Optical Sensor Keeps Tabs on Rocket Fuel

R. Winn Hardin

MARLY-LE ROI, France -- Bigger is not always better -- especially when you are building rockets.
A new optic sensor under development in France could provide flight engineers with the first pressure readings from inside the liquid gas fuel tanks that power a cryogenic rocket engine. The 1.6-mm-diameter sensor measures pressure "ripples," which can cause changes in thrust, quicker than conventional methods can. In addition, it operates comfortably in the extremely low temperatures associated with liquid gases.
Researchers at Photonetics SA based here; Photonetics in Wakefield, Mass.; and the Société Européenne de Propulsion are testing the sensor in turbo pump engines used in rocket propulsion. But despite early successes, the optical sensor will have to undergo more testing before it finds a place in Ariane, the European Space Agency's commercial satellite launch vehicle.

No loss of importance

Nevertheless, the optical sensor is likely to continue to gain importance in the European space program. Ariane 5, which destructed during its first commercial test earlier this year, relied on one cryogenic engine for its main stage and two solid-fuel boosters. Its predecessor, Ariane 4, depended on five smaller, solid-fuel rockets and a cryogenic engine for the third and final stage.
Conventional fuel pressure sensors, such as strain gauges on silicon membranes or electric differential potential sensors, cannot operate at the 20-K temperatures inside a liquid gas fuel tank, said Mark Girault, an engineer at Photonetics SA. For pressure measurements, fuel must travel outside the tank to where the 6-mm-diameter sensors are housed. A better solution would be to take measurements from inside the tank, Girault said, and that is the motivation behind the sensor.
Designed as a Fizeau microinterferometer, the device measures the white light interference pattern inside a cavity. A silicon chip with a reflective surface caps a cavity etched into a glass chip, a metal tube contains the entire probe, and an optical fiber connects the probe to a minispectrometer.
As the pressure changes in the liquid oxygen and hydrogen fuel tanks, the silicon chip flexes, modulating the incident light from a tungsten lamp. A combination minispectrometer and charge-coupled device array converts the light patterns into an analog signal and, eventually, pressure level readouts for waiting flight engineers.
The technology could have industrial applications outside the aerospace field, said Girault. "Anywhere liquid gases are manufactured or stored, they could use this kind of device," he said.
By changing the thickness of the silicon layer, the sensor can measure changes in pressure up to 400 bars. In rocket research, Girault and team leader Didier Matarin of the Société Européenne de Propulsion have optimized the sensor for 200 bars, the normal pressure inside a turbo pump. Girault hopes to increase the bandwidth of the output signal to increase sensitivity.