38mm Gorilla Motor grains

[north boy]

Hi everyone, hopefully this doesn't come under restricted topic as I dont want to talk about chemicals. As my other post today said I had fired my motor. Its was on Sugar but my burn was quite a fast burn at about a second. Is there any way I can slow it down. If its ok to talk about the grain setup I'll give the description of the grain measurements and core dia etc.

 

[CarVac]

You can make it a moonburner: have the core on the side of the grain. That approximately doubles burn time, though it is harder on the liner. I don't have any experience in making motors, though, so I don't know what other changes you would have to make, for example reducing the nozzle throat size.

 

[patelldp]

You can make it a moonburner: have the core on the side of the grain. That approximately doubles burn time, though it is harder on the liner. I don't have any experience in making motors, though, so I don't know what other changes you would have to make, for example reducing the nozzle throat size.

Remember, moon burners are typically monolithic. This presents an issue for most non-pourable propellants, especially in 38mm, as the grains are too long to drill the core. Standard procedure is to cast the offset core in.

Since there are only 2 grain faces with a monolithic grain instead of the usual 12 in a 6 grain motor, a smaller nozzle throat must be used to increase initial Kn.

Additionally, a standard propellant may not be appropriate for a moon burn motor. It needs to be ignitable at a low Kn, and not over pressure when the motor progresses through its burn.

 

[north boy]

Hi guys , its the smallest motor available and I cast 2 grains for the 38mm motor. Both grains were 84mm long as finished product. The core was 10mm in dia. Total propelent was about 240 grams / 120 grams per grain. I'm thinking there might have been a small air pocket in a grain. Maybe both which might have added to the fast burn. I had a test burn of a 1 inch strand before motor test and it burned at about 4 seconds which is fast in my book and thats at atmospheric pressure. I've got all my info of Nakka website and others he's suggested. Would casting 4 grains or 3 be a better route to illiminate air pockets or would tat increase the burn again?? A smaller nozzle would increase chamber pressure = to higher burn rate wouldn't it

 

[ClayD]

burn rates under pressure are worse if the substance is sensitive to pressure. look at different oxidizers and thier burn rate exponent, (*hint*) - trying to keep the mods off my post... but your oxidizer is certainly part of the burn rate, and more importatnly in this case burn rate exponent.

your 84mm is around 3.3125" which for your diameter would produce a progressive burn. This in turn may have induced some erosive burning.
both of these increase burn rates.

small airpockets in the propellant i make is no concern and doesnt increase burn rate. maybe a minor and mostly temporary difference in burning surface area. might make the motor sound "dirty" when it burns. but overall is a small percentage change.
from what i think i know about your propellant, is that they would be fairly airpocket sensitive, and rapidly overpressurize if it occured.

Would casting 4 grains or 3 be a better route to illiminate air pockets or would tat increase the burn again?? A smaller nozzle would increase chamber pressure = to higher burn rate wouldn't it

what your talking about, is KN the burning surface area in ratio to the throat area. Here is one of the best documents I found early on, and gave me a clear understanding of how making a nozzle smaller in diameter, may not work so well to increase the KN. since it has an exponetial effect on Cp
https://thrustgear.com/topics/Kn_Notes.htm
I usual increse the core diameter, or add grains (end grain surface) to up my kn. making a smaller nozzle, comes after you understand the pressure exponent fully for the propellant.

I would suggest, using the bates geometry formula to find the most neutral bates configuration(listed in that kn guide) fire a low end KN range, then middle, then higher. based on diffrent nozzles. if you have a way to measure the thrust or Cp, you then can determine the burn rate exponent of your propellant.

 

[Polyalcoholic]

'About a second' is the expected burn time for a 38mm cored grain. The burn time is determined by the grain web, the distance from the core to the inhibitor, and the burn rate of the propellant. To extend the burn time, you need to change the propellant or the grain geometry. A moonburner, mentioned earlier, is an effective way to increase the burn time. A larger nozzle will also extend the burn time by reducing chamber pressure but it also has a negative impact on Isp.

I believe that the 38/390 motor is designed for 3 grains. Two grains with the geometry you used are longer than ideal and the peak Kn and pressure are higher than if you used three grains of correct geometry.

If I may point you in the right direction, Loki and Tru-Core have suggested grain geometries listed on their websites for 38mm motors.

https://lokiresearch.com/techinfo.asp
https://www.rocketsaway.com/38mm_Grain_Geometries.txt

Those are good places to start.

Learn to use Burnsim or Motorsim and develop an understanding of how changes in grain geometry affect burn profile.

Above all...

BE SAFE!

HTH