5
$\begingroup$

This is about the Mig-25. This documentary on youtube, @10:03, says:

Special insulating panels had to be fitted to protect the area around the engines from overheating. The panels were silver-coated. Five kilograms of silver were used in their manufacture.

My question is, why? What does silver-coating do exactly? AFAIK you still need some kind of cooling, either air or liquid, or else the heat flux will steadily raise the temperature to the failing point.

Wikipedia lists the thermal conductivity for silver and copper as 429 W/(m·K) and 401 W/(m·K) respectively. It is only a slight difference and I can't imagine why you would absolutely need that slight difference for a massive increase in cost. Plus I'm pretty sure silver (and gold) are a lot easier to melt than copper.

The only "special" thing about silver, that I know, is that it's highly reflective and therefore is used in the mirrors of superindustrial strength telescopes. I cannot imagine you would need that inside an engine, as virtually all the heat comes from conduction via the hot engine gasses, not radiation.

Edit: The silver could be part of some silver alloy. It would still have to be a coating of this alloy.

$\endgroup$
  • $\begingroup$ Reflectivity, maybe? I have no hard data to support this but my first instinct is to check wether a silver panel would reflect more incoming radiation than a copper one, thus reducing one heat transmission vector. Conductivity only matters after the energy has been delivered to the material. $\endgroup$ – AEhere Jul 19 '17 at 10:53
  • 2
    $\begingroup$ The emissivity of polished silver is very low, the lowest of any metal. Hence, a silver-coated hot object radiates energy very weakly in the infrared (and absorb radiant heat weakly, too...). $\endgroup$ – xxavier Jul 19 '17 at 11:33
2
$\begingroup$

As you have pointed out yourself, the Silver is there because it is highly reflective. The panels around the engines were coated with silver to prevent the heating of the fuselage around it. According to this document,

The engine bays each incorporated heat shields coated with a 30-micron layer of silver (5 kg/11 lb per aircraft), which absorbed 5 percent of the heat and represented a more cost-effective solution than gold or rhodium, though these metals were tested.

The panels basically trapped the heat within the engine bay, which is then cooled using boundary layer bleed air from drain slots between the air intake sides and airframe sides.

$\endgroup$
  • $\begingroup$ Your source says the silver absorbed the heat, whereas your first paragraph talks about reflection. As per my numbers, silver is an even better conductor (of heat) than copper, so it would have a very poor heat capacity and thus not good at absorbing large heat for little change in temp. I will nonetheless make an edit to my OP because I just realized the silver could be part of a silver alloy with who-knows-what properties. $\endgroup$ – DrZ214 Jul 19 '17 at 19:12
  • $\begingroup$ @DrZ214 You should read that as "only five percent. $\endgroup$ – Sanchises Jul 20 '17 at 13:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.