Understanding and Controlling Hybrid Electric Gas Turbine Engine Transient Dynamics

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NASA/TP-20260001365

Understanding and Controlling Hybrid Electric

Gas Turbine Engine Transient Dynamics

Dennis E. Culley, Santino J. Bianco, Jonathan S. Litt, Jonathan L. Kratz, and Arman Mirhashemi Glenn Research Center, Cleveland, Ohio

February 2026

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NASA/TP-20260001365

Understanding and Controlling Hybrid Electric

Gas Turbine Engine Transient Dynamics

Dennis E. Culley, Santino J. Bianco, Jonathan S. Litt, Jonathan L. Kratz, and Arman Mirhashemi Glenn Research Center, Cleveland, Ohio National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135

February 2026

Acknowledgments This work was funded by the NASA Aeronautics Research Mission Directorate, Transformative Aeronautics Concepts Program, Transformational Tools and Technologies Project.

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NASA/TP-20260001365 iii

Understanding and Controlling Hybrid Electric

Gas Turbine Engine Transient Dynamics

Dennis E. Culley, Santino J. Bianco, Jonathan S. Litt, Jonathan L. Kratz, and Arman Mirhashemi National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135 ASMICS adaptive sliding mode impedance control with

Summary

scaling (software) The electrification of the gas turbine engine is known to DC direct current increase the flexibility of aircraft architectures by enabling the HIL hardware in the loop generation of electrical power to distribute to other electrically HPC high-pressure compressor based, thrust-producing subsystems. It also has potential for HPS high-pressure spool direct performance benefits in the gas turbine engine itself, both HPT high-pressure turbine at steady state and dynamically. Although the design focus of HyPER Hybrid Propulsion Emulation Rig the gas turbine engine performance is primarily at steady state, LPC low-pressure compressor it is often the instabilities occurring during transients that cause LPS low-pressure spool disequilibrium and constrain performance improvements. Insta- LPT low-pressure turbine bilities arise from the disequilibrium of the energy storage NEAT NASA Electric Aircraft Testbed mechanisms within the traditional engine system and likewise, OID operability-influenced design for the electrified engine system. The primary energy storage OTAC object-oriented turbomachinery analysis code mechanisms in the traditional system are the rotational inertia, SAF sustainable aviation fuels gas path volumes, and the thermal masses that make up the SM stall margin mechanical structure; and now, the electrical power system will STARC-ABL Single-aisle Turboelectric AiRCraft with an aft provide additional contributions. Understanding the effect that boundary layer propulsor each of these energy storage mechanisms has on the others and T-MATS Toolbox for the Modeling and Analysis of controlling them appropriately allows for the suppression of Thermodynamic Systems state changes within the turbomachinery components to the TEEM Turbine Electrified Energy Management degree that the components remain near steady state, thus Symbols reducing the disequilibrium within the system. The ability to tightly regulate the state changes of turbomachinery compo- α angle between the absolute flow vector and the axial line n nents, such as the compressor, minimizes the excursion of the of the compressor compressor operating point from the operating line (opera- β angle between the relative flow vector and the axial line n bility), which allows for higher performing, more efficient of the compressor compressor designs by decreasing the amount of stall margin c absolute flow vector n needed for safe engine power level changes. Preliminary studies n location within compressor stage with the electrification of the turbine engine have shown that P total pressure, psia t this is possible, and this concept can lead to design trades bene- s specific entropy, J/(kg-K) fitting engine performance, weight, and volume. This report T total temperature, ° R t explores in detail the energy storage mechanisms and control U mechanical rotor tangential velocity vector approaches for coordinating their state changes and ultimately W corrected mass flow rate, lbm/s c proposes that a higher performing, more efficient compressor w relative flow vector with respect to stage location n design can result.

1.0 Introduction

Nomenclature

Aircraft propulsion technologies are increasingly focused on Acronyms the elimination of all carbon and nitrogen oxide emissions while reducing noise and operating costs (Refs. 1 and 2). In the long AGTF30 Advanced Geared Turbofan 30,000-lbf thrust NASA/TP-20260001365 1 term, this may be accomplished using hydrogen fuel or devel- increase in weight. These weight concerns have spawned a oping an all-electric powertrain, two technologies that have tremendous amount of research to minimize the weight disadvantage of these new powertrain components in achieving intractable technical, logistical, and infrastructure hurdles to overcome. Alternative fuels such as sustainable aviation fuels feasible flight configurations (Refs. 15 and 16).

(SAF) or ammonia are being considered as nearer term oppor- The complexity of electrified propulsion is exemplified by the integration of multiple subsystems that are coupled together tunities, still with their own significant roadblocks. In all cases, there is no simple, clear path forward (Ref. 3). This reality is driv- and affect each other’s state. This introduces additional ing near-term innovation into the hybridization of highly reliable opportunities for unanticipated system interactions that can propagate faults or introduce new system instabilities.

turbomachinery-based systems with emerging technologies typically involving electrification (Refs. 4, 5, and 6). Electri- Spakovszky (Ref. 17) points out that it is instabilities induced fication in this context implies that a substantial portion of the gas by improperly coordinated changes in the engine state that are turbine engine mechanical power is being converted to, or aug- the leading cause of program delays and increased costs mented by, electric power for propulsive purposes (Refs. 7 and jeopardizing gas turbine engine certification. These conditions 8). Most of these concepts involve electric boost or power extrac- can occur over widely varying time scales that include energy tion. Boost employs the use of an electric machine to directly add conversion processes among power streams involving the shaft power to augment the thrust production of the engine during engine gas path, mechanical, thermal, and electrical systems.

Simon (Ref. 18) explores the increased complexity and critical phases like takeoff and climb. This enables the turboma- chinery to be sized for the cruise condition, thus reducing weight potential for dynamic interactions when these electrified and minimizing fuel burn. Power extraction, however, is a more systems are integrated at the vehicle level. Until now, the goal has been to identify and manage these potential instabilities in common area of research. In this configuration, large amounts of power are extracted from the mechanical shaft(s) of the engine, the interest of improving operability, and thus, safety.

converted to electric power, and typically used to drive electric The reduction of instabilities is only partially addressed by simplifying the architectural design of the hybrid propulsion sys- propulsors on the airframe (Refs. 9 and 10).

Electrification has resulted in a dramatic expansion of the tem. Actively coordinating the system state changes via control possible airframe architectures for most classes of air vehicles systems has become a necessity, as the pilot workload to manage (Ref. 11). For commercial transport-class aircraft, these the operation of the multidomain system becomes intractable. For architectures are aimed at achieving potential improvements in that reason, control systems must be considered during the design propulsive efficiency by increasing the effective fan bypass of such systems. The purpose of this report is to suggest that tur- ratio through the operation of several smaller electric fans. The bomachinery designers should embrace and leverage the ability use of distributed fans can reduce drag, and the designs often of the control system to tightly manage the system state changes.

This concept can lead to higher performing compressor designs employ boundary layer ingestion for additional propulsive benefit (Ref. 12). by minimizing the need for reserving an excessive SM to ensure Initially, electrification was not focused on the direct benefit safe power level changes.

This paradigm change requires a multidisciplinary under- of the gas turbine engine; however, it has created several interesting operational system benefits, such as the elimination standing of the cause and mitigation of these transient dynamics.

of stall margin (SM) overshoot during power level changes and The aim of this investigation is to systematically address the con- bleedless operation at low engine speeds. All these advantages cepts that chronicle our journey with electrification by connec- are being evaluated as trades with implications for system ting the large body of previous work in ensuring system stability weight and complexity (Refs. 13 and 14). The weight disad- to the greater goal of impacting system performance.

vantage of electrification is readily apparent and is a major The report is organized as follows. First is a description of obstacle to achieving a net benefit for a given system. An operability and transient off-design performance. This leads to electric subsystem for propulsion consists of electric machines an overview of energy management control, its implementation, and electrical power converters and controllers, an electrical feasibility, and methods of using fuel and electric power to distribution bus, and typically includes a gearbox to match the directly control engine shaft mechanical state. Presented next electric machine performance characteristics to the turbine are advances in modeling transient dynamics, followed by engine. In more complex architectures, a separate electrical proposed operability-influenced compressor design methods.

energy storage element is also used. With an increase in the Finally, the report presents the conclusions and potential future number of required propulsion system components, there is an research directions.

NASA/TP-20260001365 2 exchange from the turbomachinery perspective. These are also

2.0 Operability and Transient Off-

described from an engine control perspective by Jaw (Ref. 20).

Design Performance

First, the engine is a pressure vessel, and the many volumes along the gas path have time-dependent responses to distur- Turbomachinery is tested in a facility under repeatable steady-state conditions to determine its optimal performance bances individually These are the most consequential since fluid instabilities increase component losses and produce characteristics. In normal flight operation, the engine is con- tinually migrating between operating points, during which its blockages radially, circumferentially, and axially through the transient off-design behavior is exhibited. Transient off design engine, leading to stall and surge conditions. Second, turbomac- hinery unavoidably has a substantial rotational inertia that is described as momentary excursions away from engine component operating lines as shown in Figure 1. In extreme causes the mechanical system (shaft spools) to exhibit a time- cases this can lead to engine instability, damage, and failure. dependent response. These are observable as lags in thrust res- ponse because energy must be added or removed from the Transient response is generally the performance-limiting factor in operability for the high-pressure compressor. inherent flywheel as a prerequisite for a rotational speed Transient behavior is dynamical and is associated with vari- change. The last of these is the time dependencies associated with the thermal masses of the system. These have three promi- ous forms of energy being stored within the engine components.

In effect, every point on the operating line is a temporary state nent detrimental effects: (1) time-dependent energy absorption of equilibrium such that the system must undergo an energy into the structure, (2) changes in tip clearance that greatly affect component efficiency, and (3) time-dependent energy transfer rebalance to adjust to a different point within its operating enve- lope. Any input, whether from the pilot, the system, or the between the fluid and the solid surfaces that impact aerodyn- amics and the inlet flow to blade rows (stage matching). All of environment will perturb the operating state of the turboma- chinery. Transient operation persists until the energy balance is these effects have widely divergent timescales that produce a reestablished at the new operating point. complex natural transient response in the engine.

Operating transients occur during throttle accelerations or The engine itself stores a considerable amount of energy using three prominent mechanisms that affect operational thrust decelerations. Federal Regulation Title 14 CFR 33.73 (Ref. 21) production: gas path volume, rotational inertia, and thermal states that engine power must stably advance from 15% of the rated takeoff power or thrust available to 95% within 5 s mass. Kerrebrock (Ref. 19) discusses energy conversion and Figure 1.—High-pressure compressor map (Ref. 17) showing the transient off-design operation that needs protection from stall during acceleration.

NASA/TP-20260001365 3 without overtemperature, overpressure, surge, stall, or other condition. On a compressor map (refer to Fig. 1) there are detrimental factors occurring. Deceleration transients, unspec- two common definitions: the distance from the current ified by regulations, typically last significantly longer, consid- operating point to the stall line at (1) constant mass flow or (2) ering that turbomachinery can only produce power to generate at constant rotational speed. Problematically, the stall line thrust and has limited means of decelerating the shaft spools. position changes with engine degradation and under certain operating conditions.

The fundamental task for control has been to ensure that the off- design transients do not exceed the conditions that guarantee It is important to understand the physical meaning of SM stall-free operation; that is, remaining within the transient stack rather than fixating on its probabilistic value determined from a compressor map. The root physical cause of compressor margin while meeting the engine acceleration requirements.

Closed-loop control in turbomachinery has traditionally been instability and stall is flow blockage in the compressor gas path.

implemented by varying fuel flow along with scheduled (open- In the compressor, the rotor blades convert kinetic energy to loop) actuation of compressor variable geometry and operabil- thermal energy within the flow, which both the rotor and stator ity bleeds. To avoid all adverse events when throttling engine vanes diffuse and convert to potential energy, slowing the power, various scheduled maximum and minimum limits on velocity and creating a rise in pressure. Turbomachinery desig- measured engine properties are employed in the controller that ners use the concept of velocity triangles (Ref. 22), shown in constrain the fuel flow rate of change. Figure 2, to optimize the energy exchange between the Variable geometry is designed to optimize stage matching mechanical and fluid systems in the compressors and turbines.

when the compressor is not operating at its single design point, The vector U describes the mechanical rotor tangential velocity; which is generally the cruise condition. Typically, this actuation and c , w , β , and α describe the absolute flow vector, relative n n n n scheme is applied to a few of the weakest blade rows in the flow vector, the angle between the relative flow vector and the compressors that limit its performance. Compressor airflow axial line of the compressor, and the angle between the absolute bleeding is another actuation technique that increases inlet mass flow vector and axial line of the compressor. The subscript n flow through the compressor during low power conditions for denotes locations upstream of the rotor (1), rotor exit (2), and the purpose of reducing compressor blade and stage stator exit (3). Subtracting U from c yields the relative velocity aerodynamic loading. The bleed flow is generally transferred w and its angle β with respect to the rotor blade. Adding U to 1 1 into the bypass flow and results in a loss of compressor w yields the absolute flow velocity c into the stator at angle 2 2 efficiency. These actuation schedules are set during the engine α 2. The red vectors indicate reduced flow relative to blade design phase to ensure the compressor operating line holds velocity and how the flow propagates downstream, affecting sufficient SM for operability. SM is a convenient proxy for compressor performance. The velocity triangles illustrate this compressor stability, which is an unmeasurable engine direct relationship between the mechanical inertial and gas path Figure 2.—Turbomachinery velocity diagram illustrating relationship between rotating wheel speed ( U ) and the gas path velocity and incidence angle into airfoils (rotor: w , β ) and (stator: c , α ).

1 1 2 2 NASA/TP-20260001365 4 volume time dependencies, two of the fundamental energy demand in mass flow from the HPC, even while the LPC mass storage mechanisms in turbomachinery. When operating the flow begins to increase.

engine transiently, the relative change in the two vectors occurs A somewhat reverse situation develops on deceleration, at different rates, violating the steady-state relationship and when the energy held in the rotating mass of the LPS serves to altering the flow incidence angle onto the airfoils. This excites continue to drive the LPC flow into the HPC, even while the combustor fuel is being throttled down. In this situation, the entropy production and flow blockage, producing a transient flow mismatch for the following stages. LPC pressure ratio increases while the lower inertia, HPS speed Consider Figure 3, while referring to the compressor map in slows, pumping less fluid and restricting the mass flow in the gas path, which results in a reduced LPC SM. This increases Figure 1, to appreciate the complex dynamics that are initiated during acceleration by adding fuel to the combustor. As fuel is inlet pressure to the HPC while the HPC is attempting to decel- added, the flow exiting the combustor has increased enthalpy erate its pressure ratio increase. The result is an increase in HPC with a small increase in mass flow. The flow entering the com- SM and the possibility of very lean fuel-air ratio conditions in bustor, however, will not change appreciably until the high- the combustor, threatening blow out.

pressure turbine (HPT) begins extracting work from its entering It is the close coupling of mechanical and fluidic time- flow and thereby power the high-pressure compressor (HPC), dependent changes, and the indirect control of the turbomach- to which it is connected by a mechanical shaft. A similar inery shaft speeds via the fuel flow rate, that has made compres- scenario unfolds with the low-pressure spool (LPS), further sor stability the limiting factor in turbomachinery performance.

delaying the recursively changing conditions in the high- Up to half of the SM reserved in the compressor design is pressure spool (HPS). As the HPC begins to accelerate, the devoted to transient operations (Ref. 17). Electrification now provides the means for the direct, independent control of HPC inlet pressure is reduced because of the delayed supply of air from the low-pressure compressor (LPC). This phenomenon mechanical shaft time-dependent behavior. The coordination of increases the HPC pressure ratio and causes the HPC to operate the inertial and volumetric energy storage mechanisms during operation can result in turbomachinery operation that approxi- with a deficient inlet mass flow. The result is a significantly reduced HPC SM. In contrast, the LPC experiences increased mates a steady-state condition, thus reducing the disequilib- SM because its pressure rise is restricted by an increased rium, or entropy production, in the gas path.

Figure 3.—Illustration showing energy pathways between gas turbine engine components. Red and blue lines indicate hot and cold air, respectively; green lines indicate airflow transferred via bleeds; and gold lines indicate mechanical coupling. LPT is low-pressure turbine.

NASA/TP-20260001365 5 3.1 Implementation and Feasibility

3.0 Energy Management Control

The simplification of TEEM allowed for the bifurcation of Electrification of the gas turbine engine was first proposed to the control approach addressing accelerations and decelera- counteract the diminishing returns of increasing fan diameter tions. In the early simulations, there was no electrical system on propulsive efficiency (Ref. 23) by achieving a higher effect- model. Power was simply inserted via a power offtake variable tive bypass ratio with multiple smaller electric fans. Various in the model of the HPS during acceleration to reduce the concepts were explored under the umbrella term of “more dis- aerodynamic loading in the HPC. During decelerations, power tributed propulsion.” However, coupling turbomachinery with was extracted from the LPS to reduce the aerodynamic loading electric machines introduces electrical dynamics that change in the LPC. Both techniques demonstrated that the off-design several orders of magnitude faster than the turbomachinery operation of the loaded compressor could be reduced or even thrust response. Coordinating these additional dynamics with forced back to the operating line, or beyond, based on the the rest of the mechanical and thermodynamic system is neces- amount of power used.

sary to ensure engine stability. It was these concerns about the To increase the practicality of the simulations, improvements impact on engine stability that evolved into a control philos- were incorporated to accurately model the electrical systems.

ophy designated by the NASA as “Turbine Electrified Energy TEEM needed to leverage the electric machines in the existing Management (TEEM)” (Ref. 13), which leverages the electric design of the propulsion system as fast, high-control-authority machines for use as stability control actuators.

actuators for turbomachinery transient control in addition to their Initial simulation studies were focused on control methods for primary role as power conversion devices. Consideration of how the protection of SM, where transient conditions caused the rota- this would work within the existing framework of the concept tional velocity of the blading on both spools to be delayed with vehicle was important. This dual-use technique required that a respect to the gas path velocity (see Fig. 2). The first assumption machine could be rapidly switched between motoring and was that to counteract the loss of SM, both spools required power generating modes of operation by its controller, while always injection when accelerating and power extraction when deceler- operating in a single rotational direction. It also took advantage ating to maintain the appropriate relationship between spool of an electrical energy storage device that could be used as an speeds and airflow. This was soon proven to be unnecessary, as independent source of energy for operability control during the loss of SM is isolated to the HPS when accelerating and the transients. The general configuration is shown in Figure 4.

LPS when decelerating. Studies by Kratz (Refs. 24 and 25) using The independent energy source allowed for the momentary the conceptual Advanced Geared Turbofan 30,000-lbf thrust decoupling of the electric loads from the turbine-engine-driven (AGTF30) engine model (Ref. 26) demonstrated that this simpli- generator; the main task of the generator is to power those fication created turbomachinery operability benefits throughout ® ® electric loads. Energy from the energy storage device would the flight envelope. The AGTF30 is a MATLAB -Simulink supply power to the downstream loads while simultaneously (The MathWorks, Inc.) implementation of the NASA reference propulsion system (Ref. 27). It uses a numerical energy balance method at the component level and performance maps to define the nonlinear turbomachinery system. Simultaneously, the Single-aisle Turboelectric AiRCraft with an aft boundary layer propulsor (STARC-ABL), a NASA-designed conceptual electri- fied aircraft, was studied and shown to benefit from the addition of electrical energy storage and TEEM control as well (Ref. 28).

These early efforts were followed by higher fidelity simula- tions and hardware-in-the-loop (HIL) experimental studies to demonstrate the feasibility of using the electrical power system to mitigate compressor stall under large power extraction condi- tions, generally looking at two-spool power extraction config- urations on turbofan engines. What follows is a discussion of these studies for implementation and feasibility of energy Figure 4.—Electrified turbofan engine with two-spool power management control progressing toward a capability to impact extraction, a coupled energy storage device, and electrical gas turbine engine performance.

loads.

NASA/TP-20260001365 6 either sourcing power to the high spool or absorbing power electric machines can respond with sufficient speed and authority from the low spool: the former would occur during an accele- to maintain compressor stability. The machines also can easily ration, and the latter, during deceleration. Additional refine- react to throttle demands and to asynchronous variations in the ments to this approach are discussed in Section 3.2, “Torque loads supplied by the engine. In Reference 40, Kratz develops Shaping and Optimization.” The energy storage device also optimized fuel flow schedules using genetic algorithms. Kratz employed a DC-DC converter in front of the battery for three (Ref. 41) then sequentially applies genetic algorithms to develop purposes: (1) to boost battery voltage, (2) to regulate the DC controllers for implementing TEEM control in coordination with bus voltage in the interest of providing consistent motor perfor- optimized fuel schedules. In previous efforts, the use of electric mance, and (3) to allow for the energy storage device to main- power had been commanded through an inner closed-loop cor- tain a static state of charge. The energy storage element could rection to the basic outer closed-loop fuel flow rate controller.

be a battery, but it was generally considered to be more practical Using the optimization technique, a reduction in the use of elec- to use a supercapacitor to avoid the charge and discharge limita- tric machine power was realized along with a further simplifica- tions of batteries. These practicalities forced several constraints tion of the electric control via use of a scheduled approach. The on the system; specifically, electric machine power and energy transient response benefit was quantified in terms of transient storage capacity, which essentially is a weight, volume, and stack usage and transient excursion integral. The former is a mea- performance trade. Ideally, the transient control could be sure of the maximum penetration into the transient stack at any accomplished by the baseline electrified propulsion architecture point, and the latter is a measure of the overall usage of the tran- and not drive the sizing of the electrical design. There are sient stack area to perform the control. The result is shown in several studies that examine these complex trades (Refs. 29, 30, Figure 5, illustrating the increase in the distance between the 31, 32, and 33).

operating point and the stall line during an idle-max-idle throttle As the technology matured, the issue of feasibility in using the movement when TEEM is implemented as opposed to without electric power system for stability control in a hybrid propulsion TEEM.

architecture needed to be demonstrated. This involved exper- In a related study, Litt (Ref. 42) further expands the previous imental efforts in two facilities. The first was in the NASA approach by applying genetic algorithms to simultaneously Electric Aircraft Testbed (NEAT), using megawatt-scale electric optimize both the fuel flow rate and electric power. The machines. The second was in the newly constructed Hybrid AGTF30 engine model was also modified for the first time to Propulsion Emulation Rig (HyPER), using 60-kW machines.

incorporate the effects of heat soak and HPT tip clearance Both HIL facilities utilized an experimental electric power control. Although the engine control was still effective at system driven by the emulated shaft interfaces of a two-spool suppressing compressor transient off-design operation, the turbofan engine using electric machines driven in torque mode.

impact of thermal dynamics significantly changed the results.

Typically, the engine model was the AGTF30, with the TEEM controller providing closed-loop control. In all cases, the engine simulation and controller were operated at full scale while the HIL torque and speed outputs were scaled for the specific test facility hardware, while preserving the full-scale turbomachinery shaft time dependencies. This capability was provided by an adaptive sliding mode impedance control with scaling (ASMICS) software (Ref. 34). The details of NEAT and HyPER test facilities are covered in several publications (Refs. 35, 36, and 37). The results of these experimental investigations demon- strated the immense capability of the electrical power system to perform transiently for the benefit of turbomachinery stability control (Refs. 38 and 39).

3.2 Torque Shaping and Optimization The application of torque via the electric machines for control of the gas turbine engine has evolved considerably. The attempts at optimizing the application of electric machine torque through- Figure 5.—Replotted results from Kratz (Ref. 41) showing the out a propulsion system transient will be referred to in this report operating point trajectories during an idle-max-idle throttle maneuver with and without TEEM.

as “torque shaping.” The following efforts have demonstrated NASA/TP-20260001365 7 This study did not investigate deceleration or other concerns (Ref. 17). The successful demonstration of a controls-based with thermal time-dependent responses such as Bode operability improvement can lead to turbomachinery designers being able to recover this large area of the compressor map, maneuvers (repeated accelerations and decelerations) and aerodynamic effects. used only for transient off-design operation, for performance The most recent efforts at torque shaping indicate that electric gains. Even though this could help increase the HPC pressure ratio, the difficulties with decreasing the dimensions of the last machine torque could be tailored together with the fuel flow rate profile to address specific portions of the transient timeline. blade row would likely favor pursuing weight and volume This suggests that the objectives for the transient control can reductions instead of performance increases. The most likely impact of a minimal transient stack would be reduction in the change, even within a specific transient event. For example, SM protection may be traded for minimizing peak temperature number of stages and solidity (ratio of the aerodynamic chord (Ref. 43) or maintaining component efficiency. over the peripheral distance between two adjacent blades) as the The fundamental approach to controlling transient time- pressure rise is redistributed across the LPC and HPC with dependent responses is becoming clearly understood and is reduced blade losses (higher efficiency). Depending on system important for stabilizing electrified propulsion architectures. details and objectives, this could represent several percentage However, the benefits of the technology have been primarily points in fuel burn reduction over the entire mission.

applied for operability, an important safety issue, but have not yet been shown to provide an engine performance benefit. Still, the

4.0 Transient Modeling and Dynamics

potential for operability to be used specifically for engine performance enhancement was recognized, and more attention Although it is encouraging that torque shaping holds promise has been focused in that area. For example, Jing (Ref. 44) to address specific control actions that can potentially mitigate recently considered using similar technology to enhance the performance-limiting transient response, the factor limiting acceleration in low-bypass turbofan engines. the use of energy management control is our physical under- One immediate engine performance benefit that has been standing of the transient itself. The physical understanding of gas observed evolved from the low spool method of transient turbine engine transient behavior is hindered by access to empiri- control during decelerations. Because of the low spool’s large cal data. As previously mentioned, most engine component tests inertia relative to that of the high spool, significant amounts of are intended to define performance under steady-state conditions.

power can be extracted, driving the sizing capacity of the Also, obtaining high-frequency measurements of the gas path is electrical energy storage device. Under extended-duration low- extremely difficult because of the lack of access, measurement engine-power settings typical of the mission segments of techniques, facility capabilities, and especially the expense. Even approach and landing or ground operations, there is also mini- though understanding transients at the component level is impor- mal LPC SM, and more significantly, poor engine efficiency. tant, the overall system transient response is determined by each Here, a bleed is often used to increase the mass flow through component’s effect on the others. For these reasons, modeling is the LPC to push the operating point further from the stall line. the most appropriate activity for developing an improved The fluid extracted from the compressor is transferred into the physics-based understanding of the transient phenomena. This is bypass flow, resulting in a loss in power and efficiency. This likely to have the added advantage of being related to component power loss can be eliminated by replacing the bleed with an geometry. Thus, the time-dependent responses of the compo- electric machine functioning as a generator, transferring the nents and system should be considered during the design phase power through the electrical system to the HPC. The result is of the component and system.

higher efficiency operation and reduced fuel burn for the Perturbing the gas-path state must be avoided to guarantee engine. This requires the electrification of both spools. For the stable, stall-free operation of the gas turbine engine. How- extended periods of low-power operation, the benefit increases ever, it is interesting to consider what is happening with the engine efficiency, producing about a 10% fuel burn reduction mechanism that is exciting the gas-path state in this situation.

for those mission segments. This is especially impactful for Cumpsty (Ref. 45) considers compressor stall and surge in short-duration flights (Ref. 14). some detail, although as a flow disturbance not caused by the A second engine performance benefit requires a paradigm fundamental operation of the engine. If a throttle movement can shift in design philosophy: one that embraces controls to be considered a flow disturbance into a fixed spool speed, then achieve a step change in engine performance. This philosophy it would likely be modeled as a full circumferential, large broadly applies the transient control techniques described disturbance to the compressor, producing the most severe earlier to justify a large reduction in the high-pressure com- effects. In our control methodology, we are attempting to pressor transient stack margin to near zero. Roughly half of change the spool speed to minimize the magnitude of this the total compressor SM is devoted to transient operations disturbance to the flow.

NASA/TP-20260001365 8 Considering the idea of the transient as an energy imbalance,

5.0 Operability-Influenced Design

we then should be able to model the power flow through the Propulsion system electrification creates an opportunity to engine system along with time-varying changes in the stored energy in the gas turbine components. Effectiveness of the improve gas turbine performance by modifying engine dynamic control should appear as an ability to minimize the stored operation, the leading cause of an engine’s performance-limiting instabilities. This is enabling for a new compressor design pro- energy deviation in the turbomachinery components from their steady-state values. The expectation for the overall transient is cess that utilizes physics-based entropy rise models rather than described in a simple manner in Figure 6, as a timeline of the empirical pressure loss models that have traditionally been used in the design of new compressors. An overview of the energy disequilibrium recursively spiraling out from its origin in the combustor to the LPC. Still, validation of the model is methodology is presented here.

difficult except at the start and end points of the overall transient Entropy rise models refer to loss models that track entropy gen- where the steady-state and transient models must match by eration within a compressor. Given compression in the Brayton definition. cycle is ideally isentropic, the entropy generation is ideally zero.

This approach is currently being modeled using NASA’s Thus, any entropy generation relates to inefficiencies in the com- Toolbox for the Modeling and Analysis of Thermodynamic pression process. Other loss model methods rely on empirical rela- Systems (T-MATS) (Ref. 46). Here, the Volume block is tionships developed with data that are often proprietary and/or specific to a compressor design. Entropy rise models focus on ap- modified to incorporate the instantaneous, time-varying, fluidic energy storage as signified by changes in enthalpy and the mass proximating changes in thermodynamic performance based on a flow differential across each component block that persists only variety of factors such as friction, end-wall losses, clearance los- ses, shock losses, and so forth. This approach retains more phys- for the duration of the transient. This was incorporated into the AGTF30 model to get a better estimate of the transient control ical meaning and is more generic. Entropy rise models can be em- effectiveness relative to the perturbation of the gas-path state ployed to match the performance of a known design or to alter a design. When combined with a technology such as TEEM, ent- (Ref. 47).

Next steps would be to incorporate thermal, time-dependent ropy rise models play a role in the redesign of the compressor to state variables that model the phenomena of heat soak, turbine take advantage of the ability to reduce stall margin. The loosening tip clearance, and aerothermodynamic flow effects that alter the of operability requirements opens up the compressor design space.

engine performance as described in Reference 42. Gas path With the reduced need for stall margin, the compressor can be re- flow is the source of both heating and cooling effects inside designed to achieve lower entropy generation as predicted by the the engine, and it is a reasonable assumption that the use of model, which equates to higher efficiency. The design can also be electric power to alter the mechanical state will moderate peak modified to consider other potential benefits such as weight reduc- tion, for which component efficiency and weight will need to be temperature changes downstream of the combustor during accelerations. considered simultaneously under the raised operability constraint.

Figure 6.—Gas turbine engine dynamic transient response originating from the combustor and spiraling out over time to other components.

NASA/TP-20260001365 9 This process leverages NASA’s object-oriented turboma- in rotors 1, 7, and 9 and in the final stage. Estimating entropy chinery analysis code (OTAC) (Refs. 48 and 49), which was rise in rotor 1 is challenging because of the existence of shock waves and because the model used seems to overestimate modified to account for entropy rise calculations across each compressor blade row. A significant effort was expended to entropy rise due to the shock at this stage. One of the compres- identify various existing entropy rise models and assess their sor bleeds is at stage 7, causing irregularities in the estimation of end-wall losses in rotor 7. It proved to be challenging to performance when applied to the NASA Energy Efficient Engine (E3) HPC model. These models were mostly guided by estimate the entropy rise at the last stages of the HPC. This is Denton’s approach (Ref. 50) to identify loss mechanisms in most likely due to the overestimation of the end-wall losses and a shortcoming of the model used here.

turbomachines. The entropy rise models were then modified or further developed to match the known entropy rise from the The modifications to OTAC were categorized into two main extensive E3 HPC database (Refs. 51 and 52). From there, a groups. First, enhancements involved incorporating entropy rise new steady-state model of the 10-stage HPC was developed. calculations into the code, enabling users to input estimates of The off-design performance of the new HPC model was also entropy rise for each blade row either through experimental data assessed, requiring further adjustment of the entropy rise or entropy rise models. The code utilizes the user's initial estimate models to match the E3 HPC database. to solve for each blade row and compute flow characteristics at A comprehensive literature review was conducted to identify the blade row exit. Additionally, a separate module was created to integrate all entropy rise models considered in this study.

pressure loss and entropy rise models and evaluate their effect- iveness in predicting the performance of the E3 HPC. Pressure Second, adjustments and further developments were made to loss coefficients, being empirical models derived from experi- OTAC elements to facilitate the proper design of the E3 HPC.

This included developing an element to accommodate com- mentation, have limited adjustability, mainly confined to blind- ly altering the model parameters, specifically when applied to pressor bleed at stages 5 and 7 as well as introducing a blockage more modern designs. Conversely, entropy rise models are element to incorporate blockage effects in blade row calcula- tions. Also, a new expander element was created to allow users developed by simplifying the physics of loss mechanisms.

Therefore, enhancing the current entropy rise models by adding to define compressor input profiles based on Mach number, complexity to these simplified models, or adjusting the under- total pressure, and total temperature distributions.

lying assumptions, can offer a more physically meaningful Figure 8 shows total pressure, P , and total temperature, T , t t approach to match the performance of the E3 HPC (Ref. 53). versus the inlet guide vane, rotor, and stator row number, com- Four primary sources contributing to entropy rise were ana- paring the E3 test data to the OTAC model with and without the lyzed: blade profile, end-wall flows, shock, and tip leakage.

entropy rise method. The plot shows good agreement between For each category, at least two models were identified and the HPC models developed in OTAC and the values from the further refined. test data .

As an example, profile entropy rise predominantly stems The objective of the design is to minimize entropy rise, thereby from the boundary layer along the suction and pressure surfaces reducing losses throughout the machine and enhancing the com- of the blade, which correlates with the velocity distribution pressor’s efficiency. Operability-influenced design (OID) facil- across the blade. To address this, simplified—but variable— itates improvements in compressor design through two primary velocity profiles were examined for rotating and stationary avenues. First, leveraging electric machines to control the tran- blades individually. This approach allowed for the development sient performance of the engine and compressor can substantially of distinct entropy rise models for each type of blade row. One diminish the SM required, potentially leading to a more compact of the challenges with these models is their generality; rooted compressor design. Second, a more aggressive blade design can in simple physics, these models need to be tuned to the proper be integrated to notably decrease entropy rise resulting from application, and this process limits their capability. blade profile and endwalls. Therefore, redesigning the com- Figure 7(a) illustrates the estimate of the rise in specific pressor blades to decrease entropy production near the operating entropy, s , versus the stator and rotor row number calculated line at the expense of off-design operation theoretically enables from the proposed models (Total Rise Model) and the E3 test increased on-design pressure rise. This, in combination with data (Total Rise E3). Figure 7(b) shows the calculations of s minimizing excursions from the operating line demonstrates the from the E3 test data versus the stator and rotor row number, plausibility of OID. Compressor rotor blade design needs to comparing the specific entropy (Total Rise E3) test data for balance efficiency and sensitivity to the inlet flow angles. With each stator and rotor into four contributing phenomena obtained the capability of OID to control stall instances, these blades may from the model: profile rise, tip leakage rise, shock rise, and be designed with more flow angle sensitivity and higher endwall rise. The magnitude of estimated entropy rise using the efficiency. However, these competing goals define a trade space proposed models is generally lower than the E3 amount except that still needs to be fully explored.

NASA/TP-20260001365 10 Figure 7.—Specific entropy rise s at design point versus rotor and stator blade row number (a) Comparison of entropy rise model and E3 test data. (b) Comparison of E3 test data specific entropy rise and each model component that contributes to the total rise calculation.

NASA/TP-20260001365 11 Figure 8.—Total pressure and total temperature versus inlet guide vane (IGV), rotor (RTR), and stator (ST) row number, comparing the E3 test data (E3) and the OTAC model without (OTAC) and with (OTACEM) the entropy rise method. (a) Total pressure, P t . (b) Total temperature, T t .

electric powertrains are the ideal. What will also continue is an

6.0 Conclusions

increase in system complexity and a corresponding concern New aircraft and propulsion systems are leaning heavily into with potential system-wide instabilities from vehicle-level electrified architectures as a pathway for meeting government en- interactions. This will result in an increasing future reliance on vironmental regulations and the endless quest for lower operating controls. However, propulsion electrification has demonstrated and maintenance costs. Although not ideal in the sense of real benefits for compressor stability. Here, electric machines achieving zero emissions, electrified gas turbine engines are used for power conversion become dual-use actuators for enabling for transformational changes in the flexibility of aircraft protecting stall margin during transients by using bidirectional design and energy optimization leading to reduced fuel burn. power flow to affect compressor blade loading. When this The move to electrification significantly changes the propul- technique is extended beyond transient operation, it becomes a sion system powertrain, adding more components with in- method for power transfer between spools to affect the engine creased complexity and weight. There has been great progress cycle for improved performance. Furthermore, this can all be in reducing the weight and volume of the electrical systems, and accomplished using the electric machines within the existing this will continue as the long-term presumption is that all- designs; that is, they do not drive the sizing requirements.

NASA/TP-20260001365 12 The limiting factor on energy management control tech- Ultimately energy management technology, used collaborat- nology is a more detailed understanding of engine gas path ively with fuel management, has the potential to fully control dynamics and how power is converted and flows through engine shaft dynamics. This will minimize the off-incidence multiple pathways in the engine under transient conditions. flow across the gas path blading, eliminating compressor insta- Research to better understand these phenomena, such as bility. In this capacity, the high-pressure compressor transient modeling of heat transfer and volume dynamics, can provide stack will be managed to near zero margin, thus enabling insight that enables optimized power injection and/or extraction designers to achieve a step change in compressor performance schedules. through operability-influenced design.

NASA/TP-20260001365 13

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NASA/TP-20260001365 15

Understanding and Controlling Hybrid Electric Gas Turbine Engine Transient Dynamics

Overview

Document

NASA/TP-20260001365

Understanding and Controlling Hybrid Electric

Gas Turbine Engine Transient Dynamics

February 2026

NASA STI Program Report Series

NASA/TP-20260001365

Understanding and Controlling Hybrid Electric

Gas Turbine Engine Transient Dynamics

February 2026

Contents

Understanding and Controlling Hybrid Electric

Gas Turbine Engine Transient Dynamics

Summary

1.0 Introduction

Nomenclature

2.0 Operability and Transient Off-

Design Performance

3.0 Energy Management Control

4.0 Transient Modeling and Dynamics

5.0 Operability-Influenced Design

6.0 Conclusions

References

Source & rights

Document details