How reaction controls made space flight possible

As you might expect, launching humans into space required a great deal of clever engineering and innovation. But before the enormous rockets and shiny lander modules, it was a few quirky theories and outlandish inventions that really helped pave the way for space travel as we know it. New book Breaking The Chains Of Gravity tells the story of the scientific advances, personalities and rivalries that laid the foundations for the space race between America and Russia. Here, author Amy Shira Teitel explores the fascinating history of one of the most crucial space flight innovations that made the Apollo moon landings possible…

In 1956, one of the most futuristic simulators at Edwards Air Force Base in California looked like a ride in a schoolyard playground. Two I-beams crossed into an X spun freely on a universal truck joint as the pilot in an open simulated cockpit twisted a joystick. Every movement of his wrist shot compressed nitrogen gas from small rockets on the ends of the beams, and it was his challenge to keep the simulated plane flying level. Simple as it was, this simulator was teaching the nation’s best pilots to fly to the edge of space with the cutting edge technology of reaction controls.

Iron Cross simulator
The Iron Cross simulator used to train pilots for spaceflight


The idea of controlling an airplane in flight came fast on the heels of the Wright brothers’ developing the first heavier than air flying machine in 1903. Airplanes fly because the air traveling over the wing moves faster than the air under the wing, creating a pressure difference that generates lift. Controlling an aircraft in flight is as simple as making that pressure difference work for you.

The Wright brothers began controlling their flyers with a method called wing warping. By twisting their flyers’ wing tips in flight, they could get more drag on one side of the plane than the other. The effect was roll, turning the airplane wing over wing. They used an elevator at the front of the plane to pitch the nose up and down and a rudder in the rear to twist (properly called yaw) the airplane around it’s central vertical axis.

Wright Flyer
The first successful flight of the Wright Flyer


As airplanes developed, their structures changed. Wings became too rigid for warping when they began doubling as fuel tanks. This forced engineers to develop the new flight control system of control surfaces. Small, movable surfaces on the ends of the wings called ailerons control roll, a rear rudder controls yaw, and a rear elevator controls pitch. These are the same flight controls commercial jets use today.

But a problem with this system arose in the 1950s. Flight control surfaces only work because they have air to push against; it’s the action of pushing against the air that causes the opposite action of turning or pitching the aircraft. With the advent of rocket-powered flight, planes were starting to fly high above the atmosphere where the air is too thin to push against. Not only that, they were flying faster, too.

The combination of high and fast led to a phenomenon called inertial coupling wherein the inertia of an aircraft’s fuselage overpowers the abilities of its control surfaces. Planes flying too high and too fast could fall victim to inertial coupling and tumble. The pilot would be helpless until his plane reached thick enough air for his control surfaces to bite into.

The question in the era of rocket planes thus became how to control high-speed high-altitude flight. It was clear that a new means of flight control was needed to keep pace with these new aircraft. The solution was reaction controls.

Even if traditional flight control surfaces don’t work in thin air, Newton’s third law of motion still applies; for every action there is still an equal and opposite reaction. Controlling a plane at altitude, then, came down to applying the opposite force, and the means engineers developed are the aptly named reaction controls. Reaction controls are small jets that expel compressed gas in short bursts. The force is imperceptible at sea level, but at high altitudes it’s enough to nudge an aircraft into the correct orientation. This was vital on high-altitude rocket flights. The pilot had to keep the aircraft oriented correctly for a smooth reentry. If he missed, the aircraft would tumble as it fell through the increasingly thickening atmosphere, and he might not recover.

Neil Armstrong with the X-15
Neil Armstrong with the X-15 hypersonic research aircraft


The first aircraft to test reaction controls in flight was the X-1B, a later version of the supersonic X-1 that was the first to break the sound barrier in 1947. Successful testing led to reaction controls inclusion on the X-15, the hypersonic research aircraft they were really developed for. This small plane saw NASA and Air Force pilots flying higher than 280,000 feet and faster than Mach 6 in the 1960s. Using compressed hydrogen peroxide, X-15 pilots were able to keep their aircraft oriented so they reentered the atmosphere smoothly, perfectly lined up for their atmospheric reentry and eventual landing on the dry lakebed at Edwards Air Force Base.

And reaction controls use didn’t end with airplanes. The same principle applies to spaceflight flying in a near vacuum. Apollo era spacecraft had no flight control surfaces. They were designed to fly only in space and had only reaction controls for fine tuning movements in orbit and on the way to the Moon. The Apollo lunar module, too, had only reaction controls for manoeuvrability; the atmosphere on the Moon is far too thin for traditional flight controls. Only the space shuttle had traditional flight control surfaces, and it didn’t use those in orbit. After using reaction controls during a mission, shuttle astronauts used the airplane-inspired spacecraft’s traditional control surfaces for it’s reentry and runway landing.

Reaction controls on the Apollo lunar module
Reaction controls on the Apollo lunar module (centre top of the image)


Breaking The Chains Of Gravity by Amy Shira Teitel, published by Bloomsbury Publishing, is now available in hardback for £16.99 and as an ebook for £14.99. 

Breaking the chains of gravity
Breaking The Chains Of Gravity by Amy Shira Teitel


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Plus, take a look at:

Cosmonauts: How Russia kick-started the space age 

Interview: Buzz Aldrin wants an international moon base 

NASA’s Apollo 11: How the Lander Module worked