Install a Motor Hook for Any Motor Length

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NOLA_BAR

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This is a technique that I have standardized for any build where I want to install the motor hook, but leave the forward end of the motor mount open for longer motors.

I’ve just started building the Roachwerks 4x24mm Nike-Hercules that I purchased from a forum member. The kit included E size motor hooks (95mm length), but I still wanted the forward end of the motor tubes unobstructed.

First thing I always do is switch out the motor tubes to heavier wall versions. For 18mm motor tubes I switch to ST-7, and for 24mm motor tubes I switch to ST-9. These are available at Semroc/eRockets. The OD on these tubes are slightly larger, so you will have to slightly enlarged the ID of your center rings.

Below is the diagram for the motor tubes. I will be eliminating the hook slot and engine block.
28E16BD4-7489-4F12-BCBD-20536F5B6C3D.jpeg


Next step is to bend one end of the motor hook 180 degrees. Do this with a small pair of needle nose pliers. Be careful not to over-fatigue the metal. The kit included 8 BT-50 to BT-55 fiber center-rings, 2 for each motor tube. I added one more each of the green Estes type. I also added a ST-9 to LT115 CR as a hook wrap for more strength. I notched 2 of the rings and the wrap for the motor hook with a needle file. Below are the 3 CR’s and motor hook ready to mount on the motor tube.
BCFFD548-A85D-41EC-93FB-751B87B3304E.jpeg


I then dry fitted the rings and motor hook on the motor tube and marked all the locations according to the directions.
Notice that the forward end of the hook now faces upward.
82F185C7-F962-4502-833C-959D4F5516BC.jpeg

An AT 24/60 casing and aft enclosure were used to position the hook for a 0.25in overhang.

I then applied wood glue on the motor tube and a dab of JB Weld on the hook where the rings will be located.

DF1D5ED5-2A75-4FA4-AACD-5A54B9404F77.jpeg

Note, I lightly sanded the motor tube with 400 grit, and cleaned and buffed the hook with 800 grit.

Another note, I filed down the motor hook a bit so it would not extend above the OD of the green center ring.

Slide the CR’s into position and wipe off any excess glue and JB Weld.
447DC101-A468-4F92-ADF3-0D375D1A562B.jpeg

You can also just use an Estes 24mm motor and 0.25in of masking tape to position the hook. Tamiya 6mm tape is just about perfect!

That’s about it. I let the glue set before removing the motor casing.

Since the motor block is eliminated, you do need to make a aft tape thrust ring for BP motors. Composite reload casings and single use motors already have a thrust ring.

Update: You could also straighten one end of the motor hook, and snip it off. Then do a new bend at 90 degrees upward. I have done this when I thought that the metal was over-fatigued. You need to use 2 pliers to bend it , but it is still pretty easy to do. Also wear safety glasses! That piece of metal can fly when when snipped.
 
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cbrarick

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very creative.
However, this is designed to do the same thing and won't undergo the cyclic failure your motor hook will, and it guarantees the motor won't fall out.
 

lakeroadster

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very creative.
However, this is designed to do the same thing and won't undergo the cyclic failure your motor hook will, and it guarantees the motor won't fall out.
cyclic failure? Could a rocket realistically fly enough times that the hook would ever break from fatigue?
 

NOLA_BAR

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very creative.
However, this is designed to do the same thing and won't undergo the cyclic failure your motor hook will, and it guarantees the motor won't fall out.
Yes, I have the AeroPack retainer on other rockets. Excellent product. This is a 4x24mm Nike-Hercules scale kit, so I don’t really want 4 retainer assemblies on the back end. The ends of the motor hooks are flush with the backend of the booster tubes. I don’t think a retainer would fit anyway.
 

heada

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cyclic failure? Could a rocket realistically fly enough times that the hook would ever break from fatigue?
Yes. Seems like it happens about 30-40 launches for me. Much longer if I don't have to move the hook much. I've mostly switched to retainers rather than hooks but on some, you want the low-profile hook to keep closer to a scale look.
 

BEC

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I've had some interesting failures with models that have done 75-100+ flights....but I've never had a motor hook fatigue and fail. But that doesn't mean it doesn't happen, of course.
 

lakeroadster

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Yes. Seems like it happens about 30-40 launches for me. Much longer if I don't have to move the hook much. I've mostly switched to retainers rather than hooks but on some, you want the low-profile hook to keep closer to a scale look.
It's all a matter of the distance from the hook itself to the first support. A good design will maximize that distance. And a small wrap of masking tape around the end of the motor ensures no deflection of the hook during flight.

Easiest and lightest approach? No hook, no retainer, just use regular ole 3M crepe masking tape.

Motor Hook Good.jpg
 

heada

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It's all a matter of the distance from the hook itself to the first support. A good design will maximize that distance. And a small wrap of masking tape around the end of the motor ensures no deflection of the hook during flight.

Easiest and lightest approach? No hook, no retainer, just use regular ole 3M crepe masking tape.

View attachment 464574
Agree, best is no block or hook but on a 4x 24mm scale cluster, I would use a hook nearly every time. I have seen too many shot-gun ejection charges on Estes 24mm to know that 4 of them going off at once would risk spitting a motor and on a scale kit, I want the look to be as close to scale as possible and not have 4 masking rings sticking out the aft end.

As with nearly everything in the hobby, there is no hard rule about what can/should be done in every situation. NOLA_BAR found a good solution for his situation.
 

Sooner Boomer

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How I did it for 18mm injuns:




the aft centering ring was cut away above the engine hook once the glue had dried. (there's an engine in the mount in pics)
 

cbrarick

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I've had a few. I'd update my Aerotech Mustang with a aeropack, but I'm afraid it would crumble away trying to get the hook cut back enough. The hooks on my viper 4 were such a mess that I clipped them, straightened out what's left and just tape the whole mess together. Thankfully, my SHROX X-15 withstood an aeropack retrofit - but it was a pia!

I do agree, for the infrequent flyer it's a very creative and cool solution.
 

rklapp

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It happens. The C18 popped out a little after the mild Cato but has yet to happen while in flight. The Aero Pack retainer is cool but only applicable to MPR and up. I haven't had an Estes retainer fail yet. Fingers crossed...


 

tab28682

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Never had a failure of a hook type motor retainer in a lot of years of flying rockets.

I have done this hook mod for a couple of 24mm models years back, so I could use E12s and D12s without a motor spacer.
 
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prfesser

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Easiest and lightest approach? No hook, no retainer, just use regular ole 3M crepe masking tape.
I recall reading (no idea where) that a tiny drop of CA at the joint between motor and MMT has been used for serious modroc altitude attempts. Probably takes some experience to get the right amount to prevent the motor ejecting while also being easy to remove w/pliers or some such. Probably wouldn't work so well with high-power or even mid-power rockets.
 

Mike Haberer

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This is a 4x24mm Nike-Hercules scale kit, so I don’t really want 4 retainer assemblies on the back end. The ends of the motor hooks are flush with the backend of the booster tubes. I don’t think a retainer would fit anyway.
In that case I wouldn't use hooks. I would use a plywood CR on the tail end and use an appropriately sized bolt and washer in the center of the CR. The washer holds the motors in on ejection. Use a threaded insert epoxied into the CR to hold the bolt. Use masking tape on BP motors for thrust rings. It's old school but it works. I have some designs for clusters where the spacing is such that it is impossible to fit retainers onto the MMT tubes. This is really the only way to secure the multiple motors easily and cheaply.

As for motor hooks, I made the mistake on my Estes LJ II of installing the engine hook without thinking about using longer motors. It's a heavy bird and will take an F or G if well built (and I built mine like a tank). When I get ambitious I need to Dremel out the motor hook and engine block. It will do over 2000' feet on a CTI 24mm G...

I'll never use motor hooks again except on 13mm MMTs (maybe, friction fit works just fine on minis). On 18mm mounts I use Rocketarium retainers. On 24mm I use either Rocketarium or Aerotech. Aerotech on everything bigger.
 

NOLA_BAR

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Thanks for all the feedback. The kit instructions show the hooks are flush with the back end of the booster tubes, so I’m going to stay with that design.
62D6EB63-A366-4CA2-B3A7-161DF0739FDC.jpeg
 

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