Author: Anav Vaidya
Ever since we became aware of space, the planets and the stars, questions about the existence of life other than our own has boggled the minds of many. Over the past seven decades, numerous missions have been launched in search for answers. Today, space exploration has reached a new level. With great advancement in math and technology, we are able to explore regions in space that until a decade ago would have been considered improbable.
But even now, we face the pressing question: can our current propulsion systems enable long distance travel? And if not, how can we improve these systems to be able to achieve that feat?
What is propulsion?
Propulsion can be defined as the action of pushing an object forward, along a clearly defined path. In a spacecraft, the propulsion systems are involved in the rocket launch, in-space propulsion and the atmospheric re-entry.
The most commonly used propulsion technologies can be categorized into:
● Chemical Based
● Electric (Ion) Based
The basic working of any propulsion system is based on Newton’s third law of motion: Every action has an equal and opposite reaction. The force exerted by the reaction mass (fuel or ions ejected from the nozzle), will have an equal force acting on the rocket in the opposite direction, helping it to move (or accelerate).
Let us look at a few key terms in this context.
The specific impulse is a measure of how efficiently a spacecraft utilizes its propellant or fuel. Higher the specific impulse, lesser is the amount of fuel needed to reach a certain amount of thrust, making the propulsion system more efficient. From this, we infer that higher the specific impulse, more efficiently is the fuel consumed.
By experiments, it has been observed that ion propulsion has a higher Specific Impulse than chemical propulsion, meaning that the fuel used in ion propulsion systems is used more efficiently than that in chemical propulsion systems.
Thrust is basically just a push or a force. It is the very essence of propulsion. Without it, there can be no propulsion, and in turn no motion, as given by Newton in his first law: An object will remain in its current state of motion unless an external force greater in magnitude than its static frictional force acts on it. Thrust is what drives an object and propels it along a certain path.
So, in a nutshell, thrust is a measure of how hard engines can push, whereas specific impulse determines how much thrust you can get from a given amount of fuel and thus how efficient the fuel is.
There are usually two ways in which thrust is provided:
Large accelerations that run for a short time
Tiny accelerations that run for a long time
During the launch of a rocket, we need to overcome the gravitational pull of the Earth to be able to exit the atmosphere. For this we will need to give it a sufficient thrust to reach the escape velocity of 11.2 km/s.
The first method will continuously accelerate the rocket until it reaches the escape velocity, thus being more effective in overcoming the gravitational pull of the Earth. The second method cannot overcome the gravitational force and hence it cannot be used at the time of liftoff. After the spacecraft leaves the atmosphere, any of the above two methods can be used for in-space travel.
Chemical propulsion system
Combustible propulsion systems involve exothermic chemical reactions that release energy and expel the gases to produce a thrust. This thrust (by Newton’s third law of motion) is what propels the rocket forward.
An efficient propulsion system must be able to produce a large thrust using a small amount of fuel (that is the specific impulse must be high for the fuel to be used efficiently). With an energy of over 20 terajoules required in order to go past the escape velocity (that is in the approximately five minutes it takes a rocket to exit the atmosphere, over 2*10^13 joules of energy is needed) the chemical propulsion system is widely regarded as the best propulsion system for the launch.
There are primarily 2 types of fuel (or propellant) used:
● Liquid propellant
● Solid propellant
The liquid propellant is the most common in rocket launches.
Liquid Propellant: 2 types of liquid propellant are in use:
1. Hypergolic Fuels: These types of propellants are highly inflammable and do not entirely depend upon ignition to undergo combustion. They react violently and give off large amounts of heat energy. This is their crucial advantage: even if the ignition process fails, the rocket will still launch. Thus these fuels have been favored over other fuels that entirely depend upon a successful ignition. A few examples of hypergolic fuels are Hydrazine (N2H4) and dinitrogen tetroxide. 
2. Cryogenic Fuels: Cryogenic refers to very low temperatures (approximately 120K or -153 degrees C). Such fuels need to be stored at extremely low temperatures. As their boiling and vaporization temperatures are very low, on being ignited in the engine, they burn violently giving off a large amount of heat energy. Their calorific values are also high, signifying the huge amount of energy these fuels have stored in them. Liquid oxygen and liquid nitrogen are the two primary cryogenic fuels used. One important advantage of cryogenic fuels over hypergolic fuels is that while hypergolic fuels are toxic and hazardous to handle, cryogenic fuels show no such toxic characteristics.
Working of the chemical propulsion system
In this category of propulsion, the engines acquire the necessary energy to generate the thrust through chemical reactions. The propellant is first injected into the engine and stored under suitable conditions.
Once the rocket is ready to lift off, the fuel is pumped into the combustion chamber, where it is ignited, undergoing an exothermic reaction with the oxidizer. The immense amount of energy (20 terajoules) and gas is then thrown out of the nozzle, creating an upward thrust on the rocket.
(The emissions you see in the picture is the reaction mass, that is the combusting fuel and energy that is released from the nozzle of the rocket during liftoff.)
Pros and cons
A significant advantage combustible propulsion systems possess over the non-combustible propulsion systems is that it is the only system which produces enough energy to overcome the Earth’s gravitational pull and thus it is the only method of propulsion currently available that can be used at the time of liftoff.
Though there is efficient usage of the fuel, it is quite low (30% fuel efficiency) compared to other propulsion systems (like the ion propulsion systems). Another significant limitation is that the immense heat generated in the combustion chamber limits the use of a few commonly found materials as the inner walls of the chamber.
There is very limited availability of materials that can sustain such high temperatures. Titanium based alloys are the best suited materials, but they are not readily available. It is only in the presence of such materials that the chemical propulsion systems can actually serve their real purpose.
Also, large amounts of combustible fuel are needed for long distance in-space travel, making the primary, secondary and subsequent phase vehicles quite huge in size.
As a result of the inefficient fuel usage in case of chemical propulsion systems, scientists have started to experiment and have come up with alternate technologies for propulsion, which have a higher specific impulse and over an 80% improvement in efficiency of fuel usage.
Electric propulsion systems are one such possible alternate in-space propulsion system and has shown a lot of promise in recent years.
Electric propulsion systems
Such non-chemical propulsion or electric propulsion systems include colloidal thrusters, ion thrusters, Hall Effect thrusters and many more. We shall now study ion propulsion systems.
Ion Propulsion System: As the name suggests, ion propulsion involves the usage of ionized particles for propelling the spacecraft. While rockets that are propelled by the chemical based system get the thrust by the combustion and expulsion of fuels, the ion system derives the thrust from the ejection of ions.
This happens in the following way: The ion propulsion system consists of a chamber (just like the combustion chamber in case of chemical propulsion) with an inlet, at one end, to fill the propellant (in this case a gas) and 2 layers of grid, at the other, with a potential difference between them. At the very end of this chamber there is an outlet from where the ions are ejected out to produce the thrust.
Working of the ion propulsion system
The chamber is first filled with a chemically non-reactive gas (an inert gas like Xenon).
Next, by virtue of a cathode tube a strong beam of electrons is produced and is pumped into the gas chamber. This is done in such a way that the electrons collide with the already existing neutral Xenon molecules.
This process of colliding of electrons is called electron bombardment and it results in the formation of ions – positively as well as negatively charged ones. The more time the collision of electrons takes place, the more ions may be formed.
These charged as well as the uncharged Xenon atoms form what is called plasma – an electrically neutral gas that consists of positive, negative and neutral particles whose charge finally adds up to zero. Plasma is an integral part of all electrical systems of propulsion.
Due to constant electron-atom and atom-wall collision, some particles lose their charge while others gain a charge. Thus, the total charge still remains zero.
Now these charged particles accelerate towards the grid. The grid has been made in such a way that only a few particles would be able to pass through it, the others collide with the wall and lose their charge. (The ones that lose their charge once again collide with the charged particles and gain charge.)
Within the grid and outlet, a potential difference is created, so when the charged particles that pass through the grid come in this region of potential difference, a force is exerted on them, due to the effect of electric current. This force provides the thrust needed for propulsion.
Pros and cons
A significant advantage this kind of propulsion has is that its fuel consumption is very efficient with a high specific impulse. It has a fuel efficiency of 90%. Moreover, the fuel used is a gas, and thus, large amounts of it can be filled in the chamber.
The primary disadvantage it faces is the limited availability of a material that can sustain the high amount of energy and heat produced by the plasma.
Also, the low amount of thrust generated by the ion propulsion system is not enough to overcome the gravitational pull of the Earth. This is one of the limitations of the ion propulsion system that limits its usage during lift off.
While chemical based propulsion is most frequently used in the launching of satellites for e.g. sending astronauts up to the International Space Station or launching of the James Webb Telescope, there has also been tremendous progress with the development of the cryogenic engine, wherein the efficiency of fuel utilization in the chemical based propulsion has been improved by more than 30%, with scientists predicting a possibility of a further improvement.
Along with this, various prototypes and models of the non-chemical propulsion systems have also been developed, whose usage is being tested. Efforts are also being made to miniaturize the fuel payload, especially in case of the colloidal and ionic propulsion systems. Accion Systems is currently working and experimenting on a miniaturized model of payload, ‘Tiled Ionic Liquid Electrospray’ driven by non- chemical propulsion systems. 
With the likes of Musk and other propulsion professionals hard at work at SpaceX, NASA, ISRO and other space agencies world over, the non-chemical propulsions systems are undoubtedly the future of in-space propulsion for all our space missions.