L E F T :
Smoke from a
hot-fire test.
BELOW: An
engine mockup
from 1973, eight
years before its
first flight.company mothballed. It was sad, says Bradley, to
see them decommissioned when they could have
flown dozens more missions.
Now with the title of Deputy Program Man-
ager at Aerojet Rocketdyne, Bradley is overseeing
the rebirth of the SSME engine. (A 2013 merger
with Aerojet led to the company’s current name.)
And it’s quite a comeback: SLS is the custom-built,
off-planet ride for NASA’s Artemis campaign to
return humans to the lunar surface and later to land
people on Mars. That puts the SSME, originally cre-
ated with slide rules and paper, at the foundation of
21st-century human exploration.
“It’s very gratifying to see those engines resur-
rected,” Bradley says. “And what’s better than going
to the moon and Mars?”ROCKETDYNE BEGAN PRODUCTION OF THE SSME
on March 31, 1972, but it’d be another five years
before Bradley would come on board. An engineer-
ing graduate fresh from California Polytechnic
State University in San Luis Obispo, Bradley got a
tip from a neighbor and applied at Rocketdyne to
work on the shuttle program.
“I was old-school. Every design you were getting
down on a physical drawing board with pens or
pencils,” Bradley says. “They started hot-fire testing
engines in 1975, but in ’77 we were still burning up a
fair amount of engines. I was in turbomachinery and
we were taking the brunt of some of the damage, so
it was very exciting.”
Four turbopumps are at the fiery heart of the
engine’s design. These spinning, whirling fans cre-
ate the extreme pressure that carefully shoots liquid
hydrogen (LH 2 ) and liquid oxygen (LOX) to the
main combustion chamber. These machines operate
in brutal conditions, with heat as high as 6,000°F
inside the chamber—hot enough to melt an iron bar
into a puddle. But it’s the temperature swings that
introduce especially dangerous stress to the engine’s
delicately machined innards.
“Just on the turbopumps alone, you’ve got a cou-
ple of thousand degrees gradient from one end of it
to the other,” Bradley says. “The combustion cham-
ber is even worse. They’re at thousands of degrees
with (physical) tolerances of fractions of inches.”
Making central air conditioning for the main
combustion chamber required some novel thinking.
In the early 1970s, Rocketdyne researchers created
a new copper-zirconium alloy called NARloy-Z
expressly for use on the engine. The new material
could withstand pressure and temperatures that
threatened to deform the tiny channels that deliver
superchilled liquid hydrogen through the main com-
bustion chamber’s lining to keep it from burning up.
The SSME was also subject to years of testing con-ducted in an era before computer simulations. NASA
dictated that the engines endure at least 65,000 sec-
onds of flame and smoke on stands before making
the first flight, even though the engine works for just
510 seconds during a mission. The trials included
challenging the SSME to perform at power levels
greater than what the mission required. Decades
later, when NASA was evaluating SLS engines, that
extra thrust would prove invaluable.
The space shuttle launched on April 12, 1981,
becoming the world’s first reusable crewed space-
craft, and SSME the first reusable space rocket
engine. When the orbiter landed, its SSMEs were
removed, inspected, and refurbished before being
readied for another mission.
“I call these my children because I’ve lived with
all these engines from the day they were born, when
they were tested, when they were flown,” says Bill
Muddle, Aerojet Rocketdyne’s RS-25 field inte-
gration engineer, who also worked on the shuttle
program. “I got to touch and be part of all of those
motors through their history for 125 flights.”
It turns out that the reusable engines, built to
the same standards, developed what engineers call
“personalities” based on their performance and
maintenance demands. It’s a phenomenon seen
in airplane hangars and shipyards—places where
hardware is reused and the maintainers get to know
it during a service life.
And Muddle sees the existing stable of RS-25s
in a similar, more personal way.May/June 2022 55GE
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