E Engine Holder/Spacer Questions

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DynaSoar

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I'm working on some minimum diamater 24mm birds. I'm using metal engine hooks. This means they're mounted outside the tube. I have two questions related to using these:

1. Are these strong enough at the top end to handle the thrust of the largest 24mm engines without beefing them up with internal coupler tube/engine block?

2. The hooks are 3.75" long to hold Estes E engines, but many other E engines are 2.75" Is coupler tube strong enough to serve as the 1" spacer to use the shorter engines with these hooks?
 

Fore Check

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I would most definitely recommend an "engine block" above your engine hook, if you insist on using the hook. It will greatly add to the longevity of your model.

Think of it in terms of PSI on the engine tube itself.

Let's generously say that an engine hook is 1/8" wide. If the hook alone performs as the thrust block, all of the force exerted by the engine that is not absorbed by the friction fit of the engine and the tube itself will be absorbe by that 1/8" wide contact strip of the metal hook on the 0.013" inch thick tube. That's a *tiny* area to absorb a good amount of force. Expect damage.

Conversely, if you use a standard 3/16" thick wound paper centering ring as a "thrust ring" just above the insertion point of the hook, you have a holding area that is 24 times greater than that of a hook alone (in our example on a bt50 size rocket.)

For your second question, coupler stock would work just fine for the application you want (the 1" spacer above the D engine within an E engine hook.) However, I would be quite certain that you'll have a *very* difficult time getting it back out of there if you ever *did* want to use an E engine. If that's fine with you, go for it! Otherwise, I'd take a spent D or E engine casing, and saw off a 1" piece from the ejection end of the casing. Makes great D to E spacers!
 

Fore Check

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That brings me back to where I was going at the start of the post.

Why bother with the engine hook at all?

Friction fits work rather reliably, and a thrust ring will be your best bet for keeping your engine where it needs to be under power anyway.
 

DynaSoar

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Originally posted by Fore Check
That brings me back to where I was going at the start of the post.

Why bother with the engine hook at all?

Friction fits work rather reliably, and a thrust ring will be your best bet for keeping your engine where it needs to be under power anyway.
Good question. Mostly behavioral momentum. That's brain science speak for "'cause I always did". I just rmember how easy they made life when they first came out. They didn't exist when I started building.

In one case it's pretty much necessary, because the main tube is recessed over an inch from two concentric larger "fin" tubes. Reaching the engine to pull it out with pliers is likely to result in damage. However, in that case, the body where the top end of the hook goes through is reinforced with a half inch wide styrene sheet from 1" above the hook to 2" below. Besides being cemented down with plastic cement, there's a bunch of fillet all around that. I know that sounds like a huge gob of plastic, but it serves double duty as a stand-off for the launch lug. The nose is slightly convex towards the base, like an Honest John, so the lug needed to stand off a bit.

Plus, I just assumed the 24 to 18mm adapters would expect them.

Perhaps I'd best try to hack together a thrust ring for strength with a retainer hook for easy of changing engines.
 

jetra2

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Just wanted to throw in my 2 cents here.

Instead of using an engine hook on minimum diameter birds, do this. For the smaller LPR rockets (18mm and 24mm) mount your fins about a 1/2" forward of the end of the body tube and mount a thrust ring in the tube for the longest motor you plan on using, for 18mm a good distance is 2.5" and for 24mm a good distance is 3.5" - this will accomodate the longest Estes and AT motors. Then when you are prepping the rocket for flight, insert your motor in to the tube and then wrap two layers of masking tape around the end of the motor and the body. This will act as a pseudo motor hook, and will retain your motor nicely.

This is also a good procedure to use on the Fliskits Deuce's Wild! kits and the Tres.

Powderburner, since you are our resident aerospace engineer, would you explain the benefits of mounting the fins slightly forward?

Thanks,
Jason
 
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