The Metrology and Calibration Laboratory (MCL) at Marshall Space Flight Center (MSFC) has some pretty amazing capabilities, one of which is the Gilmore Force Machine. It can apply five million pounds of force in both tension and compression modes. Built in 1966, the original charter of the Gilmore was to calibrate load cells for the Apollo Program.
Large load cells are used to measure the thrust applied by rockets and rocket engines. This measurement is critical to ensure liftoff and acceleration of the spacecraft. Today, we use load cells in many other ways, including legal trade (weighing produce, fuel, trucks, even postage), and many research applications like structural testing.
The most accurate way to calibrate a load cell is by applying known mass (weight). In the industry, this is called deadweight calibration and used for calibration of primary level force measurements like load cells. The MSFC Gilmore has 500,000 pounds of deadweight capacity. The nineteen masses were calibrated and balanced by the National Institute of Standards and Technologies (NIST) before they were installed at MSFC in 1966. The value of each mass is corrected for local gravity effects, temperature, and humidity. The result is a very accurate (< 30 ppm) force measurement. In 2011, the MCL performed a complex measurement assurance process on the masses and the maximum deviation measured from the 1966 NIST values was only 20 ppm (1 lb out of 44,000 lbs). The 500,000 lbs of masses are installed in a fifty foot deep room under the Gilmore. Also in 2011, the lab developed new software, modern electronics, and state of the art processes – reducing the total uncertainty of the deadweight calibrations by half, from 50 ppm to 25 ppm.
To reach five million pounds of force, the Gilmore has a secondary system using hydraulics. Ten hydraulic rams, each producing up to 500,000 lbs-F, can apply a load in either tension or compression. The load is sensed by radially spaced load cells individually calibrated using the Gilmore’s deadweight. This approach efficiently transfers the deadweight accuracy to the hydraulic process. The end result is a hydraulic load uncertainty of < 50 ppm.
This capability does not exist anywhere else in our nation, making the Gilmore a National asset. ERC, under the METTS contract, manages and operates the Gilmore to support NASA’s missions like Space Launch Systems (SLS). The Gilmore also supports multiple Department of Defense agencies and commercial industries since the capability is unique to NASA/MSFC.
The Gilmore has seen almost fifty years of service to our nation, supporting a myriad of high visibility programs. We are sure the Gilmore is up to another fifty years of valuable service!
By: Kirk Foster, ERC Manager, Metrology and Calibration Laboratory