Reentry heating - are you kidding me?

[Superfluous J]

That picture you have of that horrendous rock with wings and a crazy-ass kerbal on top...please tell me you made that, and it actually is flyable.

Click it

[Geschosskopf]

IIRC there was a thread around showing that atmospheric density with 1.0 was actually lower in the upper atmosphere than it was prior. Atmospheric drag being different shouldn't surprise anyone given the entirely new system that doesn't just assume a coefficient of 0.2 for everything.

Well, whatever the intent originally was, the result is that atmospheres really are denser at higher alittudes than before. This is easily seen imperically in just the vastly reduced amount of wing area you need to maintain high-altitude flight these days, and the higher altitudes needed for aerocapturing. But you can do more technical investigations if you desire. Anyway, drag is largely dependent on atmospheric density, so yes, there is more drag at higher altitudes than before. The issue, however, is how that impacts the game.

The problem is, Squad seems to have spent a lot of time carefully tweaking their aero equations so that Kerbin's atmosphere gradually fades out to nearly nothing at its arbitrarily assigned 70km cut-off. This is the only planet where this seems to happen. When these equations are applied to other planets with different gravity and atmospheric parameters, to get the same gradual fading away of the atmosphere, the cut-off altitudes need to be much higher than they currently are. By keeping the tops of atmospheres approximately where they've always been, the game is cutting away the thin upper layers where aerocapturing should be done, and forcing you to go into what are effectively lower, denser regions, despite them being just barely under the cut-off value.

You can see a real symptom of this problem in how all atmospheres now end at nice, round numbers. Those numbers aren't the result of running the equations out to where the atmosphere would naturally fade away, they are just arbitrary caps. As such, they don't fit with the underlying equations, which causes the high-altitude density problems. There are 2 options for fixing this. Either let the tops of atmospheres fall where the underlying equations say they should (and thus all planets except Kerbin would have much taller atmospheres), or make a new set of equations that fits ALL planets and keeps the tops of their atmospheres more or less where they are now. Either way, the aero system right now is broken. And because it's actually the aero system that creates reentry heat (the heat system just moves heat around within the rocket after it's been created), anybody seeking to tweak reentry heating should focus on the aero system, not the heat system.

The decreasing thrust at altitude is to my knowledge approximating atleast two things. The decreasing mass flow rate and the change in compressor efficiency. Combustion engines can't run on the extremely thin air in the upper atmosphere so it needs to be run through a compression stage first to bring it into operational densities. That compressor is going to have a limit past which it can't do it's job effectively enough to feed the engine even if the total flow would be high enough.

I think we're talking past each other. It doesn't matter WHY jets these days suddenly lose all their thrust at about 25km, just that they do. The important thing is that when this happens, they atmosphere is much denser at 25km than it was before. They've still got plenty of IntakeAir even with just 1 intake per engine, and the plane is in nearly level flight, with much smaller wings. whereas before it would have had a steep AoA even with bigger wings. These things are only possible because the air is denser at 25km than it was before.

Larger objects heating up faster is actually somewhat to be expected. Larger objects generally experience a lesser drag force per unit mass (mass is ~cubic with size while frontal area is ~size squared) leading to them decelerating slower and carrying greater speed into the denser parts of the atmosphere. Heating rate is most likely proportional to atleast speed² if not a higher power, so even a slight increase in speed would have a dramatic impact on the generation of thermal energy. Focus that into only a couple of parts that receive the brunt of the airflow and you get more explosions with the heavier vessel

I believe you're mistaken here.

In the real world, the rate at which objects heat up is dependent on their specific heat, which is a measure of how much energy (kilojoules) it takes to change the temperature of 1 kilogram of the object by 1 degree (C or K, take your pick). This depends on the material the object is made of, but we can assume all rockets are made out of more or less the same stuff so have about the same average specific heat. Thus, a given amount of energy will take longer to heat up a more massive object or, if the energy is low enough, totally fail at it. This is why you can use a match to ignite a small twig but not a big log. To make the big log burn, you have to have a lot more energy, which is why when you build a fire, you start with kindling, then sticks, and finally logs, increasing the amount of energy (the size of the fire) at each step.

In this regard, the game's heat system is broken at a fundamental level. It has more massive objects heating up faster than lighter objects. That is just wrong, no 2 ways about it.

Of course, the rate at which objects heat up is also influenced by their ratio of surface area to mass. But again assuming that most rockets are shaped about the same, the bigger object has a lower ratio so should also heat up slower.

[Champ]

I tend to agree, but people who expect to aerobrake at jool or eve in one pass would scream

Don't see why it would be a problem if aerocapture at jool was impossible. I mean, aerocapture at Jupiter may be possible, but i don't think it's been done before.

[OddFunction]

If the the other planets' relative atmospheric properties are the problem when compared to Kerbin then wouldn't the obvious solution be to increase the planet size of Kerbin? That way Kerbin could have a more unforgiving atmosphere without modifying any of the other planets aero properties.

Or, does it make just as much sense to keep the planet size the same and just make it denser?

[Geschosskopf]

If the the other planets' relative atmospheric properties are the problem when compared to Kerbin then wouldn't the obvious solution be to increase the planet size of Kerbin? That way Kerbin could have a more unforgiving atmosphere without modifying any of the other planets aero properties.

Or, does it make just as much sense to keep the planet size the same and just make it denser?

Not really. Remember, 1.0(.whatever) was a total revamp of many things. Atmospheric lift and drag, every engine's basic stats and how they all work at the fundamental level, reentry heating, engine overheating, science mechanics, contract mechanics, you name it. All at once. The 1st several of those have complex interactions that have far-reaching effects all over the game. However, it seems that all this was carefully tailored fit Kerbin and the devs' idea of how SSTOs should work there, without much thought given to the effects at other planets.

So I'm guessing that then they noticed that under the rules they'd developed for Kerbin, just barely touching the atmosphere anywhere else resulted in instant destruction due to the time-honored heights of atmospheres. Oops. So they raised the atmosphere heights just a bit and toned down reentry heating quite a bit so that at least aerobraking, if not aerocapture, was theoretically possible at other planets (or returning to Kerbin). And the result was that Kerbin reentry is a doddle (how many can you do on 1 heat shield?). Which isn't in the least bit surprising given that Kerbin is the only place where the upper atmosphere tapers down to nothing instead of being cut off at the shoulders by an arbitrary setting.

Thus, to me, the easiest solution is to raise the tops of atmospheres of other planets to where they fade to nothing and let it eat any stations in low orbit. It's not like previous updates haven't broken saves. But if that makes for ridiculous atmosphere heights (and ir well could), then the only solution is to revamp the aero stuff again, and this will require revamping all the engines again.

I note, yet again, that all this is the result of trying to impose Earthly values on toy-sized Kerbin. Squad intentionally went for a tiny scale for gameplay reasons and I think they were correct in that. But when you do that, you have to throw out any and all ideas that making things just like on Earth will work at Kerbin. So trying to make Kerbin's atmosphere be Earth-like cannot work well in KSP.

[PDCWolf]

For people that base difficulty around one-pass interplanetary aerocaptures:

You are complaining that something proposed multiple times but shrugged off due to difficulty is not possible in the game. Basing difficulty around something that should be near impossible is something rather dumb to do.

Reentry heat is useless at its actual -non existant- difficulty, even when bumped to 120%. It's your fault, you want to slow down from 11km/s to 2km/s in a single pass with a 100 ton ship.

[Geschosskopf]

@ PDCWolf: You miss the point. KSP is NOT our universe. It's a toy-sized thing with all 4 of the fundamental forces radically different than what we know. NOTHING we're familiar with, right down to the very elements on our periodic table, can exist in the KSP universe. Period, end of story. Thinking "realism" = "just like on Earth" is fundamentally flawed.

[PDCWolf]

@ PDCWolf: You miss the point. KSP is NOT our universe. It's a toy-sized thing with all 4 of the fundamental forces radically different than what we know. NOTHING we're familiar with, right down to the very elements on our periodic table, can exist in the KSP universe. Period, end of story. Thinking "realism" = "just like on Earth" is fundamentally flawed.

Hey now, I'm not the one trying to make stuff works like it does on earth (gravity, atmosphere, etc), I'm just saying that that's what squad is trying to do, so expect it to work as it does.