Low mass, high drag designs?

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ThirstyBarbarian

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I like long-burn motors (3 seconds or more) but I'm not a big fan of moon-shot flights, especially those that require dual deployment and/or trackers.

Often rockets that are built around low-thrust, long-burning motors such as Estes F15 (3.5-second burn), or CTI mellow yellow loads (around 4.4 seconds) are designed for high flights. The rockets are light so they can get off the pad with enough speed to be stable, but then they are also built skinny so that they can keep climbing for the full burn.

I'm interested in building a rocket for long burn motors with a low mass that can get stable quickly, but with high drag that will hold it back once it's up to speed.

An extreme example is a saucer. Once they reach a certain speed, they don't really go any faster, and their drag balances their thrust for the remainder of the burn.

I don't want to build a saucer, but I'd like to build a rocket that has some of those same characteristics.

I'm thinking the rocket will need fat fins for drag. Back-swept fins to keep the motor forward. Large-diameter but light body tube. Lightweight nose cone. Avoid heavy materials such as plywood.

Some types of designs I think might be good for inspiration are the Mosquito, Baby Bertha, and Der Red Max. I'd like to scale the design for 29mm motors such as the Estes F15, or possibly CTI G33, H42, or H53 mellow loads. It does not need to built to withstand higher thrust motors.

Anyone have any ideas for design, materials or techniques? Thanks!
 
I'm thinking the rocket will need fat fins for drag. Back-swept fins to keep the motor forward. Large-diameter but light body tube. Lightweight nose cone. Avoid heavy materials such as plywood.

Some types of designs I think might be good for inspiration are the Mosquito, Baby Bertha, and Der Red Max. I'd like to scale the design for 29mm motors such as the Estes F15, or possibly CTI G33, H42, or H53 mellow loads. It does not need to built to withstand higher thrust motors.

Anyone have any ideas for design, materials or techniques? Thanks!

Build your fins with model-airplane-style techniques...a fat, symmetrical airfoil section with Spar, rib, and leading/trailing edge stringers. Something like this: 12662012_10205605344676268_1544437183944045512_n.jpg

Then cover it with thin balsa sheet. Canted tip plates will help induce even more drag, and if you really want to dirty it up you can brace the fins with wires or struts.

Join this Facebook group for inspiration/examples: Balsa Model Aircraft Builders Association
 
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What a great topic! I also like long burns, and also getting my rockets back afterwards. :)

My first thought, beyond what you've mentioned, is simply to think through the various high-drag accessories commonly found on low power rockets: pods, scoops, excessive fins and other junk sticking out of the body. Adding this stuff without too much weight would be the challenge I guess, although given the rocket would not be going too fast it should be possible to use lightweight techniques (like the airplane techniques above). Oh, and maybe canting the fins to induce some spin.

Unfortunately I can't experiment with a lot of this stuff in OR because it doesn't support pods, or fins attached to fins, or other kinds of stuff.
 
I think a cone design would work well in this application. I think a 4/1 ratio is a good place to start. length/width. the wider the more drag until you approach a saucer shape. Base drag causes huge penalties esp after burn out !
 
Cones and spool rockets. Spools pretty much stop going up when the thrust runs out. Hobby Lobby has lots of big Easter eggs out now. One of those as a "cone" should be draggy as heck!
 
Oh, and maybe canting the fins to induce some spin.

Hmm, OR shows no altitude loss from canted fins on one test design. That is counter to what I thought. Oh well.

Anyway, one thing that does add drag is a tail cone (larger at the back). Adding a modest tail cone (1" to 1.6" diameter, 2" long) I was able to knock 20% off the apogee, and it'll still look like a rocket (if that's what you're going for). By enlarging the back of the cone to 2.6", I cut the altitude in half. And that should weigh almost nothing....
 
a design that incorporates tube fins might work.
Rex
Completely agree. I had the same goal with the attached design. According to the Jan/Feb, Mar/Apr 2008 and Mar/Apr 2010 Sport Rocketry series on tube fin designs, tube fin rocket Cd is unusually dependent on velocity, increasing rapidly with velocity. Higher L/D designs have higher Cds that stubbies. Since the Cd increases rapidly with velocity, liftoff velocity is not impeded, but max altitude is.

The attached design uses thin wall BT80 to reduce mass with a stuffer tube used for structural reinforcement and to decrease the volume needing pressurization during parachute ejection. The centering rings are lite ply. To increase durability, the tube fins were made from thicker walled 2.56" MPR tubing. Because of the greater OD of the 2.56" thick wall tube fins vs BT80 and the resulting greater ID of the inside of the tube fin ring, the OD of the BT80 under the 2.56" tube fins was increased by gluing on a fin-length split piece of thin wall BT-80. This came very close to being right, requiring only a single 1/8" x 1/8" balsa strip between two of the tube fins to eliminate the resulting gap.

I've only flown this with E20-4s and the measured altitudes were, amazingly, within feet of predicted. It's highly stable and puts on a nice show because of the relatively low climb rate due to high drag. It's a 24mm design, but I simmed it with 29mm E16 and F15 for you. Velocity off a 48" rod is about 32.3 fps for both of those motors, a bit low, whereas it's 40.3 fps off a 36" rod for an E20-4 which, BTW, is the perfect motor match for this design.

View attachment 2.56 inch tube finned 24mm.rkt
 
This subject is right up my alley!

I'm (slowly) working on a 10" diameter paper rocket. I don't have all the pieces and parts fabricated yet, but by my initial guesstimates it should be a fuzz over 2 pounds.

Just a mockup of some fins. I think I've decided on going with 4, for two reasons, more drag, and better chances of landing without damage.

ImageUploadedByRocketry Forum1454881275.099980.jpg

https://www.rocketryforum.com/showthread.php?131623-My-10-inch-diameter-Paper-Rocket

I'm working on the final wraps on the stuffer tube...today!
 
Build your fins with model-airplane-style techniques...a fat, symmetrical airfoil section with Spar, rib, and leading/trailing edge stringers. Something like this: View attachment 281810

Then cover it with thin balsa sheet. Canted tip plates will help induce even more drag, and if you really want to dirty it up you can brace the fins with wires or struts.

Join this Facebook group for inspiration/examples: Balsa Model Aircraft Builders Association

The model airplane technique looks super light. I think I want to try something a little less involved to start, but I'll keep it in mid for the future.

The canted fin tips are a good idea. I think Estes has a newish kit that uses that.

Thanks!
 
What a great topic! I also like long burns, and also getting my rockets back afterwards. :)

My first thought, beyond what you've mentioned, is simply to think through the various high-drag accessories commonly found on low power rockets: pods, scoops, excessive fins and other junk sticking out of the body. Adding this stuff without too much weight would be the challenge I guess, although given the rocket would not be going too fast it should be possible to use lightweight techniques (like the airplane techniques above). Oh, and maybe canting the fins to induce some spin.

Unfortunately I can't experiment with a lot of this stuff in OR because it doesn't support pods, or fins attached to fins, or other kinds of stuff.

Thanks! Hopefully everyone who likes long burns and short walks can get something out of this.

I do like sci-fi rockets, so interesting details glued all over is appealing to me, but like you said, light weight is also part of the challenge. I think for the first round, I want to keep it simple, but I like the idea of a sci-fi rocket too. Maybe a simple design for the F15, and something more complex for the mellows.
 
I think a cone design would work well in this application. I think a 4/1 ratio is a good place to start. length/width. the wider the more drag until you approach a saucer shape. Base drag causes huge penalties esp after burn out !

Great point about the cone design. Maybe something along the lines of a much simplified Mars Lander might work. Hmmm...
 
Cones and spool rockets. Spools pretty much stop going up when the thrust runs out. Hobby Lobby has lots of big Easter eggs out now. One of those as a "cone" should be draggy as heck!

I guess this would be the perfect time of year to look for egg-shaped thinks to turn into rockets. That's a great point. Thanks!
 
Hmm, OR shows no altitude loss from canted fins on one test design. That is counter to what I thought. Oh well.

Anyway, one thing that does add drag is a tail cone (larger at the back). Adding a modest tail cone (1" to 1.6" diameter, 2" long) I was able to knock 20% off the apogee, and it'll still look like a rocket (if that's what you're going for). By enlarging the back of the cone to 2.6", I cut the altitude in half. And that should weigh almost nothing....

That's interesting, so you sort of "flare" the bottom of the rocket by adding a short cone to the bottom?
 
Completely agree. I had the same goal with the attached design. According to the Jan/Feb, Mar/Apr 2008 and Mar/Apr 2010 Sport Rocketry series on tube fin designs, tube fin rocket Cd is unusually dependent on velocity, increasing rapidly with velocity. Higher L/D designs have higher Cds that stubbies. Since the Cd increases rapidly with velocity, liftoff velocity is not impeded, but max altitude is.

The attached design uses thin wall BT80 to reduce mass with a stuffer tube used for structural reinforcement and to decrease the volume needing pressurization during parachute ejection. The centering rings are lite ply. To increase durability, the tube fins were made from thicker walled 2.56" MPR tubing. Because of the greater OD of the 2.56" thick wall tube fins vs BT80 and the resulting greater ID of the inside of the tube fin ring, the OD of the BT80 under the 2.56" tube fins was increased by gluing on a fin-length split piece of thin wall BT-80. This came very close to being right, requiring only a single 1/8" x 1/8" balsa strip between two of the tube fins to eliminate the resulting gap.

I've only flown this with E20-4s and the measured altitudes were, amazingly, within feet of predicted. It's highly stable and puts on a nice show because of the relatively low climb rate due to high drag. It's a 24mm design, but I simmed it with 29mm E16 and F15 for you. Velocity off a 48" rod is about 32.3 fps for both of those motors, a bit low, whereas it's 40.3 fps off a 36" rod for an E20-4 which, BTW, is the perfect motor match for this design.

Thanks for this. I'm on my iPad right now, but I'll check out the file when I'm on my desktop.
 
This subject is right up my alley!

I'm (slowly) working on a 10" diameter paper rocket. I don't have all the pieces and parts fabricated yet, but by my initial guesstimates it should be a fuzz over 2 pounds.

Just a mockup of some fins. I think I've decided on going with 4, for two reasons, more drag, and better chances of landing without damage.

View attachment 281818

https://www.rocketryforum.com/showthread.php?131623-My-10-inch-diameter-Paper-Rocket

I'm working on the final wraps on the stuffer tube...today!

Cool! I'll check this out later when I have more time. My experience with short stubby rockets is that they prefer to get off the pad in a hurry. Usually you need a motor with a hard kick to get them up to speed and flying straight. For example, a Big Daddy likes an E12 or an E20 over an E9. But maybe if it is light enough, a long burning motor can get it up to speed fast enough. I think I want the rocket to be relatively short, so a lot of weight is not spent on body tubes and stuffer tubes, but it can't be marginally stable like a lot of the very short stubby rockets are. I think my basic design would need to be different from yours, but I do want to check out your lightweight techniques. A 10" rocket under 2 pounds is incredibly light! Wow!
 
Well, thanks to you and this thread, my brain went into over drive. Create a light weight, high drag, stable design. I'll stay in my wheelhouse with a max of 24mm . We don't want a knock off of another design. Landing undamaged is a given, so lots of protuberances are a bad thing. AH HECK, Let's go for broke and make it retro to boot. Yep, the Wolf Works are back in business---more to come !
 
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Well, thanks to you and this thread, my brain went into over drive. Create a light weight, high drag, stable design. I'll stay in my wheelhouse with a max of 24mm . We don't want a knock off of another design. Landing undamaged is a given, so lots of protuberances are a bad thing. AH HECK, Let's go for broke and make it retro to boot. Yep, the Wolf Works are back in business---more to come !

Darn you!!!

I build like molasses in January! Y'all are gonna have flight videos before I even get my crap done!

Except -maybe- one difference, mine will be entirely scratch built, with no 'off the shelf' parts.


Curse you fast builders :(
 
Well, thanks to you and this thread, my brain went into over drive. Create a light weight, high drag, stable design. I'll stay in my wheelhouse with a max of 24mm . We don't want a knock off of another design. Landing undamaged is a given, so lots of protuberances are a bad thing. AH HECK, Let's go for broke and make it retro to boot. Yep, the Wolf Works are back in business---more to come !

Excellent! The more ideas and designs, the better.
 
Darn you!!!

I build like molasses in January! Y'all are gonna have flight videos before I even get my crap done!

Except -maybe- one difference, mine will be entirely scratch built, with no 'off the shelf' parts.


Curse you fast builders :(

You definitely have something special going with the extreme scratch building. I just checked out your 10" paper rocket thread, and it is very cool.

I think as I get my feet wet with this project, I might try to stick with mostly off the shelf parts, substituting lightweight materials where I can. We'll see how far I go with that. It seems like a lot of larger MPR and HPR kits use robust, but heavy materials for centering rings and fins especially, because they're designed to take high-thrust motors. But you should be able to swap those out if you commit to only using low thrust.
 
You definitely have something special going with the extreme scratch building. I just checked out your 10" paper rocket thread, and it is very cool.

Thanks! As much as I like to watch others with the Lamborghini type, neck snapping, teleport-off-the-pad Mach busters, I prefer mine to be more along the lines of an early 70's Cadillac. Low and slow, baby!

That is my goal, is to use the -smallest- motor possible, but yet be safe and stable.

My -very- preliminary OR sims say it will leave the pad at 44 ft/sec, and reach an apogee of 153 feet...on a 24mm E28 :D

The hard part? Most of the 'optimum delays' are -under- 3 seconds. I've never drilled that far down!!
 
there you have a problem, AT delays will fail if you try to drill down to under 4 seconds(it is not pretty, sure is showy though having fire from both ends).
Rex
 
there you have a problem, AT delays will fail if you try to drill down to under 4 seconds(it is not pretty, sure is showy though having fire from both ends).
Rex

Geeze Rex! You're raining on my parade! Thanks :(

I guess that much more of a reason to make it a rear eject? I've never made one before, but I understand the concept. Also part of the reason I made a larger-than-needed stuffer tube, I left rear eject open as an option.

And yet another reason to get my feet wet in deployment electronics...is there a Conman in the house? Unfortunately, the JL-CR is out of my budget right now...
 
rather not have to put out grass fires. rear eject would likely be best for this bird.
Rex
 
Pssst...Rex...

We're posting this stuff in the -wrong- thread!

Sorry ThirstyBarbarian, we will stop hijacking your thread :)
 
That's interesting, so you sort of "flare" the bottom of the rocket by adding a short cone to the bottom?

That's what I was thinking. Something like this, although this is not a working design (and still too heavy):

draggy.png

View attachment draggy.ork

That sims to 600+ feet on an F15, reaching a whopping 120 mph. :)

Seems like you could adjust the size of the cone to set whatever altitude and speed you're shooting for.

Anyway, just one approach. I've seen cone-tailed rockets, but never one that combines a tail cone and fins like this.
 
I have a cone fin rocket, interesting to watch for sure. However, while adding drag is easy, surely the ideal is to get the drag by making it huge but still light. Or huge-looking, such as by big fins. The saucer design gets drag and acceleration, but they look small so kind of defeat the point unless you just like burning big motors.
 
In addition to tube fins, ring tails have extremely high drag. I've done a couple designs with concentric ring tails. One which is a 3 oz model that (according to the sim) would require an F32 to break 700 feet. An E9 pushes it to about 525' or so.
 
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