About Me

My Photo

I previously worked on Virtual Reality and other hardware at Valve.  I currently work at Google[x].

Prior to starting at Valve, I built computer peripherals such as keyboards, mice, and joysticks that were designed to be used inside MRI machines.  My company, Mag Design and Engineering, sold these devices directly to researchers at academic institutions who used them to publish scientific papers in peer-reviewed journals.

After work, I spend time on many different types of projects that usually involve circuit design, machining, material selection, and general fabrication/hacking.  My favorite place to be is my home workshop.

ben dot krasnow at gmail




Monday, September 24, 2012

Hybrid rocket engine with acrylic and gaseous oxygen

I built a small rocket engine for demonstration purposes. The engine is built from a 2" diameter acrylic rod through which I drilled a 0.5" hole. The oxygen at 80 psi or less is passed through the hole and then is forced through a convergent-divergent nozzle at the tail end. The nozzle's throat is about 0.25" and expands to 0.625". I lit the engine by inserting a burning cotton swab (with wooden stick) while a small amount of oxygen was flowing. The acrylic catches fire very easily in a pure oxygen environment. The engine can be throttled and shut off completely, which is a major benefit to hybrid engine designs. Solid-fuel rockets cannot be throttled or shut off, which makes them difficult to control.


  1. hum, any chance the oxygen feeding tube would start burning from the inside, when the pressure is lowered but not completely shut ?

  2. Convergent/divergent nozzles are because a convergent nozzle can only accelerate a flow to sonic speed (which makes sense, if you consider that back pressure can't propagate forward faster than the speed of sound). Weirdly, once a flow becomes sonic, to speed it up more, you use a divergent nozzle.

  3. Just wanted to say that you make really great videos! I can't believe I didn't notice your Youtube-channel before.

    Do you have any plans of making a rocket and launch it?

  4. Echoing Kenneth. That deserves a correction.

    Very small nitpicks aside, this is a very neat educational tool and absolutely deserves the attention it's getting (I've already seen it on Universe Today). Maybe you should see if you can get some Schlieren photography of the interior burn dynamics :)

  5. Are you actually getting choked flow there? I don't see any Mach diamonds.

    Armadillo had a good video, a while back, of what the transition to choked flow looks like (and sounds like):


  6. Saw your video on dump.com -- awesome stuff!

    Very cool tidbit I took from a nasa video, they channel liquid oxygen along the inside of the nozzle to use evaporative cooling to prevent the nozzle from liquifying at several thousand degrees. Perhaps some sort of liquid cooling on your tailstock aluminum nozzle mount (run liquid o2 through the alum to make it gaseous for reinjection at the top = cool the aluminum and get high pressure from low volume liquid o2.

    Again, very cool stuff!

    @anonymous first comment; oxygen doesn't burn. unless you ignite the metal oxygen feed tube, there is no fuel in that hose to burn, only oxygen, and oxygen doesn't oxidize itself :P

  7. Looking at the video more closely, I think I can see a few brief bursts of choked flow, where the exhaust narrows down and the noise increases.

  8. Very nice! Ever thought of doing fusion. If you are interested, try fusor.net. Keep up the good work.