Valving for controlling a fluid-driven reciprocating apparatus
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 07/901,290, filed Jun. 19, 1992, now U.S. Pat. No. 5,222,873, issued Jun. 29, 1993, and assigned to the same assignee. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a fluid-driven apparatus, such as a free-piston system, for applications including liquid rocket propulsion; particularly to a control valve assembly for fluid-driven free-piston devices, and more particularly to a valve arrangement using a plurality of valve assemblies for each reciprocating device of an associated pair of such devices so that the devices are alternately actuated by using fluid pressure communication therebetween. 
     Liquid rocket propulsion requirements have historically fallen into two distinct categories, which correspond to two different aerospace applications. The first kind of requirement is for performance-oriented rocket stages which deliver high levels of thrust continuously with large total impulses, typically used for launch vehicles. High performance is achieved through the use of rotating propellant pumps driven by turbines, which permits high-pressure propellant delivery from relatively lightweight tankage at low pressure. The second kind of requirement is for relatively small propulsion systems, which must reliably provide thrust on demand over periods of many years. The main application of the small systems is orbital maintenance, so performance has been much less important than long-term reliability for this second kind of propulsion system. Hence, satellite liquid propulsion systems have been pressure-fed. Advantages of avoiding pumps are overall simplicity and the lack of moving seals which could wear and permit propellant leakage losses during long periods of in operation. Perhaps more importantly, turbopumps cannot support short thrust pulses, since it requires time at reduced efficiency for turbomachinery to start and stop. 
     Recently, there has been interest in developing technology for increasing the performance capabilities of small rocket propulsion systems which thrust intermittently. For example, electrically-driven rotating pumps for possible use on satellites have been demonstrated. An approach for high thrust missions has been to increase the performance of pressure-fed operation, by using state-of-the-art materials technology for otherwise mass-intensive pressure vessels and high pressure liquid tankage. 
     An alternative small propulsion system has been demonstrated that uses low pressure tankage and free-piston pumps which can start and stop rapidly to meet a demand-thrust requirement. These pumps are driven by a gas source. Free-piston devices are positive displacement fluid power machines which undergo reciprocating motion, without mechanical power transfer, such as a connecting rod to a rotating crankshaft. Earlier applications of the free-piston devices were steam-driven air compressors, compressors driven directly by oscillating electromagnetic fields and hydraulic or gas-driven intensifiers. These prior applications generally used a relatively slow-moving differential free-piston to amplify pressure. In contrast, the free-piston pump for small propulsion systems must operate as a high volume flow device, with a high power-to-weight ratio, as well as being a pressure amplifier. U.S. Pat. No. 5,026,259, issued Jun. 25, 1991, in the name of John C. Whitehead et al., exemplifies the use of free-piston devices for propulsion systems used for attitude control or maneuvering. 
     While gas-driven free-piston pumps can operate at any flow rate, prior known propulsion systems using free-piston pumps have been unable to provide continuous flow of either monopropellant or bipropellant for rocket systems without complicated electrical control systems. Thus, there has existed in the small rocket propulsion systems a need for a valve control system which provides for rapid response, as well as continuous liquid delivery for steady thrust. 
     The present invention fills this prior recognized need by providing an improved valve control assembly which alternately actuates a pair of fluid-driven pumps, using fluid pressure communication between them. This invention is an improvement over the three-way valve arrangements of the above-referenced application Ser. No. 07/901,290, now U.S. Pat. No. 5,222,873, in that the intake-exhaust valve and the signal valve are separated, which greatly simplifies the valve assembly and provides fewer critical leakage paths, among other advantages. Each control valve of one of a pair of pumps is switched by a pressure signal depending on the state of its counterpart&#39;s pump piston. The communication logic is arranged to provide overlap of the forward strokes of the pistons, so that at least one of the pair of pumps will always be pressurized. Thus, uninterrupted pumping of liquid is made possible from a pair of free piston pumps. In addition, the speed and frequency of piston stroking is entirely dependent on the mechanical power load applied. In the case of a pair of pumps, this enables liquid delivery at a substantially constant pressure over the full range of flow rates, from zero to maximum flow. 
     The present invention like that described and claimed in the above-referenced has application for both monopropellant liquid propulsion systems and for bipropellant liquid propulsion systems, each utilizing reciprocating pumps. For the bipropellant system, a total of four pumps enables continuous flow of both fuel and oxidizer liquids, while allowing a different flow rate of each of the different liquids utilized. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved valve control arrangement for a reciprocating system. 
     A further object of the invention is to provide a pump system utilizing at least one pair of fluid-driven pumps and a control valve arrangement involving a plurality of valve assemblies whereby uninterrupted pumping of liquid is made possible. 
     A still further object of the invention is to provide a pressure actuated control valve assembly which is small and light weight while having a high fluid throughput capacity. 
     A still further object of the invention is to provide a control valve assembly wherein the signal portion thereof is separated from the intake-exhaust portion thereof. 
     A still further object of the invention is to provide automatically oscillating valving for a pair of cylinders which work at any pressure because no springs are used. 
     A still further object of the invention is to provide a liquid propulsion system using at least one pair of free-piston pumps, each pump having an improved control valve arrangement which is pressure actuated by the other pump. 
     Another object of the invention is to provide a fluid pressure control valve assembly having an inlet-exhaust section and a signal/control section. 
     Still another object of the invention is to provide a spool valve control arrangement for a fluid-driven reciprocating pump system for either monopropellant or bipropellant liquid propulsion system. 
     Other objects and advantages of the invention will become apparent from the following description and accompanying drawings. 
     Briefly, the invention involves a valve arrangement of control valve assemblies operating in pairs, each valve assembly being oppressively connected to actuate a reciprocating pump device. Each pair of control valve assemblies are interconnected to alternately actuate their associated pumps by using fluid pressure communication between the control valve assemblies. Each control valve assembly is switched by a pressure signal depending on the state of its counterparts pump piston. The control valve assembly/reciprocating pump arrangement of the present invention may be effectively utilized for both monopropellant and bipropellant liquid propulsion systems. For the bipropellant system, two pair of control valve assembly/pump arrangements are used, one for the fuel and one for the oxidizer, in which the oxidizer pump control valve assemblies may be slaved to the fuel pump control valve assemblies for synchronous operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate and embodiment of the invention and, together with the written description, serve to explain the principles of the invention. 
     FIG. 1 schematically illustrates the basic features of the present invention. 
     FIGS. 2A-2F schematically illustrates the pneumatic switching of the pump valves. 
     FIG. 3 is an end view of an embodiment of a control valve assembly of the invention mounted on a free piston pump. 
     FIG. 4 is a cross-sectional view of FIG. 3 taken along the line 4--4 of FIG. 3, and with the signal valve rod and piston split to show its activated and inactivated position. 
     FIG. 5 is an enlarged view of the signal valve of the apparatus illustrated in FIGS. 3 and 4, and with the signal valve rod split as in FIG. 4. 
     FIGS. 6 and 8 are end views of the intake-exhaust valve of the FIG. 3 apparatus. 
     FIG. 7 is a cross-sectional view taken along line 7--7 of FIG. 6 with the valve in its intake position; and 
     FIGS. 9 and 10 are cross-sectional views taken along line 9--9 of FIG. 7 and illustrate the switching and exhaust position of the valve of FIG. 7. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally directed to a valving arrangement for small monopropellant or bipropellant liquid propulsion systems which use low pressure tankage and reciprocating pumps which can start and stop rapidly to meet a demand-thrust requirement. More specifically, the invention is directed to an improved valve control system for a pair of free-piston pumps which utilize valve assemblies oppressively connected to each pump and interconnected so as to have fluid pressure communication therebetween. The improved control valve arrangement of the present invention permits rapid switching between intake and exhaust of the pump, in addition to providing large cross sectional areas for the flows with a short stroke and minimal hardware mass and size. 
     As set forth in above-referenced application Ser. No. 07/901,290, now U.S. Pat. No. 5,222,873, in liquid rocket propulsion, pumps are needed to achieve the highest performance by pumping fuel stored at low pressure to the thrust chambers at high pressure, thus reducing structural mass by minimizing the volume of highly-pressurized parts. In a free-piston pump, there is no crankshaft from which valve-timing information can be mechanically obtained, yet for control it is necessary to actuate the intake and exhaust valving with a precise dependence on piston position. In applications, such as liquid rocket propulsion, there is a need for continuous liquid delivery capability for steady thrust, but also with immediate start and stop capability. 
     Thus, the present invention, which is an improvement over that described and claimed in U.S. application Ser. No. 07/901,290, now U.S. Pat. No. 5222,873, is a simpler, less expensive, means to alternatively actuate at least one pair of fluid-driven free-piston pumps or reciprocating devices, by using fluid pressure communication between them. Each free-piston pump is provided with a control valve assembly, and each control valve assembly is switched by a pressure signal depending on the state of its counterparts&#39; piston. The communication logic between the control valve assemblies is arranged to provide overlap of the forward strokes of the pistons of the pumps, so that at least one of the pair of pumps will always be pressurized. Thus, uninterrupted pumping of liquid is made possible from a pair of free-piston pumps. In addition, the speed and frequency of piston stroking is entirely dependent on the mechanical power load applied. In the case of a pair of pumps, this enables liquid delivery at a substantially constant pressure over the full range of flow rates, from zero to maximum. 
     The control valve assemblies of this invention provide significant simplifications for greater reliability, longer life, less leakage, fewer parts, lower cost, and easier assembly than the three-way valve of the above-referenced copending application. This valve assembly of this invention has particular application for use with high temperature gas. 
     Referring now to the drawings, FIG. 1 schematically illustrates the basic features of the present invention. Apparatus of FIG. 1 comprises a cylinder 1 defining therein a working chamber within which is a partition, free-piston, or movable member 2. Connected to the cylinder 1 is a intake/exhaust control valve assembly generally indicated at 3 having therein a double-headed pressure actuated piston 4 which uncovers/covers a common intake/exhaust port 5 and exhaust ports 6. Also connected to cylinder 1 is a signal valve assembly 7 having a piston 8 connected to movable member 2, piston 8 covering or uncovering a signal port 9 depending on the location of movable member 2 within cylinder 1. Signal valve assembly 7 also includes a signal vent port 9&#39;. Connected to valve assembly 3 on the side thereof adjacent intake port 5 is an intake line 5&#39; connected to a supply fluid source, not shown, while a control line 6&#39; is connected on the side of valve assembly 3 adjacent exhaust ports 6 and is connected to a control fluid source, not shown. Connected to signal port 9 is a signal output line 9&#34;. 
     With the pistons 4 and 8 of respective valve assemblies 3 and 7 positioned as shown in FIG. 1, supply fluid via line 5&#39; is directed into the working chamber of cylinder 1 moving the member 2 inwardly (toward the top as shown), and the signal line 9&#34; is in open communication with the chamber of cylinder 1. As the member 2 continues to move inwardly in cylinder 1, piston 8 of signal valve assembly 7 passes port 9 and connects signal line 9&#34; with signal vent 9&#39;. While not shown, signal line 9&#34; of one of a pair of apparatus illustrated in FIG. 1 is connected to control line 6&#39; of another of a pair of pump/valve assemblies (not shown), and vice-versa. Thus, when the signal line 9&#34; is vented, the pressurized signal line 9&#34; of the companion apparatus is connected to control line 6&#39;, whereby piston 4 is moved (to the left as shown) past intake port 5 such that fluid from the chamber of cylinder 1 is exhausted via ports 5 and 6, thereby allowing movable member 2 to return or move outward (move toward the bottom as shown). Outward movement of member 2 causes signal piston 8 to move outward, thus reconnecting signal line 9 with the chamber of cylinder 1. 
     The essence of the switch logic basically described above with respect to FIG. 1 and described in greater detail hereinafter with respect to FIGS. 2A-F, includes the following features: 
     1. Supply fluid enters the working chamber in absence of control pressure. 
     2. Control pressure causes the working chamber to be vented. 
     3. Signal output line is connected to the working chamber when the chamber is not full. 
     4. Signal output line is vented when the working chamber is nearly full. 
     5. Paired apparatus having the above features with the signal output line of each one connected to the control line or port of the other. 
     6. Apparatus will oscillate at any supply pressure due to absence of springs (which restrict operation to particular forces and pressure levels), since operation over a wide pressure range is important for bootstrapping in a propulsion system. Bootstrapping refers to the creation of a higher pressure from a lower pressure when a pump-fed rocket initially starts, without a stored high pressure source. 
     While the upper or inward end of cylinder 1 of FIG. 1 is shown as being open, it can be connected to inlet and outlet check valves, such as shown in FIG. 2 of application Ser. No. 07/901,290, now U.S. Pat. No. 5,222,873, for pumping applications, or to a plenum for pressurizing same. Also, the partition or movable member 2 of FIG. 1 may be connected to an oscillating mechanism, whereby a pair of such apparatus would alternately oscillate the mechanism. 
     For passive demand-thrust operation of a propulsion system after startup, the valve assemblies must operate so that at least one pump assembly is pressurized and ready to deliver liquid whenever a thrust control valve is opened. Therefore, the warm gas intake ports of the valve assemblies must open alternately with positive overlap. A pump assembly which is nearing the end of its stroke should be exhausted just after the other&#39;s intake opens, so it can refill immediately. These conditions are in Table I. 
     
                       TABLE I                                                     
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PUMP/VALVE CONTROL LOGIC                                                  
                     Desired                                              
Condition of First Pump                                                   
                     State of Second Pump                                 
______________________________________                                    
1. begin pumping                                                          
              (pressurized)                                               
                         open exhaust                                     
2. stroking forward                                                       
              (pressurized)                                               
                         hold exhaust open                                
3. near full stroke                                                       
              (pressurized)                                               
                         open intake                                      
4. refilling  (vented)   hold intake open                                 
______________________________________                                    
 
    
     As seen from Table I, during the first, second and fourth conditions listed, the first and second pump assemblies have opposite states of pressurization. Therefore, the pneumatic signal lines connecting the two-valve assemblies are arranged so that pressure in a pump assembly&#39;s gas cylinder simply pushes the other pump assembly&#39;s signal piston into the exhaust position. However, to accommodate the third condition in Table I, this pressure signal is shut off when the pump assembly&#39;s piston is near the end of its forward stroke. The pneumatic switching sequences is illustrated in FIGS. 2A-2F in which the sequence of operation is illustrated by the steps or stages A through F of FIGS. 2A-2F. Corresponding reference numerals to those of FIG. 1 are used in FIGS. 2A-2F. 
     Referring now to the specific embodiment of the invention illustrated in FIGS. 3-10. The control valve assembly of this invention differs from the three-way valve assembly in above-identified copending application Ser. No. 07/901,290, now U.S. Pat. No. 5,222,873, in that the signal section thereof is separate from the intake/exhaust section. The control valve assembly of the present invention compared to the above-referenced three-way valve has fewer parts, is cheaper to manufacture, has fewer critical leakage paths, and has a more robust operation with longer lifetime. Thus, by eliminating the co-axial concentric arrangement of the above-referenced three-way valve assembly, significant simplification has resulted without adverse effect on the overall pumping or propulsion system in which the valve assembly is utilized. 
     As shown in FIGS. 3 and 4, a free-piston type pump, generally indicated at 10, includes a cylinder 11 having a curved head 12 at one end and an outwardly extending flange 13 at the opposite end, and within which is a piston 14 having a head 15 with a central indented section 16, and a seal assembly 17 positioned around the periphery of the head 15. The piston 14 of pump 10 is shown in FIG. 4 in a split configuration to illustrate it in its limits of stroke (pressurized and non-pressurized) locations within cylinder 11. The cylinder 11 may be connected via flange 13 to a fluid pumping system in which a fluid is pumped via the side of the piston 14 opposite the head 12 and connected to a fluid source via appropriate intake and exhaust valves, such as illustrated in FIG. 2 of above-referenced application Ser. No. 07/901,290, now U.S. Pat. No. 5,222,873. While only one pump 10 is illustrated in FIGS. 3 and 4, it is to be understood that the invention involves the use of at least one pair of such pumps, wherein the control valve assembly of each pump controls the operation of the other pump of that pair. 
     As shown in FIGS. 3 and 4, the head 12 of pump cylinder 11 is provided with a central opening 18 and a larger opening 19 to which is respectively mounted a signal valve 20 and an intake/exhaust valve 21. The valves 20 and 21 correspond to the valves 7 and 3 of the FIG. 1 and FIGS. 2A-2F schematic illustrations, with signal valve 20 being connected to a signal line 22 (line 9&#39; of FIGS. 2A-2F), and intake/exhaust valve 21 being connected to a fluid (hot gas) supply line 23 (line 5&#39; of FIG. 2A-2F). Also, signal valve 20 is provided with an auxilliary signal line 24 which may be connected to a slave, if used, and connected to an exhaust port thereof (port 9&#34; of FIG. 1). A signal line 22&#39; shown in FIG. 3 interconnects valve 21 with a valve identical to signal valve 20 mounted on the other pump/valve assembly of the pair, as seen in FIGS. 2A-2F, such that pressure in one pump/valve assembly controls the other pump/valve assembly. 
     FIG. 5 is an enlarged view of the signal valve 20, with the signal rod shown in split (limits of stroke) positions, and includes a body 26 mounted in opening 18 of cylinder head 12 and provided with an end cap 27 having a plurality of lock wire openings 28 and a vent port 29. The lock wire openings may be omitted, but also serve as vent ports. A sleeve 30 is positioned within body 26 and includes an enlarged end section 31 located within end cap 27, with sleeve 30 having a plurality of openings 32 which align with openings 33 in body 26 to which lines 22 and 24 are connected. A rod 34 (element 8 of FIGS. 2A-2F) is located within sleeve 30 and having one end secured in indented or recessed section 16 of piston head 15. Rod 34 is provided with a land or protruding section 35 and as rod 34 is moved by piston 14 the land 35 connects or disconnects fluid flow between a region 36 within cylinder 11 and signal lines 22 or the exhaust port 29. Rod 34 is shown split such that when cylinder 12 is pressured, the land of rod 34 is in the location indicated at 35 (left end of valve 20) cylinder 12 is and when in the unpressured condition the land of rod 34&#39; is in the location indicated at 35&#39; (right end of valve 20), as seen in FIG. 5. 
     FIGS. 6-10 illustrate the components of the intake/exhaust valve 21 of FIGS. 3 and 4, which corresponds to the valve assembly 3 of FIGS. 2A-2F, with FIGS. 7, 9 and 10 illustrating the intake, switching and exhaust positions of valve 21. Valve 21 comprises a valve body 40 having a cavity 40&#39; extending therethrough, and with a pilot or signal end cap 41 and an intake end cap 42 secured to opposite ends of body 40. End cap 41 is provided with an opening or aperture 43 to which signal line 22&#39; is connected, while end cap 42 is provided with an opening or aperture 44 to which intake or supply line 23 is connected. As seen in FIGS. 8 and 9 the intake end cap 42 is secured to body 40 by screws or bolts 45 through which a wire 46 extends to maintain the screws 45 in place. Body 40 is provided with a groove or cutaway 47 at the end adjacent end cap 42 and in which is retained a poppet seat 48 having an opening therein which aligns with opening 44 of end cap 42. Located between end cap 41 and the adjacent end of body 40 is a static seal 49. A piston or spool 50 is movably located within cavity 40&#39; of body 40 and includes a pair of head sections 51 and 52 interconnected by a reduced diameter section 53, with head section 51 having a countersink 54 therein to reduce weight. A poppet seal 55 is secured around the exterior of piston head 51 and a pilot or signal seal 56 is secured around the exterior of piston head 52. Valve body 40 is provided with a plurality of holes or apertures 57 located in spaced relation therearound (also shown in FIG. 3), and provided with an enlarged diameter section of cavity 40&#39; indicated at 40&#34; in which is located an enlarged opening or slot 59. Connected to valve body 40 via enlarged opening 59, as seen in FIG. 7, is a member 60 which is mounted on cylinder head 12 around opening 19, as shown in FIG. 4, whereby fluid is directed through valve 21 into or exhausted from the region 36 within cylinder 11. 
     In operation of the valve 21, FIG. 7 illustrates the valve in its intake position, wherein signal fluid via line 22&#39; from an associated valve assembly of the pair is cut-off as being in exhaust position wherein fluid (hot gas) from an external source is directed through line 23 against head 51 of piston or spool 50 causing the piston to move to the left end of cavity 40&#39; as shown, thereby directing fluid through enlarged cavity 40&#34;, opening 59 and member 60 into the region 36 of cylinder 11 causing piston 14 to move to the left (start of piston stroke) as shown in the upper section of FIG. 4 which causes rod 34 of signal valve 20 to move to the left (into cylinder region 36). 
     FIG. 9 illustrates the valve 21 in its switching position, between intake and exhaust positions, and such is produced by signal fluid from the associated valve being directed through line 22&#39; onto head 52 of piston or spool 50 causing same to move in cavity 40&#39; toward the right end of the cavity as shown. By such movement of piston, the piston head 51 blocks passage of fluid from line 23 into enlarged cavity 40&#34; thereby cutting of fluid flow into region 36 of cylinder 11 stopping further movement of piston 14 to the left (end of piston stroke), as well as the movement of rod 34 of signal valve 20 to the left. Signal fluid via line 22&#39; continues to move the piston 50 of valve 21 into the right end of cavity 40&#39; shown in FIG. 10, thereby allowing fluid to pass from region 36 of cylinder 11 through member 60, opening 59 enlarged cavity 40&#34;, port 40&#39; and out through exhaust ports or holes 57. This allows piston 14 to move to the right as shown in FIG. 4, causing signal valve rod 34&#39; to move to the right within valve 20. 
     To enable an understanding of the operation of valves 20 and 21, which operate in pairs when mounted on pump assemblies, attention is again directed to FIGS. 2A-2F. The operation of the various components of the valves 20 and 21 can be readily seen by indicating which component of the valves are the equivalent of the components illustrated in FIGS. 1 and 2A-2F. In FIG. 1, the signal valve assembly 7 which includes components 8, 9, 9&#39; and 9&#34; correspond to components in valve 20 wherein rod 34 (34&#39;) with lands 35 (35&#39;) (component 8), port 33 and signal lines 22 and 24 (components 9 and 9&#34;), and exhaust port 29 (component 9&#39;) correspond. As to intake/exhaust valve 3 of FIG. 1, similar components of valve 21 are: piston or spool 50 (spool 4), intake line 23 (line 5&#39;), member 60 (intake port 5), ports 57 (exhaust ports 6), and signal port 22&#39; (line 6&#39;). 
     It has thus been shown that the present invention provides a simplified control valve arrangement which is particularly adapted for systems using fluid-driven pumps, with the pump/valve assembly being operable in pairs wherein the control valve assembly of one pump/valve assembly is switched by a pressure signal depending on the state of the piston of the other pump/valve assembly. Also, it has been shown how the present invention is applicable to liquid and bi-liquid applications, such as those using either free-piston or bellows type pumps, so as to provide continuous liquid pumping of both liquids, as well as providing a modification for one of the liquid pumps. 
     While the invention has been described primarily in monopropellant and bipropellant liquid propulsion systems, it is not intended to limit the use of the invention to these particular systems. The invention can be utilized for various liquid and gas systems particularly those requiring storage at a low pressure and requiring a higher pressure at the point of use, as well as for pressurizing a plenum or operating an oscillating mechanism. Also, for certain applications diaphragm type pumps can be utilized. 
     While a particular embodiment of the invention have been described and illustrated, such is not intended to limit the invention, since other embodiments and applications will become apparent to those skilled in the art. The invention is intended to be limited only by the scope of the appended.