Ion Engine

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; 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 (Isp) 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

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)[1]
Drive Power	<14.09 W (Calculated)[2]
Power Consumption	56-80 W
T/W	0.0000996 (Calculated)
Energy Source (Fuel)	Electricity
Propellant	Iodine
Reactor	Grids
Specific Density	10.9 W/kg (Calculated)

Calculated Figures:

1. Drive Power / Power Consumption (80 W) = 0.176
2. Drive Power = 1.25 mN thrust $\cdot$ 22,555 m/s exhaust velocity $/$ 2

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.

Propellant Considerations

However, ion engines suffer significant inefficiency as ionizing the propellant can 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; this allows mission planners to spend more of their launch payload on useful hardware, rather than propellant.

Note that ion engines can be designed to use a huge number of propellants, including molecules, dust particles, liquid metals and more. Ionization energy is an important parameter but many other factors come into play.

For example, many of the elements with the lowest ionization energy per kilogram are highly radioactive and solid up to extreme temperatures. They would be a pain to handle! Other options may be toxic (mercury) or carcinogenic (cadmium). Erosion from reactive species like potassium must be considered. As is the availability and ease of production of a propellant if ISRU is something you want. Xenon may be a great choice for most electric thrusters, but it is much harder to find in our solar system than nitrogen.

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.

[insert table here of propellants and specific ionization]]

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 [link], 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 [link], 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 Hall effect ion thruster [link], 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 [link], 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.

There is a form of electrostatic colloid thruster that uses the beta emissions of a radioactive substance to generate the voltages necessary to eject droplets of ionic liquid.

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 [link], 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 [link], 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 [link], 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 [link], 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 [link], 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 [link], 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 helicon double-layer thruster [link], 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 [link], 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. VASIMR has mechanisms that allow it to trade lower exhaust velocity and Isp for higher thrust.

Basic Electrostatic Ion Thruster Mathematics

An electron 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 $E$ in eV a charged particle gains is:

$E=qV$

• With $E$ the energy in joules, $q$ the particle’s charge in coulombs, and $V$ the voltage in volts.

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.

These figures are derived from the equation for kinetic energy:

$E=mv^{2}/2$

• With $E$ the particle’s energy in joules, $m$ the particle’s mass in kg, and $v$ the velocity in m/s.

Which is rearranged to give velocity:

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

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.

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

Links from ion engine design section Links from history section

https://cdn.discordapp.com/attachments/886253956777533450/887757110501318757/POTENTIALITIES_OF_THE_RADIOISQTOPE_ELECTROSTATIC_PROPULSION_SYSTEM.pdf Further reading for the radioisotope ion thruster.

Additional References

https://satsearch.co/products/busek-bit-3
http://www.projectrho.com/public_html/rocket/enginelist.php
https://www.busek.com/bet-300p

https://en.wikipedia.org/wiki/Ion_thruster
https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness
https://www1.grc.nasa.gov/space/sep/gridded-ion-thrusters-next-c/

https://en.wikipedia.org/wiki/Gridded_ion_thruster
https://en.wikipedia.org/wiki/Hall-effect_thruster
https://en.wikipedia.org/wiki/Field-emission_electric_propulsion

https://en.wikipedia.org/wiki/Pulsed_inductive_thruster
https://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster
https://en.wikipedia.org/wiki/Electrodeless_plasma_thruster
https://en.wikipedia.org/wiki/Helicon_double-layer_thruster
https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket
https://en.wikipedia.org/wiki/Microwave_electrothermal_thruster

https://en.wikipedia.org/wiki/SERT-1
https://spacemath.gsfc.nasa.gov/weekly/5Page64.pdf
Electrostatic Ion Engine Math

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 and technical details