Abstract:
The present invention provides for a pressurizer for pressurizing a fluid, comprising a pressurant entrance for the introduction of a pressurant, a fluid entrance for the fluid, a fluid exit for the fluid, and a transfer chamber movable in a cycle with respect to the fluid exit, where for a portion of a cycle the pressurant exerts a force on the fluid inside the transfer chamber. In a preferred aspect of the present invention, the pressurizer further comprises a spindle housing more than one transfer chamber, rotatable about an axis between the fluid entrance and the fluid exit. In another preferred aspect, the transfer chamber comprises either a flexible membrane or a movable piston to separate the pressurant and the fluid.

Description:
BACKGROUND 
     Rocket engines require propellants to be fed to them at very high pressures. This has historically been accomplished in two general ways: first, with the use of a pressurized fluid, such as high pressure helium; and second, with the use of a pump. 
     In the first way, a pressurized fluid is added directly to the propellant tank and exerts a force on the propellant. Nitrogen and helium, both inert gases, pressurized to a pressure as high as 50,000 PSI, have been used successfully in the past. As they are inert, there need be no barrier (e.g. membrane or piston) placed between these pressurized fluids and the propellant. The problem with this method, however, is that the pressurized fluid also exerts a force on the propellant tank. Because of the extremely high pressures required of the pressurized fluid, the walls of the propellant tank must be thick enough to withstand the pressure. The propellant tank is therefore very heavy. Rockets employing the pressurized fluid must use a greater proportion of their thrust lifting this extra weight, and therefore they are not as efficient as rockets that do not require this added weight. 
     Historically, one way to solve the above weight problem is to employ the use of a pump. Pumps (e.g. reciprocating or centrifugal pumps) are generally very complex and require their own driving means, such as an engine. Further, the engine driving the pump burns a significant percentage of the total propellant. For small rocket engine systems, since a pump is too complicated and too heavy, pressurized fluids are generally used to pressurize the propellant. However, for large rocket engine systems, pumps have the advantage that the walls of the propellant tank need not be thick, since there is little or no pressure in the tank. Therefore, the propellant tank is much lighter, and the added weight of the pump is more than offset by the reduction in propellant tank weight. 
     U.S. Pat. No. 3,213,804 to Sobey discloses fluid pressure accumulators that are connected to sources of low and high pressure by means of butterfly valves. Essentially, the pressurized fluid exerts force on the propellant in small, designated containers. While the walls of these containers must be thick in order to withstand the high pressure of the pressurized fluid, the walls of the propellant tank need not be. Therefore, the total weight of the rocket engine system employing Sobey&#39;s invention may be less than that of the previously discussed rocket engine system because these containers (fluid pressure accumulators) are small in comparison to the propellant tank. 
     A problem with Sobey&#39;s invention, however, is its complicated use of valves. In order to reduce the weight of Sobey&#39;s invention further, the sizes of the fluid pressure accumulators must decrease (thus reducing their weight). However, as they decrease, the rotation speed and precision of the butterfly valves must increase in order to accommodate the same propellant flow rate to the rocket engine. This places great stresses on the valves, and eventually a point is reached at which the valves cannot reliably rotate fast enough to provide the required timing. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a pressurizer for pressurizing a fluid, comprising a pressurant entrance for the introduction of a pressurant, a fluid entrance for the fluid, a fluid exit for the fluid, and a transfer chamber movable in a cycle with respect to the fluid exit, where for a portion of a cycle the pressurant exerts a force on the fluid inside the transfer chamber. In a preferred aspect of the present invention, the pressurizer further comprises a spindle housing more than one transfer chamber, rotatable about an axis between the fluid entrance and the fluid exit. In another preferred aspect, the transfer chamber comprises either a flexible membrane or a movable piston to separate the pressurant and the fluid. In another preferred aspect, the pressurizer further comprises a pressurant exit for a pressurant exhaust. In another preferred aspect, the pressurant exhaust is exhausted in a direction substantially opposite a direction of motion of the transfer chamber. In another preferred aspect, the pressurizer further comprises a motor to move said transfer chamber. In another preferred aspect, a cross section of the pressurant entrance is larger than a cross section of the fluid exit, and a cross section of the pressurant exit is larger than a cross section of the fluid entrance. In another preferred aspect, a cross section of the fluid entrance is greater than a cross section of the fluid exit. 
     The present invention also provides for a rocket engine system, comprising a pressurant, a pressurant container, a propellant, a propellant container, a rocket engine, and a transfer chamber movable in a cycle with respect to the rocket engine, where for a portion of a cycle the pressurant exerts a force on the propellant inside the transfer chamber. In a preferred aspect, for a portion of a cycle a bouyant force causes the propellant to flow into, and the pressurant to flow out of, the transfer chamber. In another preferred aspect, the rocket engine system further comprises a heating means for heating the pressurant, where the heating means comprises a heat conductor for conducting heat from the rocket engine to the pressurant. In another preferred aspect, a pressurant exhaust exerts a force on the propellant inside the propellant container. In another preferred aspect, the propellant comprises an oxidizer and a fuel. In another preferred aspect, the rocket engine system further comprises an engine conduit between the transfer chamber and the engine and a propellant conduit between the transfer chamber and the propellant container, where a cross section of the propellant conduit is greater than a cross section of the engine conduit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a rocket engine system employing the pressurizer described herein. 
     FIG. 2 shows a perspective view of a preferred embodiment of the pressurizer described herein. 
     FIG. 3 shows a perspective view of the pressurizer in FIG. 2 without the spindle. 
     FIG. 4 shows a perspective view of the spindle. 
     FIG. 5 shows a cut-away view of FIG.  4 . 
     FIG. 6 a  shows a top view of the top chamber separator. 
     FIG. 6 b  shows a bottom view of the bottom chamber separator. 
     FIG. 7 shows a perspective view of the pressurizer in FIG. 2 with a motor. 
     FIG. 8 shows a perspective view of another preferred embodiment of the pressurizer described herein. 
     FIG. 9 shows a perspective view of a spindle associated with the pressurizer in FIG.  8 . 
     FIG. 10 shows a schematic view of a rocket engine system with a heater for the pressurant. 
     FIG. 11 shows a schematic view of a rocket engine system with the propellant tank pressurized by the pressurant exhaust. 
     FIG. 12 shows a schematic view of a rocket engine system employing another embodiment of the pressurizer described herein. 
     FIG. 13 shows a perspective view of the pressurizer shown in FIG.  12 . 
     FIG. 14 shows a perspective view of the pressurizer in FIG. 13 without the spindle. 
     FIG. 15 is a cut-away view of a spindle with a movable piston in each transfer chamber. 
     FIG. 16 is a perspective view of a spindle with very thin transfer chambers. 
     FIG. 17 is a perspective view of a spindle with a flexible membrane in each transfer chamber. 
     FIG. 18 is a cut-away view along cross section A—A shown in FIG.  17 . 
     FIG. 19 is a schematic view of a rocket engine system employing two propellants. 
    
    
     DETAILED DESCRIPTION 
     In the following description, the use of “a,” “an,” or “the” can refer to the plural. All examples given are for clarification only, and are not intended to limit the scope of the invention. 
     Referring to FIG. 1, according to a preferred embodiment, a rocket engine system includes a propellant tank  10  connected by a propellant conduit  6  to a pressurizer  16 , a pressurant tank  18  connected by a pressurant conduit  36  to the pressurizer  16 , and a rocket engine  2  with a nozzle  4  connected by an engine conduit  32  to the pressurizer  16 . The propellant tank  10  contains a propellant  12  with meniscus  14 . Flow of the propellant  12  into pressurizer  16  is controlled by propellant valve  8 . A pressurant tank  18  contains a pressurant  20 . Flow of the pressurant  20  into pressurizer  16  is controlled by pressurant valve  22 . Flow of propellant  12  from pressurizer  16  to engine  2  is controlled by engine valve  26 . Pressurant exhaust is released from exhaust conduit  34 , and its flow is controlled by exhaust valve  24 . 
     Propellant  12  combusts in engine  2  and the resulting gas accelerates through nozzle  4 . Propellant  12  can be any monopropellant, such as a substance that decomposes by itself or in the presence of a catalyst. One example is hydrogen peroxide. Propellant  12  can also be a fuel or an oxidizer in a hybrid rocket engine system. For example, propellant  12  could be liquid oxygen and engine  2  could contain a solid resin fuel. Further, propellant  12  need not be a reacting substance at all—it could be a working medium that is heated by an external heat source. For example, propellant  12  could be liquid hydrogen and engine  2  could contain a nuclear reactor that heats the hydrogen to high pressures. 
     Pressurant  20  can be any high-pressure fluid, and the following description is meant as an example and not as a limitation. Pressurant  20 , if it comes into direct contact with the propellant  12 , should be nonreactive with propellant  12 . (An embodiment will be described later in which the pressurant  20  does not come into contact with propellant  12 .) Further, it should not react with the walls of the pressurant tank  18  or any of the components of the pressurizer herein described. For example, two fluids that meet this description are inert gases such as helium and nitrogen. However, both of these fluids are gases at room temperature (regardless of their pressure); therefore, a high density may be difficult to obtain. A high density for pressurant  20  is necessary so that a large quantity of pressurant  20  can be held in a small pressurant tank  18 . Because pressurant tank  18  is designed to withstand very high pressures, its walls may be very thick, resulting in a large weight. Therefore, the smaller the pressurant tank  18 , the better. In a preferred embodiment of the present invention, the pressurant  20  is a liquid with a very high vapor pressure. For example, liquid carbon dioxide at room temperature has a vapor pressure of approximately 750 PSI. However, 750 PSI, while high, may not be high enough. As another example, liquid nitrogen can be heated until its vapor pressure is, for example, 2000 PSI. Because of the very high vapor pressure attainable, and because liquid nitrogen is much denser than gaseous nitrogen, liquid nitrogen may be a good choice for pressurant  20 . One skilled in the art will realize that a plethora of other good choices exist for pressurant  20 . 
     Referring to FIG. 19, in another preferred embodiment of the present invention, the rocket engine system comprises two propellants, a fuel  78  contained in a fuel tank  76  and an oxidizer  80  contained in an oxidizer tank  82 . Each of the fuel and the oxidizer has its own pressurizer  16 , and the pressurizers  16  may or may not share a common pressurant  20 . In other embodiments, the rocket engine system could comprise more than two propellants, or two propellants other than a fuel and oxidizer. For example, it could comprise a fuel, an oxidizer, and a catalyst, or a decomposing propellant and a catalyst. 
     Many different potential combinations of propellant tanks and pressurizers would be apparent to one skilled in the art. 
     Referring now to FIG. 2, a pressurizer according to a preferred embodiment includes: (a) a top chamber separator  28  to which pressurant conduit  36  and exhaust conduit  34  are connected; (b) a bottom chamber separator  30  to which engine conduit  32  and propellant conduit  6  are connected; and (c) a rotatable spindle  26 . Propellant  12  flows into the spindle  26  through propellant conduit  6  and out of the spindle  26  through engine conduit  34 . Pressurant  20  flows into the spindle  26  through pressurant conduit  36  and out of the spindle through exhaust conduit  34 . Propellant  12  and pressurant  20  flow in the direction indicated by the arrow shown in each conduit. The spindle  26  in this embodiment rotates in the direction indicated by the arrow shown on the spindle  26 , although it would be obvious that it could spin in the opposite direction. 
     Referring to FIG. 3, which shows the pressurizer without the spindle  26 , the pressurizer includes a rotatable connector  38  that rotatably connects the bottom chamber separator  30  to the spindle  26 . There could also be such a connector connecting the top chamber separator  28  to the spindle  26 . The connector could comprise bearings, such as ball bearings or gas bearings. Further, there are seals (not shown) between the moving spindle  26  and selected parts of the top chamber separator  28  and the bottom chamber separator  30 . The seals should allow the spindle  26  to spin with minimal friction while preventing propellant  12  and pressurant  20  from flowing into the wrong conduits at the wrong times. By way of example and not of limitation, there could be a circular seal around the circular hole  54  on the left side of the bottom chamber separator  30  in FIG. 3, where the engine conduit  32  connects to the bottom chamber separator  30 . There could also be a seal on the top of the bottom chamber separator  30  and around its circumference, with an additional seal to separate the left and right halves of the bottom chamber separator. The placement and material composition of such seals would be obvious to one skilled in the art. 
     Referring to FIG. 4, spindle  26  includes a plurality of transfer chambers  42  and a center  40 . Each complete rotation of the spindle  26  is a complete cycle for each transfer chamber  42 . For each transfer chamber  42 , for a portion of each cycle, propellant  12  flows inward from propellant conduit  6  (in the direction of the arrow indicated as shown in FIGS. 2 and 3) and pressurant  20  flows outward to exhaust conduit  34 ; for another portion of the cycle, propellant  12  flows outward to engine conduit  32  and pressurant  20  flows inward from pressurant conduit  36 . Now referring to FIG. 5, each transfer chamber  42  is an individual chamber divided from the next, so that the meniscus of propellant  12  in each transfer chamber  42  is potentially different. First meniscus  44  is the meniscus in a transfer chamber that is just beginning the portion of the cycle in which propellant  12  flows outward to engine conduit  32  and pressurant  20  flows inward from pressurant conduit  36 . Fourth meniscus  50  is the meniscus in a transfer chamber that is just ending the portion of the cycle in which propellant  12  flows outward to engine conduit  32  and pressurant  20  flows inward from pressurant conduit  36 . In this figure, propellant  12  is flowing downward under the force of pressurant  20 . 
     Referring now to FIGS. 6 a  and  6   b , the oblong holes  52  in top chamber separator  28  of the pressurant conduit  36  and the exhaust conduit  34  are larger than the corresponding circular holes  54  in bottom chamber separator  30  of the engine conduit  32  and the propellant conduit  6 . Further, oblong holes  52  “cover” as well as “precede” the circular holes  54  in the direction of rotation of the spindle  26 , as shown in the figures. The oblong holes  52  must “cover” the circular holes  54  so that the pressurant  20  is acting on the propellant  12  in a given transfer chamber  42  at all times that the propellant  12  in the transfer chamber  42  is in pressure communication with the engine  2  via engine conduit  32 . Further, the oblong holes  52  must “precede” the circular holes  54  so that the pressurant  20  has sufficient time to pressurize the transfer chamber and provide the proper force on the propellant  12  before the propellant  12  is placed in pressure communication with the engine  2  via engine conduit  32 . There are other ways to achieve the same result and would be obvious to one skilled in the art. For example, oblong holes  52  could be thinner than shown in the figures and still achieve the same result. Further, they need not be oblong, nor need the circular holes  54  be circular. 
     As the spindle  26  (not shown in FIGS. 6 a  and  6   b ) rotates once in the counterclockwise direction relative to a top side view of the top chamber separator  28 , each transfer chamber  42  (not shown) completes a cycle. For illustrative purposes, suppose a transfer chamber is currently full with propellant  12 . As it moves inside spindle  26 , it first comes upon an oblong hole  52  of the pressurant conduit  36 . Pressurant  20  then rapidly flows into the transfer chamber  42 , due to its high pressure, and soon reaches an equilibrium pressure. Next, the transfer chamber  42  comes upon circular hole  54  of the engine conduit  32 . Propellant  12  is now in pressure communication with the engine  2 , and propellant flows to the engine  2  via engine conduit  32  under the force of pressurant  20  until most or all of propellant  12  has flowed from the transfer chamber  42 . Next, the transfer chamber  42  moves past the circular hole  54 , thus ending the pressure communication of propellant  12  with engine  2 . Next, the transfer chamber moves past the oblong hole  52  and pressurant  20  is no longer able to flow into transfer chamber  42 . The transfer chamber  42  may move past both the circular hole  54  and the oblong hole  52  roughly simultaneously. 
     Next, the transfer chamber  42  comes upon an oblong hole  52  of the exhaust conduit  34 . The pressurant  20  flows out of the transfer chamber into the exhaust conduit  34  until a near equilibrium pressure is reached between the inside of the transfer chamber  42  and the exhaust pressure of the exhaust conduit  34 . The exhaust pressure may be atmospheric pressure, or it may be a vacuum if the pressurizer herein described is used in space. Next, the transfer chamber  42  comes upon a circular hole  54  of the propellant conduit  6 . The propellant  12 , the pressure of which at its entrance into the transfer chamber  42  is higher than the exhaust pressure of the exhaust conduit  34 , flows into the transfer chamber  42  as it displaces the remaining pressurant  20 , until the propellant  12  completely or mostly fills the transfer chamber  42 . Next, the transfer chamber  42  moves past the circular hole  54 , thus ending the flow of propellant  12  into transfer chamber  42 . Next, the transfer chamber moves past the oblong hole  52  and pressurant  20  is no longer in pressure communication with exhaust conduit  34 . The transfer chamber  42  may move past both the circular hole  54  and the oblong hole  52  roughly simultaneously. 
     One full cycle has been described. After this, the cycle begins again. Because of the plurality of transfer chambers  42  in the spindle  26 , the pressurizer  16  is designed so that at any given time at least one transfer chamber  42  is in pressure communication with both the engine  2  via engine conduit  32  and the pressurant  20  via pressurant conduit  36  simultaneously, thus ensuring continuous, uninterrupted flow of propellant  12  to the engine  2 . 
     The propellant  12  at its entrance into the transfer chamber  42  is at a higher pressure than the exhaust pressure of the exhaust conduit  34  because of a pressure head due to the height of meniscus  14  relative to the entrance of the propellant  12  into the transfer chamber  42 . However, this pressure may or may not be sufficient. In order to increase this pressure, and thereby increase the flow rate of propellant  12  into transfer chamber  42 , the propellant tank  10  may be pressurized. The propellant tank  10  need not be pressurized to a very high pressure, and should be lower than the pressure of the pressurant  20 . (If the propellant  12  were pressurized to a pressure at or above the pressure of the pressurant  20 , there would be no need for the pressurizer  16 , and the walls of the propellant tank  10  would have to be very thick and heavy.) By way of example and not of limitation, the propellant tank  10  could be pressurized to between 10 and 200 PSI, or even more, if the pressurant pressure is exceedingly high. 
     Generally, the difference between the pressure of the pressurant  20  and the working (combusting) pressure of the engine  2  is significantly greater than the difference between the pressure of the propellant  12  at its entrance into the transfer chamber  42  and the exhaust pressure of the exhaust conduit  34 . The flow rate of a fluid (e.g. propellant  12 ) through a conduit (e.g. propellant conduit  6 ) generally depends on several factors, including the difference in pressure at each end of the conduit, as well as the minimum cross sectional area of the conduit. Therefore, the flow rate per cross sectional area is generally proportional to the difference in pressure at each end of the conduit A flow rate between the propellant tank  10  and the transfer chamber  42  should be equal to a flow rate between the transfer chamber  42  and the engine  2 . Otherwise, at the end of each cycle, each transfer chamber  42  would have significantly more or less propellant  12  than it did at the end of the previous cycle. If this trend continued, it would eventually result in one of two undesirable consequences: either propellant  12  would be lost directly through the exhaust conduit  34 , or else pressurant  20  would be fed directly into the engine conduit  32 . In order to set the flow rate between the propellant tank  10  and the transfer chamber  42  equal to the flow rate between the transfer chamber  42  and the engine  2 , the minimum cross sectional area of the path between the propellant tank  10  and the transfer chamber  42  (e.g. propellant conduit  6 ) should be greater than the minimum cross sectional area of the path between the transfer chamber  42  and the engine  2  (e.g. engine conduit  32 ). This is necessary to counteract the effect resulting from a difference in pressure between the pressurant  20  and the engine  2  that is higher than the difference in pressure between the propellant  12  at its entrance into the transfer chamber  42  and the exhaust pressure of the exhaust conduit  34 . 
     Therefore, one of the circular holes  54  (FIG. 6 b ) could be larger than the other, the larger one corresponding to the point of connection between the engine conduit  32  and the bottom chamber separator  30 . Further, the engine conduit  32  could have a smaller cross section than the propellant conduit  6 . It would be obvious to one skilled in the art how to adjust the dimensions of the various elements of the pressurizer described herein in order to assure proper flow rates of propellant  12  into and out of transfer chamber  42 . 
     Referring now to FIG. 7, the spindle  26 , housing a plurality of transfer chambers  42 , can be rotated by an external means of rotation, such as a motor  58  connected to the spindle  26  via motor shaft  56 . As the motor  58  spins, the spindle  26  rotates. In each rotation of the spindle  26 , each transfer chamber  42  inside is subject to a fill cycle as previously described The motor  58  could be an electric motor, powered by a battery or some other electric power supply. The motor  58  could also be a piston engine or a turbine, powered, for example, by the combustion/decomposition of propellant  12  or the expansion of the pressurant  20 . However, the motor  58  does not need to be large or to consume much energy. It needs only to overcome the friction resulting from the contact between the moving spindle  26  and the stationary chamber separators  28 ,  30  via the seal. The greater the friction and the faster the spinning of the spindle  26 , the more work the motor  58  needs to do. 
     Referring now to FIG. 8, in another preferred embodiment of the present invention, pressurizer  16  has a spindle housing  62  with a housing jet hole  60 . In this embodiment, the spindle  26  as shown in FIG. 9 is placed inside the spindle housing  62 . The housing jet hole  60  is a hole that penetrates the wall of the spindle housing  60  in a direction that is not perpendicular to the wall. Rather, the housing jet hole  60  is pointed in a direction shown by arrow “a” that is opposite the direction of rotation “b” of the spindle  26 . Of course, these directions could be reversed. Further, spindle  26  contains spindle jet holes  64  corresponding to transfer chambers  42  (i.e. one spindle jet hole  64  per transfer chamber  42 ) that are cut similarly to housing jet hole  60  in that they are not perpendicular to the wall of spindle  26 . Rather, they point in a direction shown by the arrow “a.” There is further a seal (not shown) between the outer wall of the spindle  26  and the inner wall of the spindle housing  62  so that fluid inside a transfer chamber  42  can only escape via its corresponding spindle jet hole  64  when its spindle jet hole  64  is aligned with the housing jet hole  60 . 
     Housing jet hole  60  should be located in the wall of the spindle housing  62  “after” the pressurant conduit  36 /engine conduit  32  pair in the direction of rotation of the spindle  26 . The function of the holes will now be explained. After a transfer chamber  42  has just completed the part of the cycle in which it is in pressure communication with the pressurant conduit  36 , the transfer chamber now contains some, if any, propellant  12 , and is mostly or completely full will pressurant  20 . The housing jet hole  60  is located after this part of the cycle. As the transfer chamber  42  continues in its cycle, its then comes to the housing jet hole  60 , so that its corresponding spindle jet hole  64  and housing jet hole  60  line up (or approximately line up). At this point, the high-pressure pressurant flows out of the jet holes  60 ,  64  in the direction shown by the arrow “a.” This flow of gas results in an impulse reaction acting on the spindle  26 , thus pushing it in the direction shown by the arrow “b.” The size and diameter of the jet holes  60 ,  64  has been exaggerated in the drawings, but it would be obvious to one skilled in the art how to adjust the size, shape, dimensions, direction, and location of the holes in order to effect the spinning of spindle  26  by the exhausting of jets of pressurant  20  through the holes. In this embodiment, an external driving means, such as a motor  58 , is replaced or supplemented by the impulse reaction provided by the expulsion of pressurant  20  through the jet holes  60 ,  64 . 
     Referring now to FIG. 10, in another preferred embodiment, pressurant tank  18  contains a heating element  66  to heat the pressurant  20 . If pressurant  20  is a liquid with a high vapor pressure, then as the vapor expands (corresponding with the pressurizing of the transfer chambers  42  according to the cycle previously explained), the liquid evaporates to replenish the vapor, causing the temperature of the liquid to drop, resulting in a corresponding drop in the vapor pressure. In order to assure a constant vapor pressure of the pressurant  20 , heating element  66  applies heat to pressurant  20 , keeping it at a constant temperature. The heating element  66  can be an electric resistance element or combustor in which a small quantity of propellant  12  combusts/decomposes. Further, a heat conductive lead  68  could connect the heating element  66  with the engine  2  or the nozzle  4 , thus conducting some of the heat of combustion in the rocket engine  2  to the pressurant  20 . Further, heat conductive lead  68  could consist of conduit, thus directing a small stream of combustion gases directly from the engine  2  to the heating element  66 , and then back to the engine  2 . One skilled in the art would realize the many ways possible to provide heat to pressurant  20  to keep it at a constant temperature and vapor pressure. 
     Referring now to FIG. 11, in another preferred embodiment, exhaust conduit  34  consists of two parts, only one of which is shown in FIG.  11 . The first part, shown in FIG. 11, is connected directly to propellant tank  10  in order to provide pressure to propellant tank  10 . Propellant tank  10  should be pressurized by gas, as discussed previously, if the pressure head provided by the weight of the propellant (by way of the height of meniscus  14  relative to the pressurizer  16 ) is insufficient to cause sufficient propellant flow. Propellant tank  10  can be pressurized by the unused pressurant  20  remaining in the transfer chambers  42  just before it is exhausted. So the first part of exhaust conduit  34  directs the flow of the unused pressurant  20  to propellant tank  10 , thus pressurizing the propellant  12 . The second part of the exhaust conduit  34  (not shown in FIG. 11) is similar to the exhaust conduit  34  shown in FIG. 10, in that it is not connected to the propellant tank  10 . In the cycle of a transfer chamber  42 , the transfer chamber  42  first comes upon the first part of the exhaust conduit  34 , thus pressurizing the propellant tank  10 . Next, the transfer chamber  42  moves past and ends pressure communication with the first part of the exhaust conduit  34 , and comes upon the second part of the exhaust conduit  34 , where propellant  12  can displace the remaining pressurant  20  in the transfer chamber  42  as the remaining pressurant  20  is exhausted via the second part of exhaust conduit  34 . As would be obvious to one skilled in the art, there are many ways to modify the rocket engine system described herein to make use of the unused pressurant  20  to pressurize the propellant tank  10 . Further, FIG. 11 shows an exhaust valve  24  that regulates the pressure in propellant tank  10 . Because the pressure of pressurant  20  is so high in relation to the needed pressure in propellant tank  10 , it may be necessary to evenly vent propellant tank  10  via exhaust valve  24  in order to keep the pressure in propellant tank  10  constant. 
     Referring now to FIG. 12, in another preferred embodiment of the present invention, the pressurizer  16  is built into the propellant tank bottom portion  70  of the propellant tank  10  as shown. FIG. 13 shows a close-up of the pressurizer portion of the rocket engine system shown in FIG. 12. A top chamber separator  28 ′, which is approximately half the size of the top chamber separator  28  shown in FIG.  2  and is connected on one side to pressurant conduit  36 , is connected on the other side to the propellant tank bottom portion  70 . A bottom chamber separator  30 ′, which is approximately half the size of the bottom chamber separator  30  shown in FIG.  2  and is connected on one side to engine conduit  32 , is connected on the other side to the propellant tank bottom portion  70  (as shown in FIG.  13 ). The propellant conduit  6  and exhaust conduit  34  have been replaced in this embodiment by the propellant tank bottom portion  70 . Besides these modifications, other aspects of this embodiment (e.g. the use of a seal, the use of a spindle  26 , etc.) are similar to that described previously. Referring now to FIG. 14, a rotatable connector  38 ′ is located on the bottom chamber separator  30 ′, and a similar connector could be located on the top chamber separator  28 ′. 
     Now a portion of the cycle of a spindle  26  will be described. The portion of the cycle involving pressurant conduit  36  and engine conduit  32  is similar to that described previously with regard to FIGS. 6 a  and  6   b , and will not be repeated. After a transfer chamber  42  has moved past conduits  32 ,  36 , it then comes upon the entrance to propellant tank bottom portion  70 . At this point, both the top and bottom of the transfer chamber  42  are open to and in pressure communication with—the propellant tank  10  and the propellant  12  that it contains. The high-pressure unused pressurant  20  remaining in the transfer chamber  42  then expands against the propellant  12  located in the propellant tank bottom portion  70 , resulting in a bubble that rises due to a bouyant force of the propellant  12  acting on the pressurant  20 . As the bubble of pressurant  20  rises, it is displaced in the transfer chamber  42  by propellant  12 , until the transfer chamber  42  is completely filled with propellant  12  and no pressurant  20  remains. The bubble of pressurant  20  continues rising until it breaks meniscus  14 . Pressurant  20 , because of its high pressure, serves to pressurize propellant tank  10 , and exhaust valve  24  is used to regulate the pressure inside propellant tank  10 , as previously discussed. As the transfer chamber continues in its cycle, it then comes upon the exit of propellant tank bottom portion  70 , where its pressure communication with propellant tank  10  ends. Then, the cycle ends, and a new cycle begins, the beginning of which has been described before in regard to FIGS. 6 a  and  6   b.    
     In another embodiment, not shown, the spindle  26  is rotated by an external rotation means, such as a motor, engine, or turbine, as discussed. Further, in another embodiment, the embodiment shown in FIGS. 13 and 14 is modified with jet holes  60 ,  64  shown in FIGS. 8 and 9 in order to rotate spindle  26  by means of impulse reaction. Further, in order to address the issue of flow rate discussed previously, the cross section of the engine conduit  32  may be smaller than shown in the drawings, and/or the chamber separators  28 ′,  30 ′ may be smaller so that each transfer chamber spends a greater portion of its cycle inside propellant tank bottom portion  70 . Similar modifications to achieve similar ends would be obvious to one skilled in the art. 
     Referring now to FIG. 15, each transfer chamber  42  contains a movable means for separating the pressurant  20  from the propellant  12 , such as a piston  72 . Piston  72  can move up and down inside the transfer chamber  42  while maintaining a seal with the inside walls of the transfer chamber  42 , to prevent the leak of propellant  12  into the region above the piston  72  or the leak of pressurant  20  into the region below the piston  72 . The cycle proceeds as previously described with regard to FIGS. 6 a  and  6   b , the only difference being that the pressurant  20  acts indirectly on propellant  12  via piston  72 . Piston  72  could have the added feature that it cannot move any higher than the top of the transfer chamber  42  or any lower the than the bottom of the transfer chamber  42 . This has the benefit that there would be no worry about “overfilling” each transfer chamber  42  with propellant  12 , and no propellant  12  would be directly lost through exhaust conduit  34 . It further has the benefit that there would be no worry about feeding pressurant  20  directly to the engine  2  via engine conduit  32 . Any pressurant  20  that made it to the engine  2  could interrupt combustion and possibly fail the engine. 
     Referring now to FIG. 16, in another preferred embodiment,-spindle  26  contains a plurality of very thin transfer chambers  84 . It is the same as the spindle  26  described previously, except for the existence of thin transfer chambers  84 . The thinner the thin transfer chambers  84 , the fewer the instabilities—e.g. splashing of propellant  12 , bubbles of pressurant  20  in propellant  12 , unevenness of the meniscus of propellant  12 , etc. One might conceive of a thin transfer chamber  84  so thin that propellant  12  is fed into it by means of a capillary effect. This is all within the scope of the present invention. 
     Referring now to FIGS. 17 and 18, in another preferred embodiment, spindle  26  houses several transfer chambers  42 ′, each of which contains a movable membrane  74  that is capable of separating a region above it from a region below it. The membrane  74  serves a similar purpose as piston  72  shown in FIG. 15, except that the edges of membrane  74  are permanently attached to the walls of transfer chamber  42 ′, so that there is no need to provide a moving seal between the membrane  74  and the walls of transfer chamber  42 ′. Rather, while the edges of membrane  74  stay fixed in relation to the transfer chamber  42 ′, the remainder of the membrane  74  (particularly near the center) moves up and down in the transfer chamber  42 ′ in response to the filling and draining of propellant  12  per the cycle previously described. This embodiment, like the embodiment involving piston  72 , has the advantage that membrane  74  would prevent the direct feeding of propellant  12  to exhaust conduit  34  and the direct feeding of pressurant  20  to engine conduit  32 . 
     It will be apparent to one skilled in the art that the transfer chamber  42  need not be housed in a spindle  26 , need not rotate with the spindle  26 , and need not be in a cycle of rotation at all—it could move in many other cyclical ways relative to the conduits  6 ,  32 ,  34 ,  36  and chamber separators  28 ,  30 . By way of example and not of limitation, a transfer chamber could reciprocate between the pressurant conduit  36 /engine conduit  32  pair and the exhaust conduit  34 /propellant conduit  6  pair. In order to provide constant, uninterrupted flow to the engine  2  via engine conduit  32 , several such reciprocating transfer chambers  42  could be used in parallel, each one corresponding to a different stage in the cycle. In all cases, however, at least one transfer chamber  42  moves in a cycle to transfer a propellant/fluid from a filling stage to a pressurizing/emptying stage.