# nozzle throat area calculation for choked flow

I am quite confused for my sugar propellant rocket nozzle. I have designed a CD nozzle by hit and trial method. It worked but produced so less amount of thrust .I have used 2.3 kg propellant which gave 220 newton thrust only. I m not sure about chocking in my nozzle. Since I haven't calculated chamber pressure. so I can't say about my nozzle pressure ratio is below critical pressure ratio or not. Is there any way to know whether I had chocked flow or not? or Is there way to calculate the throat area for chock flow with just knowing the dimensions of combustion chamber and propellant used? I will be really thankful the answer.

• This can be construed as a theoretical question so I'm not voting to close. But you should know that construction of amateur motors is off topic here. Feb 10, 2021 at 19:03

It's not very scientifical, but for candy fuel engines I built when I was younger, I used simple formula to compute area of the nozzle (An) from area of engine (internal diameter) (Ae):

An = Ae / K


Rn = sqrt(Re^2 / K)


or for diameters:

Dn = 2 * sqrt((De/2)^2 / K)


for candy fuels, K of around 100 worked for me.

So for example if you have engine with internal diameter of 30mm, for K=100 you should use nozzle with diameter of 2 * sqrt(15^2 / 100) = 3mm

Assuming simple "paper tube filled with fuel and then capped by plaster/wood nozzle" construction.