kramrocket Posted July 2, 2016 Share Posted July 2, 2016 Hi everyone, Since this is the 'Rocketry' area of this forum ( but not high power rockets of course) I wish to make it clear that am very passionate about proving the reasons why there are difficulties in the task of size 'Scaling up, or down' of any perfected fuel/motor one may have produced. By perfected fuel, I mean reliable and repeatable fuel.... the actual energy is not important. For anyone making small low power motors, black powder or sugar, it would be nice to know what to expect when deciding to 'up scale' or even 'down scale' a good working fuel and motor design. Lets say we have a 'baseline' cored motor with nozzle... The nozzle is 6 mm diameter (exit area of 28.27 mm2) At start of burn, the fuel core diameter is also 6 mm and at a length of 70 mm is 1,319 mm2 , so therefore a 'fuel/nozzle' ratio of 46.65 (Kn 46.65) and lets say 500 grams thrust was witnessed. At the end of burn, the core is now 18 mm diameter and also 70 mm length 3,958 mm2 , so with the same nozzle area is Kn 140 and say 3000 grams (maximum) thrust is observed. So now for the big question.If one wishes to change the scale of their great working fuel/motor design, how do they go about it with predictable results? My observations by trial and error indicates that when I scale up, the expected maximum thrust is less !!! the reason (I believe) is things aren't as simple as it seems. I think the fuel burn rate is the culprit. An inverse way to look at this is... Imagine you have a sample of the sensational space shuttle SRB fuel and make a mini version of a booster based on the Kn's it has, say a version of one twentieth scale ... what do you think will happen ? It would vaporise !!! So to summarise, in order to truly scale our motors, the fuel burn rate needs to be given much consideration. Mark Link to comment Share on other sites More sharing options...
dagabu Posted July 2, 2016 Share Posted July 2, 2016 Mark, I will leave all the science to the pros out there but I can tell you that in pyro, we almost universally lengthen the spindle (core length) to increase performance on paper but in reality, we adjust the fuel and tweak the ratio a bit to make it a tad bit hotter. Good luck. Link to comment Share on other sites More sharing options...
Redrocketman Posted July 2, 2016 Share Posted July 2, 2016 As I have progressed from E class motors, to reliable, H & J class motors ( recorded peak thrust 18 - 22kg unofficial ) i have simply used my regular "hot" fuel (1%RIO) focused on reliably casting full length single grains as opposed to bates grains, full core 80% of nozzle throat dia. No C.A.T.O. yet!!After discussing my progress with the team at SS2S, wil now change to a 6 point coring mandrel, increasing burn surface area substantially. I haven't yet done the maths but will soon figure it out after I finalise motor dimensions & post!! Link to comment Share on other sites More sharing options...
ddewees Posted July 2, 2016 Share Posted July 2, 2016 In the opposite way, I have a few motors I make that perform poorly at smaller sizes, yet work fantastic scaled up. By poorly, I mean weak... Also, I've tried scaling whistle motors up to 3" diameter... and have never got them to do anything other than RUD. I'm guessing pressure/increment size was the leading culprit. Link to comment Share on other sites More sharing options...
pyroman2498 Posted July 3, 2016 Share Posted July 3, 2016 In the opposite way, I have a few motors I make that perform poorly at smaller sizes, yet work fantastic scaled up. By poorly, I mean weak... Also, I've tried scaling whistle motors up to 3" diameter... and have never got them to do anything other than RUD. I'm guessing pressure/increment size was the leading culprit.I really love those flash rockets you brought to the club last July! Plan on coming in September ? ~Steven Link to comment Share on other sites More sharing options...
ddewees Posted July 3, 2016 Share Posted July 3, 2016 Not this year... I have to go to New York that weekend. Link to comment Share on other sites More sharing options...
OldMarine Posted July 3, 2016 Share Posted July 3, 2016 RUD? Link to comment Share on other sites More sharing options...
BlueComet24 Posted July 3, 2016 Share Posted July 3, 2016 Rapid Unscheduled Disassembly. 1 Link to comment Share on other sites More sharing options...
OldMarine Posted July 3, 2016 Share Posted July 3, 2016 Rapid Unscheduled Disassembly. My new favorite acronym! Link to comment Share on other sites More sharing options...
pyroman2498 Posted July 4, 2016 Share Posted July 4, 2016 Not this year... I have to go to New York that weekend.aww man , me an mom were gonna celebrate my birthday up at the farm in September ... Hope to get to build with ya again ~Steven 1 Link to comment Share on other sites More sharing options...
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