BP charges

[Steven88]

Hello. I am using an ejection Calculator to calculate charges for my 5.38” diameter rocket. The calculator program recommends not having a pressure of greater than 300 psi for this size of tube. I have ground tested with 2.12 grams and feel like maybe even a little more powder yet would be good. The calculator is however recommending I use no more than 1.9 grams so I am already over their recommendation without increasing it. Is it ok to use over the recommended amount as long as ground testing from a distance? Better to blow it up than not blow it out, right

 

[Steven88]

 

[cerving]

How many shear pins are you using, and what size? Is this fiberglass or paper? That might make a difference... if it's paper you definitely don't want to exceed 300 psi or you may end up with a big hole in your tube.

 

[Steven88]

It’s got a Loc paper tube and I’m using three 2-56 shear pins

 

[Rocketjunkie]

I fly a lot of LOC tube rockets. My main chute bay is 1/2 of a tube, about 20" - the nose cone shoulder and a couple inches of the coupler. I use 2.5 - 3 grams of BP. Better a little excess than too little. I use friction fit with paper tubes. Shear pins, unless you use shear plates, do not give consistent results.

 

[OverTheTop]

I also agree that using shear pins without shear plates in a cardboard airframe is a recipe for inconsistency. If the pins rotate (towards a tension break) rather then shear they are unpredictable. Do you have to use shear pins? That's the question I would be asking myself.

 

[Steven88]

I have cut circles out of the paper with an exacto knife and glued shear plates into all the paper tubes both into the avbay and body tube

 

[Nytrunner]

You only need ~90 lbs to shear three 2-56 pins. 10 psi in a 5.5" tube will give you over 200 lbs of separating force.

300 psi would be......over 3 tons. I'm going to wager thats a tad overkill

 

[Steven88]

Pics of shear plates

 

[SDramstad]

Its not 300 psi. Its 315 pound-force. Whole different animal. 13.88 psi

 

[Steven88]

Its not 300 psi. Its 315 pound-force. Whole different animal. 13.88 psi

Thanks for the clarification! You would still recommend that I keep it to only 10 psi or so? 200 pound force will easily shear three 2-56 shear pins?

 

[David Schwantz]

I just glue thin brass plates on the inside of the cardboard airframe myself.

 

[richP]

Are you testing with the chutes and shockcord in place?
Nosecone sealed for main?
Motor inserted for drogue?
I agree that you shouldn't need to go much over 90 lbs force to get thru (3) 2-56 pins. Using my calculator, 2.1 grams should be enough for (11) 2-56 shear pins.

 

[Steven88]

Yes, I am testing it as questioned above. I just like for it to kick the parachute and laundry out of the tube with some gusto so that I know my chute doesn’t stay inside the rocket and never open.

 

[SDramstad]

Thanks for the clarification! You would still recommend that I keep it to only 10 psi or so? 200 pound force will easily shear three 2-56 shear pins?

I am a firm believer in not going with the minimum charge size. Used to do that and buried one rocket and deployed mains at terminal velocity several times. If anything is off-nominal it doesnt deploy. 10 psi would be minimum for me. If running 2 altimeters the second charge is around 25% bigger.

 

[Handeman]

Yes, I am testing it as questioned above. I just like for it to kick the parachute and laundry out of the tube with some gusto so that I know my chute doesn’t stay inside the rocket and never open.

Don't depend on the ejection charge to discharge the chute and laundry. The ejection charge pushes the nose cone off and provides very little push to the chute. The ejection charge pressurizes the BT around the chute and laundry until the shear pins let go and the nose cone ejects. Once that happens, there is very little pressure left behind the chute to push it out.

It's mostly the inertia of the nose cone that pulls the chute and laundry out. You only need to get the chute into the air stream, not shock the whole system by throwing the nose cone hard against the end of the shock cord.

 

[Steven88]

I’ve done three ground tests so far, the second one I used over the recommended amount of bp and my 96” wildman chute with a nomex blanket. It only threw the nose cone about four foot and got the chute just outside of the tube. On the final test, my deployment bag with sewn in pilot chute had come in so I installed that. It filled the main compartment up fuller. When I tested it that time with only the max recommended bp, it threw the nose cone around twenty feet just coming short of its full length. Why the inconsistency with my tests? Does switching from a nomex blanket to a deployment bag make that much difference in deployment? I felt like the last test definitely had a little too much punch so I think I should scale back, but wow, what a difference! The last test I used 2.42 gram of bp in a 5.38” tube with 14.5” of usable space.

 

[jbrracer]

I’ve done three ground tests so far, the second one I used over the recommended amount of bp and my 96” wildman chute with a nomex blanket. It only threw the nose cone about four foot and got the chute just outside of the tube. On the final test, my deployment bag with sewn in pilot chute had come in so I installed that. It filled the main compartment up fuller. When I tested it that time with only the max recommended bp, it threw the nose cone around twenty feet just coming short of its full length. Why the inconsistency with my tests? Does switching from a nomex blanket to a deployment bag make that much difference in deployment? I felt like the last test definitely had a little too much punch so I think I should scale back, but wow, what a difference! The last test I used 2.42 gram of bp in a 5.38” tube with 14.5” of usable space.

The ejection charge isn't intended to push the recovery system out. If you are seeing inconsistency in results its probably the seal of the sections or stressing your shear pins by inconsistent torqueing.

 

[Steven88]

Here are the videos of my ground tests. To me the main still seems a little strong. Let me know your thots please. Thanks in advance.
View attachment trim.2D003A0F-10EF-4F7F-A8DF-8BCB3AF9E22C.MOV
View attachment trim.8F4281BF-9612-4237-85B7-720AE35BB360.MOV

 

[jbrracer]

Main looks good drogue appears weak that’s my opinion

Here’s mine and I thought about doing a bit more

 

[Steven88]

Hmmm... for some reason my phone doesn’t want to play them

 

[David Schwantz]

The difference in your tests between the bag and no bag is probably how the chute was packed. Did it fit tighter in the bag? If so, there would have been less drag in the airframe and the chute would slide out easier.

 

[Steven88]

Yes the bag and chute was tighter than just the laundry and the chute. Weird thing is, the bag was tighter and is ejected much farther...

 

[David Schwantz]

If it was tighter, then it may have held the ejection gases in better.

 

[rharshberger]

From others here on TRF, the tight bag really doesn't do much to slow the gases as they bypass around it via the path of least resistance (soft spot between airframe and bag), what probably had more effect on the nosecone was how the friction of the recovery package was distributed. Nomex burritos try and expand inside the tube, a properly fitted and packed deployment bag will slide freely inside the airframe with little friction allow the mass of the nosecone to pull the bag out of the airframe easier hence more stretch out in the harness. I prefer deployment bags now over every other type of chute packaging for several reasons, first is the fit in the airframe, second the way the chute is packed and deploys into and from the bag its orderly and less stressful, third deployment bagged chutes seem to have far fewer deployment related failures.

 

[cwbullet]

Stronger the better as long as it does not damage the nose cone or tubing or break the shock cord. Wait, that is a lot of what-ifs.

 

[jbr]

calculator on 5" diameter 8 foot long says 12G charge, I was going to start with 8

 

[rharshberger]

calculator on 5" diameter 8 foot long says 12G charge, I was going to start with 8

Thats a huge payload bay, might consider altering the design if possible to reduce the sizes of charges needed to deploy the chutes from.

 

[jbr]

the design has an interchangeable body tube, on top of the ebay it gives room for a big chute, placed below the ebay it gives room for a 60" motor

 

[jbr]

the design has an interchangeable body tube, on top of the ebay it gives room for a big chute, placed below the ebay it gives room for a 60" motor

8 grams was great