Colloid Thruster — Taylor Cone Electrospray Propulsion

Extractor Voltage

V_b (emitter → extractor) 1200 V

Propellant Emission Mode

Ionization fraction 40%

Mass Flow Rate

ṁ per emitter 0.60 µg/s

Emitter Array

Below onset field — no emission

Beam Readouts

Exhaust velocity

— km/s

Specific impulse

— s

Total thrust

— µN

Beam current

— µA

How it works

An electrospray (colloid) thruster holds a conductive propellant — typically an ionic liquid like EMI-BF₄ — at the tip of a sharp emitter. A strong electric field pulls the liquid meniscus into a cusped Taylor cone once the local field exceeds the onset value where electrostatic stress overcomes surface tension:

E_onset = √(2γ / (ε₀ r_tip))

γ  = propellant surface tension (≈ 0.045 N/m for EMI-BF₄)
ε₀ = vacuum permittivity
r_tip = emitter tip radius of curvature (≈ 2 µm)

Above onset, the cone apex sheds either charged droplets or, at higher field/lower flow, individual solvated ions. Whichever species leaves, it is accelerated across the extractor voltage V_b and gains kinetic energy equal to its charge times that voltage:

q·V_b = ½ m v²   →   v = √(2 (q/m) V_b)

F = ṁ · v            (thrust)
I_beam = ṁ · (q/m)   (emitted current)
I_sp = v / g₀        (specific impulse)

Extractor voltage — raises the field at the tip and the exhaust velocity of every emitted particle; below onset the cone collapses and emission stops.
Ionization fraction — interpolates the charge-to-mass ratio from the droplet-dominated regime (q/m ≈ 10³–10⁴ C/kg, low velocity, high thrust-per-current) toward the pure-ion regime (q/m ≈ 10⁶ C/kg, high v_e, high I_sp) — the same trade every real colloid thruster designer makes.
Mass flow rate — sets how much propellant each emitter sheds per second; more flow means more thrust but a lower specific impulse for the same voltage.
Emitter array — real colloid thrusters (e.g. MIT/Busek/ESA designs) multiplex tens to hundreds of microfabricated emitter tips in a hex-packed array to scale thrust beyond what a single Taylor cone can provide.

Colloid thrusters produce only micronewtons to millinewtons of thrust but at very high precision and Isp of 500–2000 s, which is why they fly on missions needing exquisite pointing control, such as ESA/NASA's LISA Pathfinder drag-free formation.

Charged spray Taylor cone / emitter Extractor grid

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.