GB2593969A

GB2593969A - Contra-rotating hydraulic drive system for propulsion

Info

Publication number: GB2593969A
Application number: GB2102036.7A
Authority: GB
Inventors: Dominic Garvey Seamus; Xu Weiqing
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2020-02-13
Filing date: 2021-02-12
Publication date: 2021-10-13
Anticipated expiration: 2041-02-12
Also published as: GB2593969B; GB202102036D0; GB202001988D0

Abstract

The propulsion system has a hydraulic drive 3 connecting a first propulsor 1 co-axial with a second propulsor 2, in operation the drive exerts a torque on the first propulsor and an equal and opposite torque on the second propulsor. Also claimed is a propulsor having contra-rotating first and second propulsors 1,2 with a hydraulic drive 3 which develops a pure action-reaction torque pair between the propulsors and zero net torque-to-ground. A first unit 4 transfers oil from a stationary frame into a rotating frame of the first propulsor at a higher pressure such that most of the thrust developed is reacted internally using fluid forces within the first unit. A second unit 5 transfers oil back into the stationary frame from a rotating frame of the second propulsor at a lower pressure such that most of the thrust developed is reacted internally using fluid forces within the second unit. A set of bearings 21,22 ensures the axes of the two propulsors remain aligned and capable of carrying any residual thrust loads left after the oil-transfer units 4,5 have subtracted most of these thrusts.

Description

CONTRA-ROTATING HYDRAULIC DRIVE SYSTEM FOR PROPULSION

Field of the invention

This invention relates to a drive system for propulsion, and more particularly to the field of electrical propulsion of ships and boats and electrical propulsion in aircraft

Background

Transportation is rapidly becoming electrified. We are moving away from the use of vehicles where internal combustion engines drive the propulsors into a new era in which electrical machines will drive the propulsors. For road transportation, the transformation is very substantially done: most major manufacturers of road vehicles have plans to end the production of machines using internal combustion engines and migrate towards electrically-powered vehicles. The same is true for rail. For transportation on water, the move towards electrified propulsion is at an earlier stage but it is certainly already happening. The same is true for aerospace. In both cases, many different companies with interests in the sector are developing new solutions for how to propel vehicles using electrical machines.

It is not important whether the electricity driving these propulsion machines comes from batteries, from fuel-cells, from on-board generators combusting a fuel to generate the power or from a combination of these or other technologies. In general, there is a requirement for effective conversion of electrical power into propulsive thrust for aircraft and vessels that are propelled by pushing water behind them.

Transportation on water and in the air is very different from ground-based transportation for the key reason that propulsion in air and in/on water must be achieved by driving a fluid backwards relative to the direction of travel of the vehicle. In ground-based transportation (on roads and rail), the propulsive force is most effectively achieved by turning wheels which develop a reaction force with the ground that is perpendicular to the axis of rotation of the wheels. To clarify this, a ground-based vehicle travelling North has one or more wheels turning about an axis that is aligned to the East-West direction. For air transportation and transportation in/on water, a vehicle travelling North is most effectively propelled by one or more propulsors spinning about an axis of rotation aligned to the North-South direction.

An efficient electrical propulsion system suitable for aircraft and/or water vehicles is desirable.

Summary

According to a first aspect, there is provided a propulsion system according to claim I or claim 15. Some optional features of the propulsion system are set out in the dependent claims.

According to a second aspect, there is provided a propulsion system for accelerating a fluid relative to the body of a vehicle comprises a first propeller free to rotate relative to the body of the vehicle, a second propeller co-axial with the first propeller and also free to rotate relative to the body of the vehicle and a hydraulic differential drive unit connecting the first propeller and the second propeller and exerting a torque on a first propeller always equal to the negative of the torque being exerted on the second propeller.

An external circuit of oil located largely in the body of the vehicle may include a first rotating union for transmitting oil from the vehicle body into the differential drive unit and a second rotating union for returning the oil from the differential drive unit back into the both of the vehicle.

The direction of oil flow may be reversible to provide a reverse thrust.

The first rotating union may cause a thrust force proportional to the gauge pressure of the oil passing through it and the second rotating union may cause a thrust force proportional to the gauge pressure of the oil passing through it, with the effect that the net thrust developed by each propeller is transmitted into the body of the vehicle at least partially by at least one of the rotating unions.

The external hydraulic system may contain provisions for controlling both the average value of the hydraulic pressures at the two rotating unions and the difference between those hydraulic pressures.

A first pump may be operable to control the average hydraulic pressure and a second pump may be operable to control the differential hydraulic pressure.

An electrically driven hydraulic pump may provide the main propulsive power by circulating the hydraulic fluid through the hydraulic differential drive unit.

The spin speed of the electrical machine may exceed the spin speed of the first propeller relative to the second propeller by a factor of 2 or more.

Features described with reference to the second embodiment may be combined with features of the second embodiment.

Description of the Figures

Example embodiments will be described, with reference to the accompanying drawings. The examples are not intended to limit the scope of the invention, which should be determined with reference to the appended claims.

Figure I provides an overview of the main part of the system and illustrates how the difference in oil pressures between the flow and return lines causes a direct torque to exist between the two distinct propulsor rotors. This figure also indicates how the average pressure (between oil flow and oil return lines) can be used to change the thrust forces being transferred into the stationary frame from the two rotors.

Figure 2 illustrates the possible construction of a hydraulic differential drive unit (DDU) such as appears in Figure 1. There are multiple possible designs of hydraulic DDU and the design shown in Figure 2 represents only one possible candidate design.

Figure 3 excludes the main part of the system (described in Figure 1) and provides a basic configuration option for how oil pressure may be controlled in the flow and return lines.

Figure 4 provides an alternative method by which the average oil pressure in the system may be controlled independently of the differential pressure between the oil flow and return lines.

Three important issues affect the use of electric motors for driving propellers. The first of these has to do with matching rotational speeds. Electrical machines can have good specific power levels (i.e. reasonably good power per unit of mass) but only if they spin very quickly. Propulsors that drive fluid along an axis have well-established upper and lower limits to the rotational speeds at which they operate. When an electric motor drives a propulsor, a decision may sometimes be made to drive the propulsor directly -in which case the electrical machine will normally be much larger than it might otherwise have been and the propulsor will be smaller. Alternatively, the designer may decide to deploy mechanical gearing of some sort in order to match the optimal operating speed for the electrical machine to the natural optimum value of the propulsor. This "speed matching" can be done by using a mechanical gearbox but it may also be done by introducing a hydraulic pump (connected directly to the electrical machine) and a hydraulic motor (connected directly to the propulsor). This may be referred to as "hydraulic gearing".

The second important issue affecting electrified propulsors has to do with maximising the propulsive efficiency by removing swirl from the wake. In part, maximising efficiency relates to the point above about matching speeds since it is generally better for propulsive performance to turn big propellers at low speed than to turn smaller propellers. However, there is a second dimension to this -avoidance of (significant) swirl in the wake. A single propeller or fan turning in a fluid stream to produce forward thrust receives incoming fluid that has little or no swirl but tends to impart net angular momentum to the fluid in the same direction of turn as the propeller or fan itself. There is kinetic energy associated with that swirl but that kinetic energy is wasted. Obviously some kinetic energy must be imparted to the fluid in order to produce thrust. This unavoidable power loss is connected with the additional axial velocity given to the fluid stream. Leaving the fluid behind the propulsor with zero angular velocity about the rotation axis of the propulsor minimises the consumption of power. One way to reduce the net angular momentum of the fluid stream is to use stationary guide vanes behind a single rotating propeller/fan in order to convert kinetic energy associated with swirl back into kinetic energy associated with axial flow of the fluid. Although this is mechanically simple, it is difficult or impossible to optimise this for all conditions of net speed of the propulsor itself within the fluid and net thrust settings. An alternative method to ensure that no net angular momentum is imparted to the downstream fluid is to employ two co-axial contra-rotating propellers/fans in series with zero net torque acting on the fluid. This option of using the co-axial contra-rotating pair of propellers/fans imparting zero net torque to the fluid stream can be achieved either by driving each propeller/fan relative to the stationary frame with exactly opposing torques or alternatively by providing torque only to the relative motion of the two propeller/fans. Then no control system is required to ensure that the torques from the two propellers/fans are exactly cancelled because this happens automatically and passively.

The third issue of importance relates to how thrust force is transferred from the propulsor(s) into the body of the vehicle. Thrust force can be transferred completely via ball-bearings (either "angular-contact" ball-bearings or "deep-groove" ball bearings). However, thrust loads are very significant compared with the weight of the rotor -typically many times greater than the weight load. Transmitting thrust completely through ball-bearings results in a double effect that the bearings become significantly larger than they might otherwise have been and that these bearings dissipate significantly more power as a result of the axial load than they must carry. In modern designs of gas turbine engines for aircraft propulsion, the "fan" generates most of the engine thrust but a large fraction of this thrust is transmitted into the non-rotating part of the gas-turbine engine through pressure-differences across compressor guide vanes. This enables the bearings to be rated for much lower steady thrust loads. In embodiments, two separate rotors are provided and each of these rotors provides some thrust force. In embodiments, a means may be provided by which a large part of the thrust generated by each rotor can be transferred into the stationary frame without passing through a ball-bearing. In effect, a "parallel path" for the thrust force may be provided for each rotor.

in summary, embodiments of the present invention may provide at least one of: (a) A high (and potentially adjustable) ratio between the rotational speed of an electrical motor producing the primary propulsive power and the rotational speeds of propulsors driven by that electric motor (for example, the ratio may be at least I, at least 2 or at least 5, with the electric motor rotating faster than the propulsor(s)).

(b) An net-zero torque exerted on the stream-tube of fluid passing through the propulsor arrangement by driving one rotor relative to the other and by deliberately not providing any path for the electrical motor to react torque except into the propulsors.

(c) An integral (and optionally controllable) means of managing thrust loads (e.g. from each propulsor). The means of managing thrust loads may be configured such that a relatively small fraction of the net thrust force produced by each rotor needs to be passed through a ball-bearing Figure 1 shows a propulsion system for a vehicle, comprising: first propulsor 1, second propulsor 2, hydraulic drive unit 3 (which may also be referred to as a differential drive unit, DDU). A first drive shaft couples the first propulsor 1 to a first rotor of the hydraulic drive unit 3 A second drive shaft couples the second propulsor 2 to a second rotor of the hydraulic drive unit 3.The first propulsor 1 creates thrust when it spins in one direction about a rotation axis and the second propulsor 2 creates thrust when it spins in the opposite direction about the rotation axis. The first drive shaft (of the first propulsor 1) passes inside the second drive shaft (of the second propulsor (2) and the two propulsors share a common (or almost-common) axis. The first and second two propulsors(1, 2, are caused to rotate by a hydraulic drive unit 3 which is configured to create a torque that acts between the two propulsors. importantly, there is zero net torque transmitted to the vehicle itself by the hydraulic drive unit 3. Because there is no net torque transmitted to the vehicle, it follows that there cannot be any net torque transmitted to the fluid being moved by the propulsors and this in turn must mean that zero net swirl is left in the fluid as it moves downstream from the first and second propulsors I. 2, The 3 comprises two parts (or rotors): a first rotor connected directly to the first propulsor (via the first drive shaft) and a second rotor connected directly to second propulsor (via the second drive shaft).

Each propulsor 1, 2 has one bearing supporting it relative to the vehicle that it propels. The first drive shaft, which supports the first propulsor 1, is supported relative to the vehicle structure at the end furthest from the first propulsor 1 by such bearing 31 which would normally be either an angular-contact ball-bearing or a deep-groove ball-bearing so that it is capable of carrying some thrust, in steady operation, that bearing 31 would carry only a small fraction of the total thrust force produced by the first propulsor 1. The second drive shaft, which supports the second propulsor 2, is supported relative to the vehicle structure at a point relatively close to the second propulsor by a second bearing 32. The second bearing 32 would also normally be either an angular-contact ball-bearing or a deep-groove ball-bearing so that it is capable of carrying some thrust.

Two inter-shaft bearings, 21, 22, are present between the first and second drive shafts so that they must remain co-axial. Normally, both of these inter-shaft bearings, 21, 22, would be roller-bearings -designed to accommodate small relative axial motions between the first and second drive shafts without reacting any thrust force. This feature is important because it is very desirable that there is full determinism about the levels of thrust force carried by the bearings-to-vehicle-body, 31, 32.

The propulsion system further comprises a first oil-transfer unit 4, on the "flow" side of a hydraulic circuit that circulates oil through the hydraulic drive unit 3. The first oil-transfer unit 4 is configured to communicate oil from the (non-rotating) frame of reference of the vehicle body into the rotating frame of the first propulsor 1. The first oil-transfer unit 4 is deliberately not symmetric about a plane normal to the rotation axis. The lack of symmetry has the effect that oil pressure in the -flow" line (at the input to the hydraulic drive unit 3) causes a net axial force tending to transfer forward thrust from the second propulsor 2 into the vehicle body (at least partially reacting the forward thrust from the second propulsor). The first oil-transfer unit 4 must be designed and fitted so that no direct axial contact can occur between its rotating part (fixed to the rotating frame, e.g. the second rotor of the hydraulic drive unit 3) and the non-rotating part that is mounted to the vehicle body. If such axial contact were to occur, the contact would provide a further parallel path by which axial force could be transferred and it is highly desirable that all axial forces arising between the rotating and non-rotating elements of this unit arise as a direct result of oil pressure only.

Small levels of oil leakage from the first oil-transfer unit 4 are entirely acceptable provided that the leakage flow rates are small relative to the main throughput flow rates of the oil-transfer unit. The pressure-drops associated with passing oil through the oil-first transfer unit 4 should also be small relative to the normal operating pressures on the "flow" side. There is a direct relationship between the thrust force Fri_ transferred at the first oil-transfer unit 4 and the pressure of the oil on the "flow" side of the circuit, ppm,: taking this form: Fri = Cl X PFlow [1] Where el is a constant of proportionality related to the balance of surface area at the oil transfer unit 4 facing the appropriate direction.

A second oil-transfer unit5, is present on the "return" side of the hydraulic circuit.

The second oil transfer 5 unit enables oil to pass back into the non-rotating frame of reference of the vehicle body from the rotating frame of the first propulsor 1. This second oil-transfer unit 5 is also deliberately not symmetric about a plane normal to the rotation axis. Again, the lack of symmetry has the effect that oil pressure in the "return" line causes a net axial force tending to transfer positive thrust from the first propulsor 1 into the vehicle body. This second oil-transfer unit 5 must also be designed and fitted so that no direct axial contact can occur between its rotating part (fixed to the rotating frame, e.g. the first rotor of the hydraulic drive unit) and the part that is mounted to the vehicle body.

Small levels of oil leakage from the second oil-transfer unit 5 are also entirely acceptable provided that the leakage flow rates are small relative to the main throughput flow rates of the oil-transfer unit. The pressure-drops associated with passing oil through the second oil-transfer unit 5 should also be small relative to the normal operating pressures on the "return" side of the hydraulic circuit. There is a direct relationship between the thrust force FT2 transferred at the second oil-transfer unit 5 and the pressure of the oil on the "return" side of the circuit, pRe/nn taking this form: FT2 = c2 X PReturn 121 Where c2 is a constant of proportionality related to the balance of surface area at the oil transfer unit 4 facing the appropriate direction.

Note that in normal operation, pRen",, will be significantly lower than PE/ow. However, the thrust contributed by each of the first and second propulsor 1, 2 will normally be similar. For this reason, the net axial area exposed to pft,n. in the second oil-transfer unit 5 will normally be larger than the corresponding net axial area exposed to ppm,: in the first oil-transfer unit 4.

Oil that has been fed from the stationary frame via the first oil-transfer unit 4 enters the hydraulic drive unit 3 in one or more high-pressure chambers. Setting aside very small amounts of leakage flow, oil can flow from the one or more high-pressure chambers into one or more low-pressure chambers only by causing hydraulic drive unit 3 to turn (i.e. causing relative rotation between the first rotor and second rotor and hence between the first and second propulsors 1,2). For each high-pressure chamber in the hydraulic drive unit, there is one corresponding low-pressure chamber. There is a direct relationship between the volume flowrate of oil Q through the hydraulic drive unit 3,and the difference in the rotational speed of the first propulsor ul and the second propulsor 2 according to: Q = kflay:= k(fli -112) Evidently, the constant of proportionality here, k, has units of volume and represents the volume of oil that passes through the hydraulic drive unit 3 when there has been a relative turn of I radian between the first propulsor (I) and the second propulsor (2).

The torque acting on the first propulsor 1 is the exact negative of the torque acting on the second propulsor 2 and this torque, I, is directly proportional to the difference between the oil pressures on the flow and return sides of the hydraulic drive unit 3 expressed by: F = k(Pf tow Preturn) The hydraulic power being converted in the hydraulic drive unit 3 can be equated to the mechanical power produced by it: Phydrautic: = Qx(ppow -Preturn) = (n1 -0.2) xP =--mechanical Oil returning from the hydraulic drive unit 3 returns from the one or more low-pressure chambers into the stationary frame via the second oil-transfer unit 5.

Hydraulic pressures in the -flow" and "return" lines control both the net propulsive effort and the thrust carried by each rotor (in parallel to any thrust that may pass through contact bearings, such as tapered roller bearings or ball bearings). I0

Figure 2 illustrates the possible construction of a hydraulic drive unit 3 (DDU) such as appears in Figure 1. There are multiple possible designs of hydraulic drive unit 3 and the design shown in Figure 2 represents only one possible candidate design.

importantly, the design of the DDU would normally take into account that the DDU itself should generate zero net thrust force between the part fixed to the rotating frame of the first propulsor 1 (i.e.. the first rotor) and the part fixed to the contra-rotating frame of the second propulsor 2 (i.e. the second rotor). One way that this can be done is to ensure that oil pressure distribution within the DDU is "prismatic" with the prismatic axis being coincident with the axis of rotation and then that seals between the first part and the second part of the DDU are radial seals on a single cylindrical surface.

Figure 3 shows how the pressure in the flow and return lines of the oil circuit can be controlled. In this arrangement, there is a first electrical machine 31 coupled to a first hydraulic pump 41 which draws oil from an unpressurised oil reservoir 40 and drives it into the flow line determining the pressure in that line, Pflow* A second electrical machine 32 is coupled to a second hydraulic machine 42 that can theoretically act as either a hydraulic motor or a hydraulic pump but in normal circumstances, that second hydraulic machine 42 would always operate as a hydraulic motor sucking energy out of the oil system to drive the second electrical machine 32 as a generator. With this arrangement the first electrical machine 31 puts in power into the hydraulic circuit continuously whilst the second electrical machine 42 draws power out from the hydraulic circuit continuously -controlling the pressure in the return line' b rn r eatcrtk 0 r into )' providing a restriction against the free movement of oil from the return line the oil reservoir 40.

A second embodiment uses the same mechanical arrangement as the first and differs only in the way that the pressures in the hydraulic flow and return lines are controlled.

This second embodiment is shown in Figure 4. The second embodiment resolves one important shortcoming of the first embodiment -the inefficiency of having some power converted into mechanical power, then into hydraulic power, then back into mechanical power and finally back to electrical power.

In the second embodiment, there is once again a first electrical machine 31, and an associated first hydraulic machine 41. Thc first electrical machine 31 essentially provides all of the propulsive power and no more. It channels all of that power through the first hydraulic machine 41. Then there is a second electrical machine 32, with associated second hydraulic machine 42 whose combined function is to set the oil pressure in the hydraulic system at one point in the oil circuit and only one. Only a very tiny amount of power is associated with the second electrical machine 32 and the corresponding second hydraulic machine 42, because the oil flow-rates through the second hydraulic machine 42 are negligible. if all seals were perfect, the flow rate would be precisely zero.

In the descriptions above, it has been indicated that the oil in the higher-pressure flow line is transferred into the rotating frame of the second propulsor (e.g. into the second rotor) and that oil returning into the lower-pressure return line is transferred from the rotating frame of the first propulsor (e.g. the first rotor). Figure 1 further indicates that the first propulsor is forward of the second propulsor. it is trivial to note that the invention also includes embodiments in which the first propulsor is downstream of the second propulsor, and/or in which the flow line is coupled into the rotating frame of the first propulsor and the return line is coupled to rotating frame of the sccond propulsor.

In certain embodiments a single electro-hydraulic system can simultaneously provide three desirable attributes to facilitate efficient electrical propulsion of aircraft or vehicles that run in or on water. These three attributes are: (1) a suitable speed-reduction ratio enabling the electrical drive machine(s) to run at high speed whilst still operating propulsors at speeds close to optimal, (2) a propulsor set that naturally leaves zero swirl velocity in the downstream fluid so as to minimise the expenditure of energy fruitlessly and (3) a thrust management system enabling most of the thrust of the propulsors to be reacted by fluid pressure effects rather than relying on mechanical rolling-dement bearings to transmit all of the thrust force into the vehicle.

Certain embodiments comprise at least some of: : * Two contra-rotating propulsors * A hydraulic differential drive unit that develops a pure action-reaction torque pair between the two propulsors and zero net torque-to-ground * A first oil-transfer unit transferring oil from a stationary frame into the rotating frame of the first propulsor at a higher (flow) pressure and designed such that the most of the thrust developed by the first propulsor is reacted internally using fluid forces in the first oil-transfer unit * A second oil-transfer unit transferring oil back into the stationary frame from the rotating frame of the second propulsor at a lower (return) pressure and designed such that the most of the thrust developed by the second propulsor is reacted internally using fluid forces in the second oil-transfer unit * A set of bearings ensuring that the axes of the two propulsors remain aligned and capable of carrying any residual thrust loads left after the first and second oil-transfer units have subtracted most of these thrusts Other variations arc possible, within the scope of the invention. which should be determined with reference to the appended claims.

IS

Claims

CLAIMS1. A propulsion system for accelerating a fluid relative to a body of a vehicle comprising: a first propulsor, a second propulsor co-axial with the first propulsor and a hydraulic drive unit connecting the first propulsor and the second propulsor and operable to exert a torque on the first propulsor and an equal and opposite torque on the second propulsor.
2. The propulsion system of claim I. wherein the hydraulic drive unit is configured to react torque developed by the hydraulic drive unit into only the first propulsor and second propulsor.
3. The propulsion system of claim 1 or 2, wherein the hydraulic drive unit comprises a first rotor and a second rotor that are configured for relative rotation during operation of the hydraulic drive unit, wherein the first rotor is coupled to the first propulsor to rotate at the same rate as the first propulsor, and the second rotor is coupled to the second propulsor to rotate at the same rate as the second propulsor.
4. The propulsion system of claim 3, further comprising a first drive shaft coupling the first rotor to the first propulsor, and a second drive shaft coupling the second rotor to the second propulsor, wherein the first drive shaft is concentric with the second drive shaft.
5. The propulsion system of claim 4, further comprising an inter-shaft bearing between the first and second drive shaft, wherein the inter-shaft bearing is configured to accommodate axial movement between the first and second drive shaft
6. The propulsion system of 4 or 5, further comprising a thrust bearing supported by a non-rotating journal and configured to react at least a portion of thrust load from the first propulsor and/or second propulsor.
7 The propulsion system of any preceding claim, further comprising a first oil-transfer unit for transmitting oil into the differential drive unit and a second oil-transfer unit for returning the oil from the differential drive unit.
The propulsion system of claim 7, wherein: the first oil-transfer unit comprises: a non-rotating portion for coupling to the vehicle, and a rotating portion coupled to a first portion of the hydraulic drive unit, the second oil-transfer unit comprises: a non-rotating portion for coupling to the vehicle, and a rotating portion coupled to a second portion of the hydraulic drive
9. The propulsion system of claim 8, wherein: the first oil-transfer unit is configured to impart a thrust force on the vehicle proportional to the gauge pressure of the oil passing through it; and the second oil-transfer unit is configured to impart a thrust force on the vehicle proportional to the gauge pressure of the oil passing through it.
10. The propulsion system of any preceding claim, further comprising a hydraulic pressure control system, configured to control: an average value of hydraulic pressure at the first and second oil-transfer units and/or a difference between the hydraulic pressure at the first and second oil-transfer unit.
I I. The propulsion system of claim 10, wherein the hydraulic pressure control system comprises a first pump operable to control the average hydraulic pressure and a second pump operable to control the difference in hydraulic pressure between the first and second oil-transfer unit.
12. The propulsion system of any preceding claim, further comprising an electrically driven hydraulic pump configured to circulate hydraulic fluid through the hydraulic drive unit.
13 The propulsion system of claim 12, wherein the electrically driven hydraulic pump is driven by an electrical machine, and the hydraulic drive unit and hydraulic pump are configured such that a drive ratio, defined as the number of rotations of the electrical machine and/or the hydraulic pump for each rotation of the first propulsor, is at least 2. and unit.IS
14. The propulsion system of any preceding claim, wherein the direction of oil flow can be reversed to reverse a direction of thrust from the first and second propulsors.
15. A propulsor, comprising: a first propulsor and a second contra-rotating propulsor; a hydraulic differential drive unit that develops a pure action-reaction torque pair between the first and second propulsors and zero net torque-to-ground; a first oil-transfer unit transferring oil from a stationary frame into a rotating frame of the first propulsor at a higher (flow) pressure and designed such that most of the thrust developed by the first propulsor is reacted internally using fluid forces in that the first oil-transfer unit; a second oil-transfer unit transferring oil back into the stationary frame from a rotating frame of the second propulsor at a lower (return) pressure and designed such that the most of the thrust developed by the second propulsor is reacted internally using fluid forces in that the second oil-transfer unit; a set of bearings ensuring that the axes of the first and second propulsors remain aligned and capable of carrying any residual thrust loads left after the first and second oil-transfer units have subtracted most of these thrusts.