Impossible time ground testing!!

Jeez Louise- I must be doing something wrong, and I'm hoping someone here can pick up on it and clue me in. I've read about all I can find on DD, and am hitting walls.
I'm ground testing my first dual deploy bird (3" fiberglass Adventurer 3 from Rocketry Warehouse) before flying this coming weekend, and have 8 tests on the main behind me with only two successes (big disclaimer coming). Obviously this means I'm not going to fly it like this, but I need to figure out where I'm going wrong. All tests are using 2mL centrifuge vials and 10ohm SMD initiators, with 1g to 1.7g of Hodgon 777 (3F) loaded, and then wrapped with from 2-5 turns of electrical tape, or 3-5 layers of duct tape. 7 out of 8 tests have fired, so I don't think it's the initiator- there's no unburnt powder scattered around inside either, so i don't think it's containment.
The bay I'm pressurizing is 3" diameter, 17" long. The rocketry calc I've used (only one, so maybe that's my problem) says to use 0.93g to get 15 PSI, which supplies 106 lbs of force, so 2 4-40 nylon sheer pins are called for.
Yesterday I went up steadily in powder quantity: 1g, 1.25g, 1.5g, 1.6g with the two sheer pins installed, and had no successful deployments (again, all discharged- am using a 3S lipo, same that will be doing deployment charge in fight). Each time we could see the flash, hear a "TINK" or louder (not much- like a pop in a can) but nothing happened. My 9 year old suggested we try it without the sheer pins, so I stepped back to 1.5g, prepped, and bingo, clean deployment. Didn't shoot the burrito out (60" chute in a 9x9 nomex), but nose popped about 10 feet, and burrito was right next to the end, so I think (hope) drag would've pulled it out. I tried once more with 1.75g and sheer pins, but got failed continuity, so we called it a night (this was the only one that failed to ignite). Of course for the rest of the night I got to enjoy the story as he told how he was "smarter than dad" to his yonger brother and sister (who both denied it, thankfully!) and to his mother (who agreed...)

Today I decided to try my luck with duct tape, on the theory that the vinyl electrical tape might not be constraining the blast enough. I started at 1.6g today, with two pins, and nothing - got the flash, heard the TINK, got lotsa smoke when I opened it up, but no deployment. Did a 1.6g with no sheer pins, and perfect deployment (burrito cleared it this time). Reinstalled sheer pins, 1.75g, nada. The calculator says that at 1.75g I'm putting out 28psi with nearly 200lbf on the base of the nose...I can't fathom how, if that is true, two screws that are supposed to have sheer forces of 35-40 lbs are holding me back.

I'm thinking it's got to be that I'm using an insufficient quantity of powder (and that the calculator is wrong- this is Rocket Calculator v 1.5.10 for iOS if anyone has advice), but unfortunately I'm maxing out the tubes I have. There clearly won't be enough expansion to pressurize my drogue bay, so I'm not even going to try until I get this figured out (3x22.5")

Black powder isn't really an option- it's damned impossible to find around here, and I can't order any in time to test for this Saturday's big launch.

Open questions:
1) If you think it's powder quantity- what other sorts of containment work well with 777? I have installed 1/2" PVC endcaps with a 1-2" piece of pipe glued in. Have been sticking the centrifuge tube in, but can try to pack it. I've seen other folks use glove fingers, sandwich bags, latex surgical tubing, but all (I think) with real 4F BP.
2) If you think the quantity is good (if you agree with the Rocketry Calc on my iPhone)- what else could it be? Are there different strengths of nylon screws? I'm using ones from Ace Hardware Midwest fastner bins, btw. I had thought of trying 2-56, but none are available locally, and as I said, no time to order.
3) I haven't tried, but am thinking of giving it a shot without the recovery gear (leave the cord, remove the chute) to see if it's somehow dampening the reaction. This is really ultimately not very useful, but it's all I can think of- I feel crazy stupid for not being able to break two little plastic screws!

If I can't resolve it, don't worry- I won't attempt to fly dual deploy- but that raises another question- how many successful tests are considered enough? Is 5 in a row enough? 10? What if one in 10 doesn't work, but you think you know why...still fly?

PS- while proofing the article I checked the calculator at Info Central- same amount of powder recommended (0.93g) but only 1 sheer pin (4-40)...that makes me nervous about binding, but lends credence to the hypothesis that the calculator I'm using is incorrect. However, even if that is the case, my 1.6 and greater charges should be okay with up to 3 4-40 sheer pins. Yes, these calculators are all based on 4F BP, but what I've read shows that 777 is generally suitable...but no one mentions exchange rates!

Thanks in advance!

Dave (soopirV)

Sounds like a couple issues. On the successful ignitions, and not meant to sound obvious, but do you have your motor hardware mounted or the MMT plugged? The failed ignitions could be a number of things, bad e matches, too much resistance, etc. Even if you had too much electrical tape and it does sound like a lot, the e match should of still popped. How are you testing, though the altimeter software or just a direct batter to the e match? What gauge wire?

You should be able to get 2-56 nylon shear pins from McMaster-Carr in 2 days. 4-40 are overkill for a 3" rocket. I've never used BP substitutes so can't help you with that.

perhaps I'm remembering it wrong...but I thought the 30-40 pound figure was for 2-56 nylon screws, 4-40 screws would be around 40 - 60 pounds each.
Rex

Thanks guys, for the early and prompt responses. The testing is in the main (payload) bay, which is pretty air tight. I can seat the payload airframe onto the coupler and the nose will pop up a couple of inches.
I haven't had any failures to ignite (well, one, but that was me being stupid. I got hot melt glue on the initiator, so I changed my technique). I'm testing it with the battery I intend to fly, but connected to a push button via two conductors of trailer hook up wire (pretty heavy- maybe 16-18awg?). I have the wires that connect the igniter to the flight computer (thinner, 20ga) coming out two of the three static ports, which lead internally to euro-style terminal blocks. Other side of that is my 22-24ga igniter wire (solid core cat5 cable). Pretty robust ignitions of EMatch sans charge, and when I dry test the charge (out of the rocket) it sounds louder than a firecracker, maybe like a .22?
Good point about the sheer pins, I will order some 2's tonight, but if the calcs are recommending them, I'm still perplexed why I can't get them to go. These are filament wound FG components, so are strong and sharp...
I attached a pic of my charges- one loaded but not wrapped, the other one wrapped to the nines.

Rex R said:

perhaps I'm remembering it wrong...but I thought the 30-40 pound figure was for 2-56 nylon screws, 4-40 screws would be around 40 - 60 pounds each.
Rex

Click to expand...

Hi Rex- I found this resource that said 4-40 sheer is 50-76 lbs, rock solid memory and thanks for the correction!!

https://www.feretich.com/rocketry/Resources/shearPins.html

Still begs the question why I can't sheer them at almost 200lbf tho...maybe they're not nylon? If only I had some Formic acid, could dissolve it to confirm...
Or maybe I'll try one with 1.75g to see what happens. Hate to think of flying like that tho.

Do you tap out the holes for the screws? Maybe I should mention that I did that...too perfect and secure of a fit?

After you add the special sauce to the centrifuge tube, are you topping it off with dog barf so there's no empty space in the tube? And when I say "topping it off" I mean cramming dog barf in there until you can barely snap the lid on the centrifuge tube.

o1d_dude said:

After you add the special sauce to the centrifuge tube, are you topping it off with dog barf so there's no empty space in the tube? And when I say "topping it off" I mean cramming dog barf in there until you can barely snap the lid on the centrifuge tube.

Click to expand...

I am packing barf in, but not to the extent you mention- i had no trouble with the cap, but don't see any airspace. I bet the barf fluffed to fill the gap.

So pack it in, jam the cap on, and then wrap it up?

It sounds like your issue may just simply be over-sized sheer pins as Scott and Rex pointed out.

However I would love to hear more opinions on wrapping the charge as much as you, which is much more than I do. Not to hijack your thread but it would be good to know what the effect of wrapping it "to the nines" has...hopefully someone more knowledgeable will pop in on this one.

For me I simply fill my blast cap with BP, tap it down, make sure I have a good seal if using the type you are, pack with Estes wadding then I just use some painters tape, tape up the end, then do a single wrap.

mpitfield said:

It sounds like your issue may just simply be over-sized sheer pins as Scott and Rex pointed out.

However I would love to hear more opinions on wrapping the charge as much as you, which is much more than I do. Not to hijack your thread but it would be good to know what the effect of wrapping it "to the nines" has...hopefully someone more knowledgeable will pop in on this one.

For me I simply fill my blast cap with BP, tap it down, make sure I have a good seal if using the type you are, pack with Estes wadding then I just use some painters tape, tape up the end, then do a single wrap.

Click to expand...

Interesting! What quantity of special sauce are you using typically in your situation? Is it old fashioned authentic sauce or is it new-fangled? May be obfuscating too much, hope I don't lose anyone!

soopirV said:

Interesting! What quantity of special sauce are you using typically in your situation? Is it old fashioned authentic sauce or is it new-fangled? May be obfuscating too much, hope I don't lose anyone!

Click to expand...

I think you got my post mixed up with old_due https://www.rocketryforum.com/showt...e-time-ground-testing!!&p=1437707#post1437707 In my case I do what old_dude describes, pack the wadding or dog barf in until you can't pack any more in. I was taught that it had to be tightly packed.

If anyone has an opinion on tapping, I'm all ears as well. Having some clearance seems like it may create some momentum that might help the sheer...intimate tight taps are better at distributing load it seems, but I'm a biologist not an engineer.
Would like to order taps and drills with the screws to save on shipping.

I've had a similar problem, in my cert rocket the calculators say I need just 0.05 grams of smokeless powder... Testing shows I need twice that to separate and my failed flight showed that the time isn't enoughin real life. Further testing has me using for times the calculated amount.

I think the screws are too big for that size rocket- 2 or 3 2x56 likely better

The only powder I tried that worked consistently was Pyrodex P, which is a 3f equivalent. With that I think you would be ok with your containment. The caveat is this- are you getting blow by around your electronics bay/coupler? If you are it will likely not work consistently.

Okay- so 2-56 screws, tap and appropriate drill bits have been ordered. Since the holes I've currently drilled and tapped for the too-beefy 4-40 screws are into enclosed areas (nose cone AvBay and coupler AvBay- nose has the GPS locator and the coupler has the flight computer), there should be no need for me to cover them up when I drill and tap (perhaps, depending on the received feedback prior to the arrival of the bits), right (unless I want to for aesthetics)? Related, and as yet unconsidered by me- on a sheer-pinned bird do you still include pressure relief holes? Second, I installed sheer pins in both bays- to hold the nose on for main ejection, and in the sustainer into the aft end of the coupler for drogue ejection. The payload bay is attached to the fore end of the coupler with 3x6-32 machine screws, I've had bad luck with rivets. Is it overkill to pin everything together? It's not an MD bird, so drag sep shouldn't be TOO bad, but I have the ability to send this up on VMax or similarly aggressive motors.
Once I get my 2-56 screws I'll return to ground testing both bays. I'm less than a week before launch, and am thinking I need to do some more thinking/designing and testing...hoping for light winds so I can use motor eject and at least get this beauty into the sky. It has been an iterative learning process for sure!!

I don't have personal experience using BP substitutes, so take this for what it's worth. You said that the parachute compartment was sealed tight, but you also say, "Each time we could see the flash, hear a "TINK" or louder (not much- like a pop in a can) but nothing happened." Well, that "TINK" reminds me of the sound I've heard during the many, many ground tests I've done with student launch teams when bulkheads have wire-passage holes that haven't been sealed, or bolt holes in the airframe, or similar leaks - even relatively small ones. It always surprises me, since it's a sound that makes me think that the charge didn't fire correctly - but an identical charge in a properly sealed compartment goes "boom" instead of "tink." You've probably already inspected the compartment and bulkhead, but I encourage you to do it again.

Too bad you can't get your hands on some 4F BP - that would eliminate one big variable.

Good luck, and let us know what you discover!

Mark

An alternative to screws is styrene rod it shears easier and is available at hobby stores.

markjos said:

I don't have personal experience using BP substitutes, so take this for what it's worth. You said that the parachute compartment was sealed tight, but you also say, "Each time we could see the flash, hear a "TINK" or louder (not much- like a pop in a can) but nothing happened." Well, that "TINK" reminds me of the sound I've heard during the many, many ground tests I've done with student launch teams when bulkheads have wire-passage holes that haven't been sealed, or bolt holes in the airframe, or similar leaks - even relatively small ones. It always surprises me, since it's a sound that makes me think that the charge didn't fire correctly - but an identical charge in a properly sealed compartment goes "boom" instead of "tink." You've probably already inspected the compartment and bulkhead, but I encourage you to do it again.

Too bad you can't get your hands on some 4F BP - that would eliminate one big variable.

Good luck, and let us know what you discover!

Mark

Click to expand...

This is very helpful! I'm sure my AvBays are not air tight- I tried to (and this is embarrassing, so I better post it to the forum!) simulate a flight by taping off two of the three static ports, and sucking on the third. I about inverted my lungs, and only managed to sim a flight to about 250 feet. The "ready" beep tone stopped, but I didn't pass LDA, so nothing ever happened with my xmas light loads. I understand that the gasses from ejection are corrosive, but just how air tight do the bays need to be? I have four holes through my bulkhead- one for the 1/4x20 rod, two for the 3/16" u bolt to which the recovery is attached, and a 3/16 hole for the charge wire. This one is plugged on the outside with some poster tack...the other three are native- not sure what/how to seal those. ONe end needs to be removable, but I'm perfectly happy to seal up the other in whatever way is most effective.

terryg said:

An alternative to screws is styrene rod it shears easier and is available at hobby stores.

Click to expand...

This is a good backup, Terry- I will be looking to ground test in the future, for sure!

I got to wondering about why use sheerpins so I ran a few simulations. a MC 4" phoenix (I161) at motor burnout has a decell rate of -100 ft/sec/sec(3.125G), a competitor 3(J350) about -80 f/sec/sec(-2.5G). so if one figures a max rate of say -5G for subsonic burnouts and plans accordingly, 2-56 nylon should cover most rockets up to 4" dia. if you are still popping the main at apogee then either your drogue charge is too big or your shock cord isn't long enough.
another thought
if your screws are compressing the airframe to the av-bay you are getting added friction, as mentioned earlier things need to move before the pins can sheer.
Rex

soopirV said:

This is very helpful! I'm sure my AvBays are not air tight- I tried to (and this is embarrassing, so I better post it to the forum!) simulate a flight by taping off two of the three static ports, and sucking on the third. I about inverted my lungs, and only managed to sim a flight to about 250 feet. The "ready" beep tone stopped, but I didn't pass LDA, so nothing ever happened with my xmas light loads. I understand that the gasses from ejection are corrosive, but just how air tight do the bays need to be? I have four holes through my bulkhead- one for the 1/4x20 rod, two for the 3/16" u bolt to which the recovery is attached, and a 3/16 hole for the charge wire. This one is plugged on the outside with some poster tack...the other three are native- not sure what/how to seal those. ONe end needs to be removable, but I'm perfectly happy to seal up the other in whatever way is most effective.

Click to expand...

Rather than putting your face down near the deployment charges you're hoping to ignite, just drag out the shop-vac and use the long extension wand to create the low pressure inside the av-bay. Somewhere here on TRF Chris Attebery posted a video of this technique. I tried it and it worked perfectly.

a decent shop vac can simulate an altitude around 3000' (complete with mach delay ).
Rex

Multiple Problems:
1. It's BP substitute so you need more
2. It's 3f not 4f so you need more
3. I wouldn't use 0.7-1 on a 3 inch rocket with sheer pins. I'd probably start at 2. Don't forget that the CTI 38s come with 1.3....
4. You gotta pack substitutes as tight as you possibly can.
5. Never suck on live charges! Get yourself 2 different color LEDs (while there's still a radio shack near you), then hook them up to the mains and apogee, then dump it in a large jar, with a hole in the lid and some tubing glued in place so you can see what happens......Many altimeters give you a "test" program that you just need the LEDs to light up.

Rex R said:

if your screws are compressing the airframe to the av-bay you are getting added friction, as mentioned earlier things need to move before the pins can sheer.
Rex

Click to expand...

This is a question I have as well- you said that someone mentioned it earlier, wondering if that was me, but it's a lot easier to keep something from moving than it is to stop something once it's moving, so by tapping my sheer pin holes, am I inhibiting myself? It would stand to reason that if there was some wiggle there, momentum would build, and then be more likely to snap the pin...I'd hate to drill out my holes to test only to find out that it has no effect. Picture a passenger in a car crash with a seatbelt versus without...my sheer pins are so smooth and solid that it's like having your seatbelt on.

o1d_dude said:

Rather than putting your face down near the deployment charges you're hoping to ignite, just drag out the shop-vac and use the long extension wand to create the low pressure inside the av-bay. Somewhere here on TRF Chris Attebery posted a video of this technique. I tried it and it worked perfectly.

Click to expand...

Want to clarify- I didn't do this with initiators or charges installed, just christmas light connected to the deployment channels...It was a quick and dirty attempt to see if I can do some in situ testing. I may be slow but I'm not that stupid!

cbrarick said:

Multiple Problems:
1. It's BP substitute so you need more
2. It's 3f not 4f so you need more
3. I wouldn't use 0.7-1 on a 3 inch rocket with sheer pins. I'd probably start at 2. Don't forget that the CTI 38s come with 1.3....
4. You gotta pack substitutes as tight as you possibly can.
5. Never suck on live charges! Get yourself 2 different color LEDs (while there's still a radio shack near you), then hook them up to the mains and apogee, then dump it in a large jar, with a hole in the lid and some tubing glued in place so you can see what happens......Many altimeters give you a "test" program that you just need the LEDs to light up.

Click to expand...

Thanks- just want to clarify #5- I did NOT try to do that! I used christmas lights to indicate voltage, and did the testing in a mason jar with a syringe. The mouth test was just a quick "huh, wonder if my avbay is sealed".'

I appreciate the other advice, and will be happy to do more testing. How do you contain your charges? It seems the requirement to maintain tightly packed substitute is harder to do if one uses blast-caps (PVC, other)...is it truly sufficient to pack the charge with wadding and cover with tape as others do? Or will that work only with true black powder?

Glad you didn't. Every time I see someone suggest it, i cringe...

I have used BP substitute with the Yale Undergrad Rocketry Assn. We put it and a initiator in a baggie, then use electrical tape - one wrap to keep it all in, then we wrap it as tightly as we can, multiple wraps, trying to tighten it every time. 6-10 layers. Then we tape them to the bulkhead, because I found out the hard way if you let them they will blow holes in the sides of cardboard rockets.

How are you taping the canister? Using vinyl electric tape, start near the bottom of the vial come straight up the side under tesion, over the top and down the other side. Take a second piece of tape and repeat at 90 degrees to the first (forming an x-pattern over the top. Take a third piece of tape and wrap around the sides so the underlying tape will not slip up. This will hold the top on long enough to build up several hundred psi inside the tube before it vents, insuring the pyrodex full ignites.

The wadding in the tube should be tight. The powder should not be free to move. Initial confinement is the goal. The gas will flow through the wadding, so there is no momentum argument here, you want hot gas only leaving the canister, not unburnt solid.

You are using not using BP so why are you relying on a BP calculator? Pyrodex burns 10 times slower than BP and other smokeless powders are even slower. You have multiple modes of possible failures: Not enough powder, not enough initial powder confinement, and a leaky parachute compartment are all contributing to your problem. I would start with 1.5 grams, use (3) 2-56 screws, and attach a 6" drogue directly to the NC to make sure the NC pulls out the main chute from the airframe. (I'm assuming the main is packed so it is not difficult to pull from the airframe. If it doesn't move easily, change how you pack it or get a different chute.)

Bob

1. You can't use any of the online calculators because they are all based on 4F powder. All of the other powders need to be fully contained to burn properly. Cut a section of 1/2" launch lug. Glue a short section of dowel in one end. Drill a hole for the igniter. Glue the igniter into the dowel. Add your powder. Pack with dog barf. Cap other end with dowel. Epoxy it in place. Then you're going to have to test a bunch of times until you get the size right. Sounds like a PITA to me. So scrap that idea and continue to #2.

2. You CAN get 4F if you know where to look. It isn't cheap but it is available to everyone:
https://www.valuerockets.com/product_details.aspx?pid=14&itemid=67

Once you get either #1 or #2 worked out you should be good to go. Then you can worry about testing out your altimeter. Here's how I used a shop vac to simulate a flight. Tape off all of the altimeter bay vents except the one you are using with the vacuum.

[YOUTUBE]Iwrrmp-UYGw[/YOUTUBE]