Tony, thank you for taking the time to perform the test. I agree that if this was an issue many would have experienced it long before, but your evidence bears that out.
L3 Post Mortem - Flight of the Terminator
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dshmel
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I used (3) 4-40 nylon shear screws to hold the nosecone to the payload BT of my Terminator. The (3) 4-40 screws offer about 150 pounds of total retention force. My nosecone "assembly" weighed about 3.8 pounds. This means that the force experienced by the upper section of the rocket when the shock cord reaches full extension at drogue deployment cannot exceed about 40 gees (axial). Any axial shock in excess of 40 gees may cause the weight of the nosecone to shear the screws. I have read some postings where rocketeers plan for 100 gees. Moving up to (4) 4-40 screws for the nosecone results in 200 lbs of retention force, which equates to about 53 gees of force at apogee. At the advice of my TAPs, I used (3) 4-40 screws and that worked fine. Using (3 or 2) 2-56 screws may result in you pushing the main out a apogee. This is not a cert DQ, you will just be taking a longer walk (and running the risk of recovering outside of the waiver area - which would be a DQ).
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Thanks for your note Dwayne. My nose cone assembly was at about 7 lbs, since it contained he main, harness, and a small aux AV bay. My concern was the same, not wanting to dump the main 12k ft. Of course, that may have been preferable to what I ended up with. It's all about risks and appropriate trade offs.
ChrisAttebery
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I'll weigh in on the number/size of shear pins:
1. The upper limit is how much force you can safely generate before the nose cone or bulkhead fail.
2. The lower limit bounded by the weight of the NC and recovery gear times the nominal shock load from the drogue deployment.
3. The nominal number is somewhere in between. I usually shoot for a 50G load.
The only concern you should have about using excess pins is ensuring that you have a large enough charge to shear them reliably.
Two data points:
1. My 3" Wildman Saab has 4 x 1/8" nylon rivets holding the NC in place. It has a ballasted NC (2lbs) and I didn't want to worry about the drogue charge causing an early main deployment. I used 2g in the main charge. I'm only using a glove fingertip as a charge holder and it's flown to 10k.
2. My 4" Wildman Punisher has 3 x 1/8" nylon rivets holding the NC in place. It also has a ballasted NC (3lbs), but I was more comfortable with the loads put on the system by drogue deployment. I used 1.3g in the main charge. These charges are in the .75" ID x .875"D chargewells.
From my testing the rivets I'm using need ~80lbs to shear reliably. That's similar to your 4-40 screws.
I took the dimensions of your NC and calculated that your 2.0g charge was probably about 10% less than I would have used. The 3g charge should have been more than enough though.
1. The upper limit is how much force you can safely generate before the nose cone or bulkhead fail.
2. The lower limit bounded by the weight of the NC and recovery gear times the nominal shock load from the drogue deployment.
3. The nominal number is somewhere in between. I usually shoot for a 50G load.
The only concern you should have about using excess pins is ensuring that you have a large enough charge to shear them reliably.
Two data points:
1. My 3" Wildman Saab has 4 x 1/8" nylon rivets holding the NC in place. It has a ballasted NC (2lbs) and I didn't want to worry about the drogue charge causing an early main deployment. I used 2g in the main charge. I'm only using a glove fingertip as a charge holder and it's flown to 10k.
2. My 4" Wildman Punisher has 3 x 1/8" nylon rivets holding the NC in place. It also has a ballasted NC (3lbs), but I was more comfortable with the loads put on the system by drogue deployment. I used 1.3g in the main charge. These charges are in the .75" ID x .875"D chargewells.
From my testing the rivets I'm using need ~80lbs to shear reliably. That's similar to your 4-40 screws.
I took the dimensions of your NC and calculated that your 2.0g charge was probably about 10% less than I would have used. The 3g charge should have been more than enough though.
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dshmel
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KISS. Keep It Safe and Simple.
My TAP advisors suggested the 3 versus 4 nose cone shear screws to increase the likelihood of successful deployment. They also advised on more BP than any of the calculations came up with. The rationale is that a FG airframe will withstand a very high pressure, and the back-up charges should be programed to stagger. Using more BP is better than not enough.
My TAP advisors suggested the 3 versus 4 nose cone shear screws to increase the likelihood of successful deployment. They also advised on more BP than any of the calculations came up with. The rationale is that a FG airframe will withstand a very high pressure, and the back-up charges should be programed to stagger. Using more BP is better than not enough.
RocketFeller
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Like many others, I have been reading this with a lot of interest - this is exactly the reason that we all get those butterflies. It certainly appears that you did everything "right" and yet...
It does seem that the problem was more than just insufficient BP. I've only built and flown one large rocket. In a 12.625" x 36" compartment we used 4G of FFFF to shear six 4-40 plastic screws. There are two .5" tubes that allow for pressure equalization as the rocket ascends, as well. Two ground tests and two flights have been done without issue. We do use 5G in the backup, but during both flights the first charge ejected the twelve-pound nosecone.
It does seem that the problem was more than just insufficient BP. I've only built and flown one large rocket. In a 12.625" x 36" compartment we used 4G of FFFF to shear six 4-40 plastic screws. There are two .5" tubes that allow for pressure equalization as the rocket ascends, as well. Two ground tests and two flights have been done without issue. We do use 5G in the backup, but during both flights the first charge ejected the twelve-pound nosecone.
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They also advised on more BP than any of the calculations came up with. The rationale is that a FG airframe will withstand a very high pressure, and the back-up charges should be programed to stagger. Using more BP is better than not enough.
I would usually recommend a SLIGHT upside over the amount indicated by calculators for your primary charge.
Then a significant (~50%) upside for your delayed backup charge.
This gets your chute out with minimal overstress if things go according to plan....but really gets the chute out using the backup if the primary fails.
If the primary works, then the oversized backup harmlessly fires into an open tube.
I would usually recommend a SLIGHT upside over the amount indicated by calculators for your primary charge.
Then a significant (~50%) upside for your delayed backup charge.
This gets your chute out with minimal overstress if things go according to plan....but really gets the chute out using the backup if the primary fails.
If the primary works, then the oversized backup harmlessly fires into an open tube.
I agree - good insight Fred.
Lowpuller
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Worsaer,
Any comment or insight on my post #66, basically wondering if charge wells can be too big or if the dog barf can compress on to big of a well. I am sizing wells now for my L2 so watching close and learning everything I can.
Thank you,
Any comment or insight on my post #66, basically wondering if charge wells can be too big or if the dog barf can compress on to big of a well. I am sizing wells now for my L2 so watching close and learning everything I can.
Thank you,
I'd say that based on my recent experience, I'm least qualified to provide a definitive answer. I like charge wells for the benefit of aiding directionality of the charge. Holding BP grains in close proximity for as long as possible while they combust helps ensure full burning of the BP. A lengthy charge holder can be compared to a long muzzle loader. I hope to conduct a number of tests to better understand and quantify if the difference is material between a short or long holder of the same diameter.
As long as the powder is in contact with the e-mtatch the length of the charge holder can't really be too long. Compressing the BP is good. In a black powder firearm it is very important to compress the wad and bullet against the charge.
For example my black powder pistol has a barrel of .66" by 8.25" and the typical load is about 4-5 grams under a lead ball. (One of my BP rifles has a barrel length of about 45"!) A wad is used to contain the powder and the bullet is seated against it. BP burns fairly slowly as it is ignited by hot gases so a firearm needs a long barrel to fully combust the powder. As I mentioned in an earlier post the longer the barrel the higher the velocity (to a point) due the more complete combustion of the powder. For example the following chart shows the muzzle velocity of .45" round in barrels of varying lengths:
https://www.ballisticsbytheinch.com/45auto2.html
You can clearly see that velocity increases as does barrel length. So longer is better. I base the charge holder diameter on the amount of powder. For a gram or two .45" cartridge is good - the powder fully covers the e-match. For larger amounts bigger diameters can be used as long as the depth of the powder covers the match.
The good news for us as rocketeers is that to a degree our body tubes act as barrels at most typical rocket body sizes. Which is why we can get away with using something like a glove tip to contain the powder. However I can't recommend that technique or even really surgical tubing (even though I've used it myself in the past). For the highest chance of success our ejection canisters should mimic a gun cartridge and barrel.
BP substitutes like Pyrodex and Triple7 can work just fine when used in true ejection canisters to ensure complete ignition. But since they burn slower than BP they don't work nearly as well when used in something like a glove tip. I really like Triple7 since it produces far less residue than BP and is essentially non-corrosive.
Tony
rharshberger
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Ever notice that nearly all gun powders are ignited from the back of the charge, part of this is that the flame front travels THROUGH the powder TOWARDs the muzzle allowing a quicker ignition of all the powder as the powder develops small gaps between the granuals, if ignited from the forward end the powder is not ignited as rapidly or completely as the charge doesn't receive the benefit of the slight air spaces opened up between the grains, the longer tubes are still good to help get a more complete burning of the powder. Will igniting from the forward end still work, of course as has been proven here many times. My opinion is that the ematch needs to be as near the bottom of the charge as possible, which is why is I use the aluminum charge holders I and others make to hold the eppendorf/centrifuge tube packaged charges, by doing so the ematch is at the bottom of the charge, and the plastic protects the ematch from accidental shorting on the metal. Partially too rifle and pistol are ignited from the base of the charge because its easier to design and manufacture ignition systems like modern primers easier, but its not the only reason.
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I agree with most of what you're saying. But, I did a bunch of ejection charge tests in a vacuum chamber this past summer. I got my best results (pushed my 6 pound simulated payload bay further {as completey through and hitting the top of the vacuum chamber}) up into the body tube with the ematch on top of the BP. As compared to 12"... IIRC...with the ematch at the bottom of the BP.
I really don't want to muck up Bill's thread to much. As your containment method improves and more BP is consumed don't forget to reduce the amount of BP you're using.
Tony
blackjack2564
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+1
Especially the higher [& colder] it gets. Bp must travel through the flame front to escape, rather than blowing particles out unburnt.
Ever light a book match in below freezing? Notice how long/slow the head takes to light,same thing happens with electric match. Cold takes longer for heat transfer between BP granules.
cherokeej
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You're talking pressure differential on ascent? That's why the shear pins are there.
Assuming a sealed bay... (It won't be, but let's run with it.) Surface pressure in the bay, the bird ascends, outside pressure dropping. 27Kg against the n/c base. The shear pins hold. At apogee, we fire charges to pressurize the bay to 15 PSIG. With ~12 in^2 surface area on the base of a 4" n/c, that's ~180 lbs, sure to shear the pins that require ~156 lbs.
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cherokeej
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Everyone keeps talking about charge calculators and comparing ejection charges to ammo loads.
Your ejection charge isn't in a gun barrel. So it won't burn like it is.
This may seem radical for this thread, but has anyone ever considered ground testing? Run the calc, make the charge, put the wires thru the vent hole, prep the bird, install the pins, and fire the charge. If it's anemic, up the charge a percentage, and try again.
While you will need to pack the charges properly, you won't need to worry about containment of BP until you're well above 20K MSL.
Blow it out or blow it up, but don't leave the laundry in the tube.
Your ejection charge isn't in a gun barrel. So it won't burn like it is.
This may seem radical for this thread, but has anyone ever considered ground testing? Run the calc, make the charge, put the wires thru the vent hole, prep the bird, install the pins, and fire the charge. If it's anemic, up the charge a percentage, and try again.
While you will need to pack the charges properly, you won't need to worry about containment of BP until you're well above 20K MSL.
Blow it out or blow it up, but don't leave the laundry in the tube.
Yes, and yes. I just wanted to run the numbers.
I usually go for the extremes. Of course the bay won't be completely sealed and the actual pressure will be lower. I thought that was self-explanatory.
As I said, just trying to put some numbers on it, and check it for sensibility.
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I am 100% with Tony F and CJ...
My cannons / canisters are rather long..
I always put the match head on the top of the powder...
As the ball of hot gas expands the as of yet unburned powder
has nowhere to get away from the match head and hot gas...
Teddy
My cannons / canisters are rather long..
I always put the match head on the top of the powder...
As the ball of hot gas expands the as of yet unburned powder
has nowhere to get away from the match head and hot gas...
Teddy
rocketsam2016
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Sorry if I missed it from earlier in the thread, but is it possible that 2g was borderline and 3g would have been easily enough, but the 2g first charge disrupted the containment (tape?) of the backup charge? That would explain why the g shock from the backup charge was smaller and it seems pretty conceivable.
I respectfully disagree. Containment can make a huge difference especially in larger diameter body tubes. The primary benefit will be a faster burning charge with a sharper increase in pressure. Basic thermodynamics related to propellant. A fairly narrow diameter charge canister is the same as a barrel.
And the OP links to video of his ground test so I'm not sure why you think your comment is radical. Lots of folks have talked about ground testing. It seems as though you have not read through the thread.
Tony
dshmel
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I think I will start doing that. It makes sense. I use PVC pipe end caps as charge holders. The depth to diameter ratio is about 1:1. So far I have not had an issue with unburned FFFFg. Either I have been very lucky on dozens of flights using the PVC caps, or the shallow holders are not critical to the successful combustion of BP. I admit, I have not used more than 3 to 4 grams in any set-up. So if you are using massive amounts, maybe it makes a difference. I also pack tightly and use dog barf.
If you lengthen the charge holder it shouldn't matter whether the match is on top or on bottom (as long as it's in contact). It's all about containment till total burn. I use 1/2" x 4" long copper tube with an end cap soldered to the end. Works great for charges up to 6g and the 4" travel allows the BP to completely burn.
Me too. Followed over the top with crepe paper (flame retardant type!) and then high-temperature Kapton tape to hold it together.
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I use the PVC caps.
Match goes in first so they can be tested for continuity BEFORE any BP is installed.
Then the measured BP amount.
Followed by some tightly packed pieces of paper towel.
Last taped over multiple times using green or blue painters tape.
Our whole team does it this way.
Literally over a thousand ematches consumed...
Never had a problem with unburnt charges.
Match goes in first so they can be tested for continuity BEFORE any BP is installed.
Then the measured BP amount.
Followed by some tightly packed pieces of paper towel.
Last taped over multiple times using green or blue painters tape.
Our whole team does it this way.
Literally over a thousand ematches consumed...
Never had a problem with unburnt charges.
AHansom
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The ground test video looks good. Did you ever test lower sized charges to see how close you were to a failure?
At one time i tried using 20 gauge shotgun shells for canisters. I found that it was real important where the ematch was placed. When the ematch was located from the middle down towards the bottom of the canister from it would blow unburnt powder out during expansion. When placed closer to the opening it seemed the powder burned more efficiently, probably because the powder had to pass through the flame to escape the container.
At one time i tried using 20 gauge shotgun shells for canisters. I found that it was real important where the ematch was placed. When the ematch was located from the middle down towards the bottom of the canister from it would blow unburnt powder out during expansion. When placed closer to the opening it seemed the powder burned more efficiently, probably because the powder had to pass through the flame to escape the container.
Ahhhhhh, You know, one can cut a small circular cardboard endcap/disk to lay on the powder in the charge well and put the dog barf on top of that. Like the endcap in an old shotgun shell. Kurt
Some good stuff being discussed here. I always just stick the e-match buried as far into the BP as possible. Never once did I think about the dynamics of it. And had I used dog barf I would not of thought about that issue being discussed either. But I use the Estes wadding to pack my charges and I pack them pretty tight so nothing moves around once I tape it up. None the less the conversation has inspired me to change how I pack my charges, or at least given me something to think about.
Thanks to those participating in the discussion.
Thanks to those participating in the discussion.
Like mpitfield I generally use Estes wadding which I fold over to form a flat surface.
This has been a good thread. Lots of lively discussion and it really stresses the many variables that can go into getting a good chute deployment from the ejection charge to say nothing of the many ways to rig the overall recovery system.
Tony
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cbrarick
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Because we have a closed system when the charge goes off, directionality doesn't help. Ideal gas laws state that you could point the canister at the wall of the tube and still generate the same amount of force at the nose cone end. We don't do that because we don't want the flash or the crap going at the wall of our rocket, though. Might just hurt it....
