Brooks (brooksmoses) wrote,

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Why being an engineer is fun.

Or, "Yes, this is rocket science, why do you ask?"

Brief background -- my advisor teaches a lab class in thermodynamics and design. It's a three-part lab; in the third part of it, the students work on a small rocket engine. It's a fairly simple design; a steel tube about an inch and a half in diameter and eight inches long, with a nozzle on one end and an oxygen source and an ignitor on the other. One puts a hollow cylinder of solid fuel -- either a paraffin/carbon-black mixture or polyethylene plastic -- in the steel tube, bolts everything together, feeds in oxygen, and lights it off. Nice loud roarwhoosh noise, pretty fire, collect data, ten seconds later it shuts off. I can hear the sound quite clearly in my office in the next building over.

Needless to say, this thing has some fairly substantial safety features. First off, it's not just out in the room, it's in a containment box. A two-foot by two-foot by four-foot box made of 3/16" steel plate and 3/4" bulletproof glass, framed with steel Unistrut for those who are familiar with the stuff (basically 2-inch square steel beams with pre-drilled holes for easy bolting). The front hinges open, and is held closed with some very sturdy latches. There's a ten-inch steel duct going out of the box to the roof, for the exhaust, and a vent fan as well to clear the air in the box. Beyond all this, there's another sheet of the 3/4" bulletproof glass as a partition between the box and the students running the rocket.

(That all becomes important later, as you'll see. It's a sturdy box. You'd have a hard time hurting it seriously with a sledgehammer.)

So. The first part of the rocket segment is the students running pre-existing designs for nozzles and fuel grains. The second part of the segment is that they try to improve on the designs to produce better results -- more power, more efficiency, more thrust, or suchlike. Aside from chaging the nozzle designs, they can also change the fuel grain shape -- perhaps a cylinder with four holes in it instead of one, or perhaps something more complicated -- some of them get quite complicated, really.

This afternoon, one of the runs didn't make a ten-second roarwoosh noise.

It went WHOOMP.

This, rather predictably, produced a crowd of about a dozen people (including me) outside the rocket lab in very short order.

The immediately obvious things were that everyone was ok but a bit shaken, the room smelled of half-burnt carbon, and the box sides were significantly bent. Some of the pieces of Unistrut, where the hinges were attached, had been twisted about 45 degrees, and the steel plate on the top (and its framing) had a very noticable one-inch bow it it.

A few minutes later, someone noticed that the exhaust vent on top of the roof was mangled into a pretzel as well. And, upon inspection, the vent fan blades were found to resemble a closed tulip....

As best we can figure, this team of students had designed a fuel core that was exceptionally good at vaporizing the paraffin. So good that it vaporized all of it in less than a second. Unfortunately, it didn't do very well at burning this vapor inside the rocket -- so there was a nice mixture of vaporized paraffin and oxygen being blown out into the box and exhaust duct. Nice, explosive mixture. Just about at the ratio that produces the highest pressure upon burning, too.... And it very quickly demonstrated this potential.

So, that was my excitement for the afternoon. It was deemed, by everyone present, a good demonstration of why effective safety measures are worth having....

(Oh. and we're still not sure why the power in the machine shop next door went out right then.)

- Brooks

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