Categories
Rocketry

Convolute Tubes

For my rocket motors, I’ll be using convolute paper tubes as the casing. Paper casings are weaker than aluminum or fiberglass, but have several major benefits. First and most importantly, they are much safer. They can’t withstand quite as much pressure as an aluminum tube but when they fail they don’t generate metal shrapnel. Also, they are much cheaper. All it takes to make one is a bit of kraft paper and some white glue. Lastly, paper tubes have a little bit of flexibility, which actually aids in the retention of clay nozzles. When the clay nozzle is rammed into place, it bulges the tube outward ever so slightly, creating an interference fit that keeps the nozzle from sliding out (at least in theory). In total I made about 100 tubes, split evenly between 1/2″ ID, 3/4″ ID, and 1″ ID (inner diameter). I marked them based on which batch they were just in case there are variations between the strength of each batch. I can already tell that some of them didn’t come out as well, since you can sort of tell how strong they are when you squeeze the walls with your fingers. I may later try to make an apparatus for measuring this strength in order to do some rudimentary quality control. By the way, convolute tubes are the opposite of spiral-wound tubes, which would not be suitable for the task of being a rocket motor casing, as the spiral creates a small channel on the inside that would allow the flame to shoot down, prematurely igniting fuel and causing the engine to explode. Convolute tubes are rarely used for anything else other than rockets/fireworks, and are therefore really hard to purchase, especially if you have specific size and thickness requirements.

Categories
Rocketry

Ignition System

I made a simple ignition system using two 18650 batteries and a few other switches. It’s dead simple, which is good for safety. You just have to insert and turn a key, turn on the toggle switch, and then press and hold the button to launch. I’ll be honest here: I did NOT need to spend as much time on this as I did. I spent a good couple of days looking for and sourcing the nicest-feeling most authoritative switches possible. That being said, this box is an absolute joy to use. every click and press has an excellent tactile feel and makes the process of launching a rocket that much more enjoyable. Miles better than jamming a paperclip into Estes’s little 3V ignitor system 😛

Categories
Machining Rocketry

Tamps

In order to create the clay nozzles and fuel grains, you need to hammer them into place. To do that, I’ll need to create tamps with proper shapes and clearances for the nozzle, spindles, etc. Each motor uses 4 different tamps, and they are made from nylon plastic in order to be lightweight and durable. This also makes them a hell of a lot safer than making them from steel, since in the worst possible situation (where the motor spontaneously ignites during the ramming process) the tamps will simply shoot out the top of the motor rather than exploding into metal shrapnel. These are relatively easy to make, provided you happen to have aircraft-length drill bits (18″). Also, the nylon does not make a nice chip on the lathe and turns into a mass of spaghetti, but to combat that, just stop every once in a while and don’t let it build up like I did 😛

Categories
Rocketry

Test Stand

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.

Categories
Machining Rocketry Welding

Casing Retainers

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.

Categories
Machining Rocketry

Core Spindles

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.

Categories
All Projects Machining Rocketry

Nozzle Molds

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.

Categories
All Projects Rocketry

Rockets!

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!

Categories
Machining

Slitting Saw Arbor

I needed a slitting saw arbor and didn’t really want to buy a crappy one from amazon that has tons of runout, so this made for a nice little lathe project. Nothing too fancy, just a bit of stainless on the lathe. If I do this again I’m going to take extra care to make this even more low profile, so that the slitting saw has more reach.

Categories
Machining

Learning to Make Threads

I wanted a simple project to learn how to make threads on the lathe, since I haven’t really done that yet. I made this knurled brass knob for the gibs on the mill. I’d say that this was definitely a fun project but a waste of $20 of brass for sure.