Fluid Systems

The final topic we'll cover in Spacecraft Systems is fluid systems. A fluid system is any system that uses a working fluid, liquid or gas, to do work.  Gasses and liquids are both fluids, they flow when subjected to a shear stress. 
Liquids are incompressible, their volume does not change under pressure.  This is because liquids have constant volume.  Gasses, on the other hand, are compressible.  Their density and volume change with their pressure.
Because fluids can flow under stress they will naturally move from areas of high pressure to areas of low pressure.  Because of this the pressure in a confined fluid is the same everywhere.  Known as Pascal's Law this property means that the pressure at one wall containing a fluid is the pressure at all the walls.
Pascal's law allows fluids to transmit force.  Consider the illustration below.  When piston one is pressed down pistons two and three raise up.

How far pistons two and three raise up depends on how far piston one is pressed and how wide the pistons are.  Remember that the volume of a liquid doesn't change under pressure.  So the volume that is compressed in piston one is the volume of pistons two and three that raises up.  Volume here is the cross sectional area of the piston times how far up or down it moves.
What this means is that we can construct a device that uses a fluid to take a force from one area and relay it to another.  We do so by increasing the pressure at one end of the fluid.  Remember that pressure is a force divided by the area.  So by creating a pressure at one end we create a force at the other end that equals the pressure times the area we're working over.
It also means that the working fluid can amplify our force.  If we push down on a cylinder with a small area we get a high pressure for our force.  If it's attached to another cylinder with a large area that pressure produces more force than we had to put in.  Similar to how a lever can amplify force by increasing the moment.
In a system that transmits force through a fluid we can use either a liquid or a gas as our working fluid.  Hydraulic systems use a liquid, often water or oil, as their working fluid.  Pneumatic systems use a gas, usually air or carbon-dioxide.  Spacecraft often use helium as a working fluid.  It is stored as a cryogenic liquid but expands into a gas when in use.
Pneumatic systems are usually simpler to design and control than hydraulic systems.  They also tend to be more reliable, compressible gas can absorb shocks reducing damage to machinery.  Pneumatic systems are also safer, the gasses used are nonflammable and the systems can typically fail-safe.
Hydraulic systems have their own advantages.  They are more efficient, precisely because gasses tend to absorb some energy while liquids transmit all of their applied load.  There's no need to bleed off excess pressure when disconnecting equipment from a hydraulic system.  But their biggest advantage is their carrying load.  Pneumatic systems can support about 80 - 100 psi.  Hydraulic systems typically support 1,000 - 5,000 psi and can be designed up to 10,000 psi.

Now let's consider a simple hydraulic/pneumatic system:

The various components of this system are connected by conductors.  In fluid systems a conductor is simply a pipe or hose that contains and transmits the fluid.  Conductors must be rated for the pressure that the working fluid is intended to be transmitted at.
At the start of the system is a pump or compressor.  These two devices serve the same function, they increase the pressure of the fluid.  Pumps add pressure to a liquid, while compressors add pressure to a gas.
Working fluid goes from the pump into an accumulator, a tank designed to hold pressurized gas.  The accumulator stores the fluid as the pump builds up pressure.  From the accumulator the fluid passes through a check valve.  If the pressure is correct the check valve allows fluid to pass to the loads, if the pressure grows to high the check valve sends fluid straight to the reservoir.
Loads are devices attached to the hydraulic system that do some work using the fluid pressure.  The fluid expends its pressure doing work and the now low pressure fluid passes into the reservoir.
The reservoir is a second, low pressure, holding tank that collects used fluid from the loads.  Here the working fluid is stored until it again passes through the pump and into the hydraulic system again.
Pneumatic systems using air don't have a reservoir attached.  The reservoir in this case is the atmosphere.  Air is released by the load into the atmosphere and the compressor pulls air straight from its surroundings.
Many systems are precharged, meaning that the accumulator is already pressurized.  In this kind of system there is no pump or reservoir, spent working fluid is vented overboard.  Such a system is much lighter and simpler than a full system with compressor, however they have a very limited operating life.  When the pressure of the accumulator is spent the system is no longer any use.  Many spacecraft use precharged systems.

Now let's take a closer look at the devices in a fluid system.  First consider the devices we already saw in the simple pneumatic system above.  It is composed of two pressure vessels, the reservoir and the accumulator; several valves; and a pump or compressor.
Pressure vessels are tanks designed to contain a fluid under pressure.  They are rated based on their wall thickness, which determines how much pressure they can hold.  Readily available tanks are also given in terms of standard sizes.  One design that increases the strength, and so the pressure rating, of a tank is a composite overwrap vessel.  In this design a metal liner is surrounded with adhesive and a layer of composite fiber.  This gives the tank additional strength with little increase in mass.

Valves are simple devices that either block fluid flow or divert it when closed.  They come in two types.  Type 1 valves remain in the position they were set to until they are again changed.  Type 2 valves are classifies as either normally open or normally closed.  Type 2 valves return to their normal position when force is removed.  Valves can also be one-way, allowing fluid to pass in one direction freely, but blocking all passage backwards.
Pumps and compressors take work from some outside source and convert it into fluid pressure.  The most common pumps used in fluid systems use turbomachinery, that is spinning parts, to push fluid.  The simplest example, there is probably one in the room, is a fan.  The fan's blades create an aerodynamic force which pushes on the air.  There are many different configurations of turbo- pumps and compressors.  Below is a gas compressor:

Not all pumps are turbomachines, however.  A simple bicycle pump uses a much simpler system.  There are two one-way valves, one allowing air in from the environment and one allowing it out through the hose.  Pulling on the piston creates a vacuum which draws in air, pushing it down forces the air out.
There is another type of turbomachine that does the opposite job.  A turbine uses fluid pressure to produce work.  Fluid passes through a sequence of turbine blades, imparting a force to them.  This causes a shaft to spin, which draws power from the fluid.  Energy from the fluid (which existed as pressure) is given to the shaft (as rotation) allowing us to use the fluid to produce useful power.
Pistons also remove pressure from the fluid and convert it into power.  In a piston pressure forces the plunger to move out.  The plunger is then pulled back, forcing the working fluid back out.  A full motion of the plunger up or down is called a single stroke. 
A two-stroke piston has two inlets, pressurized fluid pushes the cylinder out, expending its pressure.  Then pressurized fluid enters through the other inlet pushing the piston back in and forcing the spent working fluid on the other side of the plunger out.  This repeats.  Other designs have two or more pistons attached to a single rotating wheel, when one pushes out the wheel pushes the other back in.
In a hydraulic system turbines are used to produce rotation.  Pistons are used to produce a back and forth motion.  Pistons can produce rotation by attaching them to a crank shaft they can also produce rotation, as is done in care engines.  However turbines are far more efficient for pneumatic applications.
Heat exchangers add or remove heat from the working fluid.  Fluid passes through a series of thermal conductors, usually metal, that are connected to a source of heat or a cold sink.  Heat exchangers attached to points hotter than the fluid entering are called high temperature reservoirs.  Heat exchangers connected to points colder than the fluid are low temperature reservoirs.  Heat exchangers add or remove energy from the flow in the form of heat.
As we discussed when looking at rockets fluid devices called nozzles convert pressure into velocity.  Another device, called a diffuser, does the opposite job.  Diffusers slow down the working fluid in order to increase its pressure.  These two devices are very similar in shape and design.  For subsonic flow (which liquids must be and pneumatic systems generally are) a nozzle narrows and a diffuser expands.
Throttles also obstruct flow.  At the throttle flow suddenly narrows, drastically reducing pressure and trading it for velocity, unlike a nozzle where this change is smooth.  Throttles function much like valves, in that they open and close allowing some flow through. 

We've looked at fluid systems and devices in a very general sense so far.  But why are we even looking at them?  What are they used for on spacecraft that makes them worth considering?  Well, before looking at hydraulics on spacecraft lets consider two other kinds of fluid systems that we've already looked at.
Consider the rocket systems we saw before.  Rockets are fundamentally a fluid system.  The working fluid is the propellant.  The combustion chamber releases pressurized working fluid through a nozzle, which does work by converting pressure into velocity and causing a transfer of momentum. 
Liquid and hybrid rockets have a complex fluid system leading into the combustion chamber.  Cold propellant is sent through pumps into the combustion chamber.  In regenerative systems it passes though a heat exchanger, gaining energy from it.  The entire propulsion system is a fluid system.
Many thermal control systems are also fluid systems.  In this case the working fluid is coolant.  It passes through heat exchangers, taking heat away from systems that produce it.  The fluid is compressed and is forced to give up that heat, then passes through the system again.

Hydraulic systems are used on spacecraft because of their very high energy density.  Similar electrical systems would be needlessly bulky.  Below is a graphic of the hydraulic systems used aboard the Space Shuttle.

In general hydraulic systems are used any time a great deal of force is needed in a confined area.  Hydraulics are used for thrust vector control, shifting the orientation of heavy engines that are mounted on gimbals.  They move aerosurfaces, flaps and rudders that control the motion of launch vehicles against immense aerodynamic loads.
Hydraulic systems are also used to open propellant valves on large vehicles.  The amount of fluid and pressure involved in large engines is simply too great for a reasonably sized electric valve to be used.  In a sense the hydraulic valve amplifies the strength of the electric valve which is used to activate it.