Ion Engine

Image of NASA's Evolutionary Xenon Thruster (NEXT) operation in vacuum chamber. (Source: Wikipedia)

Ion engines are a form of electromagnetic propulsion using ions and other charged particles as propellant. They produce low, but constant, thrust for kilowatts of electrical power. The low particular mass of their propellant results in high exhaust velocities, or high specific impulse, giving them impressive amounts of delta-V for a given mass of propellant. Some more speculative designs can achieve higher thrust.

The low acceleration of ion engines means spacecraft generally have to use less efficient spiral trajectories near planets with a strong gravitational field, and have to forgo the benefits of Oberth burns. They are a common propulsion choice for satellites and as interplanetary transit engines for spacecraft. They are a poor choice for any use that requires high thrust and short impulsive burns, and cannot make Hohmann orbit transfers efficiently.

Introduction

Most ion engines’ mechanisms involve knocking electrons out from a neutral gas, ionizing it, and using electromagnetic repulsion to accelerate it out of the engine. It is a scalable technology, with no upper limit as to how big they can get and the smallest ion engines are the size of your fingertip.

Ionizing neutral gas requires significant power - a typical ratio requires kilowatts of power for a few hundred millinewtons of thrust. 100 millinewtons of thrust is comparable to the force exerted by about three grapes (10 grams) sitting on a table . Even small chemical rockets, such as the 30 kg wet-mass Super Loki sounding rocket’s motor produces a force of 9,550 Newtons, huge in comparison.

Ion drives have an advantage in exhaust velocity (20,000 to 50,000 meters per second is a useful range), which translates into specific impulse of 2039-5099 seconds. By way of comparison, the Raptor engine used by SpaceX’s Starship has an exhaust velocity of 3,200 m/sec, and an Isp of 330 seconds. Since ion drives provide higher exhaust velocities, this allows for higher delta-v from the same mass-ratio, or the same delta-v for a smaller mass-ratio.

The tradeoff of high exhaust velocity and low thrust makes ion engines ideally suited to orbit-to-orbit missions; for anything that requires higher thrust, like getting things out of even a modest gravity well, you’ll need a second rocket with higher thrust. Ion engines are well-suited for satellite orbital maneuvering, and long duration spacecraft missions between planets. Ion drives work best for Ward-spiral style interplanetary transits as lowest delta-V Hohmann transfers require a short burn of high thrust to work; Ion drives will get your payload or passengers to their destination with a smaller fuel fraction and/or less time. Ion engines do not provide enough thrust for brachistochrone transfers.

Where ion engines shine are missions where long-duration accelerations are key to mission success. Delta-V in the tens of kilometers per second are possible even with today’s technology. They can drastically reduce the travel time between planets and if given enough time, cover great distances. Their low acceleration is not a major hindrance in accelerating in interplanetary space, overcoming solar gravity, or around the reduced surface gravities of the numerous small bodies in our solar system.

Example Engine Performance

Image of Busek BIT-3 thruster in operation in vacuum chamber. (Source: Busek) Left: iodine. Right: xenon.

Example table for Engine Performance for Busek RF Gridded Electrostatic Ion Thrusters

Name	Busek BIT-3
ISP or Exhaust Velocity	<22,555 m/s <2300 s
Thrust	<1.25 mN
Mass	1.28 kg
Efficiency	17.6% (Calculated)
Drive Power	<14.09 W (Calculated)
Power Consumption	56-80 W
T/W	0.0000996 (Calculated)
Energy Source (Fuel)	Electricity
Propellant	Iodine
Reactor	Grids
Specific Power	10.9 W/kg (Calculated)

Calculated Figures:

Design and Function

There are two usual steps in an ion engine’s operation:

• Creating charged propellant
• Accelerating the charged propellant

Creating charged propellant is usually done by ionizing it, through a variety of methods like electron guns or heating the propellant to plasma temperatures. It must be accelerated through electromagnetic means such as a voltage difference between grids, or oscillating electric and magnetic fields. This is the primary working principle of ion engines.

There are multiple factors in ion engine design as well, such as reliability, cost and/or simplicity and lifetime. In particular, when long lifetimes are needed - electrode-less ion thruster designs can be used, sidestepping the problem of propellant induced erosion on the electrodes.

Propellant Considerations

Ion engines can suffer significant inefficiency as ionizing the propellant may take significant amounts of energy input, thus designers focus on minimizing the amount of electricity spent in ionizing the propellant, influencing things like what propellant to use. A thruster that aims for a higher exhaust velocity at the expense of thrust has a lower propellant flow and allows mission planners to spend more of their launch payload on useful hardware, rather than propellant.

The ideal propellant choice is easy to ionize and has a high mass to ionization energy ratio . Another consideration is erosion, which limits the life-time of thrusters with electrodes. Some otherwise excellent propellants are toxic (mercury) or toxic and carcinogenic (cadmium). Additionally, being able to source propellants without significant processing overhead may be a desirable advantage; the colloid thrusters can use finely ground regolith as a propellant.

Balancing these considerations (except the processing and availability concerns) have led to xenon and krypton being common choices for electrostatic ion thrusters. They are high Z (Z is the proton number, therefore having more mass per atom), inert and require less energy to ionize. For gridless ion thrusters, like the Hall effect ion thrusters, bismuth and iodine are also available.

Some ion engine designs are flexible enough to use multiple propellants, or mixes that meet certain specifications. For example, airbreathing electric ramjets use different nitrogen, oxygen and hydrogen ratios depending on which orbital altitude they operate at.

Propellant Type	1st State Ionization energy	Availability	Toxicity	Molar Mass	Ionization Specific Energy (Calculated)
Xenon	1170.4 kJ/mol	Uncommon	Nontoxic	131.293 g/mol	8.914 MJ/kg
Krypton	1350.8 kJ/mol	Uncommon	Nontoxic	83.798 g/mol	16.12 MJ/kg
Argon	1520.6 kJ/mol	Uncommon	Nontoxic	39.948 g/mol	38.064 MJ/kg
Mercury	1007.1 kJ/mol	Rare	Toxic	200.59 g/mol	5.021 MJ/kg
Bismuth	703 kJ/mol	Rare	Nontoxic	208.98 g/mol	3.364 MJ/kg
Iodine	1008.4 kJ/mol	Rare	Nontoxic	126.904 g/mol	7.946 MJ/kg
Caesium	375.7 kJ/mol	Rare	Toxic	132.905 g/mol	2.827 MJ/kg
Indium	558.3 kJ/mol	Rare	Nontoxic	114.818 g/mol	4.862 MJ/kg
Sodium	495.8 kJ/mol	Common	Debated	22.989 g/mol	21.567 MJ/kg
Potassium	418.8 kJ/mol	Common	Debated	39.098 g/mol	10.712 MJ/kg
Magnesium	737.7 kJ/mol	Common	Nontoxic	24.305 g/mol	30.352 MJ/kg
Cadmium	867.8 kJ/mol	Uncommon	Toxic	112.411 g/mol	7.72 MJ/kg
Iridium	880 kJ/mol	Rare	Nontoxic	192.217 g/mol	4.578 MJ/kg
Nitrogen	1402.3 kJ/mol	Common	Nontoxic	14.006 g/mol	100.121 MJ/kg
Hydrogen	1312 kJ/mol	Very common	Nontoxic	1.007 g/mol	1302.88 MJ/kg
Ammonia	1208 kJ/mol	Common	Toxic	17.03 g/mol	70.934 MJ/kg
Water	1217.61 kJ/mol	Common	Nontoxic	18.015 g/mol	67.589 MJ/kg
Helium	2372.3 kJ/mol	Very common	Nontoxic	4.002 g/mol	592.779 MJ/kg

An important consideration is that the specific ionization energy refers primarily to the first ionization energy (i.e. the energy needed to strip the first electron off the atom.) Second, third, and other ionization energies are not given.

Engine Designs

With all these options and possibilities in mind, we can reduce ion engines into two main categories:

• Electrostatic thrusters
• Electromagnetic thruster / plasma propulsion engines

Electrostatic Ion Thruster:

Electrostatic thrusters accelerate ions via large voltage differences. Charged particles feel an attraction to a charge of the opposite sign and a repulsion to a charge of the same sign, resulting in a force called the Coulomb force.

There are four types of electrostatic thruster that use this principle:

• The gridded electrostatic ion thruster, which uses electrostatic grids to accelerate the propellant. Charged particles are subject to an electric force when travelling between the grids but not outside the grids.

• The electrostatic colloid thruster, which uses droplets of ionic liquid produced by an electrospray ionization process. An electrode provides the electric field to extract the droplets and accelerate them.
  • The radioisotope electrostatic colloid thruster, which uses a beta emitter to generate huge voltages for the electrostatic colloid thruster without power conversion.
    (when the radioisotope ion thruster article is made, make sure this is still conformant with it, and use it as context for the link to encourage someone to go read that article.) Further reading on radioisotope ion thrusters: Bookmark

• The Hall effect electrostatic ion thruster, which uses a voltage between a cylindrical anode and a negatively charged plasma cathode. Propellant is introduced and ionized at the anode, and accelerates after reaching the cathode, picking up electrons and becoming a neutral gas so that it is not pulled back into the engine.

• The field emission electric propulsion thruster, which extracts ions from liquid metal using very high electric fields. Liquid metals like caesium or iridium flow into the tip of a fountain-pen-like structure to make this process easier.

Electromagnetic Ion Thruster / Plasma Propulsion Engine:

There are even more ways to accelerate ions using magnetic fields instead of electric fields. Charged particles are sensitive to the Lorentz force, the same force that ejects projectiles out of a railgun. Other electromagnetic ion thrusters use the ponderomotive force, the result of oscillating magnetic fields, or have found some other clever way to use magnetic fields.

Magnetic fields push on negatively and positively charged particles equally. This means they put on both ions and electrons, which together form a plasma, and is the reason this type of electric thruster is also called a plasma propulsion engine.

There are 8 main types of electromagnetic ion thrusters in summary:

• The pulsed plasma thruster, which uses an electric arc to vaporize propellant (usually a block of teflon or PTFE) into plasma. Due to the force of the vaporization, the propellant plasma is ejected into the gap between an electrode and anode, where an electrical current runs through it and causes it to be further accelerated by the Lorentz force.

• The ELF pulsed plasmoid thruster, which generates a plasmoid (a smoke torus of plasma) from propellant and expands it down an electrically conducting nozzle, with directed kinetic energy (KE) generated from image currents in the nozzle due to interactions with the plasmoid. It is also electrodeless.
  • The alfvenic pulsed plasmoid thruster, which uses magnetic reconnection (the same kind that causes solar flares) to spontaneously generate low-temperature plasmoids. The outgoing plasmoid is an alfvenic outflow from the reconnection site, and so depends on the magnetic field strength rather than the propellant ion species mass for thrust.

• The pulsed inductive thruster, which uses pulses instead of continuous thrust and can run on high power levels (on the order of megawatts). Using a coil and conical propellant emitter, large built up charges are released behind the coil. This creates an electric current, and induces a magnetic field in the propellant. Together, the electric and magnetic fields accelerate the propellant through the Lorentz force.

• The magnetoplasmadynamic thruster, which ionizes the propellant via the electric field between an anode and a cathode. The plasma conducts electricity between the anode and cathode, closing the circuit, and creating a new magnetic field, which with the electric field, accelerates the propellant through the Lorentz force.

• The electrodeless plasma thruster, which does not use anode or cathode electrodes. Propellant is ionized through electromagnetic waves, and oscillating electric and magnetic fields in another chamber accelerate the propellant due to the ponderomotive force. The separation of ionization and acceleration steps allows for throttling.

• The arcjet thruster, which uses an electrical arc between two electrodes to heat flowing propellant into plasma. The expansion of the plasma into the nozzle generates thrust.
  • The microwave electrothermal thruster, which is similar to the arcjet but utilizes a microwave antenna to heat the flowing propellant instead.

• The helicon double-layer thruster, which uses radio waves emitted from an antenna to ionize the propellant into a plasma and excite a helicon wave (a low frequency EM wave bounded inside the plasma), further heating it. A special kind of magnetic nozzle accelerates the propellant.

• The VASIMR, which functions similarly to the helicon double-layer thruster but uses an additional second step to heat the plasma to 1 million K through ion cyclotron resonance heating. It uses superconductors to magnetically confine the plasma, and thrust is generated when the plasma escapes confinement as the exhaust. The VASIMR has mechanisms that allow it to trade lower exhaust velocity and Isp for higher thrust.

Basic Electrostatic Ion Thruster Mathematics

An electron or negatively charged ion with a charge of -1 leaving a hot cathode to an anode with a 100 V voltage difference between them will acquire an energy of 100 electron-Volts (or eV). A nanogram sized speck of dust with a million positive charges moving up a 10 V gradient will acquire an energy of 10 million electron-Volts (or MeV).

The equation for how much energy $E$ a charged particle gains is:

$E=qV$

• Where $E$ is the particle's energy, $q$ is the particle’s charge, and $V$ is the voltage.

The electron is a lightweight subatomic particle. 100 eV is enough to get it moving at 5929 km/s. The dust particle is relatively heavy. 10 MeV will just push it to 1.79 m/s. The velocity of the propellant is the exhaust velocity of the ion engine.

These figures are derived from the equation for kinetic energy:

$E=mv^{2}/2$

• Where $E$ is the particle's energy, $m$ is the particle's mass, and $v$ is the particle's velocity.

Which is rearranged to give velocity:

$v={\sqrt {2E/m}}$

The thrust of the electrostatic ion thruster is given by:

$N=v{\dot {m}}$

• Where $N$ is the thrust of the thruster and ${\dot {m}}$ is the propellant mass flow rate.

That can be rearranged to give mass flow rate instead:
${\dot {m}}=N/v$

The drive power of the electrostatic ion thruster can be calculated two ways, most simply with 1:

1. $P=vN/2$
2. $P=AV$

• Where $A$ is the amperage

The equation for the amperage needed is:

$A={\dot {m}}/nq$

• Where $n$ is the molar mass of the propellant.

History & Development

[This section should be in its own specific history article, so a link would be provided here instead of the text]

The conceptual history of ion engines dates back to 1911, by Konstantin Tsiolkovsky, the first person to publish a paper on the subject. He recommended it for near-vacuum conditions at altitude, however thrust was also seen with ionized air streams at atmospheric pressure.

Image of the SERT-1 spacecraft. (Source: Wikipedia)

Hermann Oberth later reignited conversation with his “Ways to Spaceflight” book in 1923, talking about its advantages in mass ratios and mass savings for payloads. He predicted its usage in spacecraft propulsion and attitude control, as well advocated for the electrostatic kind of ion thruster.

By 1959, a gridded electrostatic ion thruster had been built by Harold R. Kaufman at NASA Glenn Research Facilities, using mercury as its propellant. Suborbital tests were conducted throughout the 1960s, and an orbital test was done in 1970, the SERT-1 and SERT-2 demonstration missions.

The Hall effect thruster design, studied by both the United States and the Soviet Union, had been used by the Soviets from 1972 to late 1990s for attitude control. It is estimated that 100-200 engines were used on Soviet and Russian missions. By 1992, the designs had been brought to the West, thanks to a team of electric propulsion specialists from the Ballistic Missile Defense Organization (BMDO) visiting Soviet labs.

Worked Examples

Electrostatic Ion Thruster:

Gridded Electrostatic Ion Thruster - NSTAR, NeXT

Colloid Thruster - Busek Electrospray Thruster Bookmark

Hall Effect Ion Thruster - PPS-1350, on the SMART-1 mission. AEPS

Field Emission Electric Propulsion Thruster - IFM Nano

Electromagnetic Ion Thruster / Plasma Propulsion Engine:

Pulsed Inductive Thruster - Several laboratory examples built by NGST.

Magnetoplasmadynamic Thruster - MPD thrusters flown on EPEX (Japan) mission

Helicon Double-Layer Thruster - Prototype built by Dr Christine Charles

VASIMR - VX-10, VX-50, VX-100, VX-200. Built by Ad Astra. (not flown)

Additional Reading

Spiral Trajectories
Gravity Assists and Oberth Effect
Hohmann Transfer
Super Loki
Raptor Engine
Brachistochrone Transfer
Gridded Electrostatic Ion Thruster
Electrostatic Colloid Thruster
Radoisotope Electrostatic Colloid Thruster
Hall Effect Electrostatic Ion Thruster
Field Emission Electric Propulsion Thruster
Pulsed Plasma Thruster
ELF Pulsed Plasmoid Thruster
Alfvenic Pulsed Plasmoid Thruster
Pulsed Inductive Thruster
Magnetoplasmadynamic Thruster
Electrodeless Plasma Thruster
arcjet thruster
microwave electrothermal thruster
Helicon Double Layer Thruster
Variable Specific Impulse Magnetoplasma Rocket

https://cdn.discordapp.com/attachments/886253956777533450/887757110501318757/POTENTIALITIES_OF_THE_RADIOISQTOPE_ELECTROSTATIC_PROPULSION_SYSTEM.pdf
https://www.osti.gov/biblio/4875645
Further reading for the radioisotope ion thruster.

Credit:

To Tshhmon for writing the article

• To SOPHONT SIMP for test-reading
• To KRKIIIIII for test-reading
• To AdAstraGames for test-reading and some editing
• To MatterbeamToughSF for checking sources, technical details and contributing useful information