Cold weather deployment charges

I was doing some ground testing in 25 deg F temperatures, yesterday. The rocket assembly was outside for at least an hour, so everything was at this ambient temperature. I found that my usual charge canister (centrifuge vial, ematch, hot glue, 1.0g FFF, dog barf, snap the lid closed) failed miserably. A little poof, smoke, and then nothing. The BP burned, but the nose cone did not budge.

This setup has worked fine for me for years in warmer temps, and at many winter launches. Not yesterday, though.

I suspect low temperature, low pressure, low heat, and poor burn of the BP. I read the posts from Jarvis and TFish, and will try some of their techniques used for high-altitude combustion.

Thoughts?

Buckeye said:

I was doing some ground testing in 25 deg F temperatures, yesterday. The rocket assembly was outside for at least an hour, so everything was at this ambient temperature. I found that my usual charge canister (centrifuge vial, ematch, hot glue, 1.0g FFF, dog barf, snap the lid closed) failed miserably. A little poof, smoke, and then nothing. The BP burned, but the nose cone did not budge.

This setup has worked fine for me for years in warmer temps, and at many winter launches. Not yesterday, though.

I suspect low temperature, low pressure, low heat, and poor burn of the BP. I read the posts from Jarvis and TFish, and will try some of their techniques used for high-altitude combustion.

Thoughts?

Click to expand...

Don't underestimate the shrinkage/expansion of different materials as temps drop/rise.
You did not mention what your airframe and nose cone are made of, but if dissimilar materials, you may have picked up additional resistance to nose cone separation that you were not expecting.

a

Fiberglass nose cone, fiberglass body tube, and 2 nylon shear pins. They all connect as normal
No expansion or shrinkage that I can tell.

Keep av-bay/payload warm in car till you fly OR use some of the 6 for 2 dollar hand warmer packets. I slip one under the sled.

Did you try to remove the NC after test failure? I would be suspicious that moisture condensed and froze in a critical area, like the airframe to NC shoulder connection and caused the NC to adhere much harder than usual. Everything works better in the cold....not.

Jim

Interesting thread.

I MAC Performance Radial Flyer core sampled on a very cold day. I had my usual set-up of two altimeters (apogee and apogee +1), and motor backup as a third fail-safe, drilled to apogee + 3.

Of the three charges, none of them deployed the nose, and it came down ballistic. The main charge did blow, ejecting the chute while ballistic, which destroyed much of the airframe. The rest of the airframe was buried all the way to the motor retainer in the ground. Both altimeters were destroyed, so I could not analyze what went wrong.

I have wondered to this day if the cold somehow prevented the charges from exploding.

Alkaline batteries do not like cold temperatures.

Also, heat transfer time to the bp charge is the key factor for cold weather on the ground and for low pressure at altitude. It needs to be contained longer for complete combustion. Otherwise, unburned powder is blown from the charge canister.

I wrap the 2 ml canisters in tape to ensure I get a good pop. I use 2 grams ( FFFF) for apogee, most my rockets are 3" to 5" in Dia.

No freezing or shrinkage of the airframe is taking place.

Not a battery issue. I am ground testing with a warm, 12V launch controller.

As I suspected, I am 99% sure it is poor combustion and lack of containment. I put wraps of electrical tape over the the lid of the centrifuge vial, and everything performed as expected. Next, I will try a long length of vinyl tubing for a container as Tfish described for high altitude/low pressure. I think I need to make up "regular" charges and "cold weather" charges, depending on the temps.

My neighbors must think me an idiot or worse - blowing up rockets in the backyard on a cold winter's night.....

Sadly, my neighbors don't seem to notice when I ground test. If someone were shooting up my house, they probably wouldn't notice that, either.

This setup worked very well in 22 deg F temps. More energetic than centrifuge vial charges.

https://www.rocketryforum.com/threads/going-for-100-000-feet.134626/page-11#post-1745073

We had a launch yesterday at the Bong and there was a number of deployment problems. At one time we were 3-3 with no main chute deployment. The new LOC Saturn V was all worth going to the launch. Check out TWA web site for flight pictures.

I'm not going to give this as a recommendation, but, I tend to size ejection more on maximums than minimums. I had one ballistic return sizing the normal way and then changed my philosophy! Ever tried to get a rocket out after it embedded itself solidly into a rock underground? We could not extract the nosecone from the rock, or the rock from the ground. The tracker in the nosecone survived. I repaired some cracked bulkheads and it lived to fly again. It is still flyable today. If you ever hear Mad Puppy called out on the launch line, that's the one. I doubled the charges after that flight.

When working with fiberglass components and fairly solid bulkheads, one can go with larger than minimum charges. A minimum charge - one that ground tests fine - might be ok for a normal deploy at altitudes of a few thousand feet and no horizontal velocity to speak of. But will it deploy when you get hit with some wind shear shortly after launch and end up with a couple hundred mph horizontal velocity at apogee? It might shread after deployment (when it happened to me, it shreaded but held together for a successful recovery), but not deploying is not an option! Or temperature swings, humidity issues, higher altitude, ...? Minimum can end up below minimum rather easily.

If one upsizes the charges then ideally one wants something to absorb the extra energy so the parts don't rebound back and smack into each other or increase the risk of a recovery tangle.

There are at least two methods one can use.

One is to take sections of the line, fold together, and secure with a bit of masking tape. The tape needs to be torn for that line section to unfold. If the weight of the rocket is barely sufficient to tear the tape, then you are golden. It will deploy and absorb some energy in the process. I've used this one when deploying main from intentionally fast drogue return (say, 115fps) with fairly heavy rockets.

Another method which I routinely use for all size rockets is to knot up the line. Look up Chain Sinnet. It is used occasionally for stowing rope, where it shortens the line length to 1/3 of the original length. A pull on the end successively unknots the line lengthening it. It absorbs shock in the process. It has been noted that this can cause some surface fraying of the line and therefore is not ideal. In practice, the shock line gets far more fraying at attachment points and edge of tube locations. The line would get replaced due to wear at these other points long before friction wear from unzipping the knots would become an issue. An advantage of this approach is the line for stowage is 1/3 as long so it is easier to manage neatly with reduced chance of a tangle.

https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-4#post-456460 Starting from here you can see an example of how I used these techniques for one rocket. It also has good info for chute packing for those who have interest in the subject.

Gerald

Hmm, I Z-fold with hairbands, but the knotting is an interesting approach.