Hydraulic systems are widely used in aircrafts to control actuation of landing gear, flaps, rudders and brakes. The reasons behind the usage of hydraulic system is that it has the capability to provide extremely high force and accurate control. Size is a critical factor in aircrafts and hydraulic systems provide maximum power to size ratio. With that said, it is an obvious choice to use a hydraulic system in space to control the operations of space shuttles and space station. Like aircrafts, Space shuttles use hydraulics for major operations, however, not a single system works on hydraulics in a space-station
Challenges of using hydraulics in space
Designing a system for space application is challenging, mainly because of the vacuum, enormous temperature variations and lack of gravity. The hydraulic system is dangerous too, in case the liquid particles escape out of the system, as it can contaminate the optical and electrical instruments. The liquid is hard to trap as it will be floating in the vacuum and the temperature variation in space is drastic. When the space station is facing the sun, the temperature can go as high as 1200°C and, while in the shadow, the temperature goes down by ~1500°C. With the change in temperature the liquid’s properties like bulk modulus, viscosity and lubrication/wear will change with the temperature. Electrical heaters (to heat the liquid) and servo control feedback loop is required to precisely control the movement of the actuator. Designers have to keep in mind that in the absence of gravity they will have to connect an external gaseous pressure source to the reservoir to prevent the development of a vacuum during the pump suction (imagine what will happen when the system is under free-fall).
Cost is not an issue, it is all down to technology
Though, these are cost sensitive yet technological surmountable problems when it comes to space technology cost is the last thing the world cares about. The real challenge is efficient use of the limited power available through the solar panels. The efficiency of hydraulic system is very less compared to the DC servo motor and additional energy is consumed in hydraulic systems to keep the liquid in its working temperature range. The electric motor can be engaged and disengaged by clutch depending upon the power requirement whereas in hydraulic system, the pump will be always connected and the excess liquid will return from the pressure relief valve leading to high power consumption. This challenge associated with hydraulic systems can be simply eliminated by an electrical system and the weight increase isn’t significant as the application demands only low range of loads.
For the space shuttle hydraulics are the only option
However, the space shuttle cannot function without hydraulic systems. Space Shuttles use three different, independent hydraulic systems to serve as backup power generators. These systems are interconnected in such a way that, even in the case of one system fails, the remaining two systems can operate all of the actuators at 50% power. These hydraulic systems are used on a space shuttle to accurately control the position of hydraulic actuators to:
- thrust vector control of the three space shuttle main engines
- propellant control of various valves on the main engines,
- main and nose landing gear deployment,
- main landing gear brakes and anti-skid & nose steer
The forces required to control the direction of the engine thrust (1850 kN) are tremendous and only hydraulic systems are capable of dealing with such large forces. In the same way, the landing gear deployment, hydraulic brakes and nose steering requires large forces at remote locations where large electric motors cannot be mounted. To deal with such applications a hydraulic system is the only available option. The hydraulic system used on the space shuttle weighs only 80 pounds and is capable of generating 140hp, whereas, a normal car engine weighs around 350 pounds to produce 135-150hp power.