Eyebolts vs. U-bolts

I've seen eyebolts and u-bolts used as attachment points for recovery harnesses on high power rockets. Is one better than the other? Are there times one would be preferred over the other?

For eyebolts, there are forged eyebolts and "standard" (bent wire) eyebolts. The latter are a source of problems, unless welded closed -- I've seen amazingly light rockets bend amazingly heavy eyebolts, and come in without a recovery system because of it.

A forged eyebolt is always a superior choice to a regular eyebolt.

That said, I prefer U-bolts, as they spread the load over more area. Regardless, both work, and either one should be backed up with fender washers.

-Kevin

qquake2k said:

I've seen eyebolts and u-bolts used as attachment points for recovery harnesses on high power rockets. Is one better than the other? Are there times one would be preferred over the other?

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Another alternative is eye nuts, which allow you to connect the shock cord to each end and transfer the load without relying on the strength of the bulkhead. Some form of unscrewing prevention is required, however.

Adrian A said:

Another alternative is eye nuts, which allow you to connect the shock cord to each end and transfer the load without relying on the strength of the bulkhead. Some form of unscrewing prevention is required, however.

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What is an "eye nut"?

troj said:

For eyebolts, there are forged eyebolts and "standard" (bent wire) eyebolts. The latter are a source of problems, unless welded closed -- I've seen amazingly light rockets bend amazingly heavy eyebolts, and come in without a recovery system because of it.

A forged eyebolt is always a superior choice to a regular eyebolt.

That said, I prefer U-bolts, as they spread the load over more area. Regardless, both work, and either one should be backed up with fender washers.

-Kevin

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I would think the plywood bulkhead would fail before a steel eyebolt would straighten. The only experience I have with eyebolts are 3/16" eyebolts in my Mercury Hijacker and Integrator, and two scratch built rockets. Never had a problem with any of them.

https://www.mcmaster.com/#eye-nuts/=7chhs2

Rob

I exclusively use forged eyebolts from McMaster-Carr. To avoid spinning them off, I always use at least one split washer to apply some outward force on the backing nut so it will stay securely tightened.

https://www.commonwealth.net/cgi-bin/info.pl?prodcode=CK3908&url=hardware/ck3908.htm Better eyenuts here

qquake2k said:

I've seen eyebolts and u-bolts used as attachment points for recovery harnesses on high power rockets. Is one better than the other? Are there times one would be preferred over the other?

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If you have the room on the bulkhead plate, use U bolt. As others mentioned-better ditributed load. Plus the nicer ones (SS) come with a nice rectangular backing plate.

If on the other hand, space is REALLY tight the eye nut solution works well.

What breaks first depends on the weight of the rocket, the materials you choose, and the G-loading on the recovery gear. It takes a lot less energy to bend steel than to break decent grade plywood, so you either want to use a welded eye-bolt or U-bolts. Both will work equally well provide you use a large washer or backing plate to spread out the load on the plywood. In theory, a U-bolt is held in place with two nuts, and therefore is less susceptible to a single point failure, but U-bolts take up more real estate than eye-bolts so in practice I think it's a wash.

All plywoods are not the same. Really strong plywood is very close to fiberglass in strength to weight ratio. 1/4" 12-ply aircraft grade birch plywood is comparable to 1/4" commercial G10 in ply layer thickness, and is much stronger than 1/2 4-ply standard plywood, which is way stronger than luan plywood, and I've witnessed eye-bolts in a luan plywood bulkhead with backing plates pull through.

In really big rockets, the best approach is to attach the shock cord to an eye-bolt that is screwed into the forward closure of the motor. Even if the rocket shreds, the heaviest part will still recover safely.

Bob

I agree with the u-bolt being the better choice when possible because of the larger distributed loading area. Forged eyebolts or eyenuts would be next with welded wire eye bolts and plain eyebolts next.

I think size selection is as important as type selection. An oversized wire eye bolt may do just as well or better then an undersized forged eye bolt.

qquake2k said:

I would think the plywood bulkhead would fail before a steel eyebolt would straighten. The only experience I have with eyebolts are 3/16" eyebolts in my Mercury Hijacker and Integrator, and two scratch built rockets. Never had a problem with any of them.

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You'd be surprised. I've seen moderate quality bulkheads survive, but the eyebolt straighten enough to allow the recovery system to come off.

-Kevin

Handeman said:

I think size selection is as important as type selection. An oversized wire eye bolt may do just as well or better then an undersized forged eye bolt.

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Checking working load limits, I've yet to find a wire eye bolt that could handle as much load as a forged eyebolt.

Now, that said, I'm sure they exist, but they're not likely to be standard material anywhere.

-Kevin

I am just thinking here, and if there is enough force for an eybolt to straighten, I would be more consearned about other things first..... And my personal expereience with bent eyebolts has been good, granted I fly almost strictly LPR. Also, if you think about it, a U bot would make sence to be stronger than an eyebolt, there are 2 attachment points, distributing the load, and makeing the bulkhead a happy camper.

SD

:2:

What magnitude of stress forces are we talking about here? Under what circumstances would, say, a 2 lb. rocket experience 2,000 lbs. of stress on its recovery system? Is that very likely to happen?

MK

Where do you get that 2000 lb number? A quick check of McMaster's site lists a forged 1/4-20 eyebolt at 500# working load limit. The bent eyebolts aren't even given a pound rating. They just state do not use this product for lifting.

Deployment events stress connection points a great deal. I have seen bent eyebolts fail quite a few times. I have also seen u-bolts ripped through bulkheads as well as bulkheads ripped out of couplers. The last example happened just this weekend on a fellow flier's L3 cert attempt. When the main inflated and the rest of the rocket pendulumed below it, it hit the end of the harness and the zipperless bulkhead gave out.

I would always recommend using a u-bolt or forged eyebolt over a bent eyebolt any day.

madmax said:

Where do you get that 2000 lb number? A quick check of McMaster's site lists a forged 1/4-20 eyebolt at 500# working load limit. The bent eyebolts aren't even given a pound rating. They just state do not use this product for lifting.

Deployment events stress connection points a great deal. I have seen bent eyebolts fail quite a few times. I have also seen u-bolts ripped through bulkheads as well as bulkheads ripped out of couplers. The last example happened just this weekend on a fellow flier's L3 cert attempt. When the main inflated and the rest of the rocket pendulumed below it, it hit the end of the harness and the zipperless bulkhead gave out.

I would always recommend using a u-bolt or forged eyebolt over a bent eyebolt any day.

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Yes, but how much weight are we talking about? My level 1 LOC-IV weighs in at 36oz without motor, just a little over 2 lbs. How much did the level 3 rocket weigh?

qquake2k said:

How much did the level 3 rocket weigh?

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My 10" Polecat Thumper weighed in @ 70lbs for my L3

I was exaggerating. For large Level 2 and 3 rockets, I can see your point. For smaller stuff, do you really need recovery attachment points that look like tractor hitches? Tailoring your delays and timing your events for optimal or near-optimal deployment also has to be part of the picture, too. It's not practical or even good engineering to build up the recovery harness and attachment points to survive the worst-possible scenario. The concept of a recovery system encompasses much more than just the parachute, harness and attachment points. It also involves making informed choices about motors as well as choosing appropriate delays or event timing, packing techniques, and much more, including stable and sound aerodynamic design. In fact, the entire rocket is a system, and the goal should be to design it and fly it in such a way as to minimize anomalies so that the system does not need to absorb extreme stresses.

MK

Agreed, but a small forged or welded eyebolt/eye nut is a better choice than a small to midsize bent one, even on little rockets.

Take a 1/4" diameter steel launch rod and bend it by hand. It's pretty easy. Then try to break it. It's impossible by hand.

The design factor of rated eye-bolts is not more than 20% of the ultimate material strength. For example an eye-bolt with a working load rating of 400 pounds is expected to fail at approximately 2000 pounds.

https://practicalmaintenance.net/?p=787

To answer an earlier question, to experience a 2000 pound load with a 2 pound rocket you need to develop a 1000 g load. Something else will give first.

Bob

Chuck Taylor said:

My 10" Polecat Thumper weighed in @ 70lbs for my L3

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There would be considerably less stresses involved with a 2 or 3 pound rocket, versus a 70 pound rocket.

qquake2k said:

There would be considerably less stresses involved with a 2 or 3 pound rocket, versus a 70 pound rocket.

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Agreed, but a 2 or 3 pound rocket can still bend one of the small, bent-wire eyebolts often used in small rockets.

cjl said:

Agreed, but a 2 or 3 pound rocket can still bend one of the small, bent-wire eyebolts often used in small rockets.

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Well, I put a 1/4" bent wire eyebolt in my LOC-IV, and it's too late to change it now. I hope it'll be okay.

MarkII said:

I was exaggerating. For large Level 2 and 3 rockets, I can see your point. For smaller stuff, do you really need recovery attachment points that look like tractor hitches? Tailoring your delays and timing your events for optimal or near-optimal deployment also has to be part of the picture, too. It's not practical or even good engineering to build up the recovery harness and attachment points to survive the worst-possible scenario. The concept of a recovery system encompasses much more than just the parachute, harness and attachment points. It also involves making informed choices about motors as well as choosing appropriate delays or event timing, packing techniques, and much more, including stable and sound aerodynamic design. In fact, the entire rocket is a system, and the goal should be to design it and fly it in such a way as to minimize anomalies so that the system does not need to absorb extreme stresses.

MK

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Here!!! Here!!!

You explained exactly what I've been thinking for quite some time. Great!!!

madmax said:

Deployment events stress connection points a great deal. I have seen bent eyebolts fail quite a few times. I have also seen u-bolts ripped through bulkheads as well as bulkheads ripped out of couplers. The last example happened just this weekend on a fellow flier's L3 cert attempt. When the main inflated and the rest of the rocket pendulumed below it, it hit the end of the harness and the zipperless bulkhead gave out.

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I didn't see this flight so I can't say if it was a weak bulkhead or too long of shock cord. I can say I've seen the same thing happen when the flier used a shock cord that was very long. I've seen several flights where the main opened and the fin can fell a long ways, gaining speed and energy, and snapping off when it hit the end of the cord.

I think a short shock cord with a drogue chute and short shock cord for the main puts a lot less shock and stress on the recovery system then very long shock cords. YMMV.

Eye bolt failure is most likely to occur with heavy rockets going drogueless Dual deploy. First it is stressed at apogee separation, then the rocket freefalls thousands of feet then deploys the main stressing the recovery harness and attachment points (eye, U bolts ,bulk heads) I have seen "Open eyebolts " straighten almost completely open. Not to mention most are made in China out of low quality steel. Here in Michigan we do alot of drogueless dual deploy to to fly the small launch sites we have to make do with. No "Blackrock" here.

bobkrech said:

Take a 1/4" diameter steel launch rod and bend it by hand. It's pretty easy. Then try to break it. It's impossible by hand.

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Yes, but you can bend the rod easily because it is long and you gain a good deal of leverage by placing your hands some distance apart when you grip it. But try shortening the rod to 4", then grabbing it an inch from one end, and trying to bend the tip down with your other hand. Look, I have no problem with using eye nuts or forged or welded eye bolts for the attachment points. But consider the big picture; design a recovery system for your rocket that will be able to function like you want it to, and then build the system to provide that functionality using appropriate hardware and construction techniques. qquake2k, it looks like you have done that; I very much doubt that I would do anything different. I don't think that going overboard gains anyone anything. Then focus on flying it in such a way that it will have nominal performance (in this context, that's a good thing) and a good recovery. The last thing that you want is an "exciting" flight.

In one of his first Peak of Flight newsletters, Tim Van Milliken asked the question, "How are your flying skills?" Not your rocket building skills, your motor-building skills or your launch prep skills. Most experienced rocketeers at any power level are decent fliers, and a few truly excel at it. They have used the flight experience itself as their teacher and have developed an eye for the wind, the weather and the typical performance of the motors that they will be using, and they readily adjust (or even cancel) their flights based on this judgment. They don't have a "launch it at any cost" attitude. When we* move into a new power level, the classic advice still holds: find people in your club or at a launch who are consistently successful with their flights, and then emulate them!

*I say "we" because I'm getting ready to move into the world of HPR myself.

MK

Absolutely agreed in many cases, but I still dislike bent eyebolts (of any size). Mcmaster Carr has fully closed forged eyebolts in sizes as small as #6-32, and I'd honestly trust a 6-32 or 8-32 forged eyebolt more than a 1/4" bent one.

In those situations I would simply tie the Kevlar around the motor mount and run it up through a notch in the top centering ring. That's what I do now with all of my builds. I wouldn't even bother with a hardware anchor at the lower end.

MK

That works great too. I've done that many times, and I'd trust it more than a 1/4" bent eyebolt too.


qquake: I wouldn't be too concerned about your rocket though - don't panic or anything like that. Although I would essentially never use a 1/4" bent eyebolt, it's not all that likely to fail. It's simply another failure mode that, while unlikely, I prefer to eliminate. In all likelihood, it should be fine, especially if you select appropriate delays.