Reliable DD Ejection

[Budro0]

I've got a question that's been on my mind since my last launch. I had a couple of less than stellar recoveries on a couple new rockets. I'm not unfamiliar with DD, but I'm no expert. All the airframes were under 2.6" and so I built them with smaller charge wells. The smallest was 5/8" x 1 1/4". On every flight of those rockets, one event or the other failed to cause separation (fortunately, the other did). But it wasn't consistent though - the drogue didn't fire on one flight and then the main didn't on the next. Each time, both charges did deploy (and redundant charges where I had them).

My thought is that my containment isn't long enough to get a reliable burn on the BP. The .6 g charges fill the container over 3/4. I'm also thinking that maybe the fit of the components is too loose. One is a WM Punisher SS and the nosecone wobbles without shearpins.

I was able to get consistent and energetic results in the backyard during testing so I'm not real sure what I might be doing wrong. And the other rockets I flew that weekend had good results with larger/longer charge wells.

Has anybody else had a similar issue?

 

[Antares JS]

Are you using 4F powder? Are you packing the unused volume of your charge wells with dog barf or wadding or something?

 

[Budro0]

Are you using 4F powder? Are you packing the unused volume of your charge wells with dog barf or wadding or something?

Correct - 4f and dog barf with tape over top. I had good luck with this with other rockets that weekend and before. But these couple little ones kinda gave me hell.

 

[David Schwantz]

Is the powder on top of the e match? Or do you put the e match in last, on top of the powder?

 

[TheTank]

Correct - 4f and dog barf with tape over top. I had good luck with this with other rockets that weekend and before. But these couple little ones kinda gave me hell.

Sounds like you have a gas leak. Ground tests went ok?

 

[Budro0]

Is the powder on top of the e match? Or do you put the e match in last, on top of the powder?

On top

 

[Budro0]

Sounds like you have a gas leak. Ground tests went ok?

I had some snaps with no shearing of pins, but continued to up the amount until I got consistent results

 

[David Schwantz]

Well that video showed no deployment.

 

[Budro0]

Well that video showed no deployment.

Fair. Just trying to highlight the snap - this is basically what happened in flight on it's last flight too.

I settled on .5g for the main on this HEDD rocket. Which seemed hot to me, but the video above was around .3g and I could not get it to shear pins. Video below is .5g

 

[OverTheTop]

What is the volume of the compartment the charges are blowing in to please? Just want to check what sort of charge I would use for starting to evaluate the ejection.

 

[maddmaxx11]

I found on one of my fiberglass rockets that the gas would leak so I put masking tape around the coupler/eBay and nose cone so that it was a tight fit even though I am using a shear pin. Ground test so that you get it to separate good.

 

[Budro0]

What is the volume of the compartment the charges are blowing in to please? Just want to check what sort of charge I would use for starting to evaluate the ejection.

Attaching my calcs for the main and drogue. I did the math to figure out the volume of nosecone and adjusted the tube length to match for the main. It's a 5:1 cone if you'd like to relook it.

 

[Budro0]

I found on one of my fiberglass rockets that the gas would leak so I put making tape around the coupler/eBay and nose cone so that it was a tight fit even though I am using a shear pin. Ground test so that you get it to separate good.

This is what I was thinking, and I may try it on the next flight.

 

[David Schwantz]

Second video was perfect. On first video, I saw a puff of smoke, so your fit may be too loose.

 

[OverTheTop]

Attaching my calcs for the main and drogue. I did the math to figure out the volume of nosecone and adjusted the tube length to match for the main. It's a 5:1 cone if you'd like to relook it.

Given the volumes you gave there, I get 0.12g and 0.3g as starting points for my workup of charges. So you are in the right ballpark anyway. Just need to finesse it for your actual setup, as you have been doing.

 

[SDramstad]

I see a lot of people agonizing over their charge size. I did at first too. Here are a few things I have learned. 1) Once you find the minimum charge amount that gives a clean separation, never use that amount!!! With clean couplers and perfect conditions it will work. After the first flight you will never have a perfectly clean coupler or perfect conditions ever again. Any thing that can happen to hinder the separation will result in a ballistic recovery. Every time!
2) Dont sweat the exact charge size. I never even try to measure out .257 grams of powder because thats what the calculation calls for. On a 2.6 inch fiberglass airframe I always round up the charge size. I will use .5, .75, 1.0 or 1.5 gram charges. Always round it up a bit. And even more for the back up charges. My 2.6 inch rockets usually get 1.5 gram charges. Since I started doing that I have not had another separation failure.
3) There is a big difference between blow it out and blow it up. I have never damaged a rocket yet with a too big charge.
4) Pour the powder into the cup. Place the lighter on top of the pile of powder, never on the bottom. Pack in dog barf all around the ematch and pack it in tight then tape the crap out of it. You can not use too much tape. An ematch on the bottom of the cup will blow a large percentage of the powder out of the cup before it burns.

I could probably keep going but thats my main rules. When I break these rules, I break rockets.

 

[waltr]

Good advice SDramstad.
I too found when I started doing DD that ground testing worked fine but not in the air with the same amount of BP.
I now up the charge a bit from ground test amount.
Also use lots of tape over and around the charge well. The better the BP is confined the quicker the pressure builds. It is this quick pressure build up that blows the nose off. If pressure builds too slow then it all leaks out without moving the nose.

Only time I 'blow it up' was with a Estes BT55 BT using a shear pin. This was totally experimental so not a real surprise.
With LOC cardboard BT's I've never had a 'blow it up' issue tubes and with shear pins tearing the tube.

 

[tywain]

My 2.6 fiberglass wildman rockets all run at least 1.5 with 2 shear pins. It seems like the estimate equations are a little off on the smaller rockets in my opinion.

 

[DerRedMax]

Attaching my calcs for the main and drogue. I did the math to figure out the volume of nosecone and adjusted the tube length to match for the main. It's a 5:1 cone if you'd like to relook it.

What BP Calculation model are you using for your ejection charges?

 

[Donnager]

For small charges, it may be easier to use a glove fingertip or a piece of surgical tubing rather than a charge well to contain the powder.

 

[waltr]

For small charges, it may be easier to use a glove fingertip or a piece of surgical tubing rather than a charge well to contain the powder.

Yes, I use a 2x2" piece of paper rolled around the Ematch red cap and taped on. Add measured volumn of BP fold over and tape sereval times. This works great for charges up to about 0.8gram BP. More BP than that and all the BP does not light resulting in unreliable ejection.

 

[Zeus-cat]

I use 2ml centrifuge tubes instead of charge wells. Cut the very bottom off the tube and then insert the wires of an e-match down into the tube with the wires coming out the hole. Hot glue the bottom closed and then fill the tube with the appropriate charge and fill with dog barf.

 

[Voyager1]

I use 2ml centrifuge tubes instead of charge wells. Cut the very bottom off the tube and then insert the wires of an e-match down into the tube with the wires coming out the hole. Hot glue the bottom closed and then fill the tube with the appropriate charge and fill with dog barf.

Yep!
Makes it easy to prep the avbay before the launch session. Just add the BP, dog barf and tape onsite.