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Space Shuttle Engines: Just the Stats

How I came up with the numbers that amazed.

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?”

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