Exoskeletal engine
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The exoskeletal engine (ESE) is a concept in
turbomachinery Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from ...
design. Current
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
engines have central rotating shafts and fan-discs and are constructed mostly from heavy metals. They require lubricated bearings and need extensive cooling for hot components. They are also subject to severe imbalance (or vibrations) that could wipe out the whole
rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator * Helicopter rotor, the rotary wing(s) of a rotorcraft ...
stage, are prone to high- and low-cycle fatigue, and subject to catastrophic failure due to disc bursts from high tensile loads, consequently requiring heavy containment devices.Chamis, Christos C. and Isaiah M. Blankso
"Exo-Skeletal Engine – Novel Engine Concept". NASA, 2006.
Retrieved: 5 May 2019
To address these limitations, the ESE concept turns the conventional configuration inside-out and utilizes a drum-type rotor design for the turbomachinery in which the rotor blades are attached to the inside of a rotating drum instead of radially outwards from a shaft and discs. Multiple drum rotors could be used in a multi-spool design.


Design

Fundamentally, the ESE drum-rotor configuration typically consists of four
concentric In geometry, two or more objects are said to be concentric, coaxal, or coaxial when they share the same center or axis. Circles, regular polygons and regular polyhedra, and spheres may be concentric to one another (sharing the same center point ...
open-ended drums or shells: *an outer shell (engine casing) that both supports the bearings for the drum-rotor shell and constrains it, *the drum-rotor shell that rotates within the bearings and carries the compressor- and turbine blades, *a static
stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric mot ...
shell that supports the guide vanes, *a hollow static inner shell that provides a flow path through the centre of the engine. In the ESE design, the rotating blades are primarily in radial compression as opposed to radial tension, which means that materials that do not possess high-tensile strength, such as
ceramic materials A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
, can be used for their construction. Ceramics behave well in compressive loading situations where
brittle fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
is minimized, and would provide greater operating efficiency through higher
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
s and lighter engine weight when compared to the metal
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s that typically are used in turbomachinery components. The ESE design and the use of composite materials could also reduce the part count, reduce or eliminate cooling, and result in increased component life.Roche, Joseph M., Donald T. Palac, James E. Hunter, David E. Myers, and Christopher A. Snyder
"Investigation of Exoskeletal Engine Propulsion System Concept". NASA, 2005.
Retrieved: 31 August 2009
The use of ceramics would also be a beneficial feature for
hypersonic In aerodynamics, a hypersonic speed is one that exceeds 5 times the speed of sound, often stated as starting at speeds of Mach 5 and above. The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since in ...
propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...
systems, where high
stagnation temperature In thermodynamics and fluid mechanics, stagnation temperature is the temperature at a stagnation point in a fluid flow. At a stagnation point the speed of the fluid is zero and all of the kinetic energy has been converted to internal energy and is a ...
s can exceed the limits of traditional turbomachinery materials. The cavity within the inner shell could be exploited in several different ways. In subsonic applications, venting the centre cavity with a free-stream flow could potentially contribute to a large noise reduction; while in
supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
-hypersonic applications it might be used to house a
ramjet A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to produce thrust. Since it produces no thrust when stationary (no ram air) ramjet-powered vehicles require an ass ...
or
scramjet A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully ...
(or other devices such as a pulse detonation engine) as part of a turbine-based
combined-cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turb ...
engine. Such an arrangement could reduce the overall length of the propulsion system and thereby reduce weight and drag significantly.


Summarized potential advantages

''From'' Chamis and Blankson: *Eliminate disk and bore stresses *Utilize low-stress bearings *Increase rotor speed *Reduce airfoil thickness *Increase flutter boundaries *Minimize/eliminate containment requirements *Increase high mass flow rate *Reduce weight by 50 percent *Decrease turbine temperature for same thrust *Decrease emissions *Provide higher
thrust-to-weight ratio Thrust-to-weight ratio is a dimensionless ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine that is an indicator of the performance of the engine or vehicle. The instantaneous thrust-to-w ...
*Improve specific fuel consumption *Increase blade low-cycle and high-cycle fatigue lives *Reduce engine diameter *Reduce parts count *Decrease maintenance cost *Minimize/eliminate sealing and cooling requirements *Minimize/eliminate blade-flow losses, blade and case wear *Free core for combined turboram jet cycles *Reduce noise *Expedite aircraft/engine integration *Minimize/eliminate notch-sensitive material issues


Challenges

One of the major challenges is in bearing design as there are no known lubricated systems that can handle the magnitude of velocity encountered in the ESE; foil- and
magnetic bearing A magnetic bearing is a type of bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with ve ...
s have been suggested as possible solutions to this problem. *
Foil bearing A foil bearing, also known as a foil-air bearing, is a type of air bearing. A shaft is supported by a compliant, spring-loaded foil journal lining. Once the shaft is spinning fast enough, the working fluid (usually air) pushes the foil away from ...
s are noncontacting and ride on a thin film of air, which is generated hydrodynamically by the rotational speed, to suspend and centre the shaft. Drawbacks for the foil system include the high start-up torque, the need for set-down/lift-off mechanical bearings and associated positioning hardware, and the high temperatures generated by this system. *For the large-diameter magnetic bearing system required in the ESE, stiffness and radial growth after spin-up are problems that would be encountered. Radial growth of sufficient magnitude would result in stability problems, and a magnet pole positioning system would be required to maintain the appropriate clearances for the operation of the system. This positioning system would require high-speed sensing and positioning. A passive magnetic laminate and its mounting hardware would require high structural integrity to resist the extremely high inertial forces and would most likely drive an increase in weight. Although both bearing systems theoretically meet the requirements of the exoskeletal application, neither technology is currently ready for operation at practical sizes. Developments in foil bearing technology indicate it may take 20 years to achieve foil bearings for this diameter, and magnetic bearings appear to be too heavy for this application and would also face a lengthy technology development programme.


References

{{reflist Engines Aircraft engines Jet engines Hypothetical technology