Patent Application
20090211225
This invention relates to rocket propulsion, and more particularly to controlling the thrust of a rocket engine or rocket motor and maintaining the thrust efficiency of the system.
Modern rocket propulsion systems can be classified according to the type of energy source: chemical, nuclear, and solar. Chemical rocket propulsion uses the energy from a high-pressure combustion reaction of propellant chemicals, which heats reaction product gases to very high temperatures. These gases are then expanded in a nozzle and accelerated to very high velocities, which, in turn, bring rockets to high velocities in an opposite direction. Nuclear propulsion, using a fission reactor, a fusion reactor, or directed radioactive isotope decay, has been investigated but remains largely undeveloped. Solar propulsion may use solar panels to heat a gas. The expanded gas can be expelled through an exhaust nozzle, as with chemical propulsion.
Chemical propulsion techniques are typically divided among those using liquid propellants and those using solid propellants. Gaseous propellants and hybrid propellant systems also exist. Typically, liquid propellant rocket engines feed a propellant under pressure from tanks into a combustion chamber. Solid propellant engines, in contrast, store a propellant “grain” in the combustion chamber, the exposed surface of which bums smoothly at a predetermined rate. Combustion chamber conditions therefore vary with propellant type. The techniques applied to control thrust of the various types of rocket engines historically vary to accommodate for the different mechanics of liquid versus solid propellants. Methods for optimizing nozzle efficiency are more developed in the field of liquid propellant engines than in solid propellant motors.
Methods for initiating and stopping liquid propellant rocket engines and for varying the thrust of these liquid engines during operation and flight are described in U.S. Pat. No. 3,897,008; granted Jul. 29, 1975, to Donald G. Nyberg and Ronald F. Dettling entitled “Liquid Fuel Injector System” which is hereby incorporated by reference in its entirety.
Systems providing improved efficiency for liquid rocket engines using expansion-deflection (ED) nozzles and plug nozzles are described in Huzel, Dieter K. and Huang, David H., Design of Liquid Propellant Rocket Engines. Washington D.C.: NASA Science and Technical information Office, 1967, pp. 89-95. The plug nozzle replaces a traditional nozzle exit cone with a spike centerbody. Exiting gases pass through a throat, and then travel down the surface of the spike to converge in a direction opposite that of rocket trajectory.
The use of an ED nozzle is elaborated in Sutton, George P.; Rocket Propulsion Elements, 6th Edition, John Wiley and Sons (1992). As stated therein, “[t]his behavior is desirable at low altitudes because the atmospheric pressure is high and may be greater than the pressure of the exhaust gases. When this occurs, the exhaust is forced inward and no longer exerts force on the nozzle walls, so thrust is decreased and the rocket becomes less efficient. The centerbody, however, increases the pressure of the exhaust gases by squeezing the gases into a smaller area thereby virtually eliminating any loss in thrust at low altitude.”
Liquid propellant engines have improved performance over a wide range of Nozzle Pressure Ratios (NPRs) using systems such as those described in Sutton and Huzel and Huang. A recent improvement is described in U.S. Pat. No. 6,591,603 B2, granted Mar. 13, 2003 to Gordon A. Dressler, Thomas J. Mueller, and Scott J. Rotenberger, entitled “Pintle Injector Rocket With Expansion-Deflection Nozzle” (hereinafter “Dressler”). Dressler describes a liquid rocket engine with a variable thrust injector and an ED nozzle to improve performance. In the Dressler system, a throat is formed at one end of a combustion chamber through which hot gases escape. A rod runs through the throat, and a deflector is formed at the end of the rod, downstream of the throat. A nozzle exit cone extends from the throat. Thus, exiting gases pass through the throat and are deflected by the deflector. The deflected gases then pass along the walls of the nozzle exit cone, which direct them in a direction opposite the trajectory of the rocket.
Liquid rocket engines employing efficient variable propellant flow into the combustion chamber have been used effectively for many years but have suffered from performance inefficiencies inherent in the use of cone or bell nozzles over the wide NPR range which results from the variable chamber pressure and resulting variable thrust when using a rocket engine with a fixed throat area.
While systems such as the above have improved liquid engine rocketry, no liquid rocket engine design has adequately leveraged improved techniques to provide a simple and powerful engine with both high efficiency over a wide range of backpressures and easily controlled thrust. Such an efficient and versatile rocket engine would provide significant gains in many rocketry applications.
Techniques such as those described above are less developed in the field of solid propellant rocket motors. Designs for use in future generation Army tactical missiles have been investigated and tested, as reported in Burroughs, Susan L. et al, “Pintle Motor Challenges for Tactical Missiles”, AIAA Paper 2000-3310, July 2000. These designs use a pintle that extends into the throat or just upstream of the throat of a conical expansion nozzle. The pintle is attached to a control system that can move the pintle forwards and backwards within the combustion chamber, thereby varying the throat area. The size of the throat area is related to chamber pressure and thrust of the solid rocket motor. After passing through the variable throat area, the exhaust gases are expanded in a conventional nozzle (e.g. conical, bell, Rao, etc) to produce thrust against the walls of the nozzle. The NPR is commonly used to characterize the conditions under which a rocket operates is the ratio of internal chamber pressure to external (ambient) pressure against which the rocket exhausts.
Conventional rocket nozzles must be designed to optimize nozzle efficiency at a given NPR. Nozzle performance (i.e. the efficiency with which a nozzle converts thermal energy of the heated gases in the chamber into thrust-producing, directed kinetic energy of the exhausted gases) typically degrades at NPRs other than the “design,” or optimal, pressure ratio.
As an example, consider a rocket with a constant chamber pressure, a fixed throat area and a conical nozzle which is used to launch a payload through the earth's atmosphere. As the rocket ascends, the ambient pressure into which the motor exhausts (atmospheric pressure) will decrease, thus increasing the NPR. Nozzle efficiencies at NPRs other than the design ratio will be lower than optimal, so rocket designers must choose the pressure ratio “design point” to give the best average performance over the range of expected NPRs.
A class of nozzles called “plug” nozzles or “aerospikes,” with a fixed-position centerbody, or spike, that extends downstream of the combustion chamber throat, have the characteristic that nozzle efficiency remains relatively high as a rocket motor with a constant chamber pressure moves through varying ambient pressure conditions. These nozzles are therefore known as “altitude compensating” nozzles.
Nozzles with moveable pintles affect NPR in a different way, but suffer nonetheless from loss of nozzle efficiency at “off-design” NPRs. In this class of nozzles, the pintle is used to vary the throat area, and thus the thrust of solid propellant motors. In varying throat area, these nozzles vary the chamber pressure, and thus the propellant burn rate, with the ultimate effect of varying thrust. However, because the pintle is used in combination with a cone nozzle, varying NPRs force rockets of such a design to operate at sub-optimal NPRs. Thus thrust control, or “throttling” is achieved at the cost of nozzle efficiency.
Thus, theory and test results demonstrate that the tested designs cannot maintain high performance over a wide range of NPRs. This is largely because such designs suffer from efficiency losses due to expansion problems in a fixed nozzle exit cone or bell nozzle configuration. Regardless of whether the change in NPR occurs because of decreasing exhaust pressure (increasing altitude) or decreasing chamber pressure (thrust throttling), nozzle efficiency suffers due to non-optimal nozzle expansion at off-design NPRs. Performance losses of up to 30% off of optimal efficiency can occur at off-nominal NPRs. To date, no method has been identified for maintaining near-optimal nozzle efficiency while varying thrust over a wide range.
In summary, both liquid and solid rocket motor designs have failed to realize their full potential in providing both high efficiency over a wide range of NPRs, and thrust control. Such an efficient and versatile solid, liquid, or other propellant type rocket would provide significant gains for rockets used for commercial and military spacecraft launches, as well as missile launches used for both conventional and anti-terrorism warfare.
In consideration of the above-identified aspects of rocket design, the present invention provides systems and methods for varying the thrust of a rocket while maintaining significantly higher nozzle efficiency over the thrust range. A moveable plug design is provided for use in rocket motors and engines. The plug may be part of a “moveable plug” nozzle, where a combustion chamber exit, such as a cowl, and plug are moveable with respect to one another. A plug or combustion chamber exit may be attached, or otherwise operably coupled, to an adaptive control system for modifying their position with respect to one another. The adaptive control system may thus control the thrust force and thrust direction of a rocket. At least two configurations employing a moveable plug are described: a first configuration provides a moveable plug in a plug nozzle configuration, while a second configuration provides a moveable plug in an expansion-deflection (ED) configuration. The plug and spike operate to achieve greatly improved efficiency over a wide range of NPRs. Other advantages and features of the invention are described below.
The systems and methods for improved thrust efficiency and control in accordance with the present invention are further described with reference to the accompanying drawings in which:
Certain specific details are set forth in the following description and figures to provide a thorough understanding of various embodiments of the invention. Certain well-known details often associated with the design and manufacture of rocket motors are not set forth in the following disclosure, however, to avoid unnecessarily obscuring the various embodiments of the invention. Further, those of ordinary skill in the relevant art will understand that they can practice other embodiments of the invention without one or more of the details described below. Finally, while various methods may be described with reference to steps and sequences in the following disclosure, the description as such is for providing a clear implementation of embodiments of the invention, and the steps and sequences of steps should not be taken as required to practice this invention.
The solid propellant grain 2 and its burning surface are contained within a combustion chamber 3. When ignited, the surface of the grain 2 burns, providing hot gases from the burning surface. The burn rate of the propellant 2 affects the flow rate of gas through the throat formed by the exit 5. A faster burn rate will force more gases through the throat. The burn rate is dependent on the pressure in the combustion chamber 3. At higher pressures, the propellant 2 burns faster.
The chamber pressure, in turn, is dependent on the nozzle throat area. Nozzle throat area is defined as the smallest space through which exhaust gases must pass to exit the combustion chamber 3. In the embodiment of
The thrust of the rocket motor 1 is based upon the specific impulse of the given propellant 2, the chamber 3 pressure, the area of the throat and the thrust coefficient. The thrust coefficient is the measure of efficiency of the expansion of the exhaust gases and the transfer of their energy to the rocket 1, i.e. the efficiency of the nozzle.
The thrust coefficient may change when the rocket motor 1 is operating in different ambient pressures. Differing ambient pressures will effect the ratio of the pressure inside the chamber 3 to the pressure outside the chamber (the NPR), which affects the dynamics of the gas flow exiting the rocket motor 1. For example, when the rocket motor 1 operates at higher altitudes, the atmospheric pressure decreases, changing the NPR and the corresponding thrust coefficient. Conversely when the chamber pressure is decreased by increasing the nozzle throat area and thus decreasing the propellant burn rate, the NPR will decrease, thus affecting the nozzle thrust coefficient. The thrust coefficient can be controlled, and maintained at higher levels if desired, in both the high and low backpressure situations by using a moveable plug in a plug nozzle configuration as illustrated in
In rocket motor designs contemplated by various embodiments of the invention, a plug 4 is moveable with respect to a combustion chamber exit 5. The relative change in position can be achieved either by moving the plug 4, or by moving the exit 5, or both.
The plug 4 is defined herein as a shaped object roughly in the shape illustrated in
In
The plug 4 may be manufactured as a single piece with rod 8, or may be separately fabricated and attached to rod 8. In embodiments without a rod 8, other solutions may be adapted to fit the needs of the particular configuration. The plug 4 may be made from the same material as rod 8 or from some other material; a sturdy heat-resistant material best suited to the propellant and mission is desirable.
The term “exit” as used herein refers to the sidewall substantially overlapping and adjacent to the throat. A portion of an exit 5 may form an outer boundary of a throat. In some embodiments where the exit 5 is a very thin piece, the exit 5 may form the throat without any mass upstream or downstream thereof. The exemplary exit 5 in
Referring back to
Note that rod 8 may be a single straight shaft of any suitable material, as illustrated in
In
rb=aPcn
where rb is the burn rate at the surface of the propellant, Pc is chamber pressure, and n and a are constants related to specific characteristics of the propellant selected. A decreased burning rate results in a lower flow rate of propellant and a resulting lower thrust. Naturally, reversing the direction of the control system and the movement of the plug increases chamber pressure and corresponding thrust.
Modifying the plug 4 and/or exit 5 position upstream and downstream thus controls the amount of thrust of the rocket engine 1, which as a practical matter affects rocket acceleration and velocity. Upstream and downstream position modification of the plug 4 and exit 5 with respect to one another is referred to herein as axial motion. Thus if either plug 4 or exit 5 is moved directly upstream or directly downstream, the movement is considered axial. In contrast, moving the plug 4 or exit 5 from side-to-side affects direction of thrust, which correspondingly affects the rocket direction. Such movement will be referred to herein as radial movement. Thus, modification of the axial and radial position of plug 4 and/or exit 5 can be used to alter both rocket speed and direction. Accordingly, position control system 9 and rod 8 may comprise apparatus for moving the plug 4 and/or exit 5 both axially and radially.
Position changes of the plug 4 may be accomplished via a position control system 9. The position control system 9 is depicted upstream of the combustion chamber 3 in
Examples of position control systems such as 9 are presently in use in connection with rockets that use a pintle to modify rocket thrust. Any presently used or future developed position control system 9 is considered appropriate for use in connection with practicing the invention.
The function of the position control system 9 may be simply to adapt to ambient pressures to provide a predictable rocket speed, or may be more sophisticated. Sophisticated systems might make use of computerized controls that are capable of communicating with a computer operated by a human or automated response system. In such configurations, a human might remotely control the trajectory of a rocket by sending signals to 9, which in turn modifies the position of the plug 4 to carry out the human instructions. An automated network could also perform the task of the human. Many scenarios might be constructed in which the benefits of such a system are evident. One such scenario might involve the automatic adjustment the position of plug 4 with respect to exit 5 to account for erosion of the plug 4, exit 5, or other nozzle surfaces as the propellant 2 burns, and thus compensate for changes in the nozzle throat area and contours during rocket motor operation.
Note that position control system 9 can modify the position of the plug 4 with respect to the exit 5 of the combustion chamber 3. Note that when plug 4 is moved from a larger diameter portion of the exit 5 to a smaller diameter portion of the exit 5, either by moving plug 4 downstream or exit 5 upstream, the throat area is reduced, and vice versa. Changes in throat area may be accomplished by moving the plug 4 or by moving the exit 5. Embodiments in which the exit 5 is moved while the plug 4 remains fixed with respect to the other components of the rocket, such as sidewall 1 and support brace 7 can be implemented by mounting the exit 5 to the remainder of rocket 1 via a flexible apparatus, and by controlling the motion of the exit 5 using the position control system 9.
Support brace 7 may be included in various embodiments to support the appropriate position of the moveable plug 4 with respect to the exit 5. In embodiments where the moveable plug 4 can only move axially, support brace 7 can fit around the rod 8 snugly, but not so tight as to prevent axial sliding. In embodiments where the plug 4 can move radially as well as forward and backward, support brace 7 may be outfitted with additional apparatus to support the rod 8 in the desired position. Such additional apparatus may be independent of the position control system 9 or may be operably coupled to 9 to act in concert with the positioning activities of adaptive control system 9.
The shape of spike 6 will effect the dynamics of exhaust gases and so is a feature for close consideration in practicing the invention. In particular, the plug 4 and spike portion 6 thereof may vary depending on whether a plug nozzle, truncated plug nozzle, or ED nozzle type is used. A plug nozzle configuration is illustrated in
Referring to
The plug nozzle configuration will maintain nozzle efficiency at low flow rates and/or low altitudes where relatively high back pressure causes boundary layer separation and attendant thrust loss in conventional cone and bell nozzles. By contrast, at a low chamber pressure, low thrust condition using the standard upstream pintle design, the exhaust gases do not expand fully into the nozzle but form a core in the center of the nozzle. With a moveable plug, however, the plume does not suffer from efficiency-reducing boundary-layer separation at low chamber pressure (low NPR), and thus the efficiency of the nozzle can be near-optimized at these reduced-flow conditions.
Referring to
Reversing the direction of the control system 309 and the movement of the plug 304 and tapered rod 312 reduces the combustion chamber 303 pressure and the thrust. The chamber pressure can be reduced to near extinguishment (smoldering) or to complete extinguishment by including a notched area 311 upstream of the tapered rod 312. By positioning the notched area 311 in the exit 305, the throat area may be increased to a value sufficient for complete extinguishment of a solid propellant grain. In the illustrated embodiment, the plug 304, tapered rod 312, and notched area 311 are controlled via rod 308 and adaptive control system 309, although any other available means may be used to modify the relative position of notch 311, tapered rod 312, and plug 304 with respect to exit 305, as discussed above.
In embodiments such as
A control system 309 can provide the correct positioning of the moveable plug 304, tapered rod 312, and/or notch 311 in the exit 5 to produce the desired thrust. As with the position control system 109 from
The remaining elements, e.g. sidewall 301, grain 302, and support brace 307 are generally analogous to the corresponding elements from
The liquid propellant engine 401 may be, for example, a bipropellant, monopropellant or hybrid type engine. Engine 401 may contain a propellant flow control device 402, for example a variable flow rate injector which controls the flow rate of the propellant into the combustion chamber 403. The propellant flow control device 402 can be in an annular design as depicted in
In
The thrust of the rocket engine 401 is dependent on the specific impulse of the given propellant system, the chamber 403 pressure, the area of the throat through which the gases exit, and the thrust coefficient. The thrust coefficient is a measure of the efficiency of the expansion of the exhaust gases and transfer of their energy to the rocket.
As the propellant flow rate is decreased employing the propellant flow control device 402, the moveable plug 404 can be intelligently positioned by an automated control component 412 that is communicatively coupled to the actuator 409. Automated control component 412 moves the plug 404 such that the throat area is reduced to a value that just maintains the chamber 403 pressure at a substantially constant level. This higher chamber pressure results in a higher NPR than if the chamber pressure were reduced, as in a conventional variable thrust engine. The higher NPR then provides a higher thrust coefficient, increased efficiency and higher specific impulse, and resulting higher performance of the rocket engine 401.
Automated control component 412 can intelligently position the plug, for example, if it is communicatively coupled to one or more of the propellant flow control device 402 (or some other means of measuring propellant flow), a chamber pressure gauge 410, and an altimeter or barometer 411.
In one embodiment, automated control component 412 can position the moveable plug 404 by linking movement of the plug 404 to rate of propellant flow established by the propellant flow control device 402. Thus, when flow rate is to be reduced, for example, the automated control component 412 can also instruct the actuator 409 to move the plug 404 downstream so as to maintain optimum performance NPRs.
In another embodiment, automated control component 412 can position the moveable plug 404 based on measurements of a chamber pressure gauge 410 and/or an altimeter or ambient pressure gauge 411. For example, if there is an increase in altitude or drop in pressure measured by 411, the plug 404 may be moved upstream to correspondingly decrease the chamber 403 pressure, thereby maintaining a constant NPR. Similarly, if there is an increase in chamber pressure measured by 410, the plug 404 might be advantageously moved upstream to maintain a constant NPR.
It will be appreciated that the various components 402, 410, 411, and 412 can be combined in a variety of combinations and integrated into existing control systems in a wide variety of configurations. Also, as an alternative to measuring pressures or altitude, predicted values can be used based on a known rocket starting position and trajectory.
A novel method may thus be performed utilizing a system such as
For example, in exemplary method 500, when propellant flow rate is decreased 501, a moveable plug may be moved so as to decrease throat area 502, thereby maintaining a substantially constant NPR. In exemplary method 510, when a decrease in chamber pressure is sensed or predicted 511, a moveable plug may be moved so as to decrease throat area 512, thereby maintaining a substantially constant NPR. In exemplary method 520, when an increase in ambient pressure is sensed or predicted 521, a moveable plug may be moved so as to decrease throat area 522, thereby maintaining a substantially constant NPR.
In exemplary method 550, when propellant flow rate is increased 551, a moveable plug may be moved so as to increase throat area 552, thereby maintaining a substantially constant NPR. In exemplary method 560, when an increase in chamber pressure is sensed or predicted 561, a moveable plug may be moved so as to increase throat area 562, thereby maintaining a substantially constant NPR. In exemplary method 570, when a decrease in ambient pressure is sensed or predicted 571, a moveable plug may be moved so as to increase throat area 572, thereby maintaining a substantially constant NPR.
Of course, any of the factors referred to in 501, 511, 521, 551, 561, and 571 may offset or augment certain other of such factors, thereby increasing or decreasing the extent to which the throat area is increased or decreased.
In a liquid engine, the chamber pressure (Pc) in the rocket engine is equal to the propellant flow rate, o>, times the propellant c* (measure of propellant performance) divided by At divided by the gravity constant, g, (32.174 ft/sec2) or basically:
PC=constant×eo/At
The above equation illustrates that with a constant throat area, the chamber pressure decreases with a decrease in the flow rate of propellant.
The thrust (F) of a rocket engine is equal to the Throat Area (At) times the chamber pressure (Pc) times the thrust coefficient (Cf) or:
F=At×PC×C
f
The above equation illustrates that the thrust (F) is decreased not only in proportion to the decrease in chamber pressure but also in proportion to the decrease in the thrust coefficient.
The thrust coefficient, Cf, is based on the efficiency of the expansion of the exhaust products in the nozzle and highly dependent on the NPR which is the chamber pressure divided by the exhaust pressure or: NPR=Pc/Pe. The optimum performance of a rocket engine is thus dependent on the NPR and thrust coefficient.
For a conventional cone or bell nozzle rocket thruster with a variable propellant flow rate to vary the thrust and a fixed area throat, the Cf and therefore the performance degrades as the chamber pressure moves away from the design NPR. This is illustrated in
Part of this degradation in performance has been corrected with the use of a stationary plug nozzle or expansion deflection nozzle with a fixed throat area, as described in U.S. Pat. No. 6,591,603. This results in the Cf and thus the performance following the optimum curve shown in
The performance degradation resulting from both of the above factors can be reduced using a moveable plug nozzle to vary the throat area such that a constant high chamber pressure and a constant high optimum NPR are achieved over the full range of thrust variation. This constant high chamber pressure results in a constant high Cf and a resulting optimum high Specific Impulse (ISP) for any thrust level.
The methods of
A second performance benefit is the maintenance of the oxidizer to fuel ratio over the entire thrust variation range. As mentioned previously, most variable thrust liquid bipropellant systems have a variation in the oxidizer to fuel ratio because of the variation in pressure drop from the tanks, through the metering devices, and into the combustion chamber throughout the thrust variation range when the chamber pressure is also varying. This oxidizer to fuel ratio shift is different for the various bipropellant systems based on their propellant densities and the discharge coefficients associated with the metering devices.
Although exemplary embodiments refer to utilizing the present invention in the context of solid-propellant rocket motors, the invention is not so limited, but rather may be implemented in connection with any rocket motor configuration in which thermal energy is converted to directed kinetic energy, and thus thrust, by means of a nozzle. Therefore, the present invention should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims.
