Space Shuttle Engines: Just the Stats
How I came up with the numbers that amazed.
- By William Vietinghoff
- Air & Space magazine, September 2013
Badman Production / istockphoto
A cursory review of the history of rocket propulsion turns up a long list of well-designed rocket engines that have demonstrated superb reliability, contributed significantly to space research and defense, and transported hardware into Earth orbit and beyond.
However, the performance of these engines is usually presented in forms that do not appeal to the general public. Who (other than engineers) wants to look at tables of thrust coefficients and combustion-chamber mixture ratios? And the available information focuses on the vehicle powered by the engine, the vehicle mission, and successful or failed launches, rather than the engine itself.
A notable exception is the space shuttle main engine, designed and built by Rocketdyne, at that time a division of Rockwell International. Because most photos of the shuttle show a remote view of the nozzles that protrude from the base of the orbiter, the public is aware only of the engines’ size and configuration.
Unlike expendable engines, the SSMEs did not end up at the bottom of the ocean. The SSME had a lifetime rating of 27,000 seconds, equivalent to 55 missions. The operating conditions that set it apart from other contemporary high-performance engines were the extremely high system pressures. A high combustion-chamber pressure (greater than 3,000 psi) permitted a higher expansion ratio and higher rated vacuum thrust, 470,000 pounds, from a compact combustion chamber. By comparison, each of the five F-1 engines (also designed by Rocketdyne) that powered the first stage of the Saturn V vehicle developed 1.5 million pounds of thrust. But the chamber pressure of the F-1 was much lower, about 965 psi.
For the SSME, engineers chose the more efficient closed, staged-combustion cycle. This cycle, however, amplified the demand for pressure throughout the entire system. In that configuration, the fuel-rich turbine exhaust gas was fed into the combustion chamber and burned to capture additional energy, rather than vented to the atmosphere, as was done with other engines that commonly employ the open, gas-generator cycle. So the turbine hot-gas pressure had to be significantly higher than the chamber pressure. This requirement dictated elevated propellant pump discharge pressures.
Another engineering marvel was the ability of the engine to be throttled from 65 to 109 percent of rated thrust. By contrast, most rocket engines are “calibrated”: From liftoff to cutoff, they generate a constant thrust.
In 1977, Rocketdyne, proud of developing the leap in technology the SSME embodied, wanted to broadcast its accomplishment to the public. The job fell to Joyce Lincoln in Rocketdyne’s public and customer relations department. At the time, I was a chemical engineer in the Space Shuttle Main Engine Development Group. Joyce asked to meet with me to discuss preparing descriptive material about the SSME she could distribute for publication.
“We want some gee-whiz statements about the shuttle engine,” she said. “You know, ‘faster than a speeding bullet,’ ‘can jump over tall buildings.’ We could brag that the engine has a very high specific impulse and an extremely high combustion chamber pressure, but who would know what that means?”
“Those details are what make the difference,” I said.
“I know, but we don’t want to say all that,” she said. “We want some simpler, graphic statements that emphasize how powerful the space shuttle main engine is. Can you do that?”
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.”
William Vietinghoff grew up in an age when thoughts of space travel were considered a mental disorder. Nonetheless, with a degree in chemical engineering, he ended up with a career in aerospace. He retired from Boeing in 1998.