game element. Option 1 certainly would appeal to some members of the community but what do they get out of it except fuel for their egos? It really doesn't make KSP a better game for anybody else.

Therefore, it seems to me that reentry heat, as with pretty much everything the "realism"-mongers carry on about, is something the game can really do without Certainly, imposing it at high levels just leads to workarounds that take it out of play completely, so what's the point in having it at all? But OTOH, having reentry heat at a low enough level that you can't quite ignore it, while still having tangible benefits for dealing with it, then you have a meaningful gameplay element. Folks would have a challenge they didn't have before and an incentive to face and conquer it, rather than just avoiding it altogether with minimal inconvenience.

As a result, I think 100% reentry heat in 1.0.4 is counterproductive to the game and have no qualms at all about turning it down.

What Reentry Setting to Use in 1.0.4?

There is at yet insufficient data for a final conclusion. The real Dual FOOL had even more problems, at considerably lower speed, aerobraking at Laythe than it did at Jool, and I have yet to run similar tests at Laythe (which will be difficult given the limits of HyperEdit). That said, however, it seems pretty obvious that had I used a setting greater than 20% in these tests at Jool, things would have burned up. Of course, the ship used here is about a worst-case for being heat-proof and a different arrangement might be more survivable. But again, Jool is apparently not the worst case out there.

Thus, as a preliminary figure, I would suggest no more than 20% heat. Maybe 30%, but at any higher setting you're effectively just outlawing aerobraking in your game, not giving your self a real challenge. I note that the slider in the game only moves in 10% increments but I don't know if this is just a UI thing or something required by the underlying heat model. And always note that when you take Laythe or Eve into account, or returning to Kerbin, it might turn out that only 10% turns out to be workable.

What's Next?

Well, the easy things to test are Eve and returning to Kerbin, where you have enough room within the SOI to hit Pe at approximately interplanetary speeds. Laythe, not so much. I really don't know how to set up a test for that with HyperEdit. Any suggestions there are welcome. And of course there's Duna, but I don't anticipate that to be much of a problem if you have things set to handle the denser atmospheres.

I have no idea when I'll get around to doing any more testing, but I found this batch of tests interesting so I'm more encouraged to continue than I was when I started. However, I hope others run some tests and post their results here. Peer review, confirmation of results, all that.

[Brotoro]

I ran a test by sending a payload off to Duna (I Hyperedited the ship into Kerbin orbit, then sent it to Duna as usual). When it arrived at Duna, I followed MechJeb's aerobraking advice and targeted it at a periapsis of 19.something km with the intention of capture into an orbit with a periapsis of 2,000 km.

When I had done this previously (back in 0.90) the aerocapture showed no flame effects at all. In 1.0.4 the aerocapture was very flaming. None of the parts overheated (but it's easy for me to not notice the little red indicators). The ship survived without problem, and it ended up in an orbit with an apoapsis of 1,800...so MechJeb (a recent development version) was not horribly off, but is much worse that it used to be at predicting aerocaptures. I play with stock settings, so the entry heating slider was set at whatever its default value is.

[Geschosskopf]

When I had done this previously (back in 0.90) the aerocapture showed no flame effects at all. In 1.0.4 the aerocapture was very flaming. None of the parts overheated (but it's easy for me to not notice the little red indicators). The ship survived without problem, and it ended up in an orbit with an apoapsis of 1,800...so MechJeb (a recent development version) was not horribly off, but is much worse that it used to be at predicting aerocaptures. I play with stock settings, so the entry heating slider was set at whatever its default value is.

For stock atmospheres, all job aids that try to predict trajectories after aerobraking/capturing have been off since 1.0 and probably always will be. This is because, due to drag, the results depend on the configuration of each individual ship---no more "1 size fits all" predictions like we had pre-1.0. And seemingly things like MJ and Trajectories either can't figure out how or can't be bothered to do a detailed analysis of each ship's configuration. Things are different if you use FAR. FAR has its own drag model that voxelizes the shape of each ship somehow, and that data is then available to other mods. Thus, Trajectories at least (don't know about MJ) gives accurate predictions if you use FAR.

[Geschosskopf]

CHAPTER 2: Laythe Indirect

Having decided to experiment further with 20% reentry heat, I conducted another round of testing to see what would happen at Laythe indirectly (that is, after capturing in Jool's SOI). I'm still thinking about how to set up hitting Laythe direct from Kerbin without actually having to fly it. In 1.0.2, Laythe proved even more baneful to this ship than Jool had, to the point I just turned heat off completely. This time in 1.0.4, things were much different.

0.1 SIDE NOTE: LV-N Heat in 1.0.4

Again I used the Dual FOOL but this time actually flew it out to Jool from LKO. It needs an 8-minute burn using 4x LV-Ns and I wanted to see what had changed with them in 1.0.4. Apparently their heat really has been scaled way down. They barely got warm even though the ship had no radiators.

0.2 SIDE NOTE: Avoiding Jool Aerocapture

I started a thread on 1.0.4 aerocapture in the general discussion topic to see what others were doing and the subject of using a gravity braking off Tylo to capture at Jool came up. I'd never done this before so decided to give it a go.

The efficiency of this tactic depends a lot on where Tylo is when you get to Jool, which is luck of the draw. Even if you make the effort to burn from LKO with an optimal Tylo encounter, you probably won't have that when you really get there due to inaccuracies in executing the burn, trajectory bumps at SOI changes, tractory wobbles if rotating using RCS, and of course any mid-course tweaks you do. All these things can change your arrival time at Jool's SOI by days to weeks. Anyway, when I got to Jool, Tylo wasn't in an ideal place and this was the best I could get:

dV for Tylo encounter: 38

While this encounter did capture me at Jool, the resulting orbit was highly inclined retrograde to Jool with an Ap out near Pol and a Pe inside Jool itself.

dV to fix inclination (about 120^ of change): 226

dV to raise Jool Pe to 250km: 1

dV to pull Jool Ap down from Pol's level to Tylo's (to be like after a Jool aerocapture, to lessen the speed at Laythe): 219

Total dV to get here from entering Jool's SOI: 484m/s.

OT1H, that's probably about 400 more than I'd have spent aerocapturing at Jool, but OTOH it's probably about 500-600 less than using thrust alone. I suppose this option could be called "MOAR Fuel Lite" .

Anyway, due to a lucky set of moon phases, a burn of only 14m/s at the new Ap resulted in a Laythe encounter right off the bat. So then it was time to see what would happen at Laythe.

And NOTHING happened. The ship was hitting the atmosphere at about 4200m/s, rather faster than it did in 1.0.2 (IIRC it was then about 3500m/s). But despite diving down to 34km, nothing got hot at all using only 20% heat. No thermal gauges appeared on any parts. That's a huge change from 1.0.2, where 100% heat resulted in instant vaporization with a Pe way up at 45km.

34km (with airbrakes out) wasn't quite low enough to capture but it only took a further 21m/s to close the loop. Still, the ship had nearly 900m/s left in the tanks once settled into a 100ikm orbit, whereas in 1.0.2 it was bone dry.

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CONCLUSIONS

1. 1.0.4 Treats Laythe Differently From 1.0.2

In 1.0.2, aerocapturing at Laythe was more severe than Jool. In 1.0.4, it's the other way around. To me, this makes a lot more sense so I count this as a point in 1.0.4's favor.

2. 20% Reentry Heat is Too Low for Laythe Indirect

Seriously, I might as well not have had it on at all. I withhold judment on what a viable level there is, especially for hitting it directly from Kerbin at interplanetary speed. However, in these tests, the ship arrived at Laythe doing 4200m/s and in 1.0.2, I had ships hit Laythe directly from Kerbin only doing 4500m/s, so I'm not thinking a direct approach is really that much more severe than what i did here. I hope to find out shortly, though, because now I have a save that simulates interplanetary speeds so I can change the heat level in that. Maybe next time.

3. The Validity of 20% Heat

The 20% level was based on aerocapturing this ship at Jool. These tests showed, however, that it is not prohibitively expensive in dV to avoid Jool completely, provided you can hit Tylo. I haven't thought about using Tylo enough to get a feel for what a worst-case scenario there is---would it need much more fuel than I used here, or is it possible not to be able to get a Tylo encounter at all? If you rely on a Tylo encounter and can't get one, you're hosed.

However, I think being able to get some sort of Tylo encounter is nearly certain and I doubt they get much worse than what I had here, given the retrograde orbit I ended up with. So I'm thinking that Tylo can be trusted and you just have to allow an extra 500-800dV for a bad encounter, in which case the need for setting heat down to 20% disappears. What setting it should ultimately be is not yet determined.

[Scotius]

So...what...1.0.4. thinks Jool's atmo is scorching hot for some reason? Weird.

[Psycix]

No, this entire thing is caused by temperatures raising exponentially with higher speeds. This wasn't the case in 1.0.2.

Therefor, you can break quite hard at low velocities, even in dense atmospheres; whereas a thin atmosphere at 8000 m/s is going to be destructively hot.

The low density might lower the heat flux, but the immense heat makes up for that and melts everything, especially through long exposures like skimming across a gas giant...

[Geschosskopf]

No, this entire thing is caused by temperatures raising exponentially with higher speeds. This wasn't the case in 1.0.2.

Therefor, you can break quite hard at low velocities, even in dense atmospheres; whereas a thin atmosphere at 8000 m/s is going to be destructively hot.

The low density might lower the heat flux, but the immense heat makes up for that and melts everything, especially through long exposures like skimming across a gas giant...

Ah, that explains the change between 1.0.2 and 1.0.4 re: Laythe vs. Jool. But my gut tells me that's not quite the right way to model this stuff, at least beyond a certain point. Mechanical friction isn't a very efficient way to heat stuff and it has its limits, so I'd think heat would plateau at some level instead of continuing to increase indefinitely. The shock wave in front limits how many air molecules can ram into the ship at once, after all.