Engine hook OR engine block??

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lcorinth

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I just finished reading a new rocketry book - Make: Rockets: Down-to-Earth Rocket Science by Mike Westerfield. It was really good, gave me a lot of ideas, and gave my the confidence to try my own designs for the first time.

But I noticed in some of his designs, there was no thrust ring/engine block. I thought this might just be an omission, but on careful rereading, I realized that for rockets where he used an engine hook, he had no ring. He only used the engine block ring on friction fit rockets. For hook rockets, he suggested that the tab of the hook protruding through the engine mount tube was sufficient to hold the engine in place.

Now, I'd seen that tab, and thought it might be enough, or at least I bet that it added strength to the engine block. But every kit I've built with a hook also included a thrust ring. I'm wondering - is an engine hook sufficient?

When you guys build a rocket with an engine hook, do you bother with the thrust ring, or not? Has anybody here launched a rocket with only a hook and ever had a failure?

I'm building one of his designs right now, but I decided to include an engine block anyway, just to be on the safe side.
 
For LPR where you only intend to fly a certain size motor (diameter and case length are the same) it saves you from making a thrust ring on the motor with masking tape and friction fitting it. For standard mini and 18mm motors, I would use them. Anything bigger, I would not.
 
It's really a question of how durable your motor mount is. A good quality motor tube means you don't really need both, although I've seen several designs that call for both. I have never seen a clip without a block fail.

Once you move into 29mm motors and above you won't want the block because you'll be using different length casings. You'll want something like a Kaplow clip or and Aeropack retainer to keep the motor from ejecting.
 
Ok... lets get back to basics.

We are talking low power rockets (LPR) 18mm & 24mm type Black Powder (BP) motors.

Is it a good idea to use both a clip and a thrust ring? Yes. You will find most kits having a clip will also have a thrust ring. A thrust ring is able to spread the force of a motor's thrust to a larger area. This will give your rocket a much longer lifespan. As another user posted, a quality motor mount tube (mmt) can be used with just a clip. I have noticed cheap MMT like Quest kits use seem to wear very fast and even with a thrust ring, the clip will start to tear the tube and move around. So yes, a clip by itself will work, but you might as well add a thrust ring given the choice.

If you get into mid-power rockets (mpr) or even high power rockets (hpr), motor lengths start to vary. Because of this the thrust ring falls to the actual motor where the aft end has a bigger ring on it so it cannot go through the MMT. It is then secured by a retainer that keeps it from shooting out the end during recovery deployment. This is usually a clip or screw-on type cap.

Friction fit is always an option. I actually works too!


Jerome :)
 
I use both when I'm not just using a motor block for friction fit - I have seen that tab on the clip start tearing the tube after just a few flights, especially when it is the Estes blue motor tube. The block is just a bit of insurance to keep the whole thing flying longer.
 
Generally Both are the preferred method of installation:
Why? because bend retainer hooks can and Do often break free from the Band, tape or glue used to hold it in place allowing the metal clip to slide inside the motor tube. This can shift the balance of the model enough to cause it to go unstable. Adding a thrust ring even a very thin one makes an effective stop keeping the clip in place.

Quite a few Estes kits come with only a clip and retainer rings. With these kits and anytime I'm scratch building a 3/16" to 1/4" wide piece of spent motor casing is always added to serve as a thrust ring. Our spent 13mm, 18mm and 24mm BP motor casings make outstanding thrust rings.
 
anytime I'm scratch building a 3/16" to 1/4" wide piece of spent motor casing is always added to serve as a thrust ring. Our spent 13mm, 18mm and 24mm BP motor casings make outstanding thrust rings.

I have tried cutting motor casings, to make thrust rings (and sometimes other things) and it is a pain, those puppies are thick. No doubt this will work, but IMHO I think it is overkill.

I have gone to using the same body tube as for the motor mount (BT-5 for 13 mm engines, BT-20 for 18mm, and BT-50 for 24 mm), cutting a 1/4" ring, cutting about 1/8 longitudinal section out so it just fits inside the motor mount tube, sliding it in and fixing with white glue (yellow would probably work, but might shrink it a bit, and for paper to paper isn't significantly if at all any stronger.)

It is really easy to do (certainly easier than cutting a SPENT motor casing), lightweight. Althought is is very thin, I think it is just as effective, I have never had one fail. Your motor mount will rip before this things tears out.
 
They're fine for 13 and 18mm motors; I omit them on 24mms.

Spent motor casings are a piece of cake to cut with a razor saw.


Later!

--Coop
 
I prefer both (belt and suspenders) but also feel that the typical Estes hook is not designed properly.
I think it should be longer so that the tab will be forward of the engine block, not rearward.
That way you've got the engine block to keep the engine from shooting up through the rocket, and it will also keep the engine hook from ripping downwards if there is an ejection issue (tight cone, partly blocked baffle, etc)
 
It's one of those things that isn't ALWAYS a problem, but when it is, it's pretty bad...

It kinda started with really thin, cheap motor tubes in some kits (especially Estes, but others as well). I remember building kits back in the 80's where a motor hook alone was pretty typical, especially for low power (>A, A, B, C) motors. I guess maybe the tubes were thicker or something, I dunno. Anyway, at some point it started to be more of a problem... the motor hook alone either couldn't hold up to the acceleration forces of the motor lifting the entire rocket by pushing on the motor hook front end, which would tend to rip out the side of the motor tube where it was slit to pierce the side of the tube at the upper end of the motor, therefore 1) focusing all the forces on this small spot, and 2) introducing a cut into the wall of the tube at this point, therefore weakening it, and creating a spot where the tube could tear from the ends of the cut when excessive loads were applied by the motor in flight. The other issue was that motor ejection charges got stronger (in Estes motors anyway, which accounts for most of the LPR motors flown-- the so-called "shotgun ejections") and that sudden blast could create enough pressure pulling down on the motor hook when the charge went off to "rip" the hook back down the motor tube.

Once the motor tube is torn up inside the rocket, fixing it is especially problematic. Usually the only real way to fix it is to try to rip the entire motor mount out of the rocket, centering rings and all usually, and then build a new one, and try to install it inside a body tube that has been scraped or sanded to remove as much of the old glue and mount remains as possible, without destroying the rocket in the process... Either that or just "live with it" and tape or shim the motor in place and hope it holds.

There are several "prevention strategies" that can be used... Dr. Zooch kits typically include several of them in the instructions. The old adage "an ounce of prevention is worth a pound of cure" is especially appropriate for this issue... First, since it's necessary to "slit the tube" with a hobby knife to create a penetration for the motor hook upper end to stick through the tube wall from outside to the inside just above the motor, it's recommended to "harden" the slit with a drop of ultra-thin CA glue once the motor hook is installed... this will soak into the paper tube material and "harden" it in the areas adjacent to and surrounding the hook penetration, helping to spread any loads into a broader area of the tube and strengthening the ends of the cut slit to greater resist tearing. Secondly, a strip of paper is usually glued around the motor tube immediately BELOW where the motor hook penetration cut is made, before the motor hook is installed. A roughly half-inch wide strip of printer paper or thin cardstock, with a thin, even coat of white glue applied, is wrapped around the motor tube with its upper edge just touching the bottom edge of the motor hook "slit" in the motor tube... this gives a "reinforcement band" to the tube to prevent "shotgun ejection charges" from potentially ripping the motor hook backwards in the tube when they go off. Thirdly, a "thrust block" motor block ring is usually glued into the top of the motor tube to take the acceleration forces of the motor's thrust and spread it out through the motor mount and centering rings into the rocket's airframe, rather than having all that load being taken SOLELY by the upper tab of the motor hook penetrating the wall of the motor tube, creating a focused spot of high stress.

As mentioned elsewhere, this is typical for low power rockets. HPR kits/rockets usually don't use a motor hook or block, due to the differing lengths of the casing and clearance issues with the ejection well cap and the upper end of motor hooks when installed in a rocket. Typically HPR motors have a wider ring around the bottom of the motor that acts as a motor block to transfer thrust into the airframe, and often use a rear-mounted motor retainer of various designs. Mid-power rocketry has had a lot of "overlapping" of techniques, depending on the diameter of the motor tubes and intended motor types to be used... but of course over the last decade or two we've seen a proliferation of different length motors used (more like HPR motors) so that kits built with a motor hook for a "standard" Estes D motor cannot use the one-inch longer Estes E motors without either 1) sticking out the back (which can cause stability issues by moving the CG back) or 2) removing the upper end of the motor hook, by grinding or filing it away, or 3) modifying the kit to install a longer hook for use with the longer motors when building the kit, and using a spacer ring when using the shorter D motors. Increasingly there tends to be more of a move towards rear motor retention without upper thrust blocks and motor hooks in MPR rockets anyway, and this is probably a good thing, ESPECIALLY as we see more new motors coming out departing from the "traditional" sizes of the Estes D motor and even E motors. If minimum weight is still the overriding concern, or the kit is only designed to use a specific motor (the use of a pair of Estes "D" motors in some TLP kits immediately comes to mind) where substitution or use of larger motors may make them unstable or isn't recommended, then the good old reliable motor hook and thrust block ring in the tube is STILL the best solution to the problem, with the "beef up techniques" mentioned highly recommended. Not EVERYTHING NEEDS to be capable of having different length motors installed using rear retention and thrust rings.

Later and good luck! OL JR :)
 
It's one of those things that isn't ALWAYS a problem, but when it is, it's pretty bad...

It kinda started with really thin, cheap motor tubes in some kits (especially Estes, but others as well). I remember building kits back in the 80's where a motor hook alone was pretty typical, especially for low power (>A, A, B, C) motors. I guess maybe the tubes were thicker or something, I dunno. Anyway, at some point it started to be more of a problem... the motor hook alone either couldn't hold up to the acceleration forces of the motor lifting the entire rocket by pushing on the motor hook front end, which would tend to rip out the side of the motor tube where it was slit to pierce the side of the tube at the upper end of the motor, therefore 1) focusing all the forces on this small spot, and 2) introducing a cut into the wall of the tube at this point, therefore weakening it, and creating a spot where the tube could tear from the ends of the cut when excessive loads were applied by the motor in flight. The other issue was that motor ejection charges got stronger (in Estes motors anyway, which accounts for most of the LPR motors flown-- the so-called "shotgun ejections") and that sudden blast could create enough pressure pulling down on the motor hook when the charge went off to "rip" the hook back down the motor tube.

Once the motor tube is torn up inside the rocket, fixing it is especially problematic. Usually the only real way to fix it is to try to rip the entire motor mount out of the rocket, centering rings and all usually, and then build a new one, and try to install it inside a body tube that has been scraped or sanded to remove as much of the old glue and mount remains as possible, without destroying the rocket in the process... Either that or just "live with it" and tape or shim the motor in place and hope it holds.

There are several "prevention strategies" that can be used... Dr. Zooch kits typically include several of them in the instructions. The old adage "an ounce of prevention is worth a pound of cure" is especially appropriate for this issue... First, since it's necessary to "slit the tube" with a hobby knife to create a penetration for the motor hook upper end to stick through the tube wall from outside to the inside just above the motor, it's recommended to "harden" the slit with a drop of ultra-thin CA glue once the motor hook is installed... this will soak into the paper tube material and "harden" it in the areas adjacent to and surrounding the hook penetration, helping to spread any loads into a broader area of the tube and strengthening the ends of the cut slit to greater resist tearing. Secondly, a strip of paper is usually glued around the motor tube immediately BELOW where the motor hook penetration cut is made, before the motor hook is installed. A roughly half-inch wide strip of printer paper or thin cardstock, with a thin, even coat of white glue applied, is wrapped around the motor tube with its upper edge just touching the bottom edge of the motor hook "slit" in the motor tube... this gives a "reinforcement band" to the tube to prevent "shotgun ejection charges" from potentially ripping the motor hook backwards in the tube when they go off. Thirdly, a "thrust block" motor block ring is usually glued into the top of the motor tube to take the acceleration forces of the motor's thrust and spread it out through the motor mount and centering rings into the rocket's airframe, rather than having all that load being taken SOLELY by the upper tab of the motor hook penetrating the wall of the motor tube, creating a focused spot of high stress.

As mentioned elsewhere, this is typical for low power rockets. HPR kits/rockets usually don't use a motor hook or block, due to the differing lengths of the casing and clearance issues with the ejection well cap and the upper end of motor hooks when installed in a rocket. Typically HPR motors have a wider ring around the bottom of the motor that acts as a motor block to transfer thrust into the airframe, and often use a rear-mounted motor retainer of various designs. Mid-power rocketry has had a lot of "overlapping" of techniques, depending on the diameter of the motor tubes and intended motor types to be used... but of course over the last decade or two we've seen a proliferation of different length motors used (more like HPR motors) so that kits built with a motor hook for a "standard" Estes D motor cannot use the one-inch longer Estes E motors without either 1) sticking out the back (which can cause stability issues by moving the CG back) or 2) removing the upper end of the motor hook, by grinding or filing it away, or 3) modifying the kit to install a longer hook for use with the longer motors when building the kit, and using a spacer ring when using the shorter D motors. Increasingly there tends to be more of a move towards rear motor retention without upper thrust blocks and motor hooks in MPR rockets anyway, and this is probably a good thing, ESPECIALLY as we see more new motors coming out departing from the "traditional" sizes of the Estes D motor and even E motors. If minimum weight is still the overriding concern, or the kit is only designed to use a specific motor (the use of a pair of Estes "D" motors in some TLP kits immediately comes to mind) where substitution or use of larger motors may make them unstable or isn't recommended, then the good old reliable motor hook and thrust block ring in the tube is STILL the best solution to the problem, with the "beef up techniques" mentioned highly recommended. Not EVERYTHING NEEDS to be capable of having different length motors installed using rear retention and thrust rings.

Later and good luck! OL JR :)

That was a very thorough, helpful answer. Thanks! At the very least, I'm going to be using CA on all the motor hook slits in my motor mount tubes from this point forward. And I'm sticking with thrust rings. I'd rather not have to repair a damaged motor mount if I can avoid it. Sounds tedious and hard to do well, and I'd probably rather build a whole new rocket at that point.
 
...A thrust ring is able to spread the force of a motor's thrust to a larger area. This will give your rocket a much longer lifespan...

Jerome :)

OK, that makes sense. That's kind of what I was thinking. Thanks for confirming that for me.
 
I use both, the thrust ring/motor block to stop the motor shifting upwards on ignition plus reasons already stated tat the clip can put pressure on the motor tube. I always use a clip to stop the motor blowing out backwards on ejection. Its not like its much weight.

Cutting old motor casings up is easy with a small hacksaw.
 
I have tried cutting motor casings, to make thrust rings (and sometimes other things) and it is a pain, those puppies are thick. No doubt this will work, but IMHO I think it is overkill.

A band saw gets the job done nicely too. They are only slightly thicker than the thrust rings that come in some of the Estes kits.
 
I also cut a shallow notch, the same width and depth as the top of the engine hook, in the thrust ring so the top of the motor is actually in contact with the entire ring.
 
I have a whole box full of hooks and I rarely use them. Over the years they have given me trouble after multiple flights by loosening up, so I mostly go with a block and friction. The last bit-o-trouble has cured me forever. I bought a cluster kit for 3-24mm motors and used it in my Astron Cobra upscale. Had I known what was going to happen, I wouldn't have recessed the plywood CR in as much, per the directions in the kit. Apparently that caused a sort of vacuum, sucking the burn back into the motors and mount. As you can see in the pic, it got torched pretty good. Oddly enough, all three motors were not in there when I got it back. I suppose the heat made the hooks weak and they blew out at ejection.

Cobra Burnout.jpg
 
That was a very thorough, helpful answer. Thanks! At the very least, I'm going to be using CA on all the motor hook slits in my motor mount tubes from this point forward. And I'm sticking with thrust rings. I'd rather not have to repair a damaged motor mount if I can avoid it. Sounds tedious and hard to do well, and I'd probably rather build a whole new rocket at that point.


Good for you, right way to go. A friend flew his Estes Prowler this past weekend, wanted to fly it again and the motor hook was sliding up and down. We took a look inside, no thrust ring, hook only. He said it has flown a good number of times, rocket is in good shape yet accept for the fact that the MMT gave out do to having the hook only. The thrust ring certainly helps protect that area of the tube from the heat of the ejection charge.
 
To me I guess it is a matter of preference. I now try to stay away from hooks because they make rockets harder to display if you are going to. Mechanically sound either way though.
 
As several others have said, I usually use both a clip and a block on my 13 and 18mm rockets. However, I have also wanted to use an 18mm reload in some of them and found it wouldn't fit. So, your milage may vary.
 
I always put in a thrust ring for sport models, competition models are another story, over the years I have seen a few rockets with hooks that failed the the motor cored out through the nose of the rocket, either pushing out or getting hung up and burning the upper sections of the rocket, usually not doing a lot of damage but still I would rather fly than fix. As for competition models it depends on the motors 13mm and A class 18mm are usually friction fit and taped around the tail to save weight. I also like the longer motor hooks you can buy from some hobby suppliers that fit above the thrust ring and use it to reinforce the hook pentration.
 
Some designs actually ejects the engine for flutter recover on very light kits like the mosquito.
Other designs eject the cone with a streamer and the engine to lessen the weight of tumble.
Always use a trust ring inside the mmt right in front of the hook. These keep the engine in and use mostly paracute recovery.
I've had kits where you wrap tape around the engine for friction fit that is suppose to keep the engine in, and works well if done correctly. But they too must have a trust ring.
Talking micro maxx, 13, 18 & 24mm engines.
 
Aerotech has a somewhat different method. their motor hooks are designed so that the forward tab goes just in front of the motor block, so the motor thrusts against the block (not the hook) and at ejection the motor block keeps the hook from moving aft.
Rex
 
We are talking low power rockets (LPR) 18mm & 24mm type Black Powder (BP) motors.

I have noticed cheap MMT like Quest kits use seem to wear very fast and even with a thrust ring, the clip will start to tear the tube and move around. So yes, a clip by itself will work, but you might as well add a thrust ring given the choice.

Yes, occasionally that clip can works itself loose. For LP/MP sport models, I always install a motor block. I add an engine clip if taping the motor in is not possible or easy to do.

My competition rockets almost always have no block or clip because I tape the motor in. A tape thrust ring on the aft end of the motor and a wrap or two of tape around the ring and end of motor mount works very well. I used that method all the way up to my Level 1 cert with a single-use H.

I try to avoid friction fitting a motor, because they can be a real PITA to remove.

-W
 
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