Field of the Invention

[0001] The present invention relates to a gas turbine.

Background to the Invention

[0002] Gas turbines are a widely used type of engine. A gas turbine combusts fuel to provide mechanical energy. Gas turbines are used in aerospace applications as engines to power aircraft. Gas turbines are also used in land-based applications, such as the generation of electrical energy by combusting natural gas.

[0003] Figure 1 schematically shows the main components of a prior art gas turbine. A compressor 1 is provided at an inlet side and a turbine 2 is provided at an outlet side. The compressor 1 and the turbine 2 are housed within a tube. The compressor 1 is connected to the turbine 2 by a shaft. In use, gas (air) is drawn into the engine and compressed by the compressor 1. A combustion stage combusts fuel and the compressed gas. This expands the gas within the tube. Expanded gas is expelled through the turbine 2 which, via the connecting shaft, powers the compressor 1 . The additional energy provided by the combustion causes the turbine 2 to turn the fan 1 in a feedback loop provided by the connecting shaft. The system reaches equilibrium when the energy extracted by the turbine 2 is expended completely by the compressor 1 pulling in more air (and any ancillary equipment that may be attached). Residual energy is exhausted after the turbine 2. In aircraft engines, the gas turbine is designed to provide thrust to propel the aircraft forwards. In gas turbines designed for land- based applications, such as generation of electrical energy, the gas turbine is designed to turn an output shaft.

[0004] The physical shaft connecting the compressor 1 and turbine 2 is a linear feedback loop, whereas the mathematics that defines the dynamics of the gas flow within the engine is non-linear. A consequence is that the engine is designed around operation at a single engine speed for maximum efficiency, and that the engine has reduced efficiency at other speeds.

[0005] There are a number of approaches gas turbine manufacturers take to provide engines for aircraft that are flexible across a range of operation. This can add significant complexity to the design and can reduce maximum efficiency. spec3949 [0006] One approach is to use multiple compressors. Each compressor provides a fraction of the overall required compression. This can provide a wider operating range but increases weight of the engine.

[0007] Another approach is to use multiple shafts, each concentric to the others in a Russian Doll type arrangement. This effectively provides two or more separate gas turbines in one overall engine. Again, this is optimised around a single speed, but with a wider operational range of engine speeds. Providing two or more separate shafts means that each shaft can 'find' its optimal speed for the given engine conditions, increasing overall efficiency compared to a single shaft design. In practice, all commercial aircraft engines are of a two or more shaft design. These shafts are referred as the Low Pressure (LP) shaft for the outermost compressor/fan, High Pressure (HP) shaft for the innermost, and Intermediate Pressure (IP) shafts for extra shafts in triple or more shaft applications.

[0008] One application of the gas turbine is in the commercial airliner. In this application, the first stage of the compressor of the LP shaft is a fan which provides thrust rather than compression. The use of the fan for thrust increases the efficiency and operating range of the engine, as well as reducing noise as the slower moving fan exhaust masks the inner turbine noise. In the latest designs, the fan is large compared to the core and provides most of the thrust of the engine. However, its size and operating speed are restricted to the rotational speed of the engine. The Pratt & Whitney® Geared Turbofan has a planetary gearing system to allow the LP shaft to rotate at a fixed ratio above the speed of the fan.

[0009] The design of the blades of a compressor and a turbine is optimised for one specific ratio of gas flow in to gas flow out. Anything other than this ratio will be less efficient. In small engines which are only designed to operate at a single speed this is not so much of a problem. However with larger engines, or engines that need to operate at a range of engine speeds such as in aircraft, creating an engine which is both flexible in useful envelope of operation but also as efficient as possible is a tradeoff.

Summary of the Invention

[0010] In one aspect, the present invention provides a gas turbine comprising: a compression stage comprising a compressor which is configured to rotate about an axis; a combustion stage; and a turbine stage comprising a turbine which is configured spec3949 to rotate about an axis, wherein the turbine stage comprises an electrical generator which is configured to generate electrical energy in response to rotation of the turbine, and the compression stage comprises an electrical motor which is configured to use electrical energy generated by the electrical generator to rotate the compressor and the turbine stage is not mechanically connected to the compression stage; and wherein: each of the electrical motor and the electrical generator comprises a rotor and a stator, and the compressor comprises a hub having a longitudinal axis and a hollow shaft which is rotatable about the hub and the stator of the motor forms part of the hub and the rotor of the motor forms part of the hollow shaft; and the turbine comprises a hub having a longitudinal axis and a hollow shaft which is rotatable about the hub and the stator of the generator forms part of the hub and the rotor of the generator forms part of the hollow shaft.

[0011] The gas turbine may comprise a controller, wherein the controller is configured to control the electrical motor to rotate the compressor at a rotational speed which is different to a rotational speed of the turbine.

[0012] The gas turbine may comprise an electrical energy store.

[0013] The electrical energy store may be connected to the electrical generator and be configured to store electrical energy generated by the electrical generator.

[0014] The electrical energy store may be configured to store electrical energy generated by the electrical generator when the electrical energy generated by the generator is greater than an electrical energy demand of the motor.

[0015] The electrical energy store may be connected to the electrical motor and is configured to supply electrical energy to the electrical motor.

[0016] The electrical energy store may be configured to supply electrical energy to the electrical motor when an electrical energy demand of the motor is greater than the electrical energy generated by the generator.

[0017] The gas turbine may comprise a fan and an electrical fan motor which is configured to use electrical energy generated by the electrical generator to rotate the fan.

[0018] The turbine may comprise a hollow shaft which is rotatable about an open- ended hub.

[0019] The gas turbine may comprise an axial through-flow path for gas flow through an interior of at least one of the compressor and the turbine. spec3949 [0020] The interior of at least one of the compressor and the turbine may comprise an additional combustion stage.

[0021] The interior of at least one of the compressor and the turbine may comprise at least one of: a ramjet combustion stage; a rocket combustion stage.

[0022] The compressor stage may comprise a plurality of compressors. For example, the plurality of compressors may be arranged sequentially in the direction of gas flow. Each compressor may be driven by a dedicated motor for that compressor. This can allow the possibility of driving different compressors at different speeds. Alternatively, a single motor may drive more than one compressor.

[0023] Similarly, the turbine stage may comprise a plurality of turbines. For example, the plurality of turbines may be arranged sequentially in the direction of gas flow. Each turbine may drive a dedicated generator for that turbine. Alternatively, a single generator may be driven by more than one turbine.

[0024] The gas turbine can be used in any application where a gas turbine is currently used, such as aircraft applications or land-based applications.

Brief Description of the Drawings

[0025] Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

[0026] Figure 1 is a schematic drawing of a prior art gas turbine;

[0027] Figure 2 is a schematic drawing of a gas turbine according to one example of the present invention;

[0028] Figures 3A and 3B are schematic drawings showing use of an electrical energy store;

[0029] Figures 4A-4D show transverse cross-sections through examples of a compressor and an electrical motor, or a turbine and an electrical generator;

[0030] Figure 5 shows a longitudinal cross-section through an example of the gas turbine;

[0031] Figure 6 shows an example of a gas turbine with a fan;

[0032] Figure 7 shows another example of a gas turbine with a fan;

[0033] Figure 8 shows a plurality of gas turbines with fans; spec3949 [0034] Figure 9 shows a combination of a turbofan engine with a ramjet stage;

[0035] Figure 10 shows a combination of a turbofan engine with ramjet and rocket stages;

[0036] Figure 11 shows an application with an electrical motor on an output stage, such as a marine application.

Detailed Description of the Drawings

[0037] Figure 2 is a schematic drawing of a gas turbine according to one example of the present invention. A compression stage comprising at least one compressor 1 is provided at an inlet side of the engine. The compressor 1 comprises a set of blades. The compressor 1 is configured to rotate about a longitudinal axis 6. The compressor 1 may be an axial compressor or a radial (centrifugal) compressor. In an axial compressor air is exhausted from the compressor in an axial direction. In a radial compressor air is exhausted from the compressor in a radial direction. The

compression stage may comprise multiple compressors 1. A turbine stage comprising at least one turbine 2 is provided at an outlet side of the engine. The turbine 2 comprises a set of blades. The turbine 2 is configured to rotate about a longitudinal axis 6. The turbine stage may comprise multiple turbines 2. Each of the compressor 1 and turbine 2 may comprise a plurality of sets of blades arranged along the longitudinal extent of the compressor or turbine. The compression stage and the turbine stage are housed within an outer housing, or chamber, 1 1 . The housing 1 1 provides a structure to constrain the path of gas through the gas turbine. Although the outer housing 1 1 is schematically shown as a cylinder of uniform diameter, the housing 1 1 can vary in diameter and/or shape along the axial extent of the housing. For example, the housing 1 1 can reduce in diameter downstream of the compressor.

[0038] A combustion stage 40 is located between the compressor 1 and the turbine 2. Gas flow direction through the engine is shown by arrows 7. The housing 1 1 provides a structure to constrain gas which is expanded by the combustion stage 40, causing expanded gas to flow through the turbine 2. In use, gas (e.g. air) is drawn into the engine and compressed by the compressor 1. In the combustion stage 40, fuel is combusted with the compressed gas. This heats and expands the gas. Expanded gas is expelled through the turbine 2. This causes the turbine 2 to rotate about axis 6. In contrast to a conventional gas turbine, the compressor 1 is not connected to the turbine 2 by a drive shaft. That is, the compressor 1 is not mechanically driven by the turbine spec3949 2. Another heat source may be used in addition to, or instead of, combusting fuel in the combustion stage 40 in order to heat and expand the compressed gas. For example, steam may be used to pre-heat gas prior to the combustion stage 40.

[0039] The turbine stage comprises an electrical generator 9. The electrical generator 9 is configured to generate electrical energy in response to rotation of the turbine 2. The compression stage comprises an electrical motor 8. The electrical motor 8 is configured to rotate the compressor 1 . The electrical motor 8 is configured to use electrical energy generated by the electrical generator 9.

[0040] One advantage of this gas turbine is that the electrical motor 8 can rotate the compressor 1 at a rotational speed which is different to a rotational speed of the turbine 2.

[0041] An electrical output of the generator 9 may be directly connected to the motor 8. Optionally, an electrical energy conversion unit 55 is provided in the electrical path between the electrical generator 9 and the electrical motor 8. The electrical energy conversion unit 55 can convert an electrical output of the generator 9 into a form suitable for the motor 8. For example, the electrical energy conversion unit 55 can modify a voltage and/or a current of an electrical output of the generator 9 into a voltage and/or a current which is suitable for applying to the motor 8.

[0042] The electrical energy conversion unit 55 can comprise electrical energy storage 56, such as one or more batteries or other storage device. Figures 3A and 3B show use of an electrical energy store 56. Figure 3A shows an example where the compressor 1 and motor 8 are required to operate at a rotational speed requiring less electrical energy than is generated by the turbine 2 and generator 9. The electrical energy conversion unit 55 can harvest surplus energy output by the generator 9 and store the electrical energy in store 56. The electrical energy conversion unit 55 also provides an electrical supply to the motor 8. Figure 3B shows an example where the compressor 1 and motor 8 are required to operate at a rotational speed requiring more electrical energy than is generated by the turbine 2 and generator 9. The electrical energy conversion unit 55 can supplement the energy output by the generator 9 with electrical energy stored in store 56. The electrical energy conversion unit 55 provides an electrical supply to the motor 8.

[0043] Another possibility is to electrically brake the turbine 2. An example of a need to brake the turbine is to control a speed of the turbine 2, to prevent over-speeding.

spec3949 The turbine 2 can be braked by controlling direction of current flow through the generator 9.

[0044] A controller 50 is provided to control operation of the engine. The controller can control the rotational speed of the electrical motor 8, and therefore the compressor 1 . For example, the controller can control 52 a value of voltage and/or a value of current supplied by an electrical energy conversion/storage unit 55 to the motor 8. The controller can control the rotational speed of the turbine 2 and the electrical generator 9. For example, the controller can control the rotational speed of the turbine 2 and the electrical generator 9 by controlling combustion stage 40. Controller 50 receives control inputs 51. An example of a control input 51 is a signal indicative of an amount of work (e.g. thrust or output power) required of the engine. In an aircraft application, control input 51 can be provided by a flight control system or by a control of the aircraft cockpit. The controller 50 can receive inputs from one or more sensors S located in the engine. The sensors S can include: a sensor which senses rotational speed of the compressor 1 ; a sensor which senses rotational speed of the turbine 2; a sensor which senses a parameter within the combustion chamber (e.g. airflow rate, temperature, fuel pressure). The controller 50 is configured to use the inputs from the one or more sensors S, plus inputs 51 , to determine an operating state of the engine, such as a rotational speed of the motor/compressor and a rotational speed of the

generator/turbine. The combination of the controller 50, sensors S, control inputs 51 and control outputs 52 form a control system for the engine.

[0045] The controller 50 can adjust the flow of energy between the motor 8 and the generator 9. This can allow the precise operating speed required for greatest efficiency (or power) to be set for the compressor 1 . Excess energy may be stored in the energy storage unit 56. The turbine 2 may be designed to produce extra power in order to power a fan, similar to the fan in current commercial airliners, but which can operate at a speed unrelated to that of the core engine, creating additional efficiency gains over the classical gas turbine.

[0046] There are several possible options for locating the electrical generator 9 at the turbine stage and for locating the electrical motor 8 at the compression stage. Some possible options are shown in Figures 4A to 4D. Each of Figures 4A to 4D show a transverse cross-section and a corresponding longitudinal cross-section through the compressor and motor of the engine. The figures also relate to the turbine and generator.

spec3949 [0047] In Figures 4A and 4B the compressor 1 has an axial shaft 31 . The shaft 31 may be a cylinder of uniform diameter (as shown) or the shaft 31 can vary in diameter and/or shape along the axial extent of the shaft. The shaft 31 may be a solid structure. The axial shaft 31 has a longitudinal axis 6. The compressor 1 is configured to rotate about the longitudinal axis 6. A supporting structure and bearing (not shown) are provided to allow rotation of the shaft 31. The compressor 1 is driven by an electrical motor 8. In the example of Figure 4A, the motor 8 drives the shaft 31 . The motor 8 is axially offset along the shaft 31 from the compressor 1. The electrical motor 8 comprises a rotor 8.1 , a stator 8.3 and an air-gap 8.2 between the rotor 8.1 and stator 8.3. The stator 8.3 is concentric with the rotor 8.1 . The rotor 8.1 is concentric with, and fixed to, the shaft 31 so that the rotor 8.1 rotates with the shaft 31 . Field windings can be provided on the stator 8.3 or on the rotor 8.1. The armature can be provided on the other of the stator 8.3 or the rotor 8.1 . In use, magnetic interaction between the field windings and armature causes the rotor 8.1 to rotate with respect to the stator 8.3. This causes the compressor 1 to rotate about the longitudinal axis 6 of the shaft 31.

[0048] The electrical generator 9 and turbine 2 have a similar relationship. The turbine 2 has an axial shaft 31 . The electrical generator 9 comprises a rotor 9.1 , a stator 9.3 and an air-gap 9.2 between the rotor 9.1 and stator 9.3. The stator 9.3 is concentric with the rotor 9.1. Field windings can be provided on the stator 9.3 or on the rotor 9.1 . The armature can be provided on the other of the stator 9.3 or the rotor 9.1 . In use, the turbine 2 is driven by gas expelled through the engine. This drives the rotor 9.1 with respect to the stator 9.3. Interaction between the field windings and armature generates electrical energy in the armature which is output to the electrical energy conversion unit 55, or directly to the motor 8.

[0049] Figure 4B shows another example of a compressor 1 with a shaft 31 . In this example, the compressor 1 is driven by a motor 8 positioned at the radially-outermost part of the compressor 1 . The motor 8 comprises a rotor 8.1 , a stator 8.3 and an air- gap 8.2 between the rotor 8.1 and stator 8.3. The rotor 8.1 is concentric with the stator 8.3. The stator 8.3 is concentric with the compressor 1. The rotor 8.1 forms a radially outermost part of the compressor 1. The stator 8.3 is a ring which surrounds, and is radially offset from, a rotatable element of the compressor 1. Field windings can be provided on the stator 8.3 or on the rotor 8.1. The armature can be provided on the other of the stator 8.3 or the rotor 8.1. In use, interaction between the field windings and armature rotates the rotor 8.1 with respect to the stator 8.3. This causes the compressor 1 to rotate about the longitudinal axis 6. The electrical generator 9 and spec3949 turbine 2 have a similar relationship. The electrical generator 9 comprises a rotor 9.1 , a stator 9.3 and an air-gap 9.2 between the rotor 9.1 and stator 9.3. The rotor 9.1 is concentric with the stator 9.3. The stator 9.1 is concentric with the turbine 2. The rotor 9.1 forms a radially outermost part of the turbine 2. The stator 9.3 is a ring which surrounds, and is radially offset from, a rotatable element of the turbine 2. In use, the turbine 2 is driven by gas expelled through the engine. This drives the rotor 9.1 with respect to the stator 9.3. Interaction between the field windings and armature generates electrical energy in the armature which is output to the electrical energy conversion unit 55, or directly to the motor 8.

[0050] In Figures 4A and 4B the compressor 1 is rotatable about a (solid) shaft 31 located at the centre of the compressor, when viewed in a transverse cross-section. Figures 4C and 4D show alternative examples of the compressor 1. The compressor 1 has a hollow shaft, or sleeve, 32 with an annular cross-section and a hollow interior 3. The hollow shaft 32 is rotatable about a hub 33. The term "hub" 33 means a structure which supports the hollow shaft 32. The hollow shaft is tubular. The term "tubular" can comprise a structure of constant diameter along the length of the shaft/hub, or a structure which varies in diameter along the length of the shaft/hub. The tubular shaft/hub may be tapered, or may have a more complex shape. Bearing 34 between the hollow shaft 32 and the hub 33 allows the hollow shaft, or sleeve, 32 to rotate around the hub 33. The bearing 34 can take any suitable form, such as fluid bearing, magnetic bearing or rolling element bearing. In the examples shown in Figures 4C and 4D the hub 33 is hollow (i.e. not solid when viewed in cross-section). The hollow interior 3 of the hub 33 provides space which can serve as a through-flow path for gas, or can accommodate a further combustion stage. The hub 33 can be open-ended at one axial end or at both axial ends. The compressor 1 is rotatable about a longitudinal axis 6. The compressor 1 is driven by an electrical motor 8. The motor 8 comprises a rotor 8.1 , a stator 8.3 and an air-gap 8.2 between the rotor 8.1 and stator 8.3. The rotor 8.1 is concentric with the stator 8.3. The stator 8.3 is concentric with, and fixed to, the hub 33. The rotor 8.1 is concentric with the hollow shaft 32. The rotor 8.1 forms a radially innermost part of the compressor 1 . Field windings can be provided on the stator 8.3 or on the rotor 8.1. The armature can be provided on the other of the stator 8.3 or the rotor 8.1. In use, interaction between the field windings and armature rotates the rotor 8.1 with respect to the stator 8.3. This causes the compressor 1 to rotate about the longitudinal axis 6 of the hub 33. In the example of Figure 4C, the function of the hollow shaft 32 can be combined with the rotor 8.1 of the motor 8. The function of the hub 33 can be combined with the stator 8.3 of the motor 8. Similarly, the function of spec3949 the hollow shaft 32 can be combined with the rotor 9.1 of the generator 9 and the function of the hub 33 can be combined with the stator 9.3 of the generator 9.

[0051] The electrical generator 9 and turbine 2 have a similar relationship. The turbine 2 has a hollow shaft. The tubular shaft 32 is rotatable about a hub 33. The electrical generator 9 comprises a rotor 9.1 , a stator 9.3 and an air-gap 9.2 between the rotor 9.1 and stator 9.3. The rotor 9.1 is concentric with the stator 9.3. The stator 9.1 is concentric with, and fixed to, the hub. The rotor 9.1 is concentric with the hollow shaft. The rotor 9.1 forms a radially innermost part of the turbine 2. In use, the turbine 2 is driven by gas expelled through the engine. This drives the rotor 9.1 with respect to the stator 9.3. Interaction between the field windings and armature generates electrical energy in the armature which is output to the electrical energy conversion unit 55, or directly to the motor 8.

[0052] The stator 8.3 of the motor 8 can be located entirely within the tubular structure of a hollow hub, as shown in Figure 4C. This leaves the interior 3 of the hub 33 free for other uses, such as an airflow path or a combustion stage. A further alternative (not shown) is for the motor to also occupy part of the interior 3 of the hub 33.

[0053] The compressor (turbine) of Figure 4D is similar to Figure 4C. The compressor 1 has a hollow shaft 32. The hollow shaft 32 is rotatable about a hub 33. In this example, the compressor is driven by a motor 8 positioned at the radially-outermost part of the compressor. The motor 8 comprises a rotor 8.1 , a stator 8.3 and an air-gap 8.2 between the rotor 8.1 and stator 8.3. The rotor 8.1 is concentric with the stator 8.3. The stator 8.3 is concentric with the compressor 1. The rotor 8.1 forms a radially outermost part of the compressor 1. The stator 8.3 is a ring which surrounds, and is radially offset from, a rotatable element of the compressor 1 . Field windings can be provided on the stator 8.3 or on the rotor 8.1. The armature can be provided on the other of the stator 8.3 or the rotor 8.1. In use, interaction between the field windings and armature rotates the rotor 8.1 with respect to the stator 8.3. This causes the compressor 1 to rotate about the longitudinal axis 6 of the hub. The electrical generator 9 and turbine 2 have a similar relationship. The electrical generator 9 comprises a rotor 9.1 , a stator 9.3 and an air-gap 9.2 between the rotor 9.1 and stator 9.3. The rotor 9.1 is concentric with the stator 9.3. The stator 9.1 is concentric with the turbine 2. The rotor 9.1 is concentric with, and mounted on, the rotatable part of the turbine 2. The rotor 9.1 forms a radially outermost part of the turbine 2. The stator 9.3 is a ring which surrounds, and is radially offset from, a rotatable element of the turbine 2. In use, the turbine 2 is driven by gas expelled through the engine. This drives the spec3949 rotor 9.1 with respect to the stator 9.3. Interaction between the field windings and armature generates electrical energy in the armature which is output to the electrical energy conversion unit 55, or directly to the motor 8. In Figure 4D a bearing 34 may be provided between the hollow shaft 32 and the hub 33.

[0054] A further example (not shown) is similar to Figure 4D but does not include a shaft 32 and/or a hub 33. Instead, the stator 8.3 which surrounds the

compressor/turbine serves the dual purposes of providing a stator 8.3 of the motor/generator and providing mechanical support/constraint for the

compressor/turbine. A bearing can be provided between the compressor/turbine and the outer ring. An air gap can be provided between an inner edge of the

compressor/turbine and a housing or cowling of the gas turbine.

[0055] In Figures 4C and 4D, any suitable type of motor can be used. Any

conventional type of motor can be used. Another possibility is to use a type of linear motor. This will be called a 'circulinear' motor. If a linear motor is considered as a conventional motor laid flat, with multiple motors placed one after the other, a circulinear motor is that length of linear motors formed into a ring. An advantage of using this type is that the torque of the motor may be applied at the outer edge of the rotating machinery and therefore lower torque is required (than a centrally located motor) and the magnets used be lighter. These are used in place of the conventional motors, such that the entire shaft may be omitted, with both compressor and fan assemblies in a ring shape with empty space in the middle compared to the

conventional gas turbine.

[0056] In the examples where the rotor of the motor is combined with the compressor or turbine (e.g. Figures 4B, 4C, 4D), it is possible to manufacture the combined rotor and compressor/turbine as an integrated item. For example, it is possible to vary the properties of the compressor/turbine in regions where magnets or windings are required. The compressor/turbine can comprise a magnetically loaded composite (MLC). A composite material can be formed by a resin binding particles, fibres or other elements of the material. A magnetically loaded composite can be formed by adding ferrous material (e.g. iron powder) to parts of the composite where magnetic properties are required.

[0057] Figure 5 shows a longitudinal cross-section through an example of the engine with an enlarged hub of the kind shown in Figures 4C and 4D. Figure 5 corresponds to Figure 4D, with a motor 8 positioned at the radially outermost part of the compressor 1 and a generator 9 positioned at the radially outermost part of the turbine 2. The engine spec3949 has an outer housing 1 1 and an inner housing/cowling 10. The inner housing 10 can comprise the shaft 32 and hub 33 shown in Figure 4D. The inner housing 10 defines an inner volume 3 of free space, which can serve as a through-flow path for gas.

Another example of the engine (not shown) corresponds to Figure 5, but with the motor 8 positioned at the radially innermost part of the compressor 1 and the generator 9 positioned at the radially innermost part of the turbine 2, as shown in Figure 4C. It is also possible to use different radial positions for the motor and generator. Another example of the engine (not shown) corresponds to Figure 5, but with the motor 8 positioned at the radially innermost part of the compressor 1 and the generator 9 positioned at the radially outermost part of the turbine 2. Another example of the engine (not shown) corresponds to Figure 5, but with the motor 8 positioned at the radially outermost part of the compressor 1 and the generator 9 positioned at the radially innermost part of the turbine 2.

[0058] The use of the cylindrical engine layout of the above allows for extra equipment to be situated within the core of the engine. It can also have an advantage of reduced overall weight, and a reduced aerodynamic obstruction to airflow.

[0059] Figure 6 shows another example of a gas turbine. This shows a type of turbofan engine. A gas turbine is combined with a fan 14. The gas turbine forms a core unit 12 of the overall engine. The gas turbine used in the core unit 12 can be any of the gas turbines previously described. The fan 14 is rotatable about a longitudinal axis 6 of the engine. A housing/cowling 13 surrounds the fan 14. A turbofan engine uses a fan 14 to provide additional thrust. Fan 14 draws air into the housing and expels the air. The air passing through fan 14 does not pass through the combustion stage 40 of the core gas turbine unit 12. The fan 14 is driven by a further electrical motor 15.

[0060] In the example shown in Figure 6 the fan motor 15 is positioned at the radially outermost part of the fan 14. Figure 7 shows an alternative example in which the fan motor 15 is positioned at the radially innermost part of the fan 14. The motor 8 is positioned at the radially innermost part of the compressor 1.

[0061] Figures 6 and 7 show a fan 14 surrounding a compressor 1 . This provides a compact structure. As an alternative, it is possible to position the fan 14 upstream of the compressor 1. That is, the fan is positioned before the compressor 1 in the direction 7 of gas flow. The fan 14 can have a larger diameter than the compressor so that at least part of the exhaust path of the fan 14 does not pass through the compressor 1 , similar to a conventional turbofan engine.

spec3949 [0062] In a conventional turbofan engine the fan is driven by a low pressure drive shaft which is connected to the turbine. In contrast, the fan 14 in the turbofan engine shown in Figure 6 is not mechanically linked to the turbine. Fan motor 15 receives an electrical supply from the generator 9. Optionally, the fan motor 15 is connected to the generator 9 via an electrical energy conversion/storage unit 55 as shown in Figure 2. Controller 50 shown in Figure 2 can control the electrical supply to the fan motor 15.

[0063] In the case of failure, enough design headroom may be left in the core design that in an aircraft one or more cores could run all the fans on the aircraft, providing potentially the full performance of the aircraft. Such headroom would not be possible in a conventional design as it would have a severe negative impact on engine efficiency. In these cases in the present day, the aircraft can only fly at a very low speed, extending the time passengers are at risk. In the case of the proposed design, the engine will exhibit high efficiency despite being capable of much more power than is drawn in everyday operation. Figure 8 schematically shows an aircraft with three gas turbine engines 61 , 62, 63. Each engine 61 , 62 comprises a compressor 1 , turbine 2 and a fan 14 driven by a fan motor 15. The generator 9 of one gas turbine 61 can provide an electrical output to drive the fan motors 15 of one or more other engines 62, 63. The generator 9 of gas turbine 61 may provide an electrical output to drive the fan motor 15 of engine 61 . The generator 9 of one gas turbine 61 may be connected directly, or via an electrical energy conversion unit 55. An electrical energy store 56 may provide some of the energy to drive the fan motors 15.

[0064] The concentric layout of components provides space, within the inner volume of the gas turbine, to accommodate other components or propulsion stages/systems. Figure 9 shows another example of a gas turbine. The engine of Figure 9 is similar to Figure 7, with a core gas turbine unit 12 and a fan 14. Additionally, the engine of Figure 9 has a ramjet-type combustion stage 16 concentrically within the core gas turbine unit 12. A ramjet is a type of engine in which air drawn into a combustion chamber solely by forward movement of the aircraft. The gas turbine 12, and fan 14, provide forward movement of the aircraft. The ramjet may be used, for example, to achieve supersonic transport. Figure 10 shows a combination of the engine of Figure 9 plus part of the host aircraft body 18. A movable door (cowling) 20 in the aircraft body 18 selectively allows air to reach the fan 14 and/or main gas turbine 12. A movable door (cowling) 19 in the aircraft body 18 selectively allows air to reach the ramjet stage 16. The doors 19, 20 can be operated depending on the speed of the aircraft. For example, at lower speeds the door 19 could be closed, and the gas turbine 12 and fan spec3949 14 used. At higher speeds, the door 19 can be opened to allow air to reach the ramjet stage 16. Also, at higher speeds, the door 20 can be closed, or partially closed. This could allow supersonic transport using aircraft designs not too dissimilar to present day aircraft.

[0065] The concentric layout of the engine shown in Figures 9 and 10 could be provided in an engine with a conventional shaft connection between the turbine and compressor. However, the provision of a turbine and generator, and of a compressor and motor, allows a larger inner region to accommodate the additional combustion stage, or additional combustion stages.

[0066] Figure 10 also shows a rocket stage 23 located within the inner volume of the gas turbine 12. The rocket stage 23 and exhaust 24 are for use in very high altitude or orbital applications. The core gas turbine 12 and fan 14 can provide power for take-off and acceleration to high subsonic speed. The gas turbine 12 and fan 14 provide thrust to reach supersonic speeds, with the ramjet 16 taking over at a suitable speed. As the air becomes too thin at altitude, the rocket 23 provides the final push to orbit. Such an engine is less complex overall than a current state-of-the-art gas turbine for airliner installation, and easier to build. Because the fan of a commercial airliner is used, noise levels would be low and allow take-off from any airport, as the aircraft can fly to where it needs to be to place its payload or rendezvous with another spacecraft.

[0067] A further example modifies the core gas turbine described above by moving the gas turbine to another location. For example, in an aircraft application the gas turbine cores 12 could be located within the body of the aircraft, or elsewhere on the aircraft, and the fans could be located on the wings of the aircraft. The number of fans may be different to the number of gas turbine cores. For example, there may be a number C of powerful core engines powering a larger number F of fans, where C<F.

[0068] The compressor stage may comprise a plurality of compressors. The plurality of compressors may be arranged sequentially in the direction of gas flow. Each compressor may be driven by a dedicated motor for that compressor. This can allow the possibility of driving different compressors at different speeds. Alternatively, a single motor may drive more than one compressor.

[0069] Similarly, the turbine stage may comprise a plurality of turbines. The plurality of turbines may be arranged sequentially in the direction of gas flow. Each turbine may drive a dedicated generator for that turbine. Alternatively, a single generator may be driven by more than one turbine. spec3949 [0070] Figure 1 1 schematically shows an application with a further electrical motor 73 in an output stage. In a marine application, such as a ship or boat, the motor 73 in the output stage may drive a propulsion system of the ship or boat. The propulsion system may comprise a motor-driven propeller, a motor-driven pump jet or any other motor- driven (or electrically-driven) propulsion system. Figure 1 1 shows an example with two gas turbine engines 71 , 72. Each engine 71 , 72 comprises a compressor 1 and a turbine 2 as previously described. Motor 73 is driven by electrical energy generated by one or more of the electrical generators 9. The generator, or generators, 9 may be connected directly, or via an electrical energy conversion unit 55. An electrical energy store 56 may provide some of the energy to drive the motor 73.

[0071] An advantage of at least one example of this engine is that the feedback loop provided by the shaft in the conventional gas turbine creates a linear relationship between the demand of power of the compressor from the turbine, whereas the dynamics of the airflow within the engine are very much non-linear and not trivial to model. The electrical connection allows that non-linear relationship to be mapped on- the-fly by the engine, always ensuring optimal efficiency. The programmable nature of electronics in the present day would allow essentially arbitrary selection of engine performance characteristics according to manufacturers' wishes.

[0072] At least one example of this engine may provide an advantage of reduced design and engineering complexity of a particular example. In particular it may be possible to simply scale in size one particular design to cater for many or several power class needs.

[0073] At least one example of this engine may provide an advantage of reduced development (and manufacturing) costs as complexity in the design moves to electronics as opposed to mechanical machinery.

[0074] At least one example of this engine may provide an advantage of increased reliability of the engine due to fewer mechanical components and fewer moving parts.

[0075] At least one example of this engine may provide an advantage of increased safety and reliability of an overall application (e.g. commercial airliner) due to redundancy possibilities.

[0076] At least one example of this engine may provide an advantage of higher efficiency at the same power class as present designs due to the simpler gas path through the engine.

spec3949 [0077] At least one example of this engine may provide an advantage of higher operating efficiency close to the theoretical maximum at any operating engine speed as the compressor will be rotated at precisely the required speed to generate the required compression ratio, independently of both air and turbine speed.

[0078] The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0079] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in

combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

[0080] Further aspects of the present disclosure are set out in the following numbered clauses. However, the skilled practitioner will recognise the scope of the present application is defined by the appended claims.

Clauses:

1 . A gas turbine comprising:

a compression stage comprising a compressor which is configured to rotate about an axis;

a combustion stage; and

a turbine stage comprising a turbine which is configured to rotate about an axis, wherein the turbine stage comprises an electrical generator which is configured to generate electrical energy in response to rotation of the turbine,

and the compression stage comprises an electrical motor which is configured to use electrical energy generated by the electrical generator to rotate the compressor.

2. A gas turbine according to clause 1 further comprising a controller, wherein the controller is configured to control the electrical motor to rotate the compressor at a rotational speed which is different to a rotational speed of the turbine.

3. A gas turbine according to clause 1 or 2 further comprising an electrical energy store. spec3949 4. A gas turbine according to clause 3 wherein the electrical energy store is connected to the electrical generator and is configured to store electrical energy generated by the electrical generator.

5. A gas turbine according to clause 4 wherein the electrical energy store is configured to store electrical energy generated by the electrical generator when the electrical energy generated by the generator is greater than an electrical energy demand of the motor.

6. A gas turbine according to any one of clauses 3 to 5, wherein the electrical energy store is connected to the electrical motor and is configured to supply electrical energy to the electrical motor. 7. A gas turbine according to clause 6, wherein the electrical energy store is configured to supply electrical energy to the electrical motor when an electrical energy demand of the motor is greater than the electrical energy generated by the generator.

8. A gas turbine according to any one of clauses 1 to 7, the electrical motor comprising a rotor and a stator, the rotor forming part of a rotatable element of the compressor and the stator comprising a ring around the rotatable element.

9. A gas turbine according to any one of clauses 1 to 7 wherein the compressor comprises a hub having a longitudinal axis and a hollow shaft which is rotatable about the hub, the electrical motor comprising a stator and a rotor, the stator forming part of the hub and the rotor forming part of the hollow shaft.

10. A gas turbine according to any one of the preceding clauses, the electrical generator comprising a rotor and a stator, the rotor forming part of a rotatable element of the turbine and the stator comprising a ring around the rotatable element.

1 1 . A gas turbine according to any one of clauses 1 to 9, wherein the turbine comprises a hub having a longitudinal axis and a hollow shaft which is rotatable about the hub, the electrical generator comprising a stator and a rotor, the stator forming part of the hub and the rotor forming part of the hollow shaft.

spec3949 12. A gas turbine according to any one of the preceding clauses further comprising a fan and an electrical fan motor which is configured to use electrical energy generated by the electrical generator to rotate the fan. 13. A gas turbine according to clause 12 wherein the fan is concentric with the compressor, the fan surrounding the compressor.

14. A gas turbine according to any one of the preceding clauses wherein the compressor comprises a hollow shaft which is rotatable about an open-ended hub.

15. A gas turbine according to any one of the preceding clauses wherein the turbine comprises a hollow shaft which is rotatable about an open-ended hub.

16. A gas turbine according to any one of the preceding clauses comprising an axial through-flow path for gas flow through an interior of at least one of the compressor and the turbine.

17. A gas turbine according to any one of the preceding clauses wherein an interior of at least one of the compressor and the turbine comprises an additional combustion stage.

18. A gas turbine according to clause 17 wherein the interior of the compressor and/or turbine comprises at least one of:

a ramjet combustion stage;

a rocket combustion stage.

spec3949