Low mass, high drag designs?

Here is a dry fit of the whole "skeleton." My wife said she liked how it looked without the skin and I should just fly it like that. How's that for a high-drag design!



Here's the lover section. The motor mount tube is a cardboard tube from a roll of either foil or plastic wrap. It's a tight fit for a 29mm motor. The small centering rigs at the top fit into the bottom of the parachute bay tube that is the central tube of the upper section.



Here's a detail of how the fin tabs interlock with the slots cut into the centering rings. The stack of centering rings at the top and at the bottom get glued together together.



Here is the upper section, which is built around the parachute bay. The tube is a 2' diameter tube from a roll of wrapping paper.

burkefj said:

Looks good,real similar to what I've done, are you planning any reinforcement of the fin tips?

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This design is inspired by yours --- thanks for the great idea! This is just a small rocket to try out the technique and see how it works.

I think reinforcing the fin tips would be a good idea, but I'm not sure how to do it. What do you recommend?

rstaff3 said:

Cool, this technique works great. I love them fat light rockets! You can also trim some foam from the pouter edges and glue in dowels on the edges of the fins. Not that important on a light rocket but it would help on the lower tip of those fins. I had one HPR bird with framed foam fins hit under power at a low angle and the fins were barely dinged. One fin was sheared but the fin didn't buckle.

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This one is very small, and the entire rocket only weighs about 20 ounces. I think I will definitely need to do some reinforcement on larger birds. How exactly do you do what you were describing?

Super Neon XL is a good stock Estes one (tube fin). Goes up on 24mm. I use an F44-8 often. Here's one where the chute was tangled and still it drifted down. (Into O1d Dude's car) But even trailing a chute it was slow coming down. [video=youtube;qFXMDGCrf1A]https://www.youtube.com/watch?v=qFXMDGCrf1A[/video]

Here is the upper section with the cardstock wrap applied. I used a spray on adhesive --- sprayed one side of the sheet and just wrapped it around the centering rings. The loose end of the wrap did not seem to be staying stuck down, so I used a length of sealing tape to secure it (not shown). I think if I had just waited, it would have sealed down fin without the tape. Before this, I had used wood glue to glue the centering rings in place, and I had glued the end of the motor mount tube into the parachute bay tube.



Next I dry fit the lower section. I got everything situated and then friction fit the retainer into place, which seemed to hold everything securely. I filleted the edges where the tabs meet the centering rings and the motor tube (shown). Later I filleted where the tabs meet the motor tube.



I glued some strips of foam board to the fins to provide a surface for glueing the lower skin on.



Here is the lower skin glued in place. This part did not turn out so great. I feel like the spray adhesive is not really holding the lower skins very securely. I might use some more sealing tape here just to be sure.

The nosecone is half of a Hobby Lobby Easter Egg with a 4.875" diameter. (I've got two more in the set with diameters something like 7.5" and 9.75", so if this rocket works out, I can build some much bigger ones!) I had to cut off the part where the two egg halves screw together.



I cut a 2" long piece off of an unused section of the same wrapping tube used for the parachute bay. I slit it lengthwise and reglued it so that it would fit inside the wrapping tube. Than I glued a wrap of the cardstock around it to stiffen it up a bit. I cut two disks of foam core to act as a bulkhead and glued the small section of tube to that.



The shock cord goes though the bulkhead and is secured at the back by going through a small piece of wood, tying a knot that will not pull through the wood, and adding some wood glue.



Here is the assembled nose cone.

ThirstyBarbarian said:

This one is very small, and the entire rocket only weighs about 20 ounces. I think I will definitely need to do some reinforcement on larger birds. How exactly do you do what you were describing?

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I first find dowel stock that will fit in between the paper layers on the poster board. I forget the dimension. I then take a hobby knife with a new blade and cut out a 'v' in the foam on the outer edges. I have wanted half the dowel to fit in the slot leaving a nice rouned fin edge. This takes practice and I still haven't mastered it. I then cut pieces of dowel that overhang the edges being framed and glue one piece in. After the glue has partially set, I cut it flush using a razor saw. I then move on to another edge. Some subsequent cuts are made by eyeball and others are trimmed in situ.

Man, that is clear as mud. Holler when you try this.

I used a long cardboard tube for a launch lug. I feel like the skin of the rocket is not a super strong material, so there's no way to attach a rail button to it. I also feel like a conformal guide glued to the skin would also possibly tear the skin if the rocket were mounted on the rail and something like a gust of wind were to put any side-to-side torque on the guide. It seems like a lug would not have the same problem with side-to-side torque, because the rocket can rotate around the rod. I also opted for a very long lug to avoid having too much stress on any single part of the skin. In this shot, you can see the sealing tape I used to seal down the edge of the skin. It would have looked nicer without it...



Here is the beast fully assembled.



Here I am holding it. To get a sense of the scale, I am twelve feet tall, and the rocket is 6 feet long.



I may have exaggerated my overall length and girth again...

The rocket is actually 30.25" long, 4.875" diameter, and it weights 20.30 oz without motors. The stability is 1.12 cal without motors and without the "base drag" taken into consideration. I think it's pretty stable for such a short stubby fat rocket.

rstaff3 said:

I first find dowel stock that will fit in between the paper layers on the poster board. I forget the dimension. I then take a hobby knife with a new blade and cut out a 'v' in the foam on the outer edges. I have wanted half the dowel to fit in the slot leaving a nice rouned fin edge. This takes practice and I still haven't mastered it. I then cut pieces of dowel that overhang the edges being framed and glue one piece in. After the glue has partially set, I cut it flush using a razor saw. I then move on to another edge. Some subsequent cuts are made by eyeball and others are trimmed in situ.

Man, that is clear as mud. Holler when you try this.

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I think I get it. Thanks for the tip!

ThirstyBarbarian said:

I used a long cardboard tube for a launch lug. I feel like the skin of the rocket is not a super strong material, so there's no way to attach a rail button to it. I also feel like a conformal guide glued to the skin would also possibly tear the skin if the rocket were mounted on the rail and something like a gust of wind were to put any side-to-side torque on the guide. It seems like a lug would not have the same problem with side-to-side torque, because the rocket can rotate around the rod. I also opted for a very long lug to avoid having too much stress on any single part of the skin. In this shot, you can see the sealing tape I used to seal down the edge of the skin. It would have looked nicer without it...

View attachment 311913

Here is the beast fully assembled.

View attachment 311912

Here I am holding it. To get a sense of the scale, I am twelve feet tall, and the rocket is 6 feet long.

View attachment 311914

I may have exaggerated my overall length and girth again...

The rocket is actually 30.25" long, 4.875" diameter, and it weights 20.30 oz without motors. The stability is 1.12 cal without motors and without the "base drag" taken into consideration. I think it's pretty stable for such a short stubby fat rocket.

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Oh man, dats NICE! Easter egg cones rule

I ran sims for a bunch of motor configurations.

I had originally hoped that this rocket would be light enough for Estes black powder E16 and F15 motors, but unfortunately it only leaves the rod at 35 fps for E16 and 32 fps for F15 --- a bit too slow I think. The rocket is light, but it turned out to be quite a bit heavier than I had expected. I think I had guessed it might be 11 or 12 ounces, and it turned out to be 20.

An Aerotech single-use E20-4 has it leaving the rod at 47 fps and reaching 256 feet. That's a safe speed off the rod and a very low apogee. I can actually see possibly flying that in the local schoolyard. If the grass weren't still wet, I think I'd run over there right now and try it! Two months of club launches rained out, and I've got the fever!

What I think is really interesting about this rocket is that it looks like can be safely flown on E through H motors! Four motor classes! It's light enough to get off the rod safely on an E20, but it has enough drag to keep to just under 1500 feet on an H53.

I'm really looking forward to trying some of the motors with a longer burn, like the CTI mellow yellow loads G33, H42, or H53. Unfortunately, I only have one of those in my current motor stash, and it's the H53. I'd prefer to start with one of the smaller ones, like the G33. I also have a G54, the red long-burn motor --- that is one of the fastest motors in the sims, and I'd prefer to start slower. Most likely (if I don't cheat and go to the schoolyard and use the E20), I'll start at a club launch with an Econojet F20 or F23 from the motor stash. If that works out, I'll try a G40 or G38 from the stash. And if that works out, I'll see if the vendor has a G33 or H42.

rstaff3 said:

Oh man, dats NICE! Easter egg cones rule

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Thanks!

Have you built a few using this foam board rings and skin technique or other lightweight techniques? Got any threads you can point me to or pics?

I really liked building this. This is actually my very first fully scratch built rocket, and I think it's kind of funny that it's foam board and card stock, not plywood and other HPR materials. One of the best things about it is I don't have a lot of very good power tools, so being able to cut all the parts with a hobby knife was a real plus! Maybe for the next one, I'll go heavy duty and use the 1/2" foam board!

Came out nice! What are you going to fly it on first?

Yes, it goes very quickly when you can just cut with a knife. How did you attach the rings and skin? On mine I also had some stringers that locked into notches in the centering rings to help support the thin skin, might not be needed on a rocket this short.

For mounting rail buttons, what I did was to basically glue a thin ply strip on the top and bottom of a centering ring, maybe 1/2" wide and all the way to the stuffer tube, and then glue a block right at the end of the fingers and sandwiched between them at the periphery of the centering ring, think of it like a clothes pin type of structure, the ply can glue to the stuffer tube and you drill into the block for the rail button.

Not sure if that made sense. On the picture below I didn't even go all the way to the stuffer tube and it's been very solid on my titan dyna-soar.
typically when I do mine I use a thin 1/8" ply thrust plate centering ring at the bottom and can glue a block on that plate for the lower rail button mount. But on something this light what I show could be used for both.

You do have to measure where the blocks are so after you skin it you know where to drill

Frank

neil_w said:

Came out nice! What are you going to fly it on first?

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Check out post 131 above. It might be an E20, if I can't wait and have to try it in the local schoolyard. If I go to a club launch, first flight will probably be on an Econojet F20 or F23, just to test it out on a low altitude flight. Then either a G40 or G38. After that, I want to try some long-burn motors in the G and H classes --- CTI mellow yellows probably. I have a H53, but that's a bit big for the next step. Hopefully the vendor will have some G33 or H42 motors --- he's been very low on CTI motors since the CTI accident, so I'm not sure what he will have.

burkefj said:

Yes, it goes very quickly when you can just cut with a knife. How did you attach the rings and skin? On mine I also had some stringers that locked into notches in the centering rings to help support the thin skin, might not be needed on a rocket this short.

For mounting rail buttons, what I did was to basically glue a thin ply strip on the top and bottom of a centering ring, maybe 1/2" wide and all the way to the stuffer tube, and then glue a block right at the end of the fingers and sandwiched between them at the periphery of the centering ring, think of it like a clothes pin type of structure, the ply can glue to the stuffer tube and you drill into the block for the rail button.

Not sure if that made sense. On the picture below I didn't even go all the way to the stuffer tube and it's been very solid on my titan dyna-soar.
typically when I do mine I use a thin 1/8" ply thrust plate centering ring at the bottom and can glue a block on that plate for the lower rail button mount. But on something this light what I show could be used for both.

You do have to measure where the blocks are so after you skin it you know where to drill

Frank

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I merely glue chunks of scrap (non-rocketry) wood on two of the centering rings. Buttons held in with wood screws.

burkefj said:

Yes, it goes very quickly when you can just cut with a knife. How did you attach the rings and skin? On mine I also had some stringers that locked into notches in the centering rings to help support the thin skin, might not be needed on a rocket this short.

For mounting rail buttons, what I did was to basically glue a thin ply strip on the top and bottom of a centering ring, maybe 1/2" wide and all the way to the stuffer tube, and then glue a block right at the end of the fingers and sandwiched between them at the periphery of the centering ring, think of it like a clothes pin type of structure, the ply can glue to the stuffer tube and you drill into the block for the rail button.

Not sure if that made sense. On the picture below I didn't even go all the way to the stuffer tube and it's been very solid on my titan dyna-soar.
typically when I do mine I use a thin 1/8" ply thrust plate centering ring at the bottom and can glue a block on that plate for the lower rail button mount. But on something this light what I show could be used for both.

You do have to measure where the blocks are so after you skin it you know where to drill

Frank

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Thanks for the tip on the rail buttons! That makes perfect sense.

I did not use stringers, but I think if the rocket were much bigger, it would be helpful. For mine, the rings were less than 3 inches apart, which is pretty close, so I think the skin is pretty well supported. If they were a greater distance apart, or if the rocket were much taller or much fatter, I think you would want stringers. I was actually surprised how strong the upper tube seemed for being so light.

Ah, ok, 3" spacing is pretty close, I was using 5-6" spacing so the stringers were needed to support the skin.

Your weight is pretty close to my 5" Pershing 1A that was also foam, and about the same length, it flies to about 400' on an F-39 or F-35, I built that with a 24mm mount. You have a bit more fin drag but should be similar.

Frank

More rings makes for a more robust and less squishy model. Fewer rings makes it lighter. Just gotta figure out what you want.

Yup, you can also tradeoff stringers for centering rings as well, they also add some longitudinal stiffness to the structure. I've loaded a 1' long 8" test case with 6" centering ring spacing and 8 stringers with 75 pounds and it held up without any crushing or deformation. For something this light it isn't needed most likely.

rstaff3 said:

More rings makes for a more robust and less squishy model. Fewer rings makes it lighter. Just gotta figure out what you want.

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