Power plants – Reaction motor – Electric – nuclear – or radiated energy fluid heating means
Reexamination Certificate
1998-10-21
2001-10-02
Freay, Charles G. (Department: 3746)
Power plants
Reaction motor
Electric, nuclear, or radiated energy fluid heating means
C060S202000
Reexamination Certificate
active
06295804
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a thruster system which delivers pulses of electric current to a propellant to energize the propellant to generate thrust, and in particular to the pulse forming circuitry and the structure of the thruster of such a system.
BACKGROUND OF THE INVENTION
A thruster is a device which energizes a propellant such that when the propellant is ejected from the thruster, momentum is generated to move the body to which the thruster is attached. Thrusters use many different kinds of mechanisms to energize the propellant, but one common type of thruster introduces an electric current to the propellant to energize the propellant. These electric thrusters are commonly used in man-made satellites.
Electric thrusters can generally be categorized into two groups: steady state thrusters and pulsed thrusters, Each has its advantages and disadvantages.
As the name suggests, a steady state thruster is a thruster wherein the propellant is energized by providing a steady state electrical current to the propellant. One such steady state thruster is shown in U.S. Pat. No. 5,352,861 to Steigerwald et al.
However, steady state thrusters may have several disadvantages. For instance, steady state thrusters may respond sluggishly to changes in their operational status. Steady state thrusters usually require several milliseconds for activation, and then several minutes to reach thermal equilibrium. Moreover, steady state electric thrusters are not ideal for applications requiring only a small thrust or short-duration thrust, because at power levels below a few hundred watts steady state thrusters are commonly unstable and inefficient.
The pulsed thruster applies a series of electric current pulses of limited duration (typically on the order of microseconds to milliseconds, with microseconds being common for the low energy thrusters under consideration here) to the propellant to energize the propellant. A sample schematic of a conventional pulsed thruster system
20
is shown in FIG.
1
. The system
20
includes a low DC voltage primary power supply
22
, a high DC voltage thruster power supply
24
, a control circuit
26
, an ignition circuit
28
, an ignition device
30
, a capacitor
32
, and a thruster
34
. The primary power supply
22
is coupled to the thruster power supply
24
, which in turn is coupled to the ignition circuit
28
and selectively coupled to the capacitor
32
. The ignition circuit
28
is coupled to the ignition device
30
, such as a spark plug, and receives commands from the control circuit
26
. The capacitor
32
is selectively coupleable across the thruster
34
.
In operation, the primary power supply
22
provides power to the thruster power supply
24
, which charges the capacitor
32
. The capacitor
32
, in turn, applies this voltage across the thruster
34
, which has first and second spaced electrodes
38
,
40
. In accordance with a signal received from the control circuit
26
, the ignition circuit
28
fires the ignition device
30
. The firing of the ignition device
30
provides a sufficient amount of energy to cause an arc to form on the surface of the propellant
42
between the first and second electrodes
38
,
40
, thus completing the circuit with the capacitor
32
.
The propellant
42
is introduced into the space
44
between the first and second electrodes
38
,
40
. The energy released from the arc formed between the first and second electrodes
38
,
40
may cause the propellant
42
to change into a gaseous form, and particularly an ionized gaseous form known as plasma. The plasma exits the space
44
at high velocity to provide thrust. As the propellant
42
is heated, the propellant
42
, which is in a solid or semi-solid form as shown, is advanced into the space
44
through the action of the force F
S
, which represents the force provided by a spring (not shown) which abuts the surface of the propellant
42
to urge the propellant
42
into the space
44
.
Pulsed thrusters have several advantages compared to steady state thrusters. For example, the time required to activate a pulsed thruster is generally shorter than for a steady state thruster. Pulsed thrusters may achieve thrust in a short time duration, typically microseconds, compared to the time in which a steady state thruster can be turned on and off, typically seconds. Pulsed thrusters also generally achieve a higher peak power level, resulting in high momentum impulses compared to steady state thrusters. Also pulsed thrusters can easily vary their average thrust level by varying the capacitor energy and the pulse rate (pulses per second). Further, the pulsed thruster is generally not unstable in lower power applications.
Nonetheless, pulsed thrusters have their disadvantages. For instance, the circuit elements used to provide the electrical discharge may be subjected to high stresses, and consequently may have a relatively short useful life.
Additionally, current ringing or oscillation can occur in the capacitor and the thruster. Ringing occurs when current continues to flow back and forth through the circuit after the initial discharge of the capacitor, energizing inductances in the lines connecting the capacitor
32
with the thruster
34
.
FIG. 2
shows a plot of two consecutive current oscillations (A and B) in the capacitor
32
associated with current pulse discharges at times t1 and t2, respectively for the circuit of FIG.
1
. The vertical axis represents current level and the horizontal axis represents time, and a typical pulse length T
C
is illustrated.
Ringing can cause damage to the entire system
20
. For example, ringing may result in the charging of the capacitor
32
against its normal polarity, which may increase the wear on the capacitor
32
. Additionally, current reversal through the capacitor
32
can result in considerable energy loss, which degrades overall thruster efficiency and also increases capacitor wear. Further, the corresponding current oscillations through the thruster
34
tend to increase heating of the conductors
46
,
48
which connect the capacitor
32
to the electrodes
38
,
40
within the thruster
34
and to increase heating of the electrodes
38
,
40
, the thruster insulators (not shown) and the propellant
42
. This increased heating tends to produce undesirable erosion of the electrodes
38
,
40
and insulators within the thruster
34
, potentially shortening their life. Further, ringing can result in reversal of thrust forces within the thruster, reducing both thrust and efficiency.
It has been suggested that the ringing in the system
20
may be reduced by coupling a diode in parallel with the capacitor
32
and the thruster
34
. Specifically, such a solution is suggested by Kimura et al. in Preliminary Experiment on Pulsed Plasma Thrusters with Applied Magnetic Fields, presented at the 13th International Electric Propulsion Conference (1978). In particular, Kimura et al. suggest that the diode in parallel with the capacitor and the electrodes of the thruster may eliminate the oscillatory nature of the main discharge. This solution, however, still allows some undesirable reversal of current in the system.
Furthermore, in a conventional thruster system, as is shown, the impedance of the thruster
34
is significantly larger than the impedance of the capacitor
32
to ensure that most of the energy is delivered to the thruster
34
when the capacitor
32
discharges. Simply put, the capacitor
32
and the thruster
34
will participate in the energy distribution after the capacitor discharge in proportion to their relative impedances. Given that the capacitor is typically on the order of 10 m&OHgr;, for 80% of the energy to be distributed to the thruster
34
, the impedance of the thruster
34
must be on the order of 40 m&OHgr;. The energy distributed to the capacitor is generally lost through heating of the capacitor
32
.
However, increasing the impedance of the thruster
34
decreases the efficiency of the thrust production in the thruster
34
. For a thruster
34
relying on elect
Burton Rodney L.
Willmes Gary
Freay Charles G.
Marshall Gerstein & Borun
The Board of Trustees of the University of Illinois
Torrente David J.
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