Types of propulsion

Propulsion systems are classified depending on the form of energy transferred to the propellant and converted to high speed exhaust. These include;

  • Thermodynamic propulsion- rely on thermodynamic energy(pressure and heat)
  • Electrodynamic propulsion - rely on electric charge, electric and magnetic fields.

Thermodynamic propulsion

Thermodynamic systems transfer thermodynamic energy (heat and pressure) to the propellant and then convert the energized propellant into high speed exhaust using nozzles. There are a variety of thermodynamic propulsion systems which are further classified depending on their source of energy.

cold gas thrusters – use mechanical energy of a compressed gas and thermodynamically expands the gas through a nozzle producing a high-velocity exhaust. Cold gas are extremely simple, reliable and can be turned off and on to repeatedly producing small and finely controlled thrust bits. Due to their low thrust and specific impulse, they are used for attitude control and limited orbital maneuvering on small spacecrafts. For example The manned maneuvering Unit(MMU) used by shuttle astronauts

Chemical propulsion

Majority of the propulsion in use today rely on chemical energy to produce thrust. Chemical energy is produced during combustion of fuel (such as hydrogen) plus the oxidizer(such as oxygen). The two combine, liberating a huge amount of heat and creating by-products that form the exhaust. Propellants arrive in the combustion chamber under pressure delivered by the propellant management system, the chemical reaction plus energy transfer takes place in the combustion chamber too.

Chemical propulsion is further disintegrated into three categories

  • Liquid
  • solid
  • Hybrid

Liquid chemical propulsion

Liquid chemical propulsion exhibits two types namely; monopropellant and Bipropellant.

Bipropellants – as the name implies, this type of propulsion uses two liquid propellants. One as a fuel(like liquid hydrogen l and the other as an oxidizer(liquid oxygen).

Some propellant combinations wont spontaneously combust on contact, they need an igniter just as a car needs a spark plug to get started, this increases the complexity of the system. But not for so long, because chemists came up with a combination that reacts on contact. These propellants are called hypergolic because they don’t need a separate igniter. The combination of hydrazine plus Nitrogen tetroxide is an example of hypergolic propellants. The Titan II, IIIB, IIIC, and IV NASA rockets used this combination of propellants. Hypergolic propellants are stable at room temperature for a very long time(months or even years). That is why they preferred for long term space missions, other than the cryogenic propellants

Cryogenic propellants are stable at extremely low temperatures(about hundreds of degrees below zero, centigrade), which is very difficult to fulfil by mission planners. However, cryogenic propellants are more efficient and provide a higher performance compared to hypergolic propellants, since they produce greater specific impulse

Monopropellant – as the implies, rough contact with a single propellant is used as the propellant. These propellants are relatively unstable and easily decompose through contact with a suitable catalyst. Hydrogen peroxide is a good example of a monopropellant. But the most used monopropellant is hydrazine, because it readily decomposes when exposed to a catalyst and produces a specific impulse of about 230s. Its short come is the toxicity.

Mars Reconnaissance Orbiter used a monopropellant propulsion system: there was fuel (hydrazine), but no oxidizer. Thrust was produced by passing the fuel over beds of catalyst material just before it entered the thruster, causing the hydrazine to combust. 

Solid chemical propulsion

Just as a liquid bipropellant combines fuel and an oxidize to create combustion, a solid chemical propulsion contains a mixture of fuel, oxidizer, along with a binder, blended in the correct proportion and solidified into a single package called a motor. Currently they are used as boosters on space vehicles and create thrust for intercontinental ballistic missiles. The commonly used fuel is aluminum and oxidizer is ammonium perchlorate. Together, the fuel and oxidizer comprise of about 85 – 90% of to motor’s mass, with an oxidizer/fuel ratio of about 8:1. The remaining mass is covered by the binder that holds the ingredients togethe

Solid chemical propulsion is simple, reliable, no propellant management systems required, produces high specific density impulse compared to bipropellant, no combustion chamber cooling issues. However, it is susceptible to cracks, cant restart, difficult to stop and produces a modest specific impulse.

The space shuttle used solid motors as boosters and recently the ULA’s Delta IV heavy moon mission.

Hybrid chemical propulsion

Hybrid chemical propulsion systems, combine aspects of liquid and solid systems. A typical hybrid system uses a solid as fuel and a liquid as the oxidizer. The molded fuel grain forms the combustion chamber and the oxidizer is injected into it. A properly designed hybrid propulsion system offers the flexibility of a liquid propulsion system with the simplicity of a solid system. Hybrid combustion process is similar to burning a log in the fireplace. Oxygen from the air combines with log(fuel) in a fast oxidation process and burns. They are potentially safer and have a higher specific impulse (Isp) than solid rockets. They are also less complex and cheaper than liquid rockets. An example of hybrid configuration uses high-test peroxide (HTP) oxidizer with polythene(plastic) as fuel. Unfortunately, hybrid propulsion haven’t yet been used on any launch vehicles or spacecraft though it’s a system that considered as a prospect.

The most dramatic application of hybrid propulsion system was used the Gillette Mach 3 challenger, which used the HTP/HTPB hybrid motors.

Solar electric propulsion

Solar Electric propulsion system relies on the acceleration of ions using electric energy generated by on board solar arrays.

After launch, solar arrays unfurl, capturing solar energy which is then converted into electrical power. Electrical energy is then fed into electrostatic hall thrusters with advanced magnetic shielding. The thrusters generate and trap electrons in a magnetic field. Uses the electrons to ionize the on board propellant to produce thrust. Below are the possibilities that SEP creates when used

SEP allows deep-space missions to carry more cargo and use smaller launch vehicles while reducing mission costs

SEP provides such high fuel economy that it reduces the amount of propellant required onboard vehicles for deep-space missions by as much as 90 percent

SEP will enable affordable human-crewed missions beyond low Earth orbi

SEP will be used to maintain the spaceship’s position around the moon and move it around different orbits during the construction of the first element of the Lunar Gateway

Nuclear thermal propulsion

Nuclear thermal propulsion relies on heat produced from nuclear fission inside a nuclear reactor. N.R.P uses its propellant such as liquid hydrogen to flow through a nuclear reactor, absorbing thermal energy. The propellant is delivered to the nuclear chamber where it absorbs intense heat from the nuclear reaction. A thermodynamic expansion through a nozzle produces high thrust (up to 106 N) and high specific impulse. The challenge with N.R.P, is the environmental and political problems with testing and launching nuclear reactors. Other than that, this propulsion system perfectly suites manned planetary mission because of its high thrust and efficiency compared to chemical propulsion

Future Astronauts will be able to escape the danger of space radiations by using energy from a nuclear reactor to propel them to their destination faster

Extensive research into N.R.P was done in the U.S in 1960s as part of the NERVA program. But was disabled later on due to fear of the danger it would cause during ground tests of the systems. But NASA is looking to forward to using this system for future interplanetary missions under controlled conditions

Exotic propulsion systems

Exotic propulsion systems are those far out ideas still on the drawing board. There are many exotic variations to the propulsion system that we have discussed above like N.R.P, S.E.P. But now we focusing on even more unconventional types of propulsion- ones that produce thrust without ejecting mass. They include;

  • - Solar sails
  • - Space tethers
  • - Warp frive

Solar sails

Light is made up of particles called photons. Photons don’t have any mass, but as they travel through space they do have momentum. When light hits a solar sail, which has a bright, mirror-like surface. The photons in that light bounce off the sail (i.e. they reflect off it, just like a mirror). As the photons hit the sail their momentum is transferred to it(principle of conservation of momentum), giving it a small push. As they bounce off the sail, the photons give it another small push. Both pushes are very slight, but in the vacuum of space where there is nothing to slow down the sail, each push changes the sail’s speed.

Solar sails would work best in areas close to the sun so that it receives enough radiations for it to sail.

Solar sails will offer a cheap way to get around in space, since no propellant is required so the thrust is free. Visionaries propose that solar sails can be used to maneuver mineral rich asteroids closer to our planet to allow orbital mining operations

Space tethers

A space tether is a long cable used to couple spacecraft together as they orbit the central body (i.e. Earth). Tethers are usually made of thin strands of high-strength fibers such as Spectra or Kevlar. Any space tethered system is intimately connected to the gravitational force field

The tethered system demonstrates gravity gradient attitude control. This is a very low cost attitude control system

Because the space tether makes it possible to transfer energy and momentum from one object to another, it can legitimately be called a form of space propulsion

The first orbital flight experiment with a long tether was the Tethered Satellite System (TSS) mission, launched on the Space Shuttle in July 1992. The Tethered Satellite System-1 (TSS-1) was flown during STS-46, aboard the Space Shuttle Atlantis, from July 31 to August 8, 1992