The nozzle for my MPD
thruster was probably one of the most straight-forward components to
design for this project. The basic requirements for this component were
that it could support tremendous currents flowing between the two
electrodes, that it allowed for high propellant flow rates, and that
its geometry allowed for the generation of high levels of thrust. First, let's look
at the requirements for generating high levels of
thrust. The equation that describes the thrust output of this device is
From a MPD Thruster
F is the
thrust in Newtons, J is the
total current, ra is the radius to the inside of the anode,
and rc is the radius of the cathode, and the general
configuration of the nozzle is as shown below.
1: Nozzle Schematic,
exact form of equation 1 isn't critical at this point, but let's note
that the total thrust developed by the nozzle is governed by a) the
ratio of anode to cathode radius and b) the total current flowing
through the nozzle. One possible approach to this problem is to choose
a cathode radius large enough to withstand the large currents flowing
through it, and then set the anode radius as large as is reasonably
feasible. I chose a 0.25" diameter Tungsten cathode due to Tungsten's
ability to withstand very high temperatures and my inability to find
anything larger at a reasonable price. The ratio ra / rc
was set to about 3.5, so the anode radius worked out to .45". This
ratio was chosen primarily to keep the distance between the cathode and
anode small enough to allow the ignition circuit to work.The
of the nozzle involves three major parts: a Lexan
mounting plate, a Tungsten cathode with mounting terminals, and a large
copper anode assembly. These three parts are shown individually below
with mounting hardware.
Fig. 2: Nozzle
composed of two sections of copper soldered together with
a gas injection port and terminal blocks.
Fig. 3: Rear of
The two aluminum
terminal blocks are used to make the electrical
connection to the anode. The threaded hole on the right of the nozzle
is the gas injection port. The entire assembly sits flush against the
Lexan mount, which is also the insulator that separates the anode and
Fig. 4: (a) Lexan
and (b) Mount with Cathode Attached.
Figure 4a shows
the Lexan mount. The cathode is press-fit into the
copper bar on the rear of the mount through the hole in the center of
the mount. The anode then sits snuggly in the grove around the cathode,
and is bolted in to place.
5: (a) Front View of
Nozzle Assembly (b) Side View of Nozzle Assembly.
the anode mounted to the Lexan mount, and figure 5b
shows all of the electrical and mechanical connections, including the
anode mounting bolts, the gas injection port, and both pairs of
electrical connections to the electrodes, as well as a portion of the
Tungsten cathode protruding from the copper bar into which is it
press-fit. The entire nozzle assembly is shown below.
Fig. 6: Nozzle
Mounted to Track Cart w/Electrical and Gas Connections
 R. G.
of Electric Propulsion, Mineola:
Dover Publications, Inc., 2006,
[ Back to MPD
[ Back to Main
Comments? Suggestions? E-Mail me at MyElectricEngine@gmail.com
Copyright 2007-2010 by Matthew Krolak - All Rights
Don't copy my stuff without asking first.