The development and evaluation of new hybrid and solid rocket motors requires accurate characterization of the propellant surface regression as a function of key operational parameters. These characteristics establish the propellant flow rate and are prime design drivers affecting the propulsion system geometry, size, and overall performance. There is a similar need for the development of advanced ablative materials, and the use of conventional ablatives exposed to new operational environments. The Miniature Surface Regression Sensor (MSRS) was developed to serve these applications. It is designed to be cast or embedded in the material of interest and regresses along with it. During this process, the resistance of the sensor is related to its instantaneous length, allowing the real-time thickness of the host material to be established. The time derivative of this data reveals the instantaneous surface regression rate.

The MSRS could also be adapted to perform similar measurements for a variety of other host materials when it is desired to monitor thicknesses and/or regression rate for purposes of safety, operational control, or research. For example, the sensor could be used to monitor the thicknesses of brake linings or racecar tires and indicate when they need to be replaced. At the time of this reporting, over 200 of these sensors have been installed into a variety of host materials.

An MSRS can be made in either of two configurations, denoted “ladder” and “continuous” (see Figure 1). A ladder MSRS includes two highly electrically conductive legs, across which narrow strips of electrically resistive material are placed at small increments of length. These strips resemble the rungs of a ladder and are electrically equivalent to many tiny resistors connected in parallel. A substrate material provides structural support for the legs and rungs. The instantaneous sensor resistance is read by an external signal conditioner via wires attached to the conductive legs on the non-eroding end of the sensor. The sensor signal can be transmitted from inside a high-pressure chamber to the ambient environment, using commercially available feed through connectors. Miniaturized internal recorders or wireless data transmission could also potentially be employed to eliminate the need for producing penetrations in the chamber case.

The rungs are designed so that as each successive rung is eroded away, the resistance changes by an amount that yields a readily measurable signal larger than the background noise. (In addition, signal conditioning techniques are used in processing the resistance readings to mitigate the effect of noise.) Hence, each discrete change of resistance serves to indicate the arrival of the regressing host material front at the known depth of the affected resistor rung. The average rate of regression between two adjacent resistors can be calculated simply as the distance between the resistors divided by the time interval between their resistance jumps. Advanced data reduction techniques have also been developed to establish the instantaneous surface position and regression rate when the regressing front is between rungs.

A continuous MSRS is so named because instead of discrete rungs, there is one continuous strip of resistive material across the legs. Assuming that this strip has spatially uniform thickness and resistivity and that the electrical resistance of the legs is much less than that of the strip, the electrical resistance of this MSRS is inversely proportional to the remaining length of the sensor and, hence, to the remaining thickness of the host material in which it is embedded.

A ground-based sensor package has been developed (see Figure 2). Due to its small size and low mass potential, future generations of the MSRS technology could be applied to flight applications. One eventual goal is to provide the capability to record and transmit real-time regression data from a vehicle in flight. In this capacity, the sensor could serve a dual-use role by providing engineering data under actual operating conditions, as well as health monitoring of the host material.

This work was done by Daniel J. Gramer, Thomas J. Taagen, and Anton G. Vermaak of Orbital Technologies Corp. for Stennis Space Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

: Orbital Technologies Corp. (ORBITEC) 1212 Fourier Dr. Madison
WI 53717 Phone No.: (608) 827-5000

Refer to SSC-00140, volume and number of this NASA Tech Briefs issue, and the page number.