Space propulsion

One of the courses I teach often is Elements of Spacecraft Design. It’s an orbital mechanics class combined with a survey of spacecraft subsystems. Being a propulsion enthusiast, I tend to focus a bit on the propulsion subsystem. They each have their advantages and disadvantages. I’ll summarized the general categories now, and later delve into more detail about each type.

How to get thrust: The purpose of any propulsion system is to generate thrust, or some motive force. In cars, the “thrust” is actually turning of the crankshaft which turns the wheels. In space, thrust is basically a pushing force on the spacecraft. That force is usually in the form of kinetic energy. Space engine typically use two metrics: thrust and specific impulse (Isp). Thrust is just how much force it produces, think horsepower in your car. Specific impulse is basically a space mpg. Isp is in units of seconds and tells you how long the engine can produce 1 N of thrust from 1 kg of propellant. So higher thrust means you get there faster, and higher Isp means it’s cheaper to get there.

Chemical engines

Boeing methane rocket test fire

The mainstay of our space and terrestrial propulsion systems are chemical. By chemical I mean two molecules undergo a chemical reaction, usually combustion, to generate high temperatures and pressures. Chemical engines then convert thermal energy to kinetic energy with a nozzle (a specially designed funnel basically). The high speed particles exiting the nozzle have a lot of kinetic energy, and through Newton’s second law causes an opposite reaction on the engine that pushes the spacecraft.

Historical note: Konstantin Tsiolkovsky was the first person to prove you can move around in space using a rocket. Before him everyone believe a rocket would not work in space because there was no air for the exhaust to work against.

Chemical engines are characterized by high thrust (1 to 100,000’s lbs of thrust) but lower efficiency (Isp = 200-450 sec). These are the highest thrust engines but not very efficient compared to the later options. So they tend to have short transfer times, but require the spacecraft to carry a lot of propellant, usually >50% of the total mass.

We most often see chemical engines in rockets. All types of rockets on Earth, from the Saturn V, to sidewinder missles, to minutemen cruise missions, use some kind of chemical propulsion engines that burns a highly reactive propellant and expels the hot gas (fire) out a nozzle to create thrust. For Earth launch into space, chemical engines are a modern necessity. This is due to the huge thrust levels necessary to break the pull of gravity. Only chemical engines can do this job, so I don’t think we’ll see them going away in the foreseeable future.

Electrical propulsion

Hall thruster firing at Princeton Plasma Physics Lab

These devices include ion engines, Hall effect thrusters, arcjets, pulsed plasma thrusters, and others. The “electric” comes from the use of electrical power as an integral part of the engines. In chemical engines, the amount of energy possible is based on the energy stored in the chemical bonds in the fuel. Thus there is a hard limit on the maximum amount of acceleration and thus performance from chemical engines. Electric engines decouple the acceleration from the chemistry. Some kind of electrical power input is used to control and accelerate particles. Usually this requires a charged gas, called a plasma, made up of positive ions and negative electrons. Electric propulsion can provide Isp upwards of 5000 sec, much greater than chemical engines. However the trade-off is their thrust is very very low, less than 1 N (0.2 lb). So electrical engines are take a long time to get somewhere, but burn very little fuel to do it. They are very good for spacecrafts when time is not a big concern.

Electric thrusters are used heavily on Earth orbiting satellites. There are over 200 electric thrusters flying today around the Earth. They are used to maintain the satellite’s position over the life of the satellite. In orbit, an object has to contend with perturbing forces such as solar wind, slight variation in gravity, and atmospheric drag if low enough. These forces are very small, but built up over time to move the satellite out of correct position. Thus small precise thrust manuevers are necessary, and high efficiency. So electric propulsion works very well here. Their low thrust and very high Isp makes them ideally suitable for orbit maintenance.

The other use electric thrusters have seen is in deep space robotic mission by NASA and other space agencies. The Deep Space 1 demonstration mission, the Dawn mission to the asteroids, and the Hayabusa mission to an asteroid all uses ion engines as their main propulsion. Electric thrusters are good for deep space probes as the low thrust means a long travel time (many years), but the probes don’t really care. The high Isp means the probes can carry a small amount of fuel to get to their destination, leaving more space for instruments.

Nuclear propulsion

NASA rendering of Orion docked to a nuclear thermal engine

We have yet to use a nuclear engine for actual space missions yet, but they have proponents and advantages. There are two kinds of nuclear engines: fission and fusion, the later is not theoretical at this point. Fission engines are comprised of a reactor core with fuel rods and a conventional nozzle similar to chemical engines. A gas, hydrogen is typically suggested, is passed through the very hot reactor core and picks up lots of heat from the fuel rods. The hot gas now expands through the nozzle to create thrust. So you can say nuclear engines are similar to chemical engines, except you replace the fire heat source in the chemical engine with a nuclear reactor (nuclear fire?). Fission engines can achieve 100’s lbs of thrust and Isp of ~1000-2000 sec. So they occupy a middle ground between chemical and electrical. A major drawback tends to be the extra radiation shielding required to prevent the nuclear reactor from making any astronaut glow in the dark.

Others
This is a catch all for the non-traditional systems (not sure how traditional nuclear is though)

Laser propulsion: The idea here is to aim a very powerful focused laser beam at a reflector on the back of your spacecraft. The laser light will strike the reflectors and bounce away. While photons (light particles) have no mass, they do have momentum (weird huh). So when they hit the reflector, the laser beam imparts a force on the reflector and pushes the spacecraft. It’s kind of like shooting a BB gun at the back of a toy car. The BB’s will hit the car making it move. Same idea here. The limitations here are two fold: 1) the thrust will be low, so it will have travel times similar to electric propulsion. 2) You need a very very powerful laser, and a very good aim. The distances in space are kind of mind boggling to the point where we can’t really grasp them physically. For example the distance between the Sun and Earth is about 150 million kilometers (~93 million miles). I have no physically understanding of how far that is, besides it’s a really large number. So if you wanted to use laser propulsion to push a spacecraft that far out, you need to be able to hit a target maybe a mile across, but at a range of 93 million miles. Not impossible, just hard. And as the laser beam travels through space, it slowly losses energy. So at Earth, your spacecraft gets a larger push from the laser, but out a Jupiter per se, the thrust is much much lower.

Solar sail: Similar to wind sails on Earth, a solar sail catches the solar wind to move in space. The sun is constantly emitting light (photons) and other particles. Solar sails are desired to be reflective to light and thus the photons push the sail. These have similar limitations as laser propulsion, mainly the loss of light as you move further from the sun. The sun’s energy is much larger at Mercury than at Neptune, which is why Mercury is a super hot planet, and Neptune is mostly frozen. A solar sail powered spacecraft would get alot of push and speed close to the sun, but as it goes away from the sun, the force on the sails pushing the spacecraft will decrease quickly.

Tethers: This is a strange one to explain. A tether, or more technically an electrodynamic tether, is a long (multiple kilometers) pieces of thick wire that can conduct electricity. The tether relies on the Lorentz force, which occurs when you have an electrical current in a magnetic field. There is a resulting force due to the interaction between the current and magnetic field that generates a force. Tethers take advantage of the Lorentz force by driving a current through its length, and as it interacts with a planet’s magnetic field there is a force on the tether itself. Tether have been proposed as a method for generate thrust without using fuel. By virtue of the Lorentz force, to work the tether needs a magnetic field, which happens to exist around every planet and most moons. This means tethers could be great on planet orbiting spacecrafts, but for the long distance between they’re not so useful. But as we’ve seen, no single propulsion system can do everything well. If you’re interested, there is a company called Tethers Unlimited that does a lot of work in this area. (I am not affiliated with them in any fashion.)

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