Sugar rocket fuel for Jetex-type motors?

[lincoln]

Why would a particular type of fuel require a particular size of nozzle?

[dosco]

Why would a particular type of fuel require a particular size of nozzle?

Agh, you're forcing me to knock the rust and cobwebs loose ... (lol).

As I recall, there is a relationship between combustion temperature, rate of combustion, and case pressure ... and there's a minimum case pressure to achieve sonic velocity in the nozzle throat (necessary for "rocket thrust"). The thermodynamic attributes of the propellant are characterized by "strand burning" in a vacuum chamber. Nakka does all that stuff (he's modified commercial refrigerator equipment into a strand-burning chamber).

Way back when, I downloaded his spreadsheet and RNX data ... to get the case pressure to where it needs to be for "useful thrust" (using RNX) the nozzle throat had to be quite small in comparison to sugra-based propellants.

Actually, I'm glad you asked. Maybe with a larger nozzle throat, we can "tune down" the RNX to a more Jetex-like thrust profile.

Best-
Dave

[strat-o]

I think that the nozzle size needs to be matched to the burn-rate of the fuel.  If the fuel burns quickly and the nozzle diameter is small, pressure will simply build up rapidly until the casing's strength limits are exceeded and it explodes.  I believe it is possible to limit the burn-rate of sugar rocket fuel with the addition of sodium-bicarbonate.  Take this with a grain of saltpeter!  Everything I've written is based on what I've read, haven't experimented personally.

Strat-o

[lincoln]

Dosco:
I'm pretty sure that "useful thrust" for an airplane is much less than for a rocket. They may not even overlap. I think a thrust to weight ratio of 1:3 would probably be just fine for many models.