Few questions about chute ejection systems and body modifications

Hey, folks.

I'm well into building my first high power rocket for a level 1 and 2 certification and I've run across two things that bug me. On my plans I've designed, I'd be using a CD3 CO2 ejection system for the chute and access ports on the body to the E-Bay and other various zones. So:

The CO2 system seems like the red tubular part which contains the gun mechanism and puncturing pin seems to be mounted on the outside of the bulkhead, facing down. This bothers me a little bit because it seems like that causes a risk of accidental puncture. Can anyone here comment on that and ease my concerns?

Also - How would you recommend cutting out portions of the aeroframe to allow for internal access that allows for the cut-out portion to be used as a cover plate? The issue isn't so much mounting the plate back in but cutting it out smoothly without more advanced workshop tools. Ensuring a clean and accurate cutout. Would you recommend just carefully cutting it out with an good sharp hobby knife along an outline?

I can't attest to the use of the CD3 other than it seems to be a bit over complicated for a cert attempt... however I did a 54mm minimum diameter with dual deploy for both my L1&2 so I shouldn't really be talking...

As for the access port, why would you cut a hole in the airframe when you should actually just make it so that you can disarm your altimeters and take the rocket apart (I am assuming that is what you want the access port for)

I believe it would take some force to puncture the cartridge. Maybe you can use a small spring that wont affect the puncture pin and at the same time keep it from hitting the cartridge while face up.

junkyard said:

I can't attest to the use of the CD3 other than it seems to be a bit over complicated for a cert attempt... however I did a 54mm minimum diameter with dual deploy for both my L1&2 so I shouldn't really be talking...

As for the access port, why would you cut a hole in the airframe when you should actually just make it so that you can disarm your altimeters and take the rocket apart (I am assuming that is what you want the access port for)

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I agree on the cert complexity argument. I'm primarily doing it for the experience gained from such systems. Additionally, I already had purchased it and had it laying around - given that it's intrinsically a safer alternative to a pure BP/SP charge for ejection and has a lower chance of heat related... complications..., it seemed like a good thing to toss in there. Plus, the same rocket will be used for my level 2 and that does require an electronic deploy if I remember correctly. Can't be a bad thing to already have it set up for it!

As for the cover plate idea, that was for two reasons. One: aesthetics. Pure and simple. Two: Again, experience. Such ports can be useful for later rockets which are larger and/or more complex that I don't necessarily want to have to disassemble just to reset altimeters or tweak internal electronics. Granted, for smaller rockets it's easier to just split it apart and take off a bulkhead.

SinfulDarkLord said:

I believe it would take some force to puncture the cartridge. Maybe you can use a small spring that wont affect the puncture pin and at the same time keep it from hitting the cartridge while face up.

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Fair enough. I had considered perhaps using a small rubber o-ring that would offset the difference between the pin and the canister's top by about 1.5mm. Given the force exerted by the SP/BP charge, it it might be a viable alternative to a spring. Springs bug me a little bit because I feel that it might cut down too much of the momentum on the puncture pin.

I feel that even on large rockets an access panel is largely unnecessary... just extra complexity that isn't needed except in very specific cases.

And L2 certs don't require electronic deployment, only L3 requires dual deploy (and i think for TRA you are required to have launched previously with dual deploy)

And if you do add a spring or an o - ring to the pin make sure you ground test it to make sure that the BP charge can still overcome the force required to puncture the cartrige

junkyard said:

I feel that even on large rockets an access panel is largely unnecessary... just extra complexity that isn't needed except in very specific cases.

And L2 certs don't require electronic deployment, only L3 requires dual deploy (and i think for TRA you are required to have launched previously with dual deploy)

And if you do add a spring or an o - ring to the pin make sure you ground test it to make sure that the BP charge can still overcome the force required to puncture the cartrige

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I stand corrected on the L2 electronic deployment requirement. Still - it's never a bad idea to develop that knowledge earlier than needed.

As for ground testing - isn't that kind of the general rule? Always ground test your ejection setup? I live in an apartment complex - I'm far more likely able to test a CO2 system than a pure pyro setup.

Suggestions...

Slow down. First high power rocket, and already in a hurry to do an L2 with it. In a hurry to spend a couple hundred $$ per flight on L's?

There's no way in H-E-double toothpicks you're going to pull enough G's to cause that CD3 to pierce a cartridge. Don't change it, don't put springs in it, don't modify it. Read and follow the directions carefully. If you're going to use it, use it as designed. It works.

The KISS principle works, too. Most folks cert using motor eject. Get the autograph first, then do the trickier stuff on less "critical" flights.

Just a couple things to consider.

cherokeej said:

Suggestions...

Slow down. First high power rocket, and already in a hurry to do an L2 with it. In a hurry to spend a couple hundred $$ per flight on L's?

There's no way in H-E-double toothpicks you're going to pull enough G's to cause that CD3 to pierce a cartridge. Don't change it, don't put springs in it, don't modify it. Read and follow the directions carefully. If you're going to use it, use it as designed. It works.

The KISS principle works, too. Most folks cert using motor eject. Get the autograph first, then do the trickier stuff on less "critical" flights.

Just a couple things to consider.

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I'm not in any hurry to do a level 2. I'll do it at a later date. Like I said before, I'm looking to learn the stuff that I will find useful for later level 1 and 2 rockets.

The CD3 didn't come with ANY directions and the directions on their website seem to refer to a different model of CD3. Motor ejection is not something that I a: am able to test and b: am willing to mess with as far as drilling for timing goes on a custom build. I'm not too concerned with throwing away a little cash by doing something trickier than most other people might do. It may be my first scratch built HPR, but it's not my first introduction into rocket construction or theory. I'm an engineering student - complexity is not only fun for me but challenging. I simply don't see much of a challenge in doing what everyone else does. Yeah, it might cost me a couple flights before a certification, maybe, but that's alright. This is one rocket. I'm building two - if this one goes ballistic, I've got a backup that is much more simple (and flies lower might I add). Granted, acquiring a replacement motor might be an issue though.

Anyway, thanks for your suggestions, they're noted and appreciated. I'll just fly the CD3 without anything changed about it if you say it works. It's just that my education is telling me that it seems kind of reckless to have a sharp steel pin resting on the thinnest part of a CO2 canister. That feeling will likely go away after the first flight with it.

Thanks again, folks. The bay and ejection system is done - just waiting on my BT-80 tube to come in so that I can get the main body cut and coated.

That's not how I read your first post. Sorry.

The reason people use KISS is because it works. For your cert, you will be as nervous as a one legged man signing up for a butt kicking contest. I've been signing certs since the '90's, and everyone is nervous for their cert. It can make them... Forgetful. Using a well thought out and prepared check list will help with that. Especially with a project of some complexity. Highly recommended.

90% of cert failures are recovery related. Fail to open and destabilize the airframe, and it comes back nose first, at high speed. Cuz gravity wins every time. Failing a Level 1 cert flight is no big deal, unless it goes thru the roof of an automobile on the flight line, or hits a six year old. Then it's a big deal. And you won't be alone on the field for your cert flight.

Someone remind me to razz Tom for not putting instructions with that CD3 kit.

The o-ring on the piston will hold the piston against G forces. You'll feel the resistance when you install the piston into the bore of the actuator. It won't pierce the cylinder until the device fires. I've pulled 86.3 G's with a CD3 installed, and it worked perfectly.

And don't over-do it with powder in there. The powder cavity is sealed by the o-rings on the piston and match holder, so it only takes a little bit. We had an individual prep one with a full gram of 4F in it once. Thank God we checked it before we flew. Would have been a grenade.

Good luck, and be safe.

cherokeej said:

That's not how I read your first post. Sorry.

The reason people use KISS is because it works. For your cert, you will be as nervous as a one legged man signing up for a butt kicking contest. I've been signing certs since the '90's, and everyone is nervous for their cert. It can make them... Forgetful. Using a well thought out and prepared check list will help with that. Especially with a project of some complexity. Highly recommended.

90% of cert failures are recovery related. Fail to open and destabilize the airframe, and it comes back nose first, at high speed. Cuz gravity wins every time. Failing a Level 1 cert flight is no big deal, unless it goes thru the roof of an automobile on the flight line, or hits a six year old. Then it's a big deal. And you won't be alone on the field for your cert flight.

Someone remind me to razz Tom for not putting instructions with that CD3 kit.

The o-ring on the piston will hold the piston against G forces. You'll feel the resistance when you install the piston into the bore of the actuator. It won't pierce the cylinder until the device fires. I've pulled 86.3 G's with a CD3 installed, and it worked perfectly.

And don't over-do it with powder in there. The powder cavity is sealed by the o-rings on the piston and match holder, so it only takes a little bit. We had an individual prep one with a full gram of 4F in it once. Thank God we checked it before we flew. Would have been a grenade.

Good luck, and be safe.

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Thank you very much for your response. I managed to find someone at my college that has used the CD3 and he basically said exactly what you just said. One question though: I found that when putting the O-Ring onto the piston, I literally could not get it into the bore. It was so tight that I'd essentially have to end up hammering it in - I'm pretty sure that's not supposed to happen. What would you recommend? a bit of grease perhaps?

Phoenix1583 said:

Thank you very much for your response. I managed to find someone at my college that has used the CD3 and he basically said exactly what you just said. One question though: I found that when putting the O-Ring onto the piston, I literally could not get it into the bore. It was so tight that I'd essentially have to end up hammering it in - I'm pretty sure that's not supposed to happen. What would you recommend? a bit of grease perhaps?

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Yes, grease the o-ring with some Dow 111. Same stuff used on motor o-rings.

The e-matches I use have a little silicone rubber sleeve on them that fits inside the match holder. I use a tiny dab of grease on there too, to help seal the match holder.

cherokeej said:

Yes, grease the o-ring with some Dow 111. Same stuff used on motor o-rings.

The e-matches I use have a little silicone rubber sleeve on them that fits inside the match holder. I use a tiny dab of grease on there too, to help seal the match holder.

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thanks! yeah this thing needs to come with a manual.