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  #1  
Old 04-20-2017, 08:04 AM
stefanj stefanj is offline
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Default Motor cut-away burning in slow mo

Model rocket motor cut in half, filmed at 4K slow motion. Lots of slop, because the guy just duct-taped the motor to a plastic sheet, but still interesting.

The last sequence is of an intact motor filmed in slow-mo; the ejection charge is a lot more serious that I'd expected.

https://www.youtube.com/watch?v=2sdVhrztfoU
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  #2  
Old 04-20-2017, 11:37 PM
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Quote:
Originally Posted by stefanj
Model rocket motor cut in half, filmed at 4K slow motion. Lots of slop, because the guy just duct-taped the motor to a plastic sheet, but still interesting.

The last sequence is of an intact motor filmed in slow-mo; the ejection charge is a lot more serious that I'd expected.

https://www.youtube.com/watch?v=2sdVhrztfoU


Agree with you on the ejection charge - looks like a road flare...
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  #3  
Old 04-21-2017, 11:23 AM
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luke strawwalker luke strawwalker is offline
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Ya know, if he was gonna do it, at least do it RIGHT...

With a little more effort and forethought, the video of the bisected rocket motor firing could have been SO much better...

First off, use some EPOXY to bond the motor to the Lexan... just a thin layer to seal off the grain to the lexan so it end burns as intended rather than along the entire length near instantaneously. Second, put a thick layer of epoxy over the outside of the casing, again to bond it to the lexan, provide a seal, and help it end burn as it was meant to. Third, clamp the thing up so the casing is pressed against the lexan as snugly as possible (without going overboard and cracking the propellant or something). Then, set it up and fire it...

Another way of doing it would be to get some of the clear liquid lucite that they use to encapsulate objects for displays and such... cap the nozzle and ejection charge ends with tubing and pour in lucite around the bisected motor and embed it into a lucite block, then polish the grain side so it's transparent to the camera... that way you'd ENSURE that the motor burned from one end to the other as intended...

There's actually a much better video of a pressed motor made inside a clear acrylic tube here on Youtube... https://www.youtube.com/watch?v=12fR9neVnS8 at 4:40 into the video, or here https://www.youtube.com/watch?v=4v_utrMar9Y&t=2s at 6:58 in the video...

Later! OL J R
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Old 04-21-2017, 11:28 AM
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I'm actually surprised the first test with the duct taping was not more violent. I would have gone the extra distance to glue it down for a good seal.

The ejection charge video showed more flame than I expected.

When I watched him saw the motor in half with a table saw -- well, I cringed!
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  #5  
Old 04-21-2017, 12:47 PM
kapton kapton is offline
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Quote:
Originally Posted by LeeR
When I watched him saw the motor in half with a table saw -- well, I cringed!


Was glad to see he was wearing reasonable protective equipment - heavy gloves and a full face shield.
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  #6  
Old 04-21-2017, 02:13 PM
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Quote:
Originally Posted by luke strawwalker
First off, use some EPOXY to bond the motor to the Lexan... just a thin layer to seal off the grain to the lexan so it end burns as intended rather than along the entire length near instantaneously. Second, put a thick layer of epoxy over the outside of the casing, again to bond it to the lexan, provide a seal, and help it end burn as it was meant to. Third, clamp the thing up so the casing is pressed against the lexan as snugly as possible (without going overboard and cracking the propellant or something). Then, set it up and fire it...

Another way of doing it would be to get some of the clear liquid lucite that they use to encapsulate objects for displays and such... cap the nozzle and ejection charge ends with tubing and pour in lucite around the bisected motor and embed it into a lucite block, then polish the grain side so it's transparent to the camera... that way you'd ENSURE that the motor burned from one end to the other as intended...


Shhh ...

Don't tell anyone, but a couple of years ago I tried both of those ideas several times without much success. The motors burned in a fraction of a second ... much like the one in the posted video, but not quite as well.

Gluing the motor to Lexan with epoxy didn't work as the epoxy appeared to burn then the motor burned all at once as the flame progressed along the side.

Encasing the half-motor in a block of polyester resin had about the same effect. Though, in this case, I was left with an interesting paper weight!

I also tried clamping a bisected motor inside a box with a piece of heat-resistant glass (not cheap!) held tightly against the side of the motor. That didn't work either.

At 1200fps, the motor burns for a few frames in each of my videos. In real life that means the motor burned for less than a second. I used E9s, so they should have burned for about three seconds in real life (or more than a minute on video).

For one of my videos, I created an animation of the motor burning using some of the real slo-mo video (yeah, I cheated):

https://www.youtube.com/watch?v=zKJJvAAvKnc

I didn't post my actual inside-the-motor videos because they didn't turn out very well and I didn't want to encourage others to try it. I took a lot of safety precautions including rigging up a jig to cut the motors so I wasn't close to them whuile doing it and being quite far from them when they were ignited. I was concerned that others might not be so careful. Nevertheless, after I get back from the NEFAR launch, I'll post them for purely scientific interest.

-- Roger
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  #7  
Old 04-21-2017, 04:20 PM
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Why not just buy some transparent aluminum tubing from Plexicorp and make a few motors? I think I have Dr. Nichols' cell number around here somewhere.
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  #8  
Old 04-21-2017, 05:02 PM
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Quote:
Originally Posted by tbzep
Why not just buy some transparent aluminum tubing from Plexicorp and make a few motors? I think I have Dr. Nichols' cell number around here somewhere.
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  #9  
Old 04-21-2017, 08:52 PM
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Quote:
Originally Posted by jadebox
Shhh ...

Don't tell anyone, but a couple of years ago I tried both of those ideas several times without much success. The motors burned in a fraction of a second ... much like the one in the posted video, but not quite as well.

Gluing the motor to Lexan with epoxy didn't work as the epoxy appeared to burn then the motor burned all at once as the flame progressed along the side.

Encasing the half-motor in a block of polyester resin had about the same effect. Though, in this case, I was left with an interesting paper weight!

I also tried clamping a bisected motor inside a box with a piece of heat-resistant glass (not cheap!) held tightly against the side of the motor. That didn't work either.

At 1200fps, the motor burns for a few frames in each of my videos. In real life that means the motor burned for less than a second. I used E9s, so they should have burned for about three seconds in real life (or more than a minute on video).

For one of my videos, I created an animation of the motor burning using some of the real slo-mo video (yeah, I cheated):

https://www.youtube.com/watch?v=zKJJvAAvKnc

I didn't post my actual inside-the-motor videos because they didn't turn out very well and I didn't want to encourage others to try it. I took a lot of safety precautions including rigging up a jig to cut the motors so I wasn't close to them whuile doing it and being quite far from them when they were ignited. I was concerned that others might not be so careful. Nevertheless, after I get back from the NEFAR launch, I'll post them for purely scientific interest.

-- Roger


Interesting.

I guess we underestimate the importance of the cylindrical motor casing and ramming the propellant and how much strength that gives to BOTH the casing and propellant. Sort of like how tank pressurization adds rigidity and strength to liquid rocket stage propellant tanks....

Once that has been destroyed (by cutting the motor in half), re-establishing the tension that keeps the propellant from separating from the wall of the casing (or in this case, the epoxy or Lexan sheet, or the lucite its cast into) simply isn't there and can't be re-created... Well, possibly with PRESSURE CASTING (pressurizing the mold, but since the liquid resin is incompressible, not sure how much that would help, if any... when a motor propellant powder is pressed or rammed into a casing, the casing "stretches" slightly as the powder is rammed and forced into a slug, in intimate contact with the inner wall of the casing, "pre-tensioning" the casing, which then exerts pressure back onto the propellant grain as well...)

Thats why the hand-rammed motors in the acrylic tubes that was on the 'king of random' video links I posted earlier work like normal... the rammed powder inside the acrylic tube exerts pressure on the inside walls of the tube, the propellant essentially "stretching" the tube slightly as its rammed, keeping the propellant slug under tension and keeping the slug in intimate contact with the interior wall of the clear tube. SO, the motor burns normally from one end to the other (or the core out depending on the motor design...)

I wonder how a black powder motor hand rammed into an acrylic tube would work?? I bet it'd work as advertised and could be seen in slo-mo photography...

Later! OL J R
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  #10  
Old 04-21-2017, 11:25 AM
luke strawwalker's Avatar
luke strawwalker luke strawwalker is offline
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Quote:
Originally Posted by SEL
Agree with you on the ejection charge - looks like a road flare...


That's the remainder of the delay grain burning off... the cause of the so-called "hibachi effect". It's easier for the flames from the delay grain to escape out the open end of the casing at the top, than the nozzle end at the bottom...

The ejection charge is a quick "blast" from the front end when the "loose" powder lights off, but the remaining delay grain continues to burn after that until it's all gone...

Later! OL J R
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