Rocket Propulsion Systems | Rocket Engine Work | Rocket Engine Thrust

Rocket propulsion Systems

A rocket engine is defined as an engine that expands thrust by ejecting a stream of matter (i.e. exhaust gas) backwards. Since the reaction (i.e. thrust force) principle involved assumes a self enclosed supply of energy. A rocket engine can operate in any medium including space (i.e. Outside the earth’s atmosphere), where there is no oxygen to support combustion.

Rocket Engine work

The working principles of rocket engines are primarily governed by Newton’s law of motion. Newton’s first law states that there is no change in the motion of body unless a resultant force acts on it. The governing action such as gravitational force, lift force, drag force and the thrust force of the rocket engine all proceed on the vehicle to cause the resultant motion. The net amount of the resultant force and its directions decide the acceleration on the vehicle and the path of the flight trajectory, in accordance with Newton’s second law.

Rocket Engine Thrust

A rocket engine develops its thrust by ejecting a mass backward. The mass is accelerated backward by combustion that accelerates its velocity from 0 to 1000 m/s. The Newton’s Second law states that, the force for this acceleration is proportional to the mass of the exhaust gases. The force acting on the accelerating mass and the resultant exhaust gases, produce a thrust in accordance with Newton’s third law, which states that for every action, there is an equal and opposite reaction. So the thrust force in the rocket engine is developed by internal fluids within the rocket which accelerates equal but opposite external force.

The force vector (i.e. thrust), can be determined by investigating the change of momentum in the design and the sum of the forces that act on a closed duct in a control volume. The internal flow to the rocket experiences a change of momentum that is equal to the mass flow rate and the change in velocity of the gases. When the inlet velocity is low the change in momentum considered as negligible.

Force of a Rocket:

The sum of all pressures on the surfaces perpendicular to the flow axis of the device reduces to a resultant force. Due to the pressure differential between the pressure at the nozzle exit plane and the ambient pressure that acts on the exit area of the nozzle.

Net pressure force = (Pexit – P ambient) A exit

The sum of the forces that acts on a rocket is equal to the change of momentum in accordance with Newton’s second law.

Thrust = m Vexit + (Pexit – Pambient) A exit

  • When Pexit equals Pambient expansion is optimum and performance is good.
  • When the nozzle exit pressure is less than ambient, the nozzle is said to be over-expanded.
  • When exit pressure is greater than ambient, the nozzle is said to be under-expanded.

Generally the rocket flies through the atmosphere; it experiences variations in the ambient pressure. So it operates at optimum expansion at only one altitude. Resultant design and flight conditions based on the rocket exit areas in different altitudes.

UPSC | GATE Exam for Mechanical | Objective Question and Answers | Tamilnadu Teachers Recruitment Board

Objective Question and Answers for UPSC / GATE Exams:

UPSC | GATE Exam for Mechanical | Objective Question and Answers | Tamilnadu Teachers Recruitment Board

The common soldering method used for soldering circuit components to their boards is known as:
  1. WAVE soldering
  2. Reflow soldering
  3. Furnace soldering
  4. Infrared soldering

WAVE soldering


Wave soldering is a large-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB.

Re-flow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint.

Infrared Soldering by soldering individual joints with a hot air pencil.

The heat generated in metal cutting is dissipated in different proportions in to environment, tool, chip and work piece. The correct order of this proportion in descending magnitude is (no cutting fluid is used):
  1. Tool, work, chip, environment
  2. Work, tool, chip, environment
  3. Tool, chip, environment, work
  4. Chip, tool, work, environment

Chip, tool, work, environment


The maximum temperature in the cutting zone occurs not at the tool tip but at some distance further up the rake face. Material at a point (Chip) gets heated as it passes through the shear zone and finally leaves the chip. The point at built up edge, heating continues beyond the shear plane into the frictional heat region. So tool is the next highest absorbing medium. The point nearer to work their temperature rises merely due to conduction of heat into the work piece.


Flatness of slip gauges is checked with:
  1. Optical flat
  2. Electronic comparator
  3. Interferometer
  4. Linear measuring machine

Electronic comparator


For flatness measurement of surfaces the easiest and best way to test is with an optical flat. Normally Slip gauges and angular measuring instruments are used for accurate measurements of flatness in surface. Here the work piece and Optical flats are hold each other and a monochromatic lights are passed in between the gap. If the light bands visible then the surfaces are not in perfect contact.

In this question flatness of slip gauge are checked. So we need a high precisioned electronic comparator to check the flatness surface in slip gauges.

Interferometers are non contact type flatness measurement of surfaces such as thin, transparent wafers, substrates and windows.

The flatness of a machine bed can be measured using:
  1. Auto collimator
  2. Vernier calipers
  3. Height gauge
  4. Tool maker’s microscope

Height gauge


Flatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel.

Autocollimator is an optical instrument which is used to measure small angles with very high sensitivity. Normally straightness measured with autocollimator.

Tool maker’s microscope used for length, angle, Thread, and straightness measurements.

In a turbulent boundary layer over the entire length of a plate, the boundary layer thickness increase with its distances ‘X’ from the leading edge is:
  1. X1/2
  2. X1/5
  3. X2/5
  4. X4/5



ρ α X (1 / 2)

When the fluid flows this cohesive force along with adhesion shows itself in the macroscopic scale as viscosity. As the flow proceeds downstream of the flat plate the viscosity is able to slow down more and more fluid layers above the flat plate. This is what is called momentum transfer. And hence the boundary layer thickness increases as the fluid moves downstream.


The continuity equation (du/dx)+ (dv/dy)=0 is valid for:
  1. Ideal fluid flow only
  2. In-compressible fluids, the flow may be steady (or) unsteady
  3. Steady flow, the flow may be compressible (or) in-compressible
  4. In-compressible fluids and steady flow

In-compressible fluids, the flow may be steady (or) unsteady


In-compressible flow means  du/dx+dv/dy=0, Some time du/dx = 0 and dv/dy = 0. So the flow may be steady or unsteady.