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Small BP Ejection Charges

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MCriscione

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I have a Madcow 1.6" Cardboard Frenzy that I modified to accommodate an Eggtimer Quark and DD. I've got my charges calculated, but haven't started testing yet. Tonight hopefully that will begin.

Based on my calculations, the airframe length/volume is approx. 14" / 67 in[SUP]3[/SUP] for the drogue and 6.5" / 31 in[SUP]3[/SUP] for the main. (these are not reduced for chute, recovery harness, nomex, etc. that will also be packed in there, as I understand that is typically ignored in charge size calculations. I'm pretty confident based on other posts, online calculators, and basic PV=nRT calculations, that I'm looking at about a 0.2g drogue charge and a 0.1g main charge. I think my scale can handle those amounts, but I'm less certain I can measure and contain them. That's a pretty small volume.

Thus my question(s): 1. Is containing a charge as small as 0.1g in a latex 'fingertip' workable? 2. Would a small diameter cardboard tube with hot glue caps work better? 3. With a charge this small, is there a 'wrong' way to pack it in around the e-match? 4. Does the e-match contribute any significant additional amount of charge volume/weight that I should consider when dealing with such small amounts?

Maybe most importantly: 5. Does 0.2g/0.1g seem reasonable? I seem to recall that Estes B/C motors had larger charges than that, but then they also tend to be oversized.
 

rharshberger

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I have a Madcow 1.6" Cardboard Frenzy that I modified to accommodate an Eggtimer Quark and DD. I've got my charges calculated, but haven't started testing yet. Tonight hopefully that will begin.

Based on my calculations, the airframe length/volume is approx. 14" / 67 in[SUP]3[/SUP] for the drogue and 6.5" / 31 in[SUP]3[/SUP] for the main. (these are not reduced for chute, recovery harness, nomex, etc. that will also be packed in there, as I understand that is typically ignored in charge size calculations. I'm pretty confident based on other posts, online calculators, and basic PV=nRT calculations, that I'm looking at about a 0.2g drogue charge and a 0.1g main charge. I think my scale can handle those amounts, but I'm less certain I can measure and contain them. That's a pretty small volume.

Thus my question(s): 1. Is containing a charge as small as 0.1g in a latex 'fingertip' workable? 2. Would a small diameter cardboard tube with hot glue caps work better? 3. With a charge this small, is there a 'wrong' way to pack it in around the e-match? 4. Does the e-match contribute any significant additional amount of charge volume/weight that I should consider when dealing with such small amounts?

Maybe most importantly: 5. Does 0.2g/0.1g seem reasonable? I seem to recall that Estes B/C motors had larger charges than that, but then they also tend to be oversized.
Odds are you will need at least a .5 to 1 gram charge. I will let other chime in on your method of packing.
 

Titan II

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You are going to find the amounts way too small. CTI 29mm motors have 1.2g. The amounts noted by Rich are more in the ballpark.
 
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Viking

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The replies above are interesting, considering AT Hobbyline motors only have 0.5g and have no trouble pushing cones out of 3" and 4" Estes PSII kits....
 

markg

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I've got a 1.6" DD fiberglass Frenzy. I found 0.5g works fine for the drogue, and you could probably get by with less. For the nose with two shear pins, 0.5g only worked 50% of the time. 0.7g worked all the time. YMMV.

cheers - mark
 

rharshberger

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From a post on YORF a quote was made tha 13mm motors use .4grams, 18mm's .6grams, and 24mm's 1 gram. The individual was qouting Ed Brown (a long time Estes employee iirc).
 

vance2loud

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http://www.rocketryforum.com/showthread.php?126022-motor-eject-powder-levels
I usually refer to this thread as a reference on charge sizes included with reloads.
That being said, ground test multiple times to find what suits you.
I use small centrifuge vials to contain my charges and they can be bought in quite small sizes
http://www.ebay.com/itm/1000-Pcs-Co...457807?hash=item486776658f:g:HnUAAOSwyQtVp~Tf 1000x 0.2ml
http://www.ebay.com/itm/0-2ml-Conic...437591?hash=item1e802cc917:g:ZOUAAMXQLw1R5TAu 100 coloured 0.2ml for if you want to colour code your charges.
This seller also sells 0.5ml, 1.5ml (the size I use most) and larger sizes.
fill with dog barf on top of charge,single strip of tape as the lid (remove original) and then tape to allthread on AV bay (use slightly longer threaded rod on bay so it sticks out 1/2") gives you a directional charge.
I would also like to know if the ematch contributes in any substantial way when talking charges this small.
 

CzTeacherMan

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http://www.rocketryforum.com/showthread.php?126022-motor-eject-powder-levels
I usually refer to this thread as a reference on charge sizes included with reloads.
That being said, ground test multiple times to find what suits you.
I use small centrifuge vials to contain my charges and they can be bought in quite small sizes
http://www.ebay.com/itm/1000-Pcs-Co...457807?hash=item486776658f:g:HnUAAOSwyQtVp~Tf 1000x 0.2ml
http://www.ebay.com/itm/0-2ml-Conic...437591?hash=item1e802cc917:g:ZOUAAMXQLw1R5TAu 100 coloured 0.2ml for if you want to colour code your charges.
This seller also sells 0.5ml, 1.5ml (the size I use most) and larger sizes.
fill with dog barf on top of charge,single strip of tape as the lid (remove original) and then tape to allthread on AV bay (use slightly longer threaded rod on bay so it sticks out 1/2") gives you a directional charge.
I would also like to know if the ematch contributes in any substantial way when talking charges this small.
+1 to all of this. I buy my tubes from another supplier, though.
 

MCriscione

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This is all very interesting information. I probably should have mentioned that I had planned only on friction fitting for the drogue, and hadn't decided yet for the main. (holes are drilled, but I could always forgo the shear pins)

I will of course be ground testing, but I was hoping to minimize repetitions. Regarding containment, I don't have any vials on hand and won't prior to the launch this weekend, so I'm looking for some more easily procured methods. I do like the idea of taping the charge to an extended piece of allthread for a bit of direction rather than just letting it float around in the bay.

I welcome anybody else with thoughts to speak up! I am feeling at this point that I'm on the low side, which is easy to increase. Is there a limit to how much pressure cardboard tube will take before it bursts or otherwise makes me sorry?
 

woferry

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Personally I would never go with a charge that small. My second HPR flight was in a GLR T-Bolt (38mm airframe, ~18" between the motor and NC, some of which is space taken up by the chute and shock cord), I followed the AT motor instructions which said to only use half of the supplied (~0.7g) BP for airframes ≤2.6" and the rocket never separated (motor eject charge had fired). As I was at the pad I started to have a sinking feeling about not using the full amount, when I expressed my concern to my buddy on the pad next to me he said you can never have too much BP (which of course is only true up to a point), I should have scrubbed the flight then but instead I came home with my L1 rocket in pieces. The first flight I missed that recommendation in the AT instructions and used the full load and everything went great (the L3 overseeing my motor build didn't question using the whole charge amount either).

You should always do a ground test to verify your charge sizing, you can start that small but I'd wonder if you'd even get separation, and the chances of pulling the chute out of the airframe are much lower. I wouldn't use anything less than 0.5g, really I consider 0.7g to be the minimum for my rockets and use this on my 38mm DD Go Devil 38 (admittedly a FG rocket not cardboard) with very little volume in either section (38mm, but completely filled with chute and shock cord). It's true with a smaller airframe that there's less volume to be pressurized, but there's also less square-inches on the nosecone/avionics bay bulkhead to actually create the separation force, and the chute can pack rather tightly so it may take more force to pull it out.

As far as #4 goes I don't think the match itself really generates any pressure, it's point is to create a fire not expanding gas, that's what the BP is there for.

EDIT: I probably have my units messed up. When I say "0.7g" above, what I mean is that the AT vial is filled up to the 0.7 mark. I guess this must not actually be grams if the 38mm loads come with 1.4g since mine have only had enough BP to fill them up to the 0.7 mark. I haven't weighed any of my charges I just use the markings on the AT vial.
 
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vance2loud

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Another option I have heard of for small charges is a drinking straw. Hot glue your ematch into one end, add powder a little barf and seal. I haven't tried this myself but it sounds workable for your situation. I would possibly still tape it to the allthread/eyebolt whatever with multiple layers leaving the sealed end (bit of tape or folded over) as a directional blast.
 

AlfaBrewer

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I have a 54mm rocket with ~48 in^3 for the drogue and 96 in^3 for the main. I use .25g for the drogue charge (friction fit) and 1g for the main (2 shear pins). I've never had a problem with drogue separation, even at that small of a charge.

I use a very simple method for containing the charge - tape. Just place a e-match or Q2G2 in the center of a piece of tape, seal the ends to form a pocket around the pyrogen, and pour your charge into the pocket. Seal up the tape, and wrap with an additional layer. I use the Lee dippers instead of trying to weigh.

No matter what you do, ground test your charges and containment method.
 

MCriscione

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I tried drinking both plastic (larger diameter) and paper (standard dia) drinking straw tonight in an empty aluminum can. (Rocket wasn't quite ready yet) Found that the charge was consistently ripping open the side, probably due to the long and skinny shape. After dark while working on getting the rocket totally prepped, I found that the bulkheads just barely had space for a 1/2 pvc cap. So I'm gonna use those, kind of like sl98's suggestion, as a charge well, and simply pack the powder in there. Should contain/direct the charge pretty well. I still debating using any shear pins on the nose. I don't want to go with too many pins, requiring a higher pressure, and risk bursting the tube. And when I set the screw holes, I used a 3 screw pattern (mistake). So I think I can go with either 1 or none. If none, I'll have to add some tape to tighten the friction fit a bit. There's no extra nose weight, so I don't think drag separation there is any real concern.

I dunno. I'll have 4.5 hours in the car tomorrow driving to CT and back to think about it.
 

RocketFeller

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1/2" PVC plugs are smaller than caps and have a flat bottom. We found that a 3/4" plug will hold 4 grams, so a 1/2" plug should hold more than a gram. CPVC caps are also smaller than PVC.
 

markkoelsch

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Use disposable charges instead of holders. Pratt Hobbies makes a variety of sizes, and Wildman has disposable ones made in paper tubes. I tape them to the bulkhead.
 

NateLowrie

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I use surgical tubing for everything. This is my procedure for making charges:

1. Cut surgical tubing with 5/16" opening to length (1.5" is good for up to 2g of BP)
2. Stick match head into one side about 1/2"
3. Hot glue shut that side, making sure you don't get hot glue on the pyrogen.
4. Verify the bare wire ends of the ematch are twisted together.
4. Take a piece of paper, crease it in the middle and lay it on the scale. Tare the scale.
5. Measure desired BP amount onto paper.
6. Using a funnel for essential oils, transfer the BP from the paper into the funnel and into the tubing.
7. Visually confirm the powder is surrounding the ematch pyrogen head.
8. Gently pack cellulose wadding into the tube until it reaches 3/8" from the end of the tubing.
9. Hot glue the remaining opening closed.

Going to do my L2 recert with these charges this weekend. They work really well in most cases.

Note: a few folks on here have demonstrated better enhancements to this technique. Jim Jarvis has a paper on a vacuum testing these pieces and developed a brass tubing holder with better results. Another person has substituted vinyl aquarium tubing with much better results (only caveat is don't use the vinyl tubing in cardboard/phenolic rockets as it's energetic. Also, the charge can be much lower with vinyl).
 

cerving

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I use either 1.5g charge wells filled with 1g and packed with dog barf, or soda straws depending on whether or not I have room for a charge well. I use Triple Seven, so I tape the heck out of them for containment. Works fine with 38mm FG and 2.6" cardboard rockets (no shear pins with cardboard).
 

boatgeek

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My [very limited] experience has been that you need more BP than the calculators say for charges in glove fingertips. I put this down to the charge blowing itself apart before it has a chance to burn fully, but I could be wrong.
 

NateLowrie

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My [very limited] experience has been that you need more BP than the calculators say for charges in glove fingertips. I put this down to the charge blowing itself apart before it has a chance to burn fully, but I could be wrong.
Your pretty much right about that. I have the same problem with surgical tubing, though not to the same degree as the glove. Small charge caps also fall into the trap but longer charge wells fair better. Vinyl tubing seems to work but produces a bang when it ruptures rather than a deflagration. Where I need to get 1.5g on the surgical tube I can use .7g in the vinyl. This is due to the wall strength containing the powder and as the pressure builds the powder burns at an even faster rate.

Long copper tubes also work and are based off of burning completion while the gas cloud is expanding through the chamber. Jim Jarvis found that even in vacuum with unsealed charges the copper tube style burned all the powder.
 

KenECoyote

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Many people use spent shell casings a mini charge wells. Lots of calibers to choose from for an appropriate fit and you can trim down the casing as well. Just be careful of sharp edges (maybe put epoxy on the rim...I have to test this myself).
 
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