A Happy Failure

by George Raiche

Tuesday, September 19, 2000

One of the most interesting things that can happen to a scientist is running an experiment that doesn't work. One of my recent experiments has been a complete "failure", and I couldn't be happier.

For very high-speed atmospheric entries, heat shields are designed to ablate. Ablation, the controlled decomposition of the heat shield during entry, is a very effective means of dissipating entry heating (of course, the heatshield could not be re-used); energy absorbed in breaking the physical and chemical bonds that hold the ablator together is unavailable for heating the vehicle.

But one of the questions we still don't understand is: what happens to the material that ablates away? Most ablators are hydrocarbon resin composites, which have very complex thermal decomposition chemistry. The decomposition products include everything from diatomic radicals to soot. From a vehicle design perspective, why do we care where the products go? Because, as in the rest of nature, no waste product ever really goes away. These ablation products enter the shock layer and may effect the heating of the vehicle.

We simulate entry in the arcjets by forming a hot shock layer over test models. My experiment was an attempt to detect the presence of the decomposition products in that shock layer during an ablator test. Since we use air to form the shock layer, I expected the chemistry to resemble hydrocarbon combustion chemistry; in a combustor, two common combustion products are C2 and CH radicals. In a high energy environment, both these molecules strongly emit visible light with characteristic properties.. My experiment was designed to detect that light. Since the properties of that light are well known, my data would give me a good insight about the ablation chemistry.

But I didn't see any of this light. What I did see was even more interesting: a broad yellow background glow. What causes this glow? How might it effect the amount of heat that falls on the test model, or onto a real heatshield during entry? We are trying to answer those questions now.
Stay tuned!