Ejection Charge for Large (12.75") Rocket

[RocketFeller]

I used this formula to estimate the ejection charge for our upscale Dragonfly: N = 0.006*D2L [grams] (from Vern's website).

With an ID of 12.625" and a length of 38" I came up with a 36 gram charge. Over an ounce of BP seems like a lot!

Does anyone know the density of BP? I am curious what kind of volume this would be. I plan on using charge cups and wonder how big they will need to be.

I am considering using a bulkplate to decrease the length of the parachute compartment, but I really don't want to for a couple reasons. First of all because of the recovery harness - it is designed to attach to the forward centering ring with two quicklinks/u-bolts. Any bulkhead plate would need to be removable for access to the attachment points. It would also need a way to thread the 1" nylon through the center of the bulkhead. Secondly, the rocket is getting heavier than I had originally hoped (as always) and I would just assume not add any more weight.

Thanks for any input!

 

[blackbrandt]

That honestly doesn't seem TOO terribly off. On OCF (9" diameter), we used 17 grams for drogue.

 

[RocketFeller]

I looked a bit and it seems like BP is right around one gram per cc (if I read the figures correctly) so around 36cc. That is a lot of powder, enough to fill two inches of one inch PVC...

I may have to add another bulkhead to reduce volume, I don't really want to use a Howitzer's worth of powder to blow apart our rocket!

 

[dixontj93060]

Two words, chute cannon.

 

[crossfire]

36 grams are you sure. I use 3 grams for a 4"X 36" space and have never had a problem. So am I wrong that 12 grams should work for 12"X 36" space. All your trying to do is move sections apart not use BP for a 2nd stage motor.

 

[RocketFeller]

Two words, chute cannon.

I've considered it, but with our configuration a shortened chute compartment seems simpler and lighter.

36 grams are you sure. I use 3 grams for a 4"X 36" space and have never had a problem. So am I wrong that 12 grams should work for 12"X 36" space. All your trying to do is move sections apart not use BP for a 2nd stage motor.

No, I'm not sure at all, just going by a formula I found on Vern K's website.

4" x 36" is about 450 cubic inches. 12.6" x 36" is about 4,500 cubic inches. Volume goes up to the square of the radius. So, this actually seems about right...

 

[dixontj93060]

Larger piston size will require less internal pressure (more square inches you are pushing against) to achieve the same amount of separation force. Instead of assuming the standard 15psi which (leads to over 1800lbf force), solve for enough the right amount of force to cut your shear pins, say 200lbf to 300lbf and you be back at a reasonable BP amount.

 

[Nathan]

Not unreasonable. I know Tom Cohen uses more than 30 grams of BP in some of his really big rockets. Just do some ground tests and work your way up until you get to the right amount.

 

[TOM C]

With an ID of 12.625" and a length of 38" I came up with a 36 gram charge.
Thanks for any input!

Your 36 grams for this 12" X 38" tube is in the correct range.
I am using 30cc in a 12" X 60" tube with a large chute in a deployment bag tight in the tube.
Measure in cc's and start ground testing.
Use the same charge well holder..same type of packing method and same tape method for all ground tests as these 3 factors will all change the force of the charge.

My chutes are in deployment bags and recently I have found that for a 12" diameter tube I am putting my chutes into 14" diameter bags so that the bagged chute in the tube fits tightly...like a plug..My cones have their own chutes so the main chute in the 14" bag is not connected to the cone. The main chute relies on the charge behind it to deploy, so the tight plug prevents the charge from blowing past the chute.

 

[cwbullet]

My 10 inch Nike smoke used 8 and 10 as a back up.

 

[Onebadhawk]

Dan,,
That sounded like a lot to me...
I have zero experience to say that..
I am EXTREMELY glad Tom C chimed in...
He has more experience flying super large rockets then the nearest 3 other people to him...
Put a great deal of stock in whatever Tom C says...

Teddy

 

[blackjack2564]

Sent you PM regarding this.

 

[MClark]

I have used 1 gram per 200 cubic inches with good success in large airframes.
That would come out to 22.5 grams.
Be REALLY careful ground testing ejection on large rockets. Remember Newton, parts will go both directions.

M

 

[RocketFeller]

Larger piston size will require less internal pressure (more square inches you are pushing against) to achieve the same amount of separation force. Instead of assuming the standard 15psi which (leads to over 1800lbf force), solve for enough the right amount of force to cut your shear pins, say 200lbf to 300lbf and you be back at a reasonable BP amount.

It sounds like we will be doing a lot of ground testing...

Your 36 grams for this 12" X 38" tube is in the correct range.
I am using 30cc in a 12" X 60" tube with a large chute in a deployment bag tight in the tube.
Measure in cc's and start ground testing.
Use the same charge well holder..same type of packing method and same tape method for all ground tests as these 3 factors will all change the force of the charge.

My chutes are in deployment bags and recently I have found that for a 12" diameter tube I am putting my chutes into 14" diameter bags so that the bagged chute in the tube fits tightly...like a plug..My cones have their own chutes so the main chute in the 14" bag is not connected to the cone. The main chute relies on the charge behind it to deploy, so the tight plug prevents the charge from blowing past the chute.

Good to remember, consistency definitely seems important here.

My 10 inch Nike smoke used 8 and 10 as a back up.

How long was the recovery compartment?

Dan,,
That sounded like a lot to me...
I have zero experience to say that..
I am EXTREMELY glad Tom C chimed in...
He has more experience flying super large rockets then the nearest 3 other people to him...
Put a great deal of stock in whatever Tom C says...

Teddy

I am very glad, too! Tom and Jim have been helping me out with PMs - it is good (really good) to talk to people who have done this sort of thing!

Sent you PM regarding this.

I appreciate it!

I have used 1 gram per 200 cubic inches with good success in large airframes.
That would come out to 22.5 grams.
Be REALLY careful ground testing ejection on large rockets. Remember Newton, parts will go both directions.

M

I will be, probably overboard with safety precautions as there will be kids involved!

 

[RocketFeller]

Here are some pics of the altimeter bay. It uses 1" PVC plugs as charge wells. From talking to Jim we will be reinforcing them after adding a 1" coupler for more volume and to double the wall thickness.

 

[webtech]

Looks right to me if you are trying to pressurize the whole tube. ground testing this guy will be your best friend.. I am guessing it will sounds like a canon!!!

.006 * 12.625 * 12.625 * 38 = 36.3 grams of BP

C * D * D * L = grams of BP
Where:
C - one of the values listed below
0.002 = 5 psi
0.004 = 10 psi
0.006 = 15 psi
0.0072 = 18 psi
0.008 = 20 psi
D = airframe diameter, in inches
L = length of the cavity to be pressurized, in inches

 

[blackbrandt]

When we ground tested OCF at LDRS... we had 17ish grams, packed inside an aluminum canister, packed with dog barf and sealed with craploads of masking tape. Some reports we got were "I thought I was back at McGregor for live warhead tests" and "I was in the port-a-potty 400 feet away and I'm pretty sure I felt the shock wave". Brace the HELL out of the back end of the rocket. Also make sure you're not standing behind it.

Seriously though, the large charges depend a LOT on how you pack it. Without compressing the powder like we did, we had a much smaller boom. Make sure you pack it the same way for testing and flight.

 

[Bat-mite]

Note what webtech said ... with smaller rockets, I always ground test in my yard, and it isn't too loud, and everyone knows I'm into rockets, and no one cares.

But--

When dealing with really big charges like that, you may want to inform your neighbors first, or test at a launch. Last thing you need is a neighbor calling the cops, especially if you don't have a LEUP!

 

[blackbrandt]

Note what webtech said ... with smaller rockets, I always ground test in my yard, and it isn't too loud, and everyone knows I'm into rockets, and no one cares.

But--

When dealing with really big charges like that, you may want to inform your neighbors first, or test at a launch. Last thing you need is a neighbor calling the cops, especially if you don't have a LEUP!

AND if you decide to test it at a launch... it's a good idea to inform the LD first BEFORE you test.... We got to meet the BOD while at LDRS and it wasn't on purpose.

 

[RocketFeller]

Looks right to me if you are trying to pressurize the whole tube. ground testing this guy will be your best friend.. I am guessing it will sounds like a canon!!!

.006 * 12.625 * 12.625 * 38 = 36.3 grams of BP

C * D * D * L = grams of BP
Where:
C - one of the values listed below
0.002 = 5 psi
0.004 = 10 psi
0.006 = 15 psi
0.0072 = 18 psi
0.008 = 20 psi
D = airframe diameter, in inches
L = length of the cavity to be pressurized, in inches

That is the formula we used, I used the 15 psi number. I think it should be a pretty big bang...

When we ground tested OCF at LDRS... we had 17ish grams, packed inside an aluminum canister, packed with dog barf and sealed with craploads of masking tape. Some reports we got were "I thought I was back at McGregor for live warhead tests" and "I was in the port-a-potty 400 feet away and I'm pretty sure I felt the shock wave". Brace the HELL out of the back end of the rocket. Also make sure you're not standing behind it.

Seriously though, the large charges depend a LOT on how you pack it. Without compressing the powder like we did, we had a much smaller boom. Make sure you pack it the same way for testing and flight.

I will be sure to practice compressing/packing a bunch prior to ground-testing the actual rocket.

Note what webtech said ... with smaller rockets, I always ground test in my yard, and it isn't too loud, and everyone knows I'm into rockets, and no one cares.

But--

When dealing with really big charges like that, you may want to inform your neighbors first, or test at a launch. Last thing you need is a neighbor calling the cops, especially if you don't have a LEUP!

We're lucky in that we live in the country and all the neighbors are friends. The guy across the road has a penchant for automatic weapons, so a big boom once in a while won't scare anyone.

 

[RocketFeller]

Anyone want to chime in as to which of these powders would be best? There are a ton to choose from!

https://www.sportsmanswarehouse.com/sportsmans/Powder/category/cat100151

 

[Bat-mite]

None. You want GOEX FFFFg powder, or possibly FFFg. All the powders on that site are BP substitutes. The closest is the Hodgdon Pyrodex, which is supposed to be FFFg equivalent. I'd shop around.

 

[cwbullet]

Remember to subtract the filled portion of the tubing. The boster is 10 inches in diameter and the only portion not filled with a motor or coupler is about 24 inches. The top is 30 inches long. I put 8 and 10 in the two charges in the booster section. The payload section has 10 and 12.

Sorry I had to confirm it.

Mine works and I always ground test. I have found that usually large diameter air frames take less powder than I would thing. The again, my Nike Smoke goes off like a cannon with that about of powder.

 

[Onebadhawk]

Anyone want to chime in as to which of these powders would be best? There are a ton to choose from!

https://www.sportsmanswarehouse.com/sportsmans/Powder/category/cat100151

None of those Dan,,
You want ffff black powder..
Like this,,
https://www.thomasfx.com/FFFF-Black-Powder-1-lb-can-_p_8434.html#

Be certain not to use anything but 4f black powder..

Teddy

 

[Onebadhawk]

Or this,,
https://www.basspro.com/GOEX-Black-Powder/product/120829053247144/
Do you have a Bass Pro shops by you ??

Teddy

 

[Onebadhawk]

This looks like a good place to buy online..

https://www.grafs.com/retail/catalog/product/productId/5450

Teddy

 

[rharshberger]

+1000 the FFFFg BP, its sooo much easier to get consistent results with than Pyrodex or the other substitutes. Pack it tightly and consistently and the results will be the same or nearly so each time.

 

[RocketFeller]

None of those Dan,,
You want ffff black powder..
Like this,,
https://www.thomasfx.com/FFFF-Black-Powder-1-lb-can-_p_8434.html#

Be certain not to use anything but 4f black powder..

Teddy

I think that there are about ten more pages of choices, I will look for 4F powder.

Or this,,
https://www.basspro.com/GOEX-Black-Powder/product/120829053247144/
Do you have a Bass Pro shops by you ??

Teddy

We do, but it is twenty or thirty minutes further away than the Sportsman's Warehouse.

+1000 the FFFFg BP, its sooo much easier to get consistent results with than Pyrodex or the other substitutes. Pack it tightly and consistently and the results will be the same or nearly so each time.

I will be sure to go FFFF!

 

[Onebadhawk]

I find it hard to believe Sportsmans Warehouse doesn't carry the ffff black powder...

Give them a call..

Teddy

 

[dixontj93060]

Looks right to me if you are trying to pressurize the whole tube. ground testing this guy will be your best friend.. I am guessing it will sounds like a canon!!!

.006 * 12.625 * 12.625 * 38 = 36.3 grams of BP

C * D * D * L = grams of BP
Where:
C - one of the values listed below
0.002 = 5 psi
0.004 = 10 psi
0.006 = 15 psi
0.0072 = 18 psi
0.008 = 20 psi
D = airframe diameter, in inches
L = length of the cavity to be pressurized, in inches

That is the formula we used, I used the 15 psi number. I think it should be a pretty big bang...

Again, back to post #7. Your results with 15 psi pushing on a 12.625" piston are as below. And again, I have to ask, why would anyone need nearly 1900lbf (30 to 50 shear pin equivalents) in separation force?!?

Ejection Charge

Deployment Section Measurements
Width: 12.625 (in)
Length: 38 (in)

Pressure: 15.00 (psi)
FFFF BP Amount: 36.81 (g)
Force Produced: 1877.78 (lbf)

Number of Nylon Screws (Shear pins):
49 x 2-56
or
30 x 4-40
or
20 x 6-32

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