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Sugar rocket pressure problem


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#41 NeighborJ

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Posted 23 March 2017 - 07:42 PM

The Saturn V rocket having variable thrust liquid fueled engines would have very different results, being able to compensate it's thrust to achieve an optimal power to weight ratio at any size scale. Still time would still need to be scaled down along with the rocket otherwise one could expect a scaled down rocket to achieve orbit. My head is spinning trying to understand what this would mean. Basically I would expect there to be a lot shorter burn time so I couldn't expect it to go nearly as high.
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#42 stix

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Posted 24 March 2017 - 03:20 AM

. . . Still time would still need to be scaled down along with the rocket otherwise one could expect a scaled down rocket to achieve orbit . . .

 

Yeah NJ that does seem to be the case.

 

The "scaling theory" was fleshed out a bit in this thread.

http://www.amateurpy...fuel-burn-rate/

 

Although, that thread was more about scaling UP from a smaller motor, but the issue is the same as you suggest, as in time would need to be scaled as well. The only way of doing that (when scaling up) would be to add a burn rate modifier.


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#43 NeighborJ

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Posted 24 March 2017 - 08:25 PM

I didn't read the whole thread but it all makes sense and I can see other issues. If the large rocket motors lift off with a minimal amount of g force to protect the passengers and payload then a scaled motor would lift even slower. Without any guidance system to stabilize it, then it would be almost impossible to achieve a stable flight. Essentially a smaller motor would require tuning to achieve an equal amount of g force as the big ones or have a scaled down amount of gravity and crosswind.
It is simply easier to design a motor for a specific application. I never had calculus so I wouldn't know where to start with the math but I do understand the concepts and theories which are all pointing to the impossibility of creating an exact scaled replica of a large rocket.
I enjoy these kinds of threads, they tend to open my mind to new ideas and brush the cobwebs out of my head.

#44 stix

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Posted Today, 05:17 AM

. . . It is simply easier to design a motor for a specific application . . .

 

Yes NJ, that is one way.

 

The other way/method is to design a motor, test it, evaluate the data, then extrapolate that data and apply it to your new design and then predict the outcome. "Outcome" being very subjective. When scaling up using small motors (approx. 20mm - 40mm ID) this method does seems to hold true for this size. As we go bigger then the fuel burn rate and pressure will come into play more.

 

What I'm talking about is "scaling up".

 

If you're into mucking about with various fuels, grain geometry, nozzle designs (lots of fun), then it makes sense from a cost and time point of view to start with smaller designs, then go bigger.

 

kramrocket has done some good tests and my effort will be to show the results in a way that can easily be understood.

 

Sorry that this thread has gone off too far a tangent. I'll try my best to get this together over the next week and post a new topic for those who may be interested.

 

Cheers.


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