Abstract:
A method of cooling a rocket engine that includes removal of a gaseous propellant from a tank containing liquid propellant, thereby cooling the propellant in the tank, cooling a rocket engine with a coolant, thereby heating the coolant, and transferring heat from the heated coolant into the propellant in the tank, thereby heating the propellant in the tank and maintaining the pressure in the tank. In certain embodiments, the coolant is injected into the rocket engine after transferring heat from the rocket engine to the propellant in the tank.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is divisional of U.S. patent application Ser. No. 11/509,905, filed on Aug. 25, 2006, the entire contents of which is hereby expressly incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to rocket engines, and more particularly, to a cooling system for rocket engines. 
     BACKGROUND 
     Rocket engines are used for missiles, for launching space bound vehicles, and control of spacecraft in space. Rocket engines that are used to control the attitude of a spacecraft in space are generally referred to as thrusters. Firing a thruster produces a propulsive force that is opposite to the direction of the combustion gases exiting the nozzle of the thruster. The duration of firing and the timing of firing of multiple thrusters can be adjusted to impart sufficient torque on the spacecraft to obtain the desired attitude of the spacecraft. Thus, multiple thrusters are rapidly and repeatedly fired for accurate control of spacecraft attitude. 
     Thrusters typically use liquid monopropellants such as hydrogen peroxide (H 2 O 2 ) or hydrazine (N 2 H 4 ), or room temperature storable bipropellants such as nitrogen tetroxide (N 2 O 4 ) and hydrazine. Because these thrusters operate in a zero gravity environment, the propellant can float in the propellant tank resulting in pressurizing gas or propellant vapor instead of liquid propellant being fed out of the propellant tank. In current practice, one means of zero-gravity feed is to use a flexible or thin metallic diaphragm, separating the pressurizing gas from the liquid propellant. Another currently practiced means of zero-gravity feed is the use of surface tension propellant acquisition devices, which draw liquid propellant towards the tank exit through capillary action. 
     Nitrogen tetroxide and hydrazine are toxic and environmentally hazardous. Other propellants, such as nitrous oxide (N 2 O) and liquid oxygen (O 2 ) combined with a variety of fuels do not have these drawbacks, but there is no currently developed diaphragm tank technology for liquid oxygen and surface tension devices for nitrous oxide and liquid oxygen have not yet been demonstrated. Furthermore, surface-tension devices only operate in extremely low gravity environments, and many vehicles require a propellant feed solution which works under a range of acceleration environments and directions. 
     Another means of zero-gravity feed is to store the propellant in the propellant tank in a self pressurized state so that no additional pressurizing gas is required (sometimes called VaPak in the prior literature). Self-pressurized propellant by itself does not provide zero-gravity feed; however it is possible to consistently feed the gaseous form of the propellant out of the propellant tank by means of a heat exchanger between the withdrawn propellant and the tank contents. However, as the propellant is fed out of the propellant tank, the pressure of the tank drops causing the liquid part of the propellant to boil. The boiling causes heat to be drawn from the propellant to further drop the pressure of the propellant in the tank. Accordingly, in order to withdraw gaseous propellant consistently, the pressure of the tank drops significantly during use, to a degree which makes it impractical to use more than a small fraction of the tank contents in this way. 
     Rocket engines can be radiation cooled, which means that a rocket engine can be allowed to radiate its heat to space. The use of certain propellants, such as liquid oxygen or nitrous oxide with most fuels prohibits radiation cooling because these propellants burn very hot and radiation cooling cannot provide sufficient cooling. Certain other rocket engines circulate some of the propellant in passages in the nozzle and/or the combustion chamber to cool the rocket engine, which is called regenerative cooling. The latter cooling method, however, is not applicable to small thrusters because there is not enough cooling capacity in the propellant to cool the engine. 
     A solution to the above problems that currently exists is to run a coolant through the passages in the nozzle and/or the combustion chamber. The problem with this type of system, however, is that continuation of the cooling cycle may excessively heat the coolant. 
     SUMMARY 
     In accordance with one aspect of the present disclosure, a propulsion system includes a rocket engine, a propellant tank fluidically coupled to the rocket engine, a coolant tank configured to hold a coolant, a first heat exchanger thermally coupled to the rocket engine and fluidically coupled to the coolant tank, and a second heat exchanger thermally coupled to the propellant tank and fluidically coupled to the first heat exchanger. The coolant flows from the coolant tank to the first heat exchanger. The coolant flows from the first heat exchanger to the second heat exchanger. 
     In accordance with another aspect of the present disclosure, a propulsion system includes a rocket engine, a propellant tank fluidically coupled to the rocket engine and configured to hold a pressurized propellant, a coolant tank configured to hold a coolant, a first heat exchanger thermally coupled to the rocket engine and fluidically coupled to the coolant tank, a second heat exchanger thermally coupled to the propellant tank and fluidically coupled to the first heat exchanger, and a third heat exchanger disposed inside the propellant tank and configured to thermally couple a propellant withdrawn from the tank for combustion to a propellant disposed inside the tank. The coolant flows from the coolant tank to the first heat exchanger and through the first heat exchanger to cool the rocket engine. The propellant withdrawn from the propellant tank receives heat from the propellant disposed inside the tank through the third heat exchanger to convert to a gaseous propellant when withdrawn from the propellant tank as a liquid propellant. The coolant flows from the first heat exchanger to the second heat exchanger and through the second heat exchanger to heat the propellant disposed inside the propellant tank. 
     In accordance with another aspect of the present disclosure, a method of cooling a rocket engine having a propellant tank includes withdrawing propellant from the propellant tank wherein the withdrawn propellant is thermally coupled to the propellant disposed inside the propellant tank. The withdrawing causes the withdrawn propellant to receive heat from the propellant disposed in the propellant tank to convert to a gaseous propellant when the withdrawn propellant is in a liquid form. The method further includes cooling a rocket engine with a coolant, the cooling of the rocket engine causing heating of the coolant. The method also includes transferring heat from the heated coolant to the propellant disposed in the propellant tank so that a pressure of the propellant disposed in the propellant tank is maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a propulsion system having a cooling system constructed in accordance with an embodiment the teachings of the present disclosure. 
         FIG. 2  is a schematic diagram of a propulsion system having a cooling system constructed in accordance with another embodiment of the teachings of the present disclosure. 
         FIG. 3  is a schematic diagram of a propulsion system having a cooling system constructed in accordance with another embodiment of the teachings of the present disclosure. 
         FIG. 4  is a schematic diagram of a propulsion system having a cooling system constructed in accordance with another embodiment of the teachings of the present disclosure. 
         FIG. 5  is a schematic diagram of a propulsion system having a cooling system constructed in accordance with another embodiment of the teachings of the present disclosure. 
         FIG. 6  is a schematic diagram of a propulsion system having a cooling system constructed in accordance with another embodiment of the teachings of the present disclosure. 
         FIG. 7  is a schematic diagram of a bipropellant propulsion system having a cooling system constructed in accordance with another embodiment of the teachings of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-6 , a cooling system  20  constructed in accordance with the teachings of the present disclosure for a propulsion system  21  is shown. The propulsion system  21  includes a rocket engine  22  and one or more propellant tanks  24 , which can be in fluid communication with the rocket engine  22  to supply a propellant to the rocket engine  22  for combustion. In  FIGS. 1-6 , a monopropellant propulsion system is shown with only one propellant tank  24 . In  FIG. 7 , a bipropellant propulsion system is shown having two propellant tanks  24 . Accordingly, the present disclosure is applicable to any rocket engine regardless of the number and configuration of propellant tanks and oxidizer tanks used. The propulsion system  21  may also include a first heat exchanger  26  that is thermally coupled to the rocket engine  22 . The cooling system  20  includes a second heat exchanger  28  that is thermally coupled to the propellant tank  24 , and a coolant tank  30  configured to hold a coolant. The first heat exchanger  26  may be a component of the cooling system  20 , or as noted above, a component of the propulsion system  21 . The cooling system  20  also includes a flow circuit  34 . As shown in  FIG. 1 , the closed flow circuit  34  is defined by a pump  32 , the first heat exchanger  26 , the second heat exchanger  28  and the coolant tank  30 . The coolant returns to the coolant tank  30  with the aid of the pump  32 . The pump  32  may be anywhere in the flow circuit  34 . For example, the pump  32  may be before or after the coolant tank  30 . The coolant may not be returned to the coolant tank  30 . Accordingly, the coolant flows in the flow circuit  34 , which is defined by the coolant tank  30 , first heat exchanger  26  and the second heat exchanger  28 , and is then either vented overboard, as shown in  FIG. 2 , or is dumped into the rocket engine  22 , as shown in  FIG. 3 . As shown by example in  FIGS. 4-6  and described in detail in the following, the flow circuit  34  may be constructed from a combination of the embodiments described herein. 
     In both the closed and the open flow circuits  34 , the coolant flows from the coolant tank  30  to the first heat exchanger  26 . The waste heat from the rocket engine  22  is transferred to the coolant to cool the rocket engine  22 . The heated coolant transfers the heat therein to the propellant with the second heat exchanger  28 , thereby causing a pressure of the propellant in the propellant tank  24  to be maintained at a desired level. The cooling system  20  may also include a third heat exchanger  35 , which facilitates heat exchange between propellant being withdrawn from the propellant tank  24  and the propellant inside the propellant tank  24  so that the withdrawn propellant is in gaseous form. Any resulting cooling of the propellant in the propellant tank  24  is then countered by the heat from the second heat exchanger  28  so that the pressure in the propellant tank  24  is maintained at a desired level. 
     The propulsion system  21  of  FIGS. 1-6  is shown to operate with a monopropellant, which includes both the fuel and oxidizer parts that are necessary for combustion. An example of a monopropellants is nitrous oxide (N 2 O). Accordingly, the propulsion system  21  is shown to include only one propellant tank  24 , in which a monopropellant is stored. The propulsion system  21 , however, can also operate with a fuel and an oxidizer as separate compounds (not shown) that are mixed prior to combustion. Furthermore, the propulsion system  21  can operate as a bipropellant system. An example of such a system is shown in  FIG. 7 , in which parts similar to those in  FIGS. 1-6  are denoted with the same reference numbers. Accordingly, the disclosed cooling system  20  is equally applicable to various types of propulsion systems. 
     With reference to  FIG. 1 , the cooling system  20  having the closed flow circuit  34  for the coolant will now be described in detail. The propellant tank  24  is connected to the rocket engine  22  with one or more conduits. In  FIG. 1 , a conduit  40  is schematically shown, by which the propellant tank  24  is in fluid communication with the rocket engine  22  and supplies the propellant to the rocket engine  22  for combustion. The pump  32  may be disposed at any location in the flow circuit  34  to provide pumping of the coolant in the flow circuit  34 . In  FIG. 1 , the pump  32  is shown to be disposed in the flow circuit  34  between the second heat exchanger  28  and the coolant tank  30 . The coolant flows from the coolant tank  30  to the first heat exchanger  26  through the conduit  42 . The first heat exchanger  26  may be an integral part of the rocket engine  22  or a separate component that is attached to portions of the rocket engine  22  to provide cooling thereof Typical rocket engines include a combustion chamber and a nozzle. The propellant is burned in the combustion chamber and expanded by the nozzle before being ejected from the nozzle to produce a propulsive force. In the rocket engine  22  of  FIGS. 1-7 , a combustion chamber  44 , which is also generally referred to herein as the combustor  44 , and a nozzle  46  are schematically shown. The first heat exchanger  26  may be integral with the combustion chamber  44  (not shown) or be attached to the combustion chamber  44  for cooling thereof. The heat exchanger  26  may also be an integral part of the rocket engine  22 . In the example shown in  FIGS. 1-7 , the nozzle  46  includes passages  48  therein that can be in fluid communication with the conduit  42  to receive the coolant from the flow circuit  34 . Thus, although the first heat exchanger  26 , i.e., passages  48 , is shown to be an integral part of the nozzle  46 , the first heat exchanger  26  may also include passages that are integrally coupled to the combustion chamber  44 . Alternatively, the first heat exchanger  26  may be a component that is independent from the rocket engine  22  and is a component of the cooling system  20 , such that it is attached or connected to anyone of the combustion chamber  44  and the nozzle  46 . 
     The first heat exchanger  26  facilitates transfer of heat from the combustion chamber  44  and/or the nozzle  46  to the coolant. The noted transfer of heat cools the combustion chamber  44  and/or the nozzle  46 . The heated coolant is then transferred through a conduit  50  to the second heat exchanger  28 . The second heat exchanger  28  is thermally coupled to the propellant tank  24 . This thermal coupling may be through direct connection of the second heat exchanger  28  with the walls of the propellant tank  24 , or through an indirect connection with an intermediate heat exchanger or similar heat exchange devices. The second heat exchanger  28  facilitates the transfer of heat from the heated coolant to the propellant tank  24 . The heat from the propellant tank  24  is then transferred to the propellant therein. 
     The propellant in the propellant tank  24  may be self pressurized. Accordingly, as the propellant is consumed by the rocket engine  22 , the pressure in the propellant tank  24  may drop to a level, below which flow of the propellant to the rocket engine  22  due to the pressurization of the propellant may become inconsistent. However, the heat transferred from the coolant to the propellant as described herein can maintain the pressure in the propellant tank  24  for operation of the propulsion system  21 . The heat from the coolant can boil a portion of the propellant which is in liquid form to increase the pressure of the gaseous portion of the propellant. Thus, the heat for the boiling the liquid propellant is drawn from the heated coolant and is not drawn from the propellant. After transferring heat to the propellant, the coolant can be transferred from the second heat exchanger  28  to the coolant tank  30  through the conduit  52  to re-circulate in the flow circuit  34  as described above. The pump  32  facilitates the return of the coolant to the coolant tank  30 . 
     To ensure that the propellant being withdrawn from the propellant tank  24  for combustion is in a gaseous form, the propellant tank  24  draws the propellant from the tank  24  through an outlet  53 , which may be any type of orifice or other suitable outlet. As the withdrawn propellant expands to convert to a gaseous state, it draws heat from the propellant inside the propellant tank  24  through the third heat exchanger  35 . Accordingly, the third heat exchanger  35  provides heat from the propellant inside the propellant tank  24  to the withdrawn propellant to ensure that the withdrawn propellant is in gaseous form. The heat exchange through the third heat exchanger  35  will cause a slight cooling or larger accumulative cooling of the propellant in the propellant tank. The cooling can contribute to a pressure drop in the tank  24 . Furthermore, as the propellant is withdrawn from the tank, the pressure inside the tank  24  can drop to cause a boiling of the liquid propellant and further cooling of the tank. The noted cooling of the propellant in the propellant tank  24  may cause the pressure in the tank  24  to drop below a desired or operational pressure. However, because heat is transferred to the propellant tank  24  through the second heat exchanger  28 , the pressure of the propellant in the tank  24  can be maintained at a desired level. 
     In some cases the heat provided to the propellant tank via the coolant flow may not be sufficient to completely vaporize the propellant. An economizer regulator (back pressure regulator) can bypass some of the propellant leaving the tank around the heat exchanger  35 , such that some liquid is mixed into the propellant delivered to the conduit  40 . This allows the propellant tank temperature and pressure to remain at the desired point. 
     Alternately, a control element, such as a pressure regulator, can be introduced in conduit  40  to control the propellant flow rate into the combustor  44 . Unbalanced heat exchange can be allowed to raise or lower the average temperature of the complete coolant/propellant/tank system, and thus the propellant tank pressure, but the pressure regulator would keep the combustor&#39;s operating conditions constant, and allow the system embodied in  FIG. 1  to operate for significant durations even without the extra heat rejection systems in  FIGS. 2-6 . 
     As shown in  FIG. 1 , all of the coolant flows from the second heat exchanger  28  through the flow circuit  34  to return to the coolant tank  30  with the aid of the pump  32 . The cooling system  20  of  FIG. 1  represent a scenario where the heat exchange in the flow circuit  34  is balanced. However, the heat exchange may not be balanced such that continuous circulation of the coolant in the flow circuit  34  raises the temperature of the coolant. Accordingly, some or all of the coolant may have to be removed from circulation so that warming of the coolant beyond acceptable levels is avoided. Referring to  FIGS. 2-5 , several embodiments of the disclosed cooling system  20  are shown in which the coolant may circulate through an open flow circuit  34  or a combination of an open and a closed flow circuit  34 . 
     Referring to  FIG. 2 , after the coolant passes through the second heat exchanger  28  to heat the propellant in the propellant tank  24 , the coolant may be vented overboard and not returned to the coolant tank  30 . Referring to  FIG. 3 , after the coolant passes through the second heat exchanger  28  to warm the propellant in the propellant tank  24 , the coolant may be dumped into the rocket engine  22  and not returned to the coolant tank  30 . If the coolant includes a combustible material, it can be added to the combustion chamber  44  and or the nozzle  46  for combustion. An example of a combustible coolant is alcohol and/or a mixture of alcohol with other combustible and non-combustible compounds. If the coolant includes a non-combustible material, it can be added to the combustion chamber  44  and or the nozzle  46  to increase the mass flow rate of the gases exiting the nozzle to thereby increase the propulsive force of the rocket engine  22 . An example of a non-combustible coolant is water. Thus, in the embodiments of the disclosed cooling system  20  shown in  FIGS. 2 and 3 , the coolant flows in an open flow circuit  34  where the coolant is not returned to the coolant tank  30  after passing through the second heat exchanger  28 . Accordingly, the coolant from the tank  30  may be driven by the pressure in the tank  30  without using a pump  32 . Optionally, however, a pump  32  may be added as described herein. 
     The cooling system  20  may also include a combination of any two of the embodiments shown in  FIGS. 1-3 . For example, as shown in  FIG. 4 , the cooling system may include a flow control device  60  that can dump a portion of the coolant into the rocket engine  22  and vent the remaining portion of the coolant overboard. The flow control device  60  may be a passive device, such as a T-shaped flow splitter, or an active control mechanism by which the splitting of the flow can be controlled as desired. If the control device  60  is an active control device, the cooling system  20  can include sensors and controllers by which the coolant flow can be controlled to achieve desired results. In another example, as shown in  FIG. 5 , the control device  60  may vent a portion of the coolant overboard and send the remaining portion of the coolant back to the coolant tank  30  with the aid of the pump  32 . In yet another example, as shown in  FIG. 6 , the control device  60  may return a portion of the coolant to the flow circuit  34  and route the remaining portion of the coolant to the engine  22  for combustion. 
     The cooling system  20  is described above for use with a propulsion system that uses a monopropellant. However, as described above, the cooling system  20  is equally applicable to propulsion systems that operate by one or more fuels and one or more oxidizers that are mixed prior to or during combustion. For example, in a rocket engine that uses a fuel and an oxidizer as separate stored components, the oxidizer tank can be part of the cooling system  20  as described herein, while the fuel tank may optionally be part of the cooling system  20 . In yet another example, in a propulsion system having a bipropellant, as shown in  FIG. 7 , both propellant tanks  24  can include a second heat exchanger  28  and may include a third heat exchanger  35  to provide the above-described functionality for a monopropellant system. 
     The cooling system  20  of the present disclosure may be the only cooling system used in a propulsion system, or may be an auxiliary cooling system that can provide additional cooling when necessary. The cooling system  20  can be retrofitted to existing propulsion systems, in which any one of the fuel and the oxidizer is used to cool the combustion chamber and/or the nozzle. The cooling system  20  can also be retrofitted to existing propulsion systems that use radiation cooling of the combustion chamber and the nozzle. The cooling system  20  can also be an integral part of a propulsion system that can either use the cooling system  20  of the present disclosure alone or in combination with other cooling systems. 
     From the foregoing, it will be appreciated that the cooling system  20  of the present disclosure provides cooling of the combustion chamber and/or the nozzle while heating the self pressurized propellant or fuel and/or oxidizer to maintain the pressure in the propellant or fuel and/or oxidizer tanks. 
     Although not discussed or shown herein, components such as propellant and coolant control valves; pressurization, vent, and safety relief valves; filters, sensors, and test ports; and other paraphernalia common to rocket propulsion systems of the type discussed herein may be used in the disclosed system. The placement and use of these components is known to those of ordinary skill in the art and need not be duplicated here. 
     While a particular form of the disclosure has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the disclosure be limited, except as by the appended claims.