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Short and Fat - what size motor mount?

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kjkcolorado

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My next build is going to be a short, fat rocket. I will be using 4" Blue Tube left over from my L2 scratch build, and 1/4" Baltic birch for fins/rings/bulkhead. My plans are to make it an easy sport flyer, no electronics (other than a simple Jolly Logic for altitude), and zipperless design (despite the best simulations, planning for the inevitable 'hard opening'). My current design in OR has a 54mm motor mount with enough length in the fin can for a 3-grain CTI case. With that configuration and using the nose cone space for chute/harness packing, I think I can make it as short as 60-65cm (close to a 6:1 length to diameter ratio). I recognize I will need a significant amount of nose weight to fly a 3-grain 54mm motor to get a safe stability margin, even with the increased influence of base drag. Given the penalty of needing to add a lot of nose weight, would I be better off to drop down to a 38mm motor mount? For those of you with fat and short rocket experience (and observation) let me know your thoughts.
 

FredA

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Seems like a no-brainer.....54mm MMT all the way.
If it weren't for you need for motor eject, I'd say you need a 75mm MMT.

You can always add nose weight....and you can always put a smaller motor in a larger MMT.

Nose weight does not need to be permanently glued in place.
On almost all of my rockets that need nose weight, I use a variable system that allows me to match the NW to the motor so that I can tune the Cg for any configuration.
 

kjkcolorado

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Thanks Fred. I was already thinking of trying an adjustable nose weight system. As part of this discussion, how is yours set up?
 

FredA

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My variable system uses the all-thread I usually use to keep the NC aft bulkhead in place.
Normally, I use a section of all-thread with a few nuts and washers on the forward end that get epoxied into the tip that then runs aft with enough length to protrude though the NC's aft bulkhead and gets tightened down with an eye-nut.
I put a cotter pin though the aft end of the all-thread so that the eye-nut doesn't unwind in flight.

I like to remove the NC bulkhead and put my GPS/RDF tracker in the nose. And no, I've not had issues with the all-thread interfering with the tracker operation.

When I want to implement a NC weight, I use the same setup but instead of a permanent all-thread, I glue in an extended-length coupler-nut into the forward end of the NC. This allows me to remove the all-thread when needed.
I use lead ball fishing weights that I center-drill so that they slide onto the all-thread and secure them in place with a washer and a couple of jam-nuts. Then just put the all-thread back into the NC and use as usual.

It usually takes a little tinkering to get the perfect position for the lead ball - so that it is as far forward as possible while still allowing as much all-thread as possible to be screwed into the coupler-nut. If spaced just right, the last turn or two on the all-thread forces the ball to rub the NC so that there is some friction on the sides of the NC helping to keep the all-thread from unscrewing unintentionally.

The weakness of the design is that the all-thread MAY unscrew at the coupler-nut. A drop of weak (purple or blue) thread-locker is also advisable. I usually don't bother and I've never had one un-screw.

This system allows me to change out the lead balls to add weight as needed....remove them all for no weight, use different sizes or stack multiple balls for lots of weight.
 
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kjkcolorado

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Thanks for the description. I can visualize exactly what you are doing. In this case, I will not have an aft bulkhead on my nose cone since I am planning on using it for packing the chute. I've been thinking of how to attach the nose cone to the recovery harness. My initial thought was to use a Kevlar loop attached up in the tip of the nose cone by passing the loop around a couple 'skewers' (cut and sanded flush on the outside after installing) epoxied in with needed weight. I wounder if I could somehow slide weight onto each side of the Kevlar loop as needed. Not sure how I would secure said weight from moving aft under thrust......
 

FredA

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Kendal,
The bulkhead in the NC need not be at the aft end - it's just the customary and most painless place to put the bulkhead.
You can put a bulkhead anywhere along the NC provided it fits inside the shoulder and provided you can reach inside to tighten the eye-nut.
So you can "wall off" just the first half or so....

You don't even need a bulkhead IF you can reach inside.
But if you can put your hand inside, you can just bolt the nose weight right to the tip and put the eye-nut for the harness attach point directly aft of the weight.
 

bill2654

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Thanks for the description. I can visualize exactly what you are doing. In this case, I will not have an aft bulkhead on my nose cone since I am planning on using it for packing the chute. I've been thinking of how to attach the nose cone to the recovery harness. My initial thought was to use a Kevlar loop attached up in the tip of the nose cone by passing the loop around a couple 'skewers' (cut and sanded flush on the outside after installing) epoxied in with needed weight. I wounder if I could somehow slide weight onto each side of the Kevlar loop as needed. Not sure how I would secure said weight from moving aft under thrust......
Ive used the skewers in the NC before but I used all thread instead of Kevlar. I heated up the all thread cherry red and bent a loop on the end. Then pass the skewers thru the loop and epoxy in place. then you have something more secure to attach nose weight to and adjustable too.
 

kjkcolorado

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I was just looking through my 'miscellaneous hardware' drawers and was looking at a long 1/4" bolt. I could sink that head first with a washer in the tip, put two skewers (one on each side of the bolt) to capture the washer so it can't pull through, and epoxy in a slurry of lead shot over the washer and skewers. Leaving a couple inches of the bolt exposed would be just like all thread that I can cap with an eye nut for harness attachment. Basically would function just like bill2654 described above. I would measure out the hardware and lead shot to equal the nose weight I would need for the smallest motors I would fly since that will be a permanent modification. With the extra exposed length of the bolt, I'll use something like the lead fishing weights described by Fred previously to keep my CP/CG relationship safe for larger motors.
 
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kjkcolorado

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Having a good secure plan for the nose weight makes me comfortable with the 54 mm motor mount on this project. Who else has a short and fat HPR rocket? What's the biggest motor you can fit in it?
 

jis2

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Just a thought but if 3 grain 54mm is as big as your planning to fly why not put a long 38mm motor in there ? It may move enough weight forward that nose weight may not be needed ? Good luck , Jim
 

FredA

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Who else has a short and fat HPR rocket? What's the biggest motor you can fit in it?

I have a "stock" 5-inch Gizmo that can take a 98mm M (research)....needs 24 ounces of lead in the tip of the NC.
 

MikeyDSlagle

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I have 2 short fat rockets. I love short stubbies! No reason not to go 54mm, you can always adapt down. But if you want simple sport flying, adding an adapter will add a step, lowering the simplicity.

Madcow Cowabunga (Wastelander) is set up with an Adept22 in the nose cone for altitude deploy. With some fancy packing I should be able to fly it on the 38/600 case. I will have to check that when I get a chance. Sims say I will need ~2 oz in the nose. I had an adjustable weight system "prototype" in the nose held in with epoxy. It came loose during flight. Luckily it was only an empty piece of conduit. But I know to use some means other than epoxy to hold stuff to that nose cone.

Madcow Tembo (still being built) is going to have a 54MMT in it and will (probably) have a nose cone bay to accommodate the same sled as the Wastelander. It has a longer nose cone and with the setup I have in mind it will fit the AT 54/1280 case. Don't know how that compares to CTI 3G. Again, it may take some fancy packing. Sims say ~8 oz in the nose. That weight is going to change as I get my bay added and such.

My intent is to permanently add the minimum amount of weight to the nosecones. Which is usually accomplished by adding the sled and rings and what not for recovery. Then have some sort of adjustable system for the larger flights. But in cases like my Tembo, where the needed weight difference between small motor and big motor is only 2 oz, I will probably permanently fix the entire weight in place. And that weight will likely be sand and epoxy. I have plenty of room.

Mikey D
 
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Handeman

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My thought is, where are you flying? What's the waiver and how big is the field. If a 38mm baby J will put the rocket near the waiver and/or too high to land on the field with normal wind conditions, why bother with all the trouble of nose weights to put a 54mm in there.

Isn't this supposed to be something fun you can fly at every launch? If the whole thing is going to weigh less then 3 lbs, I would stick with the 38mm. I have other rockets I can try to slam the waiver with, I don't need to do it with my fun flier.

My 4" short stubby got rebuilt with a 38mm and it still flew most times on G & H 29mm because that is the rocket that flew on windy days, days with low cloud cover, etc. It was one of my most flown rockets because is was one I could fly in almost any condition without worrying if it was going to disappear or drift off. It would do 300ft on an F52T. It ended up being retired and never had anything larger then a 4g 38 in it.

Just my :2:
 
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Rocketjunkie

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I don't get all this talk of nose weight. This is a scratch design - make the fins large enough for stability. A margin of 10% of the rocket length is enough. This allows the use of smaller motors for those not optimum conditions days.
 

caveduck

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Gorilla Glue FTW in attaching nose weight permanently. I gave up on epoxy for that, too much trouble with skewering, bolting and other hacks to make up for the epoxy just not sticking that well to plastic.
 

kjkcolorado

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Good points and things to think about. Thanks for the suggestions.

In terms of waiver concerns, I don't have any. Our summer launch site waiver is 20K, and winter site is 12K. The largest 3-grain 54mm CTI motor in this 4" airframe won't touch the waiver with the estimated weights I'm looking at (even with minimal nose weight).

In terms of design and trying to make the fins large enough for the desired stability, I have been playing with the fin design in OR. However, there is still a certain look I want that is limiting some possibilities. One of the criteria is not wanting the fins swept back extending aft of the airframe. Obviously that's not ideal in terms of stability.

Good point on the Gorilla Glue. I do have one rocket where I used that to secure nose weight, and it is a good choice for this application.

So, I'm still thinking that based on these thoughts and desires I would be best off with an adjustable nose weight system and the 54mm motor mount. I could prove myself wrong in the end, but that's where I think I'm heading.
 
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