Introduction to various spacecraft propulsion methods

ion-accelerating engine, pic from wikipedia

Space propulsion is different from regular propulsion methods that deal with situations on the ground or in the air. It’s also different from launch propulsion which space propulsion methods exclusively deal with propulsion systems used in vacuum of space.

Before introducing different propulsion methods, there are couple of concepts that are really important related to the effectiveness of a propulsion system. The purpose of propulsion in the space is to change the velocity of a spacecraft. Since obviously that the more massive an object is, the harder to change its velocity, designers of spacecraft propulsion usually use the amount of change in momentum per unit of propellant consumed to compare the effectiveness. This is called specific impulse. It’s a different concept from the thrust which is the force moving a rocket through the air.

SpaceX’s Kestrel engine, pic from wikipedia

Essentially all spacecraft propulsion systems are reaction engines. These engines provides propulsion by generating chemical or physical reaction and expelling reaction mass. The most common engines are internal combustion heat engines. These kind of engines combust either liquid, solid or gaseous fuel with oxidizer within a combustion chamber. These type of engine needs a large mass of fuels. Rockets propelled by this kind of engine, such as monopropellant or bipropellant rocket, have relatively low specific impulse but high thrust.

The other kind of system, which is actually more suitable for traveling in the space, is electromagnetic propulsion engine. They don’t rely on the high temperature and fluid dynamics to accelerate the reaction mass, but rather accelerating the reaction mass directly by electromagnetic forces. The ion accelerating engine is the most common one among this type. These kind of engines are good that they usually carry little mass of fuels and they gain much greater specific impulse in the space.

electric propulsion engine in lab, NASA, pic from wikipedia

The window of universe — Various types of Telescope

NGC 6357 under X-ray telescope by NASA. Image from Gizmodo

The science of astronomy is about the telescopes. Without them, our naked eyes can only see thousands of stars in the night sky away from city lights. We get to see galaxies, stars, nebulas and other celestial objects that are light years away from us through telescopes. Furthermore, as the technology improved over the last century, we developed various kinds of telescopes beside optical telescope. Here is some short description about them:

It is a kind of telescope to detect high energy gamma ray photons in the universe range of 50 GeV to 50 TeV. They are usually not in the space but on the Earth.

Infrared image from NASA’s Spitzer Space telescope. Image from Space

All celestial objects with a temperature above absolute zero emit some form of electromagnetic radiation. Astronomers use infrared telescope to detect celestial objects.

Radio telescopes are used to receive electromagnetic radiations from stars in the radio wave range. They are usually huge parabolic antennas. Radio telescope is essential to radio astronomy study.

Parkes CSIRO radio telescope. Image from wikipedia
  • Ultraviolet telescope, X-ray telescope and Submillimeter telescope

These are just other kinds of telescopes that astronomers used to detect electromagnetic waves in ultraviolet range, X-ray range and Submillimeter range edited by celestial objects in the universe

These various kinds of telescope allow us to picture and study universe in different perspective and enable us to see much more stuff than optical telescope can. Our development in astronomy is all based on these telescopes and that’s why people are saying the astronomy is the science of telescope.