3 replies to this topic

[jamesj01]

Good morning,

Newbie question, as i am a bit confused.  Lets say i had a 0.5cm core diameter in a rocket moto, and we were using APCP propellant. How does the throat diameter correlate at all to the core diameter of 0.5cm, and if so, in what ratio? Would i have to make the throat diameter larger or smaller than the core diameter?

 

[NeighborJ]

Generally you will want your nozzle aperture to be same or smaller than your core. Failure to do so can result in your core causing an additional choke point. This additional choke point causes almost instantaneous CATO. If that doesn't happen and the core burns clear its own choke point it suggests that your motor has untapped potential waiting to be unleshed by a smaller nozzle aperture.

This is of coarse not the only factor which contributes to a successful motor. Core length and tube ID also play a large part. Exposed burning surface area/nozzle restriction being the only other meaningful data required to maintain a given case pressure. If your core and nozzle are too small for your tube the rocket will burn progressively increasing case pressure until it blows, there will be a direct correlation between nozzle size and tube size. If your core is too long it will also cause CATO but much earlier in the burn.

[jamesj01]

Generally you will want your nozzle aperture to be same or smaller than your core. Failure to do so can result in your core causing an additional choke point. This additional choke point causes almost instantaneous CATO. If that doesn't happen and the core burns clear its own choke point it suggests that your motor has untapped potential waiting to be unleshed by a smaller nozzle aperture.

This is of coarse not the only factor which contributes to a successful motor. Core length and tube ID also play a large part. Exposed burning surface area/nozzle restriction being the only other meaningful data required to maintain a given case pressure. If your core and nozzle are too small for your tube the rocket will burn progressively increasing case pressure until it blows, there will be a direct correlation between nozzle size and tube size. If your core is too long it will also cause CATO but much earlier in the burn.

Thats brilliant, thank you for your advice

[Arthur]

If you are making a firework rocket then the usual tooling makes the nozzle initially the same size as the core. If you are making an amateur or high power rocket then the core shape will be made separately from the nozzle shape and each will be one part of the calculation for the thrust and thrust profile to enable the rocket to go as high or fast as required.