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Force plate tests ejector seats

A Kistler Instruments product story
Edited by the Engineeringtalk editorial team Feb 11, 2008

Each new ejector seat is tested using a Kistler Instruments dynamic multicomponent force plate mounted in a test enclosure to measure the duration and thrust produced by the burn.

Although most people will think of ejection seats as being a relatively recent invention for high-speed jet fighter aircraft, ejection seats have been in use for over sixty years.

The first recorded emergency ejection was in 1949 when Jo Lancaster bailed out of the experimental AW-52, jet-powered flying wing using a Martin-Baker Mk1 ejection seat.

Since the Mk 1, Martin-Baker ejection seats have saved over 7200 lives; five in the first five weeks of 2008.

All current ejection seats use a small rocket motor to propel the seat and occupant out of the aircraft fast enough to avoid collision with the fin and high enough to allow the parachute to deploy effectively even in ground level ejections.

Ensuring that the rocket burn, usually of around 400ms, provides sufficient thrust to provide a safe escape is a critical part of the design process.

Each new rocket design is tested using a Kistler Instruments dynamic multicomponent force plate mounted in a test enclosure to measure the duration and thrust produced by the burn.

Over 10,000 lbs of thrust combined with the corrosive atmosphere in the test enclosure, demands a highly reliable force measuring system able to maintain accuracy under difficult operating conditions.

When the motor under test is fired, considerable vibration levels are produced, but the Kistler force plate allows force data to be acquired at a sample rate of 25,000 per second.

The thrust against time data allows the design engineers to calculate both the speed of ejection and, most important, the maximum acceleration the occupant will experience.

To keep the acceleration within the range the human body can withstand, initial movement of the seat is created by a low powered system so the seat is already in motion when the rocket motor fires.

This limits the g-force loading to an acceptable level.

In addition to testing new rocket motor designs, the Kistler force plate is also used for propellant life testing and in-production QA sample testing for seat and canopy separation rocket motors.

According to Martin-Baker's Environmental Test Manager, Stuart Driver, the use of the Kistler dynamic force plate was developed out of the use of Kistler static force sensors, which performed reliably but were more difficult to set up.

"Mounting a rocket motor on the Kistler force plate takes only minutes", says Stuart Driver, "and, as the data connections are permanently in place, we can carry out a comprehensive test routine quickly with total confidence in the data".

"Although the rocket motor is a small part of the ejector seat, it is the one component that must operate to full specification every time".

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