Surgical Tubing Charges?

I ground tested the surgical tubing (drogue) charges today for a new 3" MD rocket I'll be flying at BALLS, and the results weren't entirely successful. With the design I'm using, I only need to pressurize about 11" of airframe. I ran these figures through the Rimworld/NASSA calculator and it estimated .6g:

11" x 3" compartment
15 psi
3 x #2-56 nylon screws

I started out with .5g and got no separation. Upped to .75g...still nothing. The shear pins didn't even show any signs of wanting to cut.

Unless I'm mistaken, I would have thought that .75g would be plenty in a 3" airframe. Hence I'm wondering, could the issue have been with the charges themselves? I used the standard setup - surgical tubing with BP/ematch inside, and cinched closed with cable ties on each end. Upon visual inspection they seemed to have worked properly, splitting open with no leftover BP inside as far as I can tell.

The only other thought I had was that my tightly-packed chutes had muffled the charges enough to keep them from cutting the shear pins?

Heading back out to the test stand tomorrow with some bigger charges, but any theories here are welcome.

Hey Wilson, in my 3" "Color of Right", I use 1.5 to 2g (1.5 to 2cc) of 3Fg Black Powder, for both charges. Sections to be pressurized, main, 10" and drogue 13". I show now problems and they definitely separate.

Assumption: Let's say you need 1,250 N of force to break the nose cone (sum of shear pins ~75 lbf/ea, plus 56 lbf to remove NC).

P=F/(pi/4*d^2)

That would equal about 274.1 KPa.

You would need about 40 PSI, with a 20% fudge factor, so 48 PSI.

That will work out to about 1 gram of BP. So I would start with 1 gram and work your way up/down.

Greg

Wilson, my advice would be to take whatever charge you determine to be correct on the ground and then increase it by perhaps a third for a flight to 30K. My best guess.

Jim

I'm not very good advice because I oversize my charges....

When I launched my 2 stage 3" diameter, i had to pressurize a 15" section. I used 3.3 grams.

What's the rule of thumb for converting grams black powder, volume and pressure?

I'm curious as to the mole and temp change that must be built into that conversion.

The above example, if I figure correctly, is about a 3 bar change for 1.3L volume.

Charles_McG said:

What's the rule of thumb for converting grams black powder, volume and pressure?

Click to expand...

The other consideration is there is less oxygen in the air at high altitudes (and black powder needs atmospheric oxygen). The reason I've seen for using surgical tubing is to get a seal so that the air from ground level is retained.

Don't forget that CO₂ ejection systems are also an option. I prefer them for apogee charges on high-altitude flights because there's no guessing.

While I don't know that atmospheric O2 is required for combustion (it shouldn't be), I think people may have overlooked how important it [air] is in the deployment canister.

At standard conditions, the deployment canister above holds about 54 millimoles of air. 1 gram of black powder will generate about 14mmoles of gaseous products.

That 54 to 68 mmoles of gas in 1.3L of canister just doesn't make for a big pressure difference. The kick comes from going from 298K (on a warmish day) to about 950K (1240F). That gets you the big kick. (That's working backwards from the values given using the ideal gas law - but roughly matches burn temps from a quick google search. This isn't my area of chemical expertise. Lots of averages munged together here.)

At high altitude, the 14mmoles of BP products(g) stays the same, but the 54mmole of air will be much smaller. So much less gas for the deltaT to work on and much less resulting force. (45mmol at 5000', 38mmol @ 10000', 25mmol@20kft, 16mmol@30kft, 10mmol@40kft, 6mmol@50kft). My back of the envelope estimate would be about 1/3 the pressure developed at 50kft than at ground level.

Wilson,

My 54mm minimum diameter project has a 2.5" long parachute bay. There are 2x 2-56 nylon screws for shear pins. Using the formulas and values I've seen on the net I calculated that I needed .25g to safely deploy. In ground testing .3 wouldn't even budge the NC. I wound up using .7g.

Good luck,


Chris

What diameter of surgical tubing do you use? Thickness?

Charles_McG said:

The kick comes from going from 298K (on a warmish day) to about 950K (1240F).

Click to expand...

I don't believe you can assume that the air in the pressurized volume is being heated to 950K, only the BP combustion products. So to zeroth order the air shouldn't matter.

See https://www.vernk.com/EjectionChargeSizing.htm for the ideal gas equations.

Oxidizers are built-in to black and smokeless powders. Atmospheric O2 is not required for them to burn. Hell, KNO2 even release O2 as a by-product of combustion. Key factors of case pressures used in ammunition loading and as they relate to rocketry are powder burn rate and case (charge well) compression (case volume). The science of internal ballistics is a fun one for all you scientists (of which I am not).

Why even bother with black powder? I use smokeless powder and experiment with sealed charge wells. Much cleaner, less powder used and easier to measure/transfer/manipulate. Trial and error monkey style until I get the pressure I want to pop a nose cone in a given volume but hey, it is all my IQ can afford

This is a really interesting question and I haven't yet seen a good analysis of the physics for rocket deployment. A ballistics simulation is not going to be too helpful because the situation in a cartridge is very high pressure (many thousand PSI), burn rate is totally governed by the burn rate/pressure curve, there is near zero air for heat transfer, and the gas system is not very lossy. The OP used one of these and got predicted BP mass that was too low by a factor of 4 or so - in my experience you need ~2.5gm in a typical 3" rocket.

In a rocket body, nearly all of the physical factors are different: we never get above a few hundred PSI, there is tons of air for heat transfer (and it's not negligible, the conversion of chemical energy to gas pressure from the BP constituents is nowhere near 100% efficient), and the system is very lossy due to vent holes.

VernK's data basically proves that a simple model based on the ideal gas law is not a valid predictor either; it's off by more than a factor of two for mid-size 3-4" tubes.

The simplest rule of thumb that I have heard suggests a starting point of about 1gm per inch of *diameter*, which aligns OK with most reports I see of what people actually use up to ~10" diameter. I think that overpredicts somewhat at the low end but is pretty decent from 3" up. I'd love to see a good math assessment of why that relation seems to work.

starting point of about 1gm per inch of *diameter*

You better multiply by the length somewhere or this is just as random a starting point as any.....you are pressurizing a volume to create a force on the bulkhead to shear the shear-pins you've chosen by design.

Also, any BP/smoke-less powder needs no oxidizer - it just needs "Air" or some "ether" to transfer the thermal energy from grain to grain.

Also, I've got a 3" rocket that with a 16" chute bay that needs 4 gms of BP to reliably blow the chute...
....but it's got a lot of nose weight....and big shear pins to hold onto it....
Anecdotal statements like "use this" or "2gms charges work for my 3" rocket" are nearly meaningless unless you know the full story which is rarely communicated.

2.5" airframe that has a drogue bay of 4" and a main parachute bay of 6", completely and utterly stuffed full needed 2 grams for the drogue and 3.5 grams for the main. I doubled these for the backup charges. Rocket flew to 27k and recovered successfully. Shear pins on both sections consisted of 3 x #6 nylon screws.

Edward

Just remember the calculators assume that the black powder is in the rocket with no containment (Surgical Tubing). Once the powder is ignited it then has to burst the surgical tubing and it uses some of its energy doing so. Then whatever energy remains is used to separate the rocket.

I have used Surgical tubing many times with success, I typical use the calculators to get a baseline then double the amount needed. For example in my Dark Star JR, the calculators estimated 0.45g; I use 0.9g for main and 1.0g for back up

Wilson,
In my L3 rocket at NXRS - minimum diameter 75mm, with 10.25" airframe for recovery, and a tightly confined parachute and shock cord, I used 4Fa contained in surgical tubing for the apogee deployment. I tested 0.5g to 3.5g in .5 gram increments and found that 2.5g gave the desired separation energy - assertive, but not destructive.

I plan to fly it again at BALLS on an M840.

Also, I used four 4-40 nylon screws for positive retention.

BLKKROW said:

Just remember the calculators assume that the black powder is in the rocket with no containment (Surgical Tubing). Once the powder is ignited it then has to burst the surgical tubing and it uses some of its energy doing so. Then whatever energy remains is used to separate the rocket.

I have used Surgical tubing many times with success, I typical use the calculators to get a baseline then double the amount needed. For example in my Dark Star JR, the calculators estimated 0.45g; I use 0.9g for main and 1.0g for back up

Click to expand...

I will have to argue with you on the containment.

When we flew OCF, we used 15 gram charges. The first time, we set up the charges where they were PACKED down into the charge wells as tight as we could get them, then taped up with tons of tape. My dad could feel the shock from the charges and he was 400 feet away in the portajohn.
They acted stronger than expected because we CONTAINED the charges. They were in an aluminum well packed with BP, dog barf, and masking tape.
Containing the charge will actually INCREASE the strength of the charge.

Unless of course, I'm having a total brain fart and I'm completely wrong.

Some reading.....
https://www.rocketryforum.com/showt...oyment-charge-containment-construction-matter

I think your PSI calculation is the problem. With a tightly packed chute. I would be using 28 or 29 PSI as a starting point. That would give you a starting point of 1.1 Grams BP.

I think packing the charge _might_ change the pressure it ignites and burns at. And burn rate is known to be pressure dependant. From what people are saying (on various threads, including the one linked above), it looks like the answer to 'Does the burn rate make a difference?' is 'It depends...'.

By the way, when the OP says 'surgical tubing' - rubber or tygon?

I would be using 28 or 29 PSI as a starting point.

Blackrock is at ~4kMSL.
Your "starting" pressure is about 12.7PSI....not the 14.7PSI at sea level....and certainly not 28 or 29PSI.

If you want to compensate for your chute, change the volume.
But remember chutes packed tight contain huge amounts of air -- if you want the real volume, put it in a vacuum bagger and see how small it gets.

Hint for you Burnsim users too - fix your starting pressure and correctly expand your motors for flying at BR.

blackbrandt said:

I will have to argue with you on the containment.

When we flew OCF, we used 15 gram charges. The first time, we set up the charges where they were PACKED down into the charge wells as tight as we could get them, then taped up with tons of tape. My dad could feel the shock from the charges and he was 400 feet away in the portajohn.
They acted stronger than expected because we CONTAINED the charges. They were in an aluminum well packed with BP, dog barf, and masking tape.
Containing the charge will actually INCREASE the strength of the charge.

Unless of course, I'm having a total brain fart and I'm completely wrong.

Click to expand...

Your logic is correct. It is the same concept as a pile of powder burning slowly on a kitchen plate vs the same amount of powder contained by a projectile within a shell casing.

I think way too many people are lost on the fact that it is not only the type of propellant or explosive that is being burned or the rate at which it burns, but also just as importantly how it is contained/sealed within a sealed chamber. One would never get 23.5 grains of powder to send a 5.56 FMJ to 3200fps by setting the bullet on a pile of gunpowder and setting it on fire.

Maybe I was not clear in my first post. I was not talking about "atmospheric" pressure...I was talking about the "deployment" pressure used to separate the airframe parts.

Surgical rubber tubing is not what you want for a high altitude flight. As the pressure is reduced in the bay, the tube will expand like a balloon and the powder will not remain compressed, and it will not burn out at high altitude. You will not see this effect at ground level in a ground test. To check this out, hook the tubing to an air compressor and measure how much it expand as you raise the internal pressure. Be careful however as you do not want it to burst while you are handling it. (The pressurizing with compressed air on the ground has the same expansion result as putting the sealed surgical tubing in a vacuum chamber and pump out the air around it. Most folks have a compressor, but few have a vacuum chamber....).

It takes about 4.5 grams of BP to over pressurize the airframe volume to 14.7 psi at room temperature. The temperature factor is 5.7~ so if the gas remains hot, you can get 14.7 psi over pressure with as little as 0.8 grams of BP. In the worst case for 3 shear pins you could need ~90 pounds to shear the 3 pins which is ~ about 13 psi before inertial losses! You also have a lot of mass within the parachute bay to cool the ejection gas so 1.0 grams of BP is not an unreasonable starting point, and I would test 1.5 grams.

References:

https://www.chuckhawks.com/blackpowder_pyrodex.htm

Pep equilibrium calculations

Old Rocketmaterials.org shear data.

Bob

Wilson-

My approach: if the math doesn't work, throw it out. Up the BP until you get the desired response (then up it some more).

The amount you're using sounds very low, in my experience.

What Fred said about examples without context is right, but I offer one more, since you've seen this rocket a couple times and can fill in your own context. My Scarab (2" dia, ~8" long compartment) uses a main charge of 1.5g in 1/4" ID latex tubing, with a backup of 2g in the same setup. That is with 3x 2-56 nylon shear pins as well. Given that your compartment is larger, I'm guessing you'll need somewhat more than that. increase diameter by 50% and volume is 225% of before, so more.

BP is a low explosive when contained, but containment is necessary to develop a shock wave, and in general to ensure complete combustion of the BP prior to dispersal. Shock wave impact on the end of the compartment (and thus causing a high strain rate on the shear pins) is your best bet to break the shear pins. They will behave quite differently depending on the strain rate. A parachute in the compartment will complicate the matter, as it contains air and will act as a baffle, deforming, compressing, and absorbing some of the work energy that would otherwise go into breaking the pins, and thus necessitating more BP than you would otherwise require to overpressurize the chamber.

Here's another thought to keep you up at night... If you're flying to ~10km, your chosen method of containment may have an additional issue: if the latex tubing is truly sealed at both ends, you've got a balloon that will blow up as your altitude increases. pressure at 10km is ~1/4 that at sea level. Would containment still be sufficient to ensure complete combustion? What about shock wave generation?

Good luck!
-e

My goodness! I turn my head away for a little while, and now look where this has gone...
I haven't re-tested anything since my original post, but will be doing so in the near future. I think I just need to "up" my charges more. Responding to a few people:

Salvage-1 said:

Hey Wilson, in my 3" "Color of Right", I use 1.5 to 2g (1.5 to 2cc) of 3Fg Black Powder, for both charges. Sections to be pressurized, main, 10" and drogue 13". I show now problems and they definitely separate.

Click to expand...

Good to know - what size/number of shear pins?

JimJarvis50 said:

Wilson, my advice would be to take whatever charge you determine to be correct on the ground and then increase it by perhaps a third for a flight to 30K. My best guess.

Jim

Click to expand...

Thanks Jim for the tip!

blackbrandt said:

I'm not very good advice because I oversize my charges....

When I launched my 2 stage 3" diameter, i had to pressurize a 15" section. I used 3.3 grams.

Click to expand...

Seems just a little heavy for a 3" airframe - why so much, exactly?

JohnCoker said:

Don't forget that CO₂ ejection systems are also an option. I prefer them for apogee charges on high-altitude flights because there's no guessing.

Click to expand...

I haven't messed with CO2 yet, but probably should since it seems like a very wise idea for higher altitudes.

ChrisAttebery said:

My 54mm minimum diameter project has a 2.5" long parachute bay. There are 2x 2-56 nylon screws for shear pins. Using the formulas and values I've seen on the net I calculated that I needed .25g to safely deploy. In ground testing .3 wouldn't even budge the NC. I wound up using .7g.

Click to expand...

I think that may be the kind of thing I'm experiencing.

cwbullet said:

What diameter of surgical tubing do you use? Thickness?

Click to expand...

Looks like mine is 1/4".

KBlack said:

Wilson,
In my L3 rocket at NXRS - minimum diameter 75mm, with 10.25" airframe for recovery, and a tightly confined parachute and shock cord, I used 4Fa contained in surgical tubing for the apogee deployment. I tested 0.5g to 3.5g in .5 gram increments and found that 2.5g gave the desired separation energy - assertive, but not destructive.

I plan to fly it again at BALLS on an M840.

Click to expand...

Thanks Kelsey for the info - much appreciated. AND I look forward to seeing that flight!

Maxter said:

I think your PSI calculation is the problem. With a tightly packed chute. I would be using 28 or 29 PSI as a starting point. That would give you a starting point of 1.1 Grams BP.

Click to expand...

I suspect you're exactly right.

Charles_McG said:

By the way, when the OP says 'surgical tubing' - rubber or tygon?

Click to expand...

Rubber

emckee said:

My approach: if the math doesn't work, throw it out. Up the BP until you get the desired response (then up it some more).

The amount you're using sounds very low, in my experience.

Click to expand...

Great info - thanks!

Wilson....

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.

It's also entertaining to see people react to the sound of the charge from 3K feet.....

bobkrech said:

Surgical rubber tubing is not what you want for a high altitude flight. As the pressure is reduced in the bay, the tube will expand like a balloon and the powder will not remain compressed, and it will not burn out at high altitude. You will not see this effect at ground level in a ground test. To check this out, hook the tubing to an air compressor and measure how much it expand as you raise the internal pressure. Be careful however as you do not want it to burst while you are handling it. (The pressurizing with compressed air on the ground has the same expansion result as putting the sealed surgical tubing in a vacuum chamber and pump out the air around it. Most folks have a compressor, but few have a vacuum chamber....).

It takes about 4.5 grams of BP to over pressurize the airframe volume to 14.7 psi at room temperature. The temperature factor is 5.7~ so if the gas remains hot, you can get 14.7 psi over pressure with as little as 0.8 grams of BP. In the worst case for 3 shear pins you could need ~90 pounds to shear the 3 pins which is ~ about 13 psi before inertial losses! You also have a lot of mass within the parachute bay to cool the ejection gas so 1.0 grams of BP is not an unreasonable starting point, and I would test 1.5 grams.

Bob

Click to expand...

Interesting comments Bob.

I've done quite a bit of testing of the surgical tube method, and one limitation is that when they were in the vacuum chamber, I couldn't see them. I was always concerned about the balloon issue, so I did testing with both sealed and "vented" charges. I don't recall any significant differences in the results (neither approach was effective at all at high vacuums). If I ever get around to more testing, I think it would be a good feature to build the chamber with a viewing port. In the meantime, using a compressor approach would provide some idea of what actually happens to the charge.

Your 5.7 temperature factor is also interesting. Some time back, I actually calculated the volume of gas produced using the actual combustion products produced by black powder. I then back calculated the temperature that would be required to get the pressures estimated from several of the black powder charge size calculators. This was about 10 years back, but my recollection is that the temperature would need to be around 2400F. So, 2860R / 528R gives a 5.4 "temperature factor". I also recall having an independent estimate of the temperature being in that range. So, I'm curious where your 5.7 temperature factor comes from.

Jim