Hello Defence Lovers, in the first part of this article, we discussed propulsion systems that operate in the atmosphere, i.e., jet propulsion. These propulsion systems use a working fluid (air) and accelerate it to produce thrust. However, in the case of rocket propulsion, this is not feasible due to the absence of a working fluid (air) in space.
Due to this constraint, rocket propulsion is achieved by physically carrying the fuel and the oxidizer and igniting them in the combustion chamber to produce thrust. Rocket engines are classified based on the fuel they use. Rocket engines can be either solid fuel or liquid fuel based. The most common oxidizer used in engines is Liquid Oxygen (LOX).
Solid Fuel Rocket Engines
The earliest solid fuel-based rockets used gunpowder and were used by the Indians as early as the 13th century by Tipu Sultan and Hyder Ali. Modern solid rocket engines are used in missiles and strap-on boosters of rockets. Solid fuel rocket engines can be of various types based on grain geometries. In simple terms, grain geometry is the geometry of how the solid fuel is packed. A major disadvantage of solid rocket engines is that they can’t be turned off and on, unlike their liquid counterparts. Once it is ignited, it only stops once it runs out of fuel or oxidizer. DRDO solved this problem by developing a Dual Pulse Rocket Motor. Essentially the fuel compartment is split into two parts, and the igniter can be brought into contact with the fuel and oxidizer mixture when desired. Solid rocket engines are mainly used on our missile systems.
Notable mentions are the Agni series of missiles, the Prithvi III, the K series of SLBMs (Submarine Launched Ballistic Missiles), and the Shaurya missile. This list is extensive, and in conclusion, most of our missiles are powered by solid-fuel rocket engines. Some missiles use solid rocket engines in the first stage, consisting of either a liquid rocket engine or a ramjet/scramjet engine in the second stage. These are hybrid engines. An example is the Akash missile which uses a ramjet-rocket motor propulsion system. After an initial solid rocket motor, burnout uses a ramjet engine for the rest of the flight path.
These systems will also power the upcoming Astra series of air-to-air missiles. A conventional solid rocket engine powers Astra mk1. A dual-thrust pulsed rocket motor will power Astra mk2. Astra mk3 will be powered by an SFDR (Solid Fuel Ducted ramjet). Overall, solid rocket engines provide large thrust with a relatively simple design.
Liquid Fuel Rocket Engines
Since solid rocket engines can’t be controlled, i.e., they burn out in one go, the development of liquid fuel-based engines was initiated. The most common fuel is liquid Hydrogen and RP-1 (Rocket propellant grade Kerosene). ISRO is working to develop a methane-based rocket engine. The Vikas engine uses Dinitrogen tetroxide (N2O4) and Unsymmetrical Dimethyl Hydrazine (UDMH) as liquid propellants. These engines are the backbone of our PSLV and GSLV launch vehicles. ISRO successfully developed its cryogenic engine technology in 2013. The cryogenic featured at LH2/LOX mixture (Liquid oxygen and liquid Hydrogen). Cryogenic technology is used to store gaseous fuel that has been liquefied and liquefied oxidiser at very low temperatures. ISROs cryogenic engine powers the upper stages of the GSLV MkIII and the GSLV MkII. ISRO is developing the SCE-200 semi-cryogenic engine for the Gaganyaan mission, and its testing has already commenced.
This was a brief overview of the types of rocket engines and which Indian systems make use of such technologies. These are the conventional types of rocket engines. However, today a new type of rocket propulsion is emerging, and that is ion propulsion.
Ion Propulsion
Ion propulsion works by ionizing the propellant by adding or removing electrons to produce ions. The most common to achieve this is to bombard an electron with an atom of the propellant, thus causing the propellant to release electrons making it positively charged. This results in the formation of a plasma. Plasma is a gas consisting of negatively and positively charged ions in proportions resulting in zero net charges. The positive charges are accelerated towards a negatively charged plate to extremely high speeds and then thrust outwards in an ion beam resulting in the formation of thrust. The commonly used propellant in this type of propulsion is Xenon due to it high atomic mass and moreover, it is ionized easily. Bellatrix Aerospace has developed India’s first privately developed ion thruster.
Many private sector companies have started venturing into the field of rocket propulsion, and massive milestones have been achieved by these companies. Overall, these propulsion systems are essential to powering both aircraft and spacecraft.
