Next-Generation Aircraft Propulsion: A Comparative Analysis of Aviation Energy Sources

Executive Summary

The aviation industry faces mounting pressure to reduce greenhouse gas emissions while maintaining safety, performance, and economic viability. This technical analysis evaluates potential energy sources for next-generation aircraft propulsion systems by comparing their gravimetric and volumetric energy densities, storage requirements, infrastructure compatibility, and airframe design implications.

Based on a target mission energy requirement of 867,529 MJ (equivalent to a typical long-range commercial flight), this study analyzes Jet-A (baseline), Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), Hydrotreated Renewable Jet fuel (HRJ), Liquefied Natural Gas (LNG), liquid hydrogen, compressed hydrogen, lithium-ion batteries, and hybrid propulsion systems.

Key Finding: Drop-in Sustainable Aviation Fuels (SAF) such as FT-SPK or HRJ represent the most practical solution for next-generation propulsion entering service within the next decade, offering emissions reduction without major airframe redesign or infrastructure overhaul.

Figure 1: Energy Density Comparison

Energy requirements based on 867,529 MJ mission profile (comparable to a long-range widebody aircraft)

Highlights

• Comprehensive Evaluation: Analysis of 7 distinct energy sources plus hybrid configurations across multiple performance metrics
• Drop-in SAF Recommendation: FT-SPK and HRJ offer ~44 MJ/kg energy density with minimal infrastructure changes
• Hydrogen Challenges: Despite 120 MJ/kg gravimetric advantage, liquid H₂ requires 4× the volume of Jet-A
• Battery Limitations: Li-ion systems require 47× the mass of Jet-A, rendering them impractical for large aircraft
• Cost Analysis: Current fuel costs range from $0.02/MJ (Jet-A) to $0.15/MJ (batteries)
• Infrastructure Compatibility: Drop-in SAF enables immediate fleet adoption without certification delays

Methodology

Energy Requirement Baseline: The analysis uses a reference mission energy of 867,529 MJ, representative of a long-range commercial aircraft mission (approximately 8,000 nautical miles with typical payload). This baseline enables direct comparison of mass and volume requirements across all fuel types.

Evaluation Criteria:

1. Gravimetric Energy Density: Energy per unit mass (MJ/kg) — critical for aircraft range and payload
2. Volumetric Energy Density: Energy per unit volume (MJ/L) — determines tank size and airframe integration
3. Storage Requirements: Temperature, pressure, and material constraints
4. Infrastructure Compatibility: Existing airport fuel systems, distribution networks, and refueling equipment
5. Design Implications: Required modifications to airframe, propulsion system, and certification basis
6. Economic Viability: Approximate fuel cost per megajoule of delivered energy

Calculation Example: For liquid hydrogen with 120 MJ/kg gravimetric density:

• Required mass = 867,529 MJ ÷ 120 MJ/kg = 7,230 kg
• Required volume = 867,529 MJ ÷ 8.5 MJ/L = 102,000 L

This represents a 4× volume increase over Jet-A despite 65% mass reduction, necessitating major fuselage or wing tank redesign.

Detailed Analysis by Energy Source

Conventional Jet Fuel (Jet-A):

• Gravimetric: 42.8 MJ/kg | Volumetric: 34.7 MJ/L
• Advantages: Established global infrastructure, proven safety record, ambient storage
• Limitations: Fossil-derived with ~89 gCO₂/MJ lifecycle emissions
• Status: Baseline for comparison; existing fleet optimized for this fuel

Sustainable Aviation Fuels (FT-SPK & HRJ):

• Gravimetric: 43.5-44.0 MJ/kg | Volumetric: 33.5-34.0 MJ/L
• Advantages: Drop-in replacement, ASTM D7566 certified, 50-80% lifecycle emissions reduction
• Limitations: Production capacity constraints, 2-3.5× cost premium over Jet-A
• Status: Currently approved for 50% blends; 100% blend certification underway
• Production pathways: Fischer-Tropsch synthesis (FT-SPK), hydroprocessed esters and fatty acids (HRJ/HEFA)

Liquefied Natural Gas (LNG):

• Gravimetric: 50.0 MJ/kg | Volumetric: 22.0 MJ/L
• Advantages: 17% better gravimetric density than Jet-A, lower CO₂ emissions per MJ
• Limitations: Cryogenic storage at 111 K, 58% larger tank volume, methane slip concerns
• Status: Limited aviation deployment; requires new fuel systems and airport infrastructure

Liquid Hydrogen (LH₂):

• Gravimetric: 120.0 MJ/kg | Volumetric: 8.5 MJ/L
• Advantages: Zero carbon emissions at point of use, 2.8× Jet-A gravimetric density
• Limitations: Cryogenic storage at 20 K, 4× tank volume, boil-off losses, embrittlement risks
• Status: Experimental; Airbus targeting 2035 entry into service for regional aircraft
• Design impact: Requires pressure vessels in fuselage or modified wing structures

Compressed Hydrogen (700 bar):

• Gravimetric: 120.0 MJ/kg | Volumetric: 5.0 MJ/L
• Advantages: No cryogenic systems required
• Limitations: 7× tank volume of Jet-A, extremely heavy pressure vessels negate mass savings
• Status: Impractical for commercial aviation; relegated to ground vehicles

Lithium-Ion Batteries:

• Gravimetric: 0.9 MJ/kg | Volumetric: 2.5 MJ/L
• Advantages: Electric propulsion efficiency gains, zero in-flight emissions
• Limitations: 47.5× mass of Jet-A, thermal management challenges, range severely limited
• Status: Viable only for small aircraft (<20 seats) on short routes (<500 km)
• Application: Urban air mobility, training aircraft, short commuter flights

Hybrid Configurations (Jet-A + Battery Assist):

• Concept: Batteries provide power during high-demand phases (takeoff, climb); turbines cruise
• Advantages: 5-15% fuel burn reduction through optimized power management
• Limitations: Added system complexity, weight penalty, marginal range benefit
• Status: Under development for regional aircraft; NASA X-57 demonstrator program

Recommendation Rationale

After comprehensive evaluation, drop-in Sustainable Aviation Fuel (SAF) emerges as the optimal choice for next-generation propulsion systems entering service within 2026-2035:

Performance Alignment: SAF fuels (FT-SPK, HRJ) provide nearly identical energy densities to Jet-A (43.5-44.0 vs 42.8 MJ/kg), enabling existing aircraft to maintain range, payload, and operational capabilities without performance degradation.

Infrastructure Compatibility: As drop-in fuels, SAF requires no modifications to airport fuel farms, hydrant systems, refueling trucks, or aircraft fuel systems. This eliminates billions in infrastructure investment and accelerates fleet-wide adoption.

Certification Pathway: SAF blends up to 50% are already ASTM D7566 approved and fly commercially today. Full 100% SAF certification is expected by 2027, leveraging decades of jet fuel safety data rather than starting from scratch.

Emissions Reduction: Depending on feedstock (waste oils, agricultural residues, synthetic pathways), SAF can reduce lifecycle CO₂ emissions by 50-80% compared to conventional Jet-A while maintaining identical combustion characteristics.

Risk Mitigation: Unlike hydrogen or battery-electric propulsion, SAF deployment carries minimal technical risk. Existing turbofan engines, fuel systems, and maintenance procedures remain valid, preserving airlines’ operational expertise.

Economic Transition: While SAF currently costs 2-4× conventional jet fuel, production scaling and policy incentives (ReFuelEU in Europe, tax credits in the US) are driving costs down. Airlines can phase in SAF procurement without grounding fleets or investing in new aircraft.

Why Not Hydrogen?

Liquid hydrogen presents an attractive long-term vision for zero-carbon aviation but faces formidable near-term obstacles:

Volume Penalty: Hydrogen’s 8.5 MJ/L volumetric density requires tanks 4-5× larger than Jet-A. For widebody aircraft, this necessitates:

• Fuselage-mounted pressure vessels (reducing passenger/cargo capacity)
• Thickened wing structures (increasing structural weight)
• Complete aerodynamic redesign (invalidating existing type certificates)

Cryogenic Complexity: Maintaining 20 K storage temperatures demands:

• Multi-layer vacuum-insulated tanks with boil-off management
• Active refrigeration systems adding weight and power consumption
• Specialized airport infrastructure (liquefaction plants, cryogenic pumps)
• Stringent safety protocols for flammable cryogenic handling

Infrastructure Investment: Global transition to hydrogen aviation requires:

• $100B+ airport infrastructure overhaul (conservative estimate)
• Hydrogen production at scale via electrolysis (requiring massive renewable energy expansion)
• New fuel distribution networks separate from existing hydrocarbon systems

Certification Timeline: Hydrogen aircraft represent a clean-sheet design requiring:

• 10-15 years development and certification (comparable to new aircraft programs)
• Extensive safety testing of novel failure modes (tank rupture, rapid decompression, combustion)
• Regulatory framework development (current standards assume hydrocarbon fuels)

Conclusion: Hydrogen is a critical technology for post-2040 aviation but cannot address the 2026-2035 emissions reduction imperative.

Why Not Batteries?

Battery-electric propulsion suffers from fundamental physics limitations:

Energy Density Gap: At 0.9 MJ/kg, current Li-ion batteries store 47× less energy per kilogram than Jet-A. Even with optimistic projections of 1.5 MJ/kg by 2035, the gap remains insurmountable for commercial aviation.

Mass Penalty: For the reference 867,529 MJ mission:

• Jet-A: 20,270 kg
• Batteries: 963,920 kg (47× heavier)

This mass exceeds the maximum takeoff weight of most commercial aircraft, rendering long-range battery-electric flight physically impossible with foreseeable technology.

Range Limitations: Battery-electric aircraft are constrained to:

• Short routes: <500 km practical range
• Small aircraft: Typically <20 passengers
• Niche applications: Urban air mobility, flight training

Charging Infrastructure: Recharging 964 metric tons of batteries requires megawatt-scale charging infrastructure and multi-hour ground times, incompatible with airline operational models (30-45 minute turnarounds).

Application: Batteries will enable urban air mobility and electrify general aviation, but cannot support commercial airline operations in the foreseeable future.

Implementation Roadmap

Phase 1: Immediate Deployment (2026-2027)

• Increase SAF blend ratios from current 50% to 100% as certification completes
• Establish long-term SAF offtake agreements between airlines and producers
• Scale production via waste-based feedstocks (used cooking oil, forestry residues)

Phase 2: Production Scaling (2028-2032)

• Deploy advanced SAF pathways: alcohol-to-jet (ATJ), power-to-liquid (PtL)
• Achieve cost parity with fossil Jet-A through economies of scale and policy support
• Mandate SAF percentages in aviation fuel supply (e.g., ReFuelEU’s 2% by 2026, 70% by 2050)

Phase 3: Hydrogen Transition (2033-2040)

• Certify first-generation hydrogen regional aircraft (100-150 seats, <2000 km range)
• Develop airport hydrogen infrastructure at key hubs
• Parallel-path battery-electric technology for urban air mobility and short-haul

Phase 4: Full Decarbonization (2040-2050)

• Hydrogen narrowbody aircraft for mainline routes
• SAF for long-haul widebody where hydrogen volumetric penalty remains prohibitive
• Portfolio approach: right fuel for right mission profile

Key Assumptions and Limitations

Energy Requirement: The 867,529 MJ baseline represents a typical long-range mission but does not account for reserves, taxi fuel, or operational contingencies. Actual fuel loads include 5-10% additional capacity per regulatory requirements.

Technology Readiness: Energy densities reflect current state-of-the-art. Battery projections assume incremental lithium-ion improvements, not breakthrough solid-state or lithium-air technologies that remain speculative.

Cost Estimates: Fuel costs are approximate 2026 values and highly sensitive to:

• Crude oil prices (Jet-A, SAF feedstock competition)
• Renewable electricity costs (hydrogen electrolysis, PtL SAF)
• Carbon pricing policies (EU ETS, potential US carbon tax)

Lifecycle Emissions: SAF emissions reduction percentages are lifecycle (well-to-wake) values, not combustion-only. Actual carbon intensity varies by feedstock, production pathway, and land-use change assumptions.

Infrastructure Compatibility: “Drop-in” assumes compliance with ASTM D7566 and compatibility with existing turbine combustors. Some SAF pathways require minor seal or gasket material changes in older aircraft.

Conclusion

The path to sustainable aviation is a phased transition, not a single technological leap. Drop-in Sustainable Aviation Fuel represents the optimal bridge technology, delivering immediate emissions reductions while preserving operational continuity and economic viability.

Hydrogen and battery-electric propulsion will play critical roles in the post-2035 landscape, but their deployment timelines, infrastructure requirements, and performance trade-offs make them unsuitable for near-term fleet-wide adoption.

For aerospace engineers developing next-generation propulsion systems today, SAF provides the highest probability of technical success, lowest certification risk, and fastest pathway to meaningful carbon reduction—making it the recommended energy source for aircraft entering service within the next decade.

References and Further Reading

• ASTM D7566: Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons
• ICAO CORSIA: Carbon Offsetting and Reduction Scheme for International Aviation
• NASA Electrified Aircraft Propulsion Program: nasa.gov/aero/eap
• Airbus ZEROe Hydrogen Aircraft Concepts: airbus.com/zeroe
• SAE International: Aerospace Fuel System Design Standards
• European Union Aviation Safety Agency (EASA): Alternative Fuels Certification Guidance

About This Analysis

This comparative study was conducted as part of an independent research project exploring sustainable aviation technologies. The analysis synthesizes publicly available data on aviation fuels, propulsion systems, and aircraft design constraints to evaluate practical pathways toward decarbonizing commercial aviation.

Methodology Notes: All energy density values are sourced from peer-reviewed aerospace engineering literature and manufacturer specifications. Cost estimates reflect 2026 market conditions and include production, distribution, and handling. Storage requirement analyses assume compliance with aviation safety regulations (14 CFR Part 25, CS-25) and industry best practices.

Contact: For questions, corrections, or collaboration opportunities, please reach out via the contact page.

Last Updated: February 2026