Sustainable Aviation

Sustainable aviation is one of the most pressing challenges in modern aerospace engineering. Aviation accounts for approximately 2-3% of global CO₂ emissions, and demand for air travel is projected to grow significantly in coming decades. The industry has committed to achieving net-zero carbon emissions by 2050, driving innovation across fuels, propulsion, airframe design, and operations.

Environmental Impact of Aviation

Direct Emissions

Aircraft engines produce several types of emissions:

The total climate impact of aviation is estimated to be 2-4 times greater than CO₂ alone due to non-CO₂ effects at altitude.

The Carbon Intensity Metric

Aviation carbon intensity is measured in grams of CO₂ per Revenue Passenger Kilometer (RPK):

Where $m_{fuel}$ is fuel mass consumed, $EF$ is the emission factor (3.16 kg CO₂ per kg of Jet-A1), and $RPK$ is the total passenger-distance.

Modern aircraft achieve approximately 80-90 g CO₂/RPK, compared to 200+ g CO₂/RPK in the 1970s.

Sustainable Aviation Fuels (SAF)

SAF are drop-in replacement fuels produced from sustainable feedstocks that can reduce lifecycle carbon emissions by 50-80% compared to conventional jet fuel.

Production Pathways

1. HEFA (Hydroprocessed Esters and Fatty Acids) — From used cooking oil, animal fats, or plant oils. Currently the most mature pathway.

2. Fischer-Tropsch — Gasification of biomass or municipal waste followed by catalytic synthesis. Can use diverse feedstocks.

3. Alcohol-to-Jet (AtJ) — Converts ethanol or isobutanol to jet-range hydrocarbons.

4. Power-to-Liquid (PtL) — Uses renewable electricity to produce hydrogen via electrolysis, then combines with captured CO₂ via Fischer-Tropsch synthesis. Potentially carbon-neutral but energy-intensive.

SAF Chemistry

SAF must meet ASTM D7566 specifications. Key properties:

The carbon intensity of SAF is calculated on a lifecycle basis:

Blending Limits

Currently, SAF can be blended up to 50% with conventional jet fuel (ASTM certified). Research is progressing toward 100% SAF operation, with successful test flights already completed.

Electric and Hybrid-Electric Propulsion

Battery-Electric Aircraft

The fundamental challenge is energy density. Current lithium-ion batteries provide approximately:

Compare this to jet fuel:

This ~48x gap means battery-electric propulsion is currently viable only for short-range flights (< 500 km) with small aircraft (< 19 passengers).

The Breguet Range Equation (Modified for Electric)

For battery-electric aircraft, range is:

Where $\eta_{total}$ is the total powertrain efficiency (motor + propeller ≈ 0.85), $E_{bat}$ is specific energy of the battery, and $m_{bat}/m_{total}$ is the battery mass fraction.

Hybrid-Electric Architectures

1. Series Hybrid — Gas turbine generates electricity → electric motor drives propeller. Decouples engine from propulsor.

2. Parallel Hybrid — Both gas turbine and electric motor can drive the propulsor. Electric motor assists during high-power phases (takeoff, climb).

3. Turboelectric — Gas turbine drives generator → distributed electric propulsors. Enables Boundary Layer Ingestion (BLI) and distributed propulsion.

Hydrogen Aviation

Hydrogen offers a potentially zero-carbon fuel pathway for aviation:

Hydrogen Properties

Hydrogen has excellent energy per unit mass (2.8x kerosene) but poor energy per unit volume (4x more volume needed), requiring significant airframe redesign.

Hydrogen Combustion

Burning hydrogen in a modified gas turbine produces only water vapor and NOₓ:

The NOₓ emissions can be minimized through lean premixed combustion or micromix burner technology.

Hydrogen Fuel Cells

An alternative to combustion is using Proton Exchange Membrane (PEM) fuel cells:

This is significantly more efficient than a gas turbine (~35-40%), but fuel cells have lower power density and are better suited for smaller aircraft or auxiliary power.

Operational Efficiency Improvements

Beyond new fuels and propulsion, significant emissions reductions come from operational improvements:

• Continuous Descent Operations (CDO) — 5-10% fuel savings on approach
• Formation Flying — 5-10% drag reduction through wake surfing
• Optimized Routing — Avoiding contrail-forming regions using weather data
• Single-Engine Taxi — 20-40% reduction in ground fuel burn
• Weight Reduction — Every 1% reduction in weight saves ~0.75% fuel

CORSIA Framework

The Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) is ICAO's market-based mechanism to stabilize net CO₂ emissions from international aviation at 2019 levels.

Airlines must:

1. Monitor and report CO₂ emissions
2. Purchase carbon offsets or SAF credits for emissions above 2019 baseline
3. Progressively reduce emissions through fleet modernization and SAF adoption

Your Challenge: SAF Lifecycle Analysis

Calculate the lifecycle carbon savings from replacing conventional jet fuel with SAF on a typical flight.

# Flight parameters
fuel_burn_kg = 5000  # Total fuel consumed (kg)
emission_factor_jetA = 3.16  # kg CO2 per kg fuel (conventional)
emission_factor_SAF = 0.95   # kg CO2 per kg fuel (HEFA SAF, lifecycle)

# Calculate emissions for different SAF blend ratios
blend_ratios = [0, 0.10, 0.25, 0.50, 1.00]

for ratio in blend_ratios:
    saf_kg = fuel_burn_kg * ratio
    conv_kg = fuel_burn_kg * (1 - ratio)
    total_co2 = (conv_kg * emission_factor_jetA) + (saf_kg * emission_factor_SAF)
    savings_pct = (1 - total_co2 / (fuel_burn_kg * emission_factor_jetA)) * 100
    print(f"SAF {ratio*100:5.1f}%: CO2 = {total_co2:,.0f} kg, Savings = {savings_pct:.1f}%")

Extend this analysis to compare HEFA, Fischer-Tropsch, and Power-to-Liquid pathways. How do feedstock availability and production costs affect scalability?