Pressure Controllers Used in Sounding Rocket Payload Attitude Control Systems
- Tuesday, 01 May 2012
Pressure controllers from Alicat Scientific have been selected for use in NASA sounding rockets to control payload attitude. The payload sections of some NASA sounding rockets are required to achieve pointing accuracies of finer than 1 arc second (1/3600 of a degree). Conventional attitude control systems (using solenoid valves) introduce too much shock and vibration, and are imprecise. NASA contractor Northrop Grumman chose the Alicat absolute pressure controllers for use with the rockets’ delicate optical instruments.
The pressure controllers are used to directly feed the payload’s attitude control jets. The four pressure controllers are fed from a common gaseous argon manifold at approximately 160 psig, with a stock multidrop RS-232 sending set-point commands to the controllers. As soon as the rocket passes 100 km in altitude, all controllers are brought up to 5 PSIA with both pitch and yaw moments balanced. The spacecraft’s attitude is then modified by modulating the pressure commands sent to the controllers.
The controllers enable commanded pressure changes in increments as small as 0.00234 psi or as large as 150 psi, with extreme speed. NASA’s testing demonstrated that the controllers accurately track a 5-Hz sign wave, correlating to a response time faster than 32 milliseconds. As there are no backup systems for attitude control, the controllers are considered to be mission-critical pieces of equipment.
To meet NASA’s requirements, the controllers were modified slightly with a different capacitor and connector hardware. To enhance durability and reduce mass, the main electronics board was potted in plastic and the bodies were fabricated from aluminum, rather than the typical stainless steel. The controllers passed NASA testing for shock, vibration, thermal cycling, and exposure to vacuum, and have completed several successful missions. Northrop Grumman has ordered 28 controllers over a five-year period for this application.
PC Series absolute pressure controllers