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)
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)