Why didn't it work.

I flew the same rocket twice at NARAM this week. It's 4" diameter fiberglass dual deploy with two altimeters. For the first flight on a K everything was fine. Drought at the top and main at 300 feet. Only one main charge fired but that's why I have two altimeters. I cleaned it up double checked everything and reprogrammed the MARSA to 600' and left the Adept 22 at 300'. The parachute compartment is about 3' tall. The chute is in a deployment bag and the recovery system fills just about the entire volume. Each charge is a full centrifuge tube which is about 1 gram. The nose cone is held on by two 2-56 nylon shear pins. All of this is exactly the same as the first flight and the black powder came out of the same can. The flight was great. The drogue came out when it was supposed to and that was it. It just fell to the ground. After carefully taking it apart because I thought the charges didn't fire I found that both had. I have no idea what went wrong and no idea how to make it not happen again. Any ideas?

A one gram charge for a 4"x36" cylinder volume seems awful "light." A quick look on the Rocket Calculator and it is saying a minimum of 6.5 lbs pressure to shear two 2-56 pins which requires 1.5g of BP. 2g would be better, at least to provide a 50% margin which is what I shoot for.

Those charges sound WAY under the needed...they may have worked once, but perhaps you are at the edge of what can work and had good fortune that day. I use this table as a starting point. https://www.vernk.com/EjectionChargeSizing.htm On that page note the actual charge table also.

For instance I use 1.4 g of FFFg in my Mega Der Red (Duke) Max. It has an internal area around 4" diameter x ~13" tall, and no shear pins. In fact I've had 1.1 g not shear 3 pins in a 3" diameter x 14" length parachute compartment, mercifully the 1.3 g backup did it.

Are you expecting the charge to push the bagged main out, and have that push out the drogue?

Or does the charge just have to pop the nose and push out the drogue, with the drogue then pulling out the bagged main?

It makes a huge difference...

-Kevin

The VernK calculator is great, I've found it is a good starting point. However, when ground testing, the "real world" charge ends up being 75% to 100% larger.


All the best, James

You should be thinking of this in terms of "Why DID it work?" (for the first flight), as opposed to "Why DIDN'T it work?" (on the second). I think you simply got lucky that a 1g charge worked the first time.

s6

Which site is this calculator on?

dixontj93060 said:

A one gram charge for a 4"x36" cylinder volume seems awful "light." A quick look on the Rocket Calculator and it is saying a minimum of 6.5 lbs pressure to shear two 2-56 pins which requires 1.5g of BP. 2g would be better, at least to provide a 50% margin which is what I shoot for.

Click to expand...

ChrisAttebery said:

Which site is this calculator on?

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It's an iPhone app: Rocket Calculator. I use this and cross-reference it to Chuck Pierce's spreadsheet when selecting an initial charge. Of course, final charge is based on ground testing.

There are lots of online calculators. I generally use the INFOcentral one, using 10PSI to start and then ground test to make sure the ejection is sufficiently energetic.

Part of the problem may be just using two pins. When Drake Dameräu did his test on shear pins on rocketmaterials.org, he found that with two pins, "1) the average peak load is higher for each pin and, 2) the spread within the average is greater". So if your charge is on the light side, you may have been caught by the variance.
Another possibility- was the rocket substantially warmer on the second flight (e.g. sitting in the sun). Nylon pins are viscoelastic materials, and so the energy required to shear the pin will depend on both the rate of shear and the temperature of the pin. We want brittle failure, which means high rates/low T. If the charge is on the low side, the shear rate will not be as high, and if the pin is warmer then it may not undergo brittle failure. As a result, it will dissipate a lot more energy, and may not break at all.

David

I've seen InfoCentral's, Vern Knowles' and and Chuck Pierce's calculators. I had not heard of the iPhone app before.

I like to make the backup charges 50% more powerful than the primary charge. If the primary fails to get the parachute out it might be because it wasn't strong enough and this boost should do the trick. If the primary does get the parachute out then the backup fires into an empty space and no harm is done.

This works best when you know the sequencing.

NARAM is in CO correct?
Possibly be the thinner air?
Did you ever calibrate your MARSA?
My last Adept 22 flight failed after being "fixed"


JD

billspad said:

I flew the same rocket twice at NARAM this week. It's 4" diameter fiberglass dual deploy with two altimeters. For the first flight on a K everything was fine. Drought at the top and main at 300 feet. Only one main charge fired but that's why I have two altimeters. I cleaned it up double checked everything and reprogrammed the MARSA to 600' and left the Adept 22 at 300'. The parachute compartment is about 3' tall. The chute is in a deployment bag and the recovery system fills just about the entire volume. Each charge is a full centrifuge tube which is about 1 gram. The nose cone is held on by two 2-56 nylon shear pins. All of this is exactly the same as the first flight and the black powder came out of the same can. The flight was great. The drogue came out when it was supposed to and that was it. It just fell to the ground. After carefully taking it apart because I thought the charges didn't fire I found that both had. I have no idea what went wrong and no idea how to make it not happen again. Any ideas?

Click to expand...

Answering questions in random order.

I didn't recalibrate the MARSA but it and the Adept 22 were beeping out the same altitude within a few feet. I'll download the MARSA data when it gets home.

All it needed to do is pop the nosecone. There's a drogue chute that's attached to the nose cone and deployment bag. If the nose cone made it off, the drogue was right on top and would have pulled everything out. As you can see from the photo below, it didn't.

The weather conditions on the day it worked properly (Monday) were the same as the day it didn't (Tuesday).

I used the iPhone app to calculate the charge but that was a few years ago and I was going by memory. Probably not a good idea. I've got to take another vial to find out exactly what it holds. I thought it was between 1 and 1.5 grams. Until this year the scale I was using had 1g resolution so I know I had more than 1g and less than 2g in a full vial. Yes, I know that's not too smart.

My thinking on the charge size, obviously flawed, was that if I needed 2g (I thought the calculator said 1.5g but that's not what I get now) for a 4" x 36" tube, then I would need less because the tube was totally full of recovery system. What I didn't count on was how much the recovery system would compress. The electronics are in a coupler. When it hit the ground the coupler was driven 6" forward so everything could easily compress quite a bit without the nose cone coming off.

I think the "why did it work before?" theory has merit. Ironically, I bought a full 1 lb. can of black powder for two flights. I was trying to be a little more precise than my usual blow it out or blow it up technique.

The good news is that the rocket is repairable. I foamed around the fins after it's last hard landing so they held together. I've got some creative tube repair to do. By the way, on it's previous bad flights a year ago neither of the main charges fired. The electronics checked out after the flights but, since I couldn't find the problem, I replaced them and the wiring.

Bill, your charges are far too small in my opinion. I have a similar sized rocket and I use about 2.4 grams. It is energetic, bit it get the laundry out. I have about 25' of tubular Kevlar bridle, and it is about right lengthwise. I am using two 2x56 nylon screws as well. In this rocket, of I were flying redundant electronics, I would probably back the first charge to about 2 grands, and then the secondary, oh crap, charge would likely be about 3 grams.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

In a 4" x 24" tube, I normally use 3.0 gm or more.
At higher than sea level altitudes, the BP will burn slower due to the lack of air density.


JD

So I took a centrifuge vial and filled it up to the same point I did when I flew the rocket and then weighted the black powder on my precise Harbor Freight gram scale. It holds 1.5 grams. Evidently I was lucky slightly less than I was unlucky. In four years I've flow the rocket seven times. I got the main out three times when the charges actually fired, not out once when they did fire and three flights when they didn't fire at all. There will be a significant increase in black powder the next time I fly it.

billspad said:

So I took a centrifuge vial and filled it up to the same point I did when I flew the rocket and then weighted the black powder on my precise Harbor Freight gram scale. It holds 1.5 grams. Evidently I was lucky slightly less than I was unlucky. In four years I've flow the rocket seven times. I got the main out three times when the charges actually fired, not out once when they did fire and three flights when they didn't fire at all. There will be a significant increase in black powder the next time I fly it.

Click to expand...

Bill, not to muddy the waters, but I have found significant variation in pressure wave with centrifuge tubes depending on the way I prep them. If not done right you often get a "side blowout" and less containment during the charge. In the past year, I have achieved consistency by doing the following:

1. Cutting off the plastic flip top if there is one.
2. Prep charge and fill as usual.
3. Cover top with good quality blue masking tape.
4. Wrap around sides of centrifuge charge to completely cover with aluminum tape.

If you do the above you always get the charge contained and, in fact, direct the charge effectively by giving a path of least resistance through the blue tape.

I got that part right. I fill up the vial almost to the top, put a small piece of wadding in and then tape it up. The vials slip into brass tubes and are taped in place. I can almost always reuse the vials but in this case they shattered. The brass tubes stayed intact.

4" diameter. (2) #2-56 pins.

2 pins = 70-80lb holding force.

1g 4f in 36" of 4" =~4.5 PSI. 4" NC base is ~12.5inches, thus, ~56.25lb. You would need an additional 50% to be close to the shear strength of the two pins (not including the coupler friction/binding effect of a 2 pin setup). To make the charge reliable, I'd double it. That'd put you at 9psi, or 112.5lb.

I'd ground test at 2g and see how it goes, and possibly go up from there...


Later!

--Coop

billspad said:

The good news is that the rocket is repairable.

Click to expand...

Sorry about the flight (but the photo was fantastic!).

GuyNoir said:

Sorry about the flight (but the photo was fantastic!).

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No big deal. If things worked every time I'd never learn anything. I'm just amazed on how far off I was on the ejection charge size and even more amazed that it worked the times that it did. I've got 3 one pound cans of bp in my magazine so you can be the next time it fails it will be something different.


I don't know if you saw the big gash in the side but I came up with a creative way to fix it. Pictures to follow.

billspad said:

No big deal. If things worked every time I'd never learn anything.

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Does that mean you are constantly learning?

BobH48 said:

Does that mean you are constantly learning?

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Every day it seems.

dixontj93060 said:

Bill, not to muddy the waters, but I have found significant variation in pressure wave with centrifuge tubes depending on the way I prep them. If not done right you often get a "side blowout" and less containment during the charge. In the past year, I have achieved consistency by doing the following:

1. Cutting off the plastic flip top if there is one.
2. Prep charge and fill as usual.
3. Cover top with good quality blue masking tape.
4. Wrap around sides of centrifuge charge to completely cover with aluminum tape.

If you do the above you always get the charge contained and, in fact, direct the charge effectively by giving a path of least resistance through the blue tape.

Click to expand...

I learned the hard way that leaving the flip top on makes for a nasty projectile that can shoot right through the side of the airframe.

Tim - to fill excess volume, do you use the earplugs?


As an experiment I tried something weird -> I dipped the whole mess in that rubber compound you use to dip pliers handles. Made for quite the impressive BANG.

H_Rocket said:

I learned the hard way that leaving the flip top on makes for a nasty projectile that can shoot right through the side of the airframe.

Tim - to fill excess volume, do you use the earplugs?


As an experiment I tried something weird -> I dipped the whole mess in that rubber compound you use to dip pliers handles. Made for quite the impressive BANG.

Click to expand...

Earplugs, no haven't tried that...

#2 includes: 1) drill 3/32" hole through bottom of vial, 2) thread e-match through hole and hot glue into bottom of vial (e-match head inside vial!), 3) measure and fill with powder, and 4) pack remaining space to top of vial with dog barf (which has to be cheaper than ear plugs).

Quick-Dip rubber... I can imagine. I wonder if the energy observed was just result of extra containment, or is there a combustive element to the dip?

H_Rocket said:

Tim - to fill excess volume, do you use the earplugs?

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Use dog barf - it's a lot cheaper.

-Kevin

troj said:

Use dog barf - it's a lot cheaper.

-Kevin

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But it's really difficult to get out of your ears.

Call me a cave man, I would have treated that rocket with 3.9 g with a 4.5 g backup.
I never have a chute stick......

billspad said:

But it's really difficult to get out of your ears.

Click to expand...

Use a leaf blower.

Unless it's Al, in which case the solution is a quart of epoxy.

-Kevin