What am i looking for in charge testing?

[Ccolvin968]

I did three ejection charge tests today on my 4" MadCow Tomach.

First with three shear pins, 1.3g BP.
Second with no shear pins 2.0g BP.
THIRD with two shear pins 2.0g BP.

The first test was a total failure.

The second pushed the top portion of the rocket off and partially pushed the chute out.

The third pushed the top portion of the rocket off, but no chute extraction.

I'm trying to do my L2 and just want to keep it simple. (Compared to my original plan.)
No DD. No chute bags or drogue deployment. Etc.
Should my charge push everything out or just enough to get the top off to pull the rest with it?

But the motors only have 1.3g BP.
If I'm going for motor only eject, how can I make it work?

Do I need shear pins if I don't do DD for the bottom half that I plan on pushing the chute out of?
I'd still use rivets and pins for the top that will not be doing anything.
Any advice helps. I'm pretty frustrated right now.

Thanks everyone.

 

[manixFan]

What size charge did your calculations show you should use? You don't say what size shear pins you are using nor the total length of the body tube you need to pressurize. I would start with about 50% more than needed to break your shear pins.

You can augment the motor ejection charge for most motors by adding additional BP in the charge well and covering it with tape. It will ignite at the same time as the underlying charge.

I think you would want everything out and the nosecone thrown pretty close to the limit of the shock cord.


Tony

 

[Ccolvin968]

I used 2.0g FFF BP. The calculator said I should use 1.75g based on a 25" body tube.
The shear pins are 2-56. Looks like I'll need some larger charge containers.
I'm maxed out at 2.10g if I pack it as tight as possible.

 

[Ccolvin968]

Correction. I can get 2.20g if I pack it tight.

 

[MikeyDSlagle]

Did the calculator say 3F or 4F, 3F may take a bit more.

What are you using for charge wells? May be possible to extend capacity with stuff lying around. Not my cup of tea, but some folks don't care how their interiors look.

Depending on the motor, speed of rocket, drag of rear section, weight of upper section, etc, you shouldn't need to pin the bottom.

As long as your chute leaves the airframe and gets into the air stream, it should deploy. Shouldn't matter how far away from the BT it goes. I try to get my harness near fully extended, well, that depends on harness length I guess. I have had a 1.0 gram charge send the nose cone all the way to the end of the harness and a 1.4 gram charge not send it as far, same rocket. So it also depends on how you pack

Mikey D

 

[mpitfield]

Personally I would go with electronic ejection as it is arguably more reliable, YMMV.

That aside if you are going with motor ejection then chose a motor that is a relatively slow and low motor and make sure you have enough friction where the rocket splits. The general advice is you should be able to pick up the rocket by the upper stage and the bottom stage should stay on but slide off slowly.

Another contributing factor is the pressure differential that may take place as the rocket ascends. Depending on the alt and velocity, you may get a higher internal pressure that needs to bleed off, and if your compartments are too tight then it can force the airframe apart. To counter this I always drill holes in my payload compartments to help equalize the pressure.

I have a calculator that I use to size these holes as you want a hole that allows the pressure to bleed off, but not so large that it effects the effective pressure of your BP. If you want some help with the calculations then post the diameter and length of the sections of the airframe you are concerned with and I, along with others, can do a quick sanity check. Posting a RS or OR file with your proposed motor would also help.

As far as what you should be looking for when ground testing, I would say a consistent clean separation and full ejection of the luggage. Personally I look for a full extension of my recovery harness, but not to the point where it snaps at the end. This is why I prefer to go on the larger length harnesses based on the rocket. In your case I would go with a 25' harness.

 

[Bkdoubleu]

I did three ejection charge tests today on my 4" MadCow Tomach.

First with three shear pins, 1.3g BP.
Second with no shear pins 2.0g BP.
THIRD with two shear pins 2.0g BP.

The first test was a total failure.

The second pushed the top portion of the rocket off and partially pushed the chute out.

The third pushed the top portion of the rocket off, but no chute extraction.

I'm trying to do my L2 and just want to keep it simple. (Compared to my original plan.)
No DD. No chute bags or drogue deployment. Etc.
Should my charge push everything out or just enough to get the top off to pull the rest with it?

But the motors only have 1.3g BP.
If I'm going for motor only eject, how can I make it work?

Do I need shear pins if I don't do DD for the bottom half that I plan on pushing the chute out of?
I'd still use rivets and pins for the top that will not be doing anything.
Any advice helps. I'm pretty frustrated right now.

Thanks everyone.

sounds very familiar. 3 things I learned through firing almost a pint of 4F one summer, in practice.
1. Put the squib at the bottom of the charge cup.
2. Pack the powder with toilet paper on top with eraser end of pencil as tight as you can get it without damaging match.
3. Use blade of exacto knife to partially cut through nylon screws, lay screw on table and roll it under blade of knive "pre-scoring" the screw.
And lots and lots of practice. That's the best advice I got from a guy.

 

[Ccolvin968]

The calculator said FFFg.

I just did two more tests.

2.2g no shear pins, ejected everything.

2.2g with shear pins, popped top half off, no chute extraction.

I'm using 2ml microcenterfuge tubes as containers.
2.2g is the max I can fit in there if I cram it in.

I'm nervous to do electronic deployment, but it's an option.
I have an Eggtimer Classic that I can put in.
Any suggestions for larger charge containers?

I think that's my only problem right now is not enough oomph for shear pins.
If not, as mentioned before I can just supplement the motor charge with more BP.
I'd probably do an additional 1.5g to be safe...

 

[Ccolvin968]

Can I wire two charges to one ignition channel of an altimeter or two channels that fire at once?
That could solve my problems right there if so!

 

[Zeus-cat]

I would not wire two charges to the same channel. The resistance of 1 igniter, or whatever you are using, can really draw down the voltage when the altimeter fires the charge. I wouldn't want to see what would happen if you double the current requirements. The altimeter/battery may not be able to handle it.

 

[manixFan]

I used 2.0g FFF BP. The calculator said I should use 1.75g based on a 25" body tube.
The shear pins are 2-56. Looks like I'll need some larger charge containers.
I'm maxed out at 2.10g if I pack it as tight as possible.

I typically use 2x the amount of BP required to shear the pins. Based on a 25" tube and 3 pins that would be 3 grams of BP. We know that 2 - 2.2 grams was not really enough so 3 grams sounds fairly reasonable, 2.75 might do it.

The best way to contain BP is in a tube with no airspace - pack it with wadding - and tightly seal it. It can be any sturdy tube. You can use a large launch lug, brass tubing from a hobby or hardware store, or even a cleaned out Estes motor tube. Don't let the size of your current device limit you from doing it right.

Jim Jarvis did some great experiments with BP containment that showed the effectiveness of using a tube.


Tony

 

[ksaves2]

Whatever you do, don't make the mistake of sizing your charges with ematches and plastics centrifuge containers and then expect that to be the "right charge" for motor ejection.

Why? Well I was afraid that the amount of powder provided with a 38mm AT reload would be "too" much for a rocket. It's a Formula 54 with a 29mm motor. I used a charge container
and threaded the ematch down an empty motor casing. O.K. .9gms worked. I then put that amount into the motor charge well and it didn't get the NC off at all. The motor ejection fired but what I think happened is some of the charge falls into the motor and the hole acts as a restrictor and that energy goes out the back of the motor. Yes, it's likely a very small amount
of energy lost that way but I believe the best way to test a charge is in the manner the charge is going to be used. Or make sure you put a little "extra" in there.

Subsequent flying after replacing the nosecone, I just toss all the powder for the 29mm reload in and call it good. It's a small rocket so the 1.38gms is good. I'd worry more with a larger rocket with more volume. Kurt

 

[Ccolvin968]

Got it Zeus. Thanks.

My wife is a Type 1 diabetic, so I have plenty of larger tubes (without the needles of course) for larger BP charges.
Plug up both ends and I should be good to go.

 

[MikeyDSlagle]

Empty rifle, pistol, shotgun rounds can work if you have the right size. Empty 20 gauge would probably be a good size. .61" inside, volume of around 40 ML. That should hold plenty of BP and wadding. .410 shell be good too. Knock the primer out, or drill it...after they have been fired. Voila, you have a charge canister with a hole on the bottom for a bolt. Don't have a 20 gauge? Need some empty shells? You can grab 100 for 17 bucks. I could probably be persuaded to send you some empties. :wink: Will give me a reason to shoot something. Can't help ya on 410, they are pricey and my 410 is in storage.

No reason to be nervous with electronic deployment. Many, including myself, trust it more than motor delays. All my HPR flights use electronic ejection. Just make sure your settings are correct on altimeter.

 

[Ccolvin968]

Thanks for that Mikey! I'll give those a shot too.

Believe me... I have WAY too many empty casings laying around of all sorts.
Guess I'm doing at least one of my hobbies right!

 

[manixFan]

Thanks for that Mikey! I'll give those a shot too.

Believe me... I have WAY too many empty casings laying around of all sorts.
Guess I'm doing at least one of my hobbies right!

Mikey is right on about using old brass cartridge cases. I find that .45 ACP and .45 Long Colt (used in Colt Single Action Army revolvers) are great for most of the charges I need. Just make sure you don't somehow short the e-match against the side of the case.

But plastic syringes also work great and it sounds like you have plenty of those as well. Cool thing about those is you can thread the wires of the e-match out the bottom and it leaves the e-match nicely centered in the BP. I run the lead back up against the side of the syringe and tape it in place.

With any charge you want to secure it so so the bottom of the charge is against a bulk head and the open end faced towards the open end of the body tube. I'll make a holder out of wood (just a square with a hole drilled in it for the charge holder to sit in) or tape it against the eyebolt.

Good luck,


Tony

 

[MikeyDSlagle]

Mikey is right on about using old brass cartridge cases. I find that .45 ACP and .45 Long Colt (used in Colt Single Action Army revolvers) are great for most of the charges I need. Just make sure you don't somehow short the e-match against the side of the case.

But plastic syringes also work great and it sounds like you have plenty of those as well. Cool thing about those is you can thread the wires of the e-match out the bottom and it leaves the e-match nicely centered in the BP. I run the lead back up against the side of the syringe and tape it in place.

With any charge you want to secure it so so the bottom of the charge is against a bulk head and the open end faced towards the open end of the body tube. I'll make a holder out of wood (just a square with a hole drilled in it for the charge holder to sit in) or tape it against the eyebolt.

Good luck,


Tony

I'm glad you brought this up. How much can you get in your empty 45 ACP with an E match and wadding? I've been meaning to try em. My dad has all my empties somewhere. Just never think about it. I have some chromed 25-06 empties I am using on my Tembo. Cutting it off right at the neck and sand smooth. Nice and shiny...til the first charge fires. It'll hold around 2 grams with E match I think. Measured it out the other day, just forgot. I also got a few empty "Ma Deuce" cut down and primer drilled out. They will hold A LOT. May use them in my Tyrannosaur but don't know. I prefer long slender cases. Gets a taller powder column. Does it help? Don't know, just makes me feel better. I have more on the business end. 357 Magnum is a bit on the small side, but will probably work in smaller birds. The 45 Long Colt seems to be ideal, same as the 410, or 45-70 (which seems to have the cheapest empties). My mind is on my Tyrannosaur mind ya. A plus for the shotgun shells: they won't cut/nick your wires and cause a short. Funny you mention the 45 Long Colt, I was just looking for a Schofield chambered in that round. They don't exactly give them away.

I have been using an old blowgun chopped up and JB Weld a nut and all thread in the bottom. 2 flights and one has failed. Guess the charge weakened the JB Weld. Don't know. Will got back with empty brass, just don't know which caliber yet.

 

[rharshberger]

I'm glad you brought this up. How much can you get in your empty 45 ACP with an E match and wadding? I've been meaning to try em. My dad has all my empties somewhere. Just never think about it. I have some chromed 25-06 empties I am using on my Tembo. Cutting it off right at the neck and sand smooth. Nice and shiny...til the first charge fires. It'll hold around 2 grams with E match I think. Measured it out the other day, just forgot. I also got a few empty "Ma Deuce" cut down and primer drilled out. They will hold A LOT. May use them in my Tyrannosaur but don't know. I prefer long slender cases. Gets a taller powder column. Does it help? Don't know, just makes me feel better. I have more on the business end. 357 Magnum is a bit on the small side, but will probably work in smaller birds. The 45 Long Colt seems to be ideal, same as the 410, or 45-70 (which seems to have the cheapest empties). My mind is on my Tyrannosaur mind ya. A plus for the shotgun shells: they won't cut/nick your wires and cause a short. Funny you mention the 45 Long Colt, I was just looking for a Schofield chambered in that round. They don't exactly give them away.

I have been using an old blowgun chopped up and JB Weld a nut and all thread in the bottom. 2 flights and one has failed. Guess the charge weakened the JB Weld. Don't know. Will got back with empty brass, just don't know which caliber yet.

Rifle brass is better than pistol brass due to its thicker walls.

 

[Ccolvin968]

This is great info everyone!

I'll make sure to mount the charge containers to a bulkhead.
Can I just aeropoxy the containers to the bulkhead?

 

[manixFan]

This is great info everyone!

I'll make sure to mount the charge containers to a bulkhead.
Can I just aeropoxy the containers to the bulkhead?

I would make them easily removable rather than epoxy them in. For that size charge as long as they don't move around in flight that's all you really need. You just don't want it pointing sideways when it fires to avoid any dings to the body tube from the charge holder. While it's never happened to me, some flyers have had body tubes punctured by a charge holder that went sideways through the tube. So as long as it's resting securely on the bulkhead you should be fine.


Tony

 

[MikeyDSlagle]

Ditto.
I bolt mine to the bulkhead, and use something like hot glue to seal around the penetration. I bought some silicone, but it didn't act right.

 

[ActingLikeAKid]

This is very relevant to my interests as I'm building an L2 rocket and doing a little hand-wringing about ejection charges myself.
I think (based on my relatively small information, please someone correct me if I'm wrong!) that you shouldn't need shear pins for a motor-eject single-deploy rocket. Unless you're doing something like a 54mm MD rocket and planning massive altitude changes really fast, drilling a vent hole or two (there are calculators out there) should be fine for keeping the NC on. Mine fits pretty well - a light shake will make the bottom fall off - so I'm thinking I may put a wipe of laminating epoxy on the inside of the body tube, then sand to where it will pop out with a vigorous shake. (Yes, you can achieve the same thing with masking tape, but why not make the solution permanent.....)

Obviously, with the amount of charge that you're talking about, you're unlikely to be concerned with this, but on my rocket, (54mm BT, about 15" of space for the charge, less chute and harness), my biggest concern was at the other end - not that I was going to blow up the rocket, but I wanted to make sure I was going to "scrub **** enough energy as the nose deployed that i wouldn't put undue stress on the recovery harness.

I was pointed to Rocket Calculator ($0.99 on the iPhone) which will give you calculations for how much BP you need to break X shear pins. (e.g. 4" tube, 20 inches of pressure chamber, 3g of FFFF will give you X PSI of pressure, putting Y pounds of force on your nose cone, and will break Z shear pins of D diameter.)

Personally, I found that the max force I'd hit on the bottom of my nose cone is around 180 pounds. Knowing that all of my recovery gear is rated for at least twice that, I'm pretty comfortable with it being sufficient. Good luck!

 

[Ccolvin968]

Thanks for that info, Kid.
I use FFFg since FFFFg is pretty much impossible for me to find.

I think I just came up with a pretty decent idea. Just bolting my charge containers to the Avionics Bay.
Less wire mess in the main airframe.
Now... just to mount my Eggtimer and everything to my sled. This could be interesting.

 

[jdbwizzard]

I use pvc caps bolted to the bulk heads of my bay. When doing this I drop a small amount of clay in the cup to cover the bolt.
Another idea if limited on space is using the cardboard that igniters come in (or 1/4 in launch lugs). Tape each end with masking tape. It seems to work much better in my experience than the plastic canisters.

 

[AlphaHybrids]

I use a container that TFish pioneered - latex tubing with zip ties to close the ends. I've used it on all my flights since learning about and never had an issue with deployment (10+ years). Another bonus is that if you buy 3 feet of tubing, you can put your e-match in one end, zip tie it off and then fill from the other end. Wham! Perfect length containers for every charge. Just zip tie above the BP and cut the tubing. With 3 feet you always have the correct charge size at your fingertips. You also don't have to worry about wadding, packing, making sure everything is in contact either. Extend the zip tie 1/2" above the zip tie and the BP envelopes your e-match.

Such a simple solution, very elegant.

Edward

 

[MALBAR 70]

Another cheap and easy method for a charge holder is to cut the fingers off of Nitrile gloves. Place the E-match in the finger tube add your calculated amount of FFF or FFFF powder, pack in a small amount of dog barf and twist tightly closed around the E-match leads making a small "balloon" and secure with masking tape. After the charge is connected, I tape it securely centered and sticking up a bit on the bulkhead, to prevent a tube "blow out". I used this method for my Level 2 flight on a 3" cardboard Madcow Areobee Hi and it has worked great.

 

[Ccolvin968]

Wow. There really are a million ways to do this.
It seems like its only limited to your own imagination!
I really appreciate the replies!

 

[Nick@JET]

Those feelings of frustration just means you're learning a lot.

One thing I would not do - don't pack the snot out of the powder, barf and tape like crazy into a hard plastic charge well - creating a small pipe b0mb of sorts - this blew my payload tube to bits by the shrapnel of the charge well, then of course then backup charge won't get the laundry out either 😟. Syringes and the like no problem, they just split - no shrapnel. So in summary- pack is ok- compress into solid pellet is not. I really like the plastic doghouse (now binder design) but the smaller ones are too thin imo, I'm sure Mike will address this. I really like the Aluminum ones from MAC Performance rocketry or using the syringe method or glove tip method - very easy and can pre make many.

I've also been told don't work in .1 increments with your charges work in .25 or .5 increments. Also that laundry has to come out!! So make it come out - but shouldn't snap the harness

Here is one of my test that I felt was on the money ( you can see the harness still has some of the Z-folds)

https://youtu.be/8TwX9Dz8cQU

Sorry I didnt imbed video, I'm on my phone

 

[AlphaHybrids]

On thing that I do is I don't work in a specific unit (grams, milliliters, etc). I have these little red caps (1000's of them, they go on 1/4-20 bolts to protect the threads) that I use to measure my charges with. Everything is measured in the number of little red caps that it takes. Why? Because they are repeatable, and I always have them at the field. It also means I don't have to try and weigh BP in the field, on a table, when it is windy and conditions aren't ideal. I've seen people at our field with milligram/grain scales trying to measure 1.25 grams and it just doesn't work well. Find something that works for you and stick with it.

Edward

 

[NateLowrie]

On thing that I do is I don't work in a specific unit (grams, milliliters, etc). I have these little red caps (1000's of them, they go on 1/4-20 bolts to protect the threads) that I use to measure my charges with. Everything is measured in the number of little red caps that it takes. Why? Because they are repeatable, and I always have them at the field. It also means I don't have to try and weigh BP in the field, on a table, when it is windy and conditions aren't ideal. I've seen people at our field with milligram/grain scales trying to measure 1.25 grams and it just doesn't work well. Find something that works for you and stick with it.

Edward

I weigh mine with a gram scale in the comfort of my shop where I am assured a windless, well-lit workspace, a large sturdy workbench, and my full inventory of parts and tools. Charges are prepped and shunted by twisting the ends together. It's a far less stressful environment to prep a rocket, which means less frustration and less human error. Other than not having the powder or other materials on hand, there is no compelling reason not to pre-prep them.