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General Questions about APCP motors

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ScrapDaddy

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I was wondering why propellant grains are the way they are in commercial motors sometimes:

1) Is there a reason the AT 29/40-120 F motors are a single ~32gram grain of propellant rather than say a pair of 16 gram grains that could be shipped USPS?

2) Is there a reason larger motors couldn't be broken down into multiples of 30 gram grains? (I know HAZMAT becomes a smaller part of the cost of larger motors, and people who fly these motors usually have access to an on site vendor, but this is just for hypothetical sake)

3) If there is a reason that propellant grains have to a certain length and this results in grains that have more than 30 grams, couldn't you make the c-slot bigger so there is less propellant to get it back under 30 grams if you were close (I'm inferring that this would increase burn rate, but I'd rather have an F30fj where i can buy without haz mat than an F22FJ where I do)

4) I get why Warp 9 motors like the H999 or I1299 don't have ejection charges(they will get snuffed out by the rapid burn), but why do the end burning ones (like an I59) not have it?

I only have a fairly basic understanding of motors(as you can tell from my questions), and was just curious about these things. If any of these questions are too specific and should be removed and belong in the research forum please let me know and I'll edit them out.
 

dhbarr

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Rather than answer questions about burning surface area and case pressure, I can super recommend any of the usual solid propellant books or websites. These are great questions, and I think you'll have a lot of fun.
 

prfesser

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When the grains are cut in half it increases the end surface area but not the core area. The result is a rather regressive-burning motor, thrust tapering off during the entire burn; not as efficient as a neutral motor. The problem can be solved by inhibiting one end of each grain with epoxy or binder+curative, but that raises the issue of "how do you do this quickly and reproducibly with hundreds or even thousands of grains?"

Grains that are too short do not have enough strength to keep the propellant from tearing under acceleration. RSD (rapid spontaneous disassembly...)

Re #3, You'd probably have to increase the c-slot rather a lot. If the c-slot is being cut with an electric saw (I assume that's how it's done in commercial production) you're removing and throwing away a lot of propellant.

As to #4, BTSOOM...

Best,
Terry
 

Alan Whitmore

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I'll have a shot at Question 4. In a BATES grain motor, the delay element is exposed to the super-hot propellant flame from the moment the motor lights up, and is absolutely certain to ignite. In an end-burner, the delay element would only be exposed at the last instant of propellant combustion, when the temperature and pressure in the combustion chamber are both on the way down. Perhaps this is enough to make ignition of the delay grain very unreliable. Also, it would need to be extremely short to get a usable delay time if it did not ignite until motor burnout. Could be there is some mechanical reason that short delay elements are unreliable.

Alan Whitmore
Carrboro, NC
 

dhbarr

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I'll have a shot at Question 4. In a BATES grain motor, the delay element is exposed to the super-hot propellant flame from the moment the motor lights up, and is absolutely certain to ignite. In an end-burner, the delay element would only be exposed at the last instant of propellant combustion, when the temperature and pressure in the combustion chamber are both on the way down. Perhaps this is enough to make ignition of the delay grain very unreliable. Also, it would need to be extremely short to get a usable delay time if it did not ignite until motor burnout. Could be there is some mechanical reason that short delay elements are unreliable.

Alan Whitmore
Carrboro, NC
Hrmm. A conical or peg delay element could protrude into an end burner and virtually guarantee ignition, maybe?
 
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