Increased efficiency arcjet thruster

There is disclosed an anode for an electrothermal arcjet thruster. The anode includes a divergent nozzle having, in tandem, a recombination portion and an expansion portion. The expansion portion has a greater rate of divergence than the recombination portion. A regeneration channel containing flowing propellent gas extends internally within the anode adjacent the recombination portion. The combination of the biangle nozzle and the regeneration channel produces an arcjet thruster having a markedly increased thrust efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to small propulsion systems for maneuvering 
spacecraft and, more particularly, to an electrothermal arcjet thruster 
having an anode with a biangle expansion portion and contoured internal 
regeneration channels to more efficiently convert thermal energy of a 
flowing propellant to kinetic energy. 
2. Description of the Prior Art 
An electrothermal arcjet thruster converts electrical energy to thermal 
energy by heat transfer from an arc discharge to a flowing propellant. The 
thermal energy is converted to directed kinetic energy by expansion of the 
heated propellant through a nozzle. 
Most electrothermal arcjet thrusters have as common features an anode in 
the form of a nozzle body and a cathode in the form of a cylindrical rod 
with a conical tip. The nozzle body has an arc chamber defined by a 
constrictor in a rearward portion of the body and a nozzle in a forward 
portion thereof. The cathode rod is aligned on the longitudinal axis of 
the nozzle body with its conical tip extending into the upstream end of 
the arc chamber in spaced relation to the constrictor so as to define a 
gap therebetween. 
When a sufficiently high current is applied, an electric arc is initiated 
between the cathode rod and the anode nozzle body at the entrance to the 
constrictor. The arc is then forced downstream through the constrictor by 
pressurized vortex-like flow of a propellant gas introduced into the arc 
chamber about the cathode rod. The arc stabilizes and attaches at the 
nozzle. The propellant gas is heated in the regions of the constrictor and 
of arc diffusion at the mouth of the nozzle downstream from the 
constrictor. The super-heated gas is exhausted out from the nozzle to 
achieve thrust. 
Historically, propellants, such as ammonia or hydrogen, have been used in 
electrothermal arcjet thrusters. More recently, hydrazine (N.sub.2 
H.sub.4) has been used. Propellants such as ammonia and hydrazine are 
preferred because these propellants are storable as a liquid without 
refrigeration while cryogenic propellants such as hydrogen and helium are 
not. The liquid storable fuels are converted to a gaseous propellent by 
passing the fuel through a gas generator. 
The specific impulse (I.sub.sp) determines the propellant mass required to 
complete a flight. I.sub.sp is denoted in pounds of force-second per pound 
of mass. The generation of a high I.sub.sp in an arcjet thruster requires 
operation of the thruster at a high specific energy (as denoted in 
watts/kg). The cryogenic propellants have a typical I.sub.sp value of up 
to 1,500 lbf-sec/lbm. The liquid storable propellants have a much lower 
specific impulse, on the order of 800-1000 lbf-sec/1 bm. 
One way to increase I.sub.sp is to increase the thrust efficiency of the 
arcjet thruster. U.S. Pat. No. 5,111,656 to Simon et al, discloses 
increasing the specific impulse of a propellant by a unique nozzle 
configuration. The exhaust portion of the nozzle has a divergent 
recombination portion in tandem with a divergent expansion portion. The 
divergence of the recombination portion is less than that of the expansion 
portion, causing a temporary delay in the pressure reduction of the 
propellant gas. This delay creates a relatively high pressure region in 
the recombination portion of the nozzle permitting a partial recombination 
of the ionized and neutral species of the propellant gas and a partial 
recovery of frozen flow losses back into the gas. 
Frozen flow losses reduce the efficiency of an arcjet thruster. Frozen flow 
losses include ionization, disassociation and deposition of energy into 
excited molecular and atomic states. These losses occur when the 
propellant gas is heated to very high temperatures by close contact with 
the electric arc and is then exhausted out the nozzle. If the propellant 
dwells for insufficient time in high pressure regions, the gas does not 
have time to recombine the ions or disassociated molecules or to relax the 
excited states. Energy locked into these processes is lost and unavailable 
for thrust. 
U.S. Pat. No. 5,111,656 is incorporated by reference in its entirety 
herein. The biangle nozzle disclosed in that patent increases the 
efficiency of the electrothermal arcjet thruster at low power levels by 
reducing frozen flow losses. However, the nozzle also generates more heat 
at the anode surface and, as the energy level (power/mass flow rate) of 
the thruster increases, the advantage over a single angle nozzle 
decreases. At relatively high specific energy levels, the efficiency of a 
biangle nozzle is inferior to that of a single angle nozzle. 
The biangle nozzle converts more chemical energy to thermal energy than a 
single angle nozzle. When operating at high specific energy, most of this 
extra energy in the form of heat is lost by conduction into the anode or 
exhausted out the nozzle and does not assist in improving the efficiency 
of the electrothermal arcjet thruster. 
It is known to preheat a propellant gas by flowing the gas through 
regeneration channels in the anode body prior to exposure to the electric 
arc. The anode body is heated by the plasma arc and by the disassociating 
propellant gas. A portion of this heat is recaptured by the propellant gas 
flowing through the regeneration channels. Preheating the propellant gas 
forms a more reactive form of propellant gas as disclosed in both U.S. 
Pat. Nos. 4,548,033 to Cann and 4,995,231 to Smith et al, both of which 
are incorporated by reference in their entireties herein. 
The regeneration channels may assist in cooling the anode, reducing thermal 
stress on the thruster materials and extending thruster life. However, 
since the propellant gas has a lower coefficient thermal conductivity than 
the anode body, the regeneration chambers may also constitute a thermal 
insulator. 
There exists, therefore, a need for a biangle nozzle for an electrothermal 
arcjet thruster that provides increased thrust efficiency at relatively 
high specific power levels. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an anode nozzle 
design for an arcjet thruster that has increased thrust efficiency. It is 
a feature of the invention that the anode nozzle comprises the combination 
of a biangle divergent portion with a regeneration system incorporated 
into the anode wall and matched to the biangle contour. 
Among the advantages of the arcjet anode of the invention are that the 
thrust efficiency and specific impulse is increased at relatively high 
specific energy levels. Another advantage is that both frozen flow losses 
and thermal losses are reduced without a corresponding increase in gas 
ionization loss. Still another advantage is that the arcjet thruster 
operates at higher voltages and lower amperages, reducing erosion of the 
walls of the anode. A further advantage is that the anode temperature is 
reduced, further reducing erosion of the anode. 
In accordance with the invention, there is provided an anode for an arcjet 
thruster. The anode has an electrically conductive body containing a 
regeneration channel. A converging upstream wall of the body has both an 
inlet and an outlet for the regeneration channel. A constrictor portion 
forms a centrally disposed wall of the electrically conductive body 
defining a cylindrical aperture. A first end of the constrictor portion is 
adjacent to the converging upstream wall portion. An opposing second end 
of the constrictor portion is adjacent a diverging nozzle portion. The 
diverging nozzle portion forms a downstream wall of the electrically 
conductive body. A diverging nozzle portion has, in tandem, a divergent 
recombination portion and a divergent expansion portion with the expansion 
portion having a greater rate of divergence than the recombination portion 
.

DETAILED DESCRIPTION 
FIG. 1 shows in cross-sectional representation an electrothermal arcjet 
thruster 10 as known from the prior art. The arcjet thruster 10 has an 
anode body 12 that is usually formed from an electrically conductive, heat 
resistant metal or metal alloy such as tungsten or a tungsten alloy. The 
anode body 12 may be arbitrarily divided into three portions. A converging 
upstream portion 14 forms one surface of the anode body 12 and directs a 
propellant 16, such as hydrazine, to a constricted portion 18. The 
constricted portion 18 is a cylindrical aperture having a first end 
adjacent the converging upstream portion and an opposing second end 
adjacent a diverging downstream portion 20. The diverging nozzle portion 
20 has, in tandem, a divergent recombination portion 22 and a divergent 
expansion portion 24. The expansion portion 24 has a greater rate of 
divergence than the recombination portion 22. 
A generally rod shaped cathode 26 is positioned within the upstream portion 
in close proximity, but spaced from, the constricted portion 18. A power 
supply 28 establishes a voltage potential between the anode 12 and cathode 
26 generating an electric arc 30. The flowing propellant gas 16 pushes the 
electric arc 30 through the constricted portion 18 to the recombination 
portion 22 where the electric arc 30 attaches to a wall of the anode 12. 
Heated propellant gases are expelled from the expansion portion 24 to 
propel the space craft. 
A portion of the propellant gas is disassociated to ions and neutral 
species without the generation of kinetic energy. The energy lost forming 
these disassociated species is referred to as "frozen flow losses". By 
providing a relatively high pressure area in the recombination portion 22 
of the arcjet thruster 10, a percentage of these dissociated and ionized 
species is recombined and generates thermal energy, thereby increasing the 
thrust achieved by a biangle nozzle over a comparable single angle nozzle. 
When the specific energy demand is increased, as manifest by a decrease in 
propellant 16 flow at constant power, the gain in thrust efficiency 
achieved by the biangle nozzle over a single angle nozzle decreases. As 
graphically illustrated in FIG. 2, at high specific energy levels the 
efficiency of a biangle nozzle is less than that of a single angle nozzle. 
With reference to FIG. 2, constant specific energy levels are denoted by 
the parallel running broken lines. The thrust efficiency (in percent) as a 
function of specific impulse (in seconds) for a conventional single angle 
nozzle is indicated by reference line 32. The same relationship for a 
biangle nozzle is denoted by reference line 34. At low specific energy 
levels, the efficiency of the biangle nozzle is greater than that of the 
single angle nozzle. However, above a threshold crossover specific energy 
level 36, which is different for each propellent and on the order of 35 
MJ/kg for hydrazine, the efficiency of the biangle nozzle is worse than 
that of the single angle nozzle. 
FIG. 3 illustrates in cross-sectional representation an electrothermal 
arcjet thruster 40 that provides increased thrust efficiency at high 
specific energy requirements. The cathode 26 is formed from any suitable 
electrically conductive material such as a copper alloy. The anode body 42 
is formed from an electrically conductive metal or metal alloy able to 
withstand relatively high temperatures, on the order of 2000.degree. C., 
with minimal distortion. Preferred materials for the anode body include 
tungsten and tungsten based alloys such as tungsten/2% by weight thorium 
and 3.5%-4.5% by weight rhenium, 0.35 molar percent hafnium carbide and 
the balance tungsten. 
The anode body 42 includes one or more regeneration channels 44. The 
regeneration channels 44 extend internally within the electrically 
conductive anode body 42 and occupy a portion of the cross-sectional area 
of the anode body 42. 
The regeneration channel 44 communicates with the anode nozzle through a 
regeneration inlet 43 and a regeneration outlet 45. The regeneration inlet 
and outlet are apertures opening generally from the wall of the upstream 
converging portion 14. The inlet is positioned so that the propellent gas 
16 passes behind the wall of the recombination portion 22 while relatively 
cool, before being excessively heated by passage through the anode body 
42. 
The preferred fuels are liquid storable propellant gases, such as hydrazine 
or ammonia. These gases have a lower coefficient of thermal conductivity 
than either tungsten or the tungsten alloy materials used in anode 
construction. If an excessive volume of the anode body is replaced with 
the regeneration channels 44, the heat generated by the plasma arc 46 and 
absorbed by the anode body 42 is inadequately dissipated. The anode body 
temperature increases until the anode body distorts, destroying the arcjet 
thruster. 
FIG. 4 illustrates in cross-sectional representation a portion 50 of the 
anode body 42 including a regeneration channel 44. Energy, in the form of 
heat generated by the plasma arc, contacts an inner surface 52 of the 
anode body 42 as denoted by reference arrow 54. The heat is conducted 
through the wall of the anode body 42 to regeneration channel 44 as 
denoted by reference line 56. The propellant 16 flowing through the 
regeneration channel 44 absorbs an amount of energy necessary to raise the 
propellant temperature to approximately equal to the temperature of the 
inner wall 58 of the regeneration channel 44. Heat transferred to an outer 
surface 60 of the anode body 42 includes heat conducted from the inner 
wall surface 58 of the regeneration channel 44 and heat thermally 
conducted through the propellant gas 16. The heat reaching the outer wall 
60 is radiated into space. 
In order to lower the inner wall 52 temperature, the regeneration 
efficiency, the amount of heat absorbed by the propellant gas that is 
retained by the gas, rather than conducted to the anode wall 42 must be 
greater than about 65% and preferably in the range of from about 80% to 
about 97%. 
To maximize the regeneration efficiency, a regeneration channel 44 is 
adjacent the wall of the recombination portion 22 and contoured to 
substantially match the angle of the recombination portion. Preferably, 
the contour also matches a portion of the expansion portion 24 as well. 
The position and contour of the regeneration channel 44 is best 
illustrated in FIG. 3. Preferably, the regeneration channel is as close to 
the surface of the recombination portion 22 as possible without weakening 
the wall strength to a point where thermal distortion occurs. Typically, 
the regeneration channel 44 is from about 1.25 mm to about 5.1 mm 
(0.05-0.20 inch) and preferably from about 1.9 mm to about 3.8 mm 
(0.075-0.15 inch) from the wall surface. 
The angle, .alpha., between the recombination portion 22 and the 
constricted portion 18 is from about 1.degree. and about 10.degree.. 
Preferably the angle, .alpha., is from about 3.degree. to about 7.degree.. 
The angle, .beta., between the expansion portion 24 and the constricted 
portion 18 is from about 10.degree. and about 40.degree.. Preferably, the 
angle, .beta., is from about 15.degree. to about 30.degree.. 
Referring back to FIG. 2, reference line 64 denotes the relationship 
between thrust efficiency and specific impulse for the nozzle 40 of FIG. 
3. It may be seen in FIG. 2 that the thrust efficiency and specific 
impulse of the anode nozzle of the invention is superior to the prior art 
nozzles denoted by reference numerals 32 and 34 at all power levels. 
The source of improved thrust efficiency is illustrated in FIGS. 5A and 5B. 
FIG. 5A illustrates the energy distribution of a conventional (single 
angle, no regeneration) 10 kilowatt, 150 MJ/kg arcjet thruster. The 
efficiency, that portion constituting thrust and identified by reference 
numeral 66 is 34.3%. The losses are comprised of thermal losses 68, 8.6% 
constituting heat absorbed by the anode; ionization losses 70, 8.5%, 
constituting energy introduced into the propellant gas and forming 
non-reactive gaseous species; and "other" denoted by reference line 72, 
and constituting 48.6% of the loss. "Other" is a combination of frozen 
flow losses and wasted heat exhausted with the propellant gas out from the 
anode nozzle. 
Referring to FIG. 5B, the energy distribution for the biangle regenerative 
anode of the invention operating at 10 KW, 150 MJ/kg has a thrust 
efficiency 66 of 39.6%. The thermal losses 68 through the anode are 5.0%. 
The ionization losses 70 are 9.1% and the "other" losses 72 are 46.3%. 
FIG. 6 illustrates graphically where the 5.3% increase in thrust efficiency 
of the arcjet thruster of the invention is constituted. There is a 2.3% 
decrease in "other" losses 72, a 3.6% decrease in thermal losses 68 and a 
0.6% increase in ionization losses constituting a net increase in thrust 
efficiency 74 of 5.3%. 
FIG. 6 illustrates that the increased efficiency was not brought about 
merely by improved performance recovery of thermal losses. Rather, 
unexpectedly in view of FIG. 2, the frozen flow losses were also reduced 
by 2.3%. 
FIG. 7 graphically illustrates that by selecting the regeneration 
efficiency to be in excess of 65% as described above, it is possible to 
operate the biangle nozzle denoted by reference line 76 at a lower 
temperature than a conventional single angle nozzle denoted by reference 
line 78. The abscissa is expressed as a function of the normalized 
distance from the constrictor exit (L/L.sub.o) such that at the 
constrictor exit L/L.sub.o equals zero. When the thermal efficiency is 
about 95%, as illustrated in FIG. 7, the temperature reduction is up to 
200.degree. K. 
With reference to FIG. 8, the voltage to sustain the arc is higher for the 
nozzle of the invention as denoted by reference line 80 than for a 
conventional biangle nozzle without a regeneration passageway as denoted 
by reference line 82 or for a single angle nozzle as denoted by reference 
line 84. Running at a higher voltage, such as from about 200 volts to 
about 250 volts and preferably from about 210 volts to about 230 volts, 
with a correspondingly lower current, reduces cathode and anode erosion, 
thereby increasing the effective thruster life. 
It is apparent that there has been provided in accordance with this 
invention a process for the manufacture of an electrothermal arcjet 
thruster that fully satisfies the objects, features and advantages set 
forth hereinbefore. While the invention has been described in combination 
with specific embodiments thereof, it is evident that many alternatives, 
modifications and variations will be apparent to those skilled in the art 
in light of the foregoing description. Accordingly, it is intended to 
embrace all such alternatives, modifications and variations as fall within 
the spirit and broad scope of the appended claims.