Space propulsion


What is space propulsion ?

Space propulsion (definition): A class of propulsion that allows to produce a propulsive force in (empty) space. It includes a.o. rocket propulsion and solar sailing.

Propulsion (definition) - The act of changing the motion of an object.
Etymology - Latin 'propellere', meaning the action or process of propelling or something that propels.


Why do we need space propulsion?

Space propulsion is needed to:

  • launch a vehicle from Earth;
  • accelerate a spacecraft to a velocity that will cary the spacecraft from one orbit to another. Depending on the velocity, the spacecraft will either remain within the influence of Earth's gravity or  is carried beyound its influence on to a distant planet or even beyond the influence of the sun and into the interstellar space. This may include one or more changes in the orbital plane
  • compensate for the influence of disturbing forces like drag, solar wind, gravity (for example due to the dynamics of a rotating, nonspherical Earth), etc. on the orbit or trajectory;
  • control the attitude of the vehicle (for example to orient antennas or sensors, or thermal control surfaces) or to compensate for disturbing torques;
  • other:
    • control spin-up/down of spacecraft
    • allow unloading of reaction or momentum wheels
    • perform separation of vehicle stages
    • perform propellant settling (liquid propellant systems sometimes require to compact the liquid inside the tank near the tank outlet).

Current space propulsion challenges (example missions requiring lots of propulsion)


How to achieve space propulsion ?

Most spacecraft (including space launchers, satellites, space probes, etc.) currently are propelled by expelling mass from within the spacecraft in a direction opposite to the direction of travel. This is referred to as rocket propulsion. The whole of hardware and software needed to generate thrust is referred to as the propulsion system. The majority of rocket propulsion systems  are chemical rocket systems, which use chemical propellants to generate the energy needed for thrust generation and as the mass to be expelled. Some spacecraft, such as the Pegasus rocket and SpaceShip One, use air-breathing propulsion on their lower stages. Most satellites have simple reliable chemical rockets to keep their station allthough lately more and more spacecraft including planetary probes are starting to use non-chemical means like electrical rockets (e.g. ion rockets and resistojets) for some of their tasks.


Propulsion systems do not come for free

For instance, to bring 7 ton of payload (the usefull stuff) into space, a giant space rocket, like the European Ariane 5, is needed. This rocket essentially is an empty hull which houses several rocket motors and the required propellant (compare the fuel needed to make a car engine perform its duty). The total mass of an Ariane 5 rocket is over 746 ton (746,000 kg) of which more than 80% (642 ton) is propellant. Given that it costs about 7,000-20,000 Euro to launch one kilogram of mass into space, one can imagine the high cost of having to carry so much propellant on board. Another example is related to system failures. Investigations have shown that over 40% of all failures of space systems are due to the propulsion system be it a catastrophic failure (explosion) or just a rocket engine that does not start. Next to these two consequences, many more exist. Such consequences should be carefully weighed against the benefits that having a means of propulsion onboard the vehicle brings.


This page is last updated 20 November 2014.


Ariane 5 Aestus stage



Nuclear electric propelled vehicle


Solar-rocket propelled space tug


Name author: B.T.C. Zandbergen
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