Spec Tech: Spacecraft Subsystems
Sometimes it helps just to know the right words when you are looking for more information on a topic. If I had only known to search for “Vortex shedding induced roll” last spring I would have saved myself many months of work. But I digress. Today’s spacecraft like the space shuttle have millions of parts. What do they all do? Like building a sailing craft or any other world building it helps to know about some of the pieces.
The rocket part of rockets. It depends on what kind of rocket you have, obviously, but generally there is some kind of propellant. Rockets today that are liquid fueled have a oxidizer tank, a fuel tank, high pressure reserves, turbo-pumps and plumbing. Lots of plumbing. The hardest part of running a liquid fueled rocket is getting enough propellant to the motor. You either have to continually pressurize the tanks, usually with helium, or you have to pump the fuel. The big rockets use high powered turbo-pumps, sometimes with their own separate fuel system (an engine within an engine!).
Then you have bleed-off lines that run extra fuel around the hot parts of the motor, or that inject extra cold fuel along the sides (called regenerative cooling and film cooling respectively). Then you have the rocket motor itself, with an injector plate, combustion chamber, throat, nozzle, and vacuum bell.
A subset of the main engine power are the tiny reaction control systems. They are a collection of self contained rockets that control the orientation and act as fine maneuvering controls during docking and such. Because of their smaller size and how numerous they are, they are almost always simple pressure driven rockets; no turbo-pumps. So at minimum you need a thruster, fuel/ox tanks a blow-down tank, and valves and, if not using hypergolic fuels, an igniter. This can be modularized and repeated throughout a ship.
Other, non rocket ways of orienting yourself include reaction wheels — large spinning masses that the vehicle can rotate around. Or magnetorquers — gigantic electromagnets that will try and force a craft to align with the magnetic field of a nearby planet of star. Careful with stars though, their magnetic fields are complex and unpredictable!
Generally the motor only moves the craft forward. You still need electricity and hydraulic power to actually run the vehicle. Power can be gleaned from the sun — but that assumes you are close to the sun and not in orbit in such a way where you are in the shadow of a planet. Fuel cells are very popular in spacecraft. Fuel cells work by combining hydrogen and oxygen in a careful manner (using tiny screens with nanometer sized holes) that creates electricity as the two elements combine. The disadvantage is that you have to take fuel with you, but the advantage is the byproduct is drinking water! This is in fact what the space shuttle does. It does not bring enough water for the astronauts during a flight, but rather creates the water they need for eating and drinking while at the same time generating power for the vehicle. Fuel cells, like RCS, are modular and contain tanks, valves, pressure vessels, such as fiber wrapped pressure vessels, plumbing and wires.
In terms of home much power you get per unit mass, nothing beats nuclear. A sufficiently advanced craft could have a nuclear power plant onboard. You need at minimum, a reactor core, control rods (or some kind of neutron moderator) containment vessel, and something to use as a heat exchanger — almost always water. On Earth we use the heat to drive a steam turbine. Then you have a condenser the drives the water back through the heat exchanger. You also need pumps to circulate the system.
Many successful deep space probes use what is called a RTG — Radioisotope thermoelectric generator. A small piece of plutonium oxide sits on one side of a heat sink and the difference in temperature between the plutonium (which will stay hot for decades due to decay heat) and the coldness of space is turned into electricity using the thermoelectric effect. This has the advantage of having no moving parts! Though it’s not very high energy unless you have a lot of them.
Once you have electricity you have run the computers and fans and pumps and such. But usually you can’t generate enough power to directly move large objects like the steering systems. This task falls to the APUs, or Auxiliary Power Units. This is a terrible name. While it is auxiliary in the sense that most of the energy of a spacecraft is concentrated in the rocket motors, APUs are absolutely critical to the functioning of the spaceship! The standard APU is some kind of fuel/oxydiser that burns in a tiny engine, often of gas turbine, that drive a hydraulic pump. The pumps run the hydraulic system of the spacecraft that gimbals the engines and moves large control surfaces (if designed for atmospheric flight). Hydraulics also make very good actuators for heavy doors and docking mechanisms. Think anything steel and industrial.
Hydraulics themselves need pumps, lines (which cannot be cut without disastrous results!) overflow and reserve tanks, and valves. Often the plumbing of the hydraulics is just as complicated as the rocket motor!
To steer and find you way you need electronics. Flight computers are usually buried deep in the ship to protect it from damage and stray radiation. There are multiple ones that act as backups and check each others answers for accuracy. To actually know where you are you have to have and IMU (inertial measurement unit) of some kind. The coolest ones involve multiple nested platforms that have gyros and accelerometers attached and the platform rotates such that it always points the same direction relative to the galaxy while to ship moves around it. There are also strap down IMUs that simply attach to the frame of the vehicle and keep track of orientation in software.
One handy device that navigation computers rely on is a star pointer. Star pointers are tiny telescopes that can pick out particular stars and calculate the orientation of the spaceship based on the angle of the stars. Often this is in a bay with other electronics and sensors.
Even if people are not expected to fly on a spacecraft, thermal management is a big problem. Most important systems want to be at one particular temperature, but the universe never seems to want it that way. If left to their own devices a static object in space will undergo radiative cooling until it gets hundreds of degrees below freezing. Fuels and hydraulic fluids must be kept above a minimum temperature so they don’t freeze, so you need heaters powered by the electrical system. But computers and electronics create their own waste heat that needs to be cooled because passive radiative cooling isn’t fast enough!
Cooling is trickier than heating. The usual way is to have a large radiator (like in a car) sticking out of the spacecraft that have a coolant like propylene glycol running through them. Real spacecraft have no need to be pretty or streamlined. There are often things sticking out of them. The large surface area of the radiator cools the coolant and then the coolant is pumped through the craft around the electronics and other heat exchangers. The again requires pumps and valves and lines.
Humans are the hardest things of all to keep happy. Environmental controls for people are hugely complex and space/power consuming. You need food, water, waste control, air, CO2 management, light, something to do with your time, windows, and a very narrow range of comfortable temperatures must be kept using heaters and radiators. Why do we insist on windows! Such a difficult thing to design into the otherwise sturdy and safe aluminum isogrid panels. We are such picky creatures.