Ever wondered what the inside of a burning rocket looks like? Well, thanks to Valve engineer Ben Krasnow, now we know. He built a homemade hybrid rocket engine that's see-through so you can actually watch how it works. And even though it's probably a really bad idea to try this at home, he made a video so you can build one, too. Just don't say I didn't warn you.
A hybrid rocket engine uses two different types of fuel—one solid and one that's either liquid or gas. As Ben explains, it can be easily throttled up or down, and you can turn it off by cutting off the oxidizer. This makes hybrid rockets much safer than their solid-fuel counterparts, which can't be shut off once they're lit because everything that's needed for combustion is contained inside the chamber. Just within the last few years, they've started using hybrid engines for real rockets to make space travel cheaper and cleaner.
For his rocket, Ben used solid acrylic for the fuel and gaseous oxygen as the oxidizer. He drilled a hole all the way through a 2" acrylic rod, then drew up aluminum end plates for it in CAD. The nozzle is made of steel because it has to withstand a much higher temperature than the end plates. Some high-temperature silicone O-rings form the seal, and four 1/4 x 20 inch threaded rods hold the whole thing together.
The rods help to make sure no oxygen escapes and maintain the pressure on the O-rings, which is one of the most important components both in terms of functionality and safety. In other words, if the pressure isn't just right, bad things happen.
To light it, he turns on the oxygen so that it flows through the nozzle at 80 pounds per square inch, then sticks a cotton swab soaked in alcohol into the chamber. This rocket was made for demonstration purposes, though, so it only burns for about ten seconds at the most.
You can find out more information on this build over at Ben's blog.
Again, I wouldn't try this at home (not that most of us have the equipment), but if you want to make your very own rocket, we've got a tutorial for just about any kind of rocket you could ever want.