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Hydraulics are usually closed, but that adds mass vs short acting open systems. F9 fins only work for 4 mins. We were ~10% off.

This actuator was designed to control the thrust vector angle on the SpaceX Falcon 9 first- and second-stage engines. It is designed for high vibration and shock loads, and has excellent frequency response characteristics. It also was designed for short-term immersion in salt water, as evidenced by the protective cover on the electro-fueldraulic servo-valve (EFSV) and hard anodic coatings. Redundancy is accomplished via dual-coils in the EFSV first-stage torque motor.
Embedded within the single-ended piston is a dual-element LVDT, which provides two independent channels of position feedback to the external controller. A single D38999 Series III connector on the EFSV housing communicates all signals to and from the controller. An optional electronic control unit (ECU) is available.
Specifications
Power 28 VDC per MIL-STD-704
Command Signal -10 to +10 mA (can be tailored to application)
Media Hydraulic Fluids or Hydrocarbon-Based Fuels
Supply Pressure 3000 psi max.
Temperature Range -40°F to +250°F
Static Force 9700/7000 lb Extension/Retraction, 2000 psi Supply
Stroke ±2.05 inches
Step Response 170/210 msec Extension/Retraction, Full Scale, 1900 psi Supply
Frequency Response 14 Hz @ -3dB, 1400 psi Supply, Unloaded
ServoValve Dual-Coil, Nozzle-Flapper, Damped First-Stage
Position Feedback Dual-Channel LVDT, 4.0V, 2.0kHz Sine
Materials of Construction 7075-T7351 Aluminum, Stainless Steel
Dimensions 17.25 L x 4.34 W x 7.57 H (overall)
Weight 13.4 lb (dry)
Part Number 101424-5