Electrodynamic Propulsion Concepts

Electrodynamic propulsion means using electromagnetic fields to exchange momentum with something real: ionised propellant carried aboard, as in ion, Hall, magnetoplasmadynamic and VASIMR thrusters; a planetary magnetic field, through a current-carrying conductor, as in electrodynamic tethers; or the solar wind, through a magnetic field, as in magnetic sails and plasma magnets. Every member expels something or pushes on something external, and the momentum bookkeeping closes. That is the load-bearing sentence separating this brief from Reactionless Propulsion and from the discredited half of Electrogravitics.

The framing under test — electrodynamic methods are an underused propulsion class — is half wrong for an interesting reason, and correcting it is the first thing this brief does. Electric propulsion is not underused. It is the dominant form of in-space propulsion by unit count and it is the entire primary propulsion system of NASA's current flagship asteroid mission. A brief that presented this class as neglected would be describing the world of 1995.

What is genuinely underused is the electrodynamic tether — the one member that needs no propellant at all, whose physics was measured in orbit in 1996, and which has never flown to produce thrust. And what limits the rest of the class is not the thruster: it is electrical power, which is why the technology tree records a dependency on Advanced Nuclear Propulsion and states it as a quantitative ladder rather than as a gesture. The brief follows that structure: mature, power-blocked, genuinely underused, and speculative-but-real-physics.

Established Start with the flight numbers, because they are the fastest way to dispose of the framing. Psyche's electric propulsion system is four Fakel SPT-140 Hall thrusters at discharge powers from 0.9 kW to 4.5 kW, and its measured in-flight thrust is 278.8 ± 0.4 millinewtons (3σ) at full power and 65.0 millinewtons at 1.0 kW. The mission team's own words: “thrust for each thruster was measured across the full range of operating powers and was in good agreement with pre-flight expectations.” Checkout alone ran 357 hours of thrusting and consumed 18.5 kg of a 1085.5 kg xenon load, against a design total impulse requirement of 16.5 MN·s. Established The electric propulsion does all of it — cruise, momentum management, science-orbit transfers and orbit maintenance. This is not a technology demonstration bolted to a chemical spacecraft; it is the spacecraft's propulsion.

Established The one anomaly is worth carrying, because it is what a real anomaly looks like when a working field encounters one. Thruster B showed a burn-in “mode-hopping” phase lasting 120 hours against an expected 24 based on prior flight history, traced to the discharge transitioning between two plasma modes, and mitigated by reducing discharge power and adjusting magnet current — with specific impulse unchanged. Frontier It was measured, published, diagnosed and fixed, and nobody described it as a breakthrough. Hold that pattern in mind for section 4.

Established The next generation is qualified rather than flown, and it sets the number the rest of this cluster should be quoted against. NASA Glenn, JPL and Aerojet Rocketdyne's 12-kilowatt Advanced Electric Propulsion System Hall thruster throttles from 300 V at 6 kW to 600 V at 12 kW, delivering 586 millinewtons at 2,736 seconds at the qualification point, with over 8,000 hours of development testing accumulated and a qualification unit approaching a 4,500-hour wear test, flight thrusters delivering from 2025. Established 586 millinewtons at 12 kW is 48.8 millinewtons per kilowatt, and it comes with a wear test, a propellant supply chain and a delivery schedule. That is the thing a mission planner would otherwise buy, and every propellantless claim in this cluster should be quoted against it as well as against the photon limit of 3.34 micronewtons per kilowatt.

Established Above that power the thruster stops being the problem, and the highest-power case has just been measured. JPL fired a lithium-fed magnetoplasmadynamic thruster prototype at up to 120 kilowatts across five ignitions on 24 February 2026, in a 26-foot water-cooled vacuum chamber, with the central tungsten electrode glowing above 5,000 °F — over 25 times the power of the thrusters on Psyche. Established And the laboratory's own framing of what is missing is entirely about power and endurance rather than physics: a Mars mission would need 2 to 4 megawatts and thrusters running more than 23,000 hours, and magnetoplasmadynamic propulsion has been researched since the 1960s and never flown operationally.

Frontier VASIMR is the vendor case and should be read as one, with the endurance result separated from the flight case. Ad Astra Rocket Company reports 88 hours of continuous operation at 80 kW in thermal steady state on 16 July 2021, with earlier benchmarks of 4,900 seconds specific impulse and 70% thruster efficiency at 200 kW on argon and a second-stage power processing unit at 98.3% radio-frequency efficiency. Established The company's own paper concedes it must overcome critical design challenges before a flight demonstration can even be designed, with transient radio-frequency issues at high power still under investigation — and states no thrust value at all. An 88-hour run at 80 kW is a real result; a flight case it is not.

Established What the power system costs is where this brief's dependency becomes quantitative, and the National Academies state it against the programmes' own interests. A 1 to 2 MWe nuclear-electric vehicle needs 20 to 35% thermal-to-electric conversion; converters of 200 to 800 kWe each at turbine temperatures of 1,100 to 1,200 K requiring at least superalloys or refractory metals; 1,500 to 3,000 square metres of single-sided radiating area at 500 K or above for two to four years; and a total system specific mass of 20 kg/kWe or less, of which electric propulsion gets under about 4.5 and everything else 15. Established Their findings are blunt: “an integrated technology development program aimed specifically toward an NEP system operating at more than 1 MWe has not been undertaken”; “radiation-hardened power electronic systems for PMAD or PPUs at megawatt electric power levels have never been developed”; high-power thruster test facilities are limited to under 50 kWe and it is unclear whether facilities exist to test a full heat-rejection subsystem at all; and “it is unclear if even an aggressive program would be able to develop an NEP system capable of executing the baseline mission in 2039”.

Established The ladder is the clearest way to carry the dependency, and it is three numbers. Flight thrusters operate at 4.5 to 7 kWe. A 100 kWe ion thruster would need an increase in grid area of an order of magnitude while holding grid spacings within less than a millimetre. The mission requirement is 1 to 2 MWe. Established The class is power-blocked, and the power is nuclear. That is the dependency in the source's own words, and section 13 records it rather than describing it.

Established Then the member that the framing gets right, and it has the most interesting measurement in the brief. A conductor moving through a planetary magnetic field develops a motional electromotive force of about 150 volts per kilometre in low Earth orbit; close the circuit through the ionospheric plasma and the resulting current crossed with the field gives a Lorentz force. Run it passively and you get drag — deorbit for free. Drive current against the electromotive force with onboard power and you get thrust with no propellant at all. The concept's own originators count 18 suborbital and orbital tether flights since 1967, of which 8 were electrodynamic; NASA's ProSEDS was built but never launched. Established The idea that made it practical is the bare tether: leave the conductor uninsulated and the positively biased segment collects electrons over its whole length “as a giant cylindrical Langmuir probe”, so no large end collector is needed, and thin foils of equal perimeter perform equivalently to round wires at much lower mass.

Established The one big flight produced a surprise in the direction nobody remembers. On TSS-1R in February 1996 the satellite current varied approximately as the square root of potential from below 10 V to nearly 1,200 V in controlled charging events, reaching as much as 1,400 V during the tether failure event. And the finding that matters: “the collected current exceeded premission expectations, based on the Parker and Murphy [1967] collection model, by factors of two to three”, across 18 current-voltage sweeps. Frontier Put that beside the EmDrive. A real electrodynamic effect, measured in orbit, came in two to three times better than theory predicted, was published in a mainstream geophysics journal within two years, and nobody called it a breakthrough — they revised the collection model. That contrast is what this brief owes the rest of the cluster.

Established The commercial line today is deorbit rather than thrust, and its headline result needs a correction that is usually left out. Tethers Unlimited's Terminator Tape is a burn-wire-released 70-metre conductive tape in two modules at 0.808 kg and 0.083 kg; the 71 kg Prox-1 flew one in 2019, giving 10.5 square metres of drag area. DragRacer was the controlled experiment: two satellites launched November 2020 into a 500 km orbit, Alchemy with the tape and Augury without, with Alchemy deorbiting in July 2021, eight months after launch, against a predicted eight to ten years for its twin. Established But AIAA's own magazine credits that outcome to “passive electrodynamic and aerodynamic drag interactions” — and at 500 km, aerodynamic drag on 10.5 square metres of deployed area is not a rounding error. The tether's deorbit case is strong; attributing this specific result purely to electrodynamics overstates it, and this brief says so rather than repeating it.

Frontier The European line treats propellantless deorbit as a product. E.T.PACK, a 45-month EU FET-Open project of about €3 million from March 2019 led by Sener Aeroespacial and Universidad Carlos III de Madrid, packs a 500-metre tape tether into a 12U cubesat volume, and projects a commercial kit under 35 kg for a 700 kg spacecraft deorbiting from 800 km sun-synchronous in under 1.5 years, with the demonstrator re-entering from 600 km in under 100 days. Electron emission uses a heaterless hollow cathode plus passive emission from a low-work-function electride coating, chosen because the power budget is tight. Speculative These are projections rather than flight results, and as of the sources this brief could obtain the demonstration had not flown.

Established Hypothesis one is the mainstream and the National Academies' chapter is effectively its statement: the class is mature and power-limited, and nothing is missing but electricity. Supported by the ladder in section 2 — 4.5 to 7 kWe flying, 100 kWe requiring an order of magnitude more grid area at sub-millimetre spacings, 1 to 2 MWe required by the mission. Frontier What would falsify it is a thruster-side limit appearing before the power-side one, and nothing in the fetched record suggests that.

Frontier Hypothesis two, and the one the framing gets right: electrodynamic tethers are genuinely underused, and the deorbit market will pull them into propulsive use. Held by the bare-tether originators' group, the E.T.PACK consortium and the commercial deorbit line. Evidence for: the bare-tether mass advantage, and eight months against eight to ten years in a controlled two-satellite comparison. Frontier Evidence against: no thrust-producing tether mission has ever flown, and TSS-1R broke at 1,400 volts.

Frontier Hypothesis three: magnetoplasmadynamic propulsion is the right answer for megawatt-class cargo and has been waiting on reactors since the 1960s. The 2026 firing at 120 kilowatts supports the thruster half. The 2 to 4 megawatt and 23,000-hour requirements are unmet by the laboratory's own statement, which is a rare case of a proponent publishing the gap rather than the promise.

Speculative Hypothesis four: VASIMR's variable specific impulse is worth its complexity. Ad Astra's position, backed by an 88-hour run at 80 kW and unbacked by any flight. Speculative The standing sceptical counter — that the power system dominates and the variable-impulse advantage is marginal — has been argued for two decades, and this brief could obtain no fetchable statement of it. That absence is reported rather than papered over, because a hypothesis space with one side missing is worse than one that says which side is missing.

Speculative Hypothesis five: magnetic sails and plasma magnets give propellantless interplanetary cruise once high-field superconducting coils are cheap. The line runs from the original superconducting-loop magnetic sail through mini-magnetospheric plasma propulsion, which injects plasma to inflate the magnetic interaction region, to the magneto-plasma sail. In the pure limit the momentum comes from the solar wind and no propellant is carried at all. Frontier This brief quotes no performance numbers for any of them, because the review it relies on resolved to abstract level and no coil currents, radii or thrust values could be obtained. Stating that is more useful than repeating figures whose provenance this page cannot vouch for.

Speculative Hypothesis six: the binding constraint on tethers is institutional rather than technical — nobody wants a kilometre of conductor on their spacecraft. Nobody's stated position. It is consistent with 18 flights in 59 years and a 1993 concept still unflown propulsively, and it is the kind of explanation that is easy to assert and hard to test.

Frontier Hypothesis seven: test-facility capacity rather than thruster physics sets the pace. Well evidenced and rarely argued: high-power thruster testing is limited to under 50 kWe, and it is unclear whether facilities exist to test a full heat-rejection subsystem at all. Speculative Hypothesis eight: propellant substitution and megaconstellation volume will drive this class's economics more than any performance advance. Plausible — the xenon figures make the cost argument visible — but this brief fetched no source that measures it, and it is flagged as a gap rather than asserted.

Established The bottleneck is electrical power, and the National Academies name every layer of it rather than the thruster. Reactor, thermal-to-electric conversion at 20 to 35%, converters at 1,100 to 1,200 K needing superalloys or refractory metals, radiators of thousands of square metres, power management and distribution, and test facilities. Established Radiation-hardened power electronics at megawatt levels have never been developed, which is a component gap rather than a physics one and is the least-discussed item on the list.

Established The second is endurance, and it is measured in tens of thousands of hours rather than in watts. A Mars-class magnetoplasmadynamic mission needs more than 23,000 hours of running; the best-supported qualification wear test in this brief is 4,500 hours on a 12 kW Hall thruster with 8,000 hours of development testing behind it. Frontier The gap between those numbers is a facility-time problem, and facility time at high power is the scarcest resource in the field.

Established The third is grid and geometry scaling, which is the one place the thruster itself is the obstacle. Taking an ion thruster to 100 kWe means an order of magnitude more grid area while holding grid spacings within less than a millimetre — a mechanical tolerance problem across a much larger structure, at temperature, after launch.

Frontier And the tether's bottlenecks are entirely different in kind, which is why it sits in a separate section. The physics is not doubted; the obstacles named in the concept's own review are dynamics and survivability — tether heating, arcing, structural bowing and breaking, deployment complexity, and for very thin foils a need for spin stabilisation. Established TSS-1R itself ended when the tether failed at 1,400 volts. The technology's problem is that it is a long thin thing in a debris environment.

Established This brief records one dependency and it is quantitative rather than rhetorical. Advanced Nuclear Propulsion owns the power source: enrichment supply, ground test stands, launch approval and appropriations. This brief owns the consequence, which is the ladder — 4.5 to 7 kWe flying today, against a 1 to 2 MWe requirement, with an intermediate 100 kWe step that needs an order of magnitude more grid area at sub-millimetre spacings.

Established The dependency is a supplier relationship, not a research one. Nothing about the thrusters waits on a discovery; a 120 kW magnetoplasmadynamic thruster has been fired and an 80 kW plasma engine has run for 88 hours. What waits is a reactor with a qualified fuel form, and that fuel form's materials question belongs to Exotic Materials for Propulsion.

Frontier Two further dependencies are real and are not briefs. High-field superconducting coils gate magnetic sails, and the 20 tesla at 20 K result belongs to High-Temperature Superconductors. Heat rejection gates everything above a few hundred kilowatts, and the panel material, working fluid and 550 K heat-pipe wall belong to Exotic Materials for Propulsion; this brief carries only the system-level area and specific-mass numbers.

Established The decisive experiment for the underused member is a thrust-producing tether mission, and it has never been flown. Drive current against the motional electromotive force with onboard power, in low Earth orbit, and measure the resulting orbit raising against the current and the field. The physics was measured in 1996; the propulsive mode has been described since 1993 and never demonstrated in flight.

Established The second is a megawatt-class integrated power and propulsion demonstration, and the National Academies say it has not been attempted. Not a thruster test and not a reactor test but the integrated system: conversion, power management, radiators and thrusters, run together for a mission duration. Frontier The facility question comes first — high-power thruster testing is capped at under 50 kWe and the heat-rejection subsystem may have no test facility at all.

Frontier The third is an endurance run at the requirement rather than at the budget. 23,000 hours is roughly two and a half years of continuous operation; the qualification standard in this brief is 4,500. Closing that is facility time and money, not invention, and it is the single clearest example in the cluster of a real technology waiting on a boring input.

Speculative And the fourth is a magnetic sail of any size at all. There is no flight, no flight path, and no fetchable performance figure. The enabling condition is a high-field superconducting coil at spacecraft mass, which now exists in the laboratory at 20 tesla and 20 K; what does not exist is any programme that would fly one.

Established The engineering requirements for the mature members are written as qualification campaigns, and that is what maturity looks like. The 12 kW Hall thruster's specification is not its 586 millinewtons but its 4,500-hour wear test with 8,000 hours of development testing behind it, throttling from 300 V at 6 kW to 600 V at 12 kW. Established A flying mission's specification is a xenon throughput of over a tonne against an erosion life, and a total impulse requirement of 16.5 MN·s.

Established The power-system requirements are the ones that decide whether a megawatt vehicle closes. 20 kg/kWe total specific mass, of which under about 4.5 goes to electric propulsion and 15 to everything else; converters of 200 to 800 kWe at 1,100 to 1,200 K; thousands of square metres of radiator at 500 K or above for two to four years. Frontier Radiation-hardened megawatt-class power electronics is the component line with no precedent, and it will be the schedule driver whether or not it is the technical one.

Established The tether's requirements are mechanical and electrical rather than thermal. Kilometre-class deployment from a small volume, high-voltage insulation and arc management to well above a kilovolt, electron emission from a heaterless hollow cathode or a low-work-function coating chosen for power budget, and survivability of a long thin conductor in a debris environment. Frontier A 500-metre tape tether in a 12U volume with a projected 35 kg kit for a 700 kg spacecraft is the current specification, and it is projections rather than flight data.

Frontier And the magnetic sail's requirement is a single component that now nearly exists. A high-field superconducting loop at spacecraft mass and cryogenic budget; 20 tesla at 20 K on a large-scale magnet is the enabling datum, and it was demonstrated for fusion rather than for propulsion. Speculative No coil radius, current or thrust figure is given here because none was obtainable.

Established The seam that matters most is with Electrogravitics, and it is drawn on a physical criterion rather than a topical one. Electroaerodynamic thrust — corona discharge accelerating ions that drag neutral air — is electrodynamic in the broad sense, and both briefs would be tempted by the same 2018 flight. The division: ion wind needs air, so it is atmospheric propulsion, and it belongs entirely to Electrogravitics — the physics, the flight, thrust density, thrust-to-power, the high-voltage converter and its industrial constraint. Established This brief owns everything that works in vacuum, and carries no ion-wind numbers at all. The clean formulation for both pages: this is the vacuum half of electrodynamic propulsion; that is the atmospheric half, plus the discredited gravitic claim that shares its name.

Established Three further boundaries are worth stating because each divides a shared object. Advanced Nuclear Propulsion owns the reactor, the fuel supply and the approvals; this brief owns the consequence for the thruster. Solar Sail Systems owns photon sails, which are radiation pressure rather than electrodynamics; magnetic sails belong here. Fusion Spacecraft owns confinement; the magnetic-nozzle and plasma physics of a thruster stays here.

Frontier And two neighbours borrow from this brief rather than the reverse. Reactionless Propulsion lists the real propellantless technologies as a signpost and takes its benchmark number from here; Mach Effect Thrusters uses the 48.8 millinewtons per kilowatt figure as the standard a claimed propellantless thrust would have to match. Established This brief supplies the number and the contrast, and does not re-run their arguments.

Established The institutional finding here is the opposite of the rest of the cluster and it is the most transferable thing on the page: this is what a field that is working looks like from the outside. Psyche's thrust agreed with pre-flight expectation and its disagreement — 120 hours of mode-hopping against an expected 24 — was published, diagnosed and mitigated. TSS-1R collected two to three times more current than the model predicted and the response was a better model. Established A field that is working produces anomalies constantly and absorbs them.

Frontier That is the contrast the whole contested cluster needs, and it should be stated as a criterion rather than as a mood. The distinguishing feature of the claims in Reactionless Propulsion, Electrogravitics and Mach Effect Thrusters is not that they produced anomalies. It is that their anomalies never became instruments, wear tests or revised models.

Established The most useful institutional document in this brief is an adversarial consensus study. The National Academies assess programmes their sponsors want to fly and report that megawatt-class integrated development has not been undertaken, that megawatt power electronics have never been developed, that test facilities cap out below 50 kWe, and that it is unclear whether even an aggressive programme could deliver the baseline mission on schedule. Frontier Four findings against interest in one chapter is a stronger evidentiary posture than any single measurement in this brief.

Speculative The tether case is where the institutional explanation is most plausible and least tested. A demonstrated physics, a favourable mass budget, a commercial deorbit product already flying, and no propulsive mission in 33 years. The technical obstacles named by the field — deployment, arcing, debris, dynamics — are real, and so is the observation that mission designers do not want a kilometre of conductor trailing behind an expensive spacecraft. Frontier Which of those dominates is not settled by anything this brief could obtain.

Frontier And the vendor question deserves an explicit rule rather than a case-by-case judgement. A company paper on the company's own engine, hosted by the company, is evidence of what was run and not of what will fly — and this brief treats the 88-hour endurance result as real while treating the flight case as the vendor's, on the vendor's own concession that critical design challenges remain.

Established The direct ethical questions in this class are mundane, and that is worth saying in a cluster where they usually are not. Electric propulsion is flying at scale on commercial constellations; its externalities are debris, spectrum and orbital congestion rather than anything exotic. The tether's most mature product exists specifically to reduce debris.

Frontier The debris argument is genuinely two-sided and should be stated that way. A conductive tape that takes a satellite from a predicted eight-to-ten-year decay to eight months is a substantial debris reduction. A 70-metre or 500-metre conductor deployed in a crowded orbit is also a large collision cross-section for its deployment duration, and tether severance is a documented failure mode. Speculative Whether the net effect is positive depends on deployment reliability figures this brief could not obtain.

Established The nuclear dependency imports a real governance load and this brief does not own it. Megawatt-class electric propulsion means a reactor, and launch approval, safeguards and ground-test containment come with it. Those belong to Advanced Nuclear Propulsion; the only point this brief adds is that the propulsion case for a reactor is stronger than the popular framing suggests, because the thrusters are already qualified and waiting.

Frontier And one epistemic obligation falls on this brief specifically. The DragRacer result is quoted throughout the tether literature as an electrodynamic success, and its own reporting attributes it to electrodynamic and aerodynamic drag. Established Repeating the stronger version because it favours a technology this brief thinks is underused would be exactly the failure mode the rest of the cluster is about.

Established Electric propulsion has already reshaped which missions are affordable, and it did so without being noticed. A flagship asteroid mission whose entire propulsion budget is four Hall thrusters and a tonne of xenon is a different economic object from a chemical spacecraft, and the change happened over roughly two decades of incremental qualification rather than in a breakthrough.

Frontier The megawatt step is the one that changes mission classes rather than mission costs. 2 to 4 megawatts with 23,000-hour thrusters is fast in-system cargo and crew transport; the thrusters for it have been fired and the reactor has not been built. Speculative That is a rare configuration — a transformative capability whose remaining obstacle is a power source somebody has to decide to fund.

Frontier The tether's civilizational case is smaller and more certain. Propellantless deorbit at tens of kilograms for a 700 kg spacecraft, if the reliability holds, makes end-of-life disposal a line item rather than a mission-design constraint, and that matters more for the sustainability of low Earth orbit than any thrust application would.

Established And the brief's most useful contribution to the rest of the map is a single number. 48.8 millinewtons per kilowatt, wear-tested and on a delivery schedule. It sits four orders of magnitude above what the Mach-effect community claims and four orders below what the EmDrive claimed. Frontier The real technology sits between the two fantasies, and that is the most compact statement of this cluster's shape available anywhere on this map.

These horizons track power, qualification campaigns and facility capacity rather than physics, because in this class the physics is not the open question:

• 10 yr: Established Expect 12 kW Hall thrusters in service and the class's flight population to keep growing on commercial constellations; nothing here waits on a result. Frontier Expect deorbit tethers to become routine before thrust tethers exist at all, and expect the propulsive tether mission to remain unflown unless a specific customer appears. Speculative A megawatt-class integrated demonstration inside ten years would require a programme that the National Academies say has not been undertaken.
• 25 yr: Frontier The decisive question is whether a space reactor above a megawatt gets built; the thrusters, at 120 kW demonstrated and 80 kW run for 88 hours, are ahead of it. Frontier Facility capacity is the quiet constraint — testing above 50 kWe and testing a full heat-rejection loop are capital decisions that gate everything above. Speculative A first thrust-producing tether mission is plausible at this horizon and has been plausible for thirty years, which is itself the evidence for the institutional reading.
• 50 yr: Speculative If megawatt power arrives, magnetoplasmadynamic and high-power plasma thrusters become the workhorses for cargo, and the 23,000-hour endurance requirement becomes the qualification problem of the era. Speculative Magnetic sails remain the most likely member to go from concept to flight without an intermediate step, because their enabling component — a high-field superconducting coil — is being developed for reasons that have nothing to do with propulsion.
• 100 / 250+ yr: Speculative Beyond useful forecasting for specific hardware, but the structural statement holds: this class scales with available electrical power and nothing else, so its long-run trajectory is the trajectory of in-space power generation. Established That is an unusually clean forecast for this map, and it is clean precisely because none of the open questions here are about whether the effect exists.

• Depends on One edge, and it is quantitative rather than rhetorical. This class waits on Advanced Nuclear Propulsion, and the ladder makes the abstraction concrete: flight thrusters operate at 4.5 to 7 kWe; a 100 kWe ion thruster needs an order of magnitude more grid area while holding grid spacings within less than a millimetre; the mission requirement is 1 to 2 MWe. The National Academies state the gap in their own words — an integrated development programme for an NEP system above 1 MWe has not been undertaken, radiation-hardened power electronics at megawatt levels have never been developed, and high-power thruster test facilities are limited to under 50 kWe. The thruster is not the obstacle: a lithium-fed magnetoplasmadynamic prototype fired at 120 kilowatts in 2026 and a plasma engine ran 88 hours at 80 kW in 2021. What is missing is the reactor, and its unqualified fuel form is a materials question owned by Exotic Materials for Propulsion.
• Enables Everything in-system that is mass-limited rather than time-limited: Asteroid Mining and Space-Based Manufacturing price their logistics in this class's specific impulse, and Deep Space Infrastructure assumes it. The tether member enables something narrower and more certain: propellantless end-of-life disposal at tens of kilograms per spacecraft, which is an orbital-sustainability capability rather than a propulsion one. This brief also supplies the cluster's benchmark number — 48.8 millinewtons per kilowatt, wear-tested and scheduled — which Reactionless Propulsion and Mach Effect Thrusters both quote against their claims.
• Adjacent Plasma physics, ionospheric physics and high-voltage engineering supply the mechanisms; thermal management and power electronics supply the constraints. Within this map: Electrogravitics owns the atmospheric member entirely — ion wind needs air — while this brief owns everything that works in vacuum; Solar Sail Systems owns photon sails and this brief owns magnetic ones; High-Temperature Superconductors supplies the coils a magnetic sail would need; and Fusion Spacecraft shares the plasma vocabulary and almost nothing else.

Established “Electrodynamic propulsion is an underused class.” This is the framing under test and it is half wrong. Electric propulsion is the default — Psyche's entire primary propulsion, cruise and momentum management and orbit transfers and maintenance, is four Hall thrusters sized for 16.5 MN·s of total impulse on 1085.5 kg of xenon, measured in flight at 278.8 ± 0.4 millinewtons. Frontier The defensible version is narrower: electrodynamic thrusters are heavily used and power-limited; electrodynamic tethers are underused relative to a demonstrated physics and a favourable mass budget, and the reason is engineering risk rather than doubt about the effect.

Established “Electric propulsion is futuristic.” It flies on flagship science missions and on commercial constellations, and its next-generation unit is qualified with a 4,500-hour wear test and a delivery schedule. Established Low thrust is not the same as immature; 586 millinewtons at 2,736 seconds is a specification, not a projection.

Established “The thruster is the bottleneck at high power.” It is not. The bottleneck is reactor, conversion, radiator, power management and test facilities — the National Academies name all five. Established A 120 kilowatt magnetoplasmadynamic firing and an 88-hour run at 80 kilowatts are both on the record; a megawatt space reactor is not.

Established “MPD or VASIMR is close to flight.” Magnetoplasmadynamic propulsion has never flown operationally and needs roughly 20 to 30 times more power and about 200 times more endurance than has been demonstrated, on its own laboratory's statement. Established VASIMR's vendor paper concedes that critical design challenges must be overcome before a flight demonstration can be designed, and states no thrust value.

Established “Electrodynamic tethers are unproven physics.” TSS-1R measured the current-voltage characteristic in orbit across 18 sweeps, from below 10 V to nearly 1,200 V, and found collected current exceeding the standard collection model by factors of two to three. Frontier The physics came in better than predicted and the field revised the model — which is precisely the response that the contested claims elsewhere in this cluster never produced.

Established “DragRacer proved electrodynamic deorbit.” The comparison is real and striking — eight months against a predicted eight to ten years for an identical satellite without the tape — but AIAA's own magazine attributes it to “passive electrodynamic and aerodynamic drag interactions”, and at 500 km the aerodynamic contribution on 10.5 square metres of deployed area is not negligible. Frontier The deorbit case for tethers is strong; the attribution of this particular result purely to electrodynamics is not, and this brief will not repeat it.

Established “A tether has flown as a thruster.” No thrust-producing electrodynamic tether mission has flown. Deorbit drag has, commercially. Established The propulsive mode — driving current against the motional electromotive force with onboard power — has been described since 1993 and demonstrated in orbit never.

Speculative “Magnetic sails have demonstrated performance.” They have not flown, and this brief could obtain no performance figures at all — no coil currents, no radii, no thrust values. Established That absence is stated rather than filled in, and it is the honest reason this member sits at speculative while the tether sits at frontier.

Established “Ion wind is part of this class.” It is electrodynamic in the broad sense and it needs air, which makes it atmospheric propulsion. Established It belongs entirely to Electrogravitics, where it is the real technology sharing that brief's name — the physics, the 2018 flight, thrust density, thrust-to-power and the high-voltage converter. This brief carries none of those numbers by design.

Established And “anything in this class is reactionless.” Nothing is. Every member expels ions or pushes on a real external field — a planet's magnetosphere, the solar wind, the ionospheric plasma. Frontier The tether is the sharpest illustration: it carries no propellant and it is emphatically not reactionless, because the equal and opposite momentum goes into the Earth. That distinction is the whole of Reactionless Propulsion, and this brief is the evidence for its positive half.