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  #1  
Old 11-29-2011, 10:51 PM
VonMises VonMises is offline
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Default Rod or Rail

At what point is a rocket too big for a launch rod?
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Old 11-29-2011, 11:00 PM
chrism chrism is offline
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Are you talking about length, width or weight?
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Old 11-30-2011, 07:02 AM
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Or engine size? They all play a part in either rod or rail..

Our club in Orangeburg has rods up to 3/4" in diameter and there are a couple members who fly their HPR rockets off this rod..
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Old 11-30-2011, 08:10 AM
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We may need to get an engineer in here to explain moments and torques. But I here is my take.

General info: When a rocket is under thrust, it is applying a vector parallel to the rod (which is obvious). There is a lever action going on (a torque), which is not that apparent at the base where it is anchored, but the effects are evident as the rocket nears the end of the rod. This is exacerbated by larger diameter rockets, since their thrust vector is further away from the rod.

As a ROT "rule of thumb", that may be helpful. Imagine a launch rod parallel with the ground and anchored firmly at one end. Note the rod position at the other end. Now put your rocket on the un-anchored end, with the lug covering the end of the rod. If the rod has fallen more than 1/20th of its length (i.e., rod without rocket vs rod with rocket), your rocket for that length may put at risk for "rod whip". That is why as rockets get bigger (usually meaning more mass) the greater the rod diameter you need to have the rod resist flexing. At some point, it is better to use a rail, either 1010 or 1515, because of their great rigidity. For some they may want a rail when a rocket has a launch mass of 2 pounds, and some will go that way at 4 pounds.

Anyway, that is an "armchair explanation" that I hope hasn't confused you.

Greg
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Old 11-30-2011, 06:31 PM
VonMises VonMises is offline
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Thanks everyone.

Last edited by VonMises : 04-21-2013 at 12:17 PM.
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Old 11-30-2011, 08:25 PM
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Quote:
Originally Posted by VonMises
Open Rocket has it at 63 inches tall and just over three pounds. Of course it’ll probably be packing a pound of my paint insanity as well.
Okay, now we're talking

You have a 3+ pound rocket, so you will very likely be flying high power which pretty much implies flying with a club. Your rocket's gonna need either 1/2" lugs (use two spaced 12" or more apart) or rail guides (two, as before). You need to figure out who you'll be flying with and see what kind of HPR support they have. If they have 1/2" rods, add lugs. If they have rails, find out what size and use those instead.

If you use rail guides, anchoring them requires some forethought - you need something solid inside the airframe for them to be anchored in, and whatever that is, it can't interfere with the recovery system - you don't want your chute getting snagged on block of wood. Anchoring them in centering rings is common, but the rings need to be a 1/2" thick or so, or have a thick spot - ie, a block of wood - built up on them.

If you try to fly this on your own, outside the purview of an organized club, keep in mind that if it exceeds 3.3 pounds (with motor, chute, etc loaded), you're supposed to get an FAA waiver.

Sorry to be so long winded. Also, if you're interested, I have some techniques for mounting rail buttons externally.

Doug

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Old 12-01-2011, 07:37 AM
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and the easiest thing for them to be screwed in to is centering rings.








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Old 12-01-2011, 08:49 PM
VonMises VonMises is offline
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Thanks Doug

Last edited by VonMises : 04-21-2013 at 12:18 PM.
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Old 12-01-2011, 09:08 PM
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Quote:
Originally Posted by VonMises
You managed to presume my next question. Is this waver a NOTAM? If so, that’s not a problem.
No. In the past, we had a step below a waiver called "FAA notification". We didn't need permission, but needed to advise them of our intent to fly a "large model rocket", a class which has been eliminated. (It was between 1 and 3.3 pounds or had more than 113g of propellant but less than 125.) When FAA notification was made, the FAA would issue a NOTAM. (For that matter, they issue a NOTAM when they grant a waiver, too.)

Anyway, to get a waiver, there's a fair amount of paperwork to do. It's best to just fly with a club that already has a waiver. Later, if you want to get your own waiver, your best bet will be someone in that club who already knows the ropes in that region. They can advise you on the details of dealing with the local FAA officials.

HTH.

Doug

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Old 12-01-2011, 09:21 PM
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Quote:
Originally Posted by VonMises
Indeed I am Doug. I’m never too proud to learn, and certainly never to proud to think I can’t learn something from everybody.
As pointed out herein, one good way to anchor the rail buttons is in the edge of the centering rings. But this requires the rings to be thick enough to not split - eg, ~1/2" thick. And it requires fairly accurate location of the rings - if you're drilling into the side of the rocket into a ring buried inside and no longer accessible, you need to know exactly where it is lest you should miss it partially (and make a mess). But if the rocket's already built, it may be too late to determine the exact location.

Or, in my case, I simply didn't want to mess with all that tedium so I came up with this. I built up a spot on the outside of the airframe, screwed in a threaded insert, and can then attach the buttons to that using an 8-32 screw (IIRC).



In this case, I used some 1/4" aircraft ply, contoured it to the airframe via sanding, then glued it in place. The idea was to get a decent sized patch to ensure the load is spread out a bit - otherwise it could tear loose from the surface of the airframe. A little sanding and filler help to fair it in nicely.

HTH.

Doug

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