Surgical Tubing Charges?

Jim

For mechanical purposes you can measure the swelling of surgical tubing either in a vacuum chamber or by internal pressurization with a compressor as the delta-p is the only environmental parameter that cause the swelling. Naturally the mechanical properties of the specific surgical tubing you use determines the extent od the swelling. The compressor method is also useful as you can measure the bursting pressure of the tubing.

I simply used ProPEP to obtain the BP combustion temperature which was 1712K (2623F). I used the BP formulation in the article I referenced, and normalize the quantities to 1 gram. Room temperature is assume to be 298 K, (25C or 77F)

You can make the hot to cold gas volume ratio vary simply by what you species you use in the calculation. ProPep correctly treats many solid as gases at the combustion temperature however as the gases don't remain that hot for long, I choose to use only the stable species and my exact ratio is 5.74 if I leave out water, and 6.38 if I include it. The hot gas volume generated is 2,103 cc/g (128 cu.in./g) and the room temperature generation is 330 cc/g (20 cu.in./g) where the water is condensed out.

The effective temperature of the ejection gas will be less than the combustion temperature but more than 298K, so that will mean the effective ratio will be less than what I calculated.

Different formulations and different charcoals will give slightly different results, but the results will not be that different.

Bob

blackbrandt said:

Because I prefer to get the chute out. I've had 2 failures now with going with the stock charge and it doesn't get the chute out. I'd rather blow the chute out than lawn dart the rocket. I'm working on chute packing so that should bring my charge size down some.

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Understood!

As usual, reading through TRF feedback has me more concerned than necessary...but a trip back out to the test stand yesterday had me reassured.

I tested 1 g and 1.5 g, and both worked perfectly resulting in good "authoritative" separation. Guess I was way off on my estimations! For the flight itself, I'll likely go with a 1.5 g primary and a 2.5 g backup, or thereabouts.

Where does one find Tygon tubing in the real world?

M

MClark said:

Where does one find Tygon tubing in the real world?

M

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Tygon tubing can be had from McMaster Carr, but you would use latex tubing for the charges we're talking about. I tested a wide range of sizes at one point, all from McMaster Carr. The original method was developed with 7/16" x 1/4" tubing.

Jim

Revisiting this thread...


As I've outlined in other posts, this was the result of aforementioned BALLS flight. Seeing as this may be the second time surgical tubing charges have failed me now, it has me wondering...

I used 1.5g and 2.5g charges, prepped the usual way (cinched shut with cable ties, then ends sealed with electrical tape)...so they really couldn't have leaked.
I highly doubt it was an altimeter issue as both main charges fired (shredding the main).
Both of these charges worked on the ground, but not at +/- 25K AGL.
I suppose bigger charges may be all that are needed...

What gives? Any other theories?

Or maybe time to switch to CO2...

JimJarvis50 said:

Tygon tubing can be had from McMaster Carr, but you would use latex tubing for the charges we're talking about. I tested a wide range of sizes at one point, all from McMaster Carr. The original method was developed with 7/16" x 1/4" tubing.

Jim

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Tygon tubing can also be had from local small engine repair shops since it is a common fuel line material on small engines.

1.5 grams might have been marginal, but 2.5 grams should have been plenty at 25K, regardless of containment method. Something else happened.

Jim

JimJarvis50 said:

1.5 grams might have been marginal, but 2.5 grams should have been plenty at 25K, regardless of containment method. Something else happened.

Jim

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My thoughts exactly. I wonder what...

AlnessW said:

My thoughts exactly. I wonder what...

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How tight was your parachute when packed?

BLKKROW said:

How tight was your parachute when packed?

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Pretty tight actually - the charges were also both "sandwiched" in between 2 chutes, just by nature of the deployment configuration. I suppose that may have had something to do with it.

Chutes that when packed are long tend to try to expand in diameter when hit by the ejection charge gas causing it to jam. The chute may slide easy going in but not when pushed out.
I did some skinny rockets with very long packed chutes (chute packed at 2.25" diameter X 24" long) and for used cardboard tube staves (tube split long ways) with a solid bulkhead bottom. The whole thing is pushed out, staves fall away, chute opens.

M

It might offend some flyers, but I calculate my charge and add 0.2 to 1 gm depending on the rocket construction materials and tube diameter.

I always add about 1 gram to whatever I ground test at. I haven't broken a rocket yet... but it does turn some heads when I put 3.3 grams of powder in a 2" diameter 6 lb. rocket.

blackbrandt said:

I always add about 1 gram to whatever I ground test at. I haven't broken a rocket yet... but it does turn some heads when I put 3.3 grams of powder in a 2" diameter 6 lb. rocket.

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That works for fiberglass but be careful how much you add to cardboard and paper.

I don't fly any cardboard anymore

What about using the glove finger tip style of charge wrapped with tape and then placed into the surgical tubing? The tubing can balloon up and not disturb the powder contain within the charge.

Chris

MClark said:

Chutes that when packed are long tend to try to expand in diameter when hit by the ejection charge gas causing it to jam. The chute may slide easy going in but not when pushed out.
I did some skinny rockets with very long packed chutes (chute packed at 2.25" diameter X 24" long) and for used cardboard tube staves (tube split long ways) with a solid bulkhead bottom. The whole thing is pushed out, staves fall away, chute opens.

M

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Interesting to know - thanks Mark for the insights.

cwbullet said:

It might offend some flyers, but I calculate my charge and add 0.2 to 1 gm depending on the rocket construction materials and tube diameter.

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I usually do too, and often times I'll add more for the backup charges.

blackbrandt said:

I always add about 1 gram to whatever I ground test at. I haven't broken a rocket yet... but it does turn some heads when I put 3.3 grams of powder in a 2" diameter 6 lb. rocket.

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A friend of mine always says "Carefully build up to a working charge - the double it!" Seems a little excessive, but again, probably less of a concern if you're using fiberglass.

blackbrandt said:

I don't fly any cardboard anymore

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Good, neither do I!

ckamila said:

What about using the glove finger tip style of charge wrapped with tape and then placed into the surgical tubing? The tubing can balloon up and not disturb the powder contain within the charge.

Chris

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I suppose this might work, though I recall containment with glove fingertips is not as good as it is with surgical tubing.

These past few weeks, I've been exchanging emails with a good friend of mine from AeroPac who has a lot of experience (successfully) getting birds open above 30K+ using BP charges. He was able to identify a potential flaw in my design:

AlnessW said:

...cinched shut with cable ties, then ends sealed with electrical tape...

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While this may have been enough to keep my charges from leaking, it may not have been enough to get them perfectly airtight...a factor I might have overlooked previously. Just cinching them tight as I had been doing likely wasn't providing a concentric seal. So with his advice, in the next few weeks I will be testing out 3/8" surgical tubing from McMaster (up from 1/4" which was a little snug, anyways), sealed with nylon plugs and red silicone RTV. We'll see how those work - more later. Couple of other notes/possible forensics from my BALLS flight:

I really don't think it was an altimeter issue - both the main charges fired (from what I can tell) and everything beeped correctly on the pad.
Both altimeters (Raven3/SL100) were being used in baro mode - i.e., no possibility for another one of those mysterious "Raven accelerometer didn't properly detect apogee" stories.
Av-bay was vented...
Since both apogee charges failed, I really think it points to some issue with how they were prepped.

My method of prepping did work once at 25K, but on 2 other occasions it didn't. Unfortunately the first lawndart was never recovered, so no forensics from that one.

Not sure what else it could be, really...

Have you thought of making a Z-Pard type device? It is an aluminum piston that houses black powder in one end and then is connected between your e-bay and nosecone. Because you don't have to pressurized the entire bay a little bit of BP can generate quite the force.

The downside is extra weight, the setup I typically use weighs about 75 grams per foot. The plus is that you get positive extraction of your nosecone and associated components out of the rocket. When I first tested these 5 or 6 years ago I had a 9 inch version and was able to loft a regular brick about 15 feet into the air.

Edward

This may be of interest on this subject:

[YOUTUBE]ftXHhscXuVw[/YOUTUBE]

Put a surgical tube deployment charge in a Vacuum chamber and sent it to 100,000 feet. I was reading the pressure off of a mechanical gauge on my chamber. I had my stratologgers in the chamber to record pressure, but after the 5 other tests I did that day I forgot to plug in the altimeters.

Interesting results, the charge wells did not expand at all. Contrary to what I and quite a few others it seems expected.

AlnessW said:

Pretty tight actually - the charges were also both "sandwiched" in between 2 chutes, just by nature of the deployment configuration. I suppose that may have had something to do with it.

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If your charges were on the end of long wires running out of the electronics bay, could the parachutes have shifted under acceleration and ripped the wires apart or out of the terminals or something?

blackbrandt said:

I always add about 1 gram to whatever I ground test at. I haven't broken a rocket yet... but it does turn some heads when I put 3.3 grams of powder in a 2" diameter 6 lb. rocket.

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Ka-blooweey!

I built a fat rocket that reliably ejected the HED nose at 6 grams Pyrodex, 4 charge tubes. So, yeah, noise is good.