Aerobraking/ReentryHeatManagement - impossible in 1.1.2?

[BoPET]

Well, that's an interesting problem (and for some reason I haven't found a comprehensive answer to it yet -maybe I just googled too bad but nevermind): since recent updates it seem that aerocapture became an extremly dangerous and almost impossible thing - especially if we are talking about gas giant aerobraking. Well, no more "Precisely aerobrake 200-tonn interstellar ship in Jool\Sarnus\Neidon(OPM FTW) atmosphere". It's either burns to charcoal or decelerates too fast and eventually burns or flips over and... well, burns to hell. 

The thin aerocapture window between "burn to hell\fall down" and "fly away forever" became way too thin. I'm not whining, it's exactly how it works IRL - too dangerous. But I want to hear your suggestions and share experience: what aerocapture profile do you use without burning\flipping over\etc?

The second question is a more general question regarding heat system and all such thing: well, I get the basic concept of "exposed to convection parts heat up, transfer some heat to other parts, etc", but I don't get some minor details:
1. Does the amount of heat generated during reentry depends on drag of detail?
2. Is it a wise idea to add a lot of radiators to heat shield\detail just above heat shield (because HS still heats up and transfer heat to other parts) and just... cool it, extending duration of reentry? 

3. Why during the reentry sometimes the TAIL of my craft heats up, while parts in the middle remain cold. Only the parts exposed to incoming air heat up, aren't they? 

4. How, in the name of all gods out there (And the Almighty Probe from Orbiter of course)  do you land a spaceplane without burning during reentry? It's a great mystery to me: no matter what do I try, I keep burning up. 

Edited May 11, 2016 by BoPET

[Streetwind]

43 minutes ago, BoPET said:

1. Does the amount of heat generated during reentry depends on drag of detail?
2. Is it a wise idea to add a lot of radiators to heat shield\detail just above heat shield (because HS still heats up and transfer heat to other parts) and just... cool it, extending duration of reentry? 

3. Why during the reentry sometimes the TAIL of my craft heats up, while parts in the middle remain cold. Only the parts exposed to incoming air heat up, aren't they? 

4. How, in the name of all gods out there (And the Almighty Probe from Orbiter of course)  do you land a spaceplane without burning during reentry? It's a great mystery to me: no matter what do I try, I keep burning up. 

1.) Yes. Parts which create a lot of drag implicitly have a lot of air hitting them, and therefore, suffer strongly from shock heating.

2.) Radiators can indeed help you manage heat. However, you should not usually need to cool heatshields. They have ablator that removes heat by burning off. Ablator isn't just a meaningless resource that's simulated to disappear to look nice, but KSP literally simulates the pyrolysis process of the ablator consuming heat energy to destroy itself. The higher the temperature of the heatshield, the faster ablator burns off, and therefore, the more heat load the heatshield can take. If you cool the heatshield with a radiator, it will not ablate as much. Which may be desirable if you want to aerobrake repeatedly, but if one round is all you need, then the radiators are useless dry mass.

3.) Yes, only the exposed parts heat up. If your tail heats up, your tail is exposed.

The part of the vessel which is pointing into the airstream creates a 'bow shock', which is kind of an umbrella that deflects the airstream to the sides, thereby shielding the stack above the nose. However, if your craft is very long, of if the rear sections grow wider than the front ones (or both), then the bow shock may not be large enough or close enough to shield the entire craft properly. Also note that if you are not pointing 100% prograde/retrograde, then your tail may simply swing out of the protected zone.

4.) A radiator panel on the top, near the nose, might help, since it should be shielded by the craft for most of the reentry (if you place it into the correct spot).

Other than that? Reentry profile makes or breaks your plane, literally. Don't spend too much time in the upper atmosphere, unless you plan to skip out again for a while, because the upper atmosphere has barely any drag but enough heat for it to accumulate. Drag increases much faster than heat buildup with air pressure, so you want to go as low as you can without exceeding the heat tolerance of your craft. If you stay high, there will be less heating, but the heat will soak into your craft over time as you wait for the near-nonexistant drag to do something, and you'll be glowing red hot before you even lost any appreciable amount of speed.

Also, take as much heat load off of your nose as possible. The nose is always the first to go, and once you get into the thicker atmosphere, the nose is naturally forced to point into the airstream (you know, as noses are designed to do). So to avoid having too much pre-soaked heat in your nose, and to spend too long in the upper atmosphere, you want to do the pancake maneuver. Orient yourself belly first into the airstream. Not just a little, but literally perpendicular, broad side of the barn, maximum profile. The Shuttle did the same! Hold that orientation as long as you can, until the airstream eventually forces you to flatten out.

At that point, you need to check your trajectory. If you continue to keep your nose up, this may lead to you gain altitude again. That's called a skip reentry,  and can help you reduce the heat load by giving you some time to radiate away some of the heat your craft has accumulated as it flies a suborbital arc before descending again. However, if you're aiming to come down in a specific area, that may not be desirable. You'll be forced to put the nose level. But that makes it bear all of the heat, which is not good for its health, and this is also the plane's lowest drag profile, so you don't slow down properly. Instead, you should roll to the side and pitch up, like you're trying to fly a curve. This will put more of your belly into the airstream again, so you get more drag and less heat on the nose. After a while, roll the other way and pitch in that direction instead, effectively flying consecutive S-turns. The Shuttle did that, too!

 

To return to the aerobraking question: that, too, boils down to essentially the same. You need to avoid putting all your head load on one "nose" part, and you want to put a large surface into the airstream to create drag. The technical term for this is "ballistic coefficient". A high ballistic coefficient means you have very little surface, and a lot of mass behind it. Such a thing will cut through air like an arrow and hardly slow down at all. A low ballistic coefficient means you have a large surface that doesn't weigh much, like a sheet of paper. It slows down very quickly. That's why you pancake your spaceplane belly-forward into the atmosphere. And that's also why your heavy interplanetary mothership fails to survive while trying to hide behind one small heat shield. The mothership either needs more area to brake with - or, it must shed some mass. Otherwise you won't be able to aerobrake with it effectively.

Increasing area can mean something like mounting the new inflatable heatshield. It's only 2.5m while stowed, but when deployed, it clocks in at a massive 10 meters. That's almost the size of a hexagon built out of seven 3.75m heatshields, all in one conveniently stored and launched part! Mind you, this one does not have ablator, and while it is very heat tolerant, you can try supporting it with radiators for added safety.

Edited May 11, 2016 by Streetwind

[Geschosskopf]

3 hours ago, BoPET said:

Well, that's an interesting problem (and for some reason I haven't found a comprehensive answer to it yet -maybe I just googled too bad but nevermind): since recent updates it seem that aerocapture became an extremly dangerous and almost impossible thing - especially if we are talking about gas giant aerobraking. Well, no more "Precisely aerobrake 200-tonn interstellar ship in Jool\Sarnus\Neidon(OPM FTW) atmosphere". It's either burns to charcoal or decelerates too fast and eventually burns or flips over and... well, burns to hell. 

Guess it depends on what you call "recent".  It's been this way for over a year,:).  Since 1.0 came out, interplanetary aerocapture has been a no-go except for between Kerbin and Duna.  Everything else, you try, you fry.

HOWEVER, now in 1.1.x we have a 10m inflatable (but not deflatable) heatshield that seems to take the heat quite well, plus has beacoup drag so slows you down quickly, so the heat goes down quickly.  It works quite well on Eve which is actually the most difficult target.  Haven't tried it on a gas giant yet but I have every confidence.

There's also a mod called Realistic Atmospheres that makes the atmospheres of all other planets gradually fade away to nothing like Kerbin's does, instead of being abruptly chopped off at relatively high density, which is the root cause of the problems with aerocapture.

But all in all, I think touching atmospheres in 1.1.2 is considerably less deadly than it was in 1.0 to 1.0.5.

 

3 hours ago, BoPET said:

The second question is a more general question regarding heat system and all such thing: well, I get the basic concept of "exposed to convection parts heat up, transfer some heat to other parts, etc", but I don't get some minor details:
1. Does the amount of heat generated during reentry depends on drag of detail?
2. Is it a wise idea to add a lot of radiators to heat shield\detail just above heat shield (because HS still heats up and transfer heat to other parts) and just... cool it, extending duration of reentry? 

Technically, radiators should be counterproductive during reentry.  Heat only moves from areas of higher temperature to areas of lower temperature, and radiators are just open doors for heat.  They only cool the ship if it (or parts of it) are hotter than the surrounding environment, but will pull heat into the ship if the surroundings  are hotter.  So I never recommend them for use in reentry and believe firmly that to the extent they actually help in the game, the game is wrong

Seriously, normal reentries to land capsules and spaceplanes are no difficulty at Kerbin, Duna, or Laythe.  The MK 1-2 pod doesn't even need a heatshield.  Just start in a low orbit and aim to come down about 1/4 to 1/3 the way around the planet.  Eve's a bit more trouble and requires a trajectory tailored to your ship.  And with the spaceplane, you need to keep it pitched up fairly steeply (like 45^ or so) with the airbrakes out to start with, so you have the most drag and slow down the fastest. 

It's really only aerocapturing through a dense atmosphere at interplanetary speeds that's a problem.  And for that we now have the big heatshield.

 

3 hours ago, BoPET said:

3. Why during the reentry sometimes the TAIL of my craft heats up, while parts in the middle remain cold. Only the parts exposed to incoming air heat up, aren't they?

It depends somewhat on what parts you're using.  Different parts can take more or less heat.  Because it's usually the nose of spaceplanes that experiences the most heat, if your tail is getting hot it's probably because it's a part that has a low maxTemp rating.  The little thermometers appear when a part reaches 1/2 of its maxTemp, so the lower the maxTemp, the sooner the thermometer will appear on that part.

 

3 hours ago, BoPET said:

4. How, in the name of all gods out there (And the Almighty Probe from Orbiter of course)  do you land a spaceplane without burning during reentry? It's a great mystery to me: no matter what do I try, I keep burning up. 

Like I said, start from just above the atmosphere and retroburn to come down about in about 1/4 of the planet's circumference.  Then keep the nose up about 45^ (more if you can) for as long as you can and have the airbrakes out.  The goal is to slow down as quickly as possible.  What kills you is stay fast a long time.  The faster you go, the more heat you make, and the longer you're going fast, the longer that heat has to heat up your parts.

[bewing]

0) As said above, the inflatable heatshield will aerocapture you just about anywhere, probably including Jool.

1) Yes, but it's complicated. Heating depends on the square root of the drag, but slowing is linear with drag. And the faster you slow, the less heat you generate overall -- if you don't explode first.

2) One small radiator on the top of your ship, attached to the 3rd part of your ship is very helpful. Radiators cool the part they are attached to, and anything attached to that. As said above, you don't want to cool the heatshield. But (if you can) having the radiator cool the part right behind the heatshield can save your ship.

3) As said above, for your tail to be heating, your tail must be exposed to the air -- or it's a bug. But anything getting hot is adding to the drag, and you need the drag to slow down. So as long as it can take it, it's a good thing.

4) There are many schools of thought on this. Some people say to come in steeply. I disagree. I am with the group that say to come in shallowly. Aim the nose of your spaceplane straight up as long as you possibly can. If you need a stronger reaction wheel, add it. The most dangerous bits of speed are the first 400 m/s -- and you can easily shed them in the upper atmosphere (above 35km) if you try. Once you get rid of them, the rest is cake.

-- Also, there is currently a bug with spaceplanes. They are not generating any "body lift". Which means that they are falling out of space faster than they should. The bug is already fixed for the next release. So with the next release, things will get a little easier.

Edited May 12, 2016 by bewing

[Sereneti]

some tipps from me, about the reentry heat:

1) Radiators can help. But: You have to put they in the slipstream of other parts, until you reach thick enough atmosphere that the atmosphere can cool him...

2) Aerobreakes help...

3)

on the way up, drag is your enemy.
on the way down, drag is your friend.
Weight, mass is always your enemy...

Mass is carrier of kinetic energy.
on the way down, you want to convert kinetic energy to heat.
every gram of weight carry a lot of energy.

on an LKO you have a delta v of 3200m/s ,
that means every gram of your vessel has the kinetic energy of 5120 joule...

4)
i will make this evening a album of screenshots from reentry, if you want.
 

Edited May 12, 2016 by Sereneti