Motor hook size and strength

Here's an odd one:

I went to grab a couple of 70mm motor hooks from my stash and was surprised to see two different sizes:

I'm not certain of the provenance of these two, but I can guess. The wider one looks to have had a finger tab cut off and filed down in the past, so I am guessing it came from an Estes kit. The narrower one must have come from an assortment of hooks I purchased quite a good long time ago. I only have one of the wide ones and a bunch of the narrow ones.

What is completely unclear to me is whether I should be concerned about the strength of the narrower ones for 24mm motors? If I do, is there a source for the wider ones other than Estes? Neither BMS nor erockets.biz specifies the width of their hooks.

A quick survey of my existing fleet of scratchbuilds shows only one with the narrower hook. I'm quite sure I didn't notice the difference when I built that one. I don't know how I acquired such a stock of the wider ones, maybe I did indeed by from Estes originally.

I know that the TLP kits came with the wider, beefier "E" engine hooks.
Don't know where they sourced it from.
Only see the thinner hooks in the after market.

Hello Neil, I don't think it would make much difference. Even a 24mm composite motor generally doesn't produce more than 50 N of thrust. I can't see that a 50 N force (~10 lb) would do anything to a steel hook, whether it's 3/32" or 1/8". Tie a string to a 10lb bag of anything, hang the string on the hook. Does it bend? If not, it ought to be good to go.

Best -- Terry

prfesser said:

Hello Neil, I don't think it would make much difference. Even a 24mm composite motor generally doesn't produce more than 50 N of thrust. I can't see that a 50 N force (~10 lb) would do anything to a steel hook, whether it's 3/32" or 1/8". Tie a string to a 10lb bag of anything, hang the string on the hook. Does it bend? If not, it ought to be good to go.

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Well... the only function of the hook is to prevent ejection of the motor, since there's a motor block at the front of the mount. I guess the likelihood of a 24mm ejection charge *bending* the end of the hook are close to nil; bigger question is whether the small hooks will grab on tightly enough or if I would benefit from a wrap of tape.

Given that the other vendors all seem to sell the narrower hooks, it's probably nothing to worry about. But I will keep my eye on those smaller ones and see how they perform with 24mm composites.

I should probably stock up on the Estes 24mm screw-on retainers, but for some of my rockets there's no good place to mount them.

I can’t find it now but recent thread on whether a forward engine block was needed with a motor hook. I think the general consensus was yes. Note: for low power you don’t need a factory made engine block. A single 1/4” piece of tubing from the motor mount, with a small slice taken out so it can roll up inside the mount tube just forward of the forward engine tab, glued in with white glue, is more than sufficient and a lot lighter and cheaper.

The main purpose of the hook is not to keep the motor from blowing forward through the motor mount, but to keep the ejection force at deployment from blowing the motor out the BACK (which for a low power casing itself isn’t that big a deal, but it is BIG deal if it does THAT in lieu of pushing the laundry out the FRONT.)

I have had a case where a D Motor ejected despite the hook. Now I put a piece of tape around the motor and hook to keep it on the motor.

Yeah, I *always* have a block at the front of my MMTs.

A wrap of tape is probably the best answer. The challenge is remembering to do it.

I think it is the material that the hook is made from which makes the most difference.

A few years ago I had a couple of rockets with engine hooks made of some metal that lost its springiness after a couple of uses. I am not sure which rockets or engine hooks had the problem, but they were likely from Estes.

I would have to bend the hook inwards before each flight to ensure that it held the motor in when the ejection charge fired. Otherwise the hook would remain bent out after I installed the motor.

I haven't had that issue with newer rockets and engine hooks.

jadebox said:

I would have to bend the hook inwards before each flight to ensure that it held the motor in when the ejection charge fired. Otherwise the hook would remain bent out after I installed the motor..

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That’s probably what happened with mine.

Two words: spring steel.

kuririn said:

Two words: spring steel.

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I know there were some common items mentioned (somewhere) that had strips of good steel for motor hooks. Anyone remember what they were?

One is steel from used auto wiper blades.

Windshield wiper blades.

[ninja'd by kuririn!]

Here is a list of Chris Michielssen's blog posts on how to make your own engine hooks from wiper blades:

https://www.rocketreviews.com/engine-hooks-190922174936.html

I've made some from wiper blade parts...but some of that spring steel is wider than others and I've run into issues with fitting the notches in reload motors. So now I check and if needed, grind the width down so it will fit. Also, if you make the "ejection end" bend too long, it can drag on the forward closure. But a couple of wipers will usually provide four pieces of steel and you can pop out a bunch of retainers!

Here's a thread where I made some hooks following Chris's advice:
https://www.rocketryforum.com/threads/spev-mk-1.141178/#post-1705609

neil_w said:

What is completely unclear to me is whether I should be concerned about the strength of the narrower ones for 24mm motors? If I do, is there a source for the wider ones other than Estes? Neither BMS nor erockets.biz specifies the width of their hooks.

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BMS hook, P/N BMSEH275, is .09 wide.. I modified one for use on my Hammerhead Shark scratch build.

The ones I use on boat tails are made from the largest bobby pins. I bend a motor hook in one end and the other end has a 90deg bend that goes into the boat tail. For flat tailed rockets I use a 6-32 T-nut mounted in the centering ring. I sometimes grind the flange on the T-Nut so it actually looks like a T. I do this stuff on my MPR's and I tend to do a basic hook on my LPR's.

I used the wiper blade springs for a long time but currently I don't have any to use.

Now that I think about it, I'd bet that piano wire, probably around 20ga, would probably do a decent job as a motor hook.

I wouldn't hesitate to use the narrow motor hook. Should be more than able to retain your motor.

prfesser said:

Now that I think about it, I'd bet that piano wire, probably around 20ga, would probably do a decent job as a motor hook.

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https://qualmanrocketry.com/qualman_clips.html

ep29030 said:

I wouldn't hesitate to use the narrow motor hook. Should be more than able to retain your motor.

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+1

ep29030 said:

I wouldn't hesitate to use the narrow motor hook. Should be more than able to retain your motor.

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I agree, and that hook is now installed in the motor mount.

The narrow one looks the ones that come in Quest kits. And they work fine with 18 mm BP engines. With 24 mm engines I wouldn't be even a little worried either.