More Half-Baked Designs Thread

[lakeroadster]

I've read, not experienced, that swing tests are really hard to perform on very long rockets. And I think this would have to be very long (as shown in the video) in order for the body lift to be much greater than nose cone lift working against you. If I can satisfy myself with calculation (which remains to be seen) then I hop to crash only one or two rockets.

The guy in the video spoke about breaking his finless rocket during a swing test, he calls it a "twirl test"... check the video at 9:30

Swing tests help the builder to determine stability... but they load the body tubes in a manner they likely will never see during actual flights.

Are we approaching thin wall body tube construction, that is pressurized? Wouldn't that be sweet.

 

[jqavins]

His body tube was extremely flimsy. It seems like he wound it with just a couple of layers of office paper, and at a couple of points in the video we see his tubes and couplers crumpling. I'd use BT-20s for a min diameter build.

 

[lakeroadster]

His body tube was extremely flimsy. It seems like he wound it with just a couple of layers of office paper, and at a couple of points in the video we see his tubes and couplers crumpling. I'd use BT-20s for a min diameter build.

His were 4 layers thick, at least that's what he said in the video.

It'll be interesting to see how the BT-20's work for you. I'm thinking the extra weight of the heavier wall construction will complicate your ability to make the rocket stable.

 

[BigMacDaddy]

His were 4 layers thick, at least that's what he said in the video.

It'll be interesting to see how the BT-20's work for you. I'm thinking the extra weight of the heavier wall construction will complicate your ability to make the rocket stable.

I am planning to rebuild my Leaner Meaner Machine so will see if I can manage a swing test without the fin section and get it stable... 3x 18" BT50 tubes w/ a weighted nose cone. I am contemplating how to distribute the weight of the string along the body without messing up the swing test... Is my thinking correct that it would not make this invalid if I had a 12" or so support taped to the body tube -- as long as I center the support at the CG it should not change where CG is and mess up the test. Correct?

 

[mjennings]

Yeah when I was a kid (middle or high school) I tried to swing test a Mean Machine. I didn't destroy it but popped at least one fin off. Swing test is dependent on small angle assumption. Otherwise you get lots of side airflow over the fins and nose which gives a different result. Max rocket length to string length is likely in the range of 10% - 15%. Although I couldn't find a number in any of my references.

 

[jqavins]

And, up to a point, you can use a longer string for longer rockets, but when the rocket is quite long it's hard to swing it on such a long string. If 15% is right (I don't know either) then a six foot rocket would need a 40 foot string, and swinging that is just not going to happen. If 15% is too conservative, well, it's not happening on a 20 foot string either, at least not if I'm the one standing in the middle and swinging it.

It'll be interesting to see how if the BT-20's work for you. I'm thinking the extra weight of the heavier wall construction will complicate your ability to make the rocket stable.

Fixed that. The body tube weight will mean more nose weight is needed to get a given C_G movement. I'm thinking it'll need a really punchy motor; the Q-Jet D20 or C18 would be good bets, or the AeroTech D21 if I can still get them. (The picture has a D12 for comparison.)

 

[lakeroadster]

I am planning to rebuild my Leaner Meaner Machine so will see if I can manage a swing test without the fin section and get it stable... 3x 18" BT50 tubes w/ a weighted nose cone. I am contemplating how to distribute the weight of the string along the body without messing up the swing test... Is my thinking correct that it would not make this invalid if I had a 12" or so support taped to the body tube -- as long as I center the support at the CG it should not change where CG is and mess up the test. Correct?

Makes sense to me. Something like a coupler inside the rocket would likely work, centered on the rockets CG

 

[BigMacDaddy]

I need to rethink my swing tests -- I mostly do it in my relatively small living room... I have occasionally done it in my front yard but I am in a pretty urban setting so it is embarrassing when people walking by to look at me like I am insane. I could move to the middle of the street but that would be even more embarrassing.

 

[jqavins]

Walk your banana.

 

[jqavins]

It's a technique I heard about somewhere for getting over being embarrassed over little things. Attach a banana to a leash and take it for a walk. People will look you funny. Might even laugh at you. Let'm. It doesn't matter. Then go home and swing test your rockets in the yard.

Also, if I remember right swinging on a short line is conservative, so you're probably safe, but you may be using too much nose weight.

 

[mjennings]

My 15% might be a bit conservative. swing testing works due to local frame of reference air flow. Sketching things out with a protractor. 20% ratio is about 6 degrees off assuming CG roughly in the center of the rocket. 40% ratio is about 12 degrees off which is getting questionable. 50% is 18 degrees which is definitely outside small angle assumptions.

For long rockets using a wind tunnel set up might be better. Hang the rocket and use a box fan or leaf blower. Now these air flows are rather turbulent so a flow straightener rig would give better data. Now we are getting into half baked support equipment!

 

[lakeroadster]

I need to rethink my swing tests -- I mostly do it in my relatively small living room... I have occasionally done it in my front yard but I am in a pretty urban setting so it is embarrassing when people walking by to look at me like I am insane. I could move to the middle of the street but that would be even more embarrassing.

 

[dhbarr]

It's a technique I heard about somewhere for getting over being embarrassed over little things. Attach a banana to a leash and take it for a walk. People will look you funny. Might even laugh at you. Let'm. It doesn't matter. Then go home and swing test your rockets in the yard.

Also, if I remember right swinging on a short line is conservative, so you're probably safe, but you may be using too much nose weight.

Plus, Banana Walker is a fine rocket name.

 

[jqavins]

For long rockets using a wind tunnel set up might be better. Hang the rocket and use a box fan or leaf blower. Now these air flows are rather turbulent so a flow straightener rig would give better data. Now we are getting into half baked support equipment!

MythBusters, Grant I think, made a flow straightener out of many boxes of straws with the ends cut off. It seemed to work pretty well.

 

[lakeroadster]

Plus, Banana Walker is a fine rocket name.

Middle name? "Sky"

 

[mjennings]

MythBusters, Grant I think, made a flow straightener out of many boxes of straws with the ends cut off. It seemed to work pretty well.
View attachment 489610

Exactly the wind tunnel where I did my under grad had a bunch of 1 inch diameter straws followed by a couple layers of window screen to act as flow straightener/debris catcher.

 

[bjphoenix]

And, up to a point, you can use a longer string for longer rockets, but when the rocket is quite long it's hard to swing it on such a long string. If 15% is right (I don't know either) then a six foot rocket would need a 40 foot string

I wonder what the practical limit is. I could see using a long string but holding the string relatively near the rocket to start with then when you get it swinging good you could let more string slide through your fingers until you get the diameter you want. If you can't swing fast enough to keep the rocket off the ground you could start by standing on a small ladder or stepstool?

 

[jqavins]

Playing out line is what I do every time on the very rare occasions that I swing test at all. Even if only a few feet of line are needed it's still much easier to start with the rocket no lower than waist level with the string held over your head. I start that way then play out to about six feet no matter the rocket size. If I think I can handle ten feet of line then, if the 15% WAG is right, I'd only be good with rockets up to 18 inches. Which is more or less^* why I rarely swing test and would rather rely on kit designers or RockSim.

* I've never actually done any calculation like this, hence "more or less". Aware that the swing test is not so good beyond a rather moderate rocket length, and having kit designers or RockSim even for the shorter ones anyway, string testing hardly ever seems worthwhile.
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OK, new crazy idea. Anyone done BP multi-engine air starts? We've seen boosted gliders with multiple gliders. I'm thinking about a central rocket, 18 or 24 mm powered, BT-70 or 80, with three min diameter or BT-20 mini engine powered rockets launching from it during the coast phase.

 

[BigMacDaddy]

Ok - another idea, Bamboo for a body tube...?



When you google "Bamboo Rockets" you find tons of videos of a festival in Thailand... I was there for a similar festival where they made immense pyrotechnics spark clouds erupt from the ground (you needed to hold a banana leaf over your head to keep from burning little holes in your clothes).

Anyway, not looking to launch a whole bamboo tree and fireworks show. What I am thinking about is simply using a couple of segments of a 1" or so bamboo stalk as the body tube and 3d Printing a nose cone (panda theme?) and motor mount and mounting fins on this thing... Just a fun little side project.

I realize this stalk is not quite as straight as the cardboard tubes I usually use but I think it is good enough (considering the odd-rocs I have seen people make fly). Anyway, grabbed a nice long 6' section of bamboo today on a hike and I think it is wide enough for a 24mm motor mount. I would probably only use around 2' or so of this since it will get way to heavy for a D or E engine if I use more (I think). Would need to drill out the middle separating segment so that ejection charge can pop the nose cone and deploy a chute.

 

[jqavins]

Gluing fins on, like a more typical build? When you say "wide enough for a 24mm motor mount", do you mean with centering rings holding a tube, or are you talking about inserting the motor directly into the bamboo? I envision a bamboo tube that's just a little smaller that the engine, being bored out to to size and round at the same time.

 

[BigMacDaddy]

Gluing fins on, like a more typical build? When you say "wide enough for a 24mm motor mount", do you mean with centering rings holding a tube, or are you talking about inserting the motor directly into the bamboo? I envision a bamboo tube that's just a little smaller that the engine, being bored out to to size and round at the same time.

The bamboo is hollow but I may need to 3D print a small centering ring / motor mount so would likely drill a hole (if needed) a bit larger than the engine so I could insert the mounts (getting engine aligned might be more difficult than anything else). I might try to slot the bamboo for mounting the fins if it does not seem too difficult. Let me get a 2' section cut out of the longer piece I grabbed to see how much it weighs first to see if this has any potential.

 

[boatgeek]

The bamboo is hollow but I may need to 3D print a small centering ring / motor mount so would likely drill a hole (if needed) a bit larger than the engine so I could insert the mounts (getting engine aligned might be more difficult than anything else). I might try to slot the bamboo for mounting the fins if it does not seem too difficult. Let me get a 2' section cut out of the longer piece I grabbed to see how much it weighs first to see if this has any potential.

If you cut slots for the fins, you might want to leave space for lashing above and below the slots. Bamboo tends to split once a cut is established along the line of the fibers. Glue might work too, but lashing would be traditional.

 

[jqavins]

Oh, this is heading down a potentially very cool road. Maybe use actual feathers for fins, with lashing for and aft.

 

[Jeff Lassahn]

I think it would be cool to do a rocket entirely made from found natural materials. I was thinking about that a while ago when looking at an acorn that looked like a good nose. Acorn nose, bamboo body, feather fins could be a good setup.

 

[dhbarr]

Bamboo comes with centering rings, you just have to cut the motor mount holes

 

[BigMacDaddy]

I was thinking maybe woven bamboo for the fins? Could coat with epoxy to make rigid...

 

[Cape Byron]

Oh cool. I have a lifetime supply of organic body tubes.

 

[BigMacDaddy]

Oh cool. I have a lifetime supply of organic body tubes.

View attachment 491254

View attachment 491255

Wow - 70mm (larger than a BT-80, no?). I think you can jam a 8x18mm + 1x 29mm cluster in there! That would move a lot of wood!

 

[Cape Byron]

Wow - 70mm (larger than a BT-80, no?). I think you can jam a 8x18mm + 1x 29mm cluster in there! That would move a lot of wood!

View attachment 491258

Oh, we have bigger.

Last bamboo harvest for temporary fencing yielded 4 x 6 metre (~20 foot) poles per stalk of bamboo.

Largest piece was 18 cm (~7") in diameter.

Great building material and a damned fine wind break Grows about a metre (3 ft) a month.

 

[lakeroadster]

I hate to be a Debbie Downer..... but Bamboo is pretty heavy.... 0.264 oz/cu-in, which isn't horrible in itself, but the wall thickness is going to be a killer. Well unless you have a gun boring machine to reduce the wall thickness.