What began as a program to tackle heat issues on stealth fighters is evolving into the centerpiece of a U.S. initiative to address aircraft energy demands, and leading the drive toward engineering methods based on dynamic models.

At the heart of the Energy Optimized Aircraft (EOA) national plan will be the U.S. Air Force Research Laboratory's (AFRL) Integrated Vehicle Energy Technology (Invent) program, which is developing adaptive, smart aircraft power systems using model-based design.

Invent will be one of four core technical products of the EOA plan. The program's goal is to integrate hybrid-electric systems to maximize energy efficiency, minimize thermal challenges, and provide power and cooling on demand, based on the duty cycles of individual aircraft systems.

The Pentagon's EOA plan will set the goals for a broader technology initiative to enable energy-efficient aircraft with enhanced operational capabilities, minimized thermal constraints and increased power growth capacity.

Invent and the EOA plan to address two interrelated challenges: increasing aircraft fuel efficiency to reduce Pentagon energy demands and managing thermal issues on stealth aircraft that have nowhere to dump waste heat but into the fuel. “More efficient engines shift the balance—25 percent less fuel burned is 25 percent less heat that can be dumped,” says Steve Iden, AFRL Invent program manager.

“And it will only get worse,” he says. The Lockheed Martin F-22 and F-35 have up to five times the heat load of the company's F-16, and the next generation of fighter will be the first with directed-energy weapons, which will require megawatt-class power and cooling. “All that power does not leave the aircraft. It turns into low-quality heat in the aircraft, which becomes a flying thermos bottle,” says Iden.

Invent got its start in 2007, after a study showed not enough attention was being paid to the energy balance in fuel systems, leading to temperature runaways that affect aircraft performance. “We've got to a position where we run out of capability” because of the need to retain fuel for cooling, he says. “We bring back hot fuel, rather than burn it. We are looking for thermal persistence to stay in the fight.”

The $150 million program has three phases. Spiral 1 focused on near-term technologies that could be spun off to the F-35 to help tackle thermal-management problems. Goals include doubling ground hold time, and increasing flight time at low altitude fourfold—both times when fuel heats up in the absence of effective cooling.

Spiral 2, now underway, is aimed at midterm requirements for next-generation energy-optimized aircraft. Goals include a 10% increase in range/endurance from integrated systems, a five-fold increase in power and cooling capacity, on demand, with no thermal restrictions.

Invent aims to cut energy demand by reducing combined subsystem weight, now twice that of a single engine, and reducing bleed-air and power-extraction to minimize the fuel-consumption and thrust penalties. The program is demonstrating three key systems for the next generation of more-electric aircraft: adaptive power and thermal management (APTMS), robust electrical power (REPS) and high-performance electromechanical actuation (HPEAS).

APTMS will adapt to the best available heat sink: fuel, ram air or the additional “third-stream” flowpath provided by the next generation of variable-bypass engines being developed under AFRL's Adaptive Versatile Engine Technology and Adaptive Engine Technology Development programs. APTMS will be able to provide transient peak cooling for high power, but low duty-cycle systems like laser weapons.

More-electric architectures have altered the dynamics of power flow. Peak-to-average ratios can exceed 5:1, across milliseconds, and power-by-wire flight-control actuators can produce regenerative loads that equal peak power. REPS will provide an on-demand power capability able to meet peak demands and manage regenerative loads while delivering high-reliability power to flight-critical systems.

Technologies include bus architectures that allow power to flow both ways, from multiple sources, and electrical generation from both spools of the engine to meet peak demand. HPEAS will provide fail-operational/fail-safe actuation for flight controls, utilities and engines.

Tackling the challenges requires a culture change, says Iden. Today, aircraft systems are designed for peak power, which results in large inefficiencies when operating at lower demand. “They are on all the time, and cannot be modulated. We use them at low-duty cycle with big waste—5-10 percent of fuel burn,” he says. “We want that energy back, not in waste heat. That means more adaptive systems that match the adaptive-cycle engines.”

Iden cites the late-1990s Joint Strike Fighter Integrated Systems Technology (J/IST) program, which took the traditional engine starter/generator, auxiliary and emergency power units, and environmental control system (ECS) and consolidated them to minimize volume for low observability. “It was a good volumetric design, but it hurt performance as it's pretty inefficient,” he says.

The problem is the starter and ECS have two different operating points, “so they designed for the starter and the ECS fell out,” Iden says. “The starter is used for 30 seconds on the ground out of a two-hour flight. It's designed for a very small part of the mission, and the fallout is the ECS runs inefficiently. It takes a lot of bleed air off the engine and rejects a lot of heat into the fuel.”

The answer is to design systems that adapt to changes in demand across an aircraft's entire flight envelope. “Don't design for the peak, design for average power and thermal loads,” he says. “Look at demand across an entire mission and design with just enough margin. And for any shortfall, use energy storage.”

An electrical accumulator, for example, can provide peak power to a system and avoid having to oversize the generator. “There is a big impact if you design for peak. When you are not able to design for an average that is a fifth to an eighth of the overall peak, you carry around a lot of dead weight,” he says.

Invent is looking at developments in component technology, such as silicon-carbon power electronics that are more efficient and operate faster. “They have to pump cold air to cool the [power-by-wire] actuators on the F-35. That's lost air and lost pressure. We would not have to actively cool the actuators with silicon carbide.”

Power electronics are fuel-cooled and the highest temperature they can withstand is a key constraint in thermal-management system design. The heat rise in the system being cooled sets the maximum fuel inlet temperature. On a 120F, the 140F temperature limit on today's electronics leaves only 20F heat capacity in the fuel. “We'd like to raise that at least 50 F without any more weight,” says Iden.

Electronics that can operate to higher temperatures would reduce the heat dumped into the fuel. “We have a goal of 190F, and we'd like it to go to 250F. There are a couple of technologies out there. It's a question of how to integrate them and make them adaptive.”

To make power and thermal systems adaptive, Invent is pioneering a rapid model-based design approach. Today, airframers ripple down requirements to suppliers as static documents. “That doesn't work. It's not how we use the systems,” says Iden. “We want to pass down dynamic models that reflect how the systems work.”

The Invent modeling requirements and implementation plan is a government/industry collaboration on a framework that defines the interfaces and interactions between models, and sets fidelity and verification requirements. The goal is to replace the steady-state lookup tables used in design with dynamic, time-accurate mathematical models that address the control interactions and transient responses in on-demand systems.

To address the simulation rates required, two different tip-to-tail model fidelities have been defined—segment and mission. “Segment-level is very detailed, with small time steps. A mission-level model is less detailed, to answer questions like 'Is there enough battery storage?” he says.

“Invent is walking industry down this path,” says Iden. Northrop Grumman is looking at how Advent and Invent integrate, while Lockheed Martin is working on the F-35, tactical transports and other platforms. “Boeing is looking at the outer envelope, at F-X,” he says, referring to the Air Force's possible 2030 tactical fighter. “What is the worst case out there? Tactical is always the toughest task—it's a do-all aircraft: air-to-air, ground attack, ISR.”

Working with Boeing is AFRL's first chance to look holistically at adaptive systems. “Can you get the systems to add, or do they work against each other?” asks Iden. “What can you do with modeling and simulation? We are starting to get answers.” Under a $56 million contract half funded by AFRL, Boeing is to lead an Invent integrated ground demonstration that will start in fiscal 2014 and be completed in 2016.

“We have to virtually link labs, which will be a challenge,” says Iden. Ultimately he expects a J/IST-type flight demonstration will be needed. The ground demo will show the benefit of adaptive systems, “but sooner or later we have to fly or no one will believe us.”