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.