How many screws?

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ActingLikeAKid

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This should be an easy one :)

Working on a Blue Tube rocket. I've attached a sled to the (solid) nose cone. Sled is plywood, holds an Eggfinder and small lipo.

I want to secure the sled/NC inside the tube (the whole top ejects, so there's a bulkhead and eyebolt below this).

I'm thinking 2 #6 screws through the shoulder of the NC, one through the bottom of the sled.
(the top 2 go into solid urethane; the bottom one into 1/4" plywood).

These will only be to secure the nose cone and eggfinder during recovery - on the way up, the top of the airframe pushes against the nose cone. During ejection and chute deployment, though, the tube will want to pull away from the nose cone, so the screws are to stop that from happening. And with a nice long shock cord, most of the energy of the ejection should be dissipated.

So the (loaded) sled and nose cone weigh about 190g. If my math is right, that means each #6 screw would be supporting 64g while gently drifting down under chute; even if it experienced 10Gs during a really hard deployment, that's still about a pound and a half of force, which a #6 screw ought to be able to hold, especially if it's only for a minute.

Thoughts? Is my math right on this?
 

Dad Man Walking

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Your math might be right, but I'm not sure I understand your design, and how the screws are loaded - shearing or tension (i.e. resisting forces that are trying to extract the screw). It sounds like the top two screws are used to connect the NC to the tube. Like shear pins only you don't want them to shear. Two #6 screws will be plenty for that. I don't understand how the sled is attached. Could you post a picture showing what you are thinking of doing?
 

ActingLikeAKid

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Yes, I can do that! :)

Here's a general layout of the rocket:
Capture5.jpg

You can see the sled and nose cone way up at the top. That's what I want to keep in place.
Here's a shot of the NC and the sled:


nc1.jpg

So to answer your question - there are actually both forces in play. Two #6 screws go through the top of the sled and into the nose cone to hold the sled in place on the way up[1]. Those would be the tension force. But for securing the sled/NC combo in the rocket, I'm planning on putting two screws through the airframe into the NC shoulder, and one into the bottom of the sled, like this (screws go where the red arrows are)
screws.png
...Does that make sense? (And yeah, I realized after I saved the pic that the bottom-right arrow is a little off, but I think you get the idea)

[1] - I wanted to make sure that these could handle the load. I drilled the same size pilot hole into 1/4" plywood and screwed a #6 machine screw in about 1/2". I grabbed it with a pair of sliding-jaw pliers and yanked. I couldn't get it out. I stopped when I was exerting enough force that if it had let go I'd probably fall over. It did not budge. I was satisfied.
 

OverTheTop

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10Gs seems a bit skinny to me. In the same situation I would use 25G as the expected acceleration.

Remember that the screws need to take the BP ejection force and the snap from the harness at full extension.

I z-fold my harnesses, and tape, to dissipate some of the energy and reduce the snap.
 

Dad Man Walking

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10Gs seems a bit skinny to me. In the same situation I would use 25G as the expected acceleration.

Remember that the screws need to take the BP ejection force and the snap from the harness at full extension...
Totally agree here...I was going to mention something along those lines. But it looks to like the OP's design, and use of #6 screws as pins through the body tube are plenty strong enough. Although we don't know how crazy they are going to get with the rocket. Screen name is "ActingLikeAKid" after all... But I'd still be willing to bet that something else is going to fail long before the attachment of the NC and sled to the body tube.
 

Tonimus

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I would do 4 screws, just because it has 4 fins.
 

ActingLikeAKid

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OK, I think I'll do 4 screws at the NC shoulder and one at the bottom of the sled (mostly because that also gives me a good place to mount my camera). Note to self - bring power screwdriver to launch ;)
 
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