Dealing with atmospheric heating

[daniel911t]

I'm trying to complete the Kerbin Circumnavigation speed challenge and I'm having persistent problems with aircraft that heat up and explode about halfway around the world.  



The part that's overheating is always the command pod, which is strange because it's in the middle of the aircraft.  The basic fuel tank in front and in back of it does't overheat. 

It always does this:


I installed some small radiators - they typically register as the coolest part of the plane, but they don't seem to make much difference.  I also adjusted the angle of incidence on the wings, which dramatically reduced fuselage heating, but for some reason the command pod still overheats, even when the rest of the craft is nominal.

I've seen people get the sustained high speeds and not melt, but I just can't seem to manage it.  What are the tricks and secrets?

[fourfa]

It's an excellent question.  I've seen some speed challenge craft put 1.25m heatshields inline, between the cockpit and other parts.  I have found service bays to work too, as well as providing a place to hide stuff like your RTG that's out in the wind generating drag.  With a bit of trial and error, you can actually fill the service bays with fuel and get rid of the big front and rear tanks (just an idea)

I downloaded someone else's speed challenge craft that was loaded with radiators, flew it, was impressed with its thermal performance.  Then I realized the radiators weren't even activated - it was the service bay they were hidden inside that was insulating the cockpit!

My most successful speed challenge craft uses the Mk1 Command Pod, which has a higher temp rating and is very low drag to boot.  I even have one where I wrap the Mk1 pod in a fairing:

Which has more than enough fuel for circumnavigation at ~1749m/s if you manage to tune and fly it right.  I gave it an abort sequence just for fun - stage fairing, decouple, deploy chute - because it's not easy to land!

[Jarin]

Try putting a heat-shield inline between your nose and the cockpit.Feel free to remove all the ablator in the SPH; it's just there as a buffer for the internal heat transmitting backwards.

[AeroGav]

I think also, you can probably get the cockpit further back, and benefit.

Like this guy - looks a bit Viper-ish (BSG) 

 

Engine nozzles are apparently good at radiating heat, maybe have the engine attach to back of cockpit?

 

Edited Saturday at 03:52 AM by HardKerbin 
 

Edited February 13, 2017 by AeroGav

[Snark]

9 hours ago, daniel911t said:

The part that's overheating is always the command pod, which is strange because it's in the middle of the aircraft.  The basic fuel tank in front and in back of it does't overheat.

My guess is that it's because of this.  Look at your parts, starting from the front end and working backwards, and their heat tolerance:

• Shock cone intake, 2400 K
• Fuel tank, 2000 K
• Command pod, 1100 K

...notice anything about these numbers? 

Note that I've quoted the max internal temperature tolerance, not skin temperature.  The command pod actually has a fairly high skin tolerance of 2000 K, which I assume is why it has a reasonable ability to tolerate atmospheric reentry from space:  it doesn't have to endure that heat a really long time, so it doesn't have time to accumulate an overall heat load.

But if you're flying all the way around the planet, you're subjecting the craft to really high temperatures for a really long time.  I assume that what's happening is that it's starting to heat up from the nose cone, since that's in front.  It gets really really hot... and stays really really hot... which means it starts conducting heat to the fuel tank, which then starts getting hot itself, which then starts heating up the pod behind it.

My guess would be that at all times, the intake is hotter than the fuel tank, which is hotter than the pod... but the pod has by far the lowest temperature tolerance of the three, so it goes kaboom first.  And the reason you make it halfway around the planet is that it takes that long for enough heat to get built up and conducted back to the pod.

4 hours ago, Jarin said:

Try putting a heat-shield inline between your nose and the cockpit.Feel free to remove all the ablator in the SPH; it's just there as a buffer for the internal heat transmitting backwards.

...which is why I'd guess that this would be exactly the way to fix the problem.  Though heat shields don't enable fuel cross-feed, so that might cause problems draining fuel from the forward tank to feed the engine (unless you've got "fuel transfer obeys crossfeed rules" turned off and manually transfer fuel back from time to time).

[Foxster]

Confirmed that moving the cockpit back means it no longer is the first part to overheat...

[Streetwind]

You could also just leave the ablator on. The heatshield doesn't have to be exposed to air to ablate. It will ablate at any time the skin temperature exceeds 500 K, which will easily happen if your internal temperatures get over 1100 K. And the ablation process consumes heat. Which means that heat gets removed from your craft and you don't have to soak and store it.

[daniel911t]

I've tried some of the solutions...
-The inline heat shield delayed heat soak and destruction, but didn't eliminate it.
-Having ablator onboard does help - although not much.  I initially added 80 units, but discovered that it was being consumed at a very low rate.  I reduced it to the 20 unit minimum and haven't run out before the explosion.
-Position of the command pod helps.  Moving it back delays heating, but it's still heat soaking and failing. The aircraft is also slower due to less optimized CG/CoL, Even with the wing moved aft to compensate.  There is a pretty significant design benefit to having balanced fuel usage at both sides of the CG.
-For much of the flight, the critical part is the intake, but very slowly heat builds in the command pod, at the rate of about 1 degree per second - eventually it becomes critical, then all I can do is watch as the temp slowly climbs to explosion - usually just as I'm passing the desert. (about 3/4 of the full lap) 

I'm still trying to work out the effects of altitude.  The old rule of thumb I know is "when heat is a problem, climb".  I've tried cruising everywhere from 21Km to 28Km, and the biggest difference I've noticed is a reduction in top speed as I climb beyond the optimal altitude (about 23Km I think), but the heat issue does not seem get better or worse with altitude changes, independent of speed changes.  (within reason - if I descend to 15km I essentially spontaneously explode) 

Any other tricks, tips, or ideas?

[AeroGav]

26 minutes ago, daniel911t said:

Position of the command pod helps.  Moving it back delays heating, but it's still heat soaking and failing. The aircraft is also slower due to less optimized CG/CoL, Even with the wing moved aft to compensate.  There is a pretty significant design benefit to having balanced fuel usage at both sides of the CG.

I'd imagine the problem if you put the command pod at the back, is that having it at the front helps to balance the engine weight. End up with a very rear CoM.     And what can you put in front, apart from a pre-cooler and ncs adapter/small nose cone.  Can't have fuel tank, because it will be far ahead of CG, and you'll end up with something with excess trim drag (very nose heavy) when full, or too unstable when empty.

If big S  strakes are your wings, could keep the fuel there I suppose, if that's enough.

deck chairs in service bays aren't allowed as per challenge rules?

[Foxster]

46 minutes ago, daniel911t said:

I've tried some of the solutions...
....
Any other tricks, tips, or ideas?

I've flown a copy of your craft with the cockpit moved to the rear twice around Kerbal without it overheating to the point of exploding.  There must be something different in the way you built it. 

Do you have the wings attached to the cockpit? I do and maybe that's soaking up some heat. 

I didn't use radiators or air brakes. 

The RTG I attached to the back of the engine and then offset it into the craft. 

[AeroGav]

37 minutes ago, Foxster said:

 

The RTG I attached to the back of the engine and then offset it into the craft. 

sounds like a great excuse to have a service bay and stick it out in front of the cockpit!

 

38 minutes ago, Foxster said:

Do you have the wings attached to the cockpit? I do and maybe that's soaking up some heat. 

I do this too but wasn't 100% it was WAI, I could tell from the thermal menu a lot of heat was flowing between the cockpit and the wings, but I couldn't work out whether it was flowing out (wings acting like radiators) or in (wings heat soaking cockpit)