MICROMO (the FAULHABER Group)
Space Exploration Technologies (SpaceX) builds economical launch vehicles like the Falcon 9 to carry a range of payloads into orbit. One way to control cost is by optimizing fuel burned during launch to minimize waste. The SpaceX team ensures top performance with the help of a special fuel-trim valve, powered by servo motors from MICROMO. Rockets like the Falcon 9 and Falcon 1 at SpaceX burn a fuel known as RP-1, a highly refined form of kerosene that must be mixed with oxygen in order to burn. On the launch vehicle, 4" pipes run from tanks of RP-1 and liquid oxygen (LOX) to combine prior to entering the combustion chamber. RP-1 fuel won’t burn without oxygen, but as long as oxygen is present, the two do not need to be combined in a precise ratio. The problem is that if the ratio of LOX to RP- 1 varies from the optimum mix, either the oxygen will run out before the fuel, or the fuel before the oxygen. Once combustion stops, the material left becomes dead weight, turning from propellant to liability. To ensure this doesn’t happen, the fuel-trim valve adjusts the mixture in real time.
The fuel-trim device consists of a servomotor-controlled butterfly valve. To achieve the proper speed and torque, the design incorporates a planetary gearbox for a roughly 151:1 reduction ratio, plus additional gearing internal to the unit. The shaft of the motor interfaces with the valve directly to make fine adjustments.
The Falcon 9 launch vehicle is a two-stage vehicle with a total of ten engines, each with its own fuel-trim valve. To ensure the proper mix, the valve operates in a double closed loop based on feedback from a triplicate feedback mechanism. The first stage features nine engines that burn for approximately three minutes, and the second stage includes one engine that burns for approximately seven minutes. Because of the duration of the burn for each stage, the control loops can actually run relatively slowly. The outer loop adjusts the angle of the valve, and the inner loop keeps the position steady in case it gets pushed around by shock or vibration.
The characteristics most applications require from servo motors tend to be high torque, high speed, or small size. In the case of the fuel-trim valve, the chief motor requirement was simple: they had to survive launch. The shock and vibration produced in the first stage, in particular, are extreme: for the three minutes of the first stage, the engine is producing 100,000 pounds of thrust.
Rocket engines produce heat as well as vibration, but thermal management does not pose a significant challenge in this application. Much of the heat is radiated and is reflected away. In addition, given the relatively brief duration of the stages, the unit’s thermal mass makes it resistant to rapid temperature swings. During the second stage, for example, the engines may fire only briefly. The vehicle can then coast for as long as 45 minutes before a second burn takes place. By this point, the rocket is outside of the atmosphere, where temperatures can be quite low.
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