Why do many ablative nozzles use phenolic resin and not some other resin?

Question

I am currently looking into ablatively cooled nozzles. Most of the nozzles made out of composite material that I have found use phenol as resin (Space Shuttle Solid Rocket Booster, ARCA Excecutor).

I was wondering why there seem to be no nozzles which use other resins (for example epoxy). It would seem that these resins would be easier to handle and produce. Are there any material properties, which make phenolic resins especially suited to be used in ablative cooling? And are there other resins, which also perform well for this task?

Answer 1

For an ablative nozzle one wants the right mix of properties: low thermal conduction, high strength, and a burn (regression) rate that produces enough gas to provide effective film cooling but not too high a burn rate so that you require a really thick liner to survive the burn time. Phenolic is well characterized by decades or research too, so it's well known and has good processes worked out for handling it. The biggest plus for Phenolic is that it is in that sweet spot for burn rate for most of a nozzle/chamber. It's a little fast burning for the throat so typically folks either alter the construction of the that section to "densify" it with additives that slow the burn, or they use a different material entirely like graphite.