Nozzle throat diameter
Posted 27 September 2018 - 06:46 AM
Posted 27 September 2018 - 07:26 AM
First of all, where are you located? If in the US, you have an active amateur rocketry community available and very likely a local club you can get some hands on mentoring with.
Second, if you haven't already, spend some time on this site. http://www.nakka-rocketry.net. Lots of info on rocketry in general, and specifically on sugar motors.
So regarding nozzle throat, and in response to your other thread, yes, you will need a larger nozzle throat if you are using more propellant than a given design you're basing your motor on. One way of modeling this is with the ratio of the propellant surface area to nozzle throat area, known as Kn. Different propellants work best in different Kn Ranges. If the Kn is to high, a motor will blow up. If the Kn is too low, it'll chuff, be hard to ignite and wont produce much thrust. I would try calculating the Kn of the motor you're basing your design on and make sure the Kn you are running at is close to the same. If you're increasing the surface area of the propellant (by using more propellant/larger grains) you will need to increase the nozzle size to keep the Kn the same.
Also keep in mind that Kn will change as the motor burns and propellant surface area changes. There is software available to model that (www.burnsim.com).
Hope that helps.
Posted 27 September 2018 - 08:12 AM
The sweet spot for a BP rocket is usually found by experiment, too small a nozzle and it goes bang, too large and it doesn't make thrust.
Try bore/3 as a start.
Research other threads about sugar rockets for information, or find and join a rocketry club -or both.
Yes you will need to keep making different sizes of tooling until you find the right one.
Posted 14 October 2018 - 12:52 AM
Sorry for the late reply. I am wondering why you would use the comparison of the ESTES motor nozzle throat when asking about the sugar rocket nozzle? Also, the slower thrust buildup in sugar rockets demands a material that won't expand when heat and pressure are present, PVC is a poor choice for rockets altogether.
I have been told that PVC is cheap. I don't think it is when the variable of expansion is present. Paper tubes are easy to make, you can even use standard copy paper if you have to and it will biodegrade where PVC will not.
You have a long painful journey ahead of you if you cannot remove as many variables as possible in your rockets.
Also, are you using inhibited grains or a filled casing? Right off the bat, you will improve your likelihood of success with inhibiting the outside of your grain/s, this will keep the fire from reaching around the outside of the fuel and over pressuring the tube as well as keep the heat centralized to the core.
Good luck, keep us posted.
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Posted 11 June 2019 - 08:40 PM
Posted 21 June 2019 - 12:22 AM
There is software available to model that (www.burnsim.com).
Hope that helps.
I posted in an earlier thread a rocket calculator designed for sugar rockets. Made it myself and Id love if you could give it a try. Kn is a great place to start but its a rough estimate for the pressure inside the chamber. My program will tell the the thrust, Pressure, and Kn over the time of your whole burn. Check it out and it should deffinaty point you in the right direction of why your rocket is failing.
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