Three SSMEs sit
prominently on
the back of the
Atlantis space
shuttle, pictured
here on its final
mission in 2011.
“I’ve got 16 unique children,” he remarks.
“I love them all to death, but they all have their
idiosyncrasies.”
As the shuttle missions changed, the engine
was asked to run at higher levels, up to 104.5
percent of its rated power, mostly to handle
greater weight for the International Space
Station construction.
“They asked it to run at those high pressures
and temperatures, and it came back every single
time,” Muddle ways. “When you started that engine
to put astronauts into space, it did.”
When the shuttle retired in 2011, so did the
SSME, it seemed.IN 2015, NASA PREPARED TO ANNOUNCE ITS
choice of engines that would be used in its new
Space Launch System. With a 30-year track record
and more than 1 million seconds of total ground test
and f light firing time, the venerable engine’s history
became a selling point, especially in a traditionally
risk-averse NASA program. SLS will be the largest
operational launch system ever built, and efficient
engines get more benefits as rockets scale up, which
was good news for the SSME’s bid.
“We measure efficiency in specific impulse (ISP),
like gas mileage for a rocket. Our ISP gets 452 sec-
onds, which is very, very hot. For other rockets, that
will be in the 300s,” Bradley says. “The more effi-
cient you are, the less propellant you have to lift.Efficiency means, in reality, cargo.” That’s crucial,
he added, for the Artemis program, which aims
to establish a sustainable human presence on the
moon and make crewed missions to Mars.
In November 2015, NASA awarded Aerojet
Rocketdyne $1.16 billion to adapt the 16 mothballed
SSMEs to the new launch system and restart the
production line for six new engines, now renamed
the RS-25. “You hear that decision, you celebrate for
10 minutes, and then you get to work,” says Brad-
ley. “It’s a different vehicle, so we got a lot to do.
The rocket’s taller, so the pressure coming into the
engines is higher when it’s started.”
The space shuttles used three SSMEs, but the
behemoth SLS demands four engines, which further
changes the operating environment. There isn’t much
room to spare under the megarocket, especially given
that people need room to work on them. “There’s
only 8 inches between the wall of that vehicle and the
edge of the engine,” Muddle says. “And I’ve got to put
a technician in there that has to torque something.”
The nearby flames from the motors of 17-story
solid rocket boosters, one affixed to each side of
the core stage, also now pose a threat. “In the shut-
tle program, our engines were 20 or 30 feet higher
than the boosters,” Bradley says. “On this rocket,
we’re adjacent to them, and those are big, powerful
beasts.” Extra thermal shielding now protects the
four core engines from the two boosters.
During initial Artemis SLS launches, RS-25
engines will have to run at the maximum the SSMEs
were designed for, 109 percent of the space shuttle’s
main engine power.
Starting with fifth Artemis mission, the engines
will be called on to reach 111 percent of the shuttle-
era output. “Without any redesign per se, we can
go ahead and throttle up a little bit higher. And we
think we can go higher than that,” Bradley says.
The enduring use of this engine speaks to the
strength of its original design, says Muddle. “You ask
something to throttle up to 111 percent, 114 percent,
and then also be able to throttle down to 65 percent,
that’s a huge range to ask a rocket engine to do.”
NASA doubled down on the RS-25 in May 2020,
when it awarded Aerojet Rocketdyne $1.79 billion to
produce 18 more engines, on top of the six already
on order. The combined price of this new RS-25
contract and the earlier one—resulting in a tab of
possibly more than $100 million for each single-use
SLS engine—has raised critical eyebrows.
After all, each SSME cost an estimated $40 mil-
lion. And in 2018, United Launch Alliance paid Blue
Origin an estimated $14 to $16 million per pair of
its new BE-4 engines to use for ULA’s Vulcan Cen-
taur rocket. The BE-4 produces similar thrust to
the RS-25, but Blue Origin has not released its ISP.
Aerojet says a per-engine calculation is unfair,
since the contract’s fee includes new hardware for56 May/June 2022
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