# Thread: thermal equilibrium and heat flow

1. Hi,

I am trying to design an exhaust system that would make it very hard for a heat-seeking missile to identify.

I have read that the F-117 stealth fighter uses "heat conducting materials" in its exhaust ducts to take some heat away from the gasses leaving the engine.

I am thinking about surrounding the exhaust with the foam that the space shuttle uses to protect itself from the high temperatures experienced during re-entry to the earth's atmosphere. This material can hold a MASSIVE amount of heat energy.

However, my thermodynamics is a little rusty, since I haven't studied it for a couple of years.

I know that the zeroth law of thermodynamics says if the temperature of two bodies in contact is the same, there is no flow of energy between them.

I know that if the exhaust gas was held in a container, over time it would reduce its temperature until it was equal to the temperature of the ambient atmosphere.

The problem is that the exhaust gas is constantly being renewed, so the temperature of the gas at the point where it leaves the turbine is always the same (about 720 decgrees C). If the space shuttle foam was next to the gas flow, I think it would just heat up until it is also at 720 degrees C, and the exhaust gas would not be able to cool down.

Am I right or wrong?

2.

3. Try

Heat seeking missile exhaust gas

and read up - I thought I already replied to this somewhere....

OK here's a thought from a lateral thinker...

Plane in the air, missile on the ground - missile can only see underbelly of plane - so either mount the engine and exhaust above the wing - whoops that has various aerodynamic problems.. or place a third wing underneathe the exhaust to shield it from the ground extend it further back as well. Some of these IR missiles use a regular sampling freequency, if you can pulse mimic that you can confuse the missile...

4. I appreciate your suggestion, althoug

I am looking for a passive system rater than an active one, and the airfaft shape has already been decided. Adding extra wings would reduce the radar-evading capability of the aircraft.

Also, air-to-air IR missiles do exist, and I need to avoid those too.

The aircraft I am designing needs to fly at a low altitude and at a fairly low speed too. The soldier on the ground would just point his missile towards the aircraft, which is designed to fly in the direction it is shot in until it finds the heat source. The missile would simply fly to a higher altitude to see the heat from above, which iss why I am trying to reduce the overall temperature.

I like the idea of having an internal heat sink, but I do not feel confident with the problem in my question.

I would think that heat would flow between the exhaust gas and the heat sink until they bith have the same internal energy. If the heat sink had a lower specific heat capacity, it would ave a higher temperature than the exhaust when the energy is balanced.

Does anybody know if this is correct?

5. Originally Posted by Megabrain
Try

Heat seeking missile exhaust gas

and read up - I thought I already replied to this somewhere....

OK here's a thought from a lateral thinker...

Plane in the air, missile on the ground - missile can only see underbelly of plane - so either mount the engine and exhaust above the wing - whoops that has various aerodynamic problems.. or place a third wing underneathe the exhaust to shield it from the ground extend it further back as well. Some of these IR missiles use a regular sampling freequency, if you can pulse mimic that you can confuse the missile...
You should also take into account missiles being fired by other aircraft (i.e. sidewinder missiles). Therefore, another solution must be conjured for that aspect.

6. Originally Posted by adam chapman

I am looking for a passive system rater than an active one, and the airfaft shape has already been decided. Adding extra wings would reduce the radar-evading capability of the aircraft.

Also, air-to-air IR missiles do exist, and I need to avoid those too.

The aircraft I am designing needs to fly at a low altitude and at a fairly low speed too. The soldier on the ground would just point his missile towards the aircraft, which is designed to fly in the direction it is shot in until it finds the heat source. The missile would simply fly to a higher altitude to see the heat from above, which iss why I am trying to reduce the overall temperature.

I like the idea of having an internal heat sink, but I do not feel confident with the problem in my question.

I would think that heat would flow between the exhaust gas and the heat sink until they bith have the same internal energy. If the heat sink had a lower specific heat capacity, it would ave a higher temperature than the exhaust when the energy is balanced.

Does anybody know if this is correct?
I was assuming you were thinking of a jet engine yet you indicate low level and slow - yet if using props then they [generally] produce a huge radar signature, can you clarify?

7. Problem: Modern heat-seeking missiles don't have to target engine exhaust, they can home in on the heat of the plane's body. You probably can't match the temperature of your entire plane's body to the temperature of the air that you are flying through.

As long as there is a temperature difference between you and the air around you, the missile's infrared tracker will identify your plane as a heat source because of your plane's black body radiation emissions. So unless you are only worried about 1960s era missiles, simply cooling your engine exhaust won't work.

You have to actively jam the missile with flares, infrared beacons, IR lasers, or whatever. Although even that is getting pretty hard, because the most recent missiles are able to distinguish between a warm object that is shaped like a plane and a warm object that is not shaped like a plane, so they are much less likely to be distracted by flares or decoys.

8. They can also use a mixture, optical, infrared and radar, using 'best guess' software and even radar frequency hopping, if they bung millimetric radar into it as well, then your only defence would be an anti-missile-missile.

Millimetric radar can [theoretically] tell the difference between a tank gun barrel and a coke can sticking a few inches out of the sand.... it can even tell different tanks apart, hit a tank parked in a pile of scrap....

9. This is probably a little late, but I don't think any form of insulation would work. I would think (I'm no genius haha) that you would want heat sinks with a very high heat transfer so it just goes to the air. Otherwise, like you said, the material will just head up to the same temperature as the exhaust.

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