I threw together a rocket motor test stand using a spare arduino uno I found in my bin, as well as a load cell and SD card shield. I think I probably spent about $20 total on this project and it looks like it’s going to be really awesome. The load cell maxes out at 20kg, so it should be safe for motors up to size F, which is the maximum that I plan to test at this point. It also has removable inserts so that it is compatible with motors of size 1/2″, 3/4″, and 1.0″ ID (1.25″) OD tubes.
When ramming the clay and fuel into the engine during assembly, it creates a lot of outward forces that have a tendency to damage the cardboard tubes. In order to abate this problem, you surround the cardboard tube with a casing retainer that will take up some of that force and prevent the tubes from deforming or bursting. This can be done with either plastic or steel, but steel allows you to use fewer clamping bolts due to rigidity so I went with that. The fabrication process basically starts with a schedule 40 steel pipe, which is cleaned up and slit down the middle with a slitting saw on the mill. Then a flange is machined and welded in place. The only really tricky part of this is that you need to be VERY careful not to overheat and warp the part when welding. The easiest way to accomplish this is to simply go very slowly, welding about an inch at a time and then giving the part time for the heat to equalize and cool a bit. Seems like everything went pretty smoothly, though for the large retainer the flange warped a tiny bit, which reduces it’s clamping range somewhat. Shouldn’t be too hard to account for this, and i can easily make another in the future if it becomes bothersome.
Now that the nozzle molds are finished, making the core spindles is a relatively easy task. These will be used to create the hollow inner core of the motor. Compared to the time spent machining the nozzle molds, this was a relatively quick task. Probably less than 6 hours in total. Simply start with a rod of stainless, bring it down to nominal size, then file a rounded point and tediously sand the surface with finer and finer grits to bring it to a smooth finish.
I’ve been working hard for the past month or so machining several different nozzle molds for different sized engines. So far I’ve made 5 of them, for engines of size C, D, and E. I plan to make one more for F-size engines. Boy have these been a huge pain on my tiny lathe! Because these parts come in contact with oxidizers, they need to be made of stainless steel, and my tiny lathe is not really rigid enough to take large cuts in such a hard material. I can normally get away with 20-30 thousandths in mild steel, but stainless maxes out at about 10, so making these parts took between 30-80 facing cuts just to rough in the features. All in all I think I spent at least 24 hours of total machining time making these! Fortunately, if I don’t accidentally drop them face-down they should last me forever.
Back when I was in grade school, I would occasionally launch model rockets with friends. Eventually I got interested in making my own small solid fuel engines, but my parents were obviously worried for my safety. I did some research and eventually found this book, Amateur Rocket Motor Construction by David Sleeter. The book is over 500 pages and lays out in extreme detail every single aspect of the process, from sourcing and making your own paper tubes for engine casing, machining nozzle molds on a lathe, milling and mixing your own fuel, loading motors, testing and iterating on motor design, and so much more. I’ve owned the book for more than 20 years, and other than some small successes in high school, I haven’t really had a great chance to explore the hobby, mostly due to living in California where this kind of thing is legally dubious due to the extremely strict and vague laws regarding explosive compounds (side note- the laws are written so expansively that owning dry ice and water could constitute a felony). I have a friend with some land in the middle of the Nevada desert who is interested in the hobby, so I’ve decided to try again and hopefully make some real working engines this time! Wish me luck!
