Cold Weather Flying - Deployment Failure

[wonderboy]

At our club launch on Saturday, my Miss Riley (the fiberglass kit from Rocketman) had a deployment failure that I wanted to discuss.

Contrary to my usual routine, I did NOT procrastinate the rocket prep until I was at the field. I actually completely prepped the rocket in the comfort of my hobby room the night before. This rocket has flown two previous flights, both perfectly normal. I built per the Rocketman instructions and use three 2-56 nylon shear pins for the av-bay to booster connection, as well as the nose cone to payload tube connection. The av-bay is connected to the payload tube using three plastic rivets. Perhaps this is overkill for a 2.6" 5 pound rocket, but it is what Buddy suggested in the instructions, and what I've flown twice successfully. All the fits seemed perfectly smooth and free, although as I've noticed, the nosecone joint was a bit looser than the av-bay to booster.

For recovery, I fly an RRC3 as the primary altimeter, and an RRC2+ as a backup. Just for kicks, I actually load the powder into the ejection well of the motor too, but have the delay trimmed to ensure the motor ejection is completely last resort (2-3 seconds after planned apogee)

Launch day was cold (Team-1 Michigan launch north of Flint, MI). Temps on the field were around 35F. I did not test the fit of any of the joints at the field (which would have required removing shear pins). The motor was an AT I357 Blue Thunder.

Boost was perfect, but the drogue failed to deploy at apogee. The rocket did not separate at the av-bay to booster joint. The rocket arced over and was coming in ballistic and at the main altitude the nosecone and main was successfully deployed, but the velocity severed the recovery harness. and the booster/av-bay/payload tube combo came in all the way ballistic. Fortunately, the impact happened in a very wet clay soil. The nosecone and main parachute landed perfectly on the field with no damage. I found the rocket embedded all the way up to the fins. The shocking thing was that I could here one of the altimeters beeping out the reported altitude. I was pretty amazed by that. After running to grab a shovel, I dug a hole next to the rocket and carefully moved dirt away and recovered it.

The rocket survived amazingly well. The only damage is to the payload tube and the av-bay. As the clay packed into the forward section of the payload tube and encountered the forward av-bay bulkhead, it forced it into the coupler that forms the actual av-bay. So, the rocket will very easily be rebuilt and I have the necessary fiberglass tubes already on order.

THE QUESTION IS: What happened?

I think it can only be that the cold affected the fit of the joint and it was too tight to separate. Inspection of all charges (including the motor charge) indicate they all fired. The av-bay to booster joint was impossible to separate at the field after removing the shear pins (on person on each end of the rocket twisting and pulling). I attributed this to the force of impact, dirt, and forward bulkhead being shoved into the coupler making it egg shaped. Once home and I separated the two by cutting a slit in the payload tube with a cutoff wheel, there was no obvious sign of something wedged into the joint causing the problem. All I can think of is that the cold made the fiberglass fit too tight.

BUT, could the cold have negatively affected the deployment charges? Does the pressure generated by the black powder charge or the burn rate change in the cold? For reference, I use the centrifuge tube method of charge prep. I use place the e-match, fill with BP, pack the open space with Estes wadding and dog barf (to fill any open space in the tube) and ram it pretty tightly with a wooden dowel. I've used this technique on charges for 1/2 dozen flights with success, but never in the cold (anything below 65F). Again, all charges fired, including the motor ejection charge.

Any thoughts?

One thing I am contemplating is eliminating the shear pins for the av-bay to booster section. It doesn't seem to me that drag separation would be that large of a concern on a rocket like this. This would perhaps eliminate one more impediment to separation at this joint.

 

[rfjustin]

How many grams did you have in the drogue side please?

I would change over to charge wells in lieu of centrifuge tubes.

 

[manixFan]

I had a similar situation happen to me not long after I steered flying, with a PML rocket that used Quantum tube and a piston. The instructions even warn you if you’re going to fly in cold weather to put the piston and quantum tube assembly in the freezer and then check the fit. Since I fly in Texas, I did not think that would ever be an issue. Of course not long after I started flying I went to a launch in the very early spring where we had temps in the low 40s. The piston was jammed tight in the tube and would not budge. I suspect you would find the same thing with your fiberglass kit, in freezing temps or near them, the tube contracts enough to greatly change the way the parts fit together.

i’m glad it sounds like you’ll be able to easily repair the kit, that’s a really great looking rocket.

Tony

 

[wonderboy]

I used 1.5grams for each charge. I know of the technique of upping the backup charge, but just didn't do it on this rocket. On the previous flights, the primary always separated the rocket (you could hear and see the backup fire). However, given this situation, I'll probably start upping the backup charges in the future.

I did test this on the ground after building and it showed great pop (good separation velocity) and has worked on the previous 2 flights. I do place the centrifuge tube into a holder made from copper tube with a cap soldered on the end. This orients the charge axially and prevents any shrapnel from directly impacting the side of the airframe.

 

[Buckeye]

BUT, could the cold have negatively affected the deployment charges? Does the pressure generated by the black powder charge or the burn rate change in the cold? For reference, I use the centrifuge tube method of charge prep. I use place the e-match, fill with BP, pack the open space with Estes wadding and dog barf (to fill any open space in the tube) and ram it pretty tightly with a wooden dowel. I've used this technique on charges for 1/2 dozen flights with success, but never in the cold (anything below 65F). Again, all charges fired, including the motor ejection charge.

Yes, the cold temp may have compromised the burn and pressure of the deployment charge. I am guessing the temps in Flint were about 35 deg F? This happened to me too with centrifuge vials in the winter, but not consistently. Nonetheless, I now tape shut and wrap the centrifuge vials for a little more containment and complete burn. For the absolute best charge packing scheme, search for @tfish and his vinyl tubing method. I use this for high altitude flights and thin air.

One thing I am contemplating is eliminating the shear pins for the av-bay to booster section. It doesn't seem to me that drag separation would be that large of a concern on a rocket like this. This would perhaps eliminate one more impediment to separation at this joint.

Yes, remove those pins. For "normal" rockets, drag separation is a rare occurrence. My poor cold weather deployments only occurred with pinned main chute compartments. Unpinned drogue compartments always separated. If a deployment failure has to happen, then this scenario is a much safer alternative.

 

[Buckeye]

How many grams did you have in the drogue side please?

I would change over to charge wells in lieu of centrifuge tubes.

Curious, why do you prefer charge wells? Do you put the ematch on top of the powder, in the middle, or on the bottom?

 

[mrwalsh85]

I apologize if this is redundant, but I do not see any mention of if the charges fired.

Might recommend saving shear pin installation until the field. It just takes a few mere minutes and you're off to the pad.

I will say, though, I keep all of my altimeter bays in the house until I head to the field. I do not want the cold weather to affect the batteries to where they won't have enough oomph to fire the ematches.

I am slowly switching over to charge wells, per @rfjustin s recommendation... Easier to load and loading those pesky rubber glove tips is kind of annoying anymore.

 

[wonderboy]

I apologize if this is redundant, but I do not see any mention of if the charges fired.

No worries, I was pretty long-winded in my post. ALL charges did in fact fire. I actually had three chances for the drogue deployment: 1.5g primary, 1.5g backup, and the motor ejection on the I357T.

After all the discussion so far, I definitely am taking away these lessons:
1. No need for shear pins on av-bay to booster on this particular rocket
2. Prep at home is fine, but check all final connections/joints/fits once rocket is adjusted to flying field conditions (temps)
3. Upsize backup charges
4 (Maybe). Upsize all charges when flying in cold weather

 

[Rocketclar]

Another consideration....talc the joints (and piston when used) especially in cold weather. Also, the typical charges that come with AT motors of that size are around 1.4g or less.

 

[Bat-mite]

I had the exact same problem years ago. Two altimeters and motor backup. All three failed to get the drogue out when it was bitter cold. Cold kills batteries, but the main charge fired, so it couldn't have been that. And that wouldn't explain the motor charge failing.

Too cold for BP? I seriously doubt that. All I could figure was that the outer tube shrunk enough to jam the coupler. Or, maybe it was totally coincidental. Perhaps a corner of the parachute/harness/blanket was stuck between the tubes. Maybe it wasn't coming out even in warm weather.

My rocket was utterly destroyed on impact. I don't know if you can do any forensics on yours. Definitely look at the alt read-outs. As for changing over to charge wells? I would need that one explained to me. My rocket had charge wells.

 

[rfjustin]

Curious, why do you prefer charge wells? Do you put the ematch on top of the powder, in the middle, or on the bottom?

This is all just my .02, so take it for its worth, but I would argue that charge wells are more repeatable, and more controlled and consistent than the centrifuge or glove fingertip method. Depending how long the leads are off the centrifuge or finger tip, you don't really see how it got packed during prep. If charge wells are used, they are in the same place each time.

I place the e-match in the middle of the powder using charge wells, add a little dog barf on top, then tape over.

YMMV.

 

[rfjustin]

No worries, I was pretty long-winded in my post. ALL charges did in fact fire. I actually had three chances for the drogue deployment: 1.5g primary, 1.5g backup, and the motor ejection on the I357T.

After all the discussion so far, I definitely am taking away these lessons:
1. No need for shear pins on av-bay to booster on this particular rocket
2. Prep at home is fine, but check all final connections/joints/fits once rocket is adjusted to flying field conditions (temps)
3. Upsize backup charges
4 (Maybe). Upsize all charges when flying in cold weather

I would not ditch the shear pins, I would up the size of the charges and switch over to charge wells. This is a perfect size rocket to re-ground test with current set up, then retest with a charge well and see if you get a different result, I suspect you will.

 

[Steven88]

No worries, I was pretty long-winded in my post. ALL charges did in fact fire. I actually had three chances for the drogue deployment: 1.5g primary, 1.5g backup, and the motor ejection on the I357T.

After all the discussion so far, I definitely am taking away these lessons:
1. No need for shear pins on av-bay to booster on this particular rocket
2. Prep at home is fine, but check all final connections/joints/fits once rocket is adjusted to flying field conditions (temps)
3. Upsize backup charges
4 (Maybe). Upsize all charges when flying in cold weather

Hello. I am wondering which type
of dual deploy rocket builds it is recommended to have sheer pins on the booster section and which ones not? All my rockets have them and I have had no problems yet… but neither do I want to

 

[wonderboy]

Hello. I am wondering which type
of dual deploy rocket builds it is recommended to have sheer pins on the booster section and which ones not? All my rockets have them and I have had no problems yet… but neither do I want to

I've thought about this a lot, and don't have any practical advice. Just a basic understanding of the concept that the back half of the rocket is "draggier" than the front half. After boost as the rocket decelerates, the back half is acted on my more drag force than the front of the rocket. I think that the mass of the two halves matters too since the inertia of the two halves determines the force that is trying to keep them moving (Force = mass x velocity).

So in my uneducated analysis, its due to a low drag/high inertia nose compared to a high drag/low inertia booster... I'm sure somebody here has a more analytical description than this.

 

[tim cubbedge]

Hello. I am wondering which type
of dual deploy rocket builds it is recommended to have sheer pins on the booster section and which ones not? All my rockets have them and I have had no problems yet… but neither do I want to

I have them on most of my rockets and you definitely want them on high acceleration motors else you stand a good chance of drag separation at burnout which is why I started using them on all my rockets since then. If you don’t plan on flying anything above 20 G’s, you can probably get away with friction fit. Don’t think there are any set rules but I have seen some RSOs tell some flyers they couldn’t fly without them.

 

[Buckeye]

I have them on most of my rockets and you definitely want them on high acceleration motors else you stand a good chance of drag separation at burnout which is why I started using them on all my rockets since then. If you don’t plan on flying anything above 20 G’s, you can probably get away with friction fit. Don’t think there are any set rules but I have seen some RSOs tell some flyers they couldn’t fly without them.

Not exactly. The maximum deceleration at burnout is the value to consider. However, even more important is the difference between the booster and sustainer in terms of their drag and mass relationship:

https://www.rocketryforum.com/threads/shear-pins-on-a-8-dia-bird.153740/post-1901747
You can have a sloppy loose coupler in a rocket accelerating/decelerating at 100000000 G, but as long as M/(1+R) < M1, then the sections won't drag separate.

I fly L1000s all day long to +30 G with nothing but a tiny vent hole in the booster for pressure equalization at altitude, and I probably don't even need that.

 

[ksaves2]

I would not ditch the shear pins, I would up the size of the charges and switch over to charge wells. This is a perfect size rocket to re-ground test with current set up, then retest with a charge well and see if you get a different result, I suspect you will.

I agree with not ditching the shear pins. One can run a looser fit and depend on the shear pins to keep things together. I think all the lessons mentioned in message #8 are valid. If one flies enough rockets, ejection failures for a variety of reasons are going to happen. I've had a few of them. Common one is not enough powder in the charge well.

Even with motor only ejection, one can thread a bare ematch down a motor casing with a charge well for a ground test. Wires coming out of the bottom and match head in the well. Measure the BP (You do have a scale that measures in grams and fractions thereof right?) Then start testing. Sometimes one finds out it takes a little more powder than what comes with a reload.

Also, watch it with the grease. I used a little bit too much on a J350 and it went over the delay grain when assembled and the grease plugged the hole. Now fire couldn't get through and the rocket zippered due to the fact it was a 4 inch diameter LOC tubed cardboard rocket when the charge didn't blow. On the death dive, nosecone down, I didn't use shear pins on the single deployment main and the weight of the recovery gear by gravity pushed the nosecone off. Was able to do a coupler repair with the resultant zipper and the rocket is still flyable.

If one flies large cardboard rockets, delay grain time is absolutely critical to avoid a zipper. Very tough ships to fly and avoid zippers. One reason I moved on to fiberglass and carbon fiber. Their resilience leads to more leeway here but one has to have a harness that can take the added stress.

Kurt Savegnago

 

[Zman1961]

Hi Wonderboy, I own the same (beautiful) kit but it is not yet built. I had been storing it in a cold (think winter in MN) part of the house. I went to check the parts and start fitting for building, and the coupler tube was stuck in the airframe as if it had been epoxied in place. I could not get it to budge even with a large dowel and my body weight. I let it warm up and the parts separated. But I did notice it seemed tighter at the ends of the tube compared to the middle. Perhaps the mandrel used to make the tubes was slightly smaller at the ends, and the temperature was enough to cause the issue. I always thought the fiberglass parts were less susceptible to temperature expansion/contraction, but maybe not. I have definitely had this issue with large polyethylene nose cones, they must always be fit tested under field conditions.

 

[mtnmanak]

Hello. I am wondering which type
of dual deploy rocket builds it is recommended to have sheer pins on the booster section and which ones not? All my rockets have them and I have had no problems yet… but neither do I want to

Not exactly. The maximum deceleration at burnout is the value to consider. However, even more important is the difference between the booster and sustainer in terms of their drag and mass relationship:

You can have a sloppy loose coupler in a rocket accelerating/decelerating at 100000000 G, but as long as M/(1+R) < M1, then the sections won't drag separate.

I fly L1000s all day long to +30 G with nothing but a tiny vent hole in the booster for pressure equalization at altitude, and I probably don't even need that.

Buckeye has the right of it here - it is the drag of the booster you need to worry about.

As noted, most of the rocket kits you buy have very streamlined fins and the booster drag is relatively minimal, so you probably don't need shear pins on the booster.

I only put shear pins on the booster for high drag designs and larger rockets. High drag designs include tube fins (wow, super "draggy" - tube fins will almost certainly separate the booster after burnout), fins with a large fin span (think Goblin size fins), fins with a high cross section (I am building a rocket right now that will have 1/2 thick honeycomb fins - lots of drag). For all rockets 6" diameter and above, I generally use 2-56 shear pins on the booster and 4-40 shear pins for the nosecone. Large diameter rockets have a lot of surface area to increase drag and the boosters tend to be really heavy. On my 8" AGM-33 Pike, the booster weighs almost 40 pounds. It wants to separate as fast as gravity can pull it.

I have to agree with Justin on using charge wells - I use centrifuge tubes on my smaller rockets frequently, but as the charges get bigger, I want consistency and the ability to pack them really tightly in the well. Charge wells give me that. I recently changed my 2.6" darkstar over to charge wells, and have been getting much better deployment consistency. For small diameter rockets, I like to use copper pipe end caps for charge wells. You can find really small ones in Home Depot. 1/4" and 1/2" copper end caps work great to hold a couple grams of BP and the walls are much thinner than PVC caps, so they take up less space on the bulkhead.

In cold weather, I usually increase the primary charges by about 20% and the backup charge by 50% over warm weather flying. Blow it out or blow it up, baby!

 

[mtnmanak]

Incidentally, I have found onboard cameras to be super helpful in after action reviews of your flights. I try to get two cameras on the payload bay - one pointed forward and one pointed aft. Besides giving you super cool footage to wow your friends and family and increase your odds of becoming a YouTube sensation, if you have good cameras (like the 4K RunCam 2's) you can slow down the video and watch what happens at each stage of flight. After a launch a couple weeks ago, I was watching the video on a 4" Darkstar (without booster shear pins) and it clearly showed that, after boost on an L1090, the booster almost separated. It shimmied down so about 90% of the coupler was showing, but held on by the skin of its teeth until the drogue deployment charge blew.

Love having onboard cameras - also gives you great views of your recovery deployment.

 

[6inchmonster]

Did someone say "piston" and "deployment failure" ...

 

[David Schwantz]

I also would keep the shear pins, but on a rocket of this size and weight I would only use 1 in each section. You can fly the same rocket with a small motor and have no drag separation. But you put in a bigger motor and now have to add nose weight to balance you can then get separation. Due to the weight added up front.
BP should have not been effected from the cold. I shoot BP, live in MN, and do not change my charge due to cold.
I use the vials for charges lots. I always tape them shut, using 3 pieces of tape. 2 over the top forming a X. 3rd around the middle. Match goes in middle of BP, barf on top to fill space. Never do I ram it down though. I also prep my vials by cutting a slot in the side just far enough for the wires to slide into.
As far as the tubes getting stuck, I always round the edges of the tubes a bit so that last lit bit does not get caught on the square edge inside the outer tube. Although this was not your problem It also saves on cuts
Hope some of what all said helps. Good luck with the repairs.

 

[manixFan]

Did someone say "piston" and "deployment failure" ...

Nope.


Tony

 

[Buckeye]

Did someone say "piston" and "deployment failure" ...

Pistons, when fit properly, enhance deployment and require much smaller BP charges. In one of my scenarios of post #5, I implemented a piston, reduced the charges from 1.2 g to 0.5 g, and had solid deployments since.

 

[wonderboy]

I went to check the parts and start fitting for building, and the coupler tube was stuck in the airframe as if it had been epoxied in place. I could not get it to budge even with a large dowel and my body weight. I let it warm up and the parts separated.

This information is extremely helpful, as is everyone else's contributions, but this really tells me what was going on. My coupler fit was very smooth, but did exhibit some friction in the warmth. It was a sort-of satisfying precise fit... but I'm sure that this precise fit turned into an interference fit in the cold. I still have the coupler section and most of the payload tube (only the top few inches of payload tube was damaged).

Just for giggles, I'm going to slide the two together and set in on the back porch for an hour and see what I find. Not to overanalyze this, but I'd really just like to know how pronounced the effect is on the particular fit of parts I had.

Thanks everyone else for the contributions too. I feel I'm sufficiently armed with enough knowledge to have a much higher likelyhood of success at my next cold weather launch (but I don't think it will be until next spring).

 

[OzHybrid]

2 things stand out. Not doing a test fit of your nosecone at the site. Not using a charge well.
The difference between a good nosecone and a tight one is 2/5 of not a lot.
Charge wells give you sufficient local compression of the hot gasses to ensure complete combustion of the BP for your ejection charge. The dimensions of the charge well are important and depend on the charge size. There are many discussions on this forum.
Once that had all occurred.....for whatever reason. Chicken/egg.
The other thing is making your recovery shockproof. You can do this with rubber-banded groups of loops of your connecting cords followed by PVC taped groups for extra high shock. You can even sew the group of loops together with a cotton or polyester thread for super high loads. These will release progressively as the load increases, but the high-strength ones should not be required to release for normal safe deployment. i.e- the extra length shouldn't cause any extra issues for you if they release and the shorter length shouldn't cause an issue if they do not release. Finally, a slide ring on your chute canopy lines prevents/reduces snap opening of the chute.
Good luck with the next launch.

 

[wonderboy]

Thanks for the advice. Regarding the charge wells, I do use them along with centrifuge tubes. What I do is make a small hole in the bottom of the centrifuge tube for the e-match leads, hot glue the hole from the inside, then pack with my powder, then wadding, then dog barf to fill the tube and snap shut. The charges are very tightly packed. This is then slid into a copper plumbing charge well (3/8" copper pipe with cap soldered on bottom). I used the copper in addition to the centrifuge tube just to control where and how the centrifuge tube was oriented (keeps it always the same) and also to somewhat protect the side of the rocket from the centrifuge tube shrapnel when it goes off.

One of our club members does exactly what you mention about the sewn accordion folding of the shock cord. The loops break free one at a time gradually slowing the rocket... it's a cool idea.

It's funny that this thread came back up as I just last week finished putting the clear on the replacement payload tube section. All I need to do is mount the batteries on the av-bay sled and Miss Riley will be back in business!

 

[OzHybrid]

Have a look at human safety fall arrest harnesses. They have a short section designed to take the shock out of a fall without killing the occupant of the harness. That section and how it's constructed should be a similar construction in the final section of your harness.

Good luck with your flight.