Crimson0087 Posted September 27, 2018 Posted September 27, 2018 Is this right or have I oversimplified it. I understand that burn surface area changes but is this initially not right? Also since I am using a bates grain shouldn't the surface area remain relatively constant? So I see that kN= burn surface area/nozzle throat area. This is my math.I have 3 motor segments. 2 inches long with a 1/4 core throughout. I calculated the circumference of the core and multiplied that by the height of the segment to get a total of 1139mm for the surface area of the Core. I multiply this by 3 due to the 3 segments. 3418. Then I calculated the area of the circle that is the top and bottom of the 3 motors minus the area of the core. That worked out to be 252 mm. I multiplied this by 6 b/c of the 3 motors with 2 burning ends. I added this to the 3418 and I came up with an INITIAL burning surface area of 4932.Since the burst pressure of schedule 40 pvc is 1550 for 3/4 I used a chart off of Richard Nakka's site for my fuel to determine that a KN of 700 would yield me a pressure of 1000. this is 550 under burst as a starting point for nozzle size. Therefore if kn=burnsurface/nozzle then Nozzle=burn surface/kn so.4932/700=7.04 mm. My 5/16 initial nozzle should not have theoretically exploded. Or is my math wrong?
gdeputy Posted September 27, 2018 Posted September 27, 2018 Well, you're going from inches to mm in your area conversions, and my brain refuses to go any further. Plus you didnt give OD, so i have no idea if your math is right or not. But you're counting the surfaces correctly anyways. 1000psi in pvc tube is WAY more than i would expect it to withstand. 1000 PSI is getting close to running hot in an APCP motor with an aluminum case. Also, how are you inhibiting your grains? You said wrapping with cardboard in another post? If even a part of the outside of the grains are burning that means your surface area his higher than you think, and thus Kn is higher than you think. 1
gdeputy Posted September 27, 2018 Posted September 27, 2018 forgot, in regards to your first question, no. BATES grains do not have constant surface area. It depends on the dimensions of the grain. The longer a bates grain is, the more progressive the burn will be. Shorter grains produce more regressive burns. There is a close to neutral in the middle range, but not perfectly flat.
Crimson0087 Posted September 27, 2018 Author Posted September 27, 2018 What is an appropriate pressure to set as my max or what is a max kn that I should be aiming for?
Crimson0087 Posted September 27, 2018 Author Posted September 27, 2018 Also. what is a good way to inhibit the outside burn besides wrapping in cardboard? If I apply adhesive to the cardboard? OR a metallic paint to the motor before wrapping it?
gdeputy Posted September 27, 2018 Posted September 27, 2018 Please keep in mind i work with Ammonium Perchlorate Composite Propellant, not sugar propellants. When casting grains its normal to cast directly into a tube to inhibit the outside of the grain. I assume the sugar propellant you're using is melted? If so, pouring it into cardboard tubes would be a good place to start. You may be able to paint it with some sort of inhibitor, maybe silicone? Nakka's site should have some info on process for that as well. Max pressure/kn for sugar propellants, i can only speculate. A few hundred PSI would be a good starting point. Try to get a motor to burn and not cato, then there start ramping up Kn from there. I dont use PVC cases for motors as they're pretty much the worst case in so many ways. Gets soft with heat, fractures into nasty shrapnel when it fails, etc... Aluminum or fiberglass are a better option, but are more expensive and require some tooling and machining ability. 1
stix Posted September 28, 2018 Posted September 28, 2018 As mentioned, you haven't given us the OD of the grains, therefore I can't confirm the math. Nevertheless, if the nozzle diameter is (5/16th" = 8mm) and the core diameter (1/4" = 6.35mm) and total fuel length (6" = 152mm) are all correct, then it would be impossible to get a KN ratio of 700. So yeah, I think your math is wrong. I used a test grain OD of 3cm which seems reasonable. Shown below, SRS grain designer calculates a relatively neutral burn profile from approx. 145 to 155. Unless I've got my inches to cms conversions wrong (easy to do) then I believe this to be correct for those size grains. As gdeputy suggests a few hundred PSI would be a good starting point and you should get a very decent workable motor, then work up from there. Cheers.
Crimson0087 Posted September 28, 2018 Author Posted September 28, 2018 Also what about an end burner. It seems that if you make an end burner you could theoretically have a Tiny nozzle despite a large amount of fuel? Ex. If i have a packed 3/4 inch pvc pipe with 6 inches of packed fuel inhibited on all but the nozzle end then the area of the burn initially is 284mm2. If I wanted a kn say around 350 (which I think is safe for my pvc after reading more) then I could have a nozzle of .8mm? Somehow that doesnt seem right.......
Crimson0087 Posted September 28, 2018 Author Posted September 28, 2018 So I came up with needing a nozzle diameter of 12mm atleast thats where I am starting. That seems really crazy big to me though.....Its .47 inch which the pvc is .75 but I guess I will start there and see how it goes.
stix Posted September 28, 2018 Posted September 28, 2018 According to my calculations 3/4" OD would give you KN ratios of approx. 93 to 139 which is respectable to start with - I'm sure it would take-off ok unless it was too heavy. Why is it you think you need 350? From memory, I think for core burners the ideal nozzle diameter would be somewhere around 1/3 to 1/4 of the grain or casing OD. So in your case, a nozzle diameter of around 6mm would be a good place to start. It's a bit trickier if using black powder compared to something like KNSU which burns slower. If you use too small of a nozzle with bp, all you are making is a salute.
Crimson0087 Posted September 28, 2018 Author Posted September 28, 2018 Stix I don't see how you got that low a know. What did you get for burn surface area I got somewhere around 4500mm. Also I tried 1/4. 5/16, and 8mm nozzles...all exploded Stix I don't see how you got that low a know. What did you get for burn surface area I got somewhere around 4500mm. Also I tried 1/4. 5/16, and 8mm nozzles...all exploded
kaotch Posted September 28, 2018 Posted September 28, 2018 Crimson, look in the member tutorial section and find Speedballs in which this kn value is easily explained by a valuable member called MIXER.Did'nt you read this or use the search function? It's for BP but this article is a very good starting point for you.
Crimson0087 Posted September 28, 2018 Author Posted September 28, 2018 Well my 12mm nozzle was still too small...it blew up again....
gdeputy Posted September 28, 2018 Posted September 28, 2018 Go bigger until it doesnt blow up. Get something functioning and then you can work from there. Few things on the above posts. RE End burner, they look great on paper. Reality is, they introduce more issues. #1, super small nozzle means if anything clogs it or you get it slightly off, or it erodes during the burn it'll have more impact on the Kn than a larger nozzle changing would. Also, if you're doing any kind of long burn in PVC, its going to give out unless you are doing some major insulation work.
Crimson0087 Posted September 28, 2018 Author Posted September 28, 2018 I feel like a one inch diameter would be easier to work with. Especially with making the fuel. Assuming I want 100g of fuel in my motor would it be better to use a shorter 1 inch pipe vs a longer 3/4 inch pipe? Are there major difference in thrust? I feel like a one inch diameter would be easier to work with. Especially with making the fuel. Assuming I want 100g of fuel in my motor would it be better to use a shorter 1 inch pipe vs a longer 3/4 inch pipe? Are there major difference in thrust?
gdeputy Posted September 28, 2018 Posted September 28, 2018 Thrust is a function of many variables, including size. If going a little bigger is easier to get something functional, then go for it. Smaller motors are actually more finicky. Just be careful, PVC shrapnel is a nightmare.
stix Posted September 30, 2018 Posted September 30, 2018 (edited) Stix I don't see how you got that low a know. What did you get for burn surface area I got somewhere around 4500mm. Also I tried 1/4. 5/16, and 8mm nozzles...all exploded If your 4500 square millimeters is worked out on the details you gave me (as shown below) then it's close to what I get. 4500mm2 is the same as 45cm2. It doesn't seem obvious so I've posted a worked through example. If you look at the core surface area (30.4) plus the Exposed Ends (15.47) we get 45.87.Divide that by the Nozzle area (0.49) and we end up with a starting Kn ratio of 93.6. The Kn ratio at the end takes into account the new length - which subtracts the fuel web (x6) from the initial length multiplied final circumference, and we end up with 140.1.I haven't taken into account the thickness of your inhibitor sleeve and have used a "case bonded" scenario. Obviously this is only theoretical, because the fuel never ignites all at once, nor burns perfect and will erode any sharp edges. Hope this helps. [EDIT] Therefore, according Richard Nakka's Chamber Pressure vs Kn (for KNSU propellant - shown below) and given my above calculations to be correct, then you are looking at a theoretical maximum pressure of around 350psi. Which is way way under the burst strength of your motor casing as you describe it. So I guess one wonders why you are having explosions. I suggest that perhaps your surface area of propellant burning is much larger due to failure of the inhibitor sleeve or even large voids in the propellant, or maybe something else. If you're increasing the burn rate via adding a modifier such as RIO, that would definitely cause over-pressurisation. Chamber Pressure vs Kn (for KNSU propellant) from Richard Nakkas Website. My acknowledgements and gratitude to Richard Nakka for doing these pressure tests and graphs, so as the rest of us don't have to Cheers. Edited October 1, 2018 by stix
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