Turning a Rocket Motor on a Lathe

First of all, I need to warn the intrepid reader. I have only been using my metal lathe for a couple of months, and I am by no means an expert, if anything I am barely a novice. Please don't take what I write here as fact, or the best way to do it. It's just the way I'm doing it now, and it's sure to change down the road.

The motor I will be illustrating on this page is a bit unusual. It is designed as a test motor for an end burning grain utilizing a propellant consisting of KNO3, Sugar, Iron Oxide and Charcoal. I am using 1.5" EMT (steel conduit) for the casing, and hot rolled steel bar stock for the nozzle and forward closure. While a 2" diameter casing would have been much preferred, I didn't have any. So I push on with the 1.5" stock.

The end result is a motor with a very small throat and exit cone. So the motor looks a little funny.

Here is my quick work up of the nozzle design. Nothing fancy, just drawing the nozzle to scale and checking my math. Notice the small exit cone, it's .56" diameter, the throat is .2" diameter, that gives me an expansion ratio of about 8:1. Note the scribbling at the top of the page, that's my math, and yes I really can do it without a calculator.

I have the bar stock cut to about 2.6" long, it's 1 7/8" diameter. Here I have the bar chucked up and I am doing a few passes to true it up. I usually take .0075" in one cut, that reduces the diameter by .015" per pass. Yes, with a small lathe it takes many, many cuts to reduce the diameter.

Here I have the drill chuck in the tail stock. Chucked up is a centering bit to get the hole started true.

Now I am boring using a .2" diameter drill bit. The tail stock allows for an inward travel of just over 1 inch, so I have to shut down, then back out the bit and move the tail stock closer to bore deep holes.

Here I am facing the end of the metal. Once the metal is trued parallel, and the end trued by facing, it's easy to remove the work piece and rechuck it in the lathe. That's really the only reason for facing, as I true up the nozzle ends anyway.


Here I am adjusting the compound slide for the 30 degree half angle of the convergent section of the nozzle.

This is the start of the convergent nozzle area. With these small lathes you need to cut bevels manually. Move the tool slide in to cut, then back out, adjust out another .005" and do again, and again, and again...

The convergent area is now done. I'm using emery cloth to polish the inside. I polish to a semi mirror surface. You could go more, it just takes longer.

Here's the nozzle polished. It's nice and shinny but you can still see some machine marks in it.

If the work was trued up and faced properly, it will be perfectly balanced when you turn it around and put the other end in the chuck. I'm starting to turn the divergent cone, but it's giving me problems. I've never turned such a small cone before and my tool is binding inside the work. I had to regrind the tool to get it to fit. It's still hard going, the 15 degree half angle is always a bear to cut.

I decided I had better face this end too, and get it to the proper length of 2.5" before I finished the divergent cone. A few passes and it was in good shape. Then I finished the divergent cone. It didn't take long as the cone was so small.

Now I am reducing the divergent cone area to reduce weight. I would normally bring the metal very thin here, and taper it to a bell shape. But for now I'm leaving just a cylinder and much thicker, just in case I want to rework this nozzle with a larger diameter throat later on.

Here is the finished nozzle. I polished the divergent cone as best I could. If I decide to fly this nozzle, I will go back and reduce the weight more by removing unneeded metal.

Here are some of the tools I use. On the top is the standard tool holder with a pointed tool to the left and a short boring tool to the right. The pointed tool is used to bore the inside tapers for convergent and divergent cones on a small nozzle. A lot of material was removed from below the tip so it wouldn't touch the work surface before the tip did. The boring tool to the right works for the cone tapers in slightly larger nozzles. The tool to the bottom left is a round nose tool for taking material off the outside diameter, the tool to the right in the quick release holder is of similar design.


Here is a side view of the same tools.

These small lathes work best at high speed taking off only about .005 to .0075" of material at a time. The tools need to be sharp and ground to the precise angle or they almost won't cut at all. Practice and get experience making/sharpening tools is really all you can do.

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