jason
New Member
Posts: 4
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Post by jason on Aug 31, 2019 4:41:59 GMT -8
Hi all, I'm designing my first batch rocket (having built a J-tube rocket before), a 4"/100mm system with a single brick bell, and I have a few questions. I would very much appreciate some advice!
I'm trying to decide between making a taller, narrower bell with a bigger riser-bell gap, but only 1.5x CSA around the exhaust, or a wider, shorter bell, with more like 3x CSA around the exhaust.
Does it matter where in the bell the riser is situated? Say, is there any advantage to it being centered vs. being right up against one of the walls of the bell? I'd like to put it right up against one of the walls so it's as far from the exhaust as possible...
I understand the gap between the top of the riser and the bell should be at least 1'/30cm. Again, is there a limit beyond which it doesn't make much difference to have more space?
And the space around the chimney exit - 1.5x CSA minimum, and more space will presumably result in less turbulence and smoother exit gas flow, but is there a known upper limit beyond which more space doesn't make any difference?
Jason
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Post by peterberg on Aug 31, 2019 7:04:08 GMT -8
Hi all, I'm designing my first batch rocket (having built a J-tube rocket before), a 4"/100mm system with a single brick bell, and I have a few questions. I would very much appreciate some advice! I'm trying to decide between making a taller, narrower bell with a bigger riser-bell gap, but only 1.5x CSA around the exhaust, or a wider, shorter bell, with more like 3x CSA around the exhaust. Hi Jason, welcome to the boards. A 4" batch rocket is tiny, better go for a 5" I would say. In my opinion a wider and shorter bell would be better because of more uniform heat distribution. Give the gases space, don't try to lead those. Does it matter where in the bell the riser is situated? Say, is there any advantage to it being centered vs. being right up against one of the walls of the bell? I'd like to put it right up against one of the walls so it's as far from the exhaust as possible... It doesn't matter much where the riser is situated in the bell, right up against the back wall is possible. Although it doesn't matter where at the perimeter the exhaust is situated, whether it is at the same side or not. I understand the gap between the top of the riser and the bell should be at least 1'/30cm. Again, is there a limit beyond which it doesn't make much difference to have more space? The top gap in a batchrocket system deserves a better definition. The same as the riser diameter could be considered as minimum, twice that is playing safe and more isn't necessary but lowers the temperature of the bell ceiling due to wider spreading of the hot gases. And the space around the chimney exit - 1.5x CSA minimum, and more space will presumably result in less turbulence and smoother exit gas flow, but is there a known upper limit beyond which more space doesn't make any difference? It depends on what you call the space around the chimney exit. The majority of the gases comes from above, so where they need to go around a 90 corner the perimeter of the exhaust opening dictates how much gas is flowing through. In case the opening is at floor level part of the gases are hindered at the bottom. Define your exhaust and its placement, measure the usable perimeter (minus the part close to the floor) and multiply it with the space in front of it. The resulting figure should be at least 1.5 times csa but 2 is better. More than that lowers friction further but effect deminishes with rising figure. For understanding the exhaust issue: see donkey32.proboards.com/thread/337
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jason
New Member
Posts: 4
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Post by jason on Sept 1, 2019 3:08:11 GMT -8
Hi Peter,
Thanks so much for the quick and informative response, not for the first time, I'm inspired :-)
In this case, inspired back to the drawing board!
Pictures to come.
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