Abstract:
An improved two-stage light gas gun for launching projectiles at high speeds. The gun consists of three tubes: the expansion, pump, and launch tubes. The expansion tube contains a close-fitting expansion piston that is propelled by an explosive charge. The expansion piston in turn drives the pump piston housed within the pump tube by means of a rod connecting the two pistons. The action of the pump piston adiabatically compresses and heats a light gas of hydrogen or helium, bursting a diaphragm at a predetermined pressure and expelling the projectile from the launch tube at a very high speed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None 
     FEDERALLY SPONSORED RESEARCH 
     None 
     SEQUENCE LISTING 
     None 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a type of gun known as a two-stage light gas gun, which is designed to fire projectiles at very high speeds. 
     2. Background of the Invention 
     A light gas gun is designed to shoot projectiles at very high speeds by utilizing a high-pressure gas of low atomic number, typically either hydrogen or helium. Used extensively for research involving hypervelocity projectiles, the use of light gases as a propelling medium has produced projectile speeds up to several times greater than the highest speed attained by guns utilizing conventional propellants such as modern gunpowders. 
     In the prior art there exists various designs of light gas guns that can generally be categorized as being of one-stage, two-stage, or three-stage design. All three types of light gas gun designs are capable of firing projectiles at hypervelocity speeds. The object of this invention relates to the two-stage design. It should be mentioned that another type of hypervelocity gun appearing in the prior art is the shock wave gun, which in some embodiments takes the form of a special type of two-stage light gas gun. 
     In the two-stage light gas gun design, either hydrogen or helium gas is initially held within a so-called pump tube. Within the pump tube is a piston called the pump piston that is used to compress the light gas. Rigidly connected to one end of the pump tube is a so-called launch tube that holds a projectile to be launched. An explosive charge, such as gunpowder or a fuel/air mixture, lies on one side of the pump piston. On the other side of the piston is the light gas along with a diaphragm that initially prevents the light gas from flowing from the pump tube into the launch tube. The diaphragm, which is placed near the junction of the pump and launch tubes, is a type of one-use valve that is designed to burst open at a preset pressure. When the explosive charge is ignited it causes the piston to accelerate towards the diaphragm, an action that quickly compresses the hydrogen (or helium). When the piston has compressed the light gas to a predetermined pressure, the diaphragm bursts open. The high-pressure, hot hydrogen (or helium) pours through the burst diaphragm and into the launch tube, which in turn causes the projectile to be expelled from the launch tube&#39;s muzzle. The launch tube is typically several times smaller in diameter than the pump tube. The pump and launch tubes together form the overall length of a conventional two-stage light gas gun. 
     In the NACA technical note 4143 by Charters et al (1957) a two-stage light gas gun is described that contains a pump piston as well as a heavier secondary piston called a valve piston. After ignition of the powder charge, the pump piston and valve piston are driven in opposite directions along the length of the pump tube. The movement of the heavy valve piston allows the delayed release of hot propellant gases from the pump tube. The pump piston is designed to ‘bounce back’ after the diaphragm is ruptured, preventing it from ramming into the end of the pump tube, which could possibly damage the gun. In spite of its positive features, this design has several drawbacks. First, between firings the gun must be partially disassembled in order to return the pump and valve pistons to the firing position. Another drawback is the residue—such as a carbon buildup—that forms due to the repeated use of a solid propellant in the pump tube, which must periodically be cleaned out. Another disadvantage is that the pump tube must be lengthened in order to accommodate the rearward movement of the valve piston. A final drawback is that a danger exists that if too much propellant is used, or an insufficient quantity of light gas is present before firing, that the freely-moving pump piston will collide with the end of the pump tube, leading to damage of the piston, the pump tube, or both. 
     In U.S. Pat. No. 2,872,846 Crozier (1959) shows an alternative embodiment that is basically identical to the Charters (1957) design described above, except that Charters&#39; valve piston, whose action allows leftover propellant to escape the pump tube, is absent. The simpler design, however, leads to its first drawback: there is no provision for the automatic and convenient venting of propellant gases once the gun has been fired. Other than that difference, Crozier&#39;s design has several distinct disadvantages in common with Charters&#39; design. First, a danger exists that if too much propellant is used, or an insufficient quantity of light gas is present before firing, the pump piston will collide with the end of the pump tube, leading to damage of the piston, the pump tube, or both. Second, between firings the gun must be partially disassembled in order to return the pump piston to the firing position. Third, due to the repeated use of a solid propellant in the pump tube, residue forms that periodically must be cleaned out. 
     In contrast to the design of Charters et al (1957) summarized above in which the pump piston bounces back from the end of the pump tube, U.S. Pat. No. 2,882,796 to Clark et al (1959) describes a pump piston designed to purposely ram into the diaphragm-end of the pump tube. The pump piston is made of a material—such as nylon—that is readily deformable under high pressures. This design has the advantage that it eliminates the concern of damage to the pump tube by the pump piston, since the pump piston is specifically design to impact and then squeeze into the constriction of the pump tube that leads into the launch tube. However, there are distinct disadvantages of this design: 1) as in the Crozier (1959) design described previously, there is no mechanism provided to automatically vent the remaining propellant gases once the gun has been fired; 2) the pump tube must be opened up so that the tightly squeezed compression piston can be extricated, considerably slowing the process of preparing the gun for another firing; 3) after each firing, residue from the propellant can contaminate the interior of the pump tube; and 4) after each firing the old pump piston is severely distorted and must be discarded, while a new pump piston must be loaded into the pump tube. Discarding the pump piston after each shot increases costs as compared to a pump piston that can be reused repeatedly. 
     In U.S. Pat. No. 4,038,903 Wohlford (1977) describes a telescoped two-stage light gas gun. The telescoped gun was intended as an anti-aircraft weapon, its design permitting a higher rate of fire as compared with previous two-stage light gas gun designs. The gun is designed so that the pump piston and launch tube always move together as a single, ridged unit. One favorable feature of the gun is that the area of the pump piston that the propellant gas pushes against is greater than the area of the pump piston that compresses the light gas; unfortunately, the ratio of propellant area to compression area is not very high, being only fractionally higher than unity, i.e., much closer to a ratio of 1 than to a ratio approaching 2 or more. In spite of a few favorable features, the telescoped design suffers from a number of drawbacks: 1) in order for there to be a good seal between the outside of the gun barrel and the inside of the pump tube opening, not only must the inside of the gun barrel be machined to a high degree of precision (which is normally the case for most gun barrels), but also both the outside of the gun barrel and the inside of the pump tube opening must be machined very close to round as well. However, repeated firing of the weapon will heat its various parts. If the gun barrel is heated more or less than the pump tube, the expansion of the two parts will also vary, which could lead to either significant loss of gas at the pump tube/launch tube seal, or to increased friction at the same seal thereby slowing the motion of the pump piston; (2) this design allows propellant residue to form on both the inside of the pump tube and the outside of the launch tube, which can lead to increased wear of those parts, as well as the need for frequent cleanings of those same parts; (3) after a projectile is fired from the gun, the reloading of another projectile is overly complicated. First the rear of the pump tube must be opened, and then the rear of the launch tube must be opened as well. After the projectile (and possibly a diaphragm) is loaded, first the launch tube must be closed, followed by closure of the pump tube. Such a procedure takes an inordinate amount of time for a gun designed to be a weapon; (5) if too little light gas is introduced into the pump tube, then the pump tube piston might violently collide with the end of the pump tube housing, damaging or destroying the gun; and (6) in the telescoped gun design, the breach end of the launch tube is rigidly connected to the pump piston. That pump piston/launch tube connection is riddled with holes that allow the hot, compressed light gas to enter from the pump tube. Such a design is structurally much weaker than in other light gas gun designs, wherein there is a simple transition from the pump tube into the launch tube, and said transition of the two tubes is very strong because it is encased within a large block of metal. 
     In U.S. Pat. No. 4,658,699 Dahm (1987) describes a two-stage light gas gun referred to as a ‘wave gun’. The wave gun uses a light and flexible pump piston that—after the projectile has exited the launch tube—is forced through the pump tube/launch tube constriction, and then travels through and out the launch tube. Higher muzzle velocities of the projectile are claimed for this design, as compared to other two-stage light gas guns. The design, however, is beset by a variety of drawbacks: (1) expulsion of the light piston entirely from the gun means that propellant residue contaminates not only the pump tube, but the launch tube as well; (2) the mechanical integrity of the pump piston is questionable because it is designed to travel back and forth within the pump tube several times before finally being expelled from the gun. Such a ‘wave’ motion with the hot, high-pressure propellant gas on one side and the hot, high-pressure light gas on its other side would put enormous stresses on such a light and deformable piston, which could well lead to a blow-by of the propellant and/or light gases and subsequent contamination of the light gas with propellant, which in turn would degrade the interior ballistic performance of the projectile; (3) increased erosion of the launch tube interior. High velocity light gas guns have traditionally suffered from erosion of the launch tube after each firing of the gun. But the wave gun not only expels the projectile and associated light gas from the launch tube, but the pump piston and the propellant as well. The additional material ejected through the launch tube at high speeds would probably increase launch barrel erosion significantly as compared to more conventional designs; and (4) a final drawback of the wave gun design is that if all or part of the deformable pump piston does not completely leave the launch tube, its presence could impede a subsequent firing with potentially catastrophic damage to the gun. 
     In the article titled “World&#39;s Largest Light Gas Gun Nears Completion at Livermore” appearing in Aviation Week and Space Technology/Aug. 10, 1992/pp 57-59, a two-stage light gas gun designed by John Hunter uses a methane/air mixture as the propellant to accelerate a heavy steel piston down a long pump tube to compress the light gas. The pump tube is at a right angle to the launch tube. Shock absorbers negate the recoil transmitted through both the pump and launch tubes. The pump and launch tubes are connected in such a way that the launch tube can be swiveled to any angle from horizontal up to vertical. A positive feature of Hunter&#39;s design is that it uses a clean-burning and inexpensive propellant source. However, the design possesses a number of disadvantages: (1) the pump tube is excessively long compared to the launch tube length; indeed, the prototype that was constructed had a pump tube nearly twice as long as the launch tube. Such a long pump tube makes for an unwieldy design, and means a much more expensive gun; (2) a right angle between the pump and launch tubes leads to large torques on each tube that are eliminated with shock absorbers, which increases complexity and the total cost of the gun. Moreover, failure of a shock absorber could lead to severe damage of the gun, especially in the vicinity where the pump and launch tubes meet; (3) even though methane is typically very clean burning as compared to, say, gunpowder, if the combustion of methane is not complete, carbon deposits could still form in the pump tube; (4) after the gun is fired the freely-moving, heavy pump piston must be returned the length of the long pump tube before another firing can take place, slowing the time between firings; and (5) the swivel connection between the pump and launch tubes, which allows a projectile is to be fired at various angles, must be made of very strong materials and to very close tolerances so that no leakage of hot gases occurs, which all translates into a significant increase in the cost of the gun. 
     In NASA Contractor Report 4491 titled “Concept Definition Study for an Extremely Large Aerophysics Range Facility” by Hallock F. Swift, dated February 1993, a two-stage light gas gun is proposed that foregoes the use of a combustible propellant to propel the pump piston, using instead helium compressed to 15,000 pounds per square inch. The helium is held within high-pressure storage tanks until it is quickly released into the pump tube, at which time the highly compressed helium accelerates a large and heavy pump piston down the pump tube, compressing low-pressure helium on the opposite side of the pump piston, which in turn launches the projectile from the launch tube. 
     A prominent feature of the proposed light gas gun is that no propellant residue should form in the pump tube since the propelling gas—namely helium—is non-combustible. In spite of that advantageous characteristic, the design has a number of other features that are decidedly disadvantageous: first, the pump piston is partially deformed on each shot, and must be either discarded completely, or repaired for subsequent use, and either option translates into increased cost per shot from the gun; second, at the end of each firing the pump tube must be opened and a device inserted in order to retrieve the used pump piston, a procedure which considerably slows the process of readying the gun for another firing; third, helium used as the propelling gas of the pump piston is rather expensive; therefore, the design calls for reuse of the helium, which entails pumping it from the pump tube back into the original storage tanks; the reuse of the helium increases the complexity of the entire gun system, and greatly delays the possible time between firings; the author cites a ballpark figure of around an hour to recompress the helium; while higher-capacity pumps could certainly decrease the time needed to recompress the helium, the higher initial and ongoing costs associated with their use would also significantly increase the overall cost of the entire system. 
     As demonstrated above, there are many different designs of two-stage light gas guns known in the prior art. Each design possesses various strengths and weaknesses, some of which were outlined above; however, the designs known heretofore all suffer from a number of drawbacks:
         (a) after the gun is fired, the pump piston cannot be quickly returned to its original start position for another firing of the gun;   (b) the length of time to reload the gun with a projectile is excessive;   (c) in the prior art a number of different types of gases have been used to propel a pump piston down the length of a pump tube, but under the right conditions any type of propelling gas is capable of leaving residues within the pump tube that build up over repeated firings of the gun;   (d) after the gun is fired, the spent propellant gas is expelled either through the use of some type of valve integrated into the pump tube, which adds cost and complexity to the gun design, or by exiting through the launch tube, which can foul the launch tube with propellant residue and/or increase interior erosion;   (e) the area of the pump piston the propelling gas pushes against versus the area of the pump piston that compresses the light gas is restricted in all previous designs known heretofore, and that restriction limits the utility of those designs; specifically, most designs in the prior art set the area of the pump piston that the propelling gas pushes against equal to the area of the pump piston compressing the light gas; but at least one design results in a ratio of propelling area to compression area slightly greater than 1; however, no known previous design allows a broad range of ratios.   (f) no design known heretofore is easily adapted to a variety of roles; a design that is well-suited for laboratory research is unwieldy when applied to a military role or space launch applications, and vice versa;       

     BACKGROUND OF INVENTION 
     Objects and Advantages 
     Several objects and advantages of my invention are:
         (a) to provide a two-stage light gas gun in which the pump piston can quickly be returned to its start position for another firing;   (b) to provide a two-stage light gas gun that can be quickly reloaded with a projectile for a subsequent firing of the gun;   (c) to provide a two-stage light gas gun that prevents any possible residue from the gas propelling the pump piston from contaminating either the pump or launch tubes;   (d) to provide a two-stage light gas gun in which the gas propelling the pump piston is quickly and automatically vented without the need for valves;   (e) to provide a variety of possible ratios, from less than one, to equal to one, to greater than one, of the area of the piston the propelling gas pushes against versus the area of the pump piston compressing the light gas;   (f) to provide a two-stage light gas gun design that performs well in a variety of roles: laboratory research, anti-armor, very long-range artillery, and shots into outer space.       

     Further objects and advantages are to provide a two-stage light gas gun in which the pump piston can be halted reliably at a predetermined position within the pump tube, which can utilize inexpensive and clean-burning propellants—such as an alcohol/air mixture—without the need for an excessively long pump tube, which can use the spent propellant gas to counteract the recoil due to firing the gun, which does not deform the pump piston as part of the gun&#39;s firing cycle, which provides for a pump tube that is considerably shorter than the launch tube, and in which the projectile can be loaded into the gun via a conventional breech block. Still further objects and advantages of my invention will become apparent upon consideration of the drawings and ensuing description. 
     SUMMARY 
     In accordance with the present invention an improved two-stage light gas gun for launching projectiles at very high speeds, and consisting of three main parts: a launch tube from which a projectile is fired; a pump tube filled with pressurized hydrogen or helium; and an expansion tube containing a propellant charge. When the propellant charge is ignited a piston in the expansion tube is driven forward and pushes on a piston in the pump tube, compressing the hydrogen or helium, which in turn expels the projectile from the launch tube at high speed. 
    
    
     
       DRAWINGS 
       Figures 
       In the drawings, closely related figures are identified by the same number but with different alphabetic suffixes. 
         FIG. 1  shows a lateral cross-sectional view of the preferred embodiment of a two-stage light gas gun constructed in accordance with the present invention. 
         FIG. 2  shows a magnified view of a more-or-less central portion of  FIG. 1 . 
         FIGS. 3A-3F  depict the steps involved in firing the two-stage light gas gun of the preferred embodiment of the invention. 
         FIG. 4  shows a lateral cross-sectional view of an alternative embodiment of a two-stage light gas gun constructed in accordance with the present invention. 
         FIGS. 5A-5E  depict the steps involved in firing the two-stage light gas gun of the alternative embodiment of the invention shown in  FIG. 4 . 
         FIG. 6  shows a cross-sectional, muzzle-end view of the expansion tube and launch tube of the alternative embodiment of  FIG. 4 . 
         FIG. 7  shows a cross-sectional, muzzle-end view of an expansion tube and launch tube; the expansion tube cross-section is an alternative to the embodiment of  FIG. 4 . 
         FIG. 8  shows a lateral cross-sectional view of an alternative embodiment of the invention. 
         FIG. 9  shows a lateral cross-sectional view of an alternative embodiment of the invention. 
         FIG. 10  shows a lateral cross-sectional view of an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     FIGS.  1  and  2 —Preferred Embodiment 
     A preferred embodiment of the two-stage light gas gun of the present invention is depicted in  FIG. 1 , which is of a lateral, cross-sectional view.  FIG. 2  shows a magnified portion of  FIG. 1 . The gun can conveniently be divided into four segments: the expansion tube  10 , pump tube  11 , connecting block  12 , and launch tube  13 . The four main segments of the gun are made out of any suitable material typically employed in producing guns, such as high strength steel. Materials lighter than steel, such as titanium, or metal matrix composites, can also be employed if their tensile and compressive strengths are adequate for the role. 
     A shoulder  14  near the middle of expansion tube  10  defines combustion region  15 . A one-way valve  16  allows an oxidizing gas, such as air, nitrous oxide, or pure oxygen, to flow into combustion chamber  15  but prevents it from passing back out. The gas is supplied from a pump or pressurized tank  17  that is connected to one-way valve  16 . 
     Fuel injector  18  is connected to fuel tank  19  by fuel line  20 , which may be of either rigid or flexible construction. Spark plug  21  is connected to power supply  22 , which is grounded to expansion tube  10  by metallic bolt  23 . Pressure relief valve  24  opens automatically if the pressure inside combustion chamber  15  exceeds a predetermined safe value; valve  24  can also be opened manually. 
     Within expansion tube  10  is expansion piston  25 , which is connected to smaller pump piston  26  within pump tube  11  by connecting rod  27 . On the piston side of shoulder  14  is o-ring  28 . Expansion tube  10  has the four removable plugs  29   t  (“t” stands for “top”),  29   b  (“b” stands for “bottom”),  30   t , and  30   b . At one end of expansion tube  10 , at the end opposite combustion chamber  15 , are end-stops  31   t  and  31   b , held in place by bolts  32   t  and  32   b , respectively. 
     Situated between expansion tube  10  and pump tube  11  are return rollers  33   t  and  33   b . At one end of pump tube  11  is end cap  34 , the inside face of which holds o-ring  35 . One-way valve  36  allows a light gas, either hydrogen or helium, to flow into cavity  39  defined by pump tube  11 , but prevents the light gas from flowing back out. Pressure tank  38  contains a light gas and is connected by high-pressure line  37  to one-way valve  36 . Connecting block  12  holds diaphragm  40 . Projectile  41  lies within launch tube  13  and adjacent to diaphragm  40 . 
     Operation 
     FIGS.  1 ,  3 A- 3 F 
     Operation of the two-stage light gas gun that is the object of this invention begins with unscrewing launch tube  13  from connecting block  12  and loading diaphragm  40  and projectile  41  ( FIG. 1 ). Diaphragm  40  may be held in place by any convenient means, such as a slight taper of its outer surface, along with a corresponding taper of the inner portion of connecting block  12  where diaphragm  40  fits (said taper is not represented in  FIG. 1 ). Plugs  30   t  and  30   b  have been removed as shown in  FIGS. 3A through 3F  in order to allow the venting of the spent propellant gas; plugs  29   t  and  29   b  remain in place, but could have been removed to allow venting of the propellant gas earlier in the firing sequence. With diaphragm  40  and projectile  41  loaded into the gun and launch tube  13  screwed back into connecting block  12 , the sequence of events leading to expulsion of the projectile from the gun appears in  FIGS. 3A through 3F  (in what follows, identifying numbers refer back to  FIG. 1  and/or  FIG. 2 ). 
     In  FIG. 3A , either hydrogen or helium gas has been supplied under pressure from tank  38 , through high-pressure line  37  and one-way valve  36  into cavity  39  of pump tube  11 . The stippling within cavity  39  indicates the presence of the hydrogen or helium gas. The pressure of the gas within cavity  39  pushes upon pump piston  26 , forcing it against end cap  34 . O-ring  35 , being squeezed between pump piston  26  and end cap  34 , forms a tight seal that prevents the pressurized gas from leaking out of pump tube  11 . The pressure exerted upon pump piston  26  by the pressurized gas in cavity  39  is also partially exerted upon expansion piston  25  by way of connecting rod  27 . The resulting force acting upon expansion piston  25  squeezes o-ring  28  up against shoulder  14 , forming a tight seal. In order to ensure that adequate force is applied to both o-ring seals  28  and  35 , the distance between pump piston  26  and expansion piston  25  may be adjusted by screwing connecting rod  27  further into, or out of, either piston individually. 
     Continuing with  FIG. 3A , combustion chamber  15  has been pressurized with air or other oxidizing gas via pressurized tank  17  and one-way valve  16 , immediately after which liquid fuel, such as alcohol, is supplied from fuel tank  19 , through fuel line  20 , and injected by fuel injector  18  into combustion chamber  15 . The stippling within combustion chamber  15  depicts the resulting fuel/air, or more broadly, the fuel/oxidizer, mixture. The pre-ignition pressure within combustion chamber  15  is held sufficiently lower than the pressure within pump tube  11  so that expansion piston  25  is held tightly against shoulder  14  and o-ring  28 . To illustrate this principle, suppose the pressure of the light gas within pump tube  11  is 1,000 pounds per square inch. If the area of expansion piston  25  that is exposed to the fuel/air mixture is equal to the area of pump piston  26  that is exposed to the high-pressure light gas within cavity  39 , then a pre-ignition fuel/air pressure of 250 pounds per square inch results in a force on the left face of expansion piston  25  that is one-fourth as large as the force pushing on the right face of pump piston  26 . As long as the larger force exerted through pump piston  26  is properly distributed by connecting rod  27 , both pistons will be firmly pressed up against their adjacent o-rings, i.e., o-rings  28  and  35 . 
       FIG. 3B  depicts ignition of the fuel-air mixture by means of spark plug  21 . Combustion of the fuel/air mixture greatly increases the pressure within combustion chamber  15  so that the force pushing expansion piston  25  to the right is considerably greater than the force pushing pump piston  26  to the left. In  FIG. 3C  both pistons, along with connecting rod  27  joining them, have moved in unison to the right. After expansion piston  25  separated from shoulder  14  a much greater surface area of expansion piston  25  was exposed to the hot combustion gases, which in turn greatly increased the force pushing expansion piston  25  to the right. In  FIG. 3C  the light gas within cavity  39  of pump tube  11  has been considerably compressed from its initial volume. 
     In  FIG. 3D  expansion piston  25  has moved past the second set of plugs,  30   t  and  30   b , but not yet met end-stops  31   t  and  31   b . The force due to the combustion gas has fallen dramatically, partly because energy has been extracted from it, and partly because the gas is being vented through the open plugs. Also in  FIG. 3D  diaphragm  40  has been breached and the hot, high-pressure light gas is shown accelerating projectile  41  down launch tube  13 . Even though the light gas is now at a much higher pressure than the combustion gas in expansion tube  10 , the piston/connecting rod structure continues to move to the right due to its momentum. Only when expansion piston  25  has impacted end-stops  31   t  and  31   b  does the entire piston/connecting rod structure come to a halt, as shown in  FIG. 3E . Note also in  FIG. 3E  that projectile  41  has completely exited launch tube  13 , and that the combustion gas and light gas have both been largely dissipated, as indicated in expansion tube  10  by the reduced amount of stippling within it, and in launch tube  13  by the complete absence of any stippling. 
       FIG. 3F  shows the piston/connecting rod structure returning to its original start position depicted in  FIG. 3A  via the impetus supplied by return rollers  33   t  and  33   b . Valve  24  has been manually opened to allow the venting of residual propellant gas trapped by the return of expansion piston  25  to its position adjacent to shoulder  14 . Valve  24  is then closed, and the process of readying the gun for launching another projectile is repeated as described at the beginning of this section. 
     FIG.  4 —First Alternative Embodiment 
     An alternative embodiment of the present invention is shown in  FIG. 4 . Like the preferred embodiment, the two-stage light gas gun shown in  FIG. 4  can be conveniently divided into the four sections of expansion tube  50 , pump tube  51 , connecting block  52 , and launch tube  53 . However, in the preferred embodiment shown in  FIG. 1  those four components—the expansion tube  10 , pump tube  11 , and launch tube  13 , as well as the connecting block  12 —are laid out linearly (that is, they share a common axis); by contrast, in the alternative embodiment of  FIG. 4 , the expansion tube  50  and pump tube  51  lie beneath launch tube  53 . It is noted that while pump tube  51  and launch tube  53  possess a cylindrical shape, expansion tube  50  has a rectangular shape, as depicted in the muzzle-end-view perspective of  FIG. 6  (only launch tube  53  and expansion tube  50  appear in  FIG. 6 ). 
     Shoulder  54  near the middle of expansion tube  50  helps define combustion chamber  55 . One-way valve  56  allows an oxidizing gas to flow into combustion chamber  55  but not back out. The oxidizing gas is supplied through high pressure line  57 . 
     Fuel injector  58  is supplied by fuel line  59 , which may be of either rigid or flexible construction. Spark plug  60  is connected to power supply  61 , which is grounded to expansion tube  50  by metallic bolt  62 . Valve  63  acts as a pressure relief valve opening automatically if the pressure inside combustion chamber  55  exceeds a predetermined safe value; valve  63  can also be opened by movement of linkage  64 . 
     Expansion tube  50  and launch tube  53  are rigidly attached to each other by connectors  65   l  and  65   r  (“l” stands for left, and “r” for right). Within expansion tube  50  is expansion piston  66 , which is connected to smaller pump piston  67  within pump tube  51  by connecting rod  68 . On the piston side of shoulder  54  is o-ring  69 . Situated between expansion tube  50  and pump tube  51  are return rollers  70   t  and  70   b  (“t” stands for top, and “b” for bottom). Idler sprocket  71  and rocker arm  72  are situated beneath return rollers  70   t  and  70   b . At one end of expansion tube  50 , opposite combustion chamber  55 , is end-stop  73 . Between end-stop  73  and expansion piston  66  is exhaust port  76 , which is threaded. 
     At the one end of pump tube  51  is end cap  74 , the inside face of which holds o-ring  75 . One-way valve  77  allows a light gas to flow into cavity  78  that is defined by pump tube  51  and connecting block  52 . High pressure line  79  supplies a light gas to one-way valve  77 . 
     Screw-type breach block  80  is screwed into connecting block  52 . Connecting block  52  holds diaphragm  81 . Projectile  82  lies within launch tube  53  and adjacent to diaphragm  81 . 
     Operation of First Alternative Embodiment 
     FIGS.  4 ,  5 A- 5 E 
     The description of the operation of the alternative embodiment will be more concise than for the preferred embodiment since the operation of the two is very similar. The sequence of events leading to expulsion of the projectile from the gun appears in  FIGS. 5A through 5E ; reference numbers refer back to  FIG. 4 . 
     Operation of the alternative embodiment begins with unscrewing breach block  80  from connecting block  52 , followed by loading projectile  82  into the breach-end of launch tube  53 , with diaphragm  81  then placed behind, and in contact with, projectile  82 . In  FIG. 5A , either hydrogen or helium gas has been supplied under pressure into cavity  78  via one-way valve  77 . The pressure of the gas within cavity  78  pushes upon pump piston  67 , forcing it against end cap  74 . O-ring  75 , being squeezed between pump piston  67  and end cap  74 , forms a tight seal that prevents the pressurized gas from leaking out of cavity  78 . The pressure exerted upon pump piston  67  by the pressurized gas in cavity  78  is also exerted upon expansion piston  66  by way of connecting rod  68 . The resulting force upon expansion piston  66  squeezes o-ring  69  up against shoulder  54 , forming a tight seal. 
     Continuing with  FIG. 5A , combustion chamber  55  has been pressurized with an oxidizing gas via one-way valve  56 , and injected with liquid fuel via fuel injector  58 . Ignition of the fuel/air mixture by means of spark plug  60  is depicted by the squiggly lines appearing in  FIG. 5A . 
     In  FIG. 5B  both pistons, along with connecting rod  68  joining them, have moved in unison to the left in response to the combustion of the fuel/air mixture originally confined in combustion chamber  55 . Movement to the left of connecting rod  68  rotates return rollers  70   t  clockwise and  70   b  counterclockwise, winding torsion springs affixed to each. Idler sprocket  71  is engaged by return roller  70   b , which in turn rotates rocker arm  72  counterclockwise, thereby shifting linkage  64  to the right. Linkage  64  pushes a lever on valve  63 , but not to the point where valve  63  is yet open. 
     In  FIG. 5C  movement of expansion piston  66  has exposed port  76 , allowing hot combustion gases to vent from expansion tube  50 . Moreover, rocker arm  72  has rotated further counterclockwise, shifting linkage  64  further to the right which opens valve  63 , thereby venting additional hot combustion gases from expansion tube  50 . Also in  FIG. 5C  diaphragm  81  has been breached and the hot, high-pressure light gas has pushed projectile  82  partway down launch tube  53 . Even though the light gas is at a higher pressure than the combustion gas in expansion tube  50 , the piston/connecting rod structure continues to move to the left due to its momentum. Only when expansion piston  66  has impacted end-stop  73  does the entire piston/connecting rod structure come to a halt, as shown in  FIG. 5D . Note also in  FIG. 5D  that projectile  82  has completely exited launch tube  53 . 
       FIG. 5E  shows return of the piston/connecting rod structure mid-way towards its original start position of  FIG. 5A  by return rollers  70   t  and  70   b  via the force applied by their embedded torsion springs. As expansion piston  66  reaches shoulder  54 , valve  63  is closed; then the process of readying the gun for launching another projectile can be repeated as described at the beginning of this section, with the caveat that once breach block  80  is unscrewed from connecting block  52 , the spent diaphragm is removed before the loading of a new diaphragm  81  and projectile  82  can commence. 
     FIG.  8 —Second Alternative Embodiment 
     The second alternative embodiment of the invention, shown in  FIG. 8 , is quite similar to the first alternative embodiment shown in  FIG. 4 , so the description of its parts and its operation will be abbreviated. The principle difference between the first and second alternative embodiments is that expansion tube  50  shown in  FIG. 4  has been eliminated. In place of an expansion tube, and the many ancillary components associated with it, there is electric motor  90 , small gear  91 , and large gear  92 . 
     In contact with large gear  92  is toothed rod  93 , near the middle of which is bar stop  94 . Attached to the threaded end of toothed rod  93  is pump piston  95 , which lies within pump tube  96 . One-way valve  97 , which is supplied through high-pressure line  98 , is attached to pump tube  96 , as are end stops  99   a  and  99   b  (“a” stands for “above”, while “b” stands for “below”). Affixed to pump piston  95 , and squeezed between pump tube  96  and pump piston  95 , is o-ring  100 . Both pump tube  96  and screw-type breach block  102  are threaded into connecting block  101 . Launch tube  103  contains projectile  104  and diaphragm  105 . 
     The operation of the two-stage light gas gun depicted in  FIG. 8  is as follows: first, screw-type breach block  102  is unscrewed and projectile  104  is loaded into launch tube  103 , followed by diaphragm  105 . Screw-type breach block  102  is then replaced. Light gas is subsequently directed from high pressure line  98 , through one-way valve  97 , and into pump tube  96  until the gas pressure reaches a predetermined level. The light gas cannot escape past pump piston  95  due to the compression seal of o-ring  100 . 
     Electric motor  90  then spins smaller gear  91  clockwise, causing the counterclockwise rotation of larger gear  92 , which in turn engages the teeth of toothed rod  93 , pushing toothed rod  93  and attached pump piston  95  down pump tube  96  in the direction of screw-type breach block  102 . Movement of pump piston  95  down pump tube  96  compresses the light gas introduced through one-way valve  97 , until sufficient pressure is attained, rupturing diaphragm  105 , and propelling projectile  104  down and out of launch tube  103 . 
     After diaphragm  105  ruptures, power to electric motor  90  is shut off; however, pump piston  95  continues to compress the light gas for a short period of time due to its own momentum, along with the combined momentum of attached toothed rod  93 , and gears  91  and  92 , and electric motor  90 . Forward motion of pump piston  95  and toothed rod  93  is finally halted by pressure of the light gas pushing on pump piston  95 , as well as by the impact of bar stop  94  with end stops  99   a  and  99   b.    
     The slow reversal of electric motor  90  reverses the rotation of gears  91  and  92 , which retracts toothed rod  93  and pump piston  95  until o-ring  100  is again compressed. A new firing cycle can then commence with opening of screw-type breach block  102  as described previously, with the single caveat that the previously-used diaphragm  105  is discarded before the loading of a new projectile  104  and new diaphragm  105 . 
     FIG.  9 —Third Alternative Embodiment 
     This embodiment of the invention, depicted in  FIG. 9 , is very similar to the second alternative embodiment shown in  FIG. 8 . What differentiates the two embodiments is that electric motor  90 , small gear  91 , large gear  92 , and toothed rod  93  have been replaced with a new set a parts; otherwise, the components of the two embodiments are identical. That new set of parts consists of electric motor  110 , pulley  111 , cable  112 , smooth rod  116 , compression springs  113   a  and  113   b  (“a” stands for “above” and “b” stands for “below”), brackets  114   a  and  114   b , and connectors  115   r  and  115   f  (“r” stands for “rear” and “f” stands for “front”). For the parts of the two embodiments that are identical, their corresponding reference numbers are the same in  FIGS. 8 and 9 . 
     Compression springs  113   a  and  113   b  are affixed at one end to brackets  114   a  and  114   b , and at the other end to bar stop  94 . Brackets  114   a  and  114   b  are each connected at one end to pump tube  96 . Connectors  115   r  and  115   f  support launch tube  103  by rigidly connecting launch tube  103  to bracket  114   a.    
     The operation of the third alternative embodiment shown in  FIG. 9  in terms of loading and firing the gun is exactly the same as the operation of the second alternative embodiment shown in  FIG. 8 , with the exception of how pump piston  95  is propelled down pump tube  96  to compress the light gas. 
     In the third alternative embodiment shown in  FIG. 9 , after projectile  104  and diaphragm  105  are loaded and the light gas is introduced into the gun, all in the manner described previously for the second alternative embodiment, the gun is ready to be fired. Initially, pulley  111  is prevented from rotating, which keeps sufficient tension on cable  112  such that compression springs  113   a  and  113   b  cannot expand and push upon bar stop  94 . The gun is fired when pulley  111  is released, allowing it to freely rotate and release cable  112 ; thereafter, compression springs  113   a  and  113   b  push upon bar stop  94 , which in turn pushes upon and accelerates both smooth rod  116  and pump piston  95 . Compression of the light gas by pump piston  95  bursts diaphragm  105 , propelling projectile  104  down launch tube  103  in exactly the same manner as described previously for the second alternative embodiment. 
     The movement of smooth rod  116  is halted by attached bar stop  94  when the later impacts end stops  99   a  and  99   b . The gun is readied for another firing by first powering up electric motor  110 , which rotates pulley  111  and rolls up cable  112  onto pulley  111 . Winching cable  112  onto pulley  111  squeezes compression springs  113   a  and  113   b  until pump piston  95  meets the closed end of pump tube  96 , squeezing o-ring  100 . The spent diaphragm  105  is removed, and a new projectile  104  and diaphragm  105  are put into place; subsequently, a new charge of light gas is introduced into the gun, as per the description of operation for the second alternative embodiment given previously. 
     FIG.  10 —Fourth Alternative Embodiment 
     Yet another alternative embodiment of the present invention is depicted in  FIG. 10 . While having several parts in common with the second and third alternative embodiments, the fourth alternative embodiment of the invention is unique in that the pump piston is actuated via a cable instead of a rigid rod. 
     The parts differentiating this fourth alternative embodiment, as depicted in  FIG. 10 , from the third alternative embodiment consist of: cable  120 , which is affixed at one end to pump piston  121 , and which passes through cable sleeve  122 , over upper pulley  123   a  and around lower pulley  123   b , past end stops  125   a  and  125   b , and terminating at its other end on flywheel  127 . Electric motor  126  shares a common axle with flywheel  127 ; cable stop  124  is firmly affixed to cable  120  and lies between lower pulley  123   b  and end stops  125   a  and  125   b . One-way valve  128 , which is supplied through high-pressure line  129 , is affixed to connecting block  131  and is situated close to cable sleeve  122 . Pump tube  130  is threaded into connecting block  131 . 
     The operation of the fourth alternative embodiment, in terms of loading and firing the gun, follows much the same procedure as the operation of the second and third alternative embodiments shown in  FIGS. 8 and 9 , respectively; however, pump piston  121  is propelled down pump tube  130  by the action of a cable, instead of a rod which is utilized in all previous embodiments of the invention. 
     For the fourth alternative embodiment, shown in  FIG. 10 , after projectile  104  and diaphragm  105  are loaded and the light gas is introduced into the gun from high-pressure line  129  and through one-way valve  128 , all in the manner described previously for the second and third alternative embodiments, the gun is ready to be fired. Electric motor  126  first spins up flywheel  127 . When flywheel  127  attains a predetermined rpm it engages cable  120 . As flywheel  127  rotates, it wraps up cable  120 . Cable  120  is pulled over lower pulley  123   b  and upper pulley  123   a , through cable sleeve  122  and through pump tube  130 , where it transmits a force to pump piston  121  to which cable  120  is affixed. Cable sleeve  122  forms a close fit with cable  120  and connecting block  101  such that the light gas is prevented from leaking past cable  120  as it slides through cable sleeve  122 . Pump piston  121  is accelerated by cable  120  down pump tube  130 , compressing the light gas to a pressure sufficient to burst diaphragm  105 . The movement of cable  120  is halted when affixed cable stop  124  meets end stops  125   a  and  125   b.    
     The gun is readied for a subsequent firing by opening screw-type breach block  102 , removing the spent diaphragm  105 , and loading new projectile  104  and diaphragm  105 . After screw-type breach block  102  is replaced and tightened, a new charge of light gas is supplied through one-way valve  128  via high-pressure line  129 . The pressurized light gas pushes pump piston  121  back to the closed end of pump tube  130  where it seats against o-ring  100 . 
     Additional Embodiments 
     The screw-type breech block  80  shown in  FIG. 4  can be replaced by a more conventional sliding-type breech block, thus significantly reducing the time required to reload the gun. 
     In  FIGS. 4 and 8  the diaphragms  81  and  99 , respectively, are valves that can be used only once and then they must be replaced. Each diaphragm can be replaced with a quick-opening valve that can be used in repeated gun firings without the need to be replaced. 
     A further additional embodiment relates to the preferred embodiment of  FIG. 1 , wherein the expansion tube is considered to be cylindrical. However, the expansion tube in the alternative embodiment of  FIG. 4  has a transverse cross-section that is rectangular, as depicted in the end-view perspective of  FIG. 6 , which shows the muzzle-end of the launch tube as well as the corresponding end of the expansion tube. Obviously, many other cross-sectional shapes are possible for the expansion tube; one alternative shape of an expansion tube is shown (along with the launch tube) in the end-view perspective of  FIG. 7 . 
     An anti-recoil mechanism is described which acts to counteract recoil when the gun is fired. For the preferred embodiment of  FIG. 1 , ports  30   t  and  30   b  can be fitted with tubes bent at right-angles so that spent propellant gas is vented in the opposite direction of the projectile motion. The same sort of anti-recoil tube can be fitted to port  76  in the alternative embodiment shown in  FIG. 4 . 
     ADVANTAGES 
     From the description provided previously, a number of advantages of my two-stage light gas gun become evident:
         (a) After the gun is fired, the pump piston can quickly be returned to its start position for another firing.   (b) Any possible residue from the gas that propels the pump piston is prevented from contaminating either the pump tube or the launch tube.   (c) Spent propellant gas is quickly and automatically vented.   (d) The pump piston can be reliably halted at a predetermined position within the pump tube; hence the pump piston can be made of a non-deformable material, such as aluminum or steel, which facilitates long piston life, but without the danger that it could ram into the end of the pump tube and damage the gun.   (e) The piston area that the propellant gas expands against can be greater than, equal to, or less than, the piston area compressing the light gas; the variety of possible ratios of propellant area to compression area means that the invention can be adapted to efficiently meet the requirements of a particular role.   (f) Making the piston area that the propellant gas expands against several times greater than the piston area compressing the light gas allows for the use of much cheaper propellants, such as an alcohol/air mixture, in place of a more-conventional, and expensive, modern gunpowder, while still allowing the pump tube to be much shorter than the launch tube.   (g) In an alternative embodiment of the invention, a projectile can be loaded into the gun via a conventional breech block, thus greatly reducing the time required to reload the gun as compared with two-stage light gas gun design seen in the prior art.   (h) Propellant gases expelled from the expansion tube can be used to counteract the recoil due to firing the gun.   (i) My design for a two-stage light gas gun can be effectively applied to a variety of roles:
           laboratory research, anti-armor, artillery, high altitude intercept, and space launches.   
               

     CONCLUSION, RAMIFICATIONS, AND SCOPE OF THE INVENTION 
     It should thus be apparent to the reader that the improved two-stage light gas gun of the invention provides, as compared to previous designs appearing in the prior art, a reliable and compact gun that can sustain a high rate of fire, while also being capable of operating with an inexpensive propellant. In addition, the invention has the following distinct advantages in regards to previous embodiments of two-stage light gas guns:
         the pump piston can quickly be returned to its start position for another firing;   it can quickly be reloaded with a new projectile after it has been fired;   it prevents any possible residue from the propellant contaminating the pump tube;   the propellant is quickly and automatically vented without the need for valves;   it provides a variety of possible ratios, from less than one, to equal to one, to greater than one, of the area of the piston the propellant pushes against versus the area of the pump piston compressing the light gas;   it performs well in a variety of roles: laboratory research, anti-armor, very long-range artillery, and shots into outer space;   the pump piston can be halted reliably at a predetermined position within the pump tube;   the spent propellant gas can be used to counteract the recoil of the gun;   the projectile can be loaded into the gun via a conventional breech block.       

     The invention has the additional positive features of providing for a pump tube that is considerably shorter than the launch tube, as well as preventing deformation of the pump piston as a normal part of the firing cycle. 
     Although several embodiments of the present invention, along with many of its advantages, have been described above in detail, it should be understood that various alterations, modifications, and alternate constructions can be made herein without departing from the spirit and scope of the invention as defined by and within the appended claims. Indeed, the scope of the present application is not intended to be limited to the particular embodiments of the machine, manufacture, composition of matter, means, methods, and steps described in the specification. Instead, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined in the appended claims.