N-Decision: 15 inch x15 foot

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Feckless Counsel

Petitio principii
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TRF,

I’ve been out of the hobby for about 20 months. Tried to restart some my jumbo project efforts but got lost in all that other stuff. Looking for a kick-start I purchased a 15-inch fiberglass nosecone and two 59-inch lengths of fiberglass airframe to match. Pictures are attached.

I’ve always respected and appreciated this forum’s membership. May I please ask for a few ideas at industrial design? The design phrase I hope to express is "Guranteed Tough."

Feckless Indeed

15inNCandTubes.jpg
 
Well you could go with a dull gray rocket that is squarish and blocky to convey the idea of iron or steel
 
Nothing says tough like 1/4" hex head bolts around the shoulder of the nose cone and just outside the fin fillets going down the tube. Of course you'd cut off the threaded part-better if you can find them in aluminium....."strength thru fasteners" kinda thing...or just galvanize it with a few strategic 'weld' seams.
 
I like the textured look in some of the Rustoleum metallic paints. But may cost you dearly to do the whole thing with rattle cans.
 
If you have any airbrushing skills or know someone that does, nothing says industrial better than a brushed steel look with rivets at every seam. ImageUploadedByRocketry Forum1398863525.167274.jpg like this


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Is this another DeWalt rocket? I'm an engineer not a marketer but I think Bosch's Brute Tough evokes that "tough" image much better than any of the yellow tools. I really like the look of the Bosch Powerbox radio. That aluminum exoskeleton just screams overkill. Perhaps you could add something like that to your rocket. A conical nosecone that looked like a step drill bit would be pretty wicked.
 
TRF,

These ideas are very much appreciated. I agree with the bolted / riveted / welded design language. Good stuff.

The tubes and cone are manufactured by Public Missiles Limited. I’ve not yet received the goods. But I am absolutely stoked with their customer service and flexibility to date. Those are good folks with a “will do” attitude, committed schedules and fair market pricing.

Feckless
 
TRF,

Our rockets ride high substantially on the humble thrust plate, a bulkhead of paper or wood or meal at the extreme aft end of the vehicle. May I invite you to treat my calculation of what is sufficient?

My first critical assumption is to IGNORE reaction of the rocket. A real rocket accelerates up the guide. Resultant forces on the thrust plate are reduced by that reaction. A conservative calculation ignores that reaction taking the static case as design margin.

The basic thrust plate is an annular disk or washer. Fortunately such plates are well studied with textbook solutions available for SIMPLE cases. A good reference would be “Roark’s Formulas for Stress and Strain” ISBN 978-0070725423. The website “roymech” has published these results as pictured in the attachment.

Design goal, given the assumption above, will be to select material thickness (t) that equals the maximum allowable material stress (rho-m) given a rocket’s peak thrust PLUS the rocket’s weight (P). Note (k2) is a loading factor based on the plate’s annular web. If you’re interested (y-m) is the plate’s deflection.

If there are no objections to this line of reasoning I will tabulate numerical results for a plywood and an aluminum thrust plate under a 120 pound rocket lifted by a CTI N3301.

Feckless

Annular Plate.jpg
 
Fleckless, great to see you back!

Only issue with above... 120lbs seems light for a 15" diameter rocket.


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Tickled pink to see you back at it! I'm a big fan of stainless steel Allen screws. Just sayin' I think it really completes the industrial look, especially if they are wired!
-Ken
 
TRF,

Our rockets ride high substantially on the humble thrust plate, a bulkhead of paper or wood or meal at the extreme aft end of the vehicle. May I invite you to treat my calculation of what is sufficient?

My first critical assumption is to IGNORE reaction of the rocket. A real rocket accelerates up the guide. Resultant forces on the thrust plate are reduced by that reaction. A conservative calculation ignores that reaction taking the static case as design margin.

The basic thrust plate is an annular disk or washer. Fortunately such plates are well studied with textbook solutions available for SIMPLE cases. A good reference would be “Roark’s Formulas for Stress and Strain” ISBN 978-0070725423. The website “roymech” has published these results as pictured in the attachment.

Design goal, given the assumption above, will be to select material thickness (t) that equals the maximum allowable material stress (rho-m) given a rocket’s peak thrust PLUS the rocket’s weight (P). Note (k2) is a loading factor based on the plate’s annular web. If you’re interested (y-m) is the plate’s deflection.

If there are no objections to this line of reasoning I will tabulate numerical results for a plywood and an aluminum thrust plate under a 120 pound rocket lifted by a CTI N3301.

Feckless


That looks good to me.
 
A rocket weighing less than 120 on an N.......nice:)

Can't wait to see construction and launch pics
 
A rocket weighing less than 120 on an N.......nice:)

Can't wait to see construction and launch pics

Welcome Back!

Please put a big mount in it and adapt down to the N. 6" core for sure. If it was my rocket, they would be a few 75mm holes too. You know, just in case...

What's PML charging for 15" components? Are these custom for your specific project?

Todd
 
Well, one could easily have 6 75mm mounts and one main 8 inch hole... :) If one wanted to be truly Macho.
 
TRF,

Attached are tabulated results for various thrust plate materials sorted by weight. That assumes a pad weight of 200 pounds and CTI N3301 maximum thrust of 1141 pounds. Note the peak thrust to weight ratio is about 6:1 but the average thrust to weight ratio is about 4:1. Plate diameter is 15.16-inch with a single central hole of 98mm diameter.

A few things to note:

1. Birch plywood 1.5-inches thick is indicated. Note the simply supported case, an annulus upon which the airframe perimeter rests.
2. Aluminum weighs 30% less than plywood but is 3 times the cost.
3. That thickness of plywood is as “strong” as 0.28-inch steel but is 60% lighter.
4. Magnesium alloy is an interesting possibility but availability appears spotty and machining could be tricky.
5. Composite materials offer perhaps the best “strength to weight” value. Note deflections near 55% thickness.
6. Titanium would deflect a full material thickness. Maybe bending stress should be considered at the airframe attachment?
7. Beryllium is awesome but can only be afforded by taxpayer’s money. It is a poisonous metal that should be treated with respect.

My choice will be aluminum. Reason is machining. I plan to cut the thrust plate for 152mm motors with an “outside” screw pattern for a 98mm adapter plate.

Feckless

Thrustplate Yields.jpg
 
Todd and David,

Would appreciate some advice on exact motor dimensions, thrust curves and potential suppliers for high impusle motors. Perhaps we could have that discussion offline?

Feckless
 
Todd,

Cost of the 3 piece set is about $2k landed in Maryland. I had contacted Frank Uroda for 22-inch tubing for the "Unfinished Business" project. He made a reasonable offer on that 22-inch tubing but also told me about an off-catalog nosecone he had in stock. Instead of the 22-inch tubing I purchased the nosecone and commissioned the airframe.

Feckless
 
TRF,

These are the hard things. In flying very-high power how do we assure our designs are safe for participant, for neighbors and for spectators alike. It is an individual responsibility. One that demands we exhaust the limits of our ability. It is generously granted and allows us, in good faith, to fly as we would.

None is so important and, arguably, as inscrutable as the issue of destructive fin flutter. A very-high power rocket without adequate control lifted upon its airframe is a dangerous thing indeed. There chance rules the outcome.

I would not delegate safety to some "high-tech composite stuff" that was "amazingly stiff compared to its weight." Instead I want to understand. So I will respectfully ask your help in reviewing an analysis of fins appropriate to this project.

If you are game I will attempt to design a set of composite fins capable to 800 mph. Your critical design review is welcome and appreciated, criticism being the thing in that most positive sense.

Feckless
 
TRF,

These are the hard things. In flying very-high power how do we assure our designs are safe for participant, for neighbors and for spectators alike. It is an individual responsibility. One that demands we exhaust the limits of our ability. It is generously granted and allows us, in good faith, to fly as we would.

None is so important and, arguably, as inscrutable as the issue of destructive fin flutter. A very-high power rocket without adequate control lifted upon its airframe is a dangerous thing indeed. There chance rules the outcome.

I would not delegate safety to some "high-tech composite stuff" that was "amazingly stiff compared to its weight." Instead I want to understand. So I will respectfully ask your help in reviewing an analysis of fins appropriate to this project.

If you are game I will attempt to design a set of composite fins capable to 800 mph. Your critical design review is welcome and appreciated, criticism being the thing in that most positive sense.

Feckless

I'm in.
 
Tim,

Your engaging this sophomore is very much appreciated. Could we please discuss some of the literature circulating about the subject?

Most frequently cited is NACA Technical Note 4197 dated 1958. The fin flutter equation derived there appears prominently in amateur publications such as “Peak of Flight” and “Info Central.” It is derived from NACA Report 685 where the authors, as part of a parametric sensitivity study, “prove” the formula “graphically” given certain constrains (Graph 1B-a).

Those constrains, important to accurate results, are:

1. Mode is bending-torsion only.
2. Material is isotropic.
3. The “aspect ratio” of fin thickness over mean cord is constant between 5% and 10%.
4. Fins are “heavy” relative to a defined volume of air.
5. Fin’s natural frequency in torsion is much larger than fin’s natural frequency in bending.

Have I cataloged all the relevant constraints or am I missing anything from those approaches?

Feckless
 
Have I cataloged all the relevant constraints or am I missing anything from those approaches?

Feckless

Yes, and in your outline fundamentally described "our world" of the flat fin interacting with the air pressure and it's disturbances. Saying that, as in most engineering disciplines, all equations are simplified to only approximate the real world.

Tim (sophomore too, but wanting to help see this beast get off the ground!)


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TRF,

My parts have arrived. I’ve a few notes for those interested:

UShip.com is the way to transport big parts. $138 for 100-pounds shipped 610 miles. Total shipping time was 2 days. That comes with $5 per-pound standard insurance. $35 additional will buy you $10k of insurance with a $500 deductable. Similar offers from FedEx Freight and UPS Freight approached $400.

Nosecone: Weight is 9.9 pounds. Exposed length is 60-inches. Shoulder is 11-inches. Construction is two-part mold. Primary material is glass cloth and epoxy resin. Carbon reinforcements are apparent through a thin but complete gel coat.

Airframe: Each 59-inch length weighs 14.6 pounds. Inside dimension is 15.10-inch with 0.93-inch wall. Construction is consistent with G10. Surface is almost ready to paint save an 800 grit wet sand. Backlighting indicates no voids or bubbles. Ends are clean and square.

Coupler: Well, there isn’t one yet. And that will be subject of a future post.

Total weight so far is 39 pounds or 0.22 pounds per-inch. A good start I think?

Feckless
 
I'm sure I'm not alone in this......

I likely won't be adding much to this thread, as I likely have little of value to add. However, I absolutely WILL be following this thread eagerly. This is a very cool project, and one that I'm sure I will be learning a lot from. Please post lots of details/pics/calculations/design tweaks/etc. and know that folks are watching - even if silently.

Would love to see this one fly. Thanks for sharing it all with us.

s6
 
s6,

Thanks for your kind regards. Please know all contributors are respected and valued. I hope each will say as their mind wanders. It could be that most important comment.

Feckless
 
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