Around 1972, NASA was soliciting proposals for a reusable space vehicle which became the Shuttle. The requirements were that the vehicle had to perform in space environment by carrying large payloads and was supposed to be as similar to an airliner as possible. Just refuel it and go back up. Lockheed, Douglas and Rockwell all had concepts of how it should be configured and especially what the reentry heat shields should look like and how they would function.
I worked at Douglas Aircraft then and recall that our best brains were put on the job to define what would work and what would last without maintenance to satisfy the stated proposal. All the airframers knew about ablatives and carbon-carbon materials. Douglas engineers considered these and recognized that any concept with bonded tiles ( invented by Lockheed) would be asking for trouble because the tiles were fragile. Yet tiles were very light weight and could withstand high thermal loads. All the other concepts were heavier which reduced payloads.
Our guys traded off weight/payloads/maintenance costs and came up with a structure that was composed of high temp metals, many refractory like Niobium, Columbium, etc. These metals could withstand 1500F+ temps and didn’t oxidize readily. To cool the surface, transpiration cooling was proposed where a liquid with high specific heat would be squirted thru microscopic holes in the skin and would absorb heat by vaporizing. Other means were suggested where engine fuel would be passed under the hot exterior skins to absorb heat, vaporize and be burned in the rocket engines. The problem was that these ideas were heavy; the positive was that they would last repeated flights without maintenance.
Rockwell proposed the bonded tile idea which had been rejected by Douglas and Lockheed. Because it looked so good on paper, NASA bought into it and awarded the Shuttle contract to them. We all groaned, realizing that tiles were too subject to damage and carbon-carbon was expensive and very fragile on any kind of impact, plus it oxidized easily and needed special oxidation coatings to give longevity.
Hindsight is easy to look back on and criticize choices made. I wonder what would have evolved had Douglas received the contract award. At least the refractory metal guys would have been prosperous.
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