What's the cheapest filament that'll survive one firing on a small aerospike?

I ran a sugar motor with a printed nozzle once and it told me the answer fast. PETG held its shape for maybe a second, then the rim went soft and the whole thing sagged out of round. ABS and PC will buy you a little more time but not enough to matter. KNSU burns around 1600C in the core, and at 22mm the gas is moving fast enough that even a couple seconds of exposure eats right through any plastic. So yeah, straight FDM is basically a lost cause for anything but a throwaway curiosity run where you accept it deforming mid burn. Resin is worse like you guessed, cured resin is brittle and it actually burns rather than melting away cleanly.

Gonna push back on the resin part. Mine didn't shatter or burn through like that, it charred on the surface and actualy protected what was underneath for a couple runs. The trick was using a high temp resin, not the cheap standard stuff, and keeping the walls thick so the char layer had something to hold onto. Thin sections are where it fails, the heat soaks through before any char builds up. For a 22mm part with beefy walls I'd try it before paying for metal. Worst case you're out one print and a motor.

For the cheap metal option, I had good luck sending a file to one of those print services that does steel bronze infiltration. A part that size came out under thirty bucks shipped, arrived in a week, and I got two static fires out of it before the spike edges started rounding off. Surface finish was rough as a cob which might actually help a plastic part too, so if you do end up running a filament version, try bumping the extrusion temp and printing with fat layer lines. More of a sacrificial texture than a smooth mold-looking surface.

I went the mold route with mine. Printed the spike shape in PLA as a one piece form, then packed it solid with Rutland furnace cement and let it cure a few days before burning out the plastic with a heat gun. The cement held up way better than any filament would have, and since you only need one run the PLA was basically sacrificial. Took a couple tries to get the packing dense enough, first one had voids that ate into the spike. Cheap enough that messing up a pour costs you nothing but time.

That mold and cement idea from the last post sounds like something I could actually pull off in my garage. Before I try it though, I'm wondering about the inside shape. If I pack cement around a printed spike, how do you get the surface smooth enough that the burning gas doesn't tear chunks out of it? Mine came out kind of grainy when I tested a sample in a cup. Also does the stuff need to bake hard first or is air drying enough for a short burn?