If we asked you to design a circuit to blink a flashing turn signal, you would probably reach for a cheap micro or a 555. But old cars used bimetallic strips in a thermomechanical design. Why? Because, initially, 555s and microcontrollers weren’t available. [Breaking Taps] has the story of NASA engineers who needed some special cooling chambers in a rocket design for the Space Shuttle. Today you’d 3D print them, but in the 70s, that wasn’t an option. So they used wax. You can see a video about the process, including a build of a model rocket engine, in the video below.
The issue is the creation of tiny cooling channels in the combustion chamber. You can use additional thin pipes brazed onto the engine. However, there are several disadvantages to doing this way, but early rocket engines did it anyway. Having the cooling path integrated into the system would be ideal, but without 3D printing, it seems difficult to do. But not impossible.
The technique uses wax and machined slots that will eventually become the tubes required. Then you electroplate material on top of the wax to form the outer shell of the tube. In fact, NASA used two electroplating steps: a thin copper layer to close the tube and a thicker nickel layer to provide strength. Copper survives contact with the cryogenic coolant better than nickel does, but nickel is much stronger, so the combination makes sense. Because electroplating needs a conductive surface, the wax needs some help. NASA used a fine silver powder coating, but [Breaking Taps] simply impregnated the wax with graphite. Clearing the wax out afterward is a bit challenging but doable.
If you think about it, electroplating might be one of the original additive manufacturing techniques. In fact, people have made printers based on the idea, but they are very slow. On the other hand, electroplating a plastic print takes less time and looks great, even if you wouldn’t use it for a rocket engine.
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