No. 2994: PUNCHING OUT

by Michael Barratt

Click here for audio of Episode 2994

Today, “punching out”. The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

A few years ago I was visiting the Ronaldsway Aircraft Company in Ballasalla on the Isle of Man. Here ejection seats are built for Martin Baker. Most of us in the party had flown aircraft equipped with these seats, and so had a keen interest in their design and construction. A big surprise came when we were asked to guess how many of these were actually used. I’ll spare you our low-balled guesses; the answer was to us a staggeringly high 10%. But thinking it through, this fact becomes more understandable.

Ejector Seat in Action
Test of an ejection seat from an F-15 cockpit; the occupant is a weight representative and fully harnessed mannequin, typically instrumented to record the acceleration forces. Photo Credit: Wikimedia Commons

Powered flight requires clean aerodynamic surfaces and adequate thrust to provide lift and control. Damage the airframe or remove the power and the day can end badly. (Under the best of circumstances it may be possible to glide to a nearby runway or flat terrain. But) a damaged, uncontrollable aircraft is a desperate situation. Early in the history of aviation it was clear that pilots flying new or dodgy aircraft needed a way out. Parachutes offered a chance to bail out and land safely. But erratic motion, structural damage, or pilot injury could prevent a ready escape. Faster and more powerful airplanes meant emergencies could unfold very quickly. A means was needed to get out and clear, and do so in a hurry.

NASA Jets
T-38 aircraft used in the NASA training fleet, flying over the Shuttle launch pad. Photo courtesy NASA. Photo Credit: Courtesy of NASA

Many mechanical escape systems were tested early on, including bungee cords and compressed air. The first modern ejection seats arose with jet aircraft during the Second World War. By the late 1940s, seats used explosive charges, typically gunpowder cartridges, to move the seat and pilot up and away from a damaged airplane. As newer aircraft brought ever increasing speed and maneuverability, even these weren’t always fast enough to remove a pilot from harm’s way. Simply using a larger explosive charge might hurl you clear, but produce spine-breaking acceleration forces. Controlled rocket propulsion offered a gentler but still rapid escape, with the added capability of flying the seat to a safe altitude and orientation for chute opening.

Modern ejection seats are technological marvels. With the pull of one handle, the overhead canopy separates while lanyards pull the pilot’s legs close in to the body. A gas charge moves the seat to the top of guide rails, where an under-seat rocket motor kicks on. The seat and pilot are flown to a safe distance and altitude, where the seat automatically separates and a parachute deploys.

In Ejector Seat
Here I am sitting in the state of the art Martin Baker ejection seat that we use in our fleet of NASA T-38 jet training aircraft. Side view. This seat is used for training purposes. The parachute is packed into the black box behind the head. Note the garters around the ankles and thighs. These are attached to lanyards that retract the legs once the ejection seat is activated and just before seat motion, to ensure the legs are restrained clear of aircraft structure. Photo Credit: Courtesy of NASA

So back to that 10%. Unlike the straight and level world of civil aviation, military and test aircraft operate at relative extremes of speed and maneuverability. Their missions are inherently dangerous, airframes stay in service as long as possible, and statistics and time eventually catch up with them. But ejection seats are a great example of a companion technology elegantly developed to serve a larger purpose. The high rate of use has built engineering heritage and helped to improve the design and operating envelopes. Though not effective in every circumstance, these seats offer a fighting chance that has saved literally thousands of lives. As such, the modern ejection seat has become one of the more reliable pieces of equipment I hope never to use.

Front View of Seat
A front view of the seat shows the yellow handle between the knees. A strong pull on this activates the ejection system. Photo Credit: Courtesy of NASA

I’m NASA astronaut Michael Barratt for the University of Houston, where we’re interested in the way inventive minds work.

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End Notes

My familiarity with ejection seats centers around both the biodynamic aspects of the ejection event in my role as an aerospace medicine specialist, and my own flight experience in the NASA T-38 fleet. By far the vast majority of ejections have been from high performance military aircraft. To add more context to the 10% use rate, in 2003 Martin Baker marked its 7000th successful ejection. So yes, well over 70,000 of these seats have been made! Each seat is made up of over 1000 parts and undergoes regular inspection and maintenance, just like any critical component of the airframe. Most of us know one or a few folks who have lived to make our acquaintances thanks to these devices.

The capabilities of ejection seat systems are matched to the aircraft they serve, and so vary quite a bit in the mechanisms of actions and timing of events. In a tandem cockpit such as ours, there is a specific sequence and directionality so that one seat ejects a half second before the other to avoid collision. The Martin Baker seats in our T-38 fleet are considered “zero-zero” seats, meaning you can escape from zero altitude (ground level) and zero forward velocity, with the underseat rocket motor lifting you quickly to a safe parachute altitude. The preference is to have some altitude and forward velocity, which implies buffer altitude for parachute operation as well as some controllability and (hopefully) ejection from an upright airplane.

Martin Baker’s site that includes words on the 7000th successful ejection, as well as milestones in the history of ejection seat development. http://www.ejectorseats.co.uk/milestones_in_ejection_seat_hist.htm

Another site run by someone who simply has a passion for history and trivia associated with ejection seats. A wealth of information: http://www.ejectionsite.com/ffacts.htm

This episode first aired on March 3, 2015.