- This topic has 36 replies, 5 voices, and was last updated 19 years, 1 month ago by
Bruce R. Schaefer.
Bruce R. Schaefer.-
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Doug Gerrard
Actually when I ground test I do a net/catch assembly with a large blanket or 2
Absolutely, always rig something to catch the potential projectile. Many ask a couple friends to hold the blanket in front of the rocket. That works well as long as their life insurance is paid up…
Doug
Conway Stevens
Actually when I ground test I do a net/catch assembly with a large blanket or 2
Absolutely, always rig something to catch the potential projectile. Many ask a couple friends to hold the blanket in front of the rocket. That works well as long as their life insurance is paid up…
Doug
Doug, That was SO funny. I couldnt stop laughing… I had some pretty funny images of friends in full football or hocky gear come to mind trying to catch the parts. Like hey Dave Go long.. Then all the sudden flash.. A nosecone is sticking through Daves mid section (kinda like the Arrow through the head gag) and he says to me that he thinks he will go home now… ROFLMAO.. sorry guess ya gotta be there.. LOL
Bruce R. Schaefer
I’ll ask my wife to hold the blanket, and our dog can hold the other end. I know hers is paid up. No, won’t work, she’ll have to hold both somehow, our dog hates loud noises. 🙂 Conway, I think you said you had calculated about 5 grams as an optimum charge to start with for yours. There are online calculators, but they always need to know the pressure needed to blow things apart. Is there a rule of thumb on calculating the pressure needed per volume, or body tube size, or anything else? I was going to start with 4 grams… I’ve used the empirical method, just took a guess, then went up or down from there depending on what happened during the initial test. Now, for a 5″ diameter G10 FB tube with 24″ effective length inside (two 6″ couplers take 12″ away from the 36″ tube), giving a volume of 150 sq. inches. I’m starting with (4) 2-56 screws, with the option of drilling larger for (4) 4-40. I love fiberglass! 😉
Conway Stevens
Using one of the calculators as a guide. Start there and use a charge for that appropiate space. See how weak or strong it is when you test it. Then make adjustments from that point. Lets say it shears the pins and blows it out with auhtority. Maybe you will try a little less and see what happens. Maybe a little less is to weak. well you now know you need to go back up. Im a firm beliver that a more aggresive charge is always better then a weaker charge. At least then you know its coming out. Then be sure to use a long harness to absorb the energy from that charge and your good to go. I normally go a little bit big.. ok maybe a fair amount bigger then what some maybe do. but I have yet “knock on wood” to ever not deploy. I am willing to bet that when I am done testeing my set ups I am close to 5 to 7 grams per event.. leaning more towards the larger myself.
Doug Gerrard
Bruce, if you haven’t check out the video I put on my site you should look at it before you test. I have 24″ of 6″ tube and the online calculators said I needed at least 3 grams just to shear the four 4-40 screws, I decided to start with 2 grams and practically put a hole in the wall. Well it was a Styrofoam wall but it was MORE than adequate to do the trick, hence the posting on the forum. The calculators are overkill. Think about it this way, if you ground test and your charge is too small, so what? You might be embarrassed by your lack of performance but if it is too large you could impale you catcher and you might have to do some explaining.
Doug
Bruce R. Schaefer
Doug, I’m going to download the video at work tomorrow. I’m still only dial-up here at home. 😳 Listen, my video of me destroying my test stand and upper section of a 3″ rocket with 2 GRAMS is a lesson I better remember. I’ll start with 2 just to be safe. I know 1.5 grams won’t do it, but higher… ? Better be safe than sorry. Ken Plattner, if you read this, you wanna hold the blanket? 🙂 Actually, my launch system is automatic. I can set it for any length of time and get an audio countdown so we both could hold it. 🙄 Got good insurance? I don’t think home owners covers this. ❓ Conway, I agree, most say it’s better to go a little higher than sufficient with a loooonnnnnng harness. The ol’ NAR axiom: blow it apart or blow it up. 😉
Ken Plattner
Bruce, I’d love to help. I’m going to be out all next week through the weekend, but anytime after I get back will work for me. Just let me know.
Ken.
Bruce R. Schaefer
You’re a brave man, Ken. I won’t be doing any ground testing until after Oktoberfest most likely. But I would really like your help in keeping things from blasing through fences. 🙂 I’m saving the electronics installation, ground testing, etc. for the cooler months. Thanks, and I’ll let you now when the time comes.
Doug Gerrard
oops, I did forget to mention that I use a piston. That will affect the charge required, all the more reason to do ground testing. Just thought you should know.
Doug
Conway Stevens
Ahhh. had me worried that i had been doing something wrong there doug. I was like how much.. wow I did that little and nothing really happens. LOL.. Yes with a piston you can use moderatly less. I on the otherhand am not using a piston. So I will need more. Thanks Doug
Doug Gerrard
Conway Stevens
Bruce R. Schaefer
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