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sled without allthread rods

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blackwing94

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I'm getting ready to build my first sled out of 1/4 birch plywood for a 4 inch fiberglass rocket. The AV-bay is 9 inches long. Do I need allthread bolts running the length of the bay or can I just connect either end of the sled to the U-bolts coming through the bulkheads? In other words, the only thing traversing the length of the AV-bay would be the sled. That way I could include a transmitter on the sled and not worry about metal rods disrupting tracking.

Is there a overriding reason (like maybe strength?) to run bolts the length of the sled other than it makes thing easier?
 

cavecentral

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How are you securing the av bay so the lids do not pull off? If that part is covered, you are good. All thread is an easy way to do this.
 

Zeus-cat

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As cavecentral alluded to, the allthreads are a structural part of the avbay. They keep the avbay in one piece. There may be other ways to do this, but allthread rods are easy to use and they work well.
 

ksaves2

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You can get nylon allthread.

Disclaimer: never used it, don't know the strutural specifics. http://www.usplastic.com/catalog/item.aspx?sku=91844&gclid=CMSBoojn2s8CFQcmhgod284JmQ
I've seen an ebay rip through when using metal all-thread. Flier should have used four pieces instead of two or thicker all-thread. It was a very large project. I'd be leery of nylon all thread and would never use it myself.
Ok, perhaps a modroc with an elastic arrangement on the harness. Kurt
 

blackwing94

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I got the idea from the pictures on this web page. Scroll down. I do not work for or know the owner of the site.
http://onebadhawk.com/stainless-steel-u--bolts.html
The 6th e-bay picture... Connect the U-bolt to an L bracket on the sled. Leave off the all-thread.
I would also key the bulkplate so it only fits on the AV-tube in one place to reduce any risk of spinning.

Again, my question, and ksaves2 may have answered it, do you need all-thread bolts for anything other than ease of assembly?
 

GregGleason

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The all-thread is providing a load path from the av bay to the airframe. If the separation event is 10 Gs for the part that is separating from the av bay, and that part weighs 5 lbs, then your av bay should be able to withstand a shock load of 50 lbs. So whatever configuration you pick needs to be able to have that factored in.

Do you have a pic or a drawing? We might be able to see if there are any issues if you can post it.

Greg
 

Bat-mite

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I got the idea from the pictures on this web page. Scroll down. I do not work for or know the owner of the site.
http://onebadhawk.com/stainless-steel-u--bolts.html
The 6th e-bay picture... Connect the U-bolt to an L bracket on the sled. Leave off the all-thread.
I would also key the bulkplate so it only fits on the AV-tube in one place to reduce any risk of spinning.

Again, my question, and ksaves2 may have answered it, do you need all-thread bolts for anything other than ease of assembly?
That's Ted Chernok. He is a frequent poser here ... excuse me, I meant poster! :grin: He is a level 3 flier and knows his stuff. You may wish to PM him for tips and advice. onebadhawk
 

kjkcolorado

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To try to answer your last question regarding the need for all-thread other than ease of assembly. Maybe.... if you are attaching your recovery harness to your AV-bay, you must have a construction technique/material able to withstand the forces of a less than optimal separation event. For a 4" diameter fiberglass rocket, those forces can be very high. I will let those with more experience (and a better understanding of the math to calculate those forces) add to this most important point. If you have a 1/4" piece of plywood as your sled with attachment points on either end, and no additional structural support for the tensile force, I am confident that even an optimal separation would tear that sled in half. Or, more likely, rip the bolts holding the L-brackets right off the end of the plywood.
 

kjkcolorado

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Of course, while I was typing, a couple of those with more experience already posted.... thanks Greg and John!
 

NateLowrie

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As an alternative to allthread, you could attach the sled directly to the bulkheads and use the sled as the coupler. Make sure that you properly design everything for the expected forces involved.

Another option is to use the airframe to transfer the load. If you attach both bulkheads with bolt circles you are essentially using the airframe to trasnfer the load.
 

dixontj93060

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There are many ways to design an av-bay to maximize space internally. For instance on some of my minimum diameter rockets I just glue coupler rods on the inside of the tube. Two is usually good enough to hold the bulkhead on with some pan head screws. For connecting the bay to the recovery gear and to transfer force, often I will just use a loop of Kevlar threaded through brass or cardboard tubing (also epoxied to inside tube typically 90 degrees from coupler nut/screw position).
 

tfish

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I'm getting ready to build my first sled out of 1/4 birch plywood for a 4 inch fiberglass rocket. The AV-bay is 9 inches long. Do I need allthread bolts running the length of the bay or can I just connect either end of the sled to the U-bolts coming through the bulkheads? In other words, the only thing traversing the length of the AV-bay would be the sled. That way I could include a transmitter on the sled and not worry about metal rods disrupting tracking.

Is there a overriding reason (like maybe strength?) to run bolts the length of the sled other than it makes thing easier?
A few questions. How high do you plan on flying? What sort of stuff can your rocket get lost in at your flying fields? When you say "transmitter", what are you actually going to use. Have you done any range testing with your transmitter with and with out metal objects (allthread) along side it?

If your flying J motors in a 4" rocket to 8K you might not need to worry about it being near allthread. On the other hand if your going to 40K you might want to rethink putting it along side allthread.

And the allthread in electronics bays... is an easy and quick way to transfer the load through the ebay... and easy to make anchor points for shock cord to..and also an easy way to make the end caps removable.

Tony

Tony
 

OverTheTop

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Have you considered using bike spokes for the tenstion members? You can get them made at many bike shops, to custom lengths. Thread is imperial 2-56. You could use spoke nipples that are used for bike wheels, or you can get some 2-56 taps and make your own end attachments.

The advantage of bike spokes is that the threads are rolled and not cut. That work hardens them making them stronger. If you cut the threads with a die it will create a stress raiser at the thread root which weakens the part.

They are normally made from stainless steel. Another option you get is that for little $ extra you can get titainium spokes :cool: They have a calculated breaking load of 280kg each!


I have a sled in my Veliciraptor that uses three rods from end to end in the avionics coupler, and fiberglass endplates. I used M4 allthreads for that one:
http://www.ausrocketry.com/forum/viewtopic.php?f=6&t=1782&hilit=velociraptor&start=66
Note that the allthread as covered with heatshrink to stop it shorting anything out. The sled is just held captive by compression between the two end plates.
If I did it again I would probably use the bike spoke idea.
 

blackwing94

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OK, I'm new and stupid. kjkcolorado (and others) said the magic words that clicked by brain into gear. A sled is not strong enough to be the only structural connection between the top half and the bottom half of a 10 lb rocket. I see that now. :facepalm:

This is my L2 rocket. I'm filling the bay with eggtimer products (I'm learning to solder). Just finished building a Eggtimer classic (for practice), Quark, Wifi switch, and a Quantum. So far everything works. Most of that goes in the AV-bay. Next I'll build the TX kit and receiver kit. I guess I'll put the TX kit in the nose cone. Will my rocket get lost? Probably not. Will it be cool to see the GPS coordinates of where it landed (even though I can see it sitting in the field over there...), YES!!

Bike spokes.... Fiberglass rods... Interesting ideas.
 

rharshberger

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OK, I'm new and stupid. kjkcolorado (and others) said the magic words that clicked by brain into gear. A sled is not strong enough to be the only structural connection between the top half and the bottom half of a 10 lb rocket. I see that now. :facepalm:

This is my L2 rocket. I'm filling the bay with eggtimer products (I'm learning to solder). Just finished building a Eggtimer classic (for practice), Quark, Wifi switch, and a Quantum. So far everything works. Most of that goes in the AV-bay. Next I'll build the TX kit and receiver kit. I guess I'll put the TX kit in the nose cone. Will my rocket get lost? Probably not. Will it be cool to see the GPS coordinates of where it landed (even though I can see it sitting in the field over there...), YES!!

Bike spokes.... Fiberglass rods... Interesting ideas.
On lighter or smaller rockets I make my own rods to connect the bulkheads on the av-bay, usually its 1/4" aluminum rod, with only the ends threaded to accept nuts and washers where they got through the bulkheads. The one pictured below is threaded quite a bit more than most of mine 4" or less av-bay's.

AvBay1.jpgAvBay2.jpg
 

OverTheTop

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Will it be cool to see the GPS coordinates of where it landed (even though I can see it sitting in the field over there...), YES!!
If you have ever hunted for rockets in big fields you have no idea how good this feels until you experience it! :)
 

watermelonman

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I'm getting ready to build my first sled out of 1/4 birch plywood for a 4 inch fiberglass rocket. The AV-bay is 9 inches long. Do I need allthread bolts running the length of the bay or can I just connect either end of the sled to the U-bolts coming through the bulkheads? In other words, the only thing traversing the length of the AV-bay would be the sled. That way I could include a transmitter on the sled and not worry about metal rods disrupting tracking.

Is there a overriding reason (like maybe strength?) to run bolts the length of the sled other than it makes thing easier?
I have run threaded rod right next to my antenna on many, many flights without any issue. As tfish says the range is an extremely important factor here. Under 10k, it is probably not a big deal. At 80k, you are probably going to suffer. Where the cutover point is, I could not tell you.

I think there is a little bit of culture of fear with many rocketry topics. I should make it a motto, try it yourself!
 

Lowpuller

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I have the same concern on my current build but Chris with Eggfinder has assured at my low projected altitude 4000', it should not be a problem.
 

ksaves2

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I got the idea from the pictures on this web page. Scroll down. I do not work for or know the owner of the site.
http://onebadhawk.com/stainless-steel-u--bolts.html
The 6th e-bay picture... Connect the U-bolt to an L bracket on the sled. Leave off the all-thread.
I would also key the bulkplate so it only fits on the AV-tube in one place to reduce any risk of spinning.

Again, my question, and ksaves2 may have answered it, do you need all-thread bolts for anything other than ease of assembly?
Teddy's ebay is built for bear no doubt. Probably for one of his aggressive projects. If one leaves out the all-thread, I'd be worried about the forces ripping
through the plywood. Some of the loads on the main chute side can be pretty hard on the bay. If one would have an anomalous flight where the ejection occurs at high speed, the sled with no all-thread is going to fail a lot sooner than the one with all-thread. G10 might be stronger than plywood but still, one
is leaving themselves open a failure mode that wouldn't be there with all-thread. I mean for a rocket with a mild flight profile with deployments occurring at
the "ideal" moments, a decent sized drogue so the descent isn't "rocket fast" I'm sure it could be pulled off.
Sort of like flying a cardboard rocket where the events have to be really close to perfect to avoid a high speed zipper sort of thing. Kurt
 
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ttabbal

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I've seen designs that swap the all thread out for running the kevlar/nylon recovery harness through the ebay. That way, the recovery forces are on the harness, not the ebay materials. And the harness already has to be able to handle the various forces involved anyway...
 

ksaves2

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I've seen designs that swap the all thread out for running the kevlar/nylon recovery harness through the ebay. That way, the recovery forces are on the harness, not the ebay materials. And the harness already has to be able to handle the various forces involved anyway...
Yeah, and Teddy Chernok has a picture of one of his customers harnesses that is still attached to the eyelet of a forged eyebolt that fractured right off the base of the bolt! That is strong sewing there!!:shock: Kurt
 
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