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Nozzle throat diameter

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#1 Crimson0087


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Posted 27 September 2018 - 06:46 AM

How can I best determine nozzle throat? I have 3/4 inch PVC with 100g melted sugar and kno3 35/65. They keep exploding. Tried 7/32 tried 1/4 was about to try 10mm and bigger but these look like huge holes compared to Estes motors...I don't want to to lose thrust but I don't want it to explode either....

#2 gdeputy



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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. 

#3 Arthur



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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.

#4 Crimson0087


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Posted 27 September 2018 - 08:20 AM

Thanks to everyone. I will look into determining kn

#5 dagabu



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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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#6 Chewychewy


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Posted 11 June 2019 - 08:40 PM

Not sure but I always use a nozzle that's 1 third the size of my tube id. But depending on cored or end burning makes a difference plus I just core into mines a little. Maybe halfway through the bp and it works good for me but it's different for ever fuel/id/and all types of factors so experiment until you get it. Make sure you have a thick enough plug also nor it'll pop out making you think it blew up when it was just a weak plug that would've actually flown good if the plug was stronger. Had so many of mines either explode or just sit there shitting fire and not moving at all lol

#7 JMan



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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.

#8 Bourbon


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Posted 17 August 2019 - 08:17 PM

Does the 1/3 nozzle rule/starting point apply to all black powder motors?

Edited by Bourbon, 17 August 2019 - 08:18 PM.

#9 Mumbles



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Posted 18 August 2019 - 09:31 AM

Core burning BP rockets will have a larger nozzle diameter than end burning rockets. I'm getting rusty, so hopefully someone will correct me if I'm wrong. I think core burners tend to use approximately a 1/3 ID nozzle, and core burners use approximately a 1/4 ID nozzle.
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