Estes engine hook; who has actually had one fail under thrust?

[prfesser]

Please, please, DO NOT provide examples from other fliers. No "I saw this happen to a friend" or similar. I'm looking for those who can say "it happened to ME" (and the circumstances).

Background: Estes rockets that have the metal engine hook also include a cardboard engine block. Redundancy is fine, but.... the biggest Estes motors have a maximum thrust of 30-35 N, less than eight pounds. I don't know about you-all (yinz, you-uns), but though my fingers aren't very strong they can apply well over eight pounds of force. But I still can't bend the forward end of the hook with bare fingers. It takes a pair of pliers (and more than a few pounds!) to straighten that hook enough to allow the motor to slide by it.

So...if you've had an Estes motor that straightened out the front of the hook enough to let the motor shoot up through the rocket, I'd like to hear the account. (Was the hook installed correctly? Other specifics?)

Best,
Terry

FWIW I routinely flatten the forward end of the hook with pliers and leave the engine block off entirely. Because I may use McMotors-brand motors, some of which are longer than Estes'

 

[kuririn]

I've never had the forward tab of an engine hook get straightened out by the force of a motor. I would think the motor mount would rip away from the airframe before that happens. However, I have had on multiple occasions seen the front end of an engine hook slide forward on the inside of the motor tube. So the hook and motor slid forward about an inch. I think this is why Estes has you glue a thrust ring in front of the engine hook in their instructions.
Of course that can be mitigated also by taping a rear flange on your motor.
Question: if you straighten the forward tab then what keeps the hook anchored at ejection?
If it's a wrap of tape then why not just eliminate the hook entirely and just wrap the motor end to the motor tube?
The only time I've seen an engine hook tab get deformed is when an E motor CATO'd and bent the rear tab of an engine hook. The motor casing was ejected.

 

[dhbarr]

I've not deformed the hook per se, but I have had a motor push the forward or the rear of the hook out away from the mount tube, allowing the motor to slide either forward or or backward.

In low power I returned to including something forward and something around the middle of the hook, haven't had a repeat offender.

 

[bjphoenix]

I've never had the forward tab of an engine hook get straightened out by the force of a motor. I would think the motor mount would rip away from the airframe before that happens.

I think that would be the failure mode. I could calculate the failure strength of the front part of the hook but I suspect it would be very high. I cut a small slot in the motor tube for the front hook to go through, maybe put some glue on the outside of the motor tube for the long part of the hook to stick to, then slide the ring over the assembly. So if the engine pushes on the front part of the hook then the load path is either to the small part of the tube in front of the slot or through the glue bond. Glue bond to smooth metal may not be very good but Titebond II seems to have pretty good stick to things that you don't want it to stick to. I put the cardboard thrust block in there anyway, and the last scratchbuilt I did I made a thrustblock for it.

 

[mikeyd]

I had it happen once, on the Large R2D2 rocket, using an 18mm D Reload engine, about 10yrs ago. The engine slipped forward, but was stopped at the rear closure, normal Aerotech engine stop, but it was enough to make the R2D2 unstable, at about 40' up. It tumbled, finished the burn, then fell back down on the LCO table, and deployed the chute, startling the LCO, and knocking his fresh soda off the table to the ground. Broke a leg off of R2D2, since repaired, and an engine block installed to prevent it next time, plus adding additional nose weight. It was a little comical, as just prior to the launch, the LCO had made some wise-crack response about the rocket, as if he did not know who R2D2 was, and was not impressed with the model.

 

[cerving]

The hook is much stronger than the thrust from any Estes BP motor... the failure mode is going to be the cardboard motor tube, and that could only happen if the hook isn't secured to the tube (a wrap of masking tape is sufficient). I could see bad things happening if you forgot to use the "E spacer" with a 24mm hook, but that would be an engine mount failure... not the hook.

 

[prfesser]

I've never had the forward tab of an engine hook get straightened out by the force of a motor. I would think the motor mount would rip away from the airframe before that happens. However, I have had on multiple occasions seen the front end of an engine hook slide forward on the inside of the motor tube. So the hook and motor slid forward about an inch. I think this is why Estes has you glue a thrust ring in front of the engine hook in their instructions.
Of course that can be mitigated also by taping a rear flange on your motor.
Question: if you straighten the forward tab then what keeps the hook anchored at ejection?
If it's a wrap of tape then why not just eliminate the hook entirely and just wrap the motor end to the motor tube?
The only time I've seen an engine hook tab get deformed is when an E motor CATO'd and bent the rear tab of an engine hook. The motor casing was ejected.

Thank you! That makes sense now. White glue doesn't hold onto the engine hook very well, so the engine block keeps the hook from moving forward.

Rather than white glue, I epoxy the forward inch or so of the engine hook onto the MMT. Epoxy holds metal much better than does wood glue. For insurance I usually top it with a strip of fiberglass cloth.

Best,
Terry

 

[tsmith1315]

That makes sense now. White glue doesn't hold onto the engine hook very well, so the engine block keeps the hook from moving forward.

I must be missing something... Is this not the typical old Estes configuration with the forward end of the hook inserted through a slit in the mmt?

 

[BEC]

I think so.

I've never had a thrust-caused failure of the forward end of the hook, but I have had hooks tear the motor tube. This is generally either because the model's fin configuration allows the hook to be the first thing that hits the ground (that finger tab that showed up in the early 1990s of course makes this much more likely), or from repeated ejection charges actually driving the motor and hook aft.