How much BP for a 38mm dia (Go Devil 38)?

Gents,
I am at that stage. How much BP for the two charges? It is a 38mm. The available volume in the sustainer is 5.5 inches long. There is a 15 ft shock cord and a 12 in chute.

In the forward body it is 6.7 inches. There is a 10 ft shock cord and a 30 in chute.

The formula in Modern High Powered Rocketry 2 says:
BP (grams) = 0.006*D_compartment^2*L_Compartment

That comes out to 0.074 g for the sustainer and 0.090 grams for the forward compartment.

Does that sound right to you?

I will begin testing tomorrow.

Hey Tom. Your calculations are correct for that formula. I've always used it and found it to be pretty accurate. I must admit that I usually use just a little more just because. There are other calc' s that are a little more accurate but I lean a little more toward the camp that dictates "Blow it apart or Blow it up". Not a widely accepted policy it would seem nowadays but it always worked for me. Good luck with your testing and subsequent flight. Oz

These formulas are always iffy. 0.07 g? How do you even measure that without a reloader's scale? That is an order of magnitude in error from reality. My WM Blackhawk 38 uses 0.75g each which I eyeball using the little red Aerotech ejection cap. The deployment gives full extension of the shock cords, but nothing violent. The main includes one shear pin as well.

Reference point: 24mm Aerotech RMS motors come with 0.4g, I believe.

Ground test. Pretty soon you will rely on experience and not the simple formulas.

My post from your other Devil 38 thread:

IIRC, MP2 book was solving for 15 psi in the bay. For a small diameter tube, especially if you are using shear pins, you will want more shear force and thus higher psi.

I would start with the amount packaged with most motors, 0.5 grams.

CTI ejection charges are:

24mm .6g
29mm 1.2g
38mm 1.3g

Titan II said:

CTI ejection charges are:

24mm .6g
29mm 1.2g
38mm 1.3g

Click to expand...

Thanks, that's good to know. I would amend my recommendation to reflect those values, ground test indicated.

I would ground test. Start somewhere, work up. 0.6g is a fine place to start. For that matter, so is 0.5.


Later!

--Coop

TomSmith58 said:

In the forward body it is 6.7 inches. There is a 10 ft shock cord and a 30 in chute.

Click to expand...

I think that this is going to be very frustrating for you. The 10 foot cord may limit things quite considerably. The smaller the tube and the shorter the bay, the more difference even .1gram in charge makes.

If you're stuck with it, it's going to be a challenge to find just the right balance of energy to pop the nose, deploy the parachute, and NOT have it all bounce back from a heavy jerk as it reaches full stretch. Make sure that the way you're packing the laundry that it slips in and out of the tube easily.

Start low, work up, and use an old t-shirt as a dummy roll the same size as your packed chute (tape it closed) INSTEAD of your parachute for your first few work up loads. Likely you'll get a few fizzles or pops without deployment if you start very low, and you don't want to char your chute. Use the parachute for final confidence test once you settle on the right charge.

For a almost identical size rocket, I use .5 apogee [20ft of 1/8 kevlar] & .7 main. [15ft of same kevlar]
NC is held on with 1 2-256 shear-pin & fit is rather snug.

PS: since there is only 1 altimeter, I use 2 matches per charge.

Ground test if you can. I marked on the inside of my rocket the charge weight but I believe it is within the realm of what Jim mentions. Mine might be higher 'cause I use two shearpins on both sections. I'll go take a look.
I had used an EggTimer rocketry remote switch to do the ground testing but I suspect a Quantum altimeter might fit and be usable too. I always thought
that one shearpin is a no-no but with so many folks using that perfectly fine on this small stuff, I'm going to change my future thinking on new small projects.
Kurt

Well I started at 0.1 gm just to see what would happen. I increased the charge by 0.1 and went up to six. I had a problem with my surrogate motor at 0.5. The duct tape ripped loose. I don't know if 0.5 will work. I found that 0.6 did work but it looks like the charge is to strong. I am using two of the 2-56 nylon pins. All of you are using only one. I now have only four e-matches left. I will use them on the main compartment. If any are left over I will try 0.5 on the drogue compartment and see if that works. In any case I need to buy more at LDRS or I can't fly.

Who knows how to upload videos to the forum? When I select a video I have to put the uploader on All Files. It then has a red exclamation point next to the file name and if you hover over that it says "invalid file type". I have a video of my failed 0.5 gm test and my successful but highly energetic 0.6 gm test.

I discovered my 30 in. chute with a 12 in. chute protector is too big to fit into my 38 mm tube. I will try another arrangement to roll them up but I fear if I make it small enough in diameter it will be too long and I won't be able to get my shock cord in. I will go experiment.

TomSmith58 said:

Well I started at 0.1 gm just to see what would happen. I increased the charge by 0.1 and went up to six. I had a problem with my surrogate motor at 0.5. The duct tape ripped loose. I don't know if 0.5 will work. I found that 0.6 did work but it looks like the charge is to strong. I am using two of the 2-56 nylon pins. All of you are using only one. I now have only four e-matches left. I will use them on the main compartment. If any are left over I will try 0.5 on the drogue compartment and see if that works. In any case I need to buy more at LDRS or I can't fly.

Click to expand...

Blew out your surrogate motor? What are you using?

Me, I throw a dowel cut to length for a case, pop an old nozzle on, and use my regular retainer (aeropack, wire clip, whatever's on the model). Except for 38--then, no dowel: I just use a case and screw in an old nozzle (CTI).


Later!

--Coop

A few suggestions:

If you are short on ematches, then use old igniters or Crapperheads for ground testing. Or, buy a bunch of cheap Chinese matches on Ebay or Amazon.

Tape the circumference of the aft closure to the rear of the airframe. This is the most important part of friction fitting a motor.

Forget about a 12x12 chute protector. That is way too big. For 38mm, just use dog barf. (I use a tiny, 6x6 blanket along with dog barf.)

TomSmith58 said:

I discovered my 30 in. chute with a 12 in. chute protector is too big to fit into my 38 mm tube. I will try another arrangement to roll them up but I fear if I make it small enough in diameter it will be too long and I won't be able to get my shock cord in. I will go experiment.

Click to expand...

Get one of the Thin Mill Nylon chutes from if you don't already have one, they are lightweigth and pack small.

TomSmith58 said:

Who knows how to upload videos to the forum? When I select a video I have to put the uploader on All Files. It then has a red exclamation point next to the file name and if you hover over that it says "invalid file type". I have a video of my failed 0.5 gm test and my successful but highly energetic 0.6 gm test.

Click to expand...

Upload to Utube and post the link here. Kurt

How do you guys record how much powder is required? After ground testing and finding the right amount on my two not-flight-tested DD birds, I've written the amount in sharpie on the lids of the Av bays. One is plywood, the other is FG; not sure the sharpie will remain legible once I get a few deployments in?

soopirV said:

How do you guys record how much powder is required? After ground testing and finding the right amount on my two not-flight-tested DD birds, I've written the amount in sharpie on the lids of the Av bays. One is plywood, the other is FG; not sure the sharpie will remain legible once I get a few deployments in?

Click to expand...

One way is to keep a log of all your flights. I've tried, but I'm too busy having fun.

I write it in sharpie on the coupler. As a back up I post it in the notes section of my OR files.

soopirV said:

How do you guys record how much powder is required? After ground testing and finding the right amount on my two not-flight-tested DD birds, I've written the amount in sharpie on the lids of the Av bays. One is plywood, the other is FG; not sure the sharpie will remain legible once I get a few deployments in?

Click to expand...

I keep all my rockets in a single "ejection charge" spreadsheet.

rharshberger said:

Get one of the Thin Mill Nylon chutes from if you don't already have one, they are lightweigth and pack small.

Click to expand...

Rich,
How do I get a chute from "from if"? Could you help a guy out and type the name of the supplier?
Thanks
Tom

TomSmith58 said:

Rich,
How do I get a chute from "from if"? Could you help a guy out and type the name of the supplier?
Thanks
Tom

Click to expand...

Sorry, Top Flight Recovery, obviously I wasn't thinking coherently when I typed that.😀

I found out how to upload videos. No time to talk because the food is on the table and the wife is glaring.

Here is the sustainer. 0.6 gm could be OK
https://www.youtube.com/watch?v=rikHt5YAXTg&feature=youtu.be
Here is the main compartment. 0.6 gm black powder is way too much.
https://www.youtube.com/watch?v=yqguXfLkjic&feature=youtu.be

I thought both of those looked good. Did you not have the aft closure on your motor?

I don't have my motor. I don't have my motor mount glued in. I have a Aero Pack MD38E mount that anchors my shock cord. I can't glue it in till I get not only my new 6G-XL motor case, but a 6G-XL propellant load. I need to screw the CTI adapter into the top of the propellant load, screw the motor mount to the top of that, insert it into my body tube and check out how far in it goes. Then I can clue my mount. That won't happen till I pick up my new case and propellant at LDRS in June.

For now I have a long allthread screwed into a turnbuckle screwed onto my mount. I have a piece of plywood attached about the right distance down the allthread to simulate the end of my motor, and I duct taped the plywood to the end of the body tube.

Cluge.

Steve
As to both looking good, the second one was too powerful. I have two 2-56 nylon screws as shear pin holding the coupler to the sustainer, and two holding the coupler to the upper body. The second charge was in the upper body. It sheared all four pins. That shouldn't have happened.

What I think I am going to do is use a single shear pin for the forward body as some of the other guys do. When I test it I will poke the nosecone into the ground and have only the coupler installed in the end. That will be much more like real flight. That way it will fire the coupler out of the end of the upper body tube. If I use one shear pin there is less force involved and it will be less likely to blow the coupler out with enough force that it will yank on the end of the shock cord and rebound and recontact the upper body. I don't want recontact.

Rich wrote "Get one of the Thin Mill Nylon chutes from if you don't already have one, they are lightweigth and pack small."

I checked mine and it is a Top Flight Standard, not a Thin Mill. I will see if I can order one from one of the vendors who are coming to LDRS.

Bay Area Rocketry should be one of the vendors at LDRS, and Mike (owner) has a good selection of TFR chutes. Below a certain diameter (≤18"?) everything he has is thin mill I believe.

On my Go Devil 38 I use a Dog House small charge well on each end, each one holds ~0.6g along with the Firewire initiator, and I've always just filled them. Has worked fine, though I was only friction-fitting the nose and finding that sometimes the main would come out at apogee, other times during descent but before the altitude set in the altimeter. I have about 15' of shock cord in each bay. So I've drilled holes for shear pins for the nose and will be using them moving forward, I've never done any ground testing with this rocket as I couldn't fit my TRS in the bay, now that I have a Quantum I really should do a test just to see if I should perhaps go down a bit on the charges.