Electrical Propulsion

Electrical propulsion is in the wind as never before. Electrical propulsion has often significant higher specific impulse levels than chemical propulsion. However, their main down side is low thrust levels. Where chemical rockets can produce tens of tons of thrust, an electrical engine may hardly lift a piece of paper! In the vacuum of space where air resistance and gravity may be low, electrical engines has a role. With time and thanks to their high specific impulse levels, the speed of a spacecraft will soon out number any thirsty chemical rocket powered spacecraft.

In electrical propulsion we have the following types:

a) Electrothermal

b) Electrostatic

c) Electromagnetic

The electrothermal engine operates on the principle of heating a chemical propellant electrically and then expand it thermodynamically. There are two types of electrothermal engines:

a1) Resistojet, electrically powered heating element for gasifying the propellant.

a2) Arcjet, electrically powered arc for heating/gasifying the propellant.

Both engines can use similar propellants like Ammonium (NH3), Nitrogen Tetroxide (N2O4), Hydrazine (N2H4), Hydrogen (H2). The Arcjet may also use nitrogen (N2).

The engines can operate for months if needed. Thrust range is typically in the order of 200 to 1000 mN. Specific impulse is 200 to 1000. The Arcjet has generally higher thrust but less efficiency compares to the resistojet.

Electrostatic engines rely on the Coulomb forces to accelerate propellant composed on non-neutral charged particles. There exist three types of electrostatic engines:

b1) Electron bombardment thrusters

b2) Ion contact thrusters

b3) Field emission thrusters

The electrostatic engines have low thrust, 0.01 to 200 mN. However, their specific impulse is higher ranging from 1500 to 5000 s. Like the electrothermal engines they can operate for months without much electrical power. Classical propellants are Xenon (Xe), Krypton (Kr) and Argon (Ar). Xenon is most frequently used. Xe is ionized by stripping off electrons. Then the electrical charged heavy ions are accelerated to very high velocities by means of electrostatic fields. The emitted ions are neutralized by electrons to prevent buildup of space charge on the vehicle.

The electromagnetic thrusters are often relatively simple. Their means of operation is based on the Faraday accelerator. Simply said, a plasma conductor carries a current in the direction of an applied electrical field but perpendicular to a magnetic field. With both of these vectors in turn normal to the direction of plasma accelerator. There are generally two types of electromagnetic thrusters:

c1) Solid Pulsed Plasma (PPT)

c2) Magnetoplasma dynamic (MPD)

PPT engines can run for a decade whereas the MPD engine can run for weeks. The PPT uses Teflon as propellant. MPD can use Argon (Ar), Xenon (Xe), Hydrogen (H2) and Lithium (Li). PPT can produce 0.05 to 10 mN thrust where MPD can produce 0.001 to 2000 mN. Specific impulse level is in the range of 600 to 2000 s for the PPT. MPD has demonstrated 2000 to 5000 s.

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This article is a part of a pre-course program used by NAROM in different courses, for example Fly a Rocket!