National Aeronautics and Space Administration

Glenn Research Center

Joseph Prahl

Joseph Prahl is an ex-astronaut core participant and payload specialist. He started off his presentation with the basics of his career. Having made his way through college and then finding himself as a professor at Case Western, he was already well set in life. A personal connection to one of the principle researchers launching an experiment on the space shuttle offered him the opportunity to enter the astronaut core. There he spent two years training and researching as a backup payload specialist for a mission on the Columbia. Unfortunately the primary payload specialist was able to proceed and Joe was left here on Earth. And while he says that he was just as proud as anyone to have been a part of the space shuttle program, I would imagine that he was somewhat dismayed when the day finally came for his launch and he wasn’t onboard.

After the brief overview of his career, Joe started into some of the finer, often overlooked details associate with space flight. To start off, the shuttle was asked to perform a particular attitude of flight so that a European crystal growth furnace could perform an experiment while under microgravity. The microgravity being caused by the 1 billion monatomic oxygen atoms that occupied each and every square centimeter at flight altitude, effecting a 1/8th pound of drag on the shuttle; as Joe explained. Anyway, a slight oversight of the payload crew in positioning the crystal furnace about 3 feet and 12 degrees of the center of gravity of the space shuttle resulted in an approximant 2*10^-9 g lateral orbital tidal force that ruined the experiment. It was even more devastating that Joe had raised that very concern to the flight directors during the payload overview, but was overruled since the problem did not fall into his area of expertise.

Furthermore, the attitude of the space shuttle during flight placed the rotation due to the gravitation gradient along the intermediate moment of inertia. This, widely known by aerospace engineers, places the spacecraft into a divergent rotation that must be constantly corrected by the Vernier thrusters. The cumulative push of the thrusters, each firing to give 25 pounds of thrust for 1 second each, actually propelled the shuttle one half mile higher into its orbit. More detrimental was the fact that the thrusters were firing at approximately 60 second intervals, which just so happens to be the third harmonic of the resonant frequency of the main shuttle spar. As anyone might imagine, having a space ship literally ring like a bell by bending in the middle might not be very good for the structural integrity. Fortunately the shuttle made it back into the atmosphere before any significant damage was done.

Having now revealed the problem of the shuttle resonating, NASA began to look for other things that might cause a similar problem. One identified was the exercise bike onboard. The physical fitness trainers had long recommended that the astronauts peddle at a steady 120 rpm, or approximately 71 Hz; once again a resonant frequency of the space shuttle. Fortunately Joe had an idea for this. Why not attach the exercise bike to the shuttle via bungee cables, rather than simply bolting it to the middeck? Sort of an absurd idea, but apparently worth enough to try it out on the well known Vomit Comet jet. Everyone at the talk got a good laugh when Joe showed the video of him trying out the bike in simulated zero gravity. But, it worked and Joe reduced the resonating amplitude from 12 to 4dB and helped improve the safety and operation of the space shuttle.

Finally, Joe came to the experiment that he was in charge of; an experiment to observe the effects of microgravity and thin atmosphere on a flame. The video of the oscillating combustion sphere was incredible and we watched it through several times. Unlike here on Earth, where flames take on the familiar teardrop form, in little to no gravity, the hot gasses from the flame have no reason to rise up; there is no up in space. So instead the flame expands outward in a sphere, powered by diffusion and molecular drift.

All told, Joseph Prahl was one of the speakers who really take hold of your attention and have the ability to change your perspective on science and engineering.