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Designing and Engineering a Reaction Mass

Continuing this month’s look at some of the unique areas our engineering division is involved in, we wanted to share a small glimpse into the world of vibration testing and reaction mass design. Ross Group’s structural engineering manager Mark McKinney gave us the rundown on what a reaction mass is, who uses them, and why the process of engineering them can be challenging.

What is a reaction mass?

A reaction mass is a part of the vibration testing equipment designed to shake a test item in a way that simulates its real-world conditions. Test items could include a variety of things such as satellites, spacecraft, aircraft, missiles, weaponry, and electronic equipment, to name a few.

Typically, the reaction mass is a cast-in-place concrete structure with a steel “table” fastened to it. The table is made of large, built-up steel plates that the test item can be attached to. When testing begins, actuators apply oscillating forces to the table to shake the test item with a set of prescribed forces and frequencies. If the test item passes, it can go on to be used in its intended application.

Who uses reaction masses?

Ross Group’s engineering division and its team members have worked on a number of reaction mass projects of various sizes all across the world. For example, the largest one was completed for NASA’s vibro-acoustic testing facility. The concrete base measured 50’x50’ and was 19’ thick! The reaction mass was designed and constructed inside an existing building. It was used to test the Orion spacecraft and included both horizontal and vertical testing tables to simulate acceleration and forces during launch. To visualize the test conditions, imagine being in an elevator when it starts and stops, but your feet are stuck to the floor and the elevator is shaken vertically and horizontally from 5 to 25 times per second. It takes intense testing to make sure spacecraft are ready for operation.

Our team designed another, smaller reaction mass for a defense contractor. Smaller is a relative term, as the concrete for this piece of equipment still measured 14’x14’ and was 12’ thick. The reaction mass was used to test a large rotary-style gun that would be mounted to the deck of a Navy ship. The weapon fires thousands of rounds per minute that are approximately 7” long and contain depleted uranium for penetrating the hulls of other ships. After a certain amount of real-world service, the guns are removed and sent to this facility for maintenance. They must pass the vibration test before being put back into use.

Another interesting project our team was involved in was the engineering of two reaction masses for a European space agency. These were both larger than the reaction mass used for rotary-gun testing, but still only about half the size of NASA’s Orion testing equipment. The two reaction masses for this project were used to test satellites before they were sent into space.

What is unique about engineering a reaction mass?

One challenge of designing a reaction mass is that the natural frequency of the concrete must be at least one octave different than the test frequencies. If there is not enough difference between the two, the actuator will work in the opposite way from what is desired, shaking the reaction mass instead of the test item.

Another delicate part of engineering this equipment is ensuring the concrete mass sufficiently dampens vibrations to prevent human perception outside acceptable limits and damage to adjacent structures. To keep the concrete from cracking, rebar is often closely spaced within the structure for support. Extra-long anchor bolts are used to fasten the table to the concrete, so the actuator doesn’t have a “jackhammer” effect on the base.

Additionally, reaction masses are often built inside an existing building. Because the top of the reaction mass is often even with the building floor, excavation and shoring systems are also a consideration during the engineering process.

While designing vibration testing equipment like reaction masses isn’t an everyday engineering task, our team has taken on the challenge to become experts in this field. The equipment tested is often vital to the increased knowledge and continued safety of our country and the world. It is an area we are proud to be part of and Ross Group looks forward to continuing to support these projects in the future.



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