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  #1  
Old 03-10-2012, 05:33 PM
rtuinila rtuinila is offline
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Default Help! Moving CP Tumble recovery

Hi Folks!

I'm trying to simulate an Estes Sprite, the one with the ring tail that utilizes the ejection charge to kick the engine back about an inch. This causes the CG to move and the whole rocket becomes unstable and tumbles for recovery. It would normally be flown with the old series 3 engines but those are no loger available. That being the case, I'm trying to make an adapter so I can use 13mm engines. I know folks have used spent 18mm engine cases to do this and they have flown without problems but, to me and my OCDness, this just doesn't seem right. Hence the need for an adapter that will end up weighing the same as a Series 3 engine when I have a 13mm engine loaded in it.

I've simulated the ascent with the adapter I put together and the motor in the forward position and the numbers look reasonable. My problem has been simulating the recovery phase. I've tried simulating with the motor/adapter combo in the rear position amd was surprised at how well it flew and more surprised when it augered in for a core sample without any tumbling. So how bad is my logic here and please be kind

Thanks

Roy
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  #2  
Old 03-10-2012, 05:49 PM
Doug Sams's Avatar
Doug Sams Doug Sams is offline
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Location: Plano, TX resident since 1998.
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Quote:
Originally Posted by rtuinila
That being the case, I'm trying to make an adapter so I can use 13mm engines. I know folks have used spent 18mm engine cases to do this and they have flown without problems but, to me and my OCDness, this just doesn't seem right. Hence the need for an adapter that will end up weighing the same as a Series 3 engine when I have a 13mm engine loaded in it.
The series 3 motors had thick-walled cases. The current 18mm motors have thin-walled cases, as do the 13mm motors. Thus, when you slide the 13mm motor into the spent 18mm case, you have very close to the same wall thickness as the series 3 motors. Hence, the weight is close as well.

I think this part of the project is fairly straightforward. The bigger challenges involve securing the hook and venting the airframe. The mini motors are notorious for potent ejection charges, so you need a bit larger vent hole in the airframe. And when the hot ejections blast the motors aft, they tend to either slip the hook and come out, or wrench the hook and tear it loose. So some reinforcement is needed over the stock hook attachment. I think a piece of glass cloth and epoxy is a good modification over the original attachment method.

The other trick is to make the motor slightly tight in the airframe by applying some tape to it (ala friction fitting). It needs to slide freely, but not too loose. It needs to have enough friction such that it doesn't slide aft, bounce off the hook and then slide back in (thus resulting in lawn dart recovery instead of tumble).

Doug

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  #3  
Old 03-10-2012, 06:04 PM
RWmarlow RWmarlow is offline
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I had a sprite...it always kicked the series three (s) motors and augured in with no damage
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  #4  
Old 03-10-2012, 06:29 PM
Jerry Irvine's Avatar
Jerry Irvine Jerry Irvine is offline
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You could approximate the proportional mass change with an ejection enabled rearward piston launcher, short throw, with a gas passage at full extension. I have not coined a name for it, but have employed it successfully.

Just Jerry

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Old 03-10-2012, 08:24 PM
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tbzep tbzep is offline
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Quote:
Originally Posted by Jerry Irvine
You could approximate the proportional mass change with an ejection enabled rearward piston launcher, short throw, with a gas passage at full extension. I have not coined a name for it, but have employed it successfully.

Just Jerry

EERPLSTGP. Nope.

Rear gas passage name? FART
I employ it successfully on a daily basis.
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Old 03-10-2012, 10:20 PM
Jerry Irvine's Avatar
Jerry Irvine Jerry Irvine is offline
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I was talking rockets not bodily functions. But what better rocket to place on your desk?

Jerry again
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