I thought the rule of thumb was 1 gram per inch of body tube dia... so 7.5 grams should be fine...no ground testing needed. Kurt told me this years ago and it has worked well. Make sure you use a long shock cord too :-) Kevin Lane HVAC Supervisor UC Irvine Facilities Management krlane@xxxxxxx (949) 824-6367 Dave McCue wrote:
If 4 grams seems not to be enough, use more! :-)I agree, with a large empty volume like you describe, that will take more BP to pressureize. However, please remember that the area of your 7.5" airframe is much more than your 4" airframe. As others have said, that will give more area for the pressure to push on.I have tested on the ground by putting the rocket on it's side, with the bottom section backstopped by placing it against a wheel of my car. I put an old blanket between the rocket and the wheel to avoid breaking things. I use an old towel (or a mound of dirt) to elevate the bottom section near the joint I am going to test. The top section is now lifted just a few inches off the ground, and won't go very far when it pops off.I prep the charge and run the match wire out through one of the bay's vent holes and add enough wire to get myself safely away. Touch the wires to a 9V battery and see what happens!I think you may be right, 7 grams or so is what you need, but try it on the ground if you can...you may be surprised at what you find out!Be sure to let us all know what you did and how it worked - By the way, I use Goex FFFFg BP for my deployment work. Good luck, Dave On Thu, 8 Dec 2011, Richard Dierking wrote: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 -0800Dave: The charges are in 3/4" PVC schedule pipe and will be taped onFrom: dmccue@xxxxxxx To: roc-chat@xxxxxxxxxxxxx CC: dmccue@xxxxxxx Subject: [roc-chat] Re: BP for Deployment of Main QuestionI'm afraid I don't have a simple number to give to you, because there aresome 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 thatdevelopes when the charge goes off, so the volume of the compartment isless than an empty recovery bay. This is good, since higher pressure willresult. The container for the BP matters because a container that offersresistance 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 peakpressure. 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 cango with the match wire coming through. I then tap the BP into a bottomcorner 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" widealuminum 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 thealuminum 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" longcompartment 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=jOiaxfGpudMYou 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@xxxxxxxxxDate: Thu, 8 Dec 2011 10:39:46 -0800 Subject: [roc-chat] Re: BP for Deployment of Main Question To: roc-chat@xxxxxxxxxxxxxDefinitely in parallel. Otherwise, when the first one burns out therest 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 at1100'.) Richard DierkingBTW: 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
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