Abstract:
A lubrication system for an expendable gas turbine engine ( 20 ) having a rotatable shaft ( 26 ) is provided. Bearings ( 28 ) journal the shaft ( 26 ) for rotation about an axis. The system includes a vessel ( 46 ) for containing lubricating oil, a conduit ( 58 ) extending from the vessel ( 46 ) to the bearings ( 28 ), and a solenoid operated valve ( 70,76 ) in the conduit ( 58 ) and operable to only either fully open or fully closed. A control circuit ( 16 ) is provided for pulsing the solenoid ( 70 ) at a controlled rate to alternatingly (a) allow oil flow and (b) halt oil flow to the bearings ( 28 ) for a time insufficient to cause oil starvation of the bearings ( 28 ).

Description:
FIELD OF THE INVENTION 
   This invention relates to lubrication systems for gas turbine engines, and even more particularly, to lubrication systems for expendable turbojet engines. 
   BACKGROUND OF THE INVENTION 
   Gas turbine engines conventionally include a rotary compressor, a turbine, and a rotary shaft interconnecting the two. As is typical in equipment of all sorts having rotatable components, except when exotic bearings, such as magnetic bearings, are employed, it is necessary to provide for lubrication of the rotary components. In typical gas turbine engines, lubricating oil is provided to bearings journalling the rotary components, recovered and then recycled. These systems require pumps for recovering the lubricating oil as well as for circulating the lubricating oil. While such systems perform quite adequately, they can be heavy and/or bulky, not to mention expensive in construction. As a consequence, they are not suitable for use in all gas turbine systems. 
   For example, cruise missiles and target drones used by the military are frequently powered by small turbojet engines. Because these airborne vehicles are intended to be used, in the case of a cruise missile, but a single time, and in the case of target drones, no more than a couple of times, the turbojet engines employed are designed to be inexpensive to thereby provide an expendable engine. It accordingly follows that it is desirable that engine supporting systems, including the lubrication system, likewise be inexpensive as well. And because such engines are frequently used in airborne vehicles, it is highly desirable to minimize weight so that payload and/or range may be maximized. 
   At the same time, the lubrication system must be capable of operating reliably for the life of the engine and over a wide range of temperatures, typically from minus 40° F. to plus 180° F. Because these engines typically operate at a high rpm, a shaft or gear driven pump system is impractical as well as expensive. 
   Typically, the engines employed are relatively small and consequently, the lubricant flow rate is similarly small. Nonetheless, the flow must be reliable and delivered within the desired range under any and all conditions of operation. Typically, oil flows in the range of 1.5 cc per minute to 2.5 cc per minute are employed. To reliably obtain such flows when the oil experiences substantial changes in viscosity, dependent upon ambient temperature, poses substantial difficulty. Too little oil flow results in bearing failure and too great of an oil flow can result in premature exhaustion of oil and bearing failure. 
   Specifically, the nature of the system is that the maximum rate for the total oil flow has to be limited to assure that lubricating oil is available at or near the end of the mission cycle. Furthermore, the maximum rate has to be limited so as to enable the minimization of the size of the oil tank. Moreover, the system additionally has to be capable of being stored in the state of non-use with its compliment of lubricating oil for up to 15 years without loss and at the same time be ready for use immediately upon demand. 
   The present invention is directed to providing a lubricating oil system meeting these and other needs. 
   SUMMARY OF THE INVENTION 
   It is the principal object of the invention to provide a new and improved lubricating system for a gas turbine engine. More specifically, it is an object of the invention to provide a new and improved lubricating system for a turbojet engine; and even more specifically, it is an object of the invention to provide a new and improved lubricating system for an expendable turbojet engine mounted on an airborne vehicle. 
   An exemplary embodiment of the invention includes a lubrication system for an expendable, gas turbine engine which includes a gas turbine engine having a rotatable shaft. Bearings journal the shaft for rotation about an axis. A vessel containing lubricating oil is provided and a conduit extends from the vessel to the bearings. A solenoid operated valve is located in the conduit and is operable only to either fully open or fully close. A control circuit is provided for pulsing the solenoid at a controlled rate to alternatingly (a) allow oil flow; and (b) halt oil flow to the bearings for a time insufficient to cause oil starvation of the bearings. 
   In one embodiment of the invention, the vessel includes a tank and a bladder is disposed within the tank. Also provided is a source of gas under pressure. One or the other of the tank and the bladder contain lubricating oil for the bearings and the other of the tank and the bladder is connectable to the source of gas under pressure. By pressurizing the other of the tank and the bladder, lubricating oil is expelled into the conduit whenever the solenoid valve opens. 
   In one embodiment, the tank contains the lubricating oil and the gas under pressure is admitted to the bladder. In another embodiment, the bladder contains the lubricating oil and the tank receives the gas under pressure. 
   In a highly preferred embodiment, the time over which the valve is closed is no more than about 3 seconds. 
   A preferred embodiment includes a metering orifice in the conduit between the bearings and the solenoid valve. 
   A highly preferred embodiment further includes a pressure regulator operatively interposed between the one of the tank and the bladder receiving gas under pressure. 
   According to the embodiment mentioned immediately preceding, the pressure regulator receives an input representative of the pressure at the bearings. 
   In a highly preferred embodiment, the engine is mounted in a vehicle and the control circuit receives inputs indicative of vehicle velocity and temperature of the lubricating oil. 
   Even more preferably, the vehicle is an airborne vehicle and the control circuit additionally receives an input representative of the altitude of the vehicle. 
   Preferably, the tank is in sufficiently close proximity to the engine so as to receive heat rejected by the engine so that the lubricating oil is warmed by engine operation to reduce its viscosity. 
   The invention also contemplates that the source of gas under pressure may be pressurized gas stored in a pressure vessel or air under pressure from the compressor section of the turbine engine. 
   Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings. 

   
     DESCRIPTION THE DRAWINGS 
       FIG. 1  is a somewhat schematic sectional view of an airborne vehicle powered by a turbojet engine and embodying a lubrication system made according to the invention; 
       FIG. 2  is a somewhat simplified schematic of part of the lubrication system; 
       FIG. 3  illustrates a form of a vessel for storing lubricating oil that may be used as an alternative to that shown in  FIG. 2 ; 
       FIG. 4  shows a pressurization system for use with the oil storage vessels shown in either  FIG. 2  or  FIG. 3 ; 
       FIG. 5  is a partial schematic illustrating an alternative for the embodiment illustrated in  FIG. 4 ; and 
       FIG. 6  is a flow diagram illustrating the use of various control parameters. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An exemplary embodiment of the invention is illustrated in the environment of a turbojet driven airborne vehicle of the expendable type, such as a cruise missile or a target drone. The vehicle includes a vehicle body, generally designated  10 , which may or may not be provided with wings  12  of conventional construction. At the rear of the body  10  is a jet nozzle  14  by which the body  10  is propelled as a result of hot gases of combustion exiting the nozzle  14 . 
   In the forward part of the body  10 , a payload  16  is located. In the case of a cruise missile, the payload would be munitions whereas in the case of a target drone, the payload  16  might include a parachute or the like to allow recovery and possible re-use of the target drone. 
   At the rear of the body  10 , a small expendable turbojet engine, generally designated  20 , is provided. In the illustrated embodiment, the turbojet engine  20  is of the radial flow type and includes a rotary compressor  22  of conventional construction coupled to a turbine wheel  24  by means of a shaft  26  journalled by bearings  28 . The turbojet engine includes an annular passage  30  including diffuser vanes  32  and anti-swirl vanes  34  and which extends to an annular combuster  36 . The annular combuster includes a nozzle  38  which directs gases of combustion against blades  40  on the turbine wheel  24  to rotate the same. The gases of combustion are expelled by the nozzle  14  while rotation of the compressor wheel  22  by reason of the coupling between the turbine  24  and the compressor wheel  22  by the shaft  26  serves to provide compressed combustion air to the combuster  36 . 
   Ram air scoops  44  may extend to just outside of the vehicle body  10  to capture ambient air and direct it to the compressor wheel  22  as is well known. 
   A source of lubricating oil, generally designated  46 , is illustrated in the drawing as being located between the compressor wheel  22  and the turbine  24 . However, there are a multitude of other locations which may be employed as well. It is highly desirable that the source of lubricating oil  46  be located in close proximity to the engine  20  so that heat rejected by the engine  20  to the interior of the vehicle body  10  will warm lubricating oil contained in the source  46  to reduce its viscosity. 
   Elsewhere within the body  10  is a source of fuel  48  for the engine  20  as well as a source of compressed gas under pressure, generally designated  50 , which may be compressed air stored in a small pressure vessel. 
   Finally, the missile  10  includes a control system, generally designated  52 . 
   Turning now to  FIG. 2 , the area of the engine  20  containing the bearings  28  is a bearing cavity  56  of conventional construction. Lubricating oil is introduced into the cavity  56  via a conduit  58 . 
   One form of the lubricating oil source  46  is shown in  FIG. 2  and is seen to include an arcuate tank  60  shaped to fit within the body  10  and containing an interior, flexible bladder  62 . A body of lubricating oil  64  is contained within the bladder  62 . An inlet to the tank  60  is shown schematically at  66  and is connected to the pressurized gas source  50  in a manner to be seen. In any event, upon the admission of pressurized gas to the interior of the tank  60  via the inlet  66 , pressure is exerted against the bladder  62  to expel the lubricant  64  via an outlet  68  connected to the conduit  58 . 
   Within the conduit  58 , between the source  50  of lubricating oil and the bearing cavity  56  is a solenoid operated valve  70 . The solenoid operated valve  70  is of the type that is either fully open or fully closed. That is to say, the valve  70  does not have an analog modulating function. It is operated by the control  16  to alternatingly open and close at a variable rate while the source  50  is being pressurized so that an intermittent flow of lubricant  64  to the bearing cavity  56  results. Preferably, a metering orifice  74  is located in the conduit  56  downstream of the valve  70  to limit the maximum flow rate. 
   It has been determined that the engine  20  may operate without damage to the bearings  28  even when the flow of lubricating oil to the bearing cavity  56  is interrupted for as long as three seconds. Consequently, the total oil flow to the bearings  28  may be regulated by appropriately energizing and de-energizing the solenoid  70  to open and close the valve  76  associated therewith to provide what might be termed a “digital modulation” of oil flow. 
   An alternative form of the source of lubricating oil  50  is illustrated in  FIG. 3 . In this embodiment, a flexible bladder  80  is disposed within an arcuate tank  82 . In this case, however, the bladder  80  is connected to a pressurized gas inlet  84  which is ultimately connected to the source  50  and an outlet  86  connected to be in fluid communication with the interior of the tank  82 , but not the bladder, is provided for connection via the conduit  58  to the bearing cavity  56 . In this embodiment, the admission of pressurized gas through the inlet  84  to the interior of the bladder  80  results in a body of oil  88  being subjected to pressure so it will flow through the outlet  86  to the bearing cavity  56 . 
   Either form of the source of lubricating oil  46  shown in  FIGS. 2 and 3  may be employed. It is noted that bladder and tank type sources are highly desirable in that they allow complete depletion of the lubricating oil  64 , 88  from the source  46  as a result of pressure applied to the exterior of the bladder  62  or to the interior of the bladder  80 , as the case may be. 
     FIG. 4  illustrates a means of providing gas under pressure to the lubricant source  40 , as well as to the source of fuel  48  which may be a similar bladder and tank construction. The gas source  50  may include a pressure bottle  90  connected by a selectively operable valve (not shown) to a pressure regulator  92  and then via a check valve  94  to a junction  96 . The compressor section of the engine  20  may be tapped via a line  98  to obtain bleed air which is then passed through a check valve  100  to the junction  98 . The junction  98  is then connected to the fuel source  48  and the oil source  46  via a pressure regulator  102 . This system allows a stored gas to be utilized for initial pressurization of both the lubricating oil source  46  and the fuel source  48  with bleed air from the engine  20  taking over the pressurization function after the engine  20  has been started and brought up to operating speed. As the bottle  90  does not need to provide pressurized gas for the entire mission, its size may be reduced. In both cases, back flow is prevented by the check valves  94  and  100  and a gas at a desired pressure, designated “P a ” in the drawings is provided to the oil and fuel sources  46  and  48  respectively. 
   As will be apparent, the embodiment illustrated in  FIG. 4  provides gas under pressure based solely on the regulated pressure of gas from either the bottle  90  or the engine  20 . In some instances, finer control of pressurization may be desired. This is due to the fact that the pressure within the bearing cavity  56  will vary dependent upon altitude and forward velocity, the latter affecting ram air pressure at the inlet to the engine  10  and thus the bearing cavity  56  as well. In such a case, it may be desirable to regulate the pressure applied to the oil source  46  as a function of the pressure from the source in the form of the air bottle  90  or the engine  20  less the opposing pressure, namely, the pressure at the bearings  56 . This latter pressure is designated “P b ” in  FIG. 5  and so the control parameter would then be based on (P a –P b ). This may be accomplished by interconnecting the bearing cavity  56  and a pressure regulator  110  which in turn interconnects the gas sources  20  or  90  and the oil source  46  as illustrated in  FIG. 5 . 
   A simplified control schematic is illustrated in  FIG. 6 . The control signal to the solenoid valve  70  is indicated by an arrow  116 . Arrows  118  and  120  indicate inputs in the form of an indication of altitude and in an indication of forward speed. However, as noted previously, these could be combined into a single input representative of the pressure P b  in the bearing cavity  56 . A third input  122  to the control  16  is based on ambient temperature or the temperature of the oil as this is a measure of viscosity. The lower the ambient air or oil temperature, the higher the viscosity, thereby necessitating a longer opening period of the valve  76  by the solenoid  70  to assure adequate flow. 
   In some instances, a feedback loop  124  may be included. This feedback loop  124  feeds back the pulse rate to compensate for the possible heating effect of the solenoid coil  70  on fuel flowing in the conduit  58 . Because of the low oil flow rates typically encountered in apparatus of this sort, rapid pulsing of the solenoid  70  could substantially heat solenoid, which heat would be transferred to the oil to reduce its viscosity and increase its flow rate. The fed back pulse rate provides a measure of possibly heating as a result of rapid pulsing. 
   From the foregoing, it will be appreciated that a lubrication system for bearings made according to the invention is simple, and consequently, highly reliable in terms of having a minimum number of components subject to failure. Moreover, through the expedient of intermittent flow of the lubricant, pumps need not be employed and yet the flow rate can be reliably controlled within a range where flow is sufficiently low that a large oil source  46  is not required. At the same time, so long as oil flow occurs at least every three seconds, adequate flow of lubricating oil to the bearings  28  is provided. 
   The use of tank and bladder oil source constructions minimizes the size of the source because they can be completely emptied and provides a means for a long term storage of lubricating oil.