Vent Hole and/or Shear Pins

[neond7]

I'll be attempting my first dual deploy soon - I'm about to install shear pins for the first time on my LOC Fantom. If I use shear pins, do I still need to drill a vent hole in the upper payload bay? Or do you want that extra pressure to help shear the pins when the main charge fires? With the motors I'll be using, it should be reaching 3 to 5k in altitude. Mostly in the 3K range.

I'll be using 3 of the nylon 2-56 pins. Still trying to figure out the amount of BP that I will need. I'll ground test when I get the shims installed and acquire some BP.

[cwbullet]

Do you have excel?

[neond7]

Do you have excel?

Yes, I have Excel. Or did you mean the EXL version of the Fantom? Yes on that one, too. ;-)

[atxcple]

Ejection_ChargeCalc_L3_project.xls

here is a good tool for getting close to your charge sizes

[Polyalcoholic]

Here's a useful calculator...
http://www.info-central.org/?article=303

another...
http://www.rimworld.com/nassarocketr...alc/index.html

[neond7]

Thank you all for providing those links & spreadsheets!

Now the question is, how many PSI do I need? Its a 4" payload bay, about 16 inches long.

Using the online calculator, assuming ~15psi, it recommends 1.56g of BP. It also recommends 5 shear pins. Doesn't that seems excessive for a 4" tube?? In practice, how many sheer pins have people been using for similar sized tubes? I was planning on using 3 pins, maybe 4 if recommended, but 5 pins just seems crazy......

[dixontj93060]

The spreadsheet is telling you that it takes five shear pins to hold the 15psi generated. Less than 5, i.e., your three, would be sheared.

Thank you all for providing those links & spreadsheets!

Now the question is, how many PSI do I need? Its a 4" payload bay, about 16 inches long.

Using the online calculator, assuming ~15psi, it recommends 1.56g of BP. It also recommends 5 shear pins. Doesn't that seems excessive for a 4" tube?? In practice, how many sheer pins have people been using for similar sized tubes? I was planning on using 3 pins, maybe 4 if recommended, but 5 pins just seems crazy......

[Polyalcoholic]

Remember that the tube length used for the calculation is the effective tube length. Volume occupied by avbay and nose cone shoulder should be subtracted.
i.e. If the avbay coupler and the nose cone shoulder each extend 4" into your 16" payload section, the effective length is now 8"

Recovery items and wadding/dog barf will reduce the volume even more.

[GregGleason]

It takes about 64.24 lbs of force to break 3 2-56 nylon shear pins (according to http://www.rocketmaterials.org/datas...Pins/index.php). IMHO, all you need are 3 x 2-56 pins.

One calculation tool I have says that you need 0.53 grams of BP to get 5.1 psi to break the pins. I like a 1.5x margin, so I would start with 0.80 grams of BP. That would be 7.7 psi and 96.5 lbs of force.

Regardless, I would do at least 2 ground tests.

Greg

[atxcple]

and no matter what the calculators say GROUND TEST... GROUND TEST... GROUND TEST...

[Salvage-1]

+1 on ground test with all items loaded, ie Chet, protector, etc. Don't forget chute protection and expect first test NOT to shear pins.
Remember that when a test doesn't shear the pins, you may end up with embers in the tube. Dissemble and shake everything out well before retesting.
Once you have a charge that shears the pins, retest that charge load at least once more.

[Salvage-1]

Answering vent question, on my rockets, all tubes have vent holes, even ones with shear pins.

[jd2cylman]

Answering vent question, on my rockets, all tubes have vent holes, even ones with shear pins.

Do you really need vent holes if using shear pins? I would think it would take a huge altitude to shear the pins, yes? As long as you stay under, say, 20,000 ft. wouldn't you be OK?

Adrian

[cwbullet]

If you use shear pins you probably do not need the vent holes to prevent premature separation from tube pressurization, but it is nice to have to make it easier to remove your motor and separate parts.

The vacuum effect makes this difficult.

[neond7]

It takes about 64.24 lbs of force to break 3 2-56 nylon shear pins (according to http://www.rocketmaterials.org/datas...Pins/index.php). IMHO, all you need are 3 x 2-56 pins.

One calculation tool I have says that you need 0.53 grams of BP to get 5.1 psi to break the pins. I like a 1.5x margin, so I would start with 0.80 grams of BP. That would be 7.7 psi and 96.5 lbs of force.

Regardless, I would do at least 2 ground tests.

Greg

Thank you, that is the exact answer I was looking for. :-) I drove around the county all day visiting various gun stores, I only found one selling BP, but it was only FFFG. I gave up and ordered two pounds of FFFFG online from Grafs.com - and OUCH on the hazmat fee!!!!!!

I've got two DD rockets pretty much built and ready to go (LOC Fantom and Binder Excel). Picked up some .010 brass shim material yesterday so I just have to install that in the nosecones. I also have one rocket in the box waiting to be built (Mad Cow Frenzy), and Binder Design is currently custom cutting a set of fins and slotting a blue tube for my custom Polaris upscale project. So I'll soon have four DD rockets all using BP..... hope the two pounds last me for awhile!!

I'm in class all this week, with an all day FITSP-A exam on Friday so my stress level will be up and available rocket play time will be low. I doubt the BP will arrive this week anyway, so ground testing is out before my L2 cert attempt next Saturday. So it looks like I'm back to my original plan of using my LOC Bruiser, motor ejection. In a way, that is probably the best, keep it simple for the cert and save the dual deployment for a later day after its properly ground tested. Nobody likes an L2CADDLD. (L2 Cert Attempt Dual Deploy Lawn Dart).

So, for testing, I'll use .8g on the Fantom and since the Excel payload bay is longer I'll probably use 1g.

Thanks everybody for the help! The online calculators worked great, I just wasn't sure of the PSI I was aiming for with my setup...... Math is NOT my best subject.

[Salvage-1]

FFFg can be used to. It provides a little less pressure than the same amount of FFFFg because it burns slightly slower. If you can find 3Fg locally then get it and save on the hazmat.

Do NOT ever try and grind or press 3F into 4F, things can go wrong there. If you use 3F, add about 5% more than you would if you used 4F.

[neond7]

FFFg can be used to. It provides a little less pressure than the same amount of FFFFg because it burns slightly slower. If you can find 3Fg locally then get it and save on the hazmat.

Do NOT ever try and grind or press 3F into 4F, things can go wrong there. If you use 3F, add about 5% more than you would if you used 4F.

Good to know I can use 3F if I needed to. The 4F is already ordered, so in a week or two when it arrives I can start testing. :-)