A coating has been developed that can be applied to spacecraft equipment to significantly increase the allowable touch temperature limit, while only marginally degrading the heat transfer to the environment.
The requirements for the coating are to:
1. Limit skin temperature levels below the pain threshold by:
- Having low thermal mass.
- Providing a resistance to heat flow when the object is handled.
2. Cause minimal degradation to nominal on-orbit heat transfer.
These requirements were met in a prototype by fabricating a layup consisting of, from top to bottom:
- A 0.004 in. (0.1 mm) thick aluminum outer layer,
- A 0.0023 in. (0.06 mm) thick high-temperature acrylic transfer adhesive,
- A 0.002 in. (0.05 mm) thick square stainless steel fin with longitudinal waves that add crush strength,
- A second layer of transfer adhesive, and
- A second aluminum facesheet.
The two aluminum facesheets act as the outer heat transfer layer and the layer that bonds to the equipment. The facesheets are identical for convenience, but are sized by the requirements for the outer heat transfer layer.
In the crew cabin of a spacecraft, the heat transfer is a combination of convection to the free-stream air and radiation to the cabin walls. Because of the lack of buoyancy-driven natural convection and the low cabin airflow, the convective heat transfer coefficients are much lower than are normally found in terrestrial applications.
A mathematical heat transfer model showed that the coating dramatically improves the allowable object temperature for a given contact time — a 20 °F (11 °C) increase for long times and as much as a 100 °F (56 °C) increase for short times. The coating also improves the allowable time of contact for a given temperature. A bare 140 °F (60 °C) aluminum item can be held for 2.5 seconds without pain, but with the coating, it can be held for more than 2 minutes before pain is felt.
The coating could be used in terrestrial applications where similar requirements exist: surface coatings that allow ungloved handling of hot surfaces but minimally impede nominal heat transfer. For example, hot surfaces of powered equipment that must provide cooling, but can be inadvertently touched or handled during maintenance could be coated for safety.
This work was done by Eugene K. Ungar and Timothy K. Brady of Johnson Space Center. MSC-24726-1