Homemade model rockets

[erid]

I want to start making my own model rockets, using store bought motors, but are there any rules for balance and height for these? I saw that most estes rockets are different lengths, so is there a rule for how long it should be, or what shape and size the fins are?

[mfstraydog1]

well first thing that comes to my mind is the rockets center of gravity and the center of pressure. here are a few links that might help

 

http://exploration.grc.nasa.gov/education/rocket/rktcp.html

http://www.apogeerockets.com/education/rocket_stability.asp

http://www.nakka-rocketry.net/fins.html

[WSM]

I want to start making my own model rockets, using store bought motors, but are there any rules for balance and height for these? I saw that most estes rockets are different lengths, so is there a rule for how long it should be, or what shape and size the fins are?

 

The model rocket kits are usually balanced and ready to go when built. There are technical reports that give guidance on designing custom model rockets and making them stable (possibly a free download).

 

NOW, when you get really serious; lets talk about making your own model rocket engines! Instead of paying a few dollars a shot, you can make your own (superior) engines for about twenty five or thirty cents each. The proper sized tubes can either be purchased, made or cut from long lengths of the same tubing used by the manufacturers. making the tubes is simplified by using gummed paper tape (I prefer the un-reinforced) and rolling it on an appropriate former after moistening.

 

The nozzle material can be as simple as dry fireclay with wax added for improved consolidation and erosion resistance. The propellant is nothing more than meal powder. If you don't have access to meal powder, a reasonable substitute is a 70:30 ratio of hand mixed BP ingredients and FFFg rifle powder.

 

The basic model rocket engine is an end burn type which has between a 3:1 and 4:1 ratio of tube ID to nozzle throat ID (try the 4:1 to start). The delay composition can be as simple as hand mixed meal powder with a little sodium bicarbonate or extra charcoal mixed in to slow the burn rate.

 

I have made many successful model rocket and other BP engines of various sizes (small to large) and will be happy to share any guidance needed. Let me know. Thanks.

 

WSM

[erid]

^^thanks for the advice, but does the nozzle have to be a certain shape, or can I just drill a hole into it?

[WSM]

^^thanks for the advice, but does the nozzle have to be a certain shape, or can I just drill a hole into it?

 

 

The most efficient nozzle I'm aware of is the de Laval nozzle, with it's convergent/divergent design. I personally believe if a straight hole is drilled through the clay plug and the powder behind it is ignited, it will begin to create a convergent/divergent form by erosion and fly somewhat less well than a rocket with a preformed nozzle. Chinese bottle rockets are made with little consideration for the form of their nozzle and they seem to fly quite well. True, the efficiency isn't all it could be, but they do fly.

 

Try the clay plug with a hole drilled through (not very deep into the powder grain) and see how it does. If need be, make a jig to keep the hole straight so you don't break the clay out as you drill it. Let us know how it performs. I expect it'll work just fine; not super, but okay.

 

I've done my best work on small BP engines using an arbor press. A small arbor press will be an investment worth the cost.

 

WSM

Edited August 2, 2010 by WSM

[dagabu]

I made my first convergent rammer from a piece of oak dowel and a sanding block to put a slight taper on it. I found that I had a LOT less CATOs when I started doing that.

 

WSM has a great point, I was at a party this weekend where the host made a bunch of motors and some were pounded and some were pressed. Care to guess which CATO'd?

 

Harbor freight has a 1 ton arbor press for $49.00.

[erid]

thanks, as soon as I get the money I will buy a arbor press. As for the nozzle, how would I make a De Laval? would I have to carve it out myself, because it curves in then out again?

[Mumbles]

It's hard to tell if you're asking if you'd have to carve the clay nozzle itself. If so, no. Generally tooling is used. You can see an image of standard BP core burning tooling here:

 

http://www.wolterpyrotools.com/image/pyrotool_202.jpg?1280784720114

 

You will notice that the longest rammer, which is used to form the clay nozzle is tapered on it's end to form a concave surface on the interior surface. It's a little hard to tell, but the spindle has a similar taper on it to form the divergent nozzle on the exterior. True de Laval nozzles are relatively hard to prepare without machining each one. Frequently they are approximated with flat surfaces. I believe the convergent is approximately 45 degrees, and the convergent is approximately 60 degrees.

[dagabu]

Not bad for a guy that hasn't eaten the cookies.

[Mumbles]

You'd be amazed at what you can learn from Steve LaDuke if you keep feeding him Stroh's. Though generally it's not about rockets after #3.

 

You'd be happy to know I've made more rockets than I can count on one hand. Barring a mysterious mutation, I intend to keep it that way. Despite some people's wishes and best efforts, rockets are still pyro, and I am a student of the complete science.

[WSM]

thanks, as soon as I get the money I will buy a arbor press. As for the nozzle, how would I make a De Laval? would I have to carve it out myself, because it curves in then out again?

 

Tools are required to make a De Laval nozzle; either bought or home made (it takes more tools to make tools, I'm afraid. Vicious circle!). It's sort of like getting a loan from the bank; first you have to prove you don't need it...

 

For an arbor press, try to size it for the bulk of the work you plan to do. If you're starting small, think small arbor press. I've found that "one size fits all" doesn't. I once had a 3 ton arbor press, thinking it'll do everything; only to discover it did the big stuff fine but noodled or destroyed smaller stuff with exceptional ease (it takes a real light touch to do smaller stuff on a larger press). I currently have a 1/2 ton, 1 ton and 2 ton arbor presses; and variously use them all depending on what I'm trying to accomplish. If you are starting with 1/2" ID stuff (model rocket engines), try a 1/2 ton arbor press. I think you'll be pleased with it. It works for all sorts of other pyrotechnic stuff, too; like spollettes and inserts and hummers and...

 

Have fun, be safe and tell us all about it.

 

WSM

Edited August 4, 2010 by WSM

[WSM]

Hi All,

 

Well, I guess it had to happen sometime. Yesterday my Grandson (8 years old) asked about rockets and we dug around in a carton marked "toys" (these were my toys) and came up with a half built model rocket for him to finish and have. It was basically finished and had a coat of white paint on it. It was sized for a 1/2" OD model rocket engine, BUT 2.25" long instead of the usual 1.75" size. I have a decent collection of these NEPT type tubes, acquired from Precocious Pyrotechnics at PGI several years ago. I also had a prototype stainless end-burn spindle mounted to an aluminum base; sized for this size engine.

 

I found a piece of CPVC pipe which just fit the NEPT casing and cut a piece long enough for a sleeve and trued the ends on a lathe. Next I cut, drilled and turned some 3/8" aluminum rod for drifts and finished the set. Now for his first foray into amateur rocketry, I handed him a pair of safety glasses and mounted a half ton arbor press to the work bench and pressed some clay into the tube. For propellant we used some sulfur-less meal made from potassium nitrate and willow charcoal. We static tested the engine out back and everything went well (he loved this part; Yup, the pyro genes were definately passed on).

 

Next we made a flight version engine. This one has a type of DeLaval nozzle formed by the tooling, Sulfur-less propellant, a slow sulfur type delay and a small bit of 4Fa for an ejection charge to pop the nosecone off and deploy the little parachute for a soft landing. He wants to rush right out and launch it but I tell him we need to clear this with his folks. My Daughter is cool with it (she appreciates my efforts to teach science while having fun), but I want to involve his Dad in the enterprize (it's a male bonding thing, three generations of men firing off a rocket; it doesn't get any better than this). We'll have to find an appropriate Saturday and get together for the event (I might even have to prepare some extra engines; I can't fire up all this enthusiasm without providing for more fun).

 

The last thing was to come up with a paint scheme. I asked him to draw an idea of what he wanted. "Well, I don't think I have that many colors of spray paint on hand, but how about this...", and we settle on a red top end and a blue tail section with white left in the middle (He liked it and it does have a patriotic look). It's ready to go. More later...

 

 

WSM

[TheSidewinder]

Now THAT'S a story to warm a pyro's heart! Good on ya!!

[WSM]

Now THAT'S a story to warm a pyro's heart! Good on ya!!

 

 

Hi SW,

 

Thanks. It certainly was fun for both of us. We still need to go out and launch it; maybe this holiday weekend ? We'll see...

 

WSM