Specific impulse (ISP), or simply impulse (change in momentum) per unit amount of propellant consumed, is a measure of rocket and jet engine efficiency. The amount of propellant, or in the case of engine testing at the Stennis Space Center (SSC), cryogen consumed during rocket engine testing must be measured to accurately quantify ISP. One way to determine the amount of cryogen used is to measure the change in cryogen fluid height within a storage/feed tank during testing and then relate the change in height to volume of cryogen consumed. A float system coupled with discrete vertically positioned Reed switches is currently used at the SSC to determine cryogen fluid height and then determine cryogen consumed during a rocket motor test firing. However, the cryogen fluid level within a run tank varies continuously and the switches are placed at discrete locations, limiting the accuracy of this method. If individual switch failures occur, the error increases due to the increased distance between switches/measurement locations. In addition, since pressurized gas is used to force the significantly cooler liquid cryogen out of the tank during a test, the liquid cryogen surface is turbulent and not flat or smooth, which can also affect accuracy.
An optical sensor was demonstrated that can accurately measure the amount of fluid in a closed vessel at heights that are in-between Reed switches. This technology incorporates the existing Reed switch ball-shaped target that floats on the fluid surface with optical fibers, a laser rangefinder, a small athermal telescope, and detailed knowledge of the vessel shape. The laser rangefinder can be located outside the tank to operate in a non-cryogenic environment. Modulated diode laser light, generated with the laser rangefinder, is brought into and out of the tank using optical fibers. Light from the fibers illuminates the current Reed switch float within the tank. The ball-shaped floats have sufficient optical cross section to serve as rangefinder targets so that the light scattered off the float can be collected with an athermal telescope. The telescope, in turn, focuses light back into optical fibers to bring the light onto the laser rangefinder detector outside the tank. This optically derived position measurement is then processed with the current discrete position measurement using a Kalman filter to improve accuracy and reduce noise and other artifacts, like fluid height fluctuations. Measurements and analysis performed indicate that a non-intrusive, commercially available laser rangefinder with fiber-optic feed through into a cryogen tank coupled with a Reed switch (or other) float system could yield extremely accurate, near-continuous fluid level measurements under many conditions. These measurements could also be combined with flowmeters and other sensors.