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Propellant is mass which is ejected from a jet engine (of which a rocket is one possible type) to produce thrust through Newton's third law of motion.

Science Background

Note that small light molecules (ones with low molar mass), make for better propellents. The reason for this is that temperature can be thought of the average momentum of a single particle of gas. Let us look at Hydrogen gas (each molecule is made up of two hydrogen atoms) and nitrogen gas (each molecule is made up of two nitrogen atoms). Hydrogen has an approximate Gram Molar Mass (GMM) of 1 each so the H2 molecule has a GMM of 2. Nitrogen atoms has 14 GMM so N2 is 28 GMM.

The temperature is proportional of the momentum of the particles of gas and momentum is equal to mass times velocity. Hydrogen's mass is significantly (14 times) lower, so the VELOCITY of the molecules must be 14 times faster if the two gasses are held at an equal temperature. So if a rocket engine can make the fuel and oxidizer burn at a given temperature (say 1,000 C), the velocity given by the propellent to the rocket is proportional to the velocity of the ejected propellent. Thus you will get better 'fuel economy' if the GMM mass is low.

In the story "Space Cadet", the rockets in this story stored hydrogen as single atoms. (We don't know how.) The hydrogen was then triggered to combine. This made it really hot, and the exhaust gas had the lowest possible GMM of 2. All the demands of the story's plot would work if this type of fuel was available.

This makes hydrogen, H2, with a GMM of 2 an ideal propellent. However, it is not very dense, so rockets must have huge tanks to hold a lot of hydrogen, so the mass of these tanks make hydrogen less attractive. Hydrogen liquid is more dense, but it has a VERY low boiling point and the insulation to keep it cold adds to the mass of the rocket. Hydrogen is not space storable (heat leaks in and boils the liquid hydrogen) which is vented and wasted. And of course, we can't store single hydrogen atoms, so the H2, must burn with (say) O2 to produce heat.

Some examples: The most energy can be made by burning 2 hydrogen atoms for each oxygen to produce H2O (water steam) and a lot of heat. If you have extra hydrogen OR extra oxygen, some of the fuel or oxidizer is wasted and the flame is a bit cooler. But the Space Shuttle's main engines ran hydrogen rich. It was basically having hydrogen rich steam as its outflow gas. The reason for this was two fold. First, the flame temperature was a bit cooler, which is important for a reusable engine. Second the GMM of the output gas was a significantly lower which improved the efficiency of the rocket.

Another example: Let us say that you have two gasses in your atmosphere, H2 and CO2. The gasses are at a given temperature at the top of the planet's atmosphere. H2 has a GMM of 2, and CO2 has one of 44 grams per mole. At the edge of space, on average, the hydrogen is travelling 22 times faster than the CO2. Some proportion of the hydrogen will be travelling faster than the escape velocity, while virtually none of the carbon dioxide will be. So hydrogen gas is slowly lost from terrestrial planets, while almost no CO2 will be. See atmospheric loss for more details.

Another example: noble gases do not chemically combine. So a particle (molecule) of hydrogen gas H2 is made up of 2 atoms, where as noble gasses have only a single atom. He gas has an excellent GMM of 4, and Ne has 20 grams per mole. These noble gasses are easily ionized so Helium would make a great propellent for an ion thruster. Electric fields can accelerate ions to VERY high velocities. However, He and Ne are rare and expensive, so argon (Ar) with a GMM of 40 is sometimes used instead. The larger atoms can lose multiple electrons which allows the electro static field can throw the gas harder (that is to say faster) than if it had just a single ionized electron. Thus the higher GMM of the argon atoms is made up for the very high velocity of the exhaust particles. Xenon (131.3 GMM) likewise can lose multiple electrons which makes up for its poorer GMM.

A propellant gets its energy from an energy source. These can be internal to the rocket, or external. In chemical propulsion, the energy source is the chemical reaction between a fuel and an oxidizer. In nuclear propulsion, the energy source is a nuclear reaction, either in a reactor or in the propellant itself (nuclear salt water rocket). In solar electric propulsion the energy source is the light from the sun, external to the rocket.

Chemical propulsion

In a chemical rocket, the propellant is the reaction product of the oxidizer and the fuel. Typical liquid combinations are hydrogen and oxygen, methane and oxygen or jet fuel and oxygen, but many mixes are available. Monopropellants are also possible combining both oxidizer and fuel in a single mixtures. Solids can also be used.

Nuclear thermal propulsion

In a nuclear thermal rocket, it is normally hydrogen, since the specific impulse is dependent on the exhaust velocity of the rocket, which is higher for propellants with low molar mass. The reactor core is made as hot as practical, and then the low GMM of the hydrogen most efficiently turns that heat into thrust.

You want the temperature of the exhaust to be as high as possible, but heat always flows from hot areas to cooler ones. This means that the temperature of the reactor, must be hotter than the exhaust reaction mass. This limits the efficiency of the engine, if it gets too hot, it melts. Some designs allow portions of the engine to melt (or even go gaseous) but those designs are strictly theoretical.

The trick is to efficiently heat the reaction mass in a small and light engine. The NERVA nuclear rockets had an ISP (a measure of efficiency) just over double the best chemical rockets. On one hand this is good (double!) On the other hand, nuclear reactions are about 1,000,000 times more powerful than chemical reactions, so obviously there is room for improvement. The Zubrin Salt Water Rocket (ZSWR: see below) is several times more efficient, but it exhausts radioactive particles. (NERVA does not, so theoretically NERVA could launch in Earth's atmosphere.)

Nuclear or Solar Electric propulsion (NEP or SEP)

For Ion thrusters, an easily ionized gas is preferred. Vaporized liquid metals are good candidates. As for nuclear rockets, the lower the molar mass the higher the exhaust velocity. However, as electric engines work on ionized gases, the ionization energy of the propellant adds complexity to the engine and favors elements that are easy to ionize.

The nobles gases, such as Argon, Neon Krypton and Xenon are all usable, with Xenon achieving the best propulsion properties for many missions. However, it is also far more common than Xenon, and much less expensive, so it is used is some low cost satellites such as the Starlink system, despite the lower performance. For example, Argon is less effective than Xenon in ion thrusters.

Argon is readily available both on Earth and on Mars. this makes it a good candidate for propulsion with Insitu production by extraction from the Martian atmosphere.

Zubrin Salt Water Rocket

In this design, salt water is stored as both fuel and reaction mass. Rather than normal salts, the salts are highly enriched Uranium (near weapons grade) which are stored in small tanks lined with neutron absorbers. This mass will be sub-critical.

When you wish to fire the rocket, the salt water is pumped into a region where a significant amount of salt water is surrounded by neutron reflectors. The salt water is now critical and heats up flashing to steam.

The radioactive salt water steam then goes thru a rocket nozzle and produces thrust. The expansion of water to steam greatly improves the thrust. This is more than 7 times more efficient than a NERVA rocket, but it exhausts highly radioactive uranium salts, so it is not suitable for firing near a biosphere.

This sort of rocket engine has never been built, but is the most efficient design conceived for producing rocket thrust. Uranium is inexpensive, but highly enriched uranium is more expensive and has many legal barriers. Thus it is unlikely to be used on civilian craft, but rather it would be used for the highest performance military vessels.

Other types of propulsion

  • A laser ablation rocket will tend to use a propellant made from a combination of metal and plastic (due to their optical properties). The energy source is external.
  • A cold gas thruster will use compressed gas to feed small thrusters used for maneuvering. The propellant is usually nitrogen. The energy source is in the initial compression of the gas.
  • A steam powered rocket uses the expansion of water into steam to provide thrust. Water is the only propellant involved. The energy source can be a nuclear reactor or solar concentrators.
  • Mass drivers can use magnetic fields to project a propellant susceptible to magnetic fields at high velocities, with no heating or ionization. The energy source can be a nuclear reactor or solar power. Alternatively, a mass driver can be used to capture high velocity mass sent from another mass driver. The second form of mass driver can sidestep the rocket equation and can be seen as a form of externally powered and externally supplied propulsion.
  • Mass drivers can be installed on planets, Moon or asteroids and send out payloads using no propellant, but only energy. These also sidestep the rocket equation.
  • Fusion rockets are theoretical rockets that use fusion energy to ionize a propellant that includes a nuclear fusion fuel, such as deuterium and helium3. The fusion is created using a driver, such as a laser system, magnetic compression or even a chemical explosion.

Propellant production on Mars

On Mars, propellant can be produced in-situ from water and/or CO2. Argon and other noble gases from the atmosphere could be used for NEP or SEP.