# What is the heat signature of a piston engine, relative to a jet engine?

I'm reading about the AIM-9 Sidewinder Missile again. I noticed this interesting photo:

Notice, that's a piston-engined aircraft, and a small one at that. So I'm asking myself, how did the missile lock onto it? The AIM-9 is a very early heat-seeking missile (~1956). I thought early heat-seekers could only lock onto a jet engine, which I believe has a much larger heat signature than a piston engine.

Now I know today that IR missiles can lock onto almost any heat signature, even the heat of the adiabatically compressed air at the wings' leading edge. But in 1956, I'm surprised that an IR missile could lock onto a piston engine's heat.

So I want to start with this question: What is the heat signature of a piston engine, in terms relative to a jet engine? 2x less? 3x less? Or somehow equivalent?

• A) Please explicitly source your photo, B) Great photo!, C) Interesting that the missile hit the rear of the plane and didn't go for the hottest part. (Unless, of course, the engine is exhausted from the rear, but I doubt that for a piston engine.) – FreeMan Jul 11 '16 at 12:27
• About the missile "hitting" the rear of the plane: Missiles miss. If they actually made contact with the plane they were trying to destroy, they'd be called "hittiles". The missile just needs to get close enough that the frag sphere of its explosion causes damage to the target. On such an early missile, the analog seeker would be pointing the missile at the heat signature, meaning it would fall behind a little as it closed to the target. Newer electronic seekers are programmed to track motion and lead the target to avoid falling behind a maneuvering target. – KeithS Jul 12 '16 at 0:31
• Actually, it seems that there is a flare attached to the tail. Perhaps the engine exhaust and/or radiant heating was insufficient. – acpilot Jul 12 '16 at 6:23
• It just looks to originate from a "bright spot" behind the tailwheel. I would expect a contrail to he much thicker and not peter out at the edge of the frame. – acpilot Jul 12 '16 at 9:57
• @DrZ214 - microwave sensor detects the object It's actually an IR laser proximity fuse, which perform better than earlier passive IR fuses. – Hephaestus Aetnaean Aug 22 '16 at 3:13

Here is a color IR image of what I believe to be another F6F, on the tarmac shortly after landing:

As you can see, the engine cowling is very noticeable, as are the tires (from touchdown and braking).

Here's a similar image of an F/A-18 performing a runup test prior to takeoff. This jet is longer and larger than the F6F (though if you've ever stood under a Hellcat you'll notice they're not exactly small planes), and as you can plainly see, the entire back half of the aircraft glows white-hot:

So clearly jets are more visible targets for an IR missile, though a prop plane wouldn't be that hard for an IR seeker to find.

The biggest issue relating to getting a good IR lock and track is contrast. Against a blue sky, almost any airplane is readily visible to the missile's seeker. The biggest problem for combat aircraft in early engagements was firing at a target below the horizon (so-called "look-down, shoot-down"). This was a problem for both radar and IR technologies. For IR, solar heating of the ground produced high enough temperatures that the aircraft could not be distinguished unless it had turned its tail to the shooting aircraft and was showing its 600-degree exhaust plume. Nothing on the ground underneath it would be that hot. Prop planes, while operating at lower temperatures, would still be warmer than the ground, and unlike a jet the aspect angles at which this heat is most noticeable is around the front half of the aircraft.

• Bubble canopy -- F8F Bearcat, perhaps? Not that this detail changes the point you're making. – Ralph J Jul 12 '16 at 1:40
• Cameras these days automatically adjust focus and contrast. The white color of the first photo is not necessarily at the same temperature as the white color in another photo. On top of this, personally I find it impossible to believe that the wheel is as hot as the engine exhaust. – DrZ214 Jul 12 '16 at 9:29
• @DrZ214 the frequency of the light and intensity is usually transformed into the false color image you see. In that the temperature of two regions having the same color does not necessarily indicate an identical temperature. As for brake temperature, it isn't unusual for brake pads to exceed engine operating temperatures. Brakes are not usually cooled once the vehicle stops moving. – jCisco Apr 19 '17 at 6:32