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OverTheTop

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Interesting... so would that mean you would recommend that if one was using the Eppendorf tube method as mccordmw was suggesting, that the e-match would be at the top of the BP if you used such a method? Or were you referring to a more cylindrical charge well?
I would put the eMatch at the top of the BP, closest to the exit. Possibly doesn't matter as much for small charges and low altitudes, but I don't have statistical data for that, just a hunch. I will keep putting my eMatch closest to the exit for future flights, even though I have previously been known to use it at the pointy end of the centrifuge vials with no failures. YMMV.
 

prfesser

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For small amounts of BP (few grams) I have soda straws in which the plastic shield around the match head is a snug fit. Calibrate a straw by filling it with 1 g/2 g/3 g BP, mark those lengths, keep that straw in the range box. For each flight cut a length of straw a bit longer than needed, stick the match head and plastic shield into one end, use the straw like a scoop to fill straw with BP to desired length, tape the end closed. A few wraps of tape around the straw-match junction doesn't hurt.

Best -- Terry
 

Bat-mite

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Another "belt and suspenders" approach would be two wire two e-matches in series, and put one at the top and one at the bottom. I might try that next time.
 

tfish

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The following 2 videos are borrowed from Mendal....Rocket Junkies
The first video is showing an 8 gram charge with the ematch at the bottom of the charge canister...
[video=youtube;4UWoBhwvuWs]https://www.youtube.com/watch?v=4UWoBhwvuWs[/video]

This next video is with the ematch on top of the charge.
[video=youtube;xW810k5AySQ]https://www.youtube.com/watch?v=xW810k5AySQ[/video]

Below is a photo that shows how I've recently been doing my high altitude charges holders. It's a take off from the Jim Jarvis method.
http://www.rocketryfiles.com/files/Technicalarticles/Jim_Jarvis_Highaltitude_deployment_2013.pdf

They are not sealed at top. That tape just holds the ematch wire in place...

switching from my normal latex tubing charge of 1.3g for a 3" rocket I now use 0.4 grams in a 3" rocket.

some things that affect charges.
Container shape and orientation, ematch placement and confinement or lack of.

Also note, switching to a baffle will reduce the amount of gasses produced by the charge.

As always,,, ground test. once you find what works for you. Don't change it. If you change anything..ground test again!

Tony
 

pondman

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deadalus52

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As far as measuring goes, 1cc of 4f black powder is close enough to 1g. That's based on 15.xx grain per gram and 1cc ~= 1cc
 

Bat-mite

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As far as measuring goes, 1cc of 4f black powder is close enough to 1g. That's based on 15.xx grain per gram and 1cc ~= 1cc
Yeah, I use measuring spoons and measure volume, not mass. Like with anything, be consistent in your method.
 

JimJarvis50

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The following 2 videos are borrowed from Mendal....Rocket Junkies
The first video is showing an 8 gram charge with the ematch at the bottom of the charge canister...


This next video is with the ematch on top of the charge.


Tony
Wow, that's a remarkable difference between the two placements. I suspect that the large quantity of powder affects this, but it is still more of a difference than I would have expected.

Jim
 

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Len B

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The following 2 videos are borrowed from Mendal....Rocket Junkies
The first video is showing an 8 gram charge with the ematch at the bottom of the charge canister...

This next video is with the ematch on top of the charge.

... As always,,, ground test. once you find what works for you. Don't change it. If you change anything..ground test again!
Tony
Impressive video. It almost looks like unburned BP is "launching" upward in the first video. Is that the difference? The bottom up blast puts unburned powder into the air and the top down blast confines it more?
Very interesting. It really does graphically show what you say in your last line.
Thanks for posting this.
 

blackjack2564

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I have been placing the match on top for several years.
I use small "gun barrels" also. They are almost 2inch long 1/2 wide & will hold 1.25-1.5 grams BP depending on whether I use 1 or 2 matches per holder.

Generally I have found .5 to .75 grams is all that's needed in 54-75-98 minimum diameter rockets.
Due to I only have 3-6inches of space to pressurize in fincan compartment.
They are VERY energetic.

Shown on 54 & 75mm size bays.

DSCN0469.jpg 20160809_172440.jpg
 

8thPaladin

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Have used the NASSA formula and rocketryfiles.com calculation 4" diameter, 35 inches long, 4 sheering pins give me a 2.72 grams of BP, in looking at how the latex tubing charge it shows that it used to use 1.3g and now 0.4 grams. Is it that much of a change? as that is almost a third of the previous charge! Am just getting ready to ground test with 2.72 grams which looks huge in vinyl tube. This is the booster stage, once I have it tested will work on main charge and see what I come up with...
 

8thPaladin

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Tested charge today, it worked very well. As I have been reading about placement of e-matches and use of more than one to insure that charge goes off, tried another one with same amount of powder but with 3 e-matches one at base, mid, and at top of charge, it was much louder than the first and where the first I found the remains with the side of the vinyl blown out, and the top where the e-match inserts was blown out. I have yet to find the second test charge remains, where I attach the charge to the heavy extension cord, the e-match wires were ripped from it. So tomorrow will lower the amount of powder and test again, and then set up the rocket for ground test.
 

jrkennedy2

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One thing to possibly consider. If you lower the charge to compensate for multiple ematches, you increase the likelihood of failure, since ALL ematches must work to get the expected pressure. So, twice the ematches = 2x the failure rate since both must work for expected pressure. Now, if you leave the charge the same you cut the failure rate in half since either ematch working will get sufficient pressure. If the BT is fiberglass or otherwise reinforced, maybe the higher pressure won't really hurt anything?

Then again, there are lies, damned lies, and statistics... :wink:
 

8thPaladin

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After reading and thinking about it, kept the initial powder charges unchanged, but did add a second e-match just as redundancy, not to improve burn. Tested charges this morning, ah the smell of blackpowder in the morning! Smells like rocket launch tomorrow!! So both charges went off and separation was perfect. So will make some more charges today and get everything ready for tomorrows launch!

Thanks all for the input, help and guidance! I need it!! [video]https://youtu.be/wfNcXwPqXyo[/video]
 

kc9qzf

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I have been reworking some old nomograms I made some years back. I will post them here for posterity. Due to the size of most peoples printers, I ended up making two different charts one for large diameter and one for small diameter rockets.
The idea behind this is the ability to refer on paper to a calculation method.
Since I have redone this, it is possible to provide copies with custom isopleths.
 

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dhbarr

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I have been reworking some old nomograms I made some years back. I will post them here for posterity. Due to the size of most peoples printers, I ended up making two different charts one for large diameter and one for small diameter rockets.
The idea behind this is the ability to refer on paper to a calculation method.
Since I have redone this, it is possible to provide copies with custom isopleths.
Well that's possibly the single most information-dense post I've run into on TRF.

Two new ( to me ) words and an associated refinement of a known concept PLUS the tool itself.

[emoji122][emoji122][emoji122][emoji122][emoji122]
 

Nytrunner

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That's a really neat resource!

How did you determine the breaking points for the number of shear pins? All the forces associated look higher than any I've seen for 2-56
 

vcp

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As stated, there are online calculators that will give you a starting point, but ground testing is always your best bet.

We found that online calculators gave us a much larger charge than necessary. Our rocket is quite large - 12.75" in diameter - which is part of [t]he reason.

In the end it boils down to a few variables:
Pressure produced by a given amount of BP in a known volume
Surface area that said pressure acts upon (that of NC bulkhead)
Amount of force needed to separate NC (calculated from shear pin size/number)

Charge calculators have always seemed odd to me. Like the rocketryfiles one mentioned below, it gives you the pressure - why doesn't it tell you the force? It has the diameter. The resulting force is closer to what you want.


Thanks! Couple more questions now. ... Do I just figure it as is, empty, or with all the shock cord, and parachute and Nomex?
Thanks again for the help!
Ignore the shock cord and chute, basically you are looking at theempty space. I would use top of CR to next solid bulkhead as length, ignoring any motor tube sticking through the CR.
I agree that you would normally ignore the decrease in volume occupied by the 'chute and rigging, but should you? How much difference would it make? If a calculator allowed you to subtract an estimate of this volume...

And the shoulder length of the NC? Shouldn't you subtract that from the length of the tube? Maybe if that were built into the calculator.

Shouldn't the calculator let you measure powder in oz., gm, grains, or volume (cc); for either FFFF or FFF? Wouldn't that be handy?


Steve’s explanation is correct. That said, for my money I really liked the explanation and calculator that existed on ROL Infocentral. So happens I grabbed the page along with the code. Username and password are both guest

http://www.rocketryfiles.com/files/FlightSimulation/BP_sizing/Black Powder Sizing.htm
I checked my calculator, VCT, against the rocketryfiles calculator. We're using the same equations. I just took it a bit further.


VCT.JPG


In the 'volume calculation' block, you have a fairly complete calculation of the volume, considering any stuffer tube and the nose cone shoulder, and subtraction of the recovery components if you want to see what that does.

Component weights are optional, but they allow a coarse calculation of the component separation velocities and accelerations. (Accelerations are quite a kick, good to know.)

In the calculation block, you can enter any one of either: powder weight, powder volume, pressure, separation force, number and type of shear pins, or separation velocity; and all of the others will be calculated.

I've always maintained that you're not really interested in pressure or force, but separation velocity. That's what you're really measuring in a ground test, isn't it? That separation velocity largely depends upon the nose cone shoulder length - what other calculator considers that? It also depends upon the relative masses of the components. For this reason, has anyone considered that a ground test does not actually simulate the free-body reactions of the model separation in flight? Are the differences negligible? (Usually yes, for practical purposes. But are you sure?)(If you leave the body weight blank, or enter zero, it assumes an 'infinite' wieght, i.e., the Earth - a ground test.)

It also has a fairly complete selection of available units - get the pressure in KiloPascals if you want.

Now the caveats. Under the 'Read Me' button of that calculator, there is a long of assumptions. Reasonable ones, but the list gets depressing. Meaning, of course, that anything like this is just a coarse estimate. But I think it's a better coarse estimate than you had before. Next, and this is the killer, I wrote this (good grief), over 15 years ago. It only runs on Win7 and earlier. It would be a good project to update to a modern language if I ever get around it. (And if I can find the source - might be a challenge.)

Anyway, here's a .zip file of it, if anyone has antique computers laying around. If I recall correctly, it also has some tube guides, shrouds, and other gadgets in it if they are functional.
 

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vcp

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Oh, while looking for the source, I actually found a setup file for VCT. Might work better. If anyone can get this to work on something later than Win7, I'd like to hear it.

Waitaminit - in my search for source, I just found it working on a Win10 computer - strange. Please let me know if you have any luck with it.
 

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markg

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Oh, while looking for the source, I actually found a setup file for VCT. Might work better. If anyone can get this to work on something later than Win7, I'd like to hear it.

Waitaminit - in my search for source, I just found it working on a Win10 computer - strange.
Yep, I just ran the setup on my Windows 10 Enterprise machine and it worked for me.

cheers - mark
 

Nytrunner

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Agreed, force would be a nice end product of the calculation (with driving pressure shown for info sake)

I added some nose cone shoulder calcs and gas expansion to Chuck Pierce's ejection charge spreadsheet so i could get a rough estimate of shock cord forces. Haven't looked at that in a few months though.

And yes, several folks notice and point out that ground testing isn't "true representation" of free body separation in flight. But in y opinion ground testing isn't about testing the recovery loads, it's about separating the body and verifying that you can break your shear pins while getting the parachute out. (It'd be nice to test against chute fouling, but there's only so much you can do)
 

kc9qzf

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That's a really neat resource!

How did you determine the breaking points for the number of shear pins? All the forces associated look higher than any I've seen for 2-56
I was concerned to make sure I had enough of a margin of error to be certain I was going to assure a break, I found a range of values for the screws that varies between 10Kpsi and 12Kpsi, so I used values that would be safe. I didn’t feel comfortable using 37lb (10Kpsi). So I used 45lb (12Kpsi). I then bumped the 12Kpsi value by 1 lbf to get 46lbf. Again I was not trying to set a safe load as much as a safe break.
If you want a chart with different values let me know and I can set those easily.
 

kc9qzf

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I did some testing on my 2-56 and 4-40 nylon screws. I am not sure of their heritage, however I suspect they are from Digikey.
I used a 6" G10 tube and coupler with a bulkhead bonded to the coupler with a bolt in the center. I drilled and tapped 2-56 and 4-40 holes then used a torque wrench applied to the bolt to "torque" the coupler against the body tube.
My expectation was to be able to increase the torque until the nylon screw failed.
I measured the radius of the coupler outer surface at 3.005" which would be 0.25ft of moment.
I tried several of the 2-56 screws and obtained torque readings of 10.5,12.5,10,13 and 14 ft-lbf.
this converted to forces of 42,50,40,52,56 lbf. The average was 48lbf.
I then tried doing 2 #2-56 screws and obtained torque readings of 24 and 25 ft-lbf, and this converted to forces of 96 and 100 lbf. (single screw forces of 43 and 50lb respectiv
Using the stress area for a 2-56 screw of 0.0037 in^2 and shear strength (assuming 6/6 Nylon) at 9600 psi this would assume the breakage to be in the 36 lbf range.

These readings were a bit higher than I expected but not out of the ball park.
I assumed at first the higher readings would also show up in the 4-40, however
I got readings for the 4-40 screws at 13,15,13,14,15.5 ft-lbf, these converted to forces of 52, 60, 52, 56 and 62 lbs with the average of 54 lbf.
If I use the Stress Area for a 4-40 screw as 0.00604 in^2 and 9600 psi, this gives us 57 lbf which is in the right region.
I then applied 2 #4-40 screws and broke them at 38.5 lbf that translates to 154 lbf or 77lb per screw.

This was a very crude test, the body tube was laying on its side and strapped down to my work bench, the coupler was sticking out past the bench and I was using a "click" type torque wrench. This experiment could be improved by using a shorter tube and coupler and mounting them vertically, mounted securely to the floor. Additionally a higher accuracy max reading torque wrench and a really large sample size would help a lot.
 

Curtis Enlow

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This is a great thread; lots of great info here. Thanks to everyone who has contributed.

I have a LOC IV that will fly in the stock configuration, as well as a stretched HP/dual-deploy version with an instrument bay coupler and forward bay extension.

I have a couple questions:

  • I was impressed with the top down/bottom up BP burn pattern video. Using centrifugal-style plastic vials with relatively small BP quantities do you think that top down/bottom up BP burn makes a difference? I would like to avoid blowing still-burning powder around for obvious reasons.
  • Could top down even be accomplished with a capped centrifugal-style plastic vial? How would you pack the BP and seal it?
  • I wonder if a small aluminum 'deflector plate' (below - pardon the crude drawing, but you get the concept) would be a good idea? I ask because, while the pressure will quickly equalize, it would seem to me that the initial blast directly against the BT could deform it enough to increase the friction fit of the BT/Coupler, as well as contribute to the wear & tear on the BT.
 

Banzai88

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For 2grams or less, invert the vial in a 1/2" CPVC cap charge holder. The cap becomes a blast plate and all your powder burns. Not had one single laundry singe since I started doing this instead of naked vials or glove tips.
 

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