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Space Shuttle Engines: Just the Stats

How I came up with the numbers that amazed.

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(Continued from page 1)

I tried to zero in on the unusually stringent conditions that the engine had to endure, or difficult functions it had to perform. My first thought was that temperature drives material selection. Using numbers from my copy of Perry’s Chemical Engineers’ Handbook, I wrote: “The Rocketdyne Space Shuttle Main Engine (SSME) operates at greater temperature extremes than any mechanical system in common use today. The fuel of the SSME is liquefied hydrogen at –423 degrees Fahrenheit, and, next to liquefied helium, is the coldest liquid on earth. The Rocketdyne SSME burns liquid hydrogen with liquid oxygen. The temperature in the main combustion chamber is 6000 degrees Fahrenheit, higher than the boiling point of iron.”

But Joyce wanted to emphasize power, so I turned to the source of that power, the propellant liquid hydrogen and its oxidizer, liquid oxygen. When they burn, they combine to form water, and the energy released translates into work that propels the vehicle. The easy step was getting the LH2 and LO2 flow rates. For the most impressive numbers, I used the combined propellant flow rates for the three engines at the full power level (109 percent of rated). Perry’s Handbook provided the Heat of Combustion for that reaction. This value, and the pounds per second of propellants, gave me the BTUs per second, which will convert to any other unit of energy, such as watts.

I wanted to make numerical comparisons, so I spent the better part of a day in the company library looking up systems that generate or use energy. I played with Boeing 747s, diesel locomotives, and automotive engines. Then I came across Hoover Dam. Using the power output of the dam, I calculated that the three space shuttle main engines developed as much power as 22.966 Hoover Dams, which I rounded to 23.

By chance I passed the desk of Dorothy Rowlands, secretary to the director of design technology. I asked if she had a swimming pool. She did. How many gallons did it hold? She gave a precise figure, explaining, “If I ever have to drain and refill it, I want to know in advance how much it will cost me.” I calculated that the three shuttle engine propellant pumps, at full power, would drain her pool in 25 seconds.

I composed 16 gee-whiz statements. Joyce sent press kits to newspapers all over the country and to NASA, which released the information to radio and TV stations.

During some late 1977 approach and landing tests, in which the orbiter was mounted on the back of a Boeing 747, taken aloft, and released, Rocketdyne issued a brochure based on my statements and those contributed by others, titled Incredible Facts: Space Shuttle Main Engines. For a month, there was a flurry of publicity. I expected the newsworthiness of the SSME facts to fade after that, but two years later, Joyce sent me recent articles about the engines. They had the usual references to the operating temperature extremes, draining the swimming pool, and of course, 23 Hoover Dams. Surely, by now I thought, everyone in America knew about SSME power.

In 1981, when shuttle launches began, Joyce sent me yet another set of clippings. (After buying Rocketdyne in 1996, Boeing used Incredible Facts for its brochures and website. United Technologies/Pratt & Whitney bought Rocketdyne in 2005, and from then on, Rocketdyne maintained the Incredible Facts website.)

During the moment-by-moment recounting of the launches on TV, the commentators must have kept an Incredible Facts sheet at their side to fill dead air. In most of the shuttle launches I caught on TV, somewhere between liftoff and engine cutoff, I heard about the engines outdoing Hoover Dam or draining a pool.

With the advent of the Internet, Incredible Facts lived on. (Enter “23 Hoover Dams” in any search engine.) Some webmasters did a poor job of copying. One site states “The three engines can drain an Olympic-size pool in 25 seconds.” Another writer, thinking about the Incredible Fact “37 million horsepower,” states that “their power is equivalent to 37 Hoover Dams.” Six minutes into the documentary “Space Shuttle Columbia” in the History Channel’s Modern Marvels series, the audience sees the words “The three engines of the Space Shuttle Main Engine….” with the swimming pool statistic.

I lost track of Dorothy Rowlands years ago, but I wish I could tell her how famous her swimming pool became (some swimming pool contractors include the pool-draining fact on their websites). When I pointed out Incredible Facts citations to my wife, she asked me if I had been paid for them. “Yes,” I said. “My salary.”

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