Laser Profilometry Inspects Shuttle Thrusters

Brent D. Johnson

After a space shuttle mission, technicians thoroughly inspect the vehicle for wear and tear, looking for the minute chips and hairline fractures that can result from supersonic impacts or from the plastic expansion and contraction of materials in the orbiter. Working with Laser Techniques Co. LLC of Bellevue, Wash., researchers at NASA's White Sands Test Facility in New Mexico are developing a robotic scanner that reaches deep into the thrusters of a craft to document and map the development of such chips there.

The space shuttle's primary reaction control system thrusters have been found to be vulnerable to this type of damage. A shuttle features 38 of these thrusters, which provide attitude adjustment and which are critical in operations such as separation from the external fuel tank and docking with the International Space Station. They are constructed of niobium steel with a chromium disilicide protective coating, and the concern is that there could be a catastrophy if one of these thrusters burned through.

To detect any chipping, inspectors currently take impressions of the thrusters using a silicon-based rubber and then section and inspect these molds to determine the surface area and depth of chips. But this manual process is time-consuming and subjective, and it risks leaving residue in the thruster. NASA desired an automated, accurate, noncontact quantitative approach that employs components small enough to reach deep into the thruster combustion chamber without touching its surface.

In the new system, which employs a laser-based optical triangulation technique, a robotic scanner follows the complex shape of the combustion chamber and nozzle throat. The scanner lowers a laser profilometry probe -- small enough to fit into a thimble -- into the thruster cone, where it rotates approximately 100 times per minute, capturing details with accuracy of better than 0.0125 mm. A 635-nm diode laser serves as the illumination source, and two aspheric lenses focus the beam to a depth of field that maintains a spot size of less than 0.05 mm over the measuring range. Receiving optics focus the returning light onto a lateral-effect photodetector, which provides high spatial resolution.

Currently, technicians use a manual approach to detect the chips that may appear in a shuttle's thrusters. The process is time-consuming and subjective, and residue can remain in the thruster.

The automated approach is faster and more accurate than the mold-based technique, and it provides a quantitative, high-resolution map of the condition of an entire thruster combustion chamber and nozzle throat. The information can be used to track the condition of each thruster's condition and to indicate when one should be taken out of service for repair. "We did a survey of different technologies for coating defect measurement," said NASA project manager Regor Saulsberry, "and this system was selected [because] it meets our requirement for high accuracy and, at the same time, provides the most performance for the developmental dollar."

A new laser profilometry system inserts a miniature probe deep into the primary reaction control system thrusters on the space shuttle.

The technique is being considered for use with the space shuttle's vernier thrusters, which provide fine reaction control, and with the gas generators on its solid rocket motors. It also has found a place in the inspection of Hall thrusters in ion engine research and other safety-critical components in nuclear and aerospace applications.