General Rocketry Discussion!

[Weldo]

Neat idea?! Sheez, back in the late 60s, we built our own almost exclusively

Haha, I guess I'm a bit spoiled with all the great kits on the market! I've been toying with the idea of doing a scratch build and I like the idea of the slow lift off.

So an Estes D12-3 recommends a max (estimated) lift weight of 14oz. Anyone know if that's the actual weight at which a rocket will no longer leave the launch pad? If lift off is too slow I imagine the rocket will become unstable and dangerous, I don't want it to go up about 12 feet and spin around!

The tube I have is 3-1/4" OD, 3" ID and 37" long. Right now it's about 17.5oz so that's about 0.473oz per inch of tube.

If I cut it down to maybe 18" it'll be about 8.5oz, giving me some allowance for fins, a motor housing and nose cone. For fins I have some 0.080" plexiglass layin' around, it's pretty light and strong.

Nose cone could be built off the tube end caps, maybe glue on an old funnel or a plastic champagne glass?



Just thinking out loud here! Chime in if this sounds like a horrible idea!

 

[Weldo]

Here's a screen cap of a rough idea. Body tube was shrunk to 16" to hit the weight. Stability's not great...

Body tube is 3.25"OD, 0.125" wall.

Centering rings and bulkhead are "plywood", motor tube and engine block are cardboard, fins are labeled as 0.080" polycarbonate.

18" mylar chute and 36" of 6mm flat elastic shock cord.

Nose cone is "polycarbonate", conical

 

[rwm]

Clever with the champagne glass, but I am not sure about the durability. How about a plastic ball cut in half? Not as aerodynamic but who's counting? I cannot answer your exact question about the D12-3 but I assume that is the recommended vehicle weight and not the max weight. I vote you go at least 24" of tube. I actually flew my bird without a launch rod and never had a problem although I don't recommend that. The launch rod makes sure the rocket is fin stabilized before it is on its own. Definitely avoid any wind on the trial flights. Do you have a safe launch site if things go sideways? (Literally)
Robert

 

[rwm]

FYI


Maximum thrust is 7 lbs! I'm pretty sure you could use the whole tube. It might just sit on the pad for .1 sec or so.
Make sure it's a high quality tube or it may fail. The G's are higher than you might think. I suppose you could test load the tube with 7 + pounds and see if it holds up.
Robert

Edit: Oh yeah, do you have a good way to glue polycarbonate? That would not be my first choice of material. Maybe balsa or ABS?
R

 

[brino]

How could you build the nose cone?

I have done a couple before by gluing a hardwood dowel about half-way into a block of balsa.
Chuck the dowel in an electric drill and sand free-hand on a belt sander.
Later the dowel shank is cut off, but the stub is used to hold a screw-eye (that would NOT grip well directly into the balsa).

You could take the balsa block thru the band saw first to remove the corners, but the balsa sands very quickly.

However, today I would simply 3D print my own plastic nose cone in what ever shape and colour I want.

-brino

 

[Weldo]

I could turn down a nose cone on the lathe. You think basswood is too heavy for a 3.25” OD nose cone? I’ve got a big ole chunk of basswood laying around the shop.

 

[Weldo]

Oh yeah, do you have a good way to glue polycarbonate? That would not be my first choice of material. Maybe balsa or ABS?

I just chose polycarbonate because it's the closest thing to plexiglass that I saw on the list. Upon further investigation I noticed that acrylic was listed as well, so I chose that.

Here's a new iteration. The fins are labeled as acrylic and the nose cone as solid balsa. I changed the motor mount around a bit so I could tape the motor onto the tube.

Stability's good and weight is a bit over the Estes max of 14oz but still well under 7lbs!



So I can't find a block of balsa big enough to turn out a 3.25" round. I'd imagine you can glue a few pieces together and make a block, right? Lamination style?

 

[ttabbal]

Keep in mind that the rule of thumb for thrust vs weight is about 5:1. While the motor would able to lift, say a 6lb rocket, it wouldn't be stable. You need enough velocity for the fins to stabilize the rocket once it leaves the guide.

 

[Weldo]

The NAR chart for the D12-3 says peak impulse is 29.7 Newtons or 6.677 lbs force.

With the 5:1 thrust ratio that would mean the max safe launch weight would be 1.335 pounds or 21.366 ounces.

According to the Open Rocket file I should be safe both in stability of design and 5:1 thrust to weight ratio. Thanks for that info about the ratio by the way! That was something that I was unaware existed.

Any other reasons I should not pursue this?

 

[ttabbal]

The rest looks fine from the text and the sim pic. For the first flight, I would try to stand back a little extra just to make sure you're clear, just for added safety. When you run the simulation, what's the speed when it leaves the guide?