Spacecraft Propulsion Systems

Spacecraft propulsion systems employ fuel and oxidizer to alter vehicle momentum. Their effectiveness can often be gauged by their specific impulse – which measures how much thrust they produce per mass flow rate rate per second.

Newton’s Law holds that for every action there is an equal and opposite reaction; however, you cannot use force against something to gain momentum in space (like when your car tires rub along asphalt), as this would accelerate it upwards.


Rockets are used when high speeds are necessary to reach space, such as intercontinental ballistic missiles or to reach orbit. Rockets rely on liquid fuel (kerosene) and an oxidizer such as liquid hydrogen fed via powerful turbine pumps into propellant tanks; these must be capable of withstanding extreme thermal and pressure stresses during flight, along with producing drag that slows their vehicle as well as carbon dioxide that pollutes the upper atmosphere.

Newton’s second law of motion states that an isolated system will remain at constant mass by maintaining momentum by throwing away part of its mass at an opposite velocity (reaction force). This phenomenon explains why rockets only travel in one direction and cannot reverse course like boats not tied to docks can. That is why spacecraft require additional reaction engines in order to orient themselves properly in space.


Earth currently hosts approximately 7,560 satellites, with more constantly being launched. They allow humans and robots alike to communicate, as well as giving meteorologists a better view of global weather patterns so we can understand what causes large storms and hurricanes.

Propulsion systems are essential in keeping satellites in their elliptical orbit, yet small satellites often lack them due to weight limitations and limited capabilities and lifespan. TNO, NanoSpace and EPFL have developed a miniaturised electric propulsion system which can easily be added onto satellites.

The technology works similarly to firework: fuel and oxidiser are pre-mixed before being burned in one large chamber, producing hot gases which propel the spacecraft forward. Its use reduces risk compared to bipropellant systems that involve two liquids being kept separate and then ignited at once; plus its cost is approximately one third lower.

Spacecraft designed to travel further

Safran Spacecraft Propulsion, a division of Safran Electronics & Defense, designs and provides advanced electrical propulsion subsystems for agencies, integrators and commercial providers to build more sustainable spacecraft. We offer products and technologies related to connectivity, propulsion, navigation, timing and optics.

Assuming that a greater change in velocity necessitates more energy from propulsion systems, then Isp (specific impulse or Isp), an energy requirement measurement tool typically utilized by propulsion concepts is proportional to their propellant mass flow rate and acceleration requirements.

Einstein’s special theory of relativity states that no object can travel faster than the speed of light in a vacuum, meaning chemical rockets used to launch satellites and spacecraft cannot come anywhere close to approaching this limit. Other propulsion methods exist that allow spaceships to attain much higher speeds, including magnetic sails which deflect charged particles in solar wind.

Spacecraft designed to land

Most spacecraft require significant amounts of energy to escape Earth’s gravity, but once free, must still slow down before impacting with another celestial body with atmospheres like planets or other celestial bodies with atmospheres. This process is known as reentry and provides the greatest braking force available – aerodynamic drag from its surrounding air.

Spacecraft designers are working hard to address two fundamental problems with spacecraft design. Boeing and SpaceX, for instance, have both created vehicles using blunt shapes with high drag coefficients in conjunction with parachutes to slow their velocity as they descend from orbital speeds of 17,000 mph down to rates human occupants can survive upon impact.

As this approach cannot alter a spacecraft’s momentum without altering something else’s momentum as well, which would violate conservation of momentum laws, certain designs utilize forces other than thrust to alter orbit or deflect Sun radiation pressure by deflecting solar sails with magnetic fields.

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