On 30 Nov 2003 09:08:15 -0800, (robert arndt) wrote:

The radar absorbing paint on the SR-71 also acted as a heat sink,
lowering the airframe temperature by nearly 100 degrees. It would have
been better, however, if the aircraft was painted white overall
instead of blue-black. This was proposed at least once (based on the
X-15 research) but rejected.

No. The SR-71 is black because that improves its ability to radiate
away heat. Remember sigma x T**4? Well, sigma is higher for this
black paint than it is for white paint.

The X-15 never did any research on white paint. If you're thinking of
the white paint used to protect the pink ablative coating on #2, that
wasn't there to improve radiation and it burned off very quickly.
There was nothing significant about the paint color.

Since the X-15 flew a really quick trajectory, rather than cruising at
high speeds, color was much less of an issue. It wasn't in the air
long enough for radiation to help. (The same is true for the Space
Shuttle Orbiter.) However, the SR-71 cruised at high speeds and
radiation helped lower the temperature, as you mention.

Where do you think the heat absorbed by the paint went? Heat can be
conducted into the airframe, convected into the air, or radiated into
space. It doesn't just vanish into the molecules of the paint. The
surface of the airplane, covered with paint, radiated that heat
outward. The heat came from more than just the paint; it also came
from the skin and structure.

Mary

--
Mary Shafer Retired aerospace research engineer