The Science of Thrust and Combustion with Sustainable Aviation Fuels

Sustainable aviation fuels (SAFs) are synthetic or bio-derived alternatives to conventional Jet A/A-1 kerosene, produced from feedstocks such as used cooking oil, agricultural residues, municipal solid waste, or captured carbon and green hydrogen (power-to-liquid). While SAFs can be blended with conventional fuel at ratios up to 50% (and soon 100% under certifications like ASTM D7566 Annex), their chemical composition affects the combustion process and, consequently, thrust generation. The energy density of a fuel determines how much heat is released per unit mass during combustion; even a 2–3% reduction in volumetric energy density can require adjustments to fuel flow rates or nozzle geometry to maintain the same turbine inlet temperature and thrust output in a given engine.

Thrust is governed by the momentum change of air through the engine: F = ṁ × (V_exit – V_inlet). For a turbofan, the core (gas generator) provides high-energy exhaust that drives the fan and propels the aircraft. SAFs typically have a slightly lower gravimetric heating value (around 42–44 MJ/kg) compared to conventional jet fuel (~43.5 MJ/kg), but the difference is small enough that engine manufacturers can compensate through fuel metering. More critical are the combustion characteristics: flame speed, ignition delay, and soot formation. Synthetic paraffinic kerosenes (SPK), a common SAF type, burn more cleanly but require redesigned fuel injectors to avoid lean blowout at low power settings. Researchers at NREL and NASA have shown that optimizing nozzle geometry and swirl angles can recover any minor thrust deficiency while simultaneously reducing particulate emissions by up to 90%.

Chemical Properties of SAFs

SAFs are primarily composed of linear and branched alkanes (paraffins), with very low aromatic content. Aromatics are naturally present in petroleum-derived kerosene and are responsible for seal swelling in legacy fuel systems, but they also contribute to soot formation and contrail ice crystals. The absence of aromatics in pure SAFs means that engine seals and O-rings must be made from compatible materials—a change already implemented in current generation engines like the LEAP and GEnx. From a thrust perspective, the lower aromatic content leads to a more uniform burn front and reduced flame temperature gradients, which can improve high-altitude relight performance and extend combustor liner life. Fuel lubricity is another concern; hydroprocessed SAFs lack the natural sulfur compounds that provide lubricity in jet fuel, so additives are required to prevent premature wear in fuel pumps and metering units.

Fuel Injection and Combustion Tuning

Modern engines use lean-premixed combustion to control NOx and soot. SAFs have a higher H/C ratio (hydrogen to carbon), which reduces the adiabatic flame temperature and suppresses thermal NOx formation. However, the reduced reactivity of some SAFs—especially fuels with high isoparaffin content—can cause ignition delays that lead to combustion instability. To counteract this, engine manufacturers are introducing variable geometry combustors that adjust air-to-fuel ratios in real time. For example, Rolls-Royce’s ALECSys demonstrator uses adaptive fuel staging to maintain stable combustion across the throttle range, regardless of fuel composition. These systems rely on advanced sensing and control algorithms to ensure that the turbine inlet temperature profile remains uniform, preserving thrust integrity.

Advancements in Engine Technology

Original equipment manufacturers (OEMs) such as GE Aerospace, Pratt & Whitney, and Rolls-Royce are investing heavily in next-generation architectures that are fully compatible with 100% SAF. The focus is not merely on drop-in substitution but on optimizing the engine cycle for the unique thermal and combustion properties of renewable fuels. Two major areas of development are high-pressure fuel systems and novel combustor designs.

High-Pressure Fuel Systems

To compensate for the lower volumetric energy density of some SAFs, fuel pumps must deliver a higher mass flow rate. This requires stronger pump drives, stiffer fuel lines, and more responsive metering valves. GE’s e-HPC (enhanced High-Pressure Compressor) integrates a higher-stage compressor that increases the overall pressure ratio of the engine, improving thermal efficiency. Combined with a fuel system capable of delivering pressures above 5,000 psi, the engine can atomize SAF droplets finer, resulting in more complete combustion and a thrust-to-weight ratio comparable to or exceeding that of conventional kerosene systems. The industry is also exploring pistonless pumps driven by electric motors, which eliminate the parasitic losses of accessory gearboxes and allow precise fuel flow modulation for each flight phase.

Combustor Design Innovations

Traditional cannular combustors are being replaced by staged lean-burn annular combustors that divide the combustion zone into pilot and main stages. In these designs, the pilot stage maintains a stable flame during idle and descent, while the main stage activates at high power to maximize efficiency and minimize emissions. SAFs, with their lower soot propensity, allow the main stage to run even leaner, pushing thermal efficiency beyond 60%. The CFM International RISE (Revolutionary Innovation for Sustainable Engines) program features an open-rotor architecture with a compact combustor that can operate on 100% SAF while reducing fuel burn by 20% compared to current engines. The combustor liner is made from ceramic matrix composites (CMCs) that can withstand temperatures exceeding 3,000 °F without active cooling, enabling higher turbine inlet temperatures and thus greater thrust.

Materials and Thermal Management

SAFs burn hotter and cleaner, which reduces carbon deposits but exposes turbine blades to higher thermal loads. To maintain thrust without derating the engine, manufacturers are adopting single-crystal superalloys with thermal barrier coatings (TBCs) in the high-pressure turbine. Pratt & Whitney’s GTF Advantage engine uses a second-generation TBC that reduces blade metal temperature by 100 °C, allowing a 5% increase in turbine inlet temperature—and therefore thrust—without sacrificing durability. Additionally, additive manufacturing (3D printing) enables complex internal cooling channels in fuel nozzles and vanes, optimizing air flow to keep hot sections within safe limits while extracting maximum work from the combustion gases.

Hybrid and Electric Propulsion

While pure electric aviation remains limited to short-range commuter aircraft due to battery energy density limitations (currently around 250–300 Wh/kg vs. jet fuel’s 12,000 Wh/kg), hybrid-electric systems can augment thrust during takeoff and climb—the most fuel-intensive phases. By using electric motors to drive a ducted fan or supplement the low-spool turbine, the core engine can operate at its most efficient point while the electric system provides the peak power needed for maximum thrust. Several demonstrator programs, including Airbus’s ZEROe and GE Aerospace’s hybrid-electric project, are testing such architectures.

Supplemental Thrust Systems

A promising approach is the turboelectric concept, where a gas turbine drives a generator that powers an electrically driven fan at the rear of the aircraft. This decouples the fan speed from the core speed, allowing the turbine to run at constant high efficiency while the fan varies its RPM to deliver thrust as needed. Hybrid series/parallel configurations use a battery pack to provide boost during takeoff and then recharge during cruise using excess turbine power. In tests conducted by NASA’s Hybrid-Electric Integrated Systems Testbed (HEIST), these configurations reduced fuel consumption by up to 30% on regional turboprops. The challenge is thermal management of the batteries—lithium-ion cells generate significant heat during high-C-rate discharge—but advanced liquid cooling loops integrated into the engine’s oil system can dissipate that heat without adding excessive weight.

Battery and Fuel Cell Integration

Fuel cells are another route to clean supplementary power. Hydrogen fuel cells can produce electricity with only water as a byproduct, and when coupled with an electric motor, they can drive a fan directly. PEM fuel cells operating at 60–80 °C are suitable for auxiliary power units (APUs) but not yet for primary propulsion due to their power density. However, solid oxide fuel cells (SOFCs) running at 800–1000 °C can be thermally integrated with the combustor to preheat incoming air, improving overall cycle efficiency. Boeing’s Phantom Works and Airbus are investigating SOFC-powered hybrid systems for long-haul aircraft, with a target of 2–3 MW power output by the late 2030s. These systems would not replace the core engine but would offload up to 20% of the thrust requirement from the turbine, significantly reducing SAF consumption and extending range.

Challenges and Opportunities

Despite the technical progress, several challenges remain before SAF engines become ubiquitous. The primary hurdle is certification—both of the fuels themselves and of the engines that burn them. The current ASTM D4054 approval process for new fuel pathways can take years and cost millions. Regulators, including the FAA and EASA, are working on streamlined “drop-in plus” categories that allow engines certified on conventional fuel to operate on up to 50% SAF blends with minimal retesting, but 100% SAF certification requires demonstration that the engine maintains thrust, durability, and operability across all flight conditions—a process that involves extensive rig, ground, and flight testing.

Drop-in vs. Non-Drop-in Fuels

Most current SAFs are “drop-in” replacements designed to be fully interchangeable with Jet A/A-1. However, non-drop-in fuels such as neat methanol, ammonia, or hydrogen require completely new engine and fuel system architectures. While hydrogen combustion is being explored by companies like CFM and Airbus, its low volumetric energy density (about 1/4 of kerosene even when cryogenically stored) means that aircraft must carry massive tanks, reducing payload and range. For short-range applications, hydrogen could offer a path to zero-emission flight, but the thrust density of a hydrogen combustor is inherently lower due to wider flammability limits and the need for larger nozzles. Fuel flexibility engines that can switch between liquid SAF and gaseous hydrogen are in early conceptual stages, but materials compatibility and safe handling are major barriers.

Lifecycle Emissions and Fuel Cost

From an environmental perspective, SAF’s advantage lies in its lifecycle carbon reduction—up to 80% compared to fossil jet fuel when using waste feedstocks. However, the current production volume is less than 0.5% of global jet fuel demand, and SAF costs 2–5 times more than conventional kerosene. To incentivize adoption, governments are implementing mandates (e.g., ReFuelEU Aviation requiring 2% SAF by 2025, rising to 70% by 2050) and offering subsidies. Engine thrust performance must be consistent across a range of SAF blends and qualities, otherwise operators may be forced to derate engines or increase maintenance intervals, negating the economic and environmental benefits. Standardization of fuel properties is critical: variations in viscosity, density, and flash point among different production pathways (HEFA, ATJ, FT-SPK, etc.) require engine control laws that adapt in real time. Engine OEMs are developing fuel composition sensors that can infer the energy content and combustion characteristics of the incoming fuel and adjust injection timing and pressure accordingly.

The Road Ahead

The future of thrust in SAF engines will be shaped by three forces: policy, investment, and technology convergence. International bodies such as ICAO have set a goal of carbon-neutral growth from 2020 onward, and the International Air Transport Association (IATA) targets net-zero emissions by 2050. These commitments are driving massive research programs, including the EU’s Clean Aviation Joint Undertaking and the U.S. Sustainable Aviation Fuel Grand Challenge. The latter aims to produce 3 billion gallons of SAF per year by 2030, rising to 35 billion gallons by 2050. At that scale, SAF costs are expected to drop below $2 per gallon, making them economically competitive with fossil kerosene.

Technical milestones already achieved include the first 100% SAF-powered flights on a commercial aircraft (a United Airlines 737 MAX in 2021 using one engine), and the first transatlantic flight on 100% SAF (Virgin Atlantic in 2023). These flights demonstrated that with proper engine adjustments—modified fuel control units, tuned nozzles, and software updates—thrust levels can be fully maintained. The next step is to certify an engine family specifically designed for 100% SAF operation. Pratt & Whitney’s GTF Advantage, entering service in 2024, will be compatible with 100% SAF from day one, and GE’s GE9X (powering the Boeing 777X) has been tested on 100% SAF at its full thrust rating of 110,000 lbf. As more engines achieve this certification, the transition will accelerate.

Looking further ahead, advanced cycles such as the intercooled recuperative engine and the constant-volume combustion cycle could extract even more thrust per unit of SAF. Constant-volume combustors (pulse detonation or rotating detonation engines) theoretically offer a 15–25% improvement in thermal efficiency, but they produce intense pressure oscillations that stress turbine blades. Recent work at the University of Michigan and the Air Force Research Laboratory shows that SAF’s slower flame speed compared to hydrogen makes it a poor candidate for detonation combustion; however, pressure-gain combustion using rotating detonations in a SAF/air mixture could become viable with active cooling. Meanwhile, the growth of synthetic kerosene from direct air capture (DAC) and electrolytic hydrogen (e-fuels) offers a fully renewable, drop-in replacement that eliminates the biomass feedstock constraints of current SAFs. The production cost of e-fuels is projected to fall to $1.50–$2.00 per liter by 2040 with cheap renewable electricity, making them a long-term solution for long-haul aviation.

In conclusion, the aviation industry is on a clear trajectory toward engines that deliver high thrust with minimal environmental impact. The combination of advanced combustors, adaptive fuel systems, hybrid-electric augmentation, and a growing SAF supply base means that aircraft of the future will not have to compromise performance for sustainability. Continued collaboration between fuel producers, engine manufacturers, regulators, and airlines will be essential to overcome the remaining economic and technical hurdles. The fundamental physics of thrust—Newton’s third law—remains unchanged; what is changing is the efficiency, cleanliness, and flexibility of the engines that apply it.

For further reading: the U.S. Department of Energy’s Sustainable Aviation Fuels page provides an overview of production pathways, and the ICAO SAF portal tracks global policy developments and demonstration flights.