There are few rockets more iconic than the RS-25, also known as the Space Shuttle main
engine.
Even though it’s been around in some form since the 1970s, it’s been improved and
upgraded several times, making it the most efficient liquid fuel rocket engine going
today.
And now the RS-25 is going to see a new chapter in its history, working to carry NASA’s
new Space Launch System back to deep space, beyond Earth orbit.
When you really think about it, the principle of a rocket is actually pretty simple.
Use a combustible fuel to blast out a jet of hot gas through a nozzle.
Rockets take advantage of Newton’s third law, you know, for every action, there’s
an equal and opposite reaction.
As material is fired out the back of the rocket at supersonic speeds, the rocket receives
a force, pushing it in the opposite direction.
Unlike jet engines, which need to bring in oxygen from the atmosphere to create the combustion,
rockets carry their own oxidizer, so they can even work in the vacuum of space where
there is no oxygen.
Rockets aren’t pushing against anything, they’re throwing material out the back so
fast that they move in the opposite direction.
There are many different kinds of rockets out there on the market.
There are the small sounding rockets that can follow a parabolic trajectory, reaching
a high altitude and then returning to Earth.
And then there are the enormous, multi-staged rockets like the Atlas or Ariane capable of
lifting tonnes of satellite into low Earth orbit and beyond.
Of course, there’s everything in between, from many different manufacturers across many
different nations.
There are fewer kinds of rocket engines, though.
You’ll see the same engine attached to different sizes of rockets.
Some of these engines have been used continuously for decades, like the Russian RD-180 engine,
which is used for Russian and even American launches.
Others are relatively brand new like SpaceX’s Merlin engine, which has already been through
many iterations, carrying Falcon rockets into space.
And the even newer Blue Origin BE-4.
Today I want to talk about an iconic rocket engine, the mighty RS-25, the Space Shuttle’s
main engine.
Let’s look at its history, capabilities and how it’ll see a whole new chapter by
helping to push Space Launch Systems rockets into orbit and beyond.
You might have problems with the space shuttle program, and disagree with NASA’s plans
for the Space Launch System, but the RS-25 is an incredible piece of space hardware,
there’s not much out there that stacks up to it in pure power and efficiency.
As the Apollo Era was winding down, NASA began investigating the technologies it would be
using for its next step in space flight: the space shuttle.
While the mighty Saturn V was a beast for hurling heavy cargo deep into the cosmos,
the space shuttle was seen as a reusable vehicle that would make spaceflight a routine experience.
It turns out, spaceflight wasn’t ready to be routine, but NASA didn’t know that yet,
balancing the various requirements from different stakeholders.
During the design process, the shuttle went through many variations.
There was a time that the space shuttle could have had 12 sea level booster engines and
then 3 orbiter engines, fixed wings versus delta wing, but they finally settled on the
modern design with the space shuttle orbiter attached to the huge external fuel tank with
its solid rocket boosters.
When it came to the engines, several new technologies were being considered, like an expanding nozzle
that would operate more efficiently at different altitudes and high-pressure engines that would
use the fuel more efficiently to produce thrust.
When the initial contracts for the Space Shuttle were awarded in 1969 - yes, work on the Space
Shuttle began back when the first astronauts were landing on the Moon - NASA requested
engine proposals that matched the capabilities they had been studying, with the ability to
throttle the engine, expanding nozzles, and a very high-pressure combustion chamber.
The contract was awarded to Aerojet to produce the space shuttle main engine, the RS-25.
The company later merged in 2013 with Pratt & Whitney Rocketdyne to become Aerojet Rocketdyne,
which I think is about the coolest name for a rocket company.
The space shuttle was equipped with three RS-25 engines which were fed by liquid oxygen
and hydrogen from the huge orange external fuel tank.
Together with the twin solid rocket boosters, the space shuttle’s 2,000 tonnes or 4.4
million pounds would be carried into orbit.
Each shuttle would would be capable of delivering 27,500 kilograms or 60,600 pounds of cargo
to low-Earth orbit.
The exact thrust of the RS-25 is actually a little difficult to pin down.
The engine was designed to be throttled, so that it could change the total amount of engine
power from 67% to 109% of its power rating.
In an emergency, it could probably hit 111%.
At sea level, and at 100% thrust, each engine could generate 380,000 pounds of thrust (or
1,670 kilonewtons).
And in a vacuum, they could produce 470,000 pounds of thrust or 2,090 kilonewtons.
But like I said, the rockets could be throttled up beyond 100%, which I know sounds kind of
crazy, but 100% was the original spec, while they were able to get the engine power higher
through improvements and modifications over the engine’s development lifetime.
Need some kind of comparison?
The SpaceX Merlin engine produces 845 kilonewtons or 190,000 pounds of thrust, rising to 914
kilonewtons or 205,500 pounds of thrust when it reaches the vacuum of space.
That’s why the Falcon 9 has the name, it carries 9 Merlin engines.
The Falcon Heavy has 27 of them.
Over the course of the shuttle program, there were a total of 46 RS-25 engines used.
And together, they successfully lifted off the space shuttle 135 times.
One RS-25 did fail on a 1985 mission with Challenger, but the shuttle was still able
to get to orbit and complete its mission with the one failed engine.
And there were a handful of times when leaks and sensors connected with the engines led
to a launch delay.
And then, one year later Challenger was destroyed along with its 7-astronaut crew during launch.
The mishap was traced back to the o-rings that helped attach the solid rocket boosters.
Just to give you a sense of scale, each RS-25 measures 4.3 meters tall and the nozzle is
2.4 meters in diameter.
It’s about the size of a compact car.
There are four turbopumps that pull in liquid hydrogen and liquid oxygen from the main fuel
tank, and force them into the combustion chamber at high pressure.
Inside the combustion chamber, the fuel and oxidizer are mixed together and ignited.
Beneath this is the huge nozzle, where the hot gases are expelled at 13 times the speed
of sound.
The RS-25 is known as staged combustion engine, where turbine exhaust is captured and fed
back into the engine.
This makes for higher performance, but also higher pressure and more danger.
During its initial development, NASA had a rough time getting the RS-25 to handle the
pressure and forces involved.
This is one of the key differences from other engines on the market, like the SpaceX Merlin,
which is open-cycle.
It’s less powerful pound for pound than the RS-25, but you get the benefits of lower
cost and higher production rates.
Each Falcon Heavy will almost have as many Merlin engines in it than the number of RS-25s
that were ever built.
The end of the shuttle program would have been the end of the RS-25 engine, but it’s
gotten a new lease on life with NASA’s Space Launch System.
I’m going to talk all about how the RS-25 fits in, but first I’d like to thank:
Howard Amos Bjørn Karlsen
Epyx911 Jonathan Stein
Anders Theodorsen
And the rest of our 802 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
Once the shuttle program wrapped up, NASA was ordered to keep the fleet’s workforce
employed developing the Ares rockets for the Constellation program.
This transitioned to the Space Launch System, which would give NASA heavy lift capabilities
again, enabling missions past low-Earth orbit, to the Moon, asteroids, and beyond.
In its initial Block 1 design, SLS will be capable of lofting 70 metric tonnes to low-Earth
orbit, and the final Block II configuration will be able to carry 130 metric tonnes.
In fact, it could end up being even more powerful than the Saturn V, making it the most powerful
rocket ever built - until the SpaceX BFR becomes operational.
And much of this depends on using the RS-25 engines for the central core of the Space
Launch System’s first stage, since they have 16 left over from the space shuttle days.
Unlike the space shuttle, the SLS will be equipped with 4 RS-25s, together contributing
9,000 kilonewtons or 2 million pounds of thrust to the system.
These will be joined by twin solid rocket boosters for a total of 32,000 kilonewtons
or 8 million pounds of thrust.
As part of the upgrade to SLS, NASA engineered an entirely new engine controller to match
the engine with the new rocket.
At the end of 2017, NASA wrapped up a 400-second test of its new RS-25 controller.
This was the eighth test of the year so far and the sixth using this new controller.
There’s already been a successful test in 2018 with more to come.
If all goes well, the RS-25 will see its return to flight when the first SLS rocket blasts
off with Exploration Mission-1, now scheduled for 2019 (or maybe even 2020).
Unlike the reusability of the shuttle, though, the core stage of SLS will be destroyed after
launch, including its RS-25s.
Once they use up the initial group of leftover engines, they’ll need to get more from Aerojet
Rocketdyne.
While the RS-25 engines were developed and built for the shuttle fleet, and now SLS.
They have been upgraded several times, with the total power output updated to 105% by
the end of the shuttle program.
SLS should get to 109% and they think they can push the engines to 111%.
Normally this would mean a decline in their reusability, but these engines won’t be
reused, so they might as well be driven to the max.
Each mission using the Space Launch System is going to cost US taxpayers $500 million
to $1 billion dollars.
A launch on the SpaceX Falcon Heavy is going to cost a mere $90 million in comparison;
although it won’t have the raw launch capacity of the SLS.
Each RS-25 probably cost NASA around $60 million.
That much money will get you a flight on a Falcon 9.
So I think there’s a pretty important argument to have about the cost effectiveness of the
SLS, and developing a rocket system that gets destroyed with every launch, now that SpaceX
and Blue Origin are demonstrating reusability.
But I’ve really got to admire the power and capability of the RS-25, and the creativity
and workmanship that went into it.
It’ll go down in the history books as one of the most important and impressive rocket
engines ever built.
And I can’t wait to see it fly again.
How do you feel about the RS-25, the space shuttle and the Space Launch System.
Did you want me to compare this engine with the more modern reusable rockets from SpaceX
and Blue Origin?
Let me know your thoughts in the comments.
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In our next episode I continue my series on the discoveries made by space missions.
What did NASA’s Curiosity Rover discover?
That’s next time.
And finally, here’s a playlist