Space plane heat shielding?

[strider3]

Continuing the discussion on trying to land and gather science on Laythe...

My next question is about "planes" and atmospheric re-entry. The experimental "work in progress" in the screenshot below was cheated into orbit around Laythe and put into re-entry...it got very hot and I had several temperature bars show up...and got high enough that destruction became a worry. Note that this craft is a "learning experience"...I expect the final craft that gets launched to Laythe to be quite different in it's final form. I don't see any way to add heat shielding to spaceplanes and I don't see any parts that include it? I'd appreciate any comments on how to handle the heat.

And while I have you...do you use SAS for planes...or turn it off and fly manually?

Edited December 18, 2020 by strider3

[antipro]

1 hour ago, strider3 said:

I'd appreciate any comments on how to handle the heat.

for the few experience I have with airplane re-entry, I suppose is better to use high heat resistant surfaces in the bottom,
and entry with an angle of attack of 15° or more, like the space shuttle did.

 




Also you can take advantage from the:
https://it.wikipedia.org/wiki/Skip_reentry
https://en.wikipedia.org/wiki/Boost-glide

Edited December 18, 2020 by antipro

[bewing]

Laythe tends to be much worse than Kerbin for heating -- for one kinda sad reason. The interesting biomes are so far apart that you end up being tempted to remain hypersonic in the very high atmosphere for a very long time, gliding to your destination.

There is no real way to "heatshield" a spaceplane. The only thing you can do is to use parts with a very high heat tolerance. Test your plane with a practical reentry profile, see what parts overheat, then look for heat-resistant alternates.

Using SAS is pretty vital, because you need to maintain a stall attitude for most of your reentry. You need to do this to increase your drag, in order to bleed off most of your reentry speed. Get your speed down to 900 to 1200 m/s as fast as you reasonable can, and then your plane won't blow up. A 90 degree AoA is the smartest thing to do, if your reaction wheels can hold you there. Maintain the highest AoA that you can, until all the plasma effects disappear.

 

Edited December 19, 2020 by bewing

[king of nowhere]

there is no heat shield part, but there are parts that have high heat resistance. you can assume they do include shielding, since they also have higher mass. they are mostly the Mk2 and Mk3 parts. but you can also do pretty well with a nose cone and a soa adapter, i've reached jool inner atmosphere with those in front.

it also helps to get some airbrakes, that would take the brunt of the deceleration. there is a part named airbrakes, but it does not work for spaceplanes because of low heat tolerance. on the other hand, though, you can increase your drag by deploying your landing gear and opening any cargo bay.

[18Watt]

Try to determine which parts are getting hot.  If the parts are moveable, put them somewhere else, preferably in a cargo bay if available.   If it’s the wings or nose then those can’t be moved, and my next step is looking at the entry flight profile.

It’s counter-intuitive, but I find a steeper entry helps minimize heat problems.   A long, shallow entry might seem gentler, but you spend a lot of time going really fast (and generating heat), with little drag to slow you down.   If you can get deeper into the atmosphere faster, you spend less time heating up, and the plane decelerates quicker.

If you can hold a very high attitude (60 deg above pro-grade or better) that helps generate drag, but not lift, which is what you want- you don’t want the lift to keep you in the upper atmosphere.

I find it difficult to construct a plane that is stable at those high pitch attitudes.   So I do two other things to get me deeper in the atmosphere quicker.  First, I make my entry trajectory steeper.  Second, I delay aggressively pitching up to allow the plane to get a little deeper before it levels off.

The other suggestions are also excellent.   Use heat resistant parts, and if you can hold a high pitch attitude do that too.  Finally, the plane is going to get hot.   If nothing actually blows up, that’s a successful entry.

[Snark]

On 12/18/2020 at 4:33 PM, strider3 said:

I don't see any way to add heat shielding to spaceplanes and I don't see any parts that include it?

As other folks have discussed in the thread, there isn't really any way to heat-shield a spaceplane.  However, there is one thing you can do that can help a lot:  design it to have a very low ballistic coefficient.  In less high-falutin' language, that means:  design it to be very lightweight with lots of wing surface.  Build an albatross, not a javelin.

This works because what fries you is not just being in a high-speed airstream, but staying fast for a really long time.  If you make a craft that has great big expanses of lightweight wings relative to the mass of the craft, then you can make it become extremely draggy by pointing the nose up and presenting the flat side of the wings to the oncoming airflow.

The combination of high drag and (relatively) low mass means that the craft will slow down very rapidly, and not have time to heat to the exploding point.

TL;DR:  Wings make extremely effective airbrakes.  And what heats you up is your craft's mass.

(The other thing you can do is to stay very aware of the max temperature of all parts exposed to the airflow.  Most parts can take 2000 degrees or more, but a few-- notably, science experiments and the lower-end probe cores-- have a much lower max temp of 1200, and are easy to go boom.  Keep those out of the airflow, either by putting them inside cargo bays or on the upper surface of the craft where the body shields them, and you'll be much more temperature resistant.)

22 hours ago, 18Watt said:

It’s counter-intuitive, but I find a steeper entry helps minimize heat problems.   A long, shallow entry might seem gentler, but you spend a lot of time going really fast (and generating heat), with little drag to slow you down.   If you can get deeper into the atmosphere faster, you spend less time heating up, and the plane decelerates quicker.

Also, this.  Very much this.  Don't make the mistake of trying to come in super-shallow; you'll fry yourself without slowing down.

[ConArt70]

Just watch out for g force on steep reentry

[Snark]

19 hours ago, ConArt70 said:

Just watch out for g force on steep reentry

True, though in my experience, this tends to be less of an issue for spaceplanes than it does for traditional capsule-style reentry.

The traditional capsule-and-a-heat-shield type of reentry just plows straight ahead as it falls, meaning that if it comes in steep, it stays steep (and indeed gets steeper).  So, such a craft ends up rapidly making its way to the lower levels of the atmosphere while it's still going really fast, so the air resistance builds up very quickly and they end up pulling a lot of G's.  The steeper the descent, the more G's.

For a spaceplane, though, holding a nose-up reentry generates a lot of lift, which bends the trajectory upwards once the craft descends far enough for the wings to be able to "bite".  So the craft doesn't suddenly end up in the lower reaches, and they tend not to hit that brutal G spike; they pull gees, yes, but they tend to be milder acceleration, stretched out over a longer period of time.

Don't get me wrong, if you do something silly like hit atmosphere going straight down, then yeah, you're not gonna have a good day.    But for most "reasonable" descent profiles, e.g. if your Pe before hitting atmosphere is above ground level, then spaceplanes tend not to hit excessively high G force.

[AHHans]

21 hours ago, ConArt70 said:

Just watch out for g force on steep reentry

In addition to what @Snark wrote, my personal experience is that the wings of a spaceplane tend to react poorly to high g-forces. And because Kerbals can withstand even fairly high forces for a bit of time, my spaceplanes tend to self-correct high g-forces by shedding wing area, even before the pilots pass out. (Well, at least during the aerobraking part. Such an event usually leads to a lithobraking event, that is typically less survivable for the pilots. )

[Snark]

40 minutes ago, AHHans said:

my personal experience is that the wings of a spaceplane tend to react poorly to high g-forces

Well, that's what "autostrut to heaviest part" is for. 

I don't build spaceplanes all that often, but when I do, I always autostrut at least the biggest, most prominent wing pieces.  (That, and avoid building long floppy multi-segment wings if they're likely to be subjected to major stress.)

(Actually, I quite often find myself designing biplanes.  Not only does this mean that the wings only need to be half as long and are therefore stiffer and stronger; but it also lets me mount them top-and-bottom, leaving space in between where I can mount engines amidships-- which I like to do, because engines are heavy, and otherwise I kinda need to put the engines at the back of the plane, where they tend to pull the CoM rearwards and make the whole thing aerodynamically unstable.  But I digress.)

[Guest]

To add to what everybody else said -- spaceplanes are much better at surviving re-entry than you'd expect if you build them with sufficient wing area. I routinely send them to Eve or do re-entries from Munar orbit for example. Also bigger wings tend to lead to better, more neutral flight attitudes which leads to higher efficiency, which offsets a lot of the added mass; IME wing mass only becomes something you need to watch out when building long-haulers –– single-stage to the Mun and back or something similar.

One really important thing though is to make them almost aerodynamically neutral so that they're capable of holding a nose-up attitude against strong aero forces. If they're nose-heavy they'll be forced to pitch down, won't slow down quickly enough, and will burn up. This way you can finesse hot re-entries by holding radial-out in thin air for maximal deceleration from the start, then pitching down when the g-forces start to get out of hand. 

So the CoM should be only ever so slightly ahead of the CoL. I try to make mine CoM-invariant also, i.e. to have the fuel distributed equally around the dry CoM so that the balance doesn't shift through the flight.

(There are other issues related to stability at high velocities, but the short version of fixing that is to make them pointy. Stubby noses are not good even if CoM is ahead of CoL.)

Edited December 21, 2020 by Guest

[AHHans]

40 minutes ago, Snark said:

Well, that's what "autostrut to heaviest part" is for. 

I use "to grandparent". But that helps you only so far, put too much load on them and they'll go their separate ways.

Actually that only happens to me when I didn't bleed off enough speed at high altitude and find myself with too much speed at low altitudes. In this situation hard maneuvers in pitch are contraindicated.

39 minutes ago, Brikoleur said:

One really important thing though is to make them almost aerodynamically neutral so that they're capable of holding a nose-up attitude against strong aero forces.

I like my MK2 parts with a long nose and a delta wing (e.g. here). So I usually need trim the plane by pumping around fuel anyhow.

[Snark]

2 hours ago, Brikoleur said:

If they're nose-heavy they'll be forced to pitch down, won't slow down quickly enough, and will burn up.

True... though I find that I can generally always mitigate this with appropriate setup on whatever pitch-control surface I have at the back of the plane (e.g. AV-R8 winglet elevators if I've got a traditional airplane-style tail in the back, or large ailerons if I've got a shuttle-style delta wing).

Just set them so that they deflect upwards when deployed, and set the deploy angle as high as it will go, and set their "toggle deploy" action to an action group.  That way, when I'm re-entering, just toggle them and they muscle the nose right up.  I've basically never had a plane that couldn't nose-up enough if I did this.

[18Watt]

Here's the entry profile which has worked for me for quite a while.

From a roughly 100X100km orbit, perform de-orbit burn about 90deg prior to KSC.  That puts you over a point slightly west of the desert airfield.  Burn retrograde until your projected flight path hits the surface at the mountain range west of KSC, or even farther west.   It looks like a very steep entry.

Enter atmosphere upright and facing pro-grade.   Descending through roughly 40km altitude, raise the nose to point about 20-40 degrees above your FPV, usually about 10-30 degrees above the horizon.   

Using that profile, I have never exceeded 3G's.   In case I make a mistake like pitching up too much, or forgetting to balance fuel (and having a plane tumble..), I make sturdy planes that can handle 9+ G's.   Building long wings with multiple sections added together seems to make a bendy and weak wing, so I try to use as few wing pieces as possible.   One big wing seems to be much stronger than a bunch of smaller wings tacked together. 

[xub313]

 

On 12/20/2020 at 8:48 AM, Snark said:

On 12/19/2020 at 10:21 AM, 18Watt said:

It’s counter-intuitive, but I find a steeper entry helps minimize heat problems.   A long, shallow entry might seem gentler, but you spend a lot of time going really fast (and generating heat), with little drag to slow you down.   If you can get deeper into the atmosphere faster, you spend less time heating up, and the plane decelerates quicker.

Also, this.  Very much this.  Don't make the mistake of trying to come in super-shallow; you'll fry yourself without slowing down.

Can anyone explain this? The craft has to bleed off the same amount of kinetic energy so the shallower trajectory ought to be better since the craft has more time to radiate heat away. Maybe it's because parts lose heat faster in a thicker atmosphere.

[bewing]

56 minutes ago, xub313 said:

 

Can anyone explain this? The craft has to bleed off the same amount of kinetic energy so the shallower trajectory ought to be better since the craft has more time to radiate heat away. Maybe it's because parts lose heat faster in a thicker atmosphere.

The physical reason is mostly a matter of where the kinetic energy gets deposited. If you come in shallow enough to  really start getting hot, but you are in a low-drag (prograde or retrograde) orientation -- then it's just the nose or engine that's absorbing all the heat. And they absorb for a long time, because you aren't slowing down. Then there's the issue of skin vs. core heating. The surface of rocket parts are designed to be very heat resistant. But if you just let them bake, then their insides melt.

But you are partially correct. Heating goes as velocity cubed. So your first couple hundred m/s of reentry velocity are best scrubbed at very high altitudes, over a longish time. If you are above, say, 50km altitude, then your craft can reradiate all that heat before you get down into the flames. Even so, you want to scrub the velocity as fast as possible -- which means maximum drag.

[Snark]

1 hour ago, xub313 said:

Can anyone explain this? The craft has to bleed off the same amount of kinetic energy so the shallower trajectory ought to be better since the craft has more time to radiate heat away. Maybe it's because parts lose heat faster in a thicker atmosphere.

When you enter atmosphere, two things happen:

• You start to heat up
• You start to slow down

The problem is that in the upper, very thin reaches of the atmosphere, the amount of heating goes up by a lot before there's any significant drag.  You need to get farther down so that you lose speed more quickly.  Stay up in the upper reaches and you'll slow-roast yourself while losing practically no speed.

[18Watt]

2 hours ago, xub313 said:

 

Can anyone explain this? The craft has to bleed off the same amount of kinetic energy so the shallower trajectory ought to be better since the craft has more time to radiate heat away. Maybe it's because parts lose heat faster in a thicker atmosphere.

A shallow trajectory keeps you higher longer.  This does several things.

• The air is thinner, so you don't have as much drag.  So you don't slow down.
• The high speed still produces friction heat though.  So you still get (very) hot, but don't slow down.
• The thin air does not pull heat away from the vessel.  Yes, you still radiate heat, but that's not very efficient.  The thicker air (at lower altitudes) will wick away the heat better, much better than the radiative transfer at high altitudes.
• Finally, the game simulates the thermal mass of the parts.  It takes time to heat up parts.  If you stay high a long time, the parts will get very hot eventually.  If you get deep into the atmosphere quickly, the parts have less time to heat up, you will slow down before they reach high temperatures.

In short, by staying higher longer, you aren't slowing down, but you still get the full effect of friction heating.  And you're subjecting your plane to those high heats for longer periods, which gives the parts more time to reach max temperature.  And again, you're still not slowing down..

[king of nowhere]

On 12/19/2020 at 6:21 PM, 18Watt said:

It’s counter-intuitive, but I find a steeper entry helps minimize heat problems.   A long, shallow entry might seem gentler, but you spend a lot of time going really fast (and generating heat), with little drag to slow you down.   If you can get deeper into the atmosphere faster, you spend less time heating up, and the plane decelerates quicker.

that's very situational.

we can distinguish two different phases of reentry.

first, you are in high atmosphere. lots of heating, little drag. you can stay there for a long time, if you survive the heat.

then you get lower, and drag goes much higher. you will slow down fast, but if you must explode, shielding won't save you.

So, a steeper entry helps you if you can't survive the first phase but you can survive the second.  actually, i've never seen that in my (admittedly limited) experience; if you cannot survive the heat in high atmosphere, then going lower will disintegrate you immediately. if you can survive the heat in high atmosphere, you still may explode later.

for example, when reentrying eve, i usually explode around 50 km of altitude. that's when drag increases a lot; sure, if i could survive a half minute i'd be fine. if.

there is, on the other hand, also a case where you want to have a steeper entry because too shallow entry will make you slow down a lot in the high atmosphere. and then you will fall down fast, and will get too deep in the atmosphere too fast. while a steeper entry will make you go faster in the middle strata, and you will spend longer decelerating there before reaching the low atmosphere. this is a case i actually witnessed, and a case where a steeper reentry will help.

basically, you have to experiment. damn, i hate spaceplanes. but somehow, i keep making them.

[18Watt]

3 minutes ago, king of nowhere said:

that's very situational.

True.  The situation the OP described is entry at Laythe.  

[Spricigo]

7 hours ago, king of nowhere said:

So, a steeper entry helps you if you can't survive the first phase but you can survive the second

As often is the case in KSP, the trick is to find  a good compromisse. Not too shallow, not too steep.

[xub313]

Thanks for the answers @bewing, @Snark, and @18Watt. I figured that since the same amount of energy is expended, the heat oughta be the same, but I think my problem was assuming that the same amount would necessarily be transferred to the craft regardless of the reentry path.  I guess I'm still not quite sure how reentry heat works physics-wise. I did the math and what you say about a steeper reentry angle accumulating less heat is true (assuming the heat flow rate is proportional to the product of the square root of the air density and the cube of the velocity.)

You still might want to take a shallow entry if your craft is good at radiating heat since a shallower angle will result in a slower heat flow rate and allow you to radiate more of the heat away. You could probably design a reentry path that keeps the craft's heat flow close to equilibrium. I dunno how well the average KSP craft radiates heat though.  That'll be a project for another time.

[bewing]

Yes, I use shallow reentries myself. So it certainly is doable, but you have to be delicate about it, and it takes a long time to land -- because you have to reenter 1/3 of the way around Kerbin and then fly all the way back to KSC in realtime.

[strider3]

OMG...what have I created?? . There is so much information here on the subject. Being a first time spaceplane builder...this is gonna take some rethinking, on my part.

Thanks for all the advice!!

[king of nowhere]

On 12/18/2020 at 10:33 PM, strider3 said:

And while I have you...do you use SAS for planes...or turn it off and fly manually?

 

hey, i was missing this key piece of question.

and the answer is: depends. planes are different.

some do require SAS on, but without pointing at any direction. some will need to point prograde, while others will become unstable if you try to do it. and others will have to be flown by hand. In any case, i've never seen stability assistance (as in, not trying to point at a specific direction, only to stand still) to do any harm