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Micropropulsion Technologies 259

11.3.2 DIGITAL PROPULSION
Digital propulsion is a very compact and low-mass system, which relies on MEMS
fabrication to provide a feasible propulsion device for small spacecraft. 43–47 A digital
propulsion system (DPS) consists of a large array of sealed plenums. These plenums
are filled with fuel or an inert substance in gas, liquid, or solid form. A thin diaphragm
acts as the sealant. By igniting the fuel or heating the inert substance the pressure
inside the plenum is increased sufficiently causing the diaphragm to rupture and
release the propellant, producing an impulse. The magnitude of the impulse depends
on the amount and kind of fuel stored inside the plenum.
While this kind of propulsion usually features small specific impulses, the
ability to define the impulse bit by varying the fuel or plenum size and the number
of plenums triggered simultaneously make this propulsion system very attractive.
MEMS technology enables large number of plenums to be placed within a small
area with low mass.

11.3.2.1 Principle of Operation

Typical MEMS-fabricated digital propulsion configurations consist of a three-
layer sandwich. The top layer contains an array of thin diaphragms (of the order
0.5 mm thick silicon nitride). The middle layer contains an array of through-holes
(often used: Schott FOTURAN 1 photosensitive glass, 1.5 mm thick, 300, 500, or
700 mm diameter holes), which are loaded with propellant. The bottom layer
employs a matching array of polysilicon microresistors for propellant heating and
fuel ignition. The bottom two layers are bonded together and then fueled. The top
layer is bonded to complete the assembly as shown in Figure 11.20.
Once current is run through a microresistor underneath the plenum, heat is
generated, which ignites the fuel (e.g., lead styphnate). The ignition increases the
pressure in the plenum until the membrane ruptures and the gas inside is released to
produce a single impulse bit. Typical pressures reach values of a few MPa. The

Top die
Diaphragms on bottom
expansion nozzles on top
Middle die

Propellant fills individual holes
Bottom die

Polysilicon “ignitors” with direct
inter-connects to bond pads (no electronics)
FIGURE 11.20 Layout of digital propulsion thruster.

© 2006 by Taylor & Francis Group, LLC