One of the concerns with an active rocket engine is what comes out the hot end. Particularly, how hot the flame is that comes out and what we have built to stand in its way. Currently, the team has designed testing and launch stands that minimize the infrastructure that the flame could reach. Just in case, though, we want a backup plan.

That backup plan comes in the form of a metal shroud — hand bent by the team — which fits the form of the stand. And just in case a sheet of aluminum alloy wasn’t good enough, we coated it in a layer of intumescent caulk.

“Intumescent” refers to a material’s ability to grow when an external force is applied. The caulk that we bought chemically breaks down and grows in volume when a flame is directed at it. The resultant polymer/carbon mass provides additional thermal insulation to the structure beneath it.

Testing Theory

In order to prove that it works — and decide how thick we needed to apply it — we ran some tests. The first round, we tested six different caulk thicknesses for a set period of time. The second round focused on the two best candidate thicknesses at various flame angles.

To judge which test worked best, the temperature on both sides of the test strip were taken every 5 seconds. An infrared thermometer gauged the temperature on the back; a thermal camera estimated the front.

The Outcome

We got a definitive answer from the test: Coat the stand shroud with 1/8″ of caulk. This thickness performed the best at all angles we tried — and leaves us more than enough caulk to coat two more shrouds, if we need to.

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The caulk certainly did its job. The photos above show what happened to uncoated versus coated aluminum alloy sheets after a minute of direct flame. The uncoated sheet… melted. The coated sheet remained rigid — and sprouted a dense forest of curly, black char.

The 0.1″ caulk thickness provided optimal thermal resistance. Directing the blow torch at the center of the sheet raised the back temperature only 6°C — not nearly enough to melt any metal we’re using.

So yeah. We feel pretty comfortable covering our test stand shroud. One step closer to testing…