# Heat absorbing materials

• November 30th, 2006, 10:00 AM
Heat absorbing materials
Hello,
I am designing a stealthy UAV (Unmanned Aerial Vehicle) for a university design project.

My team are having problems with the exhaust.

I am working the low-observability of the aircraft and I want to put radar absorbing materials inside the exhaust duct. The problem is that these materials stop working at a maximum temperature of about 94 degrees C.

I have been talking to the propulsion guy in my team, and we have agreed that increasing the ehaust ducts cross sectional area towards the end of the nozzle will result in a loss of thrust and a reduction in temperature. However, there is no way that we can reduce the temperature to 94 degrees C without loosing an unacceptable amount of thrust.

I understand thermodynamics, but I have little knowledge of materials that may be appropriate to use to extract some heat from the exhaust flow close to the engine side of the nozzle.

I read a post in a similar forum that said some waxes can absorb a lot of heat.

I think the main issue is that when an equilibrium point is reached between the exhaust temperature and the absorbing material, no further reduction in temperature will be available.

Am I right in assuming that the heat absorbing materials will not affect the flow velocity?

Thanks,
• November 30th, 2006, 10:13 AM
Firstly, eventually the aircraft will have to lose the heat in some way or another. Is there any practical way you can re-mix air with the exhaust to reduce the temp (which will of course increase the volume) is the temperature of the gas or the duct that presents the problem? - If it's the duct then an airsleeve may solve the problem as well as dilute the temperature of the gas, It's not my field but sometimes suggestions like this can spark an idea in another direction - whatever you do is likely (as you know) to degrade overall performance.
• November 30th, 2006, 10:20 AM
Thanks,

and yes that is an option, although if we mixed cool gas we would need to increase the forward intake area, which would effectively decrease the aircraft's ability to fly towards a radar site without being detected.

Im just looking for other options because the larger the intake area, the larger the radar signature.
• November 30th, 2006, 11:44 AM
How about taking air from above the engine (rather like the bulge on a drag car hood- carburetor input ) and mixing post burn? If it is above and to the rear of the engine it will present a lower radar profile. Of course the rear echo will be higher but by then you should have destroyed the 'enemy' radar. If you can keep all the leading edges and 'flat-direct return'' surfaces to less than 1/2 wavelength of the interrogator it will substantially reduce echoes.

As I see it the 'bulge' would be visible to radar when flying at lower altitudes, but the aircraft may be below the radar horizon in this mode.