Mk2 Spaceplane Cockpit - Too sensitive to reentry?

[Knautilus]

Hello good Kerbal folk!

I made a rather nice Mk2-based space shuttle in 1.1.2 (goo.gl/oP0Wt2). On Kerbin reentry the cockpit (Mk2 Standard) heats up way faster than anything else and explodes every time. I tested a shallow reentry profile with PE ~40km from a near-circular 87km orbit and it still cooks off at 33.5km (goo.gl/msjzfL). I'm using MechJeb just to keep AoA very close to zero for this test; my understanding is that an AoA of zero minimizes the stress on the craft (but does it minimize the heating?).

After the cockpit explodes the centers of lift and mass are backwards, and the craft whips around violently, but it stays together and no other parts explode (I've run this test several times). You'd think that the cockpit - with poor Jeb and Bill inside - would be more resilient, or at least as resilient as the Mk2 storage and fuel parts, plus wings and landing gear etc. To me it's a bit unrealistic like this.

Am I expecting too much of the Mk2 cockpit? Am I making an error I hadn't thought of? Or is there maybe a case to make the Mk2 cockpit a little less reentry-sensitive?

Cheers, all. (goo.gl/c1LHLW)

Edited May 2, 2016 by Knautilus
wrong altitude of failure, but doesn't really matter...

[bewing]

Having an AoA of 0 maximizes the heating of the nose! Don't do that. Make your AoA 90 degrees, and you will not have the tiniest heating problem. The aerodynamic stresses are negligible -- you don't need to worry about them.

[Foxster]

Yup, pointing the nose straight up and so using the drag of the wings etc as air brakes in the "secret" to getting SSTOs and spaceplanes down in one piece. 

If you design the craft to have some control surfaces at the front then they will help maintain the steep angle.  

If you still struggle with a monster craft then you can try putting pairs of air brakes at the front and back of the craft angled vertically at assembly such that they point in the direction of travel when the craft is at or near vertical. Kind of a rabbit's ear look. 

[mk1980]

i like to pitch my planes up and down during reentry to distribute the heat to multiple parts. mk2 parts have great heat resistance. they can easily withstand direct aerocapture from a mun or minmus return if you don't point the same part into the airstream all the time.

[AeroGav]

Extreme nose up angles like that aren't necessary unless you have ridiculously undersized wings.  It's also very unlikely you'll be able to maintain control at such pitch angles once the air gets thick - ie. when heating is most critical.

Drag can help get an SSTO down safe but LIFT is much better.     

Re-entry is a process of decelerating from orbital velocity, and as you get slower you fall lower. The more lift you have the higher the altitude you will be able to maintain for a given velocity therefore the less heating you will get.

Maximum lift occurs at 30 degrees nose up in stock aero, it's probably best to aim a bit below that because lift falls off quicker if you go past the stall angle than if you undershoot it.

Also,  I see a lot of people banking hard into S turns to "help me slow down".     Whether your wings are level or banked hard over the amount of forward velocity they are capable of turning into LIFT is the SAME.   They still max out at stall angle, 30 degrees off the prograde.   The difference is that when you are banked over that lift is being diverted away from keeping you UP  into moving you SIDEWAYS.    Therefore you will find yourself at a lower altitude, in a thicker atmosphere  for a given velocity  than if you had just kept the wings level and pitched for max lift the whole time.

Spoiler

 

Note that if avoidance of explosions is no longer the overriding priority,  you can use pitch angle to try control landing site.  Maximum lift:drag ratio occurs at about 3-8 degrees angle of attack (higher during re-entry speeds, lower when subsonic) so if you're undershooting the landing site, pitch for best lift:drag (aka "glide") ratio.    This is a bit counter intuitive because coming from the re-entry attitude that probably means pitching the nose DOWN in response to an undershoot,   and similarly you're pitching UP (to increase drag, provided you can do so without getting into a stall/spin) in response to an overshoot.     The initial trend in the flight path may also be the opposite to what you're looking for, however after 10 seconds or so the change in lift:drag ratio should be having an effect and you'll see the landing point start to move closer in the map screen.

 

 

[LameLefty]

Reentry heating effects are not as simple as indicated by @AeroGav's post indicates. This sentence, in particular: "The more lift you have the higher the altitude you will be able to maintain for a given velocity therefore the less heating you will get." is simply the flat-out wrong way to think about it. 

Ultimately, what you want to do when you enter the Kerbin's atmosphere from space is LAND the vessel. That means you have to shed, at minimum, a couple thousand meters per second of velocity and end up at zero velocity/zero altitude above ground/water level. Energy is proportional to the square of the velocity. You do NOT want to "maintain altitude" or "maintain velocity" as such, during entry from space. Rather, you want to reduce both altitude and velocity in a controlled way. For spaceplanes as well as ballistic capsules, the most efficient way to shed that velocity/kinetic energy is through atmospheric drag, which comes at the cost of frictional heating. However, heat per se isn't the only problem. 

As you may or may not remember from college physics or (better yet) an engineering thermodynamics class, heat transfer naturally goes from hotter things to colder things by way of a couple mechanisms. At very high altitudes (say, 40km - 50 km), the primary ways that matter to us are radiation and conduction. Spacecraft surfaces heat up primarily from radiation - the spacecraft forms a supersonic shockwave ahead of itself, compressing the air in front as it plows through. That air, thin as it is, is VERY hot - typically a couple thousand degrees. It doesn't actually touch the spacecraft surface directly but it DOES heat the nearby surfaces by radiation (think of standing next to a red-hot heating element in a floor heater or placing your hand next to a toaster while it's heating bread ...). 

Now, having heated up the spacecraft belly of wings/fuselage by those radiation effects, those now-hot parts of your spacecraft naturally conduct at least some of that heat away from themselves to the cooler adjacent parts. That's why we sometimes see parts mounted to our capsules or spaceplanes heatsplode even when they're not directly exposed to the entry airflow directly - they're heating up by being mounted to parts that are heating up rapidly by entry effects.

So now with all this in mind, the best way to get a spaceplane down safely and efficiently is to dump as much velocity as possible, AS QUICKLY AS POSSIBLE AND WHILE UP HIGH in the thinner air - yes, slowing your craft will require converting all that velocity/kinetic energy to heat, but doing it QUICKLY minimizes the time during which the craft is exposed to that heat, and thus reduce overall heat flux. A long, slow hot entry is the worst possible way to enter in real-life, and not a great way to do it in KSP either. That kind of entry just lets all the parts of your spacecraft get much hotter than necessary, which can lead to part failure or complete loss of vehicle.

tl;dr: do what @bewing says: entry at a very high AOA with a Pe around 50km. Slow way the heck down before you get under 39km or so, then use your AOA and 30-45º banking S-turns to manage lift and energy for your glide into your chosen landing spot. That's exactly how the shuttle did it for over 30 years, reduced to Kerbin-scale altitudes and such.

Edited May 2, 2016 by LameLefty

[AeroGav]

27 minutes ago, LameLefty said:

Reentry heating effects are not as simple as indicated by @AeroGav's post indicates. This sentence, in particular: "The more lift you have the higher the altitude you will be able to maintain for a given velocity therefore the less heating you will get." is simply the flat-out wrong way to think about it. 

Ultimately, what you want to do when you enter the Kerbin's atmosphere from space is LAND the vessel. That means you have to shed, at minimum, a couple thousand meters per second of velocity and end up at zero velocity/zero altitude above ground/water level. Energy is proportional to the square of the velocity. You do NOT want to "maintain altitude" or "maintain velocity" as such, during entry from space. Rather, you want to reduce both altitude and velocity in a controlled way. For spaceplanes as well as ballistic capsules, the most efficient way to shed that velocity/kinetic energy is through atmospheric drag, which comes at the cost of frictional heating. However, heat per se isn't the only problem. 

As you may or may not remember from college physics or (better yet) an engineering thermodynamics class, heat transfer naturally goes from hotter things to colder things by way of a couple mechanisms. At very high altitudes (say, 40km - 50 km), the primary ways that matter to us are radiation and conduction. Spacecraft surfaces heat up primarily from radiation - the spacecraft forms a supersonic shockwave ahead of itself, compressing the air in front as it plows through. That air, thin as it is, is VERY hot - typically a couple thousand degrees. It doesn't actually touch the spacecraft surface directly but it DOES heat the nearby surfaces by radiation (think of standing next to a red-hot heating element in a floor heater or placing your hand next to a toaster while it's heating bread ...). 

Now, having heated up the spacecraft belly of wings/fuselage by those radiation effects, those now-hot parts of your spacecraft naturally conduct at least some of that heat away from themselves to the cooler adjacent parts. That's why we sometimes see parts mounted to our capsules or spaceplanes heatsplode even when they're not directly exposed to the entry airflow directly - they're heating up by being mounted to parts that are heating up rapidly by entry effects.

So now with all this in mind, the best way to get a spaceplane down safely and efficiently is to dump as much velocity as possible, AS QUICKLY AS POSSIBLE AND WHILE UP HIGH in the thinner air - yes, slowing your craft will require converting all that velocity/kinetic energy to heat, but doing it QUICKLY minimizes the time during which the craft is exposed to that heat, and thus reduce overall heat flux. A long, slow hot entry is the worst possible way to enter in real-life, and not a great way to do it in KSP either. That kind of entry just lets all the parts of your spacecraft get much hotter than necessary, which can lead to part failure or complete loss of vehicle.

 

Well gee, it sounds like you are directly contradicting my advice.   

I was trying to simplify things to get the message across but it looks like there's more to it than that - maybe i don't get all the physics?

I'll explain how i think it works and let me know where i went wrong

1. re-entry is the conversion of kinetic energy of the space vehicle to heat via friction

2. in KSP, i was under the impression that the rate at which this occurs is down to dynamic pressure ie. combination of speed and altitude. the faster and lower you are the more rapidly kinetic energy is converted to heat

3. if heat generation > heat dissipation,  stuff gets hotter and can explode.     In my experience skin temperature is what makes stuff blow, so it's not really about small amounts of heat building up over a long time, just too much heat arriving at once in the very short term - before it has time to conduct away to the core of the part, a radiator , engine pre-cooler or some other heat sink.

4. so, by maximising lift (keeping the wings level and maintaining max lift AoA, but not deliberately stalling) you stay up in thin air as long as possible, slowing the process of frictional heating down.    There is still just  as much kinetic energy to convert to heat, but it's spread over a longer period so the spacecraft can cope.   

So, are you saying that 

a) in the upper atmosphere, ambient temperatures are very high so the craft cannot radiate heat away - heat flows from hot to cold as you point out - so you need to get down as fast as possible

or 

b) that lift may be better than drag, but 90 degrees AoA generates so much drag this wins even though zero lift is produced - it's a case of quantity over quality?

c) some mojo that applies to blunt body shock cones that may not apply to the KSP model?

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tl;dr: do what @bewing says: entry at a very high AOA with a Pe around 50km. Slow way the heck down before you get under 39km or so, then use your AOA and 30-45º banking S-turns to manage lift and energy for your glide into your chosen landing spot. That's exactly how the shuttle did it for over 30 years, reduced to Kerbin-scale altitudes and such

.This TLDR advice appears to be one to MINIMIZE lift - pitch up a deep stall where there is NO LIFT -  then DO VERTICALLY BANKED TURNS where ALL THE LIFT IS APPLIED SIDEWAYS NOT UP - so is the object to fall deep into the atmosphere ASAP so as to pick up as little background heat as possible from the thermosphere?

Example Scenario 

coming down from an 80km orbit with 35km periapse, same starting velocity and height.

One spaceplane rides down for 90 seconds at 25 degrees AoA and wings level.

The other rides down for 90 seconds at 25 degrees AoA but in a 90 degree left bank, attempting to turn as hard as possible.

Both have been at the same AoA for the same length of time from the same starting velocity, so will have lost the same amount of speed, but the one with level wings will surely be higher up in the atmosphere after 90 seconds ?

 

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A long, slow hot entry is the worst possible way to enter

For what it's worth though I've only ever had problems re-entering a spaceplane on Eve, never on Kerbin, Duna or Laythe.   In my experience the higher my periapsis the less heat i experience, though it's going to take a very long time to come down and i'll skip off multiple times.   Also the more lift, and the higher up i stay for a given velocity, the cooler parts stay?

OK, i know this could get heated as we're both taking time to help people but appear to be working directly against each other - hearing someone else tell a person to do the exact opposite to what you think is right is very frustrating.  Apologies also for speaking against some people who are highly knowledgeable.

 

[LameLefty]

2 minutes ago, AeroGav said:

So, are you saying that 

(SNIP)

b) that lift may be better than drag, but 90 degrees AoA generates so much drag this wins even though zero lift is produced - it's a case of quantity over quality?

 

This. ^^^

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Example Scenario 

coming down from an 80km orbit with 35km periapse, same starting velocity and height.

Setting a 35km periapsis is pretty low for spaceplane entry. Remember, the way orbits work, the velocity of a craft orbiting very high is actually quite low; that's why a tiny deltaV can produce such large changes in an orbit. The same object orbiting very low will be moving very fast. So setting a periapsis at 35km, absent any atmosphere, will mean your spacecraft will be moving THAT MUCH FASTER as it hits the lower part of the trajectory as compared to a craft with a higher periapsis, like for instance about 50km.  That means there's just that much more kinetic energy to shed via friction as compared to an entry trajectory with a higher periapsis. So if you set a low Pe and then try to use your wings to counteract the natural tendency of that initial trajectory to forcer you lower into the atmosphere, you're not only greatly increasing the stress on your craft, you're also forcing your craft to go faster, lower in the atmosphere than you otherwise would AND trying to turn that kinetic energy into heat via drag than you otherwise would as well.

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One spaceplane rides down for 90 seconds at 25 degrees AoA and wings level.

The other rides down for 90 seconds at 25 degrees AoA but in a 90 degree left bank, attempting to turn as hard as possible.

Both have been at the same AoA for the same length of time from the same starting velocity, so will have lost the same amount of speed, but the one with level wings will surely be higher up in the atmosphere after 90 seconds ?

True. However, a 90 degree bank isn't really necessary or desirable - banking during entry helps if you're trying to aim for a landing site when your entry trajectory is off-track  from the runway heading you need, and it does help shed kinetic energy (speed) so that you're not breaking Mach 1 at sea level while you try to plant your wheels on the runway! How many banking turns you need depends on how far off-track you are from your intended landing site, how fast you're going when you get down into the <30km altitude range and start really FLYING as opposed to bullying through the upper atmosphere, and how far away you are linearly from the landing site. If your craft has turbojets or something like RAPIERS in air-breathing mode and enough fuel to cruise, you can get away with a lot more slop in the precision of your entry targets, but that's another subject. Anyway, banking to steer your trajectory toward the landing site rarely needs to be greater than about 45 degrees. That's plenty for a fast, very wide turn.

[Warzouz]

Inducing flat spin is also a good way to avoid overheating, and brake very strongly. You can do that by pulling fuel at the back. The manoeuvre is less dangerous than it seems. As the air pressure rises, it's easier to regain control, especially if you put the fuel at the front (but clicking on the moving parts is not that simple).

[AeroGav]

19 minutes ago, LameLefty said:

 

True. However, a 90 degree bank isn't really necessary or desirable - banking during entry helps if you're trying to aim for a landing site

OK so 30-45 degree hypersonic turns are a technique for managing landing point, rather than minimise re-entry stress.  I usually manage this by improving / worsening my glide ratio by choosing a pitch angle closer to or further from optimal lift/drag angle of attack, as per the "spoiler" part of my post but it's all just a matter of preference.

The thing is I was fed up seeing people on Youtube use vertically banked S turns "to slow down" in totally inappropriate situations.   There was a guy coming in at 26km, "oh no we're overheating!"  and rolls 90 degrees left then yanks full back stick.   A few seconds later he blows up.   Why didn't he just yank straight up and try to get up to thinner air?   Worse, some guy in a seaplane (no landing gear) who runs out of fuel over land.   "If I can get my speed low enough he might survive this, let's do some turns".  He pulls some vertically banked left and right turns with full back stick couple of hundred metres up , which certainly scrubs off airspeed but also causes the plane to generate an enormous sink rate because no lift is being used to support the plane's weight over that time period.   The plane doesn't have enough energy to pull out of the resulting dive and slams in at 70 m/s.   

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b) that lift may be better than drag, but 90 degrees AoA generates so much drag this wins even though zero lift is produced 

There's several reasons why i don't pitch up to 90 AoA above 40km like you appear to be advocating

1) I'm not used to it and would have trouble predicting my landing point, especially if the plane starts tumbling out of control

2) I find it ungainly , and unnecessary to get in anywhere with the possible exception of Eve

Just after 1.05 patch came out I tried to re-enter a spaceplane in Eve, it was fully loaded with fuel which wouldn't have helped.  Tried my technique, tried 90 degree nose up , tried deliberate flat spin.   None worked, deliberate flat spin got us 5km lower before exploding but that was because a) we were falling more quickly, in terms of time we might not have lasted much longer  b) heat was being spread out across all parts of the ship as we tumbled.

3) I'm concerned about reaching 40km with a very high rate of sink which my wings will be unable to arrest in a timely fashion.  That sink rate built up over time because we weren't generating any lift prior to that point and because we've also lost more velocity than normal.   OK I can accept that the lower forward speed at this point compensates for the fact you're about to plunge harder, deeper into the atmosphere than if you'd been making lift the whole way down.

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Setting a 35km periapsis is pretty low for spaceplane entry

Not really.  From low Kerbin orbit in a mark 1 cockpit and FAT-455 airliner wings it's probably not worth aiming any lower, but mark 2 designs with Big-S wings can handle coming back from Minmus at that altitude.    The point is though,  with my technique you won't actually get within 10km of your periapsis before starting back up again, due to the lift.

 

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 Remember, the way orbits work, the velocity of a craft orbiting very high is actually quite low; that's why a tiny deltaV can produce such large changes in an orbit. The same object orbiting very low will be moving very fast. So setting a periapsis at 35km, absent any atmosphere, will mean your spacecraft will be moving THAT MUCH FASTER as it hits the lower part of the trajectory as compared to a craft with a higher periapsis, like for instance about 50km. That means there's just that much more kinetic energy to shed via friction as compared to an entry trajectory with a higher periapsis.

 

 

If I'm reading this correctly, you're just saying that lower PE is harsher/for impatient types who just want to get down faster,  that I agree with.   I was under the impression you were arguing the opposite, given the earlier statement ,

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A long, slow hot entry is the worst possible way to enter in real-life, and not a great way to do it in KSP either.

 

Quote

 

So if you set a low Pe and then try to use your wings to counteract the natural tendency of that initial trajectory to forcer you lower into the atmosphere, you're not only greatly increasing the stress on your craft, you're also forcing your craft to go faster, lower in the atmosphere than you otherwise would AND trying to turn that kinetic energy into heat via drag than you otherwise would as well. 

 

OK I think this sentence says the same as previous - lower PE = more stressful to your craft.

Alright finally, I think I can see where you're coming from,  it's not a million miles away from where I'm standing either :

My technique -

Retro burn to a 35km periapse

As soon as you pass below 70km, pitch for high lift and stay there, wings level.  You'll probably drop to 42km then skip back up to 55km before starting down again.  A second bounce is likely lower down and a third is possible.

Once certain of not burning up, lower nose to best glide ratio or pitch up to high drag as needed to adjust landing point.

The biggest drawback seems to be all the skipping off the atmosphere that can happen.   The periapse is on the opposite side of the planet to where i made my retro burn, and we typcially glide on for another 1/3 of the way round the planet from the periapse.   If I'm being cautious or am coming back direct from minmus or even further out , i can choose a periapse that's higher, but expect to orbit quite a few times before coming down for good.

 

Your technique

Burn to a 50km periapse

Pitch to 90 deg AoA to slow down in the upper atmosphere.  The drag created rapidly pulls the periapse well below 50 so that..

...on entering the lower atmosphere,  pitch for high lift

 

 

[Empiro]

In stock aero, I've found that one counter-intuitive thing is to avoid slowing down too much at the 50+km region. If you set your PE to about 40 km, and start slowing down in the 50+km region by pitching up, all it does is create drag without creating any lift. You'll find your PE will quickly drop and you'll only have slowed down slightly (it doesn't take much delta-V to change your PE from 40 to -40 km). Then, you'll very easily burn up as you hit the 30-40km region because you'll be falling quickly and you won't be able to get much lift to arrest your vertical velocity.

What I've done instead is to hold prograde until I start to see flames and then pitch up (which happens around the 40-50km region). This reduces the amount your PE falls (since you're closer to PE at this point), and allows you to spend more time in the 40-50km region slowing down and arresting your vertical velocity.

Alternatively, install FAR, which actually makes things easier -- pitching up in the high atmosphere will cause you to slow down AND generate lift so that your PE doesn't drop (and may even rise).

[AeroGav]

1 hour ago, Empiro said:

In stock aero, I've found that one counter-intuitive thing is to avoid slowing down too much at the 50+km region. If you set your PE to about 40 km, and start slowing down in the 50+km region by pitching up, all it does is create drag without creating any lift. You'll find your PE will quickly drop and you'll only have slowed down slightly (it doesn't take much delta-V to change your PE from 40 to -40 km). Then, you'll very easily burn up as you hit the 30-40km region because you'll be falling quickly and you won't be able to get much lift to arrest your vertical velocity.

Haha,  that's contrary to both mine and Lefty's position !  

I would say that some of that depends on how much your're pitching up.   At 90 degrees nose up there is all drag and no lift, just below the stall maybe 1.7 to 1 ratio of drag to lift (at re-entry speeds) and perhaps double that at only 10 degrees of pitch.    In stock aero,  the stall always comes at 30 degrees, but in FAR the stall angle increases with speed, so it's possible that you're stalling in stock aero and not in FAR.

Now the thing is the atmosphere is incredibly thin up at 50km,  so the amount of lift and drag being produced are tiny either way.    If you bring up the ALT-F12 menu,  go to the physics tab and check the "Display Aero Data GUI" box, you will see you might only be generating lift equivalent to a few percent of the craft's weight up there.   But the thing is, you're only very slightly below orbital velocity and >95% of the craft's weight is being cancelled out by the centrifugal effect of hurtling round the planet*

Spoiler

*horrible mangling of physics, yes i know there is no such thing as centrifugal force and what's really happening is you're going round so fast the horizon is curving away from you faster than you can drop, but it's easier to understand applying the older theoretical model

The tiny bit of lift you can generate up here can make up the little shortfall and help prevent your rate of descent from growing ever larger.  I use the Kerbal Engineer mod which I've set up for a continuous readout of  AP, PE, Mach number, Critical Thermal %, and Vertical Speed.  Vertical speed is the one i'm most interested in.   It only takes a loss of velocity of a few percent to send your Periapse well below 0km,  but if you're generating lift you may find your descent rate is nonetheless low and decreasing and that the Periapse point has moved to the other side of the planet and is running away from you constantly.      The ultimate example is a subsonic plane in level flight.   Your PE will be -500km down at the planet's core, yet you're cruising straight and level at an altitude that needs and oxygen mask.

[CSE]

22 minutes ago, AeroGav said:

Now the thing is the atmosphere is incredibly thin up at 50km,  so the amount of lift and drag being produced are tiny either way.    If you bring up the ALT-F12 menu,  go to the physics tab and check the "Display Aero Data GUI" box, you will see you might only be generating lift equivalent to a few percent of the craft's weight up there.   But the thing is, you're only very slightly below orbital velocity and >95% of the craft's weight is being cancelled out by the centrifugal effect of hurtling round the planet.

This is the heart of it: fly like a spaceship when you're above the Karman line and like a plane when you're below it.

@AeroGav: I suspect your best re-entry profile is when the arrival into the thicker atmosphere is just enough to slow your rate of descent such that you are then on your best glide slope, rather than skipping back up a few km. I'm a terrible pilot and always fail at this, but I think it's what I should be aiming for.

[numerobis]

7 hours ago, AeroGav said:

1. re-entry is the conversion of kinetic energy of the space vehicle to heat via friction

You went wrong here (assuming you meant "re-entry heating"). The vast bulk of the heat is due to compressive heating of the gases you're plowing into -- it heats up so much it becomes a plasma. Blunt noses work by keeping a boundary layer of gas between your spacecraft and the plasma, so you heat by radiation and not by conduction. Ablative heat shields use the ablating substance to thicken that layer. Reinforced carbon-carbon works by having a high melting point.

The heating is proportional to your airspeed *cubed*, and only to the square root of air density. Roughly, that means that every 5km you need to lose a sixth of your airspeed to maintain the same heat flow. If you come in from Minmus at 3200 m/s surface speed, and you'd blow up at 40km at that speed, then to survive at 35km you need to drop to 2700 m/s, at 30km you need to be going a bit under 2300 m/s, and so on.

Assuming you can maintain that boundary layer (e.g. by pitching up steeply), heat takes time to transfer to your spacecraft. So there's a tradeoff between staying up high where the plasma isn't as hot versus falling quickly where the plasma will be hotter but you'll slow down faster.

[AeroGav]

6 minutes ago, CSE said:

This is the heart of it: fly like a spaceship when you're above the Karman line and like a plane when you're below it.

The thing is in KSP there is no hard and sudden transition - other than the one at 70km.

One effect may dominate but the other still influences.   On the way up,  I typically shut down engines at something like mach 6.6 and 45km because my AP has gone over 70km.     Even up at 55km,  if I come off Prograde and let my nose rise a few degrees, i'll gain a few km on the AP,  at the expense of Periapse of course. 

And yes skipping on re-entry isn't ideal, i was just illustrating that it's possible due to wing lift.  I do my best to suppress them by monitoring vertical speed trend and thermal trend on the Kerbal engineer HUD.

[AeroGav]

3 minutes ago, numerobis said:

You went wrong here (assuming you meant "re-entry heating"). The vast bulk of the heat is due to compressive heating of the gases you're plowing into -- it heats up so much it becomes a plasma.

I was aware of that, but I thought it was an acceptable simplification to get the point across.    I mean, what's the difference between compressive heating and friction if both convert kinetic energy to heat?

Does KSP model this boundary layer/blunt body effect then?   I know that if i roll a space capsule or pitch a plane's nose up, it has an immediate, but not very long lived benefit unless it starts affecting trajectory in a noticeable way, and fast.  I always just assumed the game was calculating the heat impinging on a different part of the craft's skin, and the benefit lasts until that new piece of skin gets hot (no more than a couple of seconds).

That cubed/squared law is interesting, could come in handy for calculating the never -exceed line on SSTO ascent profile.

I aways come much closer to blowing up on ascent than re-entry.   Probably because i like to build oxidizer-free nerv + jet engine spaceplanes,  i end up chasing the optimum lift:drag ratio AoA on the F12 Aero Debug display, which keeps me deeper in the atmosphere to higher speeds than i'd really like.  The solution, always, is lower wing loading.

When I reach MECO  on nightside the whole ship is usually glowing red...

[tg626]

All this back and forth about aerodynamics is nice, but.... what about the fact that the mk1 cockpit seems like it has far less hest resistance then other mk1 parts?

I notice on a darkside reentry, the cockpit starts glowing the dullest red well before the "plasma" effects start, it does have slightly different heat specs then the mk1 passenger cabin. I wonder if that has some effect?  In any case it will overheat and 'splode whole every other part never shows a temp gauge...

Something seems off...

30 degree pitch, reentry from 200km to a starting pe of 26km.  Stock aero.  Stock "slim shuttle".

Edited May 2, 2016 by tg626
Darkside autocorrected to Darkseid. LOL

[numerobis]

4 minutes ago, AeroGav said:

Does KSP model this boundary layer/blunt body effect then

It does! Summon @NathanKell from vacation if you want to discuss his pride and joy

I used to think of it as friction until @NathanKell explained the physics. Thinking of it as friction leads to the wrong inferences that there's a fixed number of joules of heating to sustain, that higher drag means more wattage, and that a shallower trajectory is always better (because equal joules but lower watts).

With the better model, the first inference is just wrong, and the other two become "it's complicated."

More drag always means fewer joules in your trajectory. Generally it means lower wattage at each point in the trajectory. But it means your trajectory is steeper, so there can be a tradeoff (I think; I haven't done the math).

A shallower trajectory normally means more joules but lower wattage. If the wattage is still higher than what you can radiate off, this can mean you pop. If you believe in the friction model, you'll respond by trying an even shallower trajectory, whereas a steeper one might be the solution.

[Knautilus]

Wow thank you to all who contributed a response so promptly! I just performed a reentry from the same 87km orbit, and stayed prograde but pitched up to ~45° when plasma effects started up, and things went very well indeed. No changes to the craft needed at all.

I had the mistaken impression that the aero forces needed to be minimized (by having a low AoA), but of course that concentrates and prolongs all heating right on the nose of the craft! 

I've played avidly since v0.17, but have been slow to take up 1.0+ as I was still working through a huge comprehensive Jool all-science Kethane-supported mission in v0.24. I had planned on aerobraking in a number of spots, but without heatshields (they weren't a thing back then), so I didn't migrate.

Looking forward to more learning by exploding!

[bewing]

The MK1 cockpit just glows for pretty. It actually has just as good heat resistance as the inline cockpit. And better than the crew cabin.

[royying]

I also think the MK2 cockpit have low heat tolerance, sometimes the F10 thermal display turn the cockpit to red in the edge of atmosphere, way before the burning effect start.

And if I increase the AOA in that moment, the spaceplane will start to climb up, really hard to predict the landing point

[AeroGav]

I think inline cockpits do better on re-entry because they are somewhat protected by the part in front.   Note, you can build a failsafe in to save the crew by putting a heatshield in between the nose cone and cockpit.  The heatshield doesn't get consumed unless the nose burns off ,  in which case it then starts forming a protective cone shielding the parts behind it from re-entry heat , same as happens on a rocket capsule.   Your plane will have more drag coming in to land and will need a new nose, but better than total loss of vehicle and crew.

BTW  here is a Youtube I shot of one of my spaceplanes re-entering with the "continually pitched up  for lift" technique.   Held more or less constant 20 deg pitch from 70 to 30km.    Had the gear out and cargo bay open for a little more drag too, most of the way in. 

It was shot in 1.05, where re-entry heat got bumped up to present levels from the far too easy 1.04.

No drama,  cockpit reaches max temp of 754K.    

Edited May 3, 2016 by AeroGav