What’s in a thruster? Plus, a few tips to do them more efficiently
What’s in a Thruster?
A thruster is a device used to propel spacecraft through space. They are usually made out of metal or plastic, but can also be composed of other materials such as ceramics, glass, and even water. A typical thruster consists of two parts: the thrust chamber (or nozzle) and the power cell (or generator). Thrust chambers are where propellant gases are injected into the engine to produce thrust. Power cells generate electricity to turn the thruster’s generators.
The thrust chamber is typically made from a tube with a nozzle at one end and a small hole in it, which allows propellant gas to enter the engine. The exhaust gases are then expelled through the nozzle. For example, most modern thrusters use liquid hydrogen as their propellant, while some older designs used kerosene or gasoline.
Power cells are similar to a battery except they contain much smaller amounts of energy. Most thrusters use lithium ion batteries, though there have been attempts to make them using nickel cadmium batteries or lead acid batteries. Lithium ion batteries are considered safer than lead acid or nickel cadmium ones because they don’t explode when overloaded like those types do.
Most thrusters have multiple power cells connected in a series to provide more voltage to the system. For example, an electric car uses around 400 such batteries in series to create enough power to get it on the road.
The gas flow through the thrust chamber is controlled by a valve to let in a precise amount of gas at a certain pressure and flow rate. The valves on a thruster are usually opened and closed by an electric servo motor. The valves have a range of different designs, but they all serve the same function: to precisely control gas flow.
The G forces inside the thrust chamber can be very high and it’s vital that the engine is very strong as well as lightweight. Most thrusters are built out of an advanced aluminium or titanium alloy to make them tough without adding too much extra weight.
There are two main types of thrusters:
A pulse jet uses a continuous combustion process in the power cell where oxygen and fuel combine to produce a hot gas which is directed through the thrust chamber and expelled. This type of engine can be very simple to make and consists of few moving parts, which makes it very reliable.
A typical pulse jet engine is a long cylinder with holes at one end and the exhaust at the other. The gas escapes through these holes and travels down the tube, where it passes through a variable geometry nozzle adjusting the flow of the gas (and thus the engine’s thrust) and then exits out of the back of the thruster.
The gas is directed alternately on and off with a shutter mechanism to control the flow and create thrust. A small amount of gas is directed out of the top of the engine through a pipe where it mixes with air and ignites. The resulting flame travels back down the pipe and bypasses the nozzle, reheating the gas in the chamber. This loop allows the gas to be fully burnt before it is expelled from the engine, giving the pulse jet a small amount of extra thrust, around 10% more than it would get if the gas was simply burnt in the chamber. Every design is different however and some pulse jets don’t use this reheating process at all.
Pulse jet engines were used as the main engine on many German V-1 “buzz bombs” during the World War II. They have also been used in some Russian intercontinental missiles, where they were used to get the missile up to speed, then larger liquid fuel or nuclear powered engines took over for the rest of the flight.
Most pulse jet engines are no longer used for propulsion because they produce a lot of thrust but not much in the way of efficiency. They require a lot of energy to operate and are generally inefficient. For these reasons they are not widely used today.
The other type of thruster in common use is the electric plasma thruster, sometimes called an ion thruster. This type uses an electric charge to propel a stream of electrically charged gas out of the back of the engine and thus providing thrust in the same way as a water pump pushes water out of the hose.
To create the charge, electrons are stripped off of atoms. This is done with a gas called ‘argon’ which is fed into the engine and passed over a metal plate which has a positive charge. The gas itself is then negatively charged and thus the resulting gas stream has a positive charge.
However when it comes into contact with the wall of the thruster tube it gives up some of its electrons and becomes neutral again. At this point it is very difficult to keep the gas inside the engine, as the attraction of the outside electrons is stronger than the force keeping them in the stream. Even a small amount of gas leaking out means a loss of thrust. To combat this electric charge is constantly added to the stream using an acceleration grid, a set of parallel electrodes across which extra electrons are transferred.
The gas stream leaving the thruster is very thin, a few microns across, and thus extremely efficient at giving up all its energy as fast as the electrons can be stripped off the atoms. A byproduct of this is that the resulting beam of gas is so thin it barely affects the aerodynamics of the ship.
Because the gas atoms are all the same (unlike the fuel in a normal engine where the components can separate out leading to sludge building up in the engine) there is no contra-flow of inert gas to cause drag.
Electric plasma thrusters are also much cleaner, since they don’t use any chemicals which can evaporate or explode in an engine room. They also give off no radioactivity and are much simpler to construct.
The main disadvantage is that they require a great deal of power to run and so they can’t be used for maneuvering as the power required to turn the ship would be too great. Larger ships often have a bank of arc fusion reactors which provide enough power for this though and allow them to accelerate, turn and then cruise on the plasma thrusters once up to speed.
The ship trembles and you are jolted in your seat as the first stage of your launch begins. After a few minutes you hear a loud boom from behind you. You are being pushed back into your seat by acceleration and you glance at the window to see the exhaust plume from the main engines pushing the ship forward. You wait patiently for many hours as the ship ramps up to its top speed and then starts to level out. You feel your stomach drop as the G-force lessens and then disappears altogether.
You glance around the shuttle to see the others all strapped into their seats, all of them asleep and unaware of what was happening. You unbuckle your seat belt and walk over to the window. Outside all you can see is the blackness of space lit up by stars here and there. You are in orbit around the planet but it looks peaceful from here, no war, no conflict, just beauty.
You sit down and watch the stars glide past, thinking about what is out there in the blackness.
The shuttle eventually enters a large military station. You wait for everyone to wake up and then you all disembark. The shuttle pilot tells you it is possible your next ride will be here soon but if not you will all have to wait it out here. You are taken through the large space port and into a briefing room with large windows looking out into space. You watch as large ships come and go, bringing supplies and taking them off, even battling with each other at one point.
The next day your shuttle does arrive and after another long wait you are all loaded onto it and sent on your way to the planet below.
Sources & references used in this article:
- Advanced space propulsion for the 21st century (RH Frisbee – Journal of Propulsion and Power, 2003 – arc.aiaa.org)
- Misconceptions of electric propulsion aircraft and their emergent aviation markets (MD Moore, B Fredericks – 2014 – ntrs.nasa.gov)
- Hydraulic hybrid propulsion for heavy vehicles: Combining the simulation and engine-in-the-loop techniques to maximize the fuel economy and emission benefits (Z Filipi, YJ Kim – Oil & Gas Science and Technology …, 2010 – ogst.ifpenergiesnouvelles.fr)
- Turboelectric distributed propulsion engine cycle analysis for hybrid-wing-body aircraft (J Felder, H Kim, G Brown – … sciences meeting including the new horizons …, 2009 – arc.aiaa.org)