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B. Hayes/NIST

The short answer

Engineers use a special kind of scale called a load cell, which is somewhat like the bathroom scale you have at home, to measure the force exerted by the rocket &#; enough to lift tens of thousands of kilograms.

Rockets that launch into space have to be able to push a lot of weight. They have to overcome the relentless pull of gravity while carrying themselves and their payloads into orbit or beyond. Researchers who design rocket engines need to be able to test them, without actually launching the rockets, to make sure that they are producing enough thrust to accomplish their task.

NIST's 4.45-million newton (one million pound) deadweight machine when fully assembled. Credit: NIST

So how do they test them? Quite simply, they turn the rocket engine on its side and attach its body to a large immovable stand to hold it in place. Arrayed in key places around the stand are devices called load cells. As the rocket engine burns through its fuel, it creates a force in the opposite direction of the thrust. This force pushes against the load cells, which compress like springs to measure that force and give a readout.

These load cells have many different designs, but they are all pretty much based on the same simple idea: They contain devices known as strain gauges that are bonded to metal. The strain gauges convert mechanical forces on the load cell into an electrical signal.

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When an engine is being tested, it exerts forces on the metal in the load cell. When a metal is being pulled apart, it becomes more resistive to the flow of electricity, and when it&#;s being compressed, it becomes less resistive. In either case, this changes the electrical voltage of a circuit in the load cell. The strain gauge can measure with high accuracy the changes in electrical voltage and use this knowledge to determine the amount of force being put on the load cell. 

But how do you know the load cells are accurate? Even though the load cells&#; properties have been well measured before their first use, these devices still need to be periodically calibrated to ensure that they are functioning properly and giving the right readings. To do that, you need known weights that you can put on the scale, i.e., the load cell. But where do you get weights big enough to calibrate something meant to measure the thrust of a rocket engine? Not many places. One place that does have such weights is the National Institute of Standards and Technology (NIST) and its 4.45 meganewton (1 million pounds force) deadweight machine in Gaithersburg, Maryland. 

Built in , the deadweight machine consists of a stack of 20 stainless steel discs about 3 meters (a little less than 10 feet) in diameter that sit in its weight pit, spanning about 10 meters (about 35 feet) in height when assembled. Their average mass is about 22,696 kg (just over 50,000 pounds) each. The weights are picked up in a chainlike fashion using a hydraulic jack to create pushing and pulling forces.

Using this titanic machine, NIST calibrates load cells for customers who in turn use them to measure large forces like those produced by rockets and jet engines. Because NIST is the national measurement institute of the United States and responsible for making sure that measurements accurately trace back to the International System of Units (also known as the SI, or metric system), our customers know that the calibrations they are getting are meticulously accurate, ensuring that their instruments are giving them correct measurements and giving them confidence they can reach for the stars. 

Why Thrust measurement is crucial in VTOL aircraft

Propeller thrust measurement is particularly critical in the design and operation of Vertical Takeoff and Landing (VTOL) aircraft. These aircraft rely on the thrust generated by their propellers for lifting and control their vertical and horizontal movement.

In a VTOL or eVTOL (electrical VTOL) aircraft, the propellers must produce enough thrust to overcome the weight and generate lift for takeoff and landing. This requires accurate measurements of the propeller's thrust output to ensure that the aircraft is capable of safe and stable operation.

Furthermore, differently from regular helicopters, VTOL or e-VTOL aircraft often utilize multiple propellers, with each propeller contributing to the overall thrust produced by the aircraft but also interfering in the overall performance and stability of the aircraft. As such, measuring the thrust output of each propeller individually is key in maintaining the balance and stability of the aircraft during flight.

In addition to thrust, the ability to precisely measure the propeller shaft torque in a VTOL is also crucial for maintaining control of the aircraft. Torque measurement allows engineers to optimize the performance of the propellers when operating in conjunction and ensure that they are operating within safe and efficient parameters.


How to measure the thrust of a propeller

Thrust is a fundamental principle in the aerospace industry. This force is produced by the rotation of a VTOL propeller's blades, creating a flow of air that propels the aircraft forward.

To measure thrust, a load cell is mounted axially to the motor stand base to measure upward lift load over the range of propeller speeds. The load cell is then used to measure the force in Newtons (N) produced by the propeller in the axial direction, which is the direction of the propeller's axis of rotation.


How Multi-Axis Load Cells are Utilized for Propeller Thrust and Torque Measurement:

Bi-axis load cells are a valuable tool in measuring simultaneously the thrust and torque produced by a VTOL aircraft propeller. FUTEK MBA500 Thrust and Torque Sensor is capable of measuring axial load and torque, allowing for the capture of all axial forces and torques produced by the propeller.

The axial force, measured in Newtons (N), is the thrust produced by the propeller, while the torque, measured in Newton-meters (Nm), are the torque acting perpendicular to the axis of rotation.

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