16.522, Space Propulsion
-25: COLLOIDAL ENGINES
Colloidal thrusters are electrostatic accelerators of charged
They were first proposed and then intensively studied from around 1960
to 1975 as an alternative to normal ion engines. Their appeal at that time rested with the
large “molecular mass” of the droplets, which was known to increase the thrust density of
an ion engine.
This is because the accelerating voltage is
, where m is the mass
of the ion or droplet, and q its charge, and c is the final speed.
If c is pre-defined (by the
mission), then V can be increased as m/q increases;
this, in turn, increases the space
charge limited current density (as V
), and leads to a thrust density,
(d=grid spacing), which is larger in proportion to V
, and therefore to
addition to the higher thrust density,
the higher voltage also increases efficiency, since
any cost-of-ion voltage V
becomes then less significant
In a sense, this succeeded too well.
Values of droplet m/q that could be generated with
the technology of the 60’s were so large that they led to voltages from 10 to 100 KV (for
This created very difficult insulation and packaging problems,
making the device unattractive, despite its demonstrated good performance.
the droplet generators were usually composed of arrays of a large number of individual
liquid-dispensing capillaries, each providing a thrust of the order of 1
missions then anticipated, this required fairly massive arrays, further discouraging
After lying dormant for over 20 years, there is now a resurgence of interest in colloid
This is motivated by:
(a) The new emphasis on miniaturization of spacecraft.
The very small thrust per
emitter now becomes a positive feature, allowing designs with both, fine
controllability and high performance.
(b) The advances made by electrospray science in the intervening years.
been motivated by other applications of charged colloids, especially in recent years,
for the extraction of charged biological macromolecules from liquid samples, for very
detailed mass spectroscopy.
These advances now offer the potential for overcoming
previous limitations on the specific charge q/m of droplets, and therefore may allow
operation at more comfortable voltages (1-5KV).
With regard to point (a), one essential advantage of colloid engines for very small thrust
levels is the fact that no gas phase ionization is involved.
Attempts to miniaturize other
16.522, Space Propulsion
Prof. Manuel Martinez-Sanchez
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