Dave: The charges are in 3/4" PVC schedule pipe and will be taped on the business end. The parachute's 10' diameter and shouldn't take up that much room. The shock cord is 9/16" nylon, 15' long, 2,300 lb test. I really don't think that 4 grams is enough. I usually use 1.5 to 2 grams for 4" diameter with the same number of 2-56 shear pins and I've had no problems. My gut's telling me that 4 grams of BP is not enough. Richard > Date: Thu, 8 Dec 2011 13:21:02 -0800 > From: dmccue@xxxxxxx > To: roc-chat@xxxxxxxxxxxxx > CC: dmccue@xxxxxxx > Subject: [roc-chat] Re: BP for Deployment of Main Question > > I'm afraid I don't have a simple number to give to you, because there are > some variables that I don't know. (I haven't been following this thread > since the beginning, sorry if the info has been poseted.) My first two > questions are: > > How much of the volume is occupied by the parachute and deployment bag - > > How is (are) the BP charge confined (what contains the charge) - > > These are important to me because I want to know what volume is being > pressurised, and I want to know how quickly the BP charge will burn. > > A bagged parachute will not compress fully under the pressure spike that > developes when the charge goes off, so the volume of the compartment is > less than an empty recovery bay. This is good, since higher pressure will > result. > > The container for the BP matters because a container that offers > resistance to the gas being generated right after the BP is ignited will > make the BP burn faster. Taking advantage of his will give you higher peak > pressure. > > My BP charges are done on the cheap, by measuring the BP into a tiny > (1"x1.5" or slightly larger) zip seal poly bag. The head of the match is > positioned in the pile of BP and the zip seal is closed as far as it can > go with the match wire coming through. I then tap the BP into a bottom > corner and fold the bag at a 45 degree angle to trap the BP and matchhead. > A piece of masking tape keeps the bag folded. > > The final step is critical: I wrap the whole thing in a layer of 2" wide > aluminum foil adhesive tape. This step constrains the burning BP a tiny > fraction of a second after the match lights; the result is a good Bang! > instead of a sad Whoosh. Peak pressure is higher, and things come apart > with enthusiasm! > > Done this way, a 4 gram charge should work nicely (assuming a parachute > occupies most of the bay's volume) but you could go to 7 grams for a > Gugisberg-class deployment - :-) > > For other projects, I have fabricated BP holder cups out of copper pipe end > caps and mounted one or more of these in the bottom of a bay. I measure in > the BP, paper towel wadding for the remaining volume, then seal with the > aluminum foil tape. Works a peach! > > -Dave Mc, who always tests on the ground before flight! > > On Thu, 8 Dec 2011, Richard Dierking wrote: > > > So Dave, how much BP would you suggest for a 7.5" diameter x 24" long > > compartment with three 2-56 shear pins? > > Richard > > > > Sent from my iPhone > > > > On Dec 8, 2011, at 12:19 PM, "Dave McCue" <dmccue@xxxxxxx> wrote: > > > >> Please remember that electric match makers alway say to put matches in > >> series - it takes no more than a 1.5V AA cell to light one, so putting two > >> or three in series is no problem. THis also solves the problem of > >> monitoring two or more matches, sine a loss of continuity anywhere will be > >> detected by the electronics. > >> > >> I do series match wiring all the time when I use multiple ematches to > >> light big liquid propulsion engines. See links below: > >> > >> http://www.youtube.com/watch?v=FlkMiCtCxg0&feature=related > >> > >> http://www.youtube.com/watch?v=jOiaxfGpudM > >> > >> You can also see the unusual recovery system I came up with for these > >> rockets: side deployed! This is because the rocket is big and heavy, and > >> needs to land on it's side to avoid unplanned damage; we assume we will > >> break two fins upon landing. > >> > >> By the way, the big 1000 square-foot parachute is deployed by BP charges > >> that push out the pilot chute, with HPR recovery electronics making the > >> decisions. This was one of several low altitude flights to test the > >> recovery system. > >> > >> Dave McCue > >> > >> On Thu, 8 Dec 2011, Richard Dierking wrote: > >> > >>> > >>> One possible failure mode: If it's in parallel, if one of the matches is > >>> bad and doesn't have continuity, you will not know, because the altimeter > >>> will detect continuity in the second and indicate that everything looks > >>> OK. So one canister would go, but probably not be enough BP for a > >>> successful deployment. We tested two matches in series yesterday, and > >>> they both went fine. I've been using series for airstarts and haven't > >>> had a problem (up to 4 engines). I thought people were crazy when I > >>> first heard about using series, but when a match lights the "plasma ball" > >>> becomes very conductive for a moment. However, I have not used series > >>> for deployment charges, so thought it would be a good idea to hear from > >>> people that had experience with this. Richard > From: xsive.guy@xxxxxxxxx > >>>> Date: Thu, 8 Dec 2011 10:39:46 -0800 > >>>> Subject: [roc-chat] Re: BP for Deployment of Main Question > >>>> To: roc-chat@xxxxxxxxxxxxx > >>>> > >>>> Definitely in parallel. Otherwise, when the first one burns out the > >>>> rest would stop getting current. By wiring them in parallel even > >>>> after the first one goes the rest continue to get current. > >>>> > >>>> I'd like to know the answers to your other questions though. > >>>> > >>>> Thanks, > >>>> Mike > >>>> > >>>> On Thu, Dec 8, 2011 at 10:09 AM, Richard Dierking > >>>> <redierking@xxxxxxxxxxx> wrote: > >>>>> Say your main deployment section is 7.5" diameter, 24" long and using > >>>>> three > >>>>> (3) 2-56 nylon shear pins. How much BP? > >>>>> > >>>>> Also, what's the advantage of using two or more deployment canisters > >>>>> instead > >>>>> of just one? For multiple canisters, would you wire in series or > >>>>> parallel? > >>>>> (One 9 volt battery and Perfectflite WD altimeter with main deployment > >>>>> at > >>>>> 1100'.) > >>>>> > >>>>> Richard Dierking > >>>>> BTW: Kurt, we're not intending to turn the nose into a second stage :-) > >>>>> > >>>>> > >>>> > >>>> -- > >>>> ROC-Chat mailing list > >>>> roc-chat@xxxxxxxxxxxxx > >>>> //www.freelists.org/list/roc-chat > >>>> > >>> > >> > >> -- > >> ROC-Chat mailing list > >> roc-chat@xxxxxxxxxxxxx > >> //www.freelists.org/list/roc-chat > >> > > > > -- > > ROC-Chat mailing list > > roc-chat@xxxxxxxxxxxxx > > //www.freelists.org/list/roc-chat > > > > -- > ROC-Chat mailing list > roc-chat@xxxxxxxxxxxxx > //www.freelists.org/list/roc-chat >