Launch Vehicles in India

Rocket Propulsion

  • A rocket has to clear earth’s atmosphere to travel in space or go into an orbit.
  • The minimum height for a satellite to go into the Earth’s orbit is approximately 200km.
  • At a certain initial speed, as the satellite tries to go off at a tangent to the earth, the Earth’s gravity pulls it back.

Principle Behind Rocket Propulsion: Newton’s 3rd Law Of Motion

  • The only means available for any object to be put in orbit is rocket propulsion.
  • Newton’s third law of motion govern the working of a rocket engine, viz for every action, there is an equal and opposite reaction. 
  • The mass of gas escaping through a rocket’s nozzle gives a push or commonly called thrust to rocket to fly in the opposite direction.
image 129

Performance Metric Of A Rocket: Thrust And Specific Impulse

  • The power generated by the rocket engine is balanced by the thrust in the opposite direction on the rocket itself, resulting in pushing the rocket at a certain initial velocity.
  • More the power of the engine, more the thrust, more is the initial velocity.
  • The efficiency of a rocket is expressed in terms of specific impulse, just like a car’s performance expressed in km/ltr (remember mileage).
  • Specific impulse depends on two things, one, the quality of fuel used and two, the performance of the engine.
  • Specific impulse is the amount of thrust derived from each pound of propellant (rocket fuel) in one second of engine operation.
  • Higher the specific impulse means higher push to the rocket.

Initial Velocity Of PSLV And GSLV: A Comparison

  • A PSLV injects an initial velocity of 7.5 Km/sec to put earth observation satellites of nearly 1000 Kg (1 tonne class) into sun-synchronous polar orbits at a height of about 800-900 km above earth surface.
  • In contrast, a GSLV needs a velocity of 10 km/sec to take a satellite to a height of 36000 km (Geostationary orbit).


  • Polar Satellite Launch Vehicle is the workhorse launch vehicle of ISRO since 1994. It is the 1st operational launch vehicle of India.
  • The PSLV is a launch system primarily developed to launch remote sensing satellites into sun synchronous orbits.
  • PSLV is a 4-stage that uses alternate combination of liquid and solid fueled rocket stages.
  • While the 1st and 3rd stages are solid-fueled, the 2nd and the 4th stages are propelled by liquid fuel.
  • Strap-on motors are used in the 1st stage of PSLV (solid stage) in order to provide additional thrust to the rocket.
  • Additional thrust is needed in the 1st stage as the rocket has to overcome the air resistance in the atmosphere in order to be launched in the orbit.
  • PSLV can deliver payloads of up to
  • 3,250kg to Low Earth Orbit
  • 1600 kg to Sun Synchronous orbit
  • 1400 kg to Geosynchronous Transfer Orbit

PSLV’s Achievements and Its Significance

  • With its capability to put small satellites in Lower Earth Orbit (LEO), PSLV is the key to Indian presence in the Global space business.
  • ISRO has launched 237 foreign satellites from 28 countries successfully by PSLV during the period 1999-2018.
  • Further PSLV-C37 launched 104 satellites on February 15, 2017, the highest number of satellites launched in a single flight so far.

PSLV Variants

Currently PSLV rockets have 4 variants

  1. PSLV-CA (core alone)
  2. PSLV-XL – 6 strap-on motors
  3. PSLV-DL – 2 strap-on motors (PSLV-C44 was the 1st mission to use it)
  4. PSLV-QL- 4 strap-on motors


  • GSLV was conceived and developed primarily to launch communication satellites (INSAT Series) of 2.5 tonne class in Geostationary Transfer Orbit and about 4.5tonne class in Low Earth Orbit.
  • GSLV is a three stage vehicle with solid fuel in the 1st stage, liquid in the 2nd stage and cryogenic in the 3rd stage.


  • Popularly called as ISRO’s ‘Fatboy’, GSLV Mk-III is the most powerful rocket of India.
  •  GSLV MK III is a three-stage heavy-lift rocket with an indigenous cryogenic engine in the third stage.
  • GSLV Mk III rocket can carry satellites weighing more than 4 tonnes to Geosynchronous Transfer Orbit (GTO) or satellites weighing about 10,000 kg to a Low Earth Orbit (LEO).
  • The GSLV Mk III rocket was the designated launch vehicle for Chandrayaan 2.
  • Further India’s first human space flight Gaganyaan to be launched in 2022 will also use GSLV Mk-III.

Cryogenic Technology

  • The indigenous cryogenic stage on the GSLV is the third stage, and uses liquid hydrogen as fuel and liquid oxygen as oxidiser.
  • Cryogenic stage is a highly efficient rocket stage that provides more thrust for every kg of propellant it burns compared to solid and earth-storable liquid propellant stages.
  • Cryogenic technology involves extremely low temperatures. Hydrogen liquefies at extremely low temperatures at minus 253 degree centigrade.
  • Nearly 50% of the power for GSLV rockets as they push into space comes from the cryogenic stage.
  • As a result engines operating with cryogenic technology can lift heavier satellites.


  • ISRO is slated to induct the Small Satellite Launch Vehicle in early 2020.
  • SSLVs are considered small wonders capable of launching payloads of 500-700 kg in low earth orbits (1/3rd of what PSLV can carry).
  • It can reach upto heights of 500 KM in the LEO.
  • SSLVs will cost 1/10th of a PSLV and will need only 72 hours for launch in comparison to 45 days for PSLV.

Vikas Engine

  • The Vikas engine is used in
  • Second stage of the light lifting PSLV
  • Second stage and the four add-on stages of the medium-lift GSLV;
  • Twin-engine core liquid stage of Mk-III
  • Recently in 2018, Vikas engine was improved for higher thrust by 6%, enabling it to carry 70 kgs of additional payload.
  • Vikas engine uses Di-Methyl Hydrazine as a fuel and Nitrogen tetroxide as oxidizer.

LOx Methane Engine

  • Liquid Propulsion Systems Centre of ISRO is developing 2 Lox methane-powered rocket engines.
  • The ‘LOx methane’ engine uses methane as fuel and liquid oxygen as oxidizer.


  • Can be synthesized in space (Methane can be synthesised using water and carbon dioxide in space).
  • It is non-toxic. (Di-Methyl Hydrazine and Nitrogen tetroxide is said to be highly toxic)
  • Higher specific impulse (read above for Specific Impulse).
  • Easy to store
  • Does not leave a residue upon combustion
  • Less bulky


  • Ion rockets are the rockets of the future for deep space exploration.
  • They are much more efficient that conventional rockets that use chemical fuels.
  • While chemical fuels generate velocities of upto 2 to 3 km/s, ion rockets can achieve velocities of about 4400 km/s.
  • Ion rockets use electric repulsion of ions to propel the rockets.
  • Small scale ion propulsion is used in a number of missions including NASA’s Dawn mission and Deep Space mission, ESA’s LISA Pathfinder and BepiColombo and Japan’s Hayabusa Mission
Online Counselling
Table of Contents