Rocket propulsion

A rocket does not need air to push against. It sends propellant one way and gains momentum in the other direction. Chemical rockets carry the ingredients needed for their reaction. This lets them work after leaving the atmosphere, including during a landing such as Chandrayaan-3.^[1]

Thrust arises from exhaust momentum flux and the pressure difference between the nozzle exit and ambient environment. The pressure contribution changes with altitude. Engine performance therefore depends on operating conditions and nozzle design; a sea-level thrust value and a vacuum thrust value are not automatically interchangeable.^[1]

Thrust tells you how strongly an engine pushes. Specific impulse describes how effectively it uses propellant. A high-efficiency engine may still push too gently to lift a vehicle from Earth. The right choice depends on whether the mission needs rapid acceleration or small changes accumulated over a long time.^[2]

Specific impulse is thrust divided by propellant weight flow at standard gravity. For a given propellant mass flow, higher effective exhaust velocity gives greater thrust, but power and engine constraints limit feasible combinations. Thrust-to-weight ratio and specific impulse measure different aspects of suitability and should not be ranked as one metric.^[2]

Propellant must accelerate both the payload and the propellant that has not yet been used. That makes ambitious velocity changes costly in mass. Staging helps by discarding hardware that no longer serves the flight. Mission design and engine choice therefore have to be considered together.^[3]

The ideal rocket equation relates velocity change to effective exhaust velocity and the logarithm of the initial-to-final mass ratio. Its idealization neglects gravity and aerodynamic losses and assumes a suitable exhaust-velocity model. Launch performance needs trajectory analysis as well; a rocket-equation calculation alone is not a payload-to-orbit prediction.^[3]

Electric propulsion uses electrical energy to accelerate propellant. It can save propellant on missions that have time to build up speed gradually. It still needs propellant and a power supply. This makes it useful for some spacecraft journeys, but not a direct substitute for high-thrust launch engines.^[4]

Electric thrusters trade low thrust and power demand for high effective exhaust velocity. Available electrical power, conversion efficiency, erosion, and mission duration constrain useful performance. Coupling the propulsion and power systems changes the spacecraft mass trade. Technology selection must include the achievable trajectory under finite thrust, not merely nominal specific impulse.^[4]