Ma\ 2012

NASA/CR-2012-217556

Subsonic Ultra Green Aircraft Research

PhaseII: N+4 Advanced Concept Development 

Marty K. Bradley and Christopher K. Droney 

Boeing Research DQG Technology, Huntington Beach&DOLIRUQLD
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May 2012 

NASA/CR–2012-217556 

Subsonic Ultra Green Aircraft Research 

Phase II: N+4 Advanced Concept Development 

Marty K. Bradley and Christopher K. Droney 

Boeing Research and Technology, Huntington Beach, California 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

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constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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Abstract

This final report documents the work of the Boeing Subsonic Ultra Green Aircraft Research 

(SUGAR) team on Task 1 of the Phase II effort. The team consisted of Boeing Research and 

Technology, Boeing Commercial Airplanes, General Electric, and Georgia Tech. 

Using a quantitative workshop process, the following technologies, appropriate to aircraft 

operational in the N+4 2040 timeframe, were identified: Liquefied Natural Gas (LNG), 

Hydrogen, fuel cell hybrids, battery electric hybrids, Low Energy Nuclear (LENR), boundary layer 

ingestion propulsion (BLI), unducted fans and advanced propellers, and combinations. 

Technology development plans were developed.

The team generated a series of configurations with different combinations of some of these

technologies. The higher heating value of LNG reduces the weight of fuel burned, but because 

of heavier aircraft systems, more energy is used for a given flight. LNG fueled aircraft have the 

potential for significant emissions advantages and LNG enhances the integration of fuel cells 

into the aircraft propulsion and power system.

An unducted fan increases propulsive efficiency and reduces fuel burn. Adding a fuel cell and 

electric motor into the propulsion system also leads to improvements in emissions and fuel 

burn. An aft fuselage boundary layer propulsor also resulted in a fuel burn benefit. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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Foreword 

Part of the mission of Boeing Research & Technology, as the company’s advanced, central 

research and technology organization, is to help create the long-term future of aerospace by 

identifying and maturing new technologies. 

However, while Boeing is interested in developing environmentally progressive vehicles, it 

would be premature to conclude that any of the concepts studied under this contract will 

replace any of Boeing’s commercial products. 

This is an advanced concept and technology study that examines a wide variety of alternative 

fuel and energy technologies and is not an offer, commitment or promise on the performance 

or capabilities of any future Boeing product. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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Acknowledgments

This project and report reflect the combined efforts of the SUGAR Task 1 team. The team 

members for this task are Boeing Research and Technology, Boeing Commercial Airplanes, GE 

Aviation, and the Georgia Institute of Technology. The coordinated effort of this team has 

produced this report.

The team would like to thank Erik Olson and Mark Guynn of the NASA Langley Research Center 

for their guidance as the NASA Contracting Officer Technical Representative (COTR), and task 

technical advisor (TA), respectively. The team would also like to thank Gerry Brown, a NASA 

subject matter expert, for his contribution. 

Additionally, other experts from NASA, the Department of Energy, the Air Force Research Lab, 

the Federal Aviation Administration, and Virginia Tech contributed during the N+4 technology 

workshop or made suggestions for the Energy Study Outline.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II 

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Table of Contents 

Abstract ............................................................................................................................................ i

Foreword ..........................................................................................................................................ii

Acknowledgments ........................................................................................................................... iii

Table of Contents ............................................................................................................................ iv

List of Tables and Figures ................................................................................................................ vi

Tables .......................................................................................................................................... vi

Figures ......................................................................................................................................... vi

Nomenclature ................................................................................................................................. ix

1.0 Introduction ......................................................................................................................... 1

2.0 Technology Selection ........................................................................................................... 3

2.1 Process Overview and Background .................................................................................. 3

2.2 Pre-workshop activities .................................................................................................... 6

2.3 N+4 Workshop Process .................................................................................................... 7

2.4 N+4 Workshop Outcomes .............................................................................................. 10

2.4.1 Virtual East Team Summary .................................................................................... 13

2.4.2 Virtual West Team Summary .................................................................................. 16

2.4.3 Onsite Team Summary ............................................................................................ 19

2.5 N+4 Workshop General Observations, Recommendations, and Inspirations ............... 20

3.0 LENR Requirements Analysis ............................................................................................. 24

4.0 Energy Study Outline Development .................................................................................. 27

5.0 N+4 Concept Development and Analysis ........................................................................... 30

5.1 765-093 SUGAR Free (Baseline Aircraft) ........................................................................ 31

5.2 765-094-TS1 N+4 Reference Aircraft ............................................................................. 32

5.3 765-095-TS1 N+4 Truss Braced Wing ............................................................................. 38

5.4 765-095-TS2 N+4 Truss Braced Wing with LNG Gas Turbine ......................................... 44

5.5 765-095-TS3 N+4 Truss Braced Wing with LNG Unducted Fan ..................................... 51

5.6 765-095-TS4 N+4 Truss Braced Wing with LNG Fuel Cell Hybrid Gas Turbine and BLI .. 57

5.7 765-095-TS5 N+4 TBW with LNG Fuel Cell Hybrid Gas Turbine Unducted Fan ............. 65

5.8 Concept Comparisons and Summary ............................................................................. 66

6.0 Technology Development Plans ........................................................................................ 71

6.1 Technology Plan Template ............................................................................................. 71

6.2 Technology Plans ............................................................................................................ 72

6.2.1 Hybrid Engine Technologies .................................................................................... 72

6.2.2 Battery Technology ................................................................................................. 77

6.2.3 Low Energy Nuclear Reactor Technologies ............................................................. 82

6.2.4 Fuel Cell Technologies ............................................................................................. 88
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6.2.5 Boundary Layer Ingestion Propulsion..................................................................... 93

6.2.6 Advanced Unducted Fans and Propellors............................................................... 98

6.2.7 LNG and Hydrogen Gas Turbine Engines.............................................................. 103

6.2.8 LNG and Hydrogen Aircraft Systems..................................................................... 107

6.2.9 LNG and Hydrogen Infrastructure ........................................................................ 112

6.3 Technology Plans Discussion........................................................................................ 117

7.0 Conclusions and Recommendations................................................................................ 119

References .................................................................................................................................. 122

Appendix A – Propulsion Concept Information.............................................................................. A
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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List of Tables and Figures

Tables

Table 2.1 – N+4 Workshop Scoring Metrics.................................................................................... 9

Table 2.2 – N+4 Workshop Teams................................................................................................ 11

Table 2.3 – Virtual West Team Technologies per Concept........................................................... 18

Table 5.1 – Phase I Future Scenario Used to Set Payload-Range Requirements ......................... 30

Table 5.2 – N+4 Performance and Sizing Runs............................................................................. 31

Table 5.3 – 765-093 Group Weight Statement............................................................................. 32

Table 5.4 – 765-094-TS1 High Speed Build-up.............................................................................. 34

Table 5.5 – 765-094-TS1 Group Weight Statement...................................................................... 38

Table 5.6 – 765-095-TS1 High Speed Build-up.............................................................................. 40

Table 5.7 – 765-095-TS1 Group Weight Statement...................................................................... 44

Table 5.8 – 765-095-TS2 High Speed Build-up.............................................................................. 47

Table 5.9 – 765-095-TS2 Group Weight Statement...................................................................... 51

Table 5.10 – 765-095-TS3 High Speed Build-up............................................................................ 53

Table 5.11 – 765-095-TS3 Group Weight Statement.................................................................... 57

Table 5.12 – 765-095-TS4 High Speed Build-up............................................................................ 61

Table 5.13 – 765-095-TS4 Group Weight Statement.................................................................... 65

Table 5.14 – 765-095-TS5 Group Weight Statement.................................................................... 66

Table 5.15 – Configuration Performance Summary ..................................................................... 68

Table 5.16 – Segment Fuel Burn ................................................................................................... 69

Table 5.17 – N+4 Fuel, Energy, Noise, and Emissions Summary .................................................. 70

Table 6.1 – Hybrid Engine Technologies Success Criteria............................................................. 75

Table 6.2 – Battery Technology Success Criteria .......................................................................... 80

Table 6.3 – LENR Technologies Success Criteria........................................................................... 86

Table 6.4 – Fuel Cell Technologies Success Criteria...................................................................... 91

Table 6.5 – Boundary Layer Ingestion Technologies Success Criteria .......................................... 96

Table 6.6 – UDF Engine Technologies Success Criteria............................................................... 101

Table 6.7 – LNG and Hydrogen Technologies Success Criteria................................................... 105

Table 6.8 – LNG and Hydrogen Aircraft Systems Success Criteria.............................................. 110

Table 6.9 – LNG and Hydrogen Airport Infrastructure Success Criteria ..................................... 115

Table 7.1 – Task 1 Technology Summary.................................................................................... 120

Figures

Figure 1.1 – SUGAR N+4 Task Flow................................................................................................. 2

Figure 1.2 – NASA Noise, Emissions, Fuel, and Energy Goals......................................................... 2
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 2.1 – Evolution of the N+4 Reference Concept ................................................................... 4

Figure 2.2 – N+4 Configuration Analysis Flow ................................................................................ 5

Figure 2.3 – N+4 Configuration Initial Screening – Technologies and Options.............................. 6

Figure 2.4 – N+4 Workshop Process Diagram ................................................................................ 8

Figure 2.5 – N+4 Workshop Score sheet Template ...................................................................... 10

Figure 2.6 – N+4 General Assumptions ........................................................................................ 12

Figure 2.7 – N+4 Workshop Concept Overview Template ........................................................... 13

Figure 2.8 – Virtual East Team Scoring ......................................................................................... 16

Figure 2.9 – Virtual West Team Scoring........................................................................................ 19

Figure 2.10 – Onsite Team Scoring ............................................................................................... 20

Figure 2.11 – Relationship of N+4 Workshop Technologies......................................................... 23

Figure 3.1 – Potential Heat Engines for LENR Systems................................................................. 25

Figure 3.2 – Parametric LENR and Heat Engine Performance Parameters.................................. 25

Figure 3.3 – Hybrid LENR + Battery Performance Parameters..................................................... 26

Figure 5.1 – 765-094-TS1 High Speed Build-up ............................................................................ 34

Figure 5.2 – 765-094-TS1 - M * L / D Total ................................................................................... 35

Figure 5.3 – 765-094-TS1 - Low Speed Lift Curve; Free Air .......................................................... 36

Figure 5.4 – 765-094-TS1 - Low Speed Polar; Free Air.................................................................. 36

Figure 5.5 – 765-094-TS1 - Low Speed Lift / Drag; Free Air.......................................................... 37

Figure 5.6 – 765-095-TS1 – Truss Braced Wing with gFan++........................................................ 39

Figure 5.7 – 765-095-TS1 High Speed Build-up ............................................................................ 40

Figure 5.8 – 765-095-TS1 - M * L / D Total ................................................................................... 41

Figure 5.9 – 765-095-TS1 - Low Speed Lift Curve; Free Air .......................................................... 42

Figure 5.10 – 765-095-TS1 - Low Speed Polar; Free Air................................................................ 42

Figure 5.11 – 765-095-TS1 - Low Speed Lift / Drag; Free Air........................................................ 43

Figure 5.12 – Fuselage Stretched to Accommodate LNG Tankage............................................... 45

Figure 5.13 – 765-095-TS2 – Truss Braced Wing With LNG gFan++ ............................................. 46

Figure 5.14 – 765-095-TS2 High Speed Build-up .......................................................................... 47

Figure 5.15 – 765-095-TS2 - M * L / D Total ................................................................................. 48

Figure 5.16 – 765-095-TS2 - Low Speed Lift Curve; Free Air ........................................................ 49

Figure 5.17 – 765-095-TS2 - Low Speed Polar; Free Air................................................................ 49

Figure 5.18 – 765-095-TS2 - Low Speed Lift / Drag; Free Air........................................................ 50

Figure 5.19 – 765-095-TS3 - Truss Braced Wing With LNG UDF................................................... 52

Figure 5.20 – 765-095-TS3 High Speed Build-up .......................................................................... 53

Figure 5.21 – 765-095-TS3 - M * L / D Total ................................................................................. 54

Figure 5.22 – 765-095-TS3 - Low Speed Lift Curve; Free Air ........................................................ 55

Figure 5.23 – 765-095-TS3 - Low Speed Polar; Free Air................................................................ 55

Figure 5.24 – 765-095-TS3 - Low Speed Lift / Drag; Free Air........................................................ 56
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Figure 5.25 – Advanced LNG Fuel Cell Hybrid Configuration with BLI Propulsor......................... 58

Figure 5.26 – 765-095-TS4 - Truss Braced Wing with LNG Fuel Cell BLI....................................... 59

Figure 5.27 – 765-095-TS4 High Speed Build-up .......................................................................... 61

Figure 5.28 – 765-095-TS4 - M * L / D Total ................................................................................. 62

Figure 5.29 – 765-095-TS4 - Low Speed Lift Curve; Free Air ........................................................ 63

Figure 5.30 – 765-095-TS4 - Low Speed Polar; Free Air................................................................ 63

Figure 5.31 – 765-095-TS4 - Low Speed Lift / Drag; Free Air........................................................ 64

Figure 6.1 – Hybrid Engine Technologies Roadmap* ................................................................... 76

Figure 6.2 – Battery Technology Roadmap................................................................................... 81

Figure 6.3 – LENR Technologies Roadmap.................................................................................... 87

Figure 6.4 – Fuel Cell Technologies Roadmap .............................................................................. 92

Figure 6.5 – Boundary Layer Ingestion Roadmap......................................................................... 97

Figure 6.6 – UDF Technologies Roadmap* ................................................................................. 102

Figure 6.7 – LNG and Hydrogen Aircraft Propulsion System Roadmap...................................... 106

Figure 6.8 – LNG and Hydrogen Aircraft Systems Roadmap ...................................................... 111

Figure 6.9 – LNG and Hydrogen Airport Infrastructure Roadmap.............................................. 116
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Nomenclature

APU Auxiliary Power Unit

ASDL Georgia Tech Aerospace Systems Design Laboratory

BET Boeing Equivalent Thrust foe engine sizing

BLI Boundary Layer Ingestion

BOP Balance of Plant for fuel cell systems

CASES Boeing Computer Aided Sizing and Evaluation System 

CD Drag Coefficient

CFD Computational Fluid Dynamics

CH4 Methane

CL Lift Coefficient

COTR NASA Contracting Officer Technical Representative

DF Ducted Fan

DOC Direct Operating Cost

DoE Department of Energy

DP Distributed Propulsion

DTE Divergent Trailing Edge airfoil

EIS Entry Into Service date

ERA Environmentally Responsible Aviation

ERA Environmentally Responsible Aviation

FAA Federal Aviation Administration

FC Fuel Cell

FEM Finite Element Model

GE General Electric

GT Georgia Tech or Gas Turbine

H2 Hydrogen

HE Hybrid Electric propulsion

ICAC Initial Cruise Altitude Capability

ISA International Standard Atmosphere

JP Conventional Jet fuel (Jet-A, JP8, etc.)

L/D Lift to Drag ratio

LENR Low Energy Nuclear Reactor (or Reaction)

LNG Liquefied Natural Gas

LP Low Pressure spool of the engine

LRC Long Range Cruise Mach

LTO Landing and Takeoff

M Mach number

MADM Multi-Attributes Decision Making

MEW Manufacturer’s Empty Weight

MIT Massachusetts Institute of Technology

MTOW Maximum Takeoff Weight

NFPA National Fire Protection Association

NLF Natural Laminar Flow

NOx Molecules such as NO, NO2, NO3, etc.

OEW Operational Empty Weight

OML Outer Mold Line
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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RE Reynolds Number

SC Superconducting

SOFC Solid Oxide Fuel Cell

SUGAR Subsonic Ultra Green Aircraft Research

TA NASA Task Technical Advisor

TOFL Takeoff Field Length

TOGW Takeoff Gross Weight

TOPSIS Technique for Ordered Preference by Similarity to Ideal Solutions

TP Turboprop

TRL Technology Readiness Level

UDF Unducted Fan (also open fan or open rotor)

VT Virginia Tech
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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1.0 Introduction

In the SUGAR Phase I study(1), Boeing identified and analyzed advanced concepts and 

technologies for aircraft that would fly in the 2030-2035 timeframe. One of the 

recommendations from that study was to conduct a follow-on study to consider the synergistic 

benefits of methane and/or hydrogen fuel. 

Considering the Boeing results and recommendations, those of the other contractor teams, as 

well as other NASA experts, NASA developed Research Objectives for Phase II which included:

• N+4 Advanced Vehicle Concept Study to leverage the substantial investment of Phase I

and study the effect of additional technology development time beyond that assumed in 

Phase I

Boeing structured an N+4 task in the SUGAR Phase II program to address the recommendations 

from Phase I as well as the research objective provided by NASA. A summary of the N+4 task 

statement of work is included here:

1. Define advanced turbofan, hybrid electric, and open rotor engines with an Entry into 

Service (EIS) date of 2040-50

2. Study propulsion systems based on cryogenic fuels that are used to cryogenically cool 

components

• Assess how the use of cryogenic fuel enables fuel cells

• Qualitatively assess how the cryogenic technologies affect the operations, safety and 

economics of engine and aircraft designs

3. Develop a reference conventional aircraft configuration and determine its fuel burn, 

emissions, noise and takeoff performance

4. Develop an advanced unconventional aircraft configuration with an EIS date of 2040-50 

and determine its fuel burn, emissions, and noise and takeoff performance.

5. Identify advanced technologies that are most applicable to the 2040-50 timeframe and 

compatible with the reference and unconventional aircraft configurations. 

6. Using an approach similar to that in Phase I, the contractor shall evaluate and rank the 

chosen technologies and generate technology development roadmaps. 

The work in this task was structured into the subtasks shown in Figure 1.1, and the approach 

and results are discussed in the sections that follow. The results in this task are assessed against 

the updated set of NASA N+3 goals shown in Figure 1.2. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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Figure 1.1 – SUGAR N+4 Task Flow 

Figure 1.2 – NASA Noise, Emissions, Fuel, and Energy Goals
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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2.0 Technology Selection 

Based on the Phase I recommendations and the requirements of the statement of work, the 

task was begun with the following list of technologies to consider:

• Hybrid battery-gas turbine propulsion with ducted fan & open rotor

• Fuel cells

• Fuel cell-gas turbine hybrid propulsion systems

• Cryogenic fuels including methane & hydrogen

• Cryogenically cool generators, motors, converters and transmission lines

• Cryogenic fuel allowing supplemental power to be supplied by fuel cells

• Advanced batteries

• Other technologies also can get better

The Boeing Company solicited input from the Georgia Tech Aerospace Systems Design 

Laboratory (ASDL) to apply their expertise in the areas of technology planning. Working closely 

with Boeing and General Electric, ASDL modified the process utilized in Phase I to select 

advanced technologies and enhance the Phase I technology roadmaps to the extended N+4 

timeframe.

2.1 Process Overview and Background

In SUGAR Phase I, the development of the technology roadmaps was based on a clean sheet 

design. The process developed for Phase I focused on utilizing qualitative and consensus 

building techniques to identify the concepts and technologies that would be quantitatively 

analyzed. However, for SUGAR Phase II, The Boeing Company utilized a simplified spreadsheet 

based method to enable quantitative analysis of a multitude of configurations and technologies 

to help inform the N+4 workshop decisions. Based on having quantitative data on various 

concepts and technologies, a process to down select to the most promising N+4 technologies 

and concepts was developed.

The process utilized Systems Engineering techniques such as Matrix of Alternatives, MultiAttributes Decision Making (MADM) and Technique for Ordered Preference by Similarity to 

Ideal Solutions (TOPSIS) to assist in identifying promising technologies to meet the NASA goals. 

The direction provided to the team as an outcome of Phase I was to consider alternative energy 

sources and refine the N+4 configuration and technology set. The flow of the evolution of the 

SUGAR Free to the N+4 Super Refined SUGAR is depicted in Figure 2.1. 

The same Phase I Baseline SUGAR Free and Refined SUGAR configurations were used, but an 

N+4 Reference needed to be developed. This N+4 Reference configuration is an improved 

version of the conventional tube and wing Super Refined SUGAR which utilizes the gFan+ 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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engine, all applicable previous aerodynamics, subsystems, and structures N+3 technologies. It 

has a 118 foot constrained wingspan and a weight reduction relative to N+3 technologies.

Figure 2.1 – Evolution of the N+4 Reference Concept

Once the N+4 Super Refined SUGAR was established, a process was developed, Figure 2.2, to 

evaluate possible N+4 technologies which initially included:

• Hybrid battery-gas turbine propulsion (HE)

• Fuel cells (FC)

• Fuel cell-gas turbine hybrid propulsion systems

• Cryogenic fuels (e.g. methane & hydrogen)

• Cryogenically cool engines, generators, motors, converters and transmission lines (SC)

• Cryogenic fuel allowing supplemental power to be supplied by fuel cells

• Advanced batteries

• Open rotor/turboprop

• Other technologies that could get better beyond the N+3 assumed level

Refined 

SUGAR

Super

Refined 

SUGAR

SUGAR

Free

N+4 Super

Refined 

SUGAR

Span 

Trades

N+4 Super

Refined 

SUGAR

(High Fidelity) 

N+4

Structures

Tech

Final selection 

based on trade 

results
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 2.2 – N+4 Configuration Analysis Flow

A number of technology combinations were developed, a subset of which is depicted in Figure 

2.3. The spreadsheet analysis described previously was used to quantitatively assess these 

various technology and vehicle options with respect to the NASA goals. This acted as a

screening exercise which filtered the concepts and technologies to be scored during the 

workshop. A set of metrics was established to quantitatively compare different concepts which

included block energy, global emissions, landing and takeoff (LTO) emissions, noise, cost, and 

technology maturation risk. As a result of the pre-screening, 4 major configurations were 

identified and included a reference N+4 system, a conventional fuel system, a hydrogen fueled 

system, and a methane-natural gas system. Consideration was also given to how to incorporate 

more noise shielding as the configuration was refined.

N+4 Super

Refined 

SUGAR

Methane

N+4 Super

Refined 

SUGAR

(High Fidelity) 

N+4

HE

N+4

SUGAR

Freeze?

(High Fidelity) 

H2

H2/JP

Tail

BLI 

Device

#1 Noise

Shielded

#2 Noise

Shielded

SC FC +

Batteries

H2/JP

HE SC

#3 Noise

Shielded

Other

Combinations

Select
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 2.3 – N+4 Configuration Initial Screening – Technologies and Options

2.2 Pre-workshop activities

The team developed a number of pre-workshop activities that were conducted over a series of 

Webex teleconferences. An initial pre-workshop kickoff Webex was held on May 18, 2011 to 

frame the workshop context and introduce NASA personnel and the team to prior analysis and 

the game plan moving forward. The agenda of the kickoff included: 

• Futurist vision presentation on different energy scenarios

• Review of initial concepts and technology screening work and technology performance 

assumptions for propulsion core and fan efficiencies, laminar flow, and structural 

weight, cryo tanks, fuel cells. Metrics for concept evaluation were also discussed.

• Overview of the process approach during the workshop

• Identify pre-workshop assignments/actions for the participants

• Recommended adjustments to technology assumptions

• Information on alternate configurations to share 

As a follow up to the kickoff Webex, two subsequent Webex meetings were held on June 1 and 

June 15, 2011. The primary focus of these telecons was to: 

• Review of any pre-work from the participants

Propulsion Technologies

Increased Fan and

Core Efficiency

Boundary

Layer

Ingestion

Hybrids,

Gas Turbine

& Electric

Fuel

Cells

Advanced

Batteries, Ultra

Capacitors

Energy

Source:

Energy

Storage on

Aircraft:

Thrust:

Fuel Cell & Electric

Gas Turbine Motor

Conversion

to Mechanical

Energy: Electric Motor

Jet Fuel Liquid Hydrogen Battery

Biomass, Open

Tank Algae

Closed

Algae

Oil

Well

Wind, Solar

and Nuclear

Open Rotor

and Propfan

Geared

Fan

Fan and

Nacelle

Turboporp

Energy Flow Options

CH4

Liquid Methane

Natural Gas

Fields

LH2

Blended

Wing

Bodies,

Truss and

Strut

Bracing

Airfoil

Technology

- Slotted

- Suction

- Laminar

Advanced

Materials,

Carbon

Nanotubes

Active

Aeroelastics,

Reduced

Maneuvering

Loads

Reduced

Tail Size

Other Technologies and Design Variables

Cryo cooled

generators,

motors, etc.

Fuel cell APU

or Battery

supplimental

power

Riblets

Operational Variables and Technologies

Reduced

Cruise Speed

Frequent Cruise

Altitude Changes

NextGen, Reduced

Taxi time, etc.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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o Recommended adjustments to technology assumptions including propulsion 

system performance (GE)

o Technology discussions– Presentations were made that covered hydrogen 

technology (NASA, GT, and Boeing), distributed propulsion (Boeing and GE), and 

Low Energy Nuclear Reactors (Boeing)

o New configurations to consider – Configuration with noise shielding (Boeing) and 

previous work on configurations with hydrogen tank integration (Boeing).

• Updated detailed plan for the workshop, including definitions of metrics and scales for 

scoring during the workshop

• Updated technologies and configurations assumptions and results including more 

information of battery, fuel cell, boundary layer ingestion propulsion, and cruise Mach 

sensitivity

2.3 N+4 Workshop Process

The general process for the two day workshop is depicted in Figure 2.4. To accomplish the 

desired goal of the workshop an agenda was developed to facilitate group discussions and the 

breakouts for three sub-teams: Onsite, Virtual East, and Virtual West. The agenda for the 

workshop was:

• Workshop logistics

• Workshop process overview and objectives

• Pre-workshop activities update

• Debrief of Purdue/FAA meeting (a separate advanced aviation technology meeting)

• Group breaks into sub-teams to score and refine each concept

o Review initial concepts

o Review technology assumptions

o Individuals score each concept with respect to the metrics

o Email file to facilitator

o Facilitator will compile all scores

o Group discussion on results (play with weightings on metrics)

o Identify what technology gaps may exist

o Identify how the noise can be improved

o Refine the prioritized concept

o Create out-brief

• Onsite team discusses concepts and scoring

• Virtual East discusses concepts and scoring

• Virtual West discusses concepts and scoring

• Identification of common elements between concepts
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• All concepts discussion and ranking

• Identification of most promising concept/technology to take forward in the analysis

• Workshop wrap up and next steps

Figure 2.4 – N+4 Workshop Process Diagram 

To facilitate the sub-team scoring, a spreadsheet template was developed and included a 

qualitative scale for the metrics under consideration and each team would independently score 

each concept. A snapshot of the template is depicted in Figure 2.5. Each team was instructed to 

score each concept against the metrics. The scales utilized for the metrics were developed by 

the whole team prior to the workshop and are defined in Table 2.1. The concepts to be scored 

included:

0. Scoring is relative to SUGAR Free Baseline (737NG Equivalent)

1. Reference airplane

2. Conventional fuel/hybrid electric concept

3. Hydrogen fuel concept (H2 Burning)

4. Methane-natural gas concept (CH4 Burning)

5. Fuel cell concept (H2/FC Battery Hybrid)

6. Team selected alternate concepts, including:

a. Distributed Propulsion

b. Low Energy Nuclear Reactor (LENR)

c. H2/FC Gas Turbine Hybrid

d. Dual fuel H2/Jet-A burner

Pre-Workshop 

Tasks

Reference N+4 

Aircraft

Conventional 

Fuel 

Hydrogen Fuel 

MethaneNatural Gas

Review 

Technology 

Assumptions and 

Identify Gaps

Score Concept 

/ Technology 

Combination

Concept 

Refinement Final Concept

Concepts Under 

Consideration

Repeat for Each Concept Under 

Consideration

Noise Shielded 

Concept

Other concept?
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e. Other combinations

f. Other ideas

Table 2.1 – N+4 Workshop Scoring Metrics

Min Max

Block Energy

Fuel Burn

Electricity Used

Nuclear Power

-10 10

Global Emissions

Fuel Burn

Life Cycle CO2

Scenario 1: Current Fuel Process & Power Grid

Scenario 2: Biofuels, H2 from Water, Nuclear Power

NOx

Other Cruise Emissions

-10 10

Emissions

LTO Emissions

NOx & Other Emissions

-10 10

Noise

Takeoff Thrust

Shielding

-10 10

Cost

Energy Cost (Fuel + Electricity)

Total Cost (includes DOC)

-10 10

Technology Maturation Risk Low Very 

High
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Figure 2.5 – N+4 Workshop Score sheet Template

As part of the sub-team activities each team would need to: sketch each of the concepts, 

develop scoring rationale and any potential issues, identify any problems with determining 

individual scores and any wide differences of opinion, identify any key technologies needed to 

enable and enhance concept, and recommend an approach for the integration of noise 

shielding. To communicate the results to the entire team, each of the 3 teams would prepare 

and brief one or more slides for each concept considered. Finally, the teams would identify 

commonality amongst views and downselect to a handful of concepts and technologies to carry 

forward for the higher fidelity analysis after the workshop.

2.4 N+4 Workshop Outcomes

The work prepared prior to the workshop created tools and resources to facilitate a more 

streamlined execution of the workshop. Participation in the workshop was both in person and 

virtual; it was conducted on June 22 and 23, 2011 and consisted of personnel from Boeing, FAA, 

GE, GT, NASA, and VT. The participants were divided into three teams: Onsite, Virtual East, and 
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Virtual West, the members of each are listed in Table 2.2. The Onsite team was facilitated by 

Jimmy Tai (GT), the Virtual East by Marty Bradley (Boeing) and Michelle Kirby (GT), and the 

Virtual West by Blaine Rawdon (Boeing).

Table 2.2 – N+4 Workshop Teams

On Site Team Virtual West Team Virtual East Team

Bradley, Marty (Boeing)

Daggett, David (Boeing)

Droney, Christopher(Boeing)

Hoisington, Zachary (Boeing)

Kirby, Michelle (GT)

Murrow, Kurt (GE)

Ran, Hongjun (GT)

Nam, Teawoo (GT)

Tai, Jimmy (GT)

Hammel, Jeff (GE)

Perullo, Chris (GT)

Guynn, Mark (NASA)

Olson, Erik (NASA)

Leavitt, Larry (NASA)

Allen, Timothy (Boeing)

Cotes, Dwaine (Boeing)

Guo, Yueping (Boeing)

Foist, Brian (Boeing)

Rawdon, Blaine (Boeing)

Wakayama, Sean (Boeing)

Dallara, Emily (Boeing)

Kowalski, Ed (Boeing)

Wat, Joe (Boeing)

Robbana, Ismail (Boeing)

Barmichev, Sergey (Boeing)

Fink, Larry (Boeing)

Sankrithi, Mithra (Boeing)

White, Edward (Boeing)

Gowda, Srini (GE)

Brown, Gerald (NASA)

Wahls, Richard (NASA)

Wells, Doug (NASA)

Jeffries, Rhett (FAA)

Felder, James (NASA)

Schetz, Joe (VT)

Burley, Casey (NASA)

Sequiera, Christopher (FAA)

Martin, John (NASA)

Kapania, Rakesh (VT)

The workshop began with an overview of the process that would be used for its duration and to 

put everyone on the same page as to what their roles and expectations for participation were. 

A review of the definitions of the metrics to score was discussed and clarification questions 

were asked by a few participants to gain a clear understanding of what each metric implied. 

Subsequently, to facilitate an understanding of the concepts to score in the workshop, Boeing 

reviewed the general assumptions of the N+4 reference concept (Figure 2.6) and then each of 

the advanced concepts to be scored within the workshop. This information provided a common 

understanding for each team and an opportunity to ask any clarification questions before the 

larger group broke into sub-team activities.
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Figure 2.6 – N+4 General Assumptions

Next, the larger group was provided an overview of each of the main concepts brought to the 

workshop based on the spreadsheet analysis tool. A sample of the template used for each 

concept is provided in Figure 2.7. The modeling and technology assumptions were then 

discussed for each concept and the larger group then broke into the sub-teams to conduct the 

scoring.

Structural Assumptions (Wing) Stress at Limit Load, psi (upper) 70,000

 density (lb/in^3, upper) 0.07

 Stress at Limit Load (lower) 64,000

 density (lb/in^3, lower) 0.07

 Wing E 14,000,000

 Strut E 20,000,000

 Min Gauge Inboard (in) 0.20

Min Gauge Outboard (in) 0.10

Aerial Weight C1 (flat area) 3.45

 Aerial Weight C2 (thickness) 2.6

 Weight Assumptions Fuse Weight Factor 0.82

 Horizontal Tail Overall Wt Factor 0.825

 Vertical Tail Overall Wt Factor 0.825

Landing Gear Overall Wt Factor 0.825

Fixed Equip, operation, misc 0.95

 

 

• N+3 Technology suite (NLF, Riblets, Adv Composites, 

N+3 mission)

• N+4 structural weight factors are a minor improvement 

over N+3 assumptions:

• 7.6% improvement in wing bending material strength/weight

• 2.3% Fuselage, Landing gear and Tail weight improvements

• No change to propulsion weights

• 5.0% Reduction in all other miscellaneous items

• gFan+ turbine engine technology

• Natural laminar flow on wing upper surface, vertical and 

horizontal tails with limits for sweep, RE and shocks

• 1000 Wh/kg batteries (N+3 assumption was 750)

• Jet-A: $4.00/Gallon

• No cost complexity factors used 

• Hybrid and LH2 Production electricity cost: $.12/kWh

• Cost Outputs done for scenario 2.

• Cruise Mach: .70

Aerodynamic Assumptions Korn K (.95 adv supercrit) 0.93 

Laminar Flow Level (0-4) 2 

Wetted Area Roughness Factor 1.03 

Upsweep Drag, ft^2 0.5 

Flap Tracks, ft^5 0.5 

Gear Pods, ft^2 0.5 

Misc Base Drag, ft^2 1 

 

Propulsion Assumptions 

Electric Power Source Battery 

Battery Wh/KG 1000

Elec Motor Peak Efficiency 0.95

Wire Loss 0.99 

Motor Contoller Loss 0.98 

Gear Reduction Loss 0.985 

Generator Efficiency 0.985 

 

Configuration Fuselage Vertical Diameter, feet 12.74

Fuselage Width, feet 12.74
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Figure 2.7 – N+4 Workshop Concept Overview Template

2.4.1 Virtual East Team Summary

The participants of the Virtual East Team conducted individual scoring of each of the concepts, 

added other concepts as they saw fit, and compiled an average score for each concept against 

the metrics. Subsequently, the participants discussed the ranking results for each concept. 

The members generally agreed on the scoring for the N+4 reference concept. The primary 

technology assumptions were accepted however, additional structural efficiency was assumed 

for the N+4 timeframe. With Concept 2 (hybrid electric) the group had some problems 

determining scores based on variations of where the control volume could be drawn for the 

block energy metric. This implied that a life cycle energy study might be needed. Consensus was 

drawn on the key technologies to enable and/or enhance the concept which included: 

• Enhancing – tail cone BLI thruster

• Battery technology – some discussion that the development of high performance 

batteries would also have wide and earlier application to ground transportation

• Recommended integration of noise shielding

• Candidate for distributed propulsion (DP)

Concept 3 (hydrogen powered) also had some issues on scoring due to where the boundary of 

energy was drawn. A great deal of discussion also surrounded the costs associated with the 

infrastructure for delivering hydrogen to aircraft. The key technologies discussed included 

1.0
 N + 4 Ref, 118-ft Span

Configuration Ref Wing Area(ft^2) 1150

Wing Sweep ° 10

Tip Taper Ratio 0.173

Wing root t/c 0.178

Wing Outboard t/c 0.158

Aspect Ratio w/o wingles or rake 12.1

Fuselage Length, feet 127

Span (aero ref) (ft) 118.0

Wing MAC (ft) 11.36

Horizontal Tail Volume 1.09

Vertical Tail Volume 0.076

Initial Cruise Altitude, ft 38,000

Cruise e, total airplane, (inviscid) 1.04

Propulsion TOGW, lbs 132,280

Number of Engines 2 Operating Empty Weight (lb) 74,841

Fan Diameter, in 77 Initial Cruise Altitude, feet 38,000

Ref Mission Fuel Start Jet_A Mid-Mission Cruise L/D 21.0

Ref Mission Fuel Finish Jet_A Fuel Burn Reduction from Baseline 51%

Sea Level Static Thrust, lbs (all eng) 41,191

Electric Motor HP 0 Global Emissions

Life Cycle CO2 (Scenario 1) 25,898 Noise Considerations Core to Shaft Therm Efficiency, Max 0.470 Relative CO2 (scenario 1) 0.50 Takeoff Thrust to Meet TOFL, lbs 31,811 

Core to Shaft Therm Efficiency, cruise 0.4696 Life Cycle CO2 (Scenario 2) 12,949 Relative Takeoff Thrust 0.65 

Fan Efficiency (Fan, Inner Nacelle, End Eff.) 0.82 Relative CO2 (scenario 2) 0.50 Shielding Potential N

Propulsion System Efficiency 0.38 NOx (Cruise and LTO) 102 Leading Edge Devices Y

Propulsion Efficiency - Nacelle Drag 0.36 Relative NOx 0.40 

Cruise SFC 0.432 Water, Vapor or Contrails (lbs) 7,690 Fuel Burn and Energy Relative Water 0.50 Fuel Burn (lbs) 6,991.01 

Take-off EI NOx, 100% Power 24.72 Electricity (kWh) - 

Climbout EI NOx, 85% Power 18.96 LTO Emissions Total Energy (BTU) 129,899,986 

Approach EI NOx, 30% Power 8.8 NOx, lbs (LTO Only) 7.1 Total Energy (KWH) 38,068.16 

Idle EI NOx, 7% Power 3.84 Relative NOx 0.37 Relative Total Energy 0.50 

300

100

100

300

500

700

900

1100

1300

300 100 100 300 500 700 900 1100 1300 1500

0.0 0.2 0.4 0.6 0.8 1.0

Cost

Takeoff Thrust

LTO NOx

Water (Vapor or Contrails)

Global NOx

CO2 (Scenario 2)

CO2 (Scenario 1)

Energy in Flight

Metrics, Relative to SUGAR Free

Cost Key:
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controlling the droplet size of water emissions and locations of the hydrogen tanks for 

safety/certification issues.

Concept 4 (methane powered) also had some issues on scoring due to where the boundary of 

energy was drawn; this aspect emerged as a consistent theme. Again, a great deal of discussion 

also surrounded the costs associated with the infrastructure for delivering methane as 

compared to hydrogen to the aircraft and also of the risk between the two concepts. Methane 

was deemed to carry less risk. No key technologies were identified. 

Concept 5 (hydrogen/fuel cell hybrid) also had some issues on scoring due to where the 

boundary of energy was drawn. Again, a great deal of discussion also surrounded the costs 

associated with the infrastructure for delivering hydrogen to the aircraft. Consensus was drawn 

on the key technologies to enable and/or enhance the concept and included:

• Fuel cell efficiency = 50% Water produced can be stored and dumped rather than put 

into contrails

• Enhancing – tail cone BLI thruster

Of the first 5 concepts, 2, 3, and 5 all scored about the same with the assumption that the 

delivery of the energy from the source was “green”. However, the “green” assumption also 

suggested the need for an energy life cycle study that extends the control volume for energy

beyond the vehicle. The methane concept (#4) was the best cost solution but had an overall 

lower score.

As a result, the team went through a discriminator discussion to identify the real differences 

between each of the concepts in terms of advantages and concerns, which included aspects 

listed below. General consensus could not be reached amongst the group and a straw poll vote 

was conducted; the hybrid electric concept was the winner of the first five concepts.

• Conventional fuel/hybrid electric concept

o Concerns:

 How to recharge the batteries at gate

 Battery lifetime

 Battery performance (can it be achieved?) 

o Advantages

 Energy conversion better over hydrogen from the gate

 Asks least from grid? Depends on flight patterns and hybrid usage?

 Better global efficiency over others in terms of fleet wide load over time

• Hydrogen fuel concept (pure H2 burner)

o Concerns:
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 Conversion of electricity to shaft power (50% at gate then 50% on shaft)

o Advantages

 Pumping system in place at gate

 Easier averaging of the power load on grid: off peak storage – cheaper

 Less technology risk once we know how to store safely on airplane

 If electricity were green and free, this might be the best?

• Fuel cell concept (H2/FC Battery Hybrid)

o Concerns:

 Battery life time

 Battery performance (can it be achieved)

 Weight

 Requires development of two different energy source technologies

o Advantages

 Easier averaging of the power load on grid (using H2): off peak storage – 

cheaper

 Easier to capture water at altitude

The group continued the discussion of the individual scoring for additional concepts added by 

group members: Concepts 7, 8, and 9, which were the distributed propulsion (DP), low energy 

nuclear reactor (LENR), and the turboprop concepts respectively. For Concept 7, the group 

assumed incremental improvements over the N+4 reference concept and identified that there 

may exist some technical risks associated with the DP implementation. Consensus was drawn 

on the key technologies to enable and/or enhance the concept and included:

• BLI – Some concern over technology risk (how well will it really work?) 

• Wing tip propulsor integration to reduce induced drag

• Low loss mechanical or electrical power distribution

Concept 8 (LENR) had the same issue with being able to draw the boundary on energy. The 

group identified that the LENR concept could have tremendous benefits, but the technical risks 

are extremely high. Lastly, Concept 9 (turboprop) also showed some benefit over the N+4 

reference concept, but the group identified that a low noise propeller design was needed. The 

team then compared the three concepts side by side and concluded:

• LENR nuclear has important advantages, but extremely high risk – if it works, 

revolutionary to World energy 

• DP distributed propulsion is enhancing to multiple concepts if it works as advertised

• TP turboprop scorers were worried about noise
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As a result of the Virtual East breakout team, the group provided the scores and rankings (with 

and without risk included) of each concept to the larger group as depicted in Figure 2.8. 

Figure 2.8 – Virtual East Team Scoring

2.4.2 Virtual West Team Summary

The participants of the Virtual West Team also conducted individual scoring of each of the 

concepts and then added other concepts as they saw fit. They compiled the results as an 

average score for each concept against the metrics. Subsequently, the participants discussed 

the ranking results for each concept. Virtual West scored the required 5 concepts and then 

added additional ideas from the group. The list of concepts scored included:

• N+4 Reference Airplane

• Conventional fuel/hybrid electric concept

• Hydrogen fuel concept (pure H2 burner)

• Methane-natural gas concept (pure CH4 burner)

• Fuel cell concept (H2/FC Battery Hybrid)

• SUGAR High TurboProps:

o Jet A

o Pure H2 burner

o H2/FC Battery Hybrid

o Pure battery-electric

Person 1 Concept Names Block 

Energy

Global 

Emissions

LTO 

Emissions Noise Cost Technology Maturity Risk

Concept # Weighting factor 1 1 1 1 1 1

0 Sugar Free 0 0 0 0 0 15

1 N+4 Reference Airplane 4.83 4.83 5.83 2.83 2.83 8.67

2 Conventional fuel/hybrid electric concept 6.43 7.43 7.00 3.14 2.71 3.86

3 Hydrogen fuel concept (pure H2 burner) 5.00 6.83 9.17 3.17 2.67 3.50

4 Methane-natural gas concept (pure CH4 burner) 5.00 4.17 8.17 3.00 3.00 4.83

5 Fuel cell concept (H2/FC Battery Hybrid) 5.17 7.33 9.83 3.33 1.83 2.67

6 Distributed propulsion (DP) 5.60 7.20 7.00 4.30 2.60 3.20

7 LENR 7.00 9.80 9.80 3.00 0.60 -2.40

8 Turboprop 4.80 4.80 5.00 0.00 2.00 9.80

Final 

Score 

with risk

k

15.0

 29.8

 30.6

 30.3

 28.2

 30.2

 24.3

20.8

21.6

Person 

1 Concept Names Block 

Energy

Global 

Emission

s

LTO 

Emission

s

Noise Cost

Technolog

y Maturity 

Risk

Final 

Score 

w/o 

Risk

Concept 

# Weighting factor 1 1 1 1 1 1

0 Sugar Free 0 0 0 0 0 15 0.0

1 N+4 Reference Airplane 4.83 4.83 5.83 2.83 2.83 8.67 21.2

2 Conventional fuel/hybrid electric concept 6.43 7.43 7.00 3.14 2.71 3.86 26.7

3 Hydrogen fuel concept (pure H2 burner) 5.00 6.83 9.17 3.17 2.67 3.50 26.8

4 Methane-natural gas concept (pure CH4 burner) 5.00 4.17 8.17 3.00 3.00 4.83 23.3

5 Fuel cell concept (H2/FC Battery Hybrid) 5.17 7.33 9.83 3.33 1.83 2.67 27.5

6 Distributed propulsion (DP) 5.60 7.20 7.00 4.30 2.60 3.20 26.7

7 LENR 7.00 9.80 9.80 3.00 0.60 -2.40 30.2

8 Turboprop 4.80 4.80 5.00 0.00 2.00 9.80 16.6
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• LENR-powered via heat turbines

• Distributed Propulsion Hybrid-Electric

• Dual Fuel H2/Jet-A 

For the Virtual West team there was a slight deviation in how the scoring was conducted, which 

was later streamlined with the approach taken by the Onsite and Virtual East teams. However, 

the team members generally agreed on the combined scores. 

The Virtual West team identified a number of additional enhancing technologies for each of the 

concepts they scored that could be considered going forward. The list of potential 

enhancing/required technologies for each concept is listed in Table 2.3. 
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Table 2.3 – Virtual West Team Technologies per Concept

Concept Technologies

N+4 Reference Airplane

Composite structure

Laminar flow

Riblets

Efficient engines

Quiet landing gear and high lift system

Conventional fuel/hybrid 

electric concept

N+4 Reference technologies

Strut braced wing

Batteries

Hybrid-electric-gas-turbine engines

Use more battery power for takeoff noise & LTO emissions

Hydrogen fuel concept

(pure H2 burner)

N+4 Reference technologies

Hydrogen propulsion system

Clean, large-scale hydrogen production

Could be strut-braced high wing

Methane-natural gas 

concept

(pure CH4 burner)

N+4 Reference technologies

Methane-natural gas propulsion system

Methane storage infrastructure

Could be strut-braced high wing

Fuel cell concept

(H2/FC Battery Hybrid)

N+4 Reference technologies

Hydrogen propulsion system

Fuel cells

Electric motors

Batteries

Clean, large-scale hydrogen production

Could be strut-braced high wing

SUGAR High TurboProps 

with Jet A

N+4 Reference technologies

High-speed propellers

Quiet propellers

Efficient turboshaft engine

Strut-braced wing

SUGAR High TurboProps 

with Pure H2 burner

SUGAR High Turboprop technologies

Hydrogen Fuel Concept technologies

SUGAR High TurboProps 

with H2/FC Battery Hybrid

Hydrogen Fuel Cell Concept technologies

SUGAR High Turboprop technologies

Variable speed propellers because of electric motor drive* 

SUGAR High TurboProps 

with Pure battery-electric 

SUGAR High Turboprop technologies

Electric motors

Batteries (especially important for this concept)

Variable speed propellers because of electric motor drive* 

LENR-powered via heat 

turbines

LENR

Flight weight

Conversion of heat to mechanical power

Electric generation via gas or steam turbine?

Hot fluid transfer to heat exchanger in core?

Possible need for radioactive shielding
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Concept Technologies

Distributed Propulsion 

Hybrid-Electric

Hybrid Electric Concept

Propulsion integration

Efficient flight weight electric generator

Explore more battery power to reduce LTO emissions and 

noise

Explore reduced fan pressure ratio

Explore reduced mixing length from small diameter nacelles

Dual Fuel H2/Jet-A 

N+4 Reference technologies

Hydrogen / Jet-A propulsion system

Clean, large-scale hydrogen production

* propulsive efficiency and acoustic benefit

The Virtual West team also identified the same general issues as the Virtual East team in the 

understanding of the control volume for the block energy scoring. The West team also

identified that a life cycle energy study should be conducted for the various energy sources.

As a result of the Virtual West breakout team, the group provided the scores and rankings (with 

risk included) of each concept to the larger group as depicted in Figure 2.9. Concepts that had 

only 1 scorer were eliminated since there was insufficient input. As with the Virtual East team, 

the West team identified that the LENR concept provided the highest payoff.

Figure 2.9 – Virtual West Team Scoring

2.4.3 Onsite Team Summary

The participants of the Onsite Team conducted a group scoring of each of the concepts and 

then added other concepts as they saw fit and then compiled as an average score for each 

concept against the metrics. Subsequently, the participants discussed the ranking results for 

each concept. 

As a result of the Onsite breakout team, the group provided the scores and rankings (with risk 

included) of each concept to the larger group as depicted in Figure 2.10. During the outbrief, 

the Onsite team suggested the possibility of a hybrid between concepts 4, 7, and 8 might be a 

viable option. The Onsite team also identified the LENR concept as the highest payoff, but with 

an associate high risk.

Person 1 Concept Names Block Energy Global 

Emissions

LTO 

Emissions Noise Cost Technology Maturity Risk

Final 

Score with 

risk

Concept # Weighting factor 3 3 1 1 1 0.666666667

0 Sugar Free 0

1 N+4 Reference Airplane 4.50 5.05 5.38 1.13 2.17 10.33 44.22

2 Conventional fuel/hybrid electric concept 6.03 6.62 6.33 1.55 0.75 6.33 50.81

3 Hydrogen fuel concept (pure H2 burner) 4.22 5.80 8.42 1.68 1.32 5.17 44.91

4 Methane-natural gas concept (pure CH4 burner) 4.37 5.17 8.12 1.58 2.22 6.67 44.96

5 Fuel cell concept (H2/FC Battery Hybrid) 3.80 5.33 9.00 1.88 -0.35 3.67 40.38

6 SUGAR High TurboProp (Jet A) 6.00 6.00 7.00 2.00 2.67 8.67 52.33

7 LENR-powered via heat turbines 5.50 8.50 8.00 2.50 0.00 5.50 57.50

8 (6a DP) Distributed propulsion 6.50 7.00 6.50 2.00 2.00 4.00 48.50
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Figure 2.10 – Onsite Team Scoring

2.5 N+4 Workshop General Observations, Recommendations, and 

Inspirations

After each sub-team conducted the breakout sessions and then presented the outbriefs to the 

whole group, the group identified some common themes amongst the sub-team observations 

that evolved into general observations of the entire concept scoring activity, specifically:

• Hybrid electric scored high from each team, which confirmed the selection of the 

concept for the current work scope in Phase II, Task 2.2

• General concern over the definition of control volume with block energy

• LENR high payoff, but high risk

• Methane concept identified as a low risk by all groups

• Participants identified that a struggle of the scoring of the concepts really revolved 

around:

o Source of power

o How it is converted

o How to use that power

As a result of the group discussion, the workshop focus shifted the expected outcome to picking 

a concept and then subsequently identifying what power application should be used; a 

summary of the result and recommendations from the group is outlined below:

1) LENR – Very high payoff/very high risk. Recommend small study to set goals and watch 

tech feasibility and development

2) Positive consensus on Hybrid Electric – validation of Phase I selection. Already covered 

in SUGAR Tasks 2.2 and 3.3 (except see energy study)

3) Energy study – Life Cycle source to use (H2 or electricity). Estimate electricity use at 

typical airport. Supports both electric battery charging and H2 production.

4) Hydrogen – Significant benefits and challenges

• Because H2 aircraft have been studied extensively in the past, we recommend 

expanding other areas of the technology space 

• H2 infrastructure and some technologies should be worked outside of this study 

Concept Names Block Energy Global 

Emissions LTO Emissions Noise Cost Technology Maturity Risk

Final 

Score

Concept # Weighting factor 2 10 5 4 8 0

0 Sugar Free 0 0 0 0 0 15 0

1 N+4 Reference Airplane 5 5 6 2 3 9 122

2 Conventional fuel/hybrid electric concept 7 8 7 3 3 5 165

3 Hydrogen fuel concept (pure H2 burner) 5 6 9 2 3 5 147

4 Methane-natural gas concept (pure CH4 burner) 5 5 9 2 4 9 145

5 Fuel cell concept (H2/FC Battery Hybrid) 5 6 10 3 1 3 140

6 Low Energy Nuclear Reactor 2 10 10 3 5 1 206

7 GT w/ SOFC Topping Cycle 6 7 9 2 3 3 159

8 Noise Optimized Propeller 6 6 7 4 3 9 147
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• Many H2 cryo aspects will be covered in recommended LNG/methane work below 

• See also energy study above

5) Methane – Low cost and possible early deployment of cryo techs

• Methane GT SOFC driving a generator with variable speed pitch low noise props ... or 

... Methane GT SOFC Hybrid with low noise turboprop 

• Methane as first step on a roadmap for a cryo fuel / superconducting

• GE to check on providing Methane GT and Methane GT SOFC cycle for N+4 task

6) Combined Approach to N+4 technology/config assessment: 

• Adv. Tech Configuration with integrated synergistic technologies

• Aft fuselage BLI integration – synergy with methane GT SOFC to drive aft electric 

fan (Goldschmied-like device)

• Technologies that are evaluated separately and could be combined into the Adv. 

Tech Configuration (or others)

• Low noise props – investigate variable RPM and shape memory alloys, plasma 

actuators?

As a result of the workshop recommendations, a number of side studies were identified to help 

the group conclude on a possible N+4 concept to pass to the higher fidelity analysis. The group 

called these inspiration ideas that composed a wish list of research that could possibly be 

conducted within the scope of the current SOW: 

1) LENR 

• Study to set goals

• Watch tech feasibility and development

• Investigate system architecture options

• Develop baseline system design and system performance targets

2) Hybrid Electric 

• Life cycle energy study

• Follow and encourage battery tech and system community

• Multiple parallel battery technology developments

3) Methane – Low cost and possible early deployment of cryo techs

• Gas turbine design issues

• Aircraft system issues & techs

• Infrastructure issues & techs

• Synergistic technologies

• Methane GT SOFC driving a generator

• Methane GT SOFC Hybrid 

• Cryo fuel / superconducting
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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4) Hydrogen

• Leverage multiple previous studies

• Life cycle energy study

• Build on methane work (GT, system, infrastructure, cryo, FC’s) 

• Gas turbine design issues & techs

• Aircraft system issues & techs

• Infrastructure issues & techs

• Synergistic technologies

• GT FC Hybrid 

• Cryo fuel / superconducting

5) Other Techs

• BLI integration

• Current BLI investigation/validation

• Aft fuselage BLI – Goldschmied-like device

• CFD, wind tunnel, and flight validation

• Low noise high cruise speed (Mach 0.65-0.7) props

• Leverage existing design tools

• Investigate variable RPM, shape memory alloys, plasma actuator technologies, 

techs from rotorcraft

From the results of the N+4 workshop, the team defined specific products to create and 

subtasks to conduct as part of the N+4 study task. 

• Figure 2.11 was developed to show how the technologies from the workshop are 

related and to illustrate the breakthrough technologies that can reduce emissions and 

environmental impacts.

• A subtask was defined to do a requirements analysis for Low Energy Nuclear Reactor 

technology (see Section 3.0) 

• A subtask was defined to develop a outline for an energy study to investigate life cycle 

energy usage for alternative fuel and energy sources for aviation ( see Section 4.0) 

• An advanced technology airplane concept was selected to be used in evaluating key N+4 

technologies including methane, boundary layer ingestion, and a fuel cell hybrid 

propulsion system (see Section 5.6). Other variations were considered including the use 

of an unducted fan/propeller (see Section 5.7).

• The list of technologies for roadmapping were selected (see Section 6.0) 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 2.11 – Relationship of N+4 Workshop Technologies

N+3 

Advanced 

Aero, 

Structures, 

Propulsion,

Subsystems,

and 

Operations 

Technologies

Hybrid 

Electric

Advanced 

Batteries

H2

Low Cost 

Clean 

Energy

Flying

LENR

High power 

density 

LENR

LNG

FC Battery 

Hybrid

FC GT 

Hybrid

Breakthrough 

Energy Storage 

(Supercaps?) 

Pure 

Electric

Lightweight 

Fuel Cells

Other Techs that could be 

integrated in multiple concepts:

• Quiet high cruise speed 

propeller design

• BLI

Breakthrough Techs

Reduced emissions & 

environmental impact
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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3.0 LENR Requirements Analysis

The idea of using a Low Energy Nuclear Reactor (LENR) was discussed at the N+4 Workshop, 

both as a ground-based source of energy to create electricity or hydrogen, and an aircraftcarried power source for primary propulsion. Given the potential of clean zero-emissions 

energy, further work was identified for both applications. Nuclear energy is a potential source 

of clean low cost energy that should be considered in a detailed energy study (see Section 4.0). 

In this section we will discuss the potential and requirements for a flying LENR application for 

aviation.

Since a LENR is essentially a source of heat, a heat engine of some kind is needed to produce 

useful work that can create an integrated propulsion system for an aircraft. It was decided to do 

a relatively simple study to determine the range of LENR and heat engine performance that 

would produce an aircraft competitive to a conventional fueled aircraft.

Some potential heat engine cycles with representative engine power to weight ratios are shown 

in Figure 3.1. Heat engine power to weight is a strong function of delta temperature from the 

LENR. Achievable LENR delta temperature is not known at this time and is beyond the scope of 

this current investigation. Nevertheless, we decided to parametrically vary the LENR and heat 

engine power per weight and apply a top level operating cost model. Even though we do not 

know the specific cost of the LENR itself, we assumed a cost of jet fuel at $4/gallon and weight 

based aircraft cost. We were able to calculate cost per mile for the LENR equipped aircraft 

compared to a conventional aircraft (Figure 3.2). Looking at the plots, one could select a point 

where the projected cost per mile is 33% less than a conventionally powered aircraft (Heat 

engine > 1 HP/lb & LENR > 3.5 HP/lb). Since the power requirements are significantly different 

at cruise compared to takeoff and climb, we also investigated a hybrid case where batteries and 

an electric motor are used to supplement the heat engine + LENR at takeoff. This yielded 

significantly improved results (Figure 3.3) which required lower LENR and heat engine 

performance levels (Heat engine > 0.4 HP/lb, LENR > 1 HP/lb, & Batteries > 225 Wh/kg).

These numbers are illustrative only, as other combinations could yield useful propulsion and 

power systems, and the results are dependent on cost and performance assumptions. 

However, the numbers should be useful in establishing initial system goals for LENR concepts.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 3.1 – Potential Heat Engines for LENR Systems

Figure 3.2 – Parametric LENR and Heat Engine Performance Parameters 

0.01

0.1

1

10

100

Wärtsilä 

tw o-stroke

Turbo Diesel

Suzuki

538 cc V2

4-stroke gas

Otto engine

502 cc gas

(petrol)

Stirling

engine

GM 6.6 L

Duramax

Turbo Diesel

engine

Mazda 13BMSP 1.3 L

Wankel

BMW V10 

gas Otto

engine

GE90-115B

Brayton

turbofan

Shuttle

Heat Engine / Heat Pump

Power to Weight (hp/lb)

Starting point for LENR 

trades

Heat engine Power to Weight (hp/lb) 

possibilities are a strong function of 

delta T from the LENR

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14 16 18 20

Reactor HP/lb

Cost Per Mile

Heat Engine HP/lb=0.6

Heat Engine HP/lb=1.0

Heat Engine HP/lb=1.5

N+4 Sugar High, Jet Fuel at $4.00/gal 

 

 

 

 

1.0

0

Requirements:

• Heat engine > 1 HP/lb

• LENR > 3.5 HP/lb
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 3.3 – Hybrid LENR + Battery Performance Parameters 

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14 16 18 20

Reactor HP/lb

Cost Per Mile

LENR + Heat Engine Only (no hybrid)

Heat Engine HP/lb: 0.6

LENR / Battery Hybrid

Heat Engine: 0.4 hp/lb Batt Wh/kg: 150, 250, 500, 1000

Heat Engine HP/lb: 1.0

Heat Engine HP/lb: 1.5

1.0

0

Requirements w/

Hybrid:

• Heat engine>

0.4 HP/lb

• LENR> 1 HP/lb

• Batt > 225 Wh/kg

Requirements w/o Hybrid:

• Heat engine> 1 HP/lb

• LENR> 3.5 HP/lb
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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4.0 Energy Study Outline Development

The idea of needing to do a life cycle energy study to answer questions and help make decisions 

about alternative aviation fuels and energy sources came out of the N+4 Workshop. Making 

some gross assumptions, we were able to determine the potential performance of aircraft 

powered by various methane, hydrogen, and electricity energy architectures. However, we had 

no way to determine if any of these approaches were beneficial when the entire aircraft and 

energy production lifecycles were considered. To answer these kinds of questions, it was 

decided that a life cycle energy study, focused on aviation specific questions, was needed. 

Doing such a study is beyond the scope of the current effort, but it was decided that it was 

worthwhile to develop an outline of what should be contained in such a comprehensive study, 

with the hope that the study will be conducted sometime in the near future. 

After discussion among the team members, we are recommending a study that includes various 

world energy scenarios to cover the likely range of possible futures and the sensitivity of the 

results to input assumptions. This study should answer questions about the life cycle usage of 

natural gas, the production of electricity and hydrogen for aviation, the best use of biofuels, 

and the impact of a breakthrough in low cost energy generation. We drew upon the results of a 

student study at Georgia Tech, unpublished work conducted at Boeing, and information from 

many public sources.(2)(3)(4)(5)(6)

We developed an initial outline and shared it outside the SUGAR team with contacts at NASA, 

the FAA (Federal Aviation Administration), MIT (Massachusetts Institute of Technology), and 

the DoE (Department of Energy). We incorporated comments and the final version of the 

Energy Study Outline is as follows:

1. Background and Motivation

a. SUGAR N+4 Results

i. Candidate alternative energy concepts

ii. Questions about supply, cost, and environmental benefits/impacts

iii. Questions about uncertainty in assumptions

2. World Energy Assumptions

a. Energy scenarios and sources

i. Scenarios should capture – High & low oil price, high and low carbon 

taxes, cheap electricity, etc. 

b. Approach to handling uncertainties in world energy supply, make-up, and cost

c. Approach to handling interactions between energy streams and impact of 

diverting energy streams to aviation

3. Metrics
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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a. Cost 

i. Methods used for aircraft, ground infrastructure, and energy costs

b. Resource availability

i. % of existing resource supply needed to supply aviation

ii. Feasibility to expand supply to meet aviation demand

c. Carbon Dioxide

i. Complete life cycle

ii. Operational use

iii. Energy production 

d. NOx

i. Methods used for local and global emissions

e. Other Emissions

i. List of other emissions (including methane released)

ii. Methods for calculating

f. Noise

i. Method for assessing and data sources

g. Health Impacts

i. Types of health impacts

ii. Methods for calculating and sources

h. Climate impacts

i. Methods for calculating

4. Case Studies

a. Natural gas/methane for ground transportation, electricity, or aviation

i. Approach with flow chart

ii. Input data and sources

iii. Quantification of methane leakage into the atmosphere

iv. Results with uncertainties and sensitivities to assumptions

b. Liquid Hydrogen, Electric battery/Jet Hybrids, at airports

i. Approach with flow chart

ii. Input data and sources

iii. Results with uncertainties and sensitivities to assumptions

c. Biomass for electricity, ground transportation, or aviation 

i. Approach with flow chart

ii. Input data and sources

iii. Results with uncertainties and sensitivities to assumptions

d. Sustainable, low-cost electricity

i. Approach with flow chart

ii. Input data and sources
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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iii. Results with uncertainties and sensitivities to assumptions

5. Conclusions

a. Discussion of case study results

b. Recommendations for aviation

6. Recommendations for future work

a. Gaps and Unknowns

b. Next steps

The SUGAR team recommends that such a study be considered for future joint funding by NASA 

and the Department of Energy or other interested parties.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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5.0 N+4 Concept Development and Analysis

The same approach used in Phase I was used to define, analyze, and compare SUGAR N+4 

concepts. The same reference mission for a medium-sized (737 class) aircraft was used (Table 

5.1). A detailed discussion of the future scenario is contained within Section 2.0 of the Phase I 

report(1). 

Table 5.1 – Phase I Future Scenario Used to Set Payload-Range Requirements

Regional Medium Large

Number of Aircraft 2,675 22,150 7,225

Family Midpoint # of Seats 70 154 300

Avg Distance 575 900 3,300

Max Distance 2,000 3,500 8,500

Avg Trips/day 6.00 5.00 2.00

Avg MPH 475 500 525

Fleet Daily Air Miles (K) 8,500 100,000 55,000

Daily Miles 3,200 4,500 7,600

Daily Hours 6.92 9.23 13.96

SUGAR Phase I and Phase II Focus

A progression of concepts were selected that will allow the quantitative evaluation of 

methane/LNG fuel, unducted fans, an LNG hybrid fuel cell, and fuselage boundary layer 

ingestion. Specific performance cases are listed in Table 5.2 and described in Sections 5.1 to 5.7. 

Results are compared and summarized in Section 5.8. 
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Table 5.2 – N+4 Performance and Sizing Runs

Case Config. 

Number Name Start Config. Fuel Engine Propulsor

1 765-093 SUGAR Free (Baseline) 765-093 JP CFM-56 Ducted 

Fan

2 765-094-TS1 N+4 Reference 765-094 JP JP+2045GT+DF Ducted 

Fan

3 765-095-TS1 N+4 High Wing 

Reference

765-095-RC 

(Task 2.1) JP JP+2045GT+DF Ducted Fan

4 765-095-TS2 SUGAR Freeze (LNG) 765-095-TS1 LNG LNG+2045GT+DF Ducted 

Fan

5 765-095-TS3 SUGAR Freeze

(LNG UDF) 765-095-TS2 LNG LNG+2045GT+UDF Unducted Fan

6 765-095-TS4 SUGAR Freeze

(LNG FC Hybrid BLI) 765-095-TS2 LNG LNG+2045GT+SOFC+BLI DF + BLI

7 765-095-TS5 SUGAR Freeze

(LNG FC Hybrid UDF) 765-095-TS3 LNG LNG+2045GT+SOFC+UDF

Unducted 

Fan

JP – Conventional Jet Fuel (Jet-A)

2045GT – N+4 Gas Turbine technology

DF – Ducted Fan

LNG – Liquified Natural Gas (Mostly Methane)

SOFC or FC – Solid Oxide Fuel Cell

UDF – Unducted Fan

BLI – Boundary Layer Ingestion

5.1 765-093 SUGAR Free (Baseline Aircraft) 

A conventional tube and wing aircraft with CFM-56 engines representative of the “N” 

timeframe of approximately 2008. See the Phase I final report(1) configuration 765-093 (Section 

5.3.1) for detailed information. This aircraft is used as the baseline for the fuel burn, energy, 

and cruise emissions goals. A minor adjustment to correct a Phase I payload sizing inconsistency 

was made to the group weight statement. The result was a 0.9% reduction in fuel burn for the 

sized configuration as compared to Phase I. Table 5.3 contains the modified group weights 

statement. All other vehicle data is the same as it was in Phase I.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Table 5.3 – 765-093 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 18,728 10.7%

BENDING MATERIAL 9,621 5.5%

SPAR WEBS 1,290 0.7%

RIBS AND BULKHEADS 1,226 0.7%

AERODYNAMIC SURFACES 3,351 1.9%

SECONDARY STRUCTURE 3,240 1.8%

TAIL 3,779 2.2%

FUSELAGE 17,597 10.0%

LANDING GEAR 6,712 3.8%

NACELLE & PYLON 5,548 3.2%

PROPULSION 11,181 6.4%

ENGINES 10,664 6.1%

FUEL SYSTEM 518 0.3%

FLIGHT CONTROLS 3,084 1.8%

COCKPIT CONTROLS 252 0.1%

SYSTEM CONTROLS 2,832 1.6%

POWER SYSTEMS 4,483 2.6%

AUXILIARY POWER UNIT 1,032 0.6%

HYDRAULICS 894 0.5%

ELECTRICAL 2,557 1.5%

INSTRUMENTS 686 0.4%

AVIONICS & AUTOPILOT 1,533 0.9%

FURNISHINGS & EQUIPMENT 10,866 6.2%

AIR CONDITIONING 1,678 1.0%

ANTI-ICING 118 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 85,993 49.0%

OPERATIONAL ITEMS 7,342 4.2%

OPERATIONAL EMPTY WEIGHT (OEW) 93,335 53.1%

USABLE FUEL 51,500 29.3%

DESIGN PAYLOAD 30,800 17.5%

TAKEOFF GROSS WEIGHT (TOGW) 175,635 100.0%

5.2 765-094-TS1 N+4 Reference Aircraft

The 765-094 is a span constrained conventional tube and wing configuration. A three view 

drawing for this configuration is included in the Phase I Final Report (1). For this study, the 

configuration was modified with a new N+4 gFan++ engine. The configuration also utilizes the

same N+3 advanced technologies used in Phase I. The gFan++ Advanced Turbofan 

(JP+2045GT+DF) engine is summarized in Appendix A. The new engine requires a new drag 

buildup and mass properties buildup for the configuration. Other changes were also rolled into 

this phase of the study including new laminar flow accounting to become consistent with the 

latest results from the recently completed Environmentally Responsible Aviation (ERA) study 

contract(7). ERA took credit for 70% (30% operational knockdown) of the calculated passive
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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laminar flow reduction and included drag increases for the passive system. For SUGAR, N+3 

aircraft are taking 85% credit and N+4 aircraft are taking 92.5% credit which represents a 

progression in the technology. The SUGAR aircraft are also focusing on natural laminar flow and 

do not take any additional penalty for a passive system. Additionally, laminar flow credit is 

carried on the horizontal, vertical, and nacelles for Phase II.

The high-speed aerodynamic buildup for the Refined SUGAR configuration is summarized in 

Table 5.4 and Figure 5.1. It should be noted that all the drag buildups in this study are 

calculated at drag divergence, not at maximum long range cruise Mach number.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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Table 5.4 – 765-094-TS1 High Speed Build-up

CONFIGURATION 765-094-TS1

AIRFOIL TYPE SUPERCRIT. DTE

F BUILD-UP (FT2

) 

FUSELAGE 9.2153

WING 8.1036

WINGLET 0.2173

HORIZONTAL 1.4215

VERTICAL 1.2158

N&P 1.8980

CANOPY 0.0405

GEAR PODS 0.0000

ETC BEFORE SUB -4.8831

EXCRESCENCE 1.6376

UPSWEEP 0.6012

WING TWIST 0.3948

ETC AFTER SUB -0.3986

FUSELAGE BUMP 0.5430

F-TOTAL (FT2

) 20.0070

E-VISC 1.00952

CRUISE CD BUILD-UP

M-CRUISE 0.74

CRUISE ALTITUDE 38408

CL-CRUISE 0.675

CD0 0.01556

CDI 0.01235

CDC 0.001245

CDTRIM 0.000595

CDTOT 0.02975

L/D 22.68952

ML/D 16.790

Figure 5.1 – 765-094-TS1 High Speed Build-up

The resulting high speed data is shown in Figure 5.2. The figure illustrates the maximum 

aerodynamic efficiency (M*L/D) occurring at a cruise Mach of 0.74 and a CL of 0.700. This is 

slightly higher than the efficiency at the Mach 0.7 cruise condition.

CD0

52%

CDI

42%

CDC

4%

CDTRIM

2%
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 5.2 – 765-094-TS1 - M * L / D Total

Figure 5.3 through Figure 5.5 show the low speed aerodynamic characteristics for the N+4 

Reference aircraft with advanced 2045 technology engines. Low speed data are trimmed as a 

function of angle of attack, lift coefficient, and drag coefficient at each flap detent. Low speed 

high lift devices on wing leading and trailing edges are deployed. As with other low speed 

buildups in this study, these polars are based on an empirical database.

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

0.200 0.300 0.400 0.500 0.600 0.700 0.800

M * L / D TOTAL

LIFT COEFFICIENT (CL) 

0.2

0.4

0.5

0.6

0.65

0.67

0.68

0.69

0.7

0.71

0.72

0.73

0.74

0.75

0.76

0.78

MACH NUMBER
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 5.3 – 765-094-TS1 - Low Speed Lift Curve; Free Air

Figure 5.4 – 765-094-TS1 - Low Speed Polar; Free Air

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

3.5000

-5.0000 0.0000 5.0000 10.0000 15.0000 20.0000 25.0000

LIFT COEFFICIENT (CL) 

ANGLE OF ATTACK (ALPHA)

0 

5 

10

15

20

25

30

35

FLAP DEFLECTION

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500

LIFT COEFFICIENT SQUARED (CL

2) 

DRAG COEFFICIENT (CD) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 5.5 – 765-094-TS1 - Low Speed Lift / Drag; Free Air

The N+4 Reference configuration weight was estimated by applying N+3 weight reduction 

factors to SUGAR Free and updating the engine component weights to be consistent with the 

2045 gFan++ engine. Table 5.5 shows the resulting group weight statement which includes each 

group’s percentage of TOGW. This weights breakdown is for an unsized configuration and is 

used to feed the sizing process.

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

0.000 0.500 1.000 1.500 2.000 2.500

LIFT TO DRAG RATIO (L/D)

LIFT COEFFICIENT (CL) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
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Table 5.5 – 765-094-TS1 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 13,780 10.1%

BENDING MATERIAL 5,754 4.2%

SPAR WEBS 994 0.7%

RIBS AND BULKHEADS 1,091 0.8%

AERODYNAMIC SURFACES 3,151 2.3%

SECONDARY STRUCTURE 2,791 2.0%

TAIL 2,676 2.0%

FUSELAGE 14,946 11.0%

LANDING GEAR 5,052 3.7%

NACELLE & PYLON 5,392 4.0%

PROPULSION 9,898 7.3%

ENGINES 9,280 6.8%

FUEL SYSTEM 618 0.5%

FLIGHT CONTROLS 3,106 2.3%

COCKPIT CONTROLS 252 0.2%

SYSTEM CONTROLS 2,853 2.1%

POWER SYSTEMS 4,211 3.1%

AUXILIARY POWER UNIT 1,014 0.7%

HYDRAULICS 901 0.7%

ELECTRICAL 2,297 1.7%

INSTRUMENTS 773 0.6%

AVIONICS & AUTOPILOT 1,504 1.1%

FURNISHINGS & EQUIPMENT 9,115 6.7%

AIR CONDITIONING 1,441 1.1%

ANTI-ICING 112 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 72,006 52.8%

OPERATIONAL ITEMS 7,207 5.3%

OPERATIONAL EMPTY WEIGHT (OEW) 79,213 58.1%

USABLE FUEL 26,399 19.4%

DESIGN PAYLOAD 30,800 22.6%

TAKEOFF GROSS WEIGHT (TOGW) 136,412 100.0%

5.3 765-095-TS1 N+4 Truss Braced Wing

This configuration, illustrated in Figure 5.6, is a high-span truss-braced wing configuration with 

the N+4 gFan++ engine and other advanced N+3 technologies. The configuration draws from 

the truss braced wing knowledge generated under the SUGAR Phase II contract Task 2.1. The 

aerodynamic, structural, and weight attributes of the configuration are currently being 

explored and the results shown in this document reflect Boeing’s current understanding of the 

aircraft. An aeroelastic FEM and high fidelity CFD are currently being run on a similar 

configuration under Task 2.1. The gFan++ Advanced Turbofan (JP+2045GT+DF) engine is 

summarized in Appendix A. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Figure 5.6 – 765-095-TS1 – Truss Braced Wing with gFan++

35’ – 0.09”

(420.09)

OVERALL

169’ – 11.50”

(2,039.50)

SPAN

193.27 188.23

~118’ – 0.00”

(1,416.00)

FOLDED SPAN

116.11

~517”

272.29 612.35

1,497

139’ – 8.68”

(1,676.68)

OVERALL

11.11°

TAIL DOWN

GEAR EXTENDED 11.5°

GEAR EXTENDED

38’ – 5.83”

(461.83)

SPAN

148.7

FS 130.00

FLOOR AT WL 200.00

LH = 863.64 

Lv = 707.12

MAC

FS 822.58

PROJECTED CHARACTERISTICS

WING

TOTAL

V-TAIL

TRAP

H-TAIL

TRAP

AREA (SQ. FT) 1477.11 297.68 296.23

ASPECT RATIO 19.55 1.00 5.00

SPAN (INCHES) 2039.30 207.04 461.83

TAPER RATIO 0.35 1.00 0.35

MAC (INCHES) 110.29 207.04 99.50

DIHEDRAL (DEG.) -1.50 -- -3.00

1⁄4 CHORD SWEEP (DEG.) 12.52 41.00 25.30

ROOT CHORD (INCHES) 130.31 207.04 136.84

TIP CHORD (INCHES) 45.13 207.04 47.89

TAIL VOLUME COEFFICIENT -- 0.07 1.57
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

40

The high-speed drag buildup for SUGAR N+4 TBW configuration is shown in Table 5.6 and Figure 

5.7. This buildup is assisted by the work completed under Task 2.1, however, the CASES 

empirical database being used to generate the data is still being extrapolated.

Table 5.6 – 765-095-TS1 High Speed Build-up

CONFIGURATION 765-095-TS1

AIRFOIL TYPE SUPERCRIT. DTE

F BUILD-UP (FT2) 

FUSELAGE 8.8378

WING 10.3240

STRUT 2.7291

JURY STRUT 0.2519

HORIZONTAL 1.9266

VERTICAL 1.7487

N&P 1.9020

CANOPY 0.0405

GEAR PODS 3.1393

ETC BEFORE SUB -7.9462

EXCRESCENCE 1.9947

UPSWEEP 0.3414

WING TWIST 0.1640

ETC AFTER SUB -1.4622

FUSELAGE BUMP 0.3675

F-TOTAL (FT2) 24.3590

E-VISC 0.93071

CRUISE CD BUILD-UP

M-CRUISE 0.73

CRUISE ALTITUDE 44000

CL-CRUISE 0.775

CD0 0.01649

CDI 0.01048

CDC 0.002058

CDTRIM 0.000592

CDTOT 0.02962

L/D 26.16257

ML/D 19.099

Figure 5.7 – 765-095-TS1 High Speed Build-up

The ETC BEFORE SUB category includes technology projections for natural laminar flow over a 

portion of the wing, strut, and vertical tail, horizontal tail, nacelles as well as riblets applied to 

CD0

56%

CDI

35%

CDC

7%

CDTRIM

2%
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

41

the turbulent portion of the wing and the fuselage. ETC AFTER SUB includes a technology 

projection for advanced supercritical airfoils with divergent trailing edge. In addition, 

technologies for low interference nacelles and strut/brace were included in the parasite 

buildup.

The resulting high speed data is shown in Figure 5.8. The figure illustrates the maximum 

aerodynamic efficiency (M*L/D) occurring at a cruise Mach of 0.73 and a CL of 0.775. 

Figure 5.8 – 765-095-TS1 - M * L / D Total

Figure 5.9 through Figure 5.11 show the low speed characteristics for the 765-095-TS1. Low 

speed data are trimmed as a function of angle of attack, lift coefficient, and drag coefficient at 

each flap detent. Low speed high lift devices on wing leading and trailing edges are deployed.

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0.200 0.300 0.400 0.500 0.600 0.700 0.800

M * L / D TOTAL

LIFT COEFFICIENT (CL) 

0.2

0.4

0.5

0.6

0.65

0.67

0.68

0.69

0.7

0.71

0.72

0.73

0.74

0.75

0.76

0.78

MACH NUMBER
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

42

Figure 5.9 – 765-095-TS1 - Low Speed Lift Curve; Free Air

Figure 5.10 – 765-095-TS1 - Low Speed Polar; Free Air

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

3.5000

-5.0000 0.0000 5.0000 10.0000 15.0000 20.0000 25.0000

LIFT COEFFICIENT (CL) 

ANGLE OF ATTACK (ALPHA)

0 

5 

10

15

20

25

30

35

FLAP DEFLECTION

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000

LIFT COEFFICIENT SQUARED (CL

2) 

DRAG COEFFICIENT (CD) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

43

Figure 5.11 – 765-095-TS1 - Low Speed Lift / Drag; Free Air

The weight for the SUGAR N+4 TBW configuration was estimated by applying N+3 weight 

reduction factors to a calibrated model. The wing was weighed using a station based analysis 

with Task 2.1 MDO generated loads and empirical allowables adjusted for technology. No 

penalty was applied for flutter. Table 5.7 shows the subsystem weights and their percentages of 

TOGW for the as-drawn analyzed weight.

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

18.000

0.000 0.500 1.000 1.500 2.000 2.500 3.000

LIFT TO DRAG RATIO (L/D)

LIFT COEFFICIENT (CL) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

44

Table 5.7 – 765-095-TS1 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 19,940 14.2%

BENDING MATERIAL 7,399 5.3%

SPAR WEBS 1,850 1.3%

RIBS AND BULKHEADS 1,850 1.3%

AERODYNAMIC SURFACES 4,993 3.6%

SECONDARY STRUCTURE 3,849 2.7%

TAIL 3,157 2.3%

FUSELAGE 16,554 11.8%

LANDING GEAR 4,706 3.4%

STRUT, JURY STRUT, AND INSTALLATION 5,392 3.9%

NACELLE & PYLON 2,399 1.7%

PROPULSION 10,008 7.1%

ENGINES 9,280 6.6%

FUEL SYSTEM 728 0.5%

FLIGHT CONTROLS 2,683 1.9%

COCKPIT CONTROLS 252 0.2%

SYSTEM CONTROLS 2,431 1.7%

POWER SYSTEMS 4,078 2.9%

AUXILIARY POWER UNIT 1,014 0.7%

HYDRAULICS 767 0.5%

ELECTRICAL 2,297 1.6%

INSTRUMENTS 773 0.6%

AVIONICS & AUTOPILOT 1,504 1.1%

FURNISHINGS & EQUIPMENT 9,115 6.5%

AIR CONDITIONING 1,441 1.0%

ANTI-ICING 123 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 81,873 58.5% 

OPERATIONAL ITEMS 7,207 5.1% 

OPERATIONAL EMPTY WEIGHT (OEW) 89,080 63.6%

USABLE FUEL 20,120 14.4%

DESIGN PAYLOAD 30,800 22.0% 

TAKEOFF GROSS WEIGHT (TOGW) 140,000 100.0%

5.4 765-095-TS2 N+4 Truss Braced Wing with LNG Gas Turbine

This configuration is the same as the 765-095-TS1 but includes a fuselage stretch (Figure 5.12) 

to accommodate LNG tanks in front of and behind the passenger section. The forward LNG tank 

is cylindrical to allow passage with minimum clearances to the flight deck. Safety and 

certification of the installation may be a challenge and could drive significant configuration 

changes. At a minimum, a vapor barrier would be required to prevent methane leakage into the 

passenger cabin. A second pressure bulkhead may be required between the methane tank and 

the passenger cabin. The configuration is currently assumed to take minimal penalty for the 

integration of the forward tank. Further research is required to understand the tank integration 

penalties.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

45

The forward tank integration requires a forward constant section extension. The aft tank 

requires lengthening and reshaping of the upsweep region. The aft constant section cannot be 

stretched because the airplane would no longer conform to the tail strike requirement carried 

by the other concepts. The overall stretch required is illustrated in Figure 5.12. 

Figure 5.12 – Fuselage Stretched to Accommodate LNG Tankage 

The increased fuselage length penalty is partially offset by a reduction in tail area for a given tail 

volume coefficient. The final N+4 integrated solution is shown in Figure 5.13. 

For a description of the LNG propellant system, see Section 6.2.8, LNG and Hydrogen Aircraft 

Systems. 

An overview of the LNG fueled gFan++ advanced turbofan (LNG+2045GT+DF) is provided in 

Appendix A. 

1,497.00

69.57

Constant Section Stretch

1,671.57

(174.57 Overall Stretch)

105.00

Aft Fuselage Stretch

Reshaped Aft Fuselage

765-095-TS1

765-095-TS2
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

46

Figure 5.13 – 765-095-TS2 – Truss Braced Wing With LNG gFan++

34’ – 9.48”

(417.48)

OVERALL

681.88

1,671.57

153’ – 1.93”

(1,837.93)

OVERALL

11.5°

GEAR EXTENDED

169’ – 11.50”

(2,039.50)

SPAN

193.27 188.23

~118’ – 0.00”

(1,416.00)

FOLDED SPAN

116.11

~517”

11.11°

TAIL DOWN

GEAR EXTENDED

36’ – 5.54”

(437.54)

SPAN

148.7

FS 60.43

FLOOR AT WL 200.00

LH = 962.20 

Lv = 816.66

272.29

MAC

FS 822.58

PROJECTED CHARACTERISTICS

WING

TOTAL

V-TAIL

TRAP

H-TAIL

TRAP

AREA (SQ. FT) 1477.11 257.76 265.89

ASPECT RATIO 19.55 1.00 5.00

SPAN (INCHES) 2039.30 192.66 437.54

TAPER RATIO 0.35 1.00 0.35

MAC (INCHES) 110.29 192.66 94.27

DIHEDRAL (DEG.) -1.50 -- -3.00

1⁄4 CHORD SWEEP (DEG.) 12.52 41.00 25.30

ROOT CHORD (INCHES) 130.31 192.66 129.64

TIP CHORD (INCHES) 45.13 192.66 45.37

TAIL VOLUME COEFFICIENT -- 0.07 1.57
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

47

The high-speed drag buildup, a small change from the 765-095-TS1, is shown in Table 5.8 and 

Figure 5.14. 

Table 5.8 – 765-095-TS2 High Speed Build-up

CONFIGURATION 765-095-TS2 

AIRFOIL TYPE SUPERCRIT. DTE

F BUILD-UP (FT2) 

FUSELAGE 9.4840

WING 10.3240

STRUT 2.7291

JURY STRUT 0.2519

HORIZONTAL 1.7482

VERTICAL 1.5327

N&P 1.9020

CANOPY 0.0405

GEAR PODS 3.1393

ETC BEFORE SUB -7.9839

EXCRESCENCE 1.9809

UPSWEEP 0.4870

WING TWIST 0.1640

ETC AFTER SUB -1.4622

FUSELAGE BUMP 0.3675

F-TOTAL (FT2) 24.7049

E-VISC 0.95206

CRUISE CD BUILD-UP

M-CRUISE 0.73

CRUISE ALTITUDE 44000

CL-CRUISE 0.775

CD0 0.01673

CDI 0.01025

CDC 0.002018

CDTRIM 0.000592

CDTOT 0.02958

L/D 26.19685

ML/D 19.124

Figure 5.14 – 765-095-TS2 High Speed Build-up

The resulting high speed data is shown in Figure 5.8. The figure illustrates the maximum 

aerodynamic efficiency (M*L/D) occurring at a cruise Mach of 0.73 and a CL of 0.775. 

CD0

56%

CDI

35%

CDC

7%

CDTRIM

2%
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

48

Figure 5.15 – 765-095-TS2 - M * L / D Total

Figure 5.16 through Figure 5.18 show the low speed characteristics for the 765-095-TS2. Low 

speed data are trimmed as a function of angle of attack, lift coefficient, and drag coefficient at 

each flap detent. Low speed high lift devices on wing leading and trailing edges are deployed.

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0.200 0.300 0.400 0.500 0.600 0.700 0.800

M * L / D TOTAL

LIFT COEFFICIENT (CL) 

0.2

0.4

0.5

0.6

0.65

0.67

0.68

0.69

0.7

0.71

0.72

0.73

0.74

0.75

0.76

0.78

MACH NUMBER
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

49

Figure 5.16 – 765-095-TS2 - Low Speed Lift Curve; Free Air

Figure 5.17 – 765-095-TS2 - Low Speed Polar; Free Air

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

3.5000

-5.0000 0.0000 5.0000 10.0000 15.0000 20.0000 25.0000

LIFT COEFFICIENT (CL) 

ANGLE OF ATTACK (ALPHA)

0 

5 

10

15

20

25

30

35

FLAP DEFLECTION

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000

LIFT COEFFICIENT SQUARED (CL

2) 

DRAG COEFFICIENT (CD) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

50

Figure 5.18 – 765-095-TS2 - Low Speed Lift / Drag; Free Air

The weight (Table 5.9) for this configuration was generated starting from the 765-095-TS1 and 

adding a fuselage stretch and the cryogenic fuel system.

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

18.000

0.000 0.500 1.000 1.500 2.000 2.500 3.000

LIFT TO DRAG RATIO (L/D)

LIFT COEFFICIENT (CL) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

51

Table 5.9 – 765-095-TS2 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 21,330 13.7%

BENDING MATERIAL 8,197 5.3%

SPAR WEBS 2,049 1.3%

RIBS AND BULKHEADS 2,049 1.3%

AERODYNAMIC SURFACES 5,113 3.3%

SECONDARY STRUCTURE 3,921 2.5%

TAIL 2,852 1.8%

FUSELAGE 19,304 12.4%

LANDING GEAR 5,300 3.4%

NACELLE & PYLON 5,392 3.5%

STRUT, JURY STRUT, AND INSTALLATION 2,399 1.5%

PROPULSION 15,753 10.1%

ENGINES 9,280 5.9%

FUEL SYSTEM 6,473 4.1%

FLIGHT CONTROLS 2,753 1.8%

COCKPIT CONTROLS 254 0.2%

SYSTEM CONTROLS 2,500 1.6%

POWER SYSTEMS 4,276 2.7%

AUXILIARY POWER UNIT 1,039 0.7%

HYDRAULICS 789 0.5%

ELECTRICAL 2,447 1.6%

INSTRUMENTS 818 0.5%

AVIONICS & AUTOPILOT 1,603 1.0%

FURNISHINGS & EQUIPMENT 10,300 6.6%

AIR CONDITIONING 1,564 1.0%

ANTI-ICING 123 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 93,765 60.1% 

OPERATIONAL ITEMS 7,803 5.0% 

OPERATIONAL EMPTY WEIGHT (OEW) 101,569 65.1%

USABLE FUEL 23,631 15.1% 

DESIGN PAYLOAD 30,800 19.7% 

TAKEOFF GROSS WEIGHT (TOGW) 156,000 100.0%

5.5 765-095-TS3 N+4 Truss Braced Wing with LNG Unducted Fan

This configuration (Figure 5.19) is the same as the 765-095-TS2, but with an unducted fan. The 

LNG fueled gFan++ powerplant with an unducted fan propulsor (LNG+2045GT+UDF) is 

discussed in Appendix A. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

52

Figure 5.19 – 765-095-TS3 - Truss Braced Wing With LNG UDF

34’ – 9.48”

(417.48)

OVERALL

681.88

1,671.57

153’ – 1.93”

(1,837.93)

OVERALL

11.5°

GEAR EXTENDED

169’ – 11.50”

(2,039.50)

SPAN

193.27 188.23

~118’ – 0.00”

(1,416.00)

FOLDED SPAN

99.84

~517”

11.11°

TAIL DOWN

GEAR EXTENDED

36’ – 5.54”

(437.54)

SPAN

148.7

FS 60.43

FLOOR AT WL 200.00

LH = 962.20 

Lv = 816.66

272.29

MAC

FS 822.58

PROJECTED CHARACTERISTICS

WING

TOTAL

V-TAIL

TRAP

H-TAIL

TRAP

AREA (SQ. FT) 1477.11 257.76 265.89

ASPECT RATIO 19.55 1.00 5.00

SPAN (INCHES) 2039.30 192.66 437.54

TAPER RATIO 0.35 1.00 0.35

MAC (INCHES) 110.29 192.66 94.27

DIHEDRAL (DEG.) -1.50 -- -3.00

1⁄4 CHORD SWEEP (DEG.) 12.52 41.00 25.30

ROOT CHORD (INCHES) 130.31 192.66 129.64

TIP CHORD (INCHES) 45.13 192.66 45.37

TAIL VOLUME COEFFICIENT -- 0.07 1.57
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

53

The aerodynamic buildup (Figure 5.20 and Table 5.10) accounts for a decreased portion of wing 

laminar flow due to the propulsion system wake.

Table 5.10 – 765-095-TS3 High Speed Build-up

CONFIGURATION 765-095-TS3 

AIRFOIL TYPE SUPERCRIT. DTE

F BUILD-UP (FT2) 

FUSELAGE 9.4840

WING 10.3240

STRUT 2.7291

JURY STRUT 0.2519

HORIZONTAL 1.7482

VERTICAL 1.5327

N&P 1.9520

CANOPY 0.0405

GEAR PODS 3.1393

ETC BEFORE SUB -6.6657

EXCRESCENCE 2.0978

UPSWEEP 0.4870

WING TWIST 0.1640

ETC AFTER SUB -1.4622

FUSELAGE BUMP 0.3675

F-TOTAL (FT2) 26.1901

E-VISC 0.95206

CRUISE CD BUILD-UP

M-CRUISE 0.73

CRUISE ALTITUDE 44000

CL-CRUISE 0.775

CD0 0.01773

CDI 0.01025

CDC 0.002018

CDTRIM 0.000612

CDTOT 0.03061

L/D 25.31906

ML/D 18.483

Figure 5.20 – 765-095-TS3 High Speed Build-up

The resulting high speed data is shown in Figure 5.21. The figure illustrates the maximum 

aerodynamic efficiency (M*L/D) occurring at a cruise Mach of 0.73 and a CL of 0.775. 

CD0

58%

CDI

33%

CDC

7%

CDTRIM

2%
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

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Figure 5.21 – 765-095-TS3 - M * L / D Total

Figure 5.22 through Figure 5.24 show the low speed characteristics for the 765-095-TS3. Low 

speed data are trimmed as a function of angle of attack, lift coefficient, and drag coefficient at 

each flap detent. Low speed high lift devices on wing leading and trailing edges are deployed.

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0.200 0.300 0.400 0.500 0.600 0.700 0.800

M * L / D TOTAL

LIFT COEFFICIENT (CL) 

0.2

0.4

0.5

0.6

0.65

0.67

0.68

0.69

0.7

0.71

0.72

0.73

0.74

0.75

0.76

0.78

MACH NUMBER
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

55

Figure 5.22 – 765-095-TS3 - Low Speed Lift Curve; Free Air

Figure 5.23 – 765-095-TS3 - Low Speed Polar; Free Air

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

3.5000

-5.0000 0.0000 5.0000 10.0000 15.0000 20.0000 25.0000

LIFT COEFFICIENT (CL) 

ANGLE OF ATTACK (ALPHA)

0 

5 

10

15

20

25

30

35

FLAP DEFLECTION

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000

LIFT COEFFICIENT SQUARED (CL

2) 

DRAG COEFFICIENT (CD) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

56

Figure 5.24 – 765-095-TS3 - Low Speed Lift / Drag; Free Air

The weight (Table 5.11) for this configuration was generated starting from the 765-095-TS2 and 

adjusting propulsion system weight.

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

18.000

0.000 0.500 1.000 1.500 2.000 2.500 3.000

LIFT TO DRAG RATIO (L/D)

LIFT COEFFICIENT (CL) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

57

Table 5.11 – 765-095-TS3 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 21,330 13.7%

BENDING MATERIAL 8,197 5.3%

SPAR WEBS 2,049 1.3%

RIBS AND BULKHEADS 2,049 1.3%

AERODYNAMIC SURFACES 5,113 3.3%

SECONDARY STRUCTURE 3,921 2.5%

TAIL 2,852 1.8%

FUSELAGE 19,304 12.4%

LANDING GEAR 5,300 3.4% 

NACELLE & PYLON 5,012 3.2% 

STRUT, JURY STRUT, AND INSTALLATION 2,399 1.5%

PROPULSION 19,083 12.2% 

ENGINES 12,610 8.1%

FUEL SYSTEM 6,473 4.1%

FLIGHT CONTROLS 2,753 1.8%

COCKPIT CONTROLS 254 0.2%

SYSTEM CONTROLS 2,500 1.6%

POWER SYSTEMS 4,276 2.7%

AUXILIARY POWER UNIT 1,039 0.7%

HYDRAULICS 789 0.5%

ELECTRICAL 2,447 1.6%

INSTRUMENTS 818 0.5%

AVIONICS & AUTOPILOT 1,603 1.0%

FURNISHINGS & EQUIPMENT 10,300 6.6%

AIR CONDITIONING 1,564 1.0%

ANTI-ICING 123 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 96,719 62.0% 

OPERATIONAL ITEMS 7,803 5.0% 

OPERATIONAL EMPTY WEIGHT (OEW) 104,519 67.0%

USABLE FUEL 20,681 13.3% 

DESIGN PAYLOAD 30,800 19.7% 

TAKEOFF GROSS WEIGHT (TOGW) 156,000 100.0%

5.6 765-095-TS4 N+4 Truss Braced Wing with LNG Fuel Cell Hybrid Gas 

Turbine and BLI

This configuration, also derived from 765-095-RC (the Task 2.1 aeroelastic FEM configuration), 

is an N+4 Truss Braced Wing configuration but with a LNG fuel cell hybrid propulsion system 

and electric aft fuselage boundary layer ingestion propulsor. This configuration uses the fuel 

cell in a topping cycle configuration as illustrated in Figure 5.25. Details about this propulsion 

system are discussed in Appendix A. This configuration has been denoted 765-095-TS4. The 

configuration, shown in Figure 5.26, required additional aft fuselage refinement to 

accommodate the BLI propulsor without incurring a ground angle limit reduction.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

58

Figure 5.25 – Advanced LNG Fuel Cell Hybrid Configuration with BLI Propulsor

LNG, SOFC Topping Cycle Powering Unducted

and Goldschmeid (Fuselage Wake) Fan

SOFC

SOFC

Cryo Power 

Conditioner

Electric

Motor

LP

Gearbox

Superconducting Motor 

and Cryo Power 

Conditioning per SUGAR 

HE Meeting 09/20/2011
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

N+4 Advanced Concept Development

59

Figure 5.26 – 765-095-TS4 - Truss Braced Wing with LNG Fuel Cell BLI

PROJECTED CHARACTERISTICS

WING

TOTAL

V-TAIL

TRAP

H-TAIL

TRAP

AREA (SQ. FT) 1477.11 257.76 265.89

ASPECT RATIO 19.55 1.00 5.00

SPAN (INCHES) 2039.30 192.66 437.54

TAPER RATIO 0.35 1.00 0.35

MAC (INCHES) 110.29 192.66 94.27

DIHEDRAL (DEG.) -1.50 -- -3.00

1⁄4 CHORD SWEEP (DEG.) 12.52 41.00 25.30

ROOT CHORD (INCHES) 130.31 192.66 129.64

TIP CHORD (INCHES) 45.13 192.66 45.37

TAIL VOLUME COEFFICIENT -- 0.07 1.57

34’ – 9.48”

(417.48)

OVERALL

681.88

1,671.57

153’ – 1.93”

(1,837.93)

OVERALL

11.5°

GEAR EXTENDED

169’ – 11.50”

(2,039.50)

SPAN

193.27 188.23

~118’ – 0.00”

(1,416.00)

FOLDED SPAN

130.41

~517”

11.11°

TAIL DOWN

GEAR EXTENDED

36’ – 5.54”

(437.54)

SPAN

148.7

FS 60.43

FLOOR AT WL 200.00

272.29

Lv = 816.66

MAC

FS 822.58

LH = 962.20 
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The aerodynamic buildup of the 765-095-TS4 is similar to other LNG truss braced 

configurations. The portions of the fuselage that are internal to the aft BLI propulsor flowpath 

are book kept in thrust. The OML of the aft BLI nacelle is also booked in the fuselage drag. 

Despite the increased aft fuselage upsweep, the BLI system is assumed to aid in keeping the 

flow attached and the upsweep drag increment is held constant as compared to other LNG 

powered configurations in this study. Additional more detailed analysis would be needed to 

assess the accuracy of this assumption.
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Table 5.12 – 765-095-TS4 High Speed Build-up

CONFIGURATION 765-095-TS4 

AIRFOIL TYPE SUPERCRIT. DTE

F BUILD-UP (FT2

) 

FUSELAGE 9.6005

WING 10.3240

STRUT 2.7291

JURY STRUT 0.2519

HORIZONTAL 1.7482

VERTICAL 1.5327

N&P 1.4270

CANOPY 0.0405

GEAR PODS 3.1393

ETC BEFORE SUB -7.9093

EXCRESCENCE 1.9856

UPSWEEP 0.4870

WING TWIST 0.1640

ETC AFTER SUB -1.4622

FUSELAGE BUMP 0.3675

F-TOTAL (FT2) 24.4036

E-VISC 0.95206

CRUISE CD BUILD-UP

M-CRUISE 0.73

CRUISE ALTITUDE 44000

CL-CRUISE 0.775

CD0 0.01652

CDI 0.01025

CDC 0.002022

CDTRIM 0.0005876

CDTOT 0.02938

L/D 26.38216

ML/D 19.259

Figure 5.27 – 765-095-TS4 High Speed Build-up

The resulting high speed data is shown in Figure 5.8. The figure illustrates the maximum 

aerodynamic efficiency (M*L/D) occurring at a cruise Mach of 0.73 and a CL of 0.775. 

CD0

57%

CDI

34%

CDC

7%

CDTRIM

2%
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Figure 5.28 – 765-095-TS4 - M * L / D Total

Figure 5.29 through Figure 5.31 show the low speed characteristics for the 765-095-TS4. Low 

speed data are trimmed as a function of angle of attack, lift coefficient, and drag coefficient at 

each flap detent. Low speed high lift devices on wing leading and trailing edges are deployed.

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0.200 0.300 0.400 0.500 0.600 0.700 0.800

M * L / D TOTAL

LIFT COEFFICIENT (CL) 

0.2

0.4

0.5

0.6

0.65

0.67

0.68

0.69

0.7

0.71

0.72

0.73

0.74

0.75

0.76

0.78

MACH NUMBER
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Figure 5.29 – 765-095-TS4 - Low Speed Lift Curve; Free Air

Figure 5.30 – 765-095-TS4 - Low Speed Polar; Free Air

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

3.5000

-5.0000 0.0000 5.0000 10.0000 15.0000 20.0000 25.0000

LIFT COEFFICIENT (CL) 

ANGLE OF ATTACK (ALPHA)

0 

5 

10

15

20

25

30

35

FLAP DEFLECTION

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000

LIFT COEFFICIENT SQUARED (CL

2) 

DRAG COEFFICIENT (CD) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
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Figure 5.31 – 765-095-TS4 - Low Speed Lift / Drag; Free Air

The weight (Table 5.11) for the configuration was generated starting from the 765-095-TS2 and 

adjusting propulsion system weight and accounting for the additional fuselage structure to 

support the BLI device.

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

18.000

0.000 0.500 1.000 1.500 2.000 2.500 3.000

LIFT TO DRAG RATIO (L/D)

LIFT COEFFICIENT (CL) 

0.0

5.0

10.0

15.0

20.0

25.0

35.0

FLAP DEFLECTION
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Table 5.13 – 765-095-TS4 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 21,330 13.7%

BENDING MATERIAL 8,197 5.3%

SPAR WEBS 2,049 1.3%

RIBS AND BULKHEADS 2,049 1.3%

AERODYNAMIC SURFACES 5,113 3.3%

SECONDARY STRUCTURE 3,921 2.5%

TAIL 2,852 1.8%

FUSELAGE 19,433 12.5% 

LANDING GEAR 5,300 3.4% 

NACELLE & PYLON 6,813 4.4% 

STRUT, JURY STRUT, AND INSTALLATION 2,399 1.5%

PROPULSION 18,746 12.0% 

ENGINES 12,271 7.9%

FUEL SYSTEM 6,475 4.2% 

FLIGHT CONTROLS 2,753 1.8%

COCKPIT CONTROLS 254 0.2%

SYSTEM CONTROLS 2,500 1.6%

POWER SYSTEMS 4,276 2.7%

AUXILIARY POWER UNIT 1,039 0.7%

HYDRAULICS 789 0.5%

ELECTRICAL 2,447 1.6%

INSTRUMENTS 818 0.5%

AVIONICS & AUTOPILOT 1,603 1.0%

FURNISHINGS & EQUIPMENT 10,300 6.6%

AIR CONDITIONING 1,564 1.0%

ANTI-ICING 123 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 98,309 63.0% 

OPERATIONAL ITEMS 7,803 5.0% 

OPERATIONAL EMPTY WEIGHT (OEW) 106,112 68.0%

USABLE FUEL 19,088 12.2% 

DESIGN PAYLOAD 30,800 19.7% 

TAKEOFF GROSS WEIGHT (TOGW) 156,000 100.0%

5.7 765-095-TS5 N+4 TBW with LNG Fuel Cell Hybrid Gas Turbine Unducted 

Fan

The 765-095-TS5 configuration was intended to be a derivative version of the 765-095-TS4 with 

unducted fan propulsion. Early prolusion system trades suggested that the unducted fan was 

more efficient than the BLI device, so the BLI device was eliminated from the configuration. The 

solid oxide fuel cell topping cycle was retained but the extracted electrical energy is redirected 

to an electric motor on the low pressure spool of the engine. The propulsion system is 

documented in Appendix A. 
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The OML aerodynamic data for this configuration is identical to the 765-095-TS3 with exception 

of the reference thrust. Please refer to Section 5.5 for a 3-View drawing and aerodynamic data.

The change in propulsion system weight relative to the 765-095-TS3 requires a new weight 

estimate which is shown in Table 5.14. 

Table 5.14 – 765-095-TS5 Group Weight Statement

GROUP WEIGHT (LB) % TOGW

WING 21,330 13.7%

BENDING MATERIAL 8,197 5.3%

SPAR WEBS 2,049 1.3%

RIBS AND BULKHEADS 2,049 1.3%

AERODYNAMIC SURFACES 5,113 3.3%

SECONDARY STRUCTURE 3,921 2.5%

TAIL 2,852 1.8%

FUSELAGE 19,304 12.4% 

LANDING GEAR 5,300 3.4% 

NACELLE & PYLON 5,742 3.7%

STRUT, JURY STRUT, AND INSTALLATION 2,399 1.5%

PROPULSION 23,945 15.3% 

ENGINES 17,472 11.2% 

FUEL SYSTEM 6,473 4.1% 

FLIGHT CONTROLS 2,753 1.8%

COCKPIT CONTROLS 254 0.2%

SYSTEM CONTROLS 2,500 1.6%

POWER SYSTEMS 4,276 2.7%

AUXILIARY POWER UNIT 1,039 0.7%

HYDRAULICS 789 0.5%

ELECTRICAL 2,447 1.6%

INSTRUMENTS 818 0.5%

AVIONICS & AUTOPILOT 1,603 1.0%

FURNISHINGS & EQUIPMENT 10,300 6.6%

AIR CONDITIONING 1,564 1.0%

ANTI-ICING 123 0.1%

MANUFACTURER'S EMPTY WEIGHT (MEW) 102,307 65.6%

OPERATIONAL ITEMS 7,803 5.0% 

OPERATIONAL EMPTY WEIGHT (OEW) 110,110 70.6% 

USABLE FUEL 15,090 9.7% 

DESIGN PAYLOAD 30,800 19.7% 

TAKEOFF GROSS WEIGHT (TOGW) 156,000 100.0%

5.8 Concept Comparisons and Summary 

The missions used for sizing and performance calculations are the same as in Phase I and are 

documented in Section 5.1 and 6.1.2 of the SUGAR Phase I Final Report(1). 
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Sized vehicle performance results for all the configurations are listed in Table 5.15. The benefit 

of going to an advanced air traffic management system (see Phase I report for details, equates 

to approximately 17% fuel burn reduction) and the advanced technologies is shown by the 

difference in performance between the 765-093 and the 765-094-TS1, approximately 50,000 

pounds of TOGW. The high span truss braced wing adds 10,000 pounds to TOGW but still shows 

a reduction in fuel burn. The extra weight of the cryogenic systems causes the sized TOGW to 

increase 10,000 to 20,000 pounds for all LNG configurations. All of the truss braced wing 

aircraft are flying at higher cruise lift coefficients than the wing design point. As mentioned 

previously, the aerodynamic buildups are still relying on extrapolated empirical databases; CFD 

efforts will clarify the wings actual design characteristics in later contract deliverables for Task 

2.1. 

Unducted aircraft show smaller wings relative to their ducted fan counterparts. This is partially 

due to their decreased TOGW but mostly due to a mismatch in thrust at takeoff and top of 

climb (TOGW / Wing Area Increases ~20 lb / sq. ft. for both cases). The high lapse 

characteristics of the unducted fan system are causing the takeoff constraints to be easily met 

with lower wing areas. This causes the aircraft to fly at lower altitudes for a given optimum lift 

coefficient. These unducted configurations also show significantly lower L/D due to their loss of 

laminar flow over a portion of the wing and their smaller wingspans. 

Mission segment fuel burn for all configurations is shown in Table 5.16. 

Fuel, energy, noise, and emissions results are summarized in Table 5.17. For easy comparison, 

the fuel burn and energy results are repeated from Table 5.15. Noise and emissions were 

assessed qualitatively by subject matter experts at General Electric. A color coding was used to 

indicate status toward the NASA defined goals. Several of the LNG configurations come close to 

meeting, meet, or exceed the NASA N+3 goals for fuel burn, energy, and emissions. No 

configuration meets the noise goals.

Compared to a conventionally fueled aircraft, using LNG reduces the weight of fuel burned. 

However, because of the integration of the cryogenic tanks and systems, the total energy used

is increased. Use of LNG enables the design of low emission combustors as well as the potential 

use of fuel cells. 

Even though it increases engine weight and adds to the noise challenge, use of an unducted fan 

propulsor reduces fuel burn and energy use. Integrating a fuel cell into the propulsion cycle is 

also shown to produce significant benefits. An aft BLI propulsor improves fuel burn and energy 

use and has some potential for reduced noise. 
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Table 5.15 – Configuration Performance Summary

Model

Sizing Level

765-093 

SUGAR FREE

765-094-TS1

N+4 REFINED 

SUGAR

765-095-TS1

N+4 SUGAR 

HIGH

765-095-TS2

SUGAR 

FREEZE

765-095-TS3

SUGAR

FREEZE UDF

765-095-TS4

SUGAR 

FREEZE 

HYBRID BLI

765-095-TS5

SUGAR 

FREEZE 

HYBRID UDF

PASSENGERS / CLASS 154 / DUAL 154 / DUAL 154 / DUAL 154 / DUAL 154 / DUAL 154 / DUAL 154 / DUAL

MAX TAKEOFF WEIGHT LB 182,600 131,500 140,200 153,300 148,500 158,800 154,200 

MAX LANDING WEIGHT LB 149,400 128,500 138,600 153,200 150,700 161,300 158,000 

MAX ZERO FUEL WEIGHT LB 140,400 120,500 130,600 145,200 142,700 153,300 150,000 

OPERATING EMPTY WEIGHT LB 94,400 74,500 84,600 99,200 96,700 107,300 104,000 

FUEL CAPACITY REQ USG 9,633 4,748 4,526 7,359 6,749 6,697 6,348 

ENGINE MODEL Scaled 

CFM56-7B27 gFan++ gFan++ LNG gFan++ LNG UDF LNG Hybrid DF + BLI

LNG Hybrid 

UDF

FAN DIAMETER IN 62 62 65 68 128 59 132 

BOEING EQUIVALENT THRUST (BET) LB 27,900 17,200 19,200 20,600 21,700 20,800 22,000 

WING AREA / SPAN FT2 / FT 1,406 / 121 1,347 / 125 1,306 / 160 1,462 / 169 1,203 / 153 1,624 / 178 1,309 / 160

ASPECT RATIO (EFFECTIVE) 10.41 11.63 19.56 19.56 19.56 19.56 19.56 

OPTIMUM CL 0.584 0.604 0.773 0.763 0.780 0.771 0.776 

CRUISE L/D @ OPT CL 17.997 21.632 25.556 26.505 24.33 27.399 24.977 

DESIGN MISSION RANGE NMI 3,680 3,500 3,500 3,500 3,500 3,500 3,500 

PERFORMANCE CRUISE MACH 0.79 0.70 0.70 0.70 0.70 0.70 0.70 

LONG RANGE CRUISE MACH (LRC) 0.79 0.70 0.70 0.70 0.70 0.70 0.70 

THRUST ICAC (MTOW, ISA) FT 36,200 36,800 40,600 40,800 37,200 42,600 38,300 

TIME / DIST (MTOW, 35k FT, ISA) MIN / NMI 23 / 148 28 / 181 28 / 180 28 / 180 28 / 180 28 / 180 28 / 180

OPTIMUM ALTITUDE (MTOW, ISA) FT 34,900 36,700 39,700 39,900 36,900 41,500 37,700 

BUFFET ICAC (MTOW, ISA) FT 36,200 46,700 44,600 45,000 41,600 46,400 42,500 

TOFL (MTOW, SEA LEVEL, 86 DEG F) FT 8,190 8,190 8,190 8,190 8,190 8,190 8,190 

APPROACH SPEED (MLW) KT 126 117 120 120 131 116 128 

BLOCK FUEL / SEAT (900 NMI) LB 91.51 (Base) 42.53 (-53.5%) 41.62 (-54.5%) 39.21 (-57.2%) 34.66 (-62.1%) 35.88 (-60.8%) 33.26 (-64.1%)

BTU / SEAT (900 NMI) 1,000 BTU 1,700 (Base) 790 (-53.5%) 773 (-54.5%) 816 (-52.0%) 721 (-57.6%) 746 (-56.1%) 683 (-59.8%)
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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Table 5.16 – Segment Fuel Burn

Fuel Burn (lb)

765-093 

SUGAR 

FREE

765-094-

TS1

N+4 

REFINED 

SUGAR

765-095-

TS1

N+4 SUGAR 

HIGH

765-095-

TS2

SUGAR 

FREEZE

765-095-

TS3

SUGAR 

FREEZE UDF

765-095-

TS4

SUGAR 

FREEZE 

HYBRID BLI 

765-095-

TS5

SUGAR 

FREEZE 

HYBRID 

UDF

Taxi-Out 525 62 62 56 56 56 56 

Takeoff / Climbout 490 286 311 311 303 329 320 

Climb 3,719 1,970 1,984 1,881 1,633 1,913 1,494 

Cruise 7,463 3,478 3,090 2,857 2,632 2,283 2,455 

Descent 466 540 740 720 524 724 516 

Loiter 1,081 0 0 0 0 0 0 

Approach / Landing 224 152 161 157 133 164 160 

Taxi-In 125 62 62 56 56 56 56 

Total 14,093 6,550 6,410 6,038 5,337 5,525 5,057 
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Table 5.17 – N+4 Fuel, Energy, Noise, and Emissions Summary

Case 1 2 3 4 5 6 7 

Config. 

Number 765-093 765-094-TS1

765-095-

TS1

765-095-

TS2

765-095-

TS3

765-095-

TS4

765-095-

TS5

Name SUGAR 

Free 

N+4 

Reference

N+4

High Wing 

Reference

SUGAR

Freeze

SUGAR 

Freeze

SUGAR 

Freeze 

SUGAR 

Freeze

Fuel JP JP JP LNG LNG LNG LNG

Engine CFM-56 

JP+

2045GT+ 

DF

JP+

2045GT+ 

DF

LNG+

2045GT+ 

DF

LNG+

2045GT+ 

UDF

LNG+

2045GT+ 

SOFC+

BLI

LNG+

2045GT+ 

SOFC+

UDF

Propulsor Ducted Fan Ducted Fan Ducted Fan Ducted Fan Unducted 

Fan DF + BLI Unducted Fan

Quantitative Scoring Goal

Block Fuel / 

Seat

(900 NMI)

(Base) -53.5% -54.5% -57.2% -62.1% -60.8% -64.1% -60%*

BTU / Seat

(900 NMI) (Base) -53.5% -54.5% -52.0% -57.6% -56.1% -59.8% -60%*

Qualitative Scoring Goal

Noise +3 0 0 0 +1 -2 +1 -71 dB†

LTO NOx 

Emissions +3 0 0 -1 -3 -2 -4 -80%‡

Cruise NOx 

Emissions +3 0 0 -1 -2 -3 -4 -80%*

Color Legend to 

NASA’s Goal Qualitative Ranking System

*

Relative to Baseline SUGAR Free Far From Goal Acoustics Emissions

†

Cum Margin Relative to Stage 4 Does Not Meet Goal Quietest -4 Least 

‡

Relative to CAEP/6 Nearly or Meets Goal 765-094-TS1 0 765-094-TS1

Exceeds Goal Loudest 4 Most
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6.0 Technology Development Plans

Based on team discussions at and after the N+4 Workshop, the team identified a list of 

technology roadmaps that would be developed. Technology plans have been developed for the 

following N+4 technologies: 

• Hybrid Electric

• High Performance Batteries

• Low Energy Nuclear Reactor

• Fuel Cells

• Boundary Layer Ingestion

• Low-Noise High Cruise Speed Unducted Fans and Propellers 

• LNG & Hydrogen Engines 

• LNG & Hydrogen Aircraft Systems

• LNG & Hydrogen Infrastructure

Each plan follows a template described in Section 6.1. The template includes an assessment of 

technical risk, a listing of tasks to improve technical maturity, and estimates for when tasks 

leading to jumps in technology readiness level (TRL) could be completed. The progression of risk 

with technical maturity is also outlined in the template.

Specific technology plans are presented in Section 6.2. 

6.1 Technology Plan Template

The technology plans in this document are presented in a standardized template. A series of 

sections provide information on the technologies as described below.

Goals and Objectives – A short description is given on what the technology plan is trying to do.

Performance Area and Impact – Short descriptions are given on the benefits of the technology. 

The descriptions indicate what the technology does to realize the benefits.

Technical Description – A longer description is given on what needs to be developed, with 

some indication of why it is needed. This section introduces the major components of the 

technology. This section may elaborate on how the technology works.

Risk Assessment – A risk grid is presented along with a statement of the risk that was assessed. 

The current assessment of risk is indicated along with the progression of risk as major 

Technology Readiness Level (TRL) milestones are achieved. Labels on the risk progression 

correspond to labels on the major milestones presented in the next section. 
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Major Milestones – A chart is shown that indicates the dates when TRL jump milestones are 

expected to be reached. When multiple tasks are planned as part of achieving a particular TRL, 

the TRL jump occurs on completion of the last task. Synergistic technologies may be presented 

in the chart.

Maturation Plan – The tasks involved in reaching each major milestone are listed. Each TRL 

heading has the corresponding milestone label in parentheses.

Dependency – Short descriptions are given for any dependencies associated with the 

technology or the maturation plan.

Success Criteria – A table is presented to describe the success criteria for each TRL milestone 

and alternate steps if the criteria are not met.

Notes – This section provides information that is not otherwise captured in the preceding 

sections.

Roadmap – A chart showing the technology development tasks verses time.

6.2 Technology Plans

6.2.1 Hybrid Engine Technologies

Goals and Objectives:

Develop high performance, flight weight, and prime-reliable electric power components 

suitable for flight propulsion applications.

Performance Area and Impact:

Life-cycle fuel burn and emissions could be reduced by using energy stored in batteries that is 

generated from alternative energy sources, such as solar, wind, or nuclear.

Noise will be reduced by using stored battery energy to replace some of the energy generated 

by gas engines, thereby reducing the production of noise from the core of the engines. Hybrid 

electric systems also could enable distributed propulsion architectures which could enable BLI 

technology.

Technical Description:

Using energy stored on batteries has the potential to reduce fuel burn, emissions, and noise. 

Savings are dependent on battery energy density as well as the performance, efficiency, and 

weight of the electric power components. Efficient, high power, and light weight motors and 

motor controllers need to be developed. Light weight radiators and surface coolers are also 

needed to maintain the electric power components at temperatures conducive to high 

efficiency. A sustained program to develop high voltage conductors and insulators is also 

needed to support development of the necessary electric power components. A variable core 
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nozzle needs to be developed to allow the engine to operate with more widely varying levels of 

load introduced by the option to switch to electric power. These components need to be 

integrated in a hybrid engine system that can be demonstrated in flight. Superconducting 

components should be considered as possible system enhancing technologies and this would 

require additional development, design, and testing of cryocooler systems for aircraft.

Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If hybrid engine performance and weight do not reach the levels assumed in the vehicle analysis, 

this technology will not contribute the projected benefits in fuel burn, emissions, and noise.

Major Milestones:

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year

N+3 

Battery 

Development 

Pathway
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Maturation Plan:

TRL 2 (a) Current

Some analysis of the engine system has been performed. 

Some mission and sizing analysis has been conducted to assess fuel burn, energy, and global life 

cycle emissions benefit.

TRL 3 (b)

A life-cycle energy study will examine net benefit to fuel burn and emissions including generation of 

energy on the ground

A study will assess the potential reductions in airport noise and emissions 

3 motor design cycles

3 surface cooler/radiator design cycles

3 motor controller/power electronics design cycles

Sustained program for lightweight high voltage conductors and insulators, with off-ramps every ~18 

months

Lightweight variable core exhaust nozzle design

A design developed for a small-scale hybrid electric propulsion system (optional)

TRL 4 (c)

3 motor build, test, report-out cycles

3 surface cooler/radiator build, test, report out cycles

3 motor controller/power electronics build, test, report out cycles

Sustained program for lightweight high voltage conductors and insulators, with off ramps every ~18 

months

Lightweight variable core exhaust nozzle build, test

An integrated ground test of a small scale hybrid engine (optional)

TRL 5 (d)

Integration of components into 1st full-scale demonstration engine

1st demonstration engine test

Flight test of a small scale hybrid engine integrated into a small aircraft (optional)

TRL 6 (e)

Integration of components into 2nd full-scale demonstration engine

2nd demonstration engine test

Dependency:

High energy density battery technology is required to harness the benefit of hybrid engine 

technology.

A suitable off-the-shelf engine asset is needed to support testing.
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Success Criteria:

Table 6.1 – Hybrid Engine Technologies Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows hybrid engine system will 

have performance (fuel burn, emissions, 

noise) and weight consistent with meeting 

goals

Continue design of system and components

Switch to alternative technology option

Consider application to smaller, shorter 

range aircraft

4 

Tests of hybrid engine system components 

show performance (fuel burn, emissions, 

noise) and weight consistent with goals

Redesign components with shortfalls

Switch to alternative technology option

5 

Hybrid engine system components 

integrated and successfully tested

Initial system performance (fuel burn, 

emissions, noise) and weight indicates goals 

can be met with some redesign

Redesign system to meet goals

Accept meeting reduced goals

Switch to alternative technology option

6 

Hybrid engine system demonstrates 

performance (fuel burn, emissions, noise) 

and weight consistent with goals

Accept meeting reduced goals

Switch to alternative technology option

Notes:

Sustained base technology program for flight-worthy motors, conductors and insulators

2 builds for demo engine

Base engine is off-the-shelf

Yields TRL6 by 2027 

3.5 design/build/test cycles for motor, motor controller, and associated cooling system hardware 

yields TRL3+ by 2020 

Base engine is off-the-shelf

Yields TRL6 by 2027 

Assumes battery technology development program separate from this plan

Ongoing engine design refinement studies

If mission performance (fuel, emissions, noise, cost) improvements are not sufficient for a medium sized 

aircraft, consider application to smaller shorter range aircraft. This decision is based on assumptions for 

future energy cost, regulatory environment (noise, emissions), as well as judgment as to achievable 

battery technology and timeframe.

Superconducting components are a potentially enhancing technology which should be considered. They 

allow for an improvement in the efficiency of the electric machines; however, current superconductors 

must be cryocooled to less than 100 deg K while operating. Significant development would be needed in 

this area. Use of cryogenic fuels (LNG or Hydrogen) is synergistic with superconducting technologies.
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Figure 6.1 – Hybrid Engine Technologies Roadmap*

* The roadmap schedule shown is notional, suitable for overall program planning purposes only, with no implied guarantee or 

commitment on the part of GE Aviation

TRL Task

4

4.2

2-3-4 5

6

7 Small Scale Demo (Optional) Small Plane/UAV

3 7.1 Integrated Ground Demo

4 7.2 Flight Demo

Business Jet

8

8.1

8.2

5 8.3

8.4

8.5

6 8.6

Demo Build 2 Component Fabrication & Assembly

Demo Build 2 Test

Engine Design Studies

Full Scale Demo

Demo Engine Design & Integration

Demo Build 1 Component Fabrication & Assembly

Demo Build 1 Test

Demo Build 2 Design & Integration

Variable Core Nozzle

5.1 Design Modeling & Analysis

5.2 Nozzle Fabrication & Component Tests

Design Modeling & Analysis

4.3 Design Fabrication & Bench Test

3.2 Controller Fabrication & Bench Test

2-3-4 Lightweight low loss cooler/radiator

2-3-4 3 Motor Controller/Power Electronics

3.1 Design Modeling & Analysis

2.2 Controller Fabrication & Bench Test

2-3-4 2 Flight weight prime-reliable motor

2.1 Design Modeling & Analysis

3 1.3 Lightweight Magnetics & Support Structure

3 1.1 Insulator Materials Program

3 1.2 Conductors and Connectors Program

2028

1 Lightweight flightworthy high voltage enabling materials

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027

GEN1 GEN2 GEN3

GEN1 Design GEN2 GEN3 GEN3.5

GEN1 Test GEN2 Test GEN3 Test 

GEN1 Design GEN2 GEN3 GEN3.5

GEN1 Test GEN2 Test GEN3 Test 

GEN1 Design GEN2 GEN3 GEN3.5

GEN1 Test GEN2 Test GEN3 Test 

Ph I Ph Ph III II Ph IV

Test Complete
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6.2.2 Battery Technology

Goals and Objectives:

Foster development of high energy density modular batteries. Work with one or more battery 

manufacturers to produce batteries that achieve aviation safety requirements and are tailored 

for aviation performance requirements and usage patterns. Integrate batteries in flight 

propulsion applications when the batteries are at an appropriate level of development.

Performance Area and Impact:

Life-cycle fuel burn and emissions could be reduced by using energy stored in batteries that is 

generated from alternative energy sources, such as solar, wind, or nuclear. High efficiency of 

electrical components may reduce total energy usage relative to conventional liquid fuels. Life 

cycle studies will be needed to confirm these savings.

Technical Description:

Multiple battery technologies have potential to produce the energy densities needed to reduce 

fuel burn and emissions in an aircraft application. Low-level studies are needed to produce 

requirements and data that can be provided to battery manufacturers to encourage the 

development of battery technology that can support such application. Once suitable batteries 

are available, a substantial development program will be required to integrate and test these 

batteries in combination with hybrid-electric engine technology and aviation specific 

requirements.
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Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If battery energy densities do not reach the levels assumed in the vehicle analysis, this technology 

will not contribute the projected benefits in fuel burn, emissions, and noise.

Major Milestones:

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year

N+3 

Battery

Development

N+4

Battery 

Development
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Maturation Plan:

TRL 2 (a) Current

Theoretical estimates and some small-scale experiments indicate feasibility of reaching the needed 

energy density

TRL 3 (b)

A life-cycle energy study will examine net benefit to fuel burn and emissions including generation of 

energy on the ground

Aircraft system studies will define requirements for battery technology (including safety and 

charge/discharge rate)

Battery manufacturers will develop the basic technology to achieve the required energy density. 

Develop approach to achieve aviation specific battery life, charge/discharge rate, and safety.

TRL 4 (c)

Battery components will be tested for meeting aircraft power, life, charge/discharge rates, and 

safety requirements including operation in a relevant environment

TRL 5 (d)

Battery components will be integrated and packaged for testing in flight

A battery package of representative size will be tested in flight or simulated flight conditions

TRL 6 (e)

A battery power system suitable for a demonstrator aircraft will be assembled

The battery power system will be integrated with a hybrid-electric engine

The combined hybrid-electric engine and battery power system will be tested in flight

Dependency:

Aviation batteries are dependent on dramatic improvements in battery technology for other 

applications such as ground transportation. Hybrid-electric or all-electric propulsion is required 

to harness the benefit of aviation battery technology.
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Success Criteria:

Table 6.2 – Battery Technology Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows battery technology will result 

in fuel burn and emissions reductions through 

a complete energy life-cycle

Battery energy density reaches required levels

Continue basic battery development

Switch to alternative battery option*

Switch to alternative technology option*

Accept meeting reduced goals**

4 

Component testing of batteries show 

suitability for aircraft application (including 

life, charge/discharge rate, and safety)

Resume basic battery development

Switch to alternative battery option*

Switch to alternative technology option*

5 

Batteries successfully packaged for use in 

flight

Battery package successfully tested in flight or 

simulated flight conditions

Redesign battery packaging

Switch to alternative technology option*

6 Battery power system successfully tested with 

hybrid-electric engine in flight

Accept meeting reduced goals**

Switch to alternative technology option*

* Baseline battery technology is assumed to be Lithium-Ion, but this chemistry may reach a 

plateau in performance before needed levels are reached. Alternative battery options include 

Lithium-Air and liquid electrolyte slurries which would require additional systems. Alternative 

technology options include hybrid batteries (multiple chemistries), capacitors, hybrid battery 

capacitor, and flywheels. Lithium-air batteries require design of air induction and exhaust 

system which would require updated roadmap tasks to be added. Quick modular battery swap 

out or mechanically rechargeable components could be used if charge rates are not fast enough 

for quick gate turn requirements.

** Lower performance batteries could be suitable for smaller and especially shorter range 

aircraft
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Figure 6.2 – Battery Technology Roadmap

TRL Task

Generation 1 Generation 2 Generation 3 Product Improvement

Test Cycle 1 Test Cycle 2 Test Batteries for Life, etc. Test Batteries for Life, etc.

Update Requirements Update Requirements

Product Improvement N+4.5?

3-4-5

6

N+3 Battery Pathway N+4 Battery Pathway

3.2 Define Battery Requirements

2033 2034

3

2.1 Perform Life-Cycle Study

2.2 Down-select final batteries

2035 2036 2037 2038

5.2 Integrate Battery Power System 

on Aircraft

3.1 Perform Aircraft System Studies

4 Integrated Power System Testing

3 Aircraft System Studies

5.3 In Flight Testing

2031 2032

4.2 Hardware buildup

4.3 Perform Integrated Testing

5 Aircraft Demonstrator

5.1 Design Battery Power System

4.1 Design and Modeling for 

Integration and Packaging

3 2 Life Cycle Study

1.2 Aviation Req. Testing

2-3-4 1 Battery Development

1.1 Basic Technology Development

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030
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6.2.3 Low Energy Nuclear Reactor Technologies

Goals and Objectives:

Develop technologies for Low Energy Nuclear Reaction (LENR) propulsion systems.

Performance Area and Impact:

Traditional fuel burn and emissions will be reduced or eliminated by using LENR energy.

Noise may be reduced by using LENR heat instead of combustion in the engines.

Technical Description:

LENR energy has the potential to eliminate traditional fuel burn and associated emissions. In 

the current concept, a LENR reactor generates heat that is distributed to heat engines that use 

the LENR heat instead of combustion. This concept is dependent on successful development of 

LENR technology, which has reportedly had some success in generating heat in a catalytic 

process that combines nickel (Ni) with hydrogen (H) gas(8). This process is reported to produce 

safe byproducts, such as copper, with no radioactive materials used and no long-lasting 

radioactive byproducts generated. Upon further investigation, it is thought that low level 

radiation may be generated during active energy cycles, but that it could be easily shielded and 

would stop quickly after reactor shutdown. Further development of LENR would be required to 

produce heat at a high enough temperature to support heat engines in a flight-weight 

installation. LENR physics analysis and evidence of high temperature pitting in LENR metal 

substrates indicate that temperatures appropriate for heat engines may have been achieved. It 

is thought that LENR would use very small amounts of fuel. 

Initial LENR testing and theory have suggested that any radiation or radio-isotopes produced in 

the LENR reactions are very short lived and can be easily shielded. In addition, some 

prototypes(9) that may be harnessing the LENR process can be controlled safely within designed 

operating parameters and the reaction can be shut down in acceptable time frames. This heat 

generating process should reduce radiological, shielding and hazardous materials barriers to 

entry of aviation LENR systems.

Should LENR development prove successful, a few technology components will need to be 

developed for LENR-based aircraft propulsion. Heat engines, which run a thermodynamic cycle 

by adding heat via heat transfer instead of combustion, need to be developed. A system for 

distributing heat from the LENR core to the heat engines also needs to be developed. Additional 

systems may need to be developed for supporting the LENR core, including systems to deliver 

reactants and remove byproducts. The Ni-H LENR system would use pure hydrogen and a 

proprietary nickel and catalyst substrate. Hydrogen usage would be small compared to systems 

that combust hydrogen. Initially, hydrogen storage might involve cryogenics. The cold liquid 

hydrogen (LH2) fluid might be used in a regenerative system whereby cooling is supplied to 

super-conducting generators, electric feeders, and motors while the gas would be used as a fuel 
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in the LENR reactor. The primary LENR byproducts that would require periodic removal from 

the aircraft are the catalyst and nickel that are contained within the reactor core. Through 

thoughtful design of the reactor core, preliminary information suggests that these can be easily 

removed and replaced. The reactor core might then be recycled at low cost, due to the absence 

of toxic products in the core.

Technology Status: 

Multiple coherent theories that explain LENR exist which use the standard Quantum 

Electrodynamics & Quantum Chromodynamics model. The Widom-Larson(10) theory appears to 

have the best current understanding, but it is far from being fully validated and applied to 

current prototype testing. Limited testing is ongoing by NASA and private contractors of nickelhydrogen LENR systems. Two commercial companies (Leonardo Corp. & Defkalion) are reported 

to be offering commercial LENR systems. Those systems are advertised to run for 6 months with 

a single fueling cycle. Although data exists on all of these systems, the current data in each case 

is lacking in either definition or 3rd party verification. Thus, the current TRL assessment is low.

In this study the SUGAR Team has assumed, for the purposes of technology planning and 

establishing system requirements that the LENR technology will work. We have not conducted 

an independent technology feasibility assessment. The technology plan contained in this 

section merely identifies the steps that would need to take place to develop a propulsion 

system for aviation that utilizes LENR technology.
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Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If development of LENR, heat engines, or heat distribution systems is not successful, this 

technology will not contribute the projected benefits in fuel burn or emissions.

Major Milestones:

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year
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Maturation Plan:

TRL 2 (a) Current

A concept for a LENR propulsion system has been generated

Basic principles of LENR are reported to have been demonstrated

TRL 3 (b)

Definitive laboratory test data released and validated showing that the concept works

System level goals (power/weight, etc.) for LENR and heat engine established using a sensitivity 

study

A conceptual design of a LENR propulsion aircraft and its systems will be performed

Heat engine will be designed and analyzed, based on expected LENR temperature differential 

achievable

Heat distribution system will be designed and analyzed

Design and analysis will be performed on other systems to support LENR

TRL 4 (c)

A basic heat engine will be built and tested

A basic heat distribution system will be built and tested

Supporting LENR system components will be built and tested

LENR core reactor technology is demonstrated (external development)

TRL 5 (d)

LENR propulsion components will be integrated in a working system

LENR propulsion system will be demonstrated in ground test

Critical LENR propulsion system components will be tested in flight

TRL 6 (e)

LENR propulsion system will be demonstrated in flight

Dependency:

Development of LENR reactor technology is assumed to be developed successfully in an 

external program. An initial requirements assessment indicates that it is beneficial to develop a 

hybrid system to augment thrust at takeoff, so as not to oversize the LENR system for cruise 

conditions
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Success Criteria:

Table 6.3 – LENR Technologies Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows LENR propulsion system can 

meet aircraft propulsion requirements 

(including safety)

Switch to alternative technology option or 

abandon concept if feasibility cannot be 

clearly established. 

4 

Tests of LENR propulsion system components 

show performance and weight consistent with 

successful system operation and safety

Redesign components with shortfalls

Switch to alternative technology option

5 LENR propulsion system components 

integrated and successfully tested

Redesign system for successful operation

Switch to alternative technology option

6 LENR propulsion system demonstrates 

successful in-flight operation Switch to alternative technology option

Notes:

Alternate technologies include other types of self contained nuclear reactors such as thorium, 

cold fusion, traveling wave, etc.

Alternate heat engines include Sterling, Diesel, Wankel, Otto, and Brayton cycles.

If a safe flight-weight system is not judged to be achievable, the alternative approach is to keep 

the reactor on the ground and use it to produce electricity or hydrogen for use in aircraft (see 

other roadmaps).
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Figure 6.3 – LENR Technologies Roadmap

TRL Task

Gen 1 Gen 2

Gen 1 Gen 2 Gen 3

Gen 1 Gen 2 Gen 3

Gen 1 Gen 2 Gen 3

5

6

Critical Components

Critical Components Complete System

2035

1.1 Define System LENR Goals

LENR Technical Feasibility Testing

2015 2016 2017 2018 2019 2021 2022 2023 2024 2025 2026 2027 2033 2034

2-3-4 1 Aircraft Systems Study

2020 2028 2029 2030 2031 2032

1.2 Conceptual Design of Aircraft

2-3-4 2 Heat Distribution System

2.1 Development and Modeling

2.2 Testing

4 Heat Engine

2-3-4 3 Balance of Plant

3.1 Development and Modeling

3.2 Testing

4.1 Development and Modeling

4.2 Testing

Ground Test

6 Ground Test

6.1 Develop Integrated System

2-3-4

2013 2014

2-3 0

7 Flight Test

7.1 Plan Test and Develop Hardware

7.2 In Flight Demonstration

6.2 Plan Test and Develop Hardware

6.3
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6.2.4 Fuel Cell Technologies

Goals and Objectives:

Develop technologies for LNG and hydrogen fuel cells.

Performance Area and Impact:

Fuel burn and emissions will be reduced by using fuel cells in a hybrid system with either a gas 

turbine or batteries.

Noise may be reduced by using electric motors and fuel cell waste heat instead of combustion 

in the engines.

Technical Description:

Fuel cells have the potential to drastically increase the thermodynamic efficiency of the aircraft 

propulsion system. Fuel cells are capable of using both pure hydrogen and hydrocarbon fuels. 

Hydrocarbon fuels, such as liquid natural gas (LNG), offer the advantage of high energy density. 

Pure hydrogen has associated issues with fuel storage in terms of volume limitations due to its 

low energy density, but advanced hydrogen storage techniques may be available in the future 

to reduce the volume required to store hydrogen onboard the aircraft. Solid Oxide Fuel Cells 

(SOFC) have shown potential to allow for fuel flexibility and also generate high quality waste 

heat that can be combined with a gas turbine bottoming cycle to maximize system efficiency. 

Using SOFC technology creates a large amount of waste heat that can be recovered using a 

combined cycle setup, such as with a Brayton cycle. This could allow for flexibility when 

designing the balance of plant (BOP) in order to maximize the power-to-weight ratio of the 

overall system. This could also lead to a noise reduction by substituting some of the combustion 

noise with “quiet” heat from the SOFC system. Electric power generated from the fuel cell will 

allow for smaller gas turbine generators which may lead to less noise and fewer emissions. 

SOFC will also allow for fuel flexibility which allows other development programs to continue 

independently of the SOFC development plan.

Fuel cell technology will need to be developed to increase the system specific power at least 

one order of magnitude over current systems in order to make it viable on future medium to 

long range aircraft. Since fuel cell transients can be dependent on fuel cell chemistry and 

operating conditions, power conversion electronics will need to be developed and tested to 

provide clean, constant power to the aircraft propulsor. Start-up times will need to be 

improved. Also, a highly integrated fuel cell system will need to be developed to reduce aircraft 

empty weight. Several useful references were used to help compile the technical plan.

(11)(12)(13)
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Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If fuel cell performance and weight do not reach the required levels, this technology will not 

contribute to the projected benefits in fuel burn, emissions, and noise.

Major Milestones:

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year

Hybrid Electric

Technology
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Maturation Plan:

TRL 2 (a) Current

Perform analysis to design system and balance of plant components, show benefit and assess lifecycle study

Perform analysis to define fuel cell specific power goals for continued development

Identify, develop, and demonstrate feasibility of advanced fuel cell materials enabling high specific 

powers

TRL 3 (b)

Laboratory testing of fuel cell stack technology to achieve high specific power

Design, modeling, and validation of thermal management system, reformer, and other balance of 

plant including heat exchangers, steam generators, and balance of plant (BOP) system integration

Design, model, and test power conversion electronics

TRL 4 (c)

A prototype power conversion system will be built and tested

A prototype highly integrated thermal management system will be built and tested with a 

representative low power fuel cell and appropriate balance of plant

Supporting BOP components will be built and tested

Fuel Cell Stack technology will be demonstrated on a small scale to determine if specific power 

targets have been met

All major components of the fuel cell system will be integrated into a low power configuration and 

tested

TRL 5 (d)

Demonstrate high life-cycle of high output power fuel cell stack with high robustness to thermal 

cycling

Ground test BOP system with full power fuel cell stack

Demonstrate and validate full-scale prototype of integrated stack and balance of plant

Demonstrate operability of fuel cell system at relevant operating conditions such as high altitude 

and low ambient temperatures

TRL 6 (e)

Integrate power conversion equipment into prototype aircraft power management and distribution 

system

Demonstrate system integrated with electric propulsion system

Dependency:

LNG aircraft systems are required to support gas turbine / SOFC hybrid architecture. Hydrogen 

storage and aircraft system integration is required for hydrogen powered fuel cell. If a battery is 

used in place of a gas turbine to supply additional system power, then battery technology and 

electric propulsion system technology must also be matured.
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Success Criteria:

Table 6.4 – Fuel Cell Technologies Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows both stack and system provide 

specific power required to meet goals and 

design shows aircraft level goals can be met

Continue design of system and components

Switch to alternative technology option

Consider application to smaller, shorter range 

aircraft

4 

Tests of the fuel cell stack and BOP 

components show performance and weight 

consistent with goals. Power conversion 

equipment shows ability to control power 

fluctuations to provide safe and reliable 

aircraft power

Redesign components with shortfalls

Switch to alternative technology option

Consider application to smaller, shorter range 

aircraft

5 

Prototype systems successfully tested in 

relevant operating environments and 

successfully integrated

Redesign system for successful operation

Switch to alternative technology option

6 

Full scale prototype tested including in-flight 

operation and partial integration into aircraft 

subsystems 

Switch to alternative technology option

Notes:

If sufficient specific power goals are not achieved system could still be used to provide 

supplemental engine power or provide enhanced APU operations to increase overall aircraft 

energy efficiency. System could also be used on the ground to create electrical power for 

battery powered aircraft.
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Figure 6.4 – Fuel Cell Technologies Roadmap

TRL Task

Develop advanced fuel cell materials and design Design low power fuel cell stack Design high power stack

Bench testing Low power stack test Endurance Testing

Design Integrated System

Component Testing

Design Integrated System Design integration for aircraft test

Component Testing Test Integrated System

Highly integrated system

5

6

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028

2-3-4 1 Fuel Cell Stack Development

1.1 Development and Modeling

1.2 Fuel Cell Stack Testing

2-3-4 3 Power Conversion

2-3-4 2 Balance of Plant

2.1 Development and Modeling

2.2 Testing

5.2 Balance of Plant

Testing

6.4 Integrated Test w/ Electric 

Propulsion

6 Full-Scale Prototype

6.1 Ground Testing

6.2 Integration with Power 

Conversion on Aircraft

6.3 Test Operation in Relevent Env.

3.1 Development and Modeling

3.2 Testing

4.2

5.1 Plan Test and Develop Hardware

Development and Modeling

5 Ground Testing

2-3-4 4 Thermal Management

4.1
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6.2.5 Boundary Layer Ingestion Propulsion

Goals and Objectives:

Develop boundary layer ingestion (BLI) engine installations and aircraft configurations as a 

means to reduce fuel burn.

Performance Area and Impact:

BLI potentially reduces the power required to produce thrust with corresponding reductions in 

fuel burn and emissions. BLI has potential to reduce the weight and drag of engine installations, 

which also helps to achieve desired reductions in fuel burn and emissions.

Upper-surface BLI configurations allow placement of engines closer to the wing surface than 

non-BLI engine installations, potentially resulting in better acoustic shielding for reduced noise.

Technical Description:

In a BLI configuration, skin friction drag on bodies ahead of the engine create boundary layers 

that slow down the net flow ingested by the engine or the thrust-producing fan. Using BLI to 

slow the flow entering the fan reduces engine ram drag and offers the potential to reduce the 

power needed to produce thrust. The reduced ram drag allows the engine to produce less gross 

thrust for the same net thrust. The same net thrust can be achieved with lower fuel burn and 

corresponding reductions in emissions.

By locating engines closer to the surface structures on which they are mounted, BLI has the 

potential to reduce engine installation weight and drag. The strut or pylon, which enables the 

nacelle to be separated from the surface in a conventional engine installation, is reduced or 

eliminated, saving weight and drag. Part of the exterior surface of the nacelle gets buried in the 

surface structure, providing further drag reduction from BLI. Weight and drag reductions would 

result in reduced fuel burn with corresponding reductions in emissions.

There are numerous difficulties with achieving BLI benefits. The reduced velocity from the 

boundary layer flow results in a drop in total pressure that will reduce engine cycle efficiency. It 

is important to configure the engine such that the distorted boundary layer air passes only 

through the fan and does not enter the engine core. In a common BLI installation with an 

engine placed over a planar surface, the boundary layer flow tends to collect on one edge of 

the fan face, which creates significant distortion. Such distortion negatively affects fan 

performance, so a means for reducing the distortion needs to be developed. Ideally, the lowspeed boundary layer flow would be distributed evenly around the fan rim by some means, 

possibly vortex generators or active flow control. Finally, there are challenges to configuring a 

vehicle to ingest enough drag to capture a large BLI benefit. Numerous considerations limit the 

placement of engines, meaning that only a fraction of the airplane skin friction drag can be 

captured. Trades need to be evaluated between inlet drag and BLI benefit for approaches using 

inlet shapes and ducting to capture more boundary layer air. Hybrid-electric systems could 
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enable distributed propulsion configurations that would allow more of the boundary layer air to 

be captured, realizing further fuel burn savings.

Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If challenges with distortion, amount of boundary layer ingestion, and total pressure loss into 

engine cores are not addressed, BLI will not contribute reductions in fuel burn, emissions, and 

noise. Furthermore, if the design is not well integrated the system may contribute to increases in 

fuel burn or reduction in engine operability.

Major Milestones:

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year
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Maturation Plan:

TRL 2 (a) Current

Concepts for reducing distortion have been studied.

Some BLI configurations have been conceived.

Some studies have shown benefits for BLI.

TRL 3 (b)

A conceptual BLI aircraft configuration will be developed as a focal point for more detailed 

development and as target for assessment of system-level benefits.

A BLI engine installation will be designed and analyzed with goals of ingesting substantial 

boundary layer flow while keeping the boundary layer flow away from the engine core.

Approaches for reducing distortion from ingested boundary layer flows will be analyzed.

The BLI aircraft aerodynamic lines will be adjusted for the BLI engine installation.

Aerodynamic analysis of the integrated BLI configuration will be performed for cruise and 

significant off-design conditions.

BLI-compatible engines will be designed for best efficiency given the anticipated engine flows.

A concept for BLI engine structural integration will be developed and analyzed.

A system-level assessment of the benefits of BLI will be made from the results of the analysis 

studies.

TRL 4 (c)

Wind tunnel tests of a BLI aircraft configuration with unpowered nacelles will validate aerodynamic 

performance predictions and measure boundary layer characteristics entering the inlets.

Wind tunnel tests of BLI engine installations with simulated onset boundary layer flows and 

simulated fan flows will be performed to validate predictions of inlet flows.

Inlet flow distortion will be measured in BLI engine installation wind tunnel tests to validate 

performance of any approaches applied to address distortion from BLI.

Tests of BLI-compatible engine components (fans and cores) will be performed, including 

simulated BLI onset flow conditions.

Structural components for a BLI engine installation will be constructed and tested.

The system-level assessment of the benefits of BLI will be updated based on the results of testing.

TRL 5 (d)

Wind tunnel tests of a BLI aircraft configuration with powered nacelles will further validate 

aerodynamic performance predictions and boundary layer characteristics entering the inlets.

A BLI-compatible engine will be integrated and tested.

BLI engine installation structure will be integrated and tested.

Wind tunnel tests of a BLI engine installation, complete with engine, will be performed with 

simulated onset boundary layer flows to validate engine operability and BLI performance benefits.

Flight tests of a complete BLI engine installation may be performed for further validation of engine 

operability and BLI performance benefits.
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TRL 6 (e)

A BLI aircraft demonstrator will be developed.

Flight tests of the BLI aircraft demonstrator will validate engine operability and BLI performance 

benefits for a specially-designed BLI aircraft.

Dependency:

Although not assumed in the plan, BLI technology would be improved by developments in 

hybrid-electric engine and distributed propulsion technology. Separating a BLI fan from its 

engine core provides a means to avoid ingestion of boundary layer air into the core. This 

separation could be accomplished with mechanical or electrical drive systems. An electrical 

drive system would benefit from technologies developed for the hybrid-electric engine. The 

hybrid-electric engine could also enable distributed propulsion systems that allow for increased 

boundary layer ingestion, which could lead to increased fuel burn benefits.

Success Criteria:

Table 6.5 – Boundary Layer Ingestion Technologies Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows reasonable means for 

addressing BLI concerns (distortion, total 

pressure loss) and suitable performance 

benefits (fuel burn, emissions, noise, drag, 

weight)

Continue design of system and components

4 

Tests of BLI system components confirm 

needed BLI flow characteristics (reasonable 

levels of distortion, low total pressure loss 

into engine core) and indicate performance 

benefits can be achieved (fuel burn, 

emissions, noise, drag, weight)

Redesign components with shortfalls

5 

BLI system components are integrated and 

successfully tested with results indicating 

performance benefits can be achieved (fuel 

burn, emissions, noise, drag, weight)

Redesign system to meet goals

Accept meeting reduced goals

6 

A BLI aircraft demonstrator is developed and 

demonstrates BLI performance benefits (fuel 

burn, emissions, noise, drag, weight)

Accept meeting reduced goals
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Figure 6.5 – Boundary Layer Ingestion Roadmap

TRL Task

5.2 Flight Test

2.5 Wind Tunnel Test with Engine

2.6

3.1 Integrated Structure Tests

2.1 Design & Analysis

1.2

3.1 Component Tests

3.2 Integrated Engine Tests

3-4-5 4 Installation Structure

3.1 Component Tests

5-6 5 Demonstrator

5.1 Development

2.3 Wind Tunnel Test

3-4-5 3 BLI Fan & Engine

3.3 Flight Test

Structural Design

Flight Test

2.2 Distortion Reduction Approach

3-4-5 2 Engine Installation

3-4-5 1 BLI Aircraft Configuration

1.1 Conceptual Design

1.2 Aerodynamic Lines & Analysis

1.5 System-Level Assessment

1.4 Powered Wind Tunnel Test

1.3 Unpowered Wind Tunnel Test

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028
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6.2.6 Advanced Unducted Fans and Propellors 

Goals and Objectives:

Develop high performance, light weight, and prime-reliable UDF Engine components suitable 

for flight propulsion applications.

Performance Area and Impact:

The unducted fan provides a fuel burn improvement by achieving propulsive efficiencies over 

the course of a mission well beyond those achievable with a conventional turbofan. Recent 

advances in blade design and acoustic analysis enable this benefit while meeting or exceeding 

next generation acoustic signature requirements.

Technical Description:

Current unducted turbofans are based on a set of counter-rotating fan bladerows. The counterrotating bladerows make it possible to generate thrust without leaving any substantial swirl in 

the fan exhaust airflow. While the pressure ratio of the unducted fan propulsor is well above 

that of a conventional propeller, the absence of swirl in the exhaust flow allows it to be 

competitive with a conventional propeller in terms of propeller efficiency while occupying a 

smaller fan diameter.

Even with this smaller diameter, a key technological challenge for the UDF engine architecture 

is to efficiently provide power to the low speed fans. To accomplish this, one implementation 

showing promise is a high speed power turbine driving a counter-rotating differential gearbox. 

This architecture allows the diameter of the fan to increase for improved propulsive efficiency 

and acoustic characteristics while keeping the power turbine size and weight in check.

In order to operate over the wide range of subsonic flight mach numbers a commercial 

turbofan experiences during a flight, the fan bladerows must be capable of variable pitch. At 

takeoff, the blading needs to be relatively closed. At cruise, the blading needs to be relatively 

open. During landings, one option for thrust reverse is to rotate the fan blades through the 

closed position and into reversed flow. The pitch change mechanism to achieve this must be 

reliable and light weight, and is another key technological challenge on the road to a viable UDF 

product.

As part of task 1 activities, an evolution of the unducted fan to extreme diameters in the 20 ft 

class was considered for narrowbody propulsion. At this diameter, it is thought that the exit 

swirl produced by a single fan bladerow might be small enough to enable a high solidity single 

stage, low tip speed design. Since fan pressure ratio and tip speed are two main design factors 

influencing the noise of the propulsor, it is thought that a high diameter single stage propeller 

at low tip speed might yield an acoustically attractive propulsor. The requirement to operate in 

a high subsonic flight regime would lead the fan to remain highly swept. This, in conjunction 

with a relatively high solidity would likely cause the design to more closely resemble its 
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counter-rotating cousin than the propellers of prior generations. The team has identified this as 

a potential topic for further investigation. At present it is beyond the scope of program funding.

Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If UDF engine performance, weight, noise and safety do not reach the levels assumed in the 

vehicle analysis, this technology will not contribute the projected benefits in fuel burn and/or meet 

noise and certification requirements 

Major Milestones:

Maturation Plan:

TRL 2 (a) Current

The following analyses of the engine system have been performed:
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• Engine architecture study

Pusher vs. Puller

Direct Drive vs. Geared

• Performance/Aero definition

Hot flowpath layout

• Propulsor design

PCM design and integration

Fan blade mechanical design

• Engine dynamics

• Engine controls integration

• Engine layout

• Engine weight

Some mission and sizing analysis has been conducted to assess fuel burn.

TRL 3 (b)

A set of new design and safety requirements need to be established to address the airworthiness 

regulations related to this new type of propulsion system.

TRL 4 (c)

Design the UDF engine component and system integration. Evaluate and test new technologies. 

Provide innovate solutions to reduce the weight and the noise. Improve the reliability and reduce 

the maintenance

TRL 5 (d)

Integrate the components into a full-scale demonstration engine/ground test. Evaluate the engine 

component integration and assess the impact on the SFC

TRL 6 (e)

Integrate the components into a full-scale demonstration engine/flight test. Evaluate the 

engine/aircraft integration and assess the impact on fuel burn and noise.

Dependency:

A reliable and quiet fan blade is required to fully benefit from this UDF engine technology.
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Success Criteria:

Table 6.6 – UDF Engine Technologies Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 Engine technical and safety requirements 

meet airworthiness regulations. 

Identify and conduct tests and analysis to 

resolve certification issues. 

4 

Analyses show UDF engine systems will have 

performance (fuel burn, emissions, noise) 

and weight consistent with meeting next 

generation goals

Redesign components with shortfalls

Switch to alternative PCM. Fan Blade 

technology option Consider different engine 

architecture

5 

UDF engine system components are 

integrated and successfully tested.

Initial system performance (SFC, emissions, 

noise) and weight indicate next generation 

goals can be met with some redesign.

Redesign system to meet goals

Accept meeting reduced goals

6 

UDF engine system components are 

integrated into aircraft and successfully 

tested

Initial system performance (fuel burn, 

emissions, noise) and weight indicate next 

generation goals can be met with some 

redesign.

Redesign system to meet goals

Accept meeting reduced goals
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Figure 6.6 – UDF Technologies Roadmap*

* The roadmap schedule shown is notional, suitable for overall program planning purposes only, with no implied guarantee or 

commitment on the part of GE Aviation

Task Name H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2 H1 H2

Establish Technical Requirement 

Functional 

Safety 

Certification 

Airframe integration 

Engine Architecture 

Propulsor Design 

Certification 

Propulsor Blade Development 

Performance 

Acoustics 

Mechanical 

Gas Generator Development 

Advanced core design 

Core Test 

Pitch Change Mechanism 

Controls Integration 

Mechanical Design 

Engine Systems integration 

Demo Engine Test 

Flight Demo 

2025 2026 2027 2028

UDF Technology Development 

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024

e

b

c

b

d

d

e

b

c

b

c

d

c
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6.2.7 LNG and Hydrogen Gas Turbine Engines 

Goals and Objectives:

Develop technologies for LNG and hydrogen aircraft propulsion systems.

Performance Area and Impact:

Fuel burn and emissions will be reduced by using LNG and hydrogen aircraft propulsion 

systems.

Technical Description:

LNG and hydrogen fuel systems for aircraft consist of storage tanks, a feed and distribution 

system and an interface fuel panel. Hydrogen is stored as a liquid at -423 deg F and LNG at -258 

deg F. LNG may be stored at nearly ambient pressure owing to its high condensation

temperature relative to other cryogenic fluids. From the tank, the cryogenic fuel will feed 

through a low pressure fuel pump in proximity to the tank for delivery from the tank to the 

engine main fuel pump. A relatively low pressure delivery system is desirable to avoid heavy 

weight piping and reduce risk to the surrounding aircraft. The vapor pressure of the fuel will 

also play a key role in determining the delivery pressure. At the main fuel pump the fuel is 

elevated to a pressure in excess of engine overall pressure prior to delivery to the combustor 

fuel nozzles. Cryogenic fuel provides an excellent cooling source for the propulsion system. 

Cooled cooling, intercooling and recuperation are examples of processes that may be employed 

to improve engine performance and raise the temperature of the fluid to a desirable sensible 

enthalpy for introduction into the combustor. At present, LNG storage temperatures are 

beyond the high temperature superconducting (HTS) material range. However, in the N+4 

timeframe HTS materials may enable the use of LNG as a cryogenic coolant with little or no 

refrigeration energy. 

Current marine and industrial gas turbines utilize two sets of nozzles to facilitate the 

introduction of a variety of fuels. One set of nozzles is used for liquid fuels such kerosene and 

biofuels, and the other is used for gaseous fuels such as natural gas or hydrogen. Natural gas 

and hydrogen based propulsion systems for aviation applications may utilize a similar dual fuel 

nozzle combustor to resort to operation on jet fuel at airports/bases where natural gas or 

hydrogen is not readily available. 

The use of hydrogen as a fuel brings additional complications for the turbomachinery designer. 

Hydrogen is a highly reactive gas, and as such it tends to react with the metals commonly used 

in engine design. This reaction leads to a phenomenon known as hydrogen embrittlement, 

which can substantially reduce the life of the hot section of the turbomachinery. Alternative 

materials capable of sustaining similar gas path temperatures, or alternative lifing strategies for 

existing materials will be required as a result.
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Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

b

c

a

The N+4 concept engines are based on the utilization of liquefied natural gas. As natural gas fuel 

systems and combustors are already commercially available, the main challenges associated with 

utilizing natural gas are associated with establishing viable flight weight designs.

Major Milestones:

a 

b 

c 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year
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Maturation Plan:

TRL 4 (a) Current

Detailed design and test of flight worthy weight LNG main fuel pump

Detailed design and test of flight worthy heat exchangers, fuel manifold, fuel nozzles and 

combustor

Detailed design and test of flight worthy LNG fuel control system

TRL 5 (b) 

Integration and test of fuel control system, fuel pump and combustor module

TRL 6 (c) 

Integration and test of LNG based propulsion system

Integration of a flight LNG and hydrogen propulsion system on a full-scale aircraft with a full-scale 

demonstrator engine 

Dependency:

LNG and hydrogen compatible aircraft and infrastructure are required to enable the benefits of 

LNG and hydrogen fueled propulsion systems.

Success Criteria:

Table 6.7 – LNG and Hydrogen Technologies Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

4 

Design and analysis of LNG and hydrogen 

based propulsion system components show 

performance (emissions) and weight 

consistent with goals

Redesign components with shortfalls

Switch to alternative technology option

5 

LNG and hydrogen based propulsion system 

components integrated and successfully 

tested

Path to meet initial system performance 

(emissions) and weight goals is visible with 

redesign

Redesign system to meet goals

Accept meeting reduced goals

Switch to alternative technology option

6 

LNG and hydrogen based propulsion system 

demonstrates performance (emissions) and 

weight consistent with goals

Accept meeting reduced goals

Switch to alternative technology option
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Figure 6.7 – LNG and Hydrogen Aircraft Propulsion System Roadmap

2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026

1 Fuel Control System

1.1 Design and Analysis

1.2 Component Test

1.3 Subsystem Test

2 Main Fuel Pump

2.1 Design and Analysis

2.2 Component Test

2.3 Subsystem Test

3 Fuel Manifolds and Nozzles

3.1 Design and Analysis

3.2 Component Test

3.3 Subsystem Test

4 Combustor

4.1 Design and Analysis

4.2 Component Test

4.3 Subsystem Test

5 Propulsion System HX

5.1 Design and Analysis

5.2 Component Test

5.3 Subsystem Test

6 Certification

7 Ground Demonstrator Engine

8 Flight Demonstrator

9 Commercial Product
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6.2.8 LNG and Hydrogen Aircraft Systems

Goals and Objectives:

Develop technologies for LNG and hydrogen aircraft systems.

Performance Area and Impact:

Fuel burn and emissions will be reduced by using LNG and hydrogen aircraft systems.

Technical Description:

LNG and hydrogen fuel systems for aircraft consist of storage tanks, a feed and distribution 

system and an interface fuel panel. Hydrogen is stored as a liquid at -423 deg F and LNG at -258 

deg F.

Storage tanks are generally spherical or cylindrical in shape and operate at relatively low 

pressure (15 – 50 psia) to minimize fuel tank weight. The tanks can be foam insulated or 

vacuum jacketed. Foam insulation has a tendency to degrade over time with cracking due to 

thermal expansion/contraction and water absorption while vacuum jacketed tanks need 

periodic vacuum maintenance. The fuel tanks will have a quantity gauging system (capacitance 

gauge or point sensors) and may have internal baffles. At a minimum each tank will have fluid 

penetrations for filling/draining, venting and may have a separate penetration for the feed gas 

to the engines. The tanks will also have electrical penetrations for the quantity gauging system, 

pressure or temperature sensors and heaters if needed inside of the tanks. The selected 

insulation system will be the primary trade study. Newer technologies like vacuum insulated 

panels, microspheres and others will also be evaluated. 

The feed and distribution system consists of all of the lines, valves, pressure and temperature 

sensors, heat exchangers, pumps and regulators needed to create a safe cryogenic fuel system. 

The feed and distribution system also allows the tanks to be filled, drained, and delivers LNG 

and hydrogen to the engines at the required pressure, temperature and flow rate. Anywhere 

that the fluid is cold, the system will use vacuum jacketed lines to eliminate frost/ice buildup 

and subsequent water inside the fuselage when the frost/ice melts between flights. Where 

vacuum jacketed lines transition to valves or other components, the non-vacuum jacketed 

areas will need to be foam insulated to prevent frost/ice buildup.

In general, fuel is stored at low pressure to minimize tank weight. As a result, a pump or 

compressor will be required to raise the pressure of the LNG or hydrogen going to the engine to 

provide sufficient flow. A cryogenic pump that is light weight (for flight applications) and has 

high reliability (long lifetime, long mean times between service, repair or replacement, and 

reliable operation) is an item for technology development.
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The interface fuel panel will consist of disconnects for filling and draining the fuel tanks, for 

purging the fuel system with gaseous helium (hydrogen) or nitrogen (LNG), and any other fluid 

or electrical interfaces needed.

Cryogenic systems (liquid hydrogen and oxygen) have flown on the NASA Space Shuttle, 

delivering hydrogen and oxygen to fuel cells for the creation of electrical power and drinking 

water. Oxygen is used in the crew cabin environment for spacecraft applications. For aircraft 

applications, many military planes use oxygen for personal pilot breathing equipment, and 

some experimental aircraft have been fitted with liquid hydrogen tanks to allow them to run on 

hydrogen for short periods of time. LNG has been used in experimental aircraft and helicopters 

as an alternative fuel, but has not yet been used in normal service or operations. There is 

extensive experience operating large and aeroderivative power generation gas turbines using 

natural gas; however, it must be evaluated how much of that knowledge is directly applicable 

to aviation applications.

Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c

If LNG and hydrogen aircraft systems weight do not reach the levels assumed in the vehicle 

analysis, then LNG or hydrogen aircraft will not be practical and will not contribute the projected 

benefits in emissions.
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Major Milestones:

Maturation Plan:

TRL 2 (a) Current

Analysis of lightweight LNG and hydrogen fuel and oxidizer systems has been performed for space 

vehicles (launch vehicles, upper stages, spacecraft)

Large scale liquid hydrogen composite tanks have been designed, built and tested in the space 

industry

Large scale liquid LNG and liquid hydrogen composite tanks are being designed with up to 10 

meter diameters (cylindrical tanks with hemispherical heads) for space applications

TRL 3 (b)

A study of composite liquid hydrogen and LNG tanks to operational aircraft system requirements

Preliminary studies of conformal cryogenic tankage with operational aircraft system requirements

LNG and hydrogen pump technology plan defined

Preliminary design, analysis and test of LNG and hydrogen pump

TRL 4 (c)

Design, fabrication of large test fuel systems using cryogenic tank technology

Detailed design of conformal cryogenic tankage with operational aircraft system requirements

Detailed design and test of LNG and hydrogen pump

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year
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TRL 5 (d)

Integration and test of a large test fuel system using cryogenic tank technology

Design and fabrication of a full-scale LNG and hydrogen aircraft system

Fabrication and test of sub-scale conformal cryogenic tankage with operational aircraft system 

requirements

TRL 6 (e)

Integration of a flight LNG and hydrogen aircraft system on a full-scale aircraft with a full-scale 

demonstration engine

Design, fabrication and laboratory test of a full-scale conformal cryogenic tank fuel system

Integration plan for conformal cryogenic tankage 

Dependency:

LNG and hydrogen aircraft infrastructure and engine are required to enable the benefits of LNG 

and hydrogen fuels. LNG and hydrogen engines are needed.

Success Criteria:

Table 6.8 – LNG and Hydrogen Aircraft Systems Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows LNG and hydrogen fuel system 

will have performance (emissions) and weight 

consistent with meeting goals

Continue design of system and components

Switch to alternative technology option

Consider application to smaller, shorter range 

aircraft

4 

Design and analysis of LNG and hydrogen fuel 

system components show performance 

(emissions) and weight consistent with goals

Redesign components with shortfalls

Switch to alternative technology option

5 

LNG and hydrogen fuel system components 

integrated and successfully tested

Initial system performance (emissions) and 

weight indicates goals can be met with some 

redesign

Redesign system to meet goals

Accept meeting reduced goals

Switch to alternative technology option

6 

LNG and hydrogen fuel system demonstrates 

performance (emissions) and weight 

consistent with goals

Accept meeting reduced goals

Switch to alternative technology option

Notes:

Implementation of operational LNG and hydrogen tankage is dependent on integration plan 

results and acceptance.

Hydrogen, due to increased leakage potential, may require somewhat more development time 

or effort. Additionally, hydrogen use will require development of a cost effective and 

environmentally friendly process for hydrogen production.
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Figure 6.8 – LNG and Hydrogen Aircraft Systems Roadmap

TRL Task

Aircraft Systems Study

Includes Large Scale and Conformal

Sub-Scale Full-Scale

Build & Test

5

6

4

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028

2-3-4 1 Lightweight Sys Research

1.1 Composite Tank Study

1.2 Design & Fabrication

2-3-4 2 Cryogenic Pump Plan

2.1 Pump Technology Design

2.2 Pump Test

2-3-4 3 Fuel Systems

3.1 Design & Analysis

3.2 Testing

5 Flight Test

5.1 Design Demonstration System

4.1 Design & Fab. Of Cryo Technology

4.2 Design & Fab. Of Full Scale Aircraft 

System

4.3 Design and Fab of Conformal 

Tankage

Integrated Testing

5.2
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6.2.9 LNG and Hydrogen Infrastructure

Goals and Objectives:

Develop technologies for LNG and hydrogen airport infrastructures.

Performance Area and Impact:

LNG and hydrogen airport infrastructure will enable the use of N+4 generation aircraft using 

LNG and hydrogen fuels to reduce fuel burn, and emissions.

Technical Description:

Airport infrastructure is the system of equipment employed at the airport used to fill the 

onboard fuel tanks with the needed commodity, in this case either LNG or liquid hydrogen. This 

also includes all delivery lines, valves, instrumentation, etc needed to make a safe cryogenic 

system to deliver fuel to the aircraft. Fuel storage on-board the aircraft is assumed to be a low 

pressure (15-50 psia) liquid in order to reduce fuel storage system weight.

A facility is required on the airport property that can store large quantities of LNG or liquid 

hydrogen and meet safety standards (National Fire Protection Association (NFPA) or National 

Aeronautics and/or Space Administration (NASA)). Commonly a 600 ft radial distance 

requirement between the LNG and/or liquid hydrogen storage vessels and any facilities that 

contain people and property lines. Usually, this would mean moving the commodity storage 

facility to a remote corner of the airport

Trade Studies to be conducted include the following:

1. The most important study to be answered is: does the LNG or liquid hydrogen get 

transported from an on-airport storage site to the airplane or does the airplane get 

transported to the storage area for fueling?

To move LNG or liquid hydrogen from the storage site to the airplane would involve a 

study of trucking the commodity from the storage area to the gate, common to what is 

currently done today with jet fuel. An alternative would be to build underground 

distribution systems to each parking stand. LNG and liquid hydrogen tanker trucks exist 

today and can already operate on any USA highway and industrialized foreign nations 

highways. Liquid hydrogen today is piped underground over miles of distance between 

plants.

2. Pumping technology must also be matured. Cryogenic pumps for LNG and liquid 

hydrogen need to be built to the unique requirements of an airport infrastructure and 

must be very reliable over long term use. 

3. Insulation is needed for the commodity storage vessels and the piping to move the 

commodity. There are commonly used insulations like foam, vacuum jackets (dewars), 

and perlite that each have their own advantages and disadvantages. Foam is subject to 

cracking over time and moisture degradation. Dewars need periodic vacuum 
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maintenance and perlite will create local heat leak paths over time. There are also 

newer technologies to be considered like aerogels, vacuum insulated panels and 

microspheres. 

4. Another trade study that needs to be conducted is whether a commodity production 

facility should be operated on site or if commodity should be trucked/piped directly to 

the airport storage facility. There are four main methods of obtaining a hydrogen 

supply. First is the merchant delivery of LH2. In this method, the airport would contract 

with a commodity supplier like Air Products or Praxair and have them deliver a known 

quantity of commodity each day, week or month. The commodity would be delivered by 

truck or pipeline depending on the quantity. The second method is steam reforming of 

methane (CH4). In this process, methane is cracked with steam and hydrogen gas is 

formed. This would require a methane supply usually found in large quantities at oil 

refineries. Using this process to form hydrogen, would mean that both commodities 

would be available at the airport. The third method of producing hydrogen gas is to use 

jet fuel reforming. In this process, jet fuel is cracked to form hydrogen. And finally, there 

is electrolysis. This process requires large quantities of water and electricity. Based on 

initial environmental assessments, it is likely that only hydrogen production based on 

green energy sources such as wind, solar, or nuclear will be environmentally acceptable. 

Additionally, significant improvements in efficiency and the cost of green energy will be 

needed to make hydrogen a practical aviation fuel source.

Risk Assessment:

5

Consequence

Likelihood

4

3

2

1

1 2 3 4 5

a

b

d

e

c
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If LNG and hydrogen airport infrastructure development does not reach the levels assumed in 

the vehicle analysis, concurrent with the N+4 vehicle development, LNG or hydrogen fueled 

aircraft can not be placed in operation.

Major Milestones:

Maturation Plan:

TRL 2 (a) Current

Studies have been conducted of hydrogen airport infrastructure

TRL 3 (b)

Conduct study of LNG and hydrogen airport infrastructure

Trade study to determine if fuel is delivered to aircraft or if aircraft is brought to fueling station

Design, analysis, and fabrication study of large commercial LNG and hydrogen cryogenic pumps

Commodity (LNG and hydrogen) delivery study

TRL 4 (c)

Design, analysis fabrication and test of small-scale LNG and hydrogen airport infrastructure facility

Design study of medium-scale LNG and hydrogen infrastructure

TRL 5 (d)

Design, analysis fabrication and test of medium-scale LNG and hydrogen airport infrastructure 

facility

Design study of full-scale LNG and hydrogen infrastructure

a 

b 

c 

d 

e 

0 

1 

2 

3 

4

5 

6 

7 

2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060

TRL

Year
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TRL 6 (e)

Fabrication and operation of full-scale LNG and hydrogen airport infrastructure facility 

Dependency:

LNG and hydrogen aircraft systems and engines are required to harness the benefit of N+4 

aircraft technologies that reduce fuel burn and emissions. For hydrogen, a cost effective and 

environmentally friendly fuel production process will be required.

Success Criteria:

Table 6.9 – LNG and Hydrogen Airport Infrastructure Success Criteria

TRL Success Criteria Alternate Steps if Unsuccessful

3 

Analysis shows LNG and hydrogen 

infrastructure will have performance 

consistent with meeting goals

Continue design of system and components

Switch to alternative technology option

Consider application to smaller, shorter range 

aircraft

4 

Design and analysis of LNG and hydrogen 

airport infrastructure system components 

show performance consistent with goals

Redesign components with shortfalls

Switch to alternative technology option

5 

LNG and hydrogen airport infrastructure 

system components integrated and 

successfully tested

Initial system performance indicates goals can 

be met with some redesign

Redesign system to meet goals

Accept meeting reduced goals

Switch to alternative technology option

6 

LNG and hydrogen airport infrastructure 

system demonstrates performance consistent 

with goals

Accept meeting reduced goals

Switch to alternative technology option

Notes:

Hydrogen use will require development of a cost effective and environmentally friendly process 

for hydrogen production.
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Figure 6.9 – LNG and Hydrogen Airport Infrastructure Roadmap 

TRL Task

Aircraft Systems Study

Includes Large Scale and Conformal

Sub-Scale Full-Scale

Build & Test

5

6

4

2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028

2-3-4 1 Lightweight Sys Research

1.1 Composite Tank Study

1.2 Design & Fabrication

2-3-4 2 Cryogenic Pump Plan

2.1 Pump Technology Design

2.2 Pump Test

2-3-4 3 Fuel Systems

3.1 Design & Analysis

3.2 Testing

5 Flight Test

5.1 Design Demonstration System

4.1 Design & Fab. Of Cryo Technology

4.2 Design & Fab. Of Full Scale Aircraft 

System

4.3 Design and Fab of Conformal 

Tankage

Integrated Testing

5.2
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6.3 Technology Plans Discussion

As we project technologies further into the future, all dates become more uncertain. 

Additionally, many non-technical outside factors, such as research funding levels, competing 

energy prices, government actions and incentives, and even public acceptance could have 

significant influences on the pace and success of technology development.

Generally, we have used a TRL 6 date of 2025 with a corresponding operational date of 2030-

2035 for N+3 technologies and a TRL 6 date of 2030-2035 with a corresponding operational 

date of 2040-2050 for N+4 technologies. We also have assumed that technologies are 

developed as soon as practical and with robust funding. Therefore, the development plans will 

tend to be optimistic compared to what will actually occur. The technology plans that have 

resulted from this effort indicate both N+3 and N+4 timeframes.

Hybrid electric propulsion was identified in Phase I as an N+3 technology and a technology plan 

was developed. This plan has been updated in this report and adds a specific development plan 

for the needed high performance modular batteries. Depending on the pace of battery 

development, they could be an N+3 or an N+4 technology. Also, because of their modularity, it

may be possible to develop an aircraft with one kind of battery technology and replace it with 

another generation of batteries or even a different battery technology during the operational 

lifetime of the system. Even if batteries of sufficient performance are not ready in the N+3 

timeframe for the assumed medium sized commercial airliner, there are likely to be other 

aircraft applications. Smaller general aviation, business jets, and even regional jets will likely 

benefit from hybrid electric technologies even at lower battery performance levels.

LNG gas turbine technology could be developed for the N+3 timeframe. The aviation 

infrastructure change required is very significant and likely to be the dominant influence on the 

timeline which could stretch into the N+4 timeframe. Hydrogen technology development is 

essentially similar to LNG technology development, but includes somewhat more difficult 

technology challenges due to lower cryogenic temperatures, material compatibility issues, and

greater leakage potential. Additionally, successful development of hydrogen requires 

improvements in hydrogen production technology to reduce cost and environmental impact 

before it is a viable option for aviation. So, it is likely hydrogen is an N+4 technology, even 

though the hydrogen gas turbine could be developed earlier.

The general viability of LENR technology is still an issue of active research. A breakthrough in 

nuclear technology would have a significant impact on the entire worldwide energy structure. 

The technology plan assumes a reasonable “waiting period” to establish viability before 

beginning development of the technology for aviation.
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All concepts in this report also assume the use of various N+3 technologies that were identified 

in Phase I. Technology plans for these other propulsion, structures, noise, and aerodynamic 

technologies can be found in the Phase I final report(1). 
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7.0 Conclusions and Recommendations

Using a quantitative workshop process, the following promising technologies were identified in 

the N+4 study: Methane/LNG, Hydrogen, Fuel Cell Hybrids, Battery Electric Hybrids, Low Energy 

Nuclear Reactors (LENR), Boundary Layer Ingestion (BLI), unducted fans and advanced 

propellers, and combinations. Technology development plans have been developed for these 

promising technologies and for the required systems and infrastructure development for 

cryogenic propellants.

An aviation specific life cycle energy study is needed, so the team developed an outline and 

recommend conducting the full study.

As an advanced technology aircraft for more detailed analysis, the team selected an LNG fueled 

gas turbine fuel cell hybrid configuration with an aft fuselage boundary layer ingestion 

propulsor.

The team then generated weight, aerodynamic, and propulsion data for a series of 

configurations with different combinations of N+4 technologies. Performance and sizing has 

been conducted for these configurations to allow comparisons on a common basis. Looking at 

the differences between the configurations allows quantification of the payoff of many of the 

N+4 technologies identified during the workshop (LNG, fuel cell topping cycle, aft fuselage 

boundary layer propulsor, and unducted fan).

• LNG fueled aircraft require heavier aircraft systems and larger propellant tankage 

compared to conventionally fueled aircraft. The higher heating value of LNG reduces the 

weight of fuel burned (-5.8%), but the heavier aircraft requires more total energy 

(+5.6%) for a given flight.

• LNG fueled aircraft have the potential for significant emissions advantages over 

conventionally fueled aircraft. LTO and cruise NOx are lower and less carbon dioxide is 

produced when it is burned. 

• Use of an unducted fan increases propulsive efficiency and reduces fuel burn (-11.6%). 

• Adding a topping cycle fuel cell and an aft fuselage boundary layer propulsor driven by 

an electric motor leads to reductions in emissions and fuel burn (-8.6%). 

• The best performing architecture analyzed used LNG, a fuel cell topping cycle, an 

unducted fan, and an electric motor augmenting fan shaft power. Relative to the SUGAR 

Free Baseline aircraft, this configuration achieved a 64.1% reduction in fuel burn, 

beating the 60% N+3 goal. The 59.8% reduction in total energy used, effectively meets 

the 60% energy reduction goal. This architecture is also estimated to beat the N+3 LTO 

and cruise NOx emissions goals.
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A summary of the technologies investigated in this study is shown in Table 7.1. 

Table 7.1 – Task 1 Technology Summary

Technology Impact Goals Relationships Major Concerns

LNG Very 

Significant

Fuel Burn, 

Emissions, 

(Fuel Cost), 

(Fuel Supply)

Enabling to Fuel Cells and 

Low Emission Combustors

Methane Emissions, 

Safety, Infrastructure

Unducted Fan Very 

Significant Fuel Burn Enhancing Noise, Safety

Engine Fuel 

Cell Significant Fuel Burn, Emissions 

Enhancing, Dependent on 

LNG or Hydrogen

BLI Aft 

Propulsor Significant

Fuel Burn, 

Emissions, 

Noise

Enhancing, Dependent on 

power source (fuel cell or 

batteries) for electric motor

LENR Game 

Changing

Fuel Burn, 

Energy Use, 

Emissions, 

Noise

Dependent on Hybrid 

Technology (gas turbine or 

electric hybrid)

Feasibility, Safety, 

Weight, Customer 

Acceptance

Hydrogen Very 

Significant

Fuel Burn, 

Emissions 

Enabling to Fuel Cells and 

Low Emission Combustors, 

Dependent on Production 

Technology

Low Cost Green 

Production, Safety, 

Customer Acceptance, 

Infrastructure

LNG technologies should continue to be investigated as there are significant potential emissions 

advantages, as well as advantages in cost and energy availability. However adding LNG to the 

aviation propellant infrastructure would be a significant challenge. Also, active research into 

methane leakage during natural gas extraction, processing, storage, and use should be 

monitored, as this could have an additional negative environmental impact.

Unducted fans, fuel cells, and BLI are potential enhancing technologies that offer significant 

improvements.

LENR technology is potentially game-changing to not just aviation, but the worldwide energy 

mix as well. This technology should be followed to determine feasibility and potential 

performance.

Hydrogen technology also has potential benefits, but widespread aviation use of hydrogen 

requires large infrastructure changes as well as significant improvements to produce hydrogen 

in a low cost environmentally friendly process.

As identified in Phase I, hybrid electric propulsion with high performance batteries offers 

significant fuel burn, energy, and emissions advantages if large battery technology 
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improvements occur and the technology can be adapted to aviation requirements. Hybrid 

electric technologies are potentially synergistic with fuel cell, BLI, and LENR technologies. 

Additionally, using superconducting, the cryogenic characteristics of LNG and hydrogen could 

be synergistic with hybrid electric technology.
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References

1. Bradley, Marty K. and Droney, Christopher K. Subsonic Ultra Green Aircraft Research: Phase I 

Final Report. s.l. : NASA, 2011. CR-2011-216847.

2. Lawrence Livermore National Laboratory. Energy Flow. Estimated Energy Use in 2010: 98

Quads. [Online] [Cited: December 1, 2011.] 

https://flowcharts.llnl.gov/content/energy/energy_archive/energy_flow_2010/LLNLUSEnerg

y2010.png.

3. Energy Information Administration (EIA). Annual EnergyOutlook 2011: with Projections to 

2035. s.l. : Department of Energy, 2011. DOE/EIA-0383.

4. Argonne National Laboratory. Argonne GREET Publication: Case Simulation: Natural Gas and 

Electricity Infrastructure. [Online] December 8, 2011. [Cited: February 25, 2012.] 

http://greet.es.anl.gov/publication-2011ws-case-simu-fc3.

5. Morris, R. E., et al. Impact of Biodiesel Fuels on Air Quality and Human Health. National 

Renewable Energy Laboratory (NREL). Golden : U.S Department of Energy, 2003. NERL/SR530-33793.

6. Stratton, Russell W., Wong, Hsin Min and Hileman, James I. Life Cycle Greenhouse Gas 

Emissions from Alternative Jet Fuels. s.l. : Partnership for AiR Transportation Noise and 

Emissions Reduction, 2010. PARTNER-COE-2010-001.

7. Bonet, John T., et al. Environmentally Responsible Aviation (ERA) Project – N+2 Advanced 

Vehicle Concepts Study and Conceptual Design of Subscale Test Vehicle (STV) . s.l. : NASA, 

2011. Pending.

8. Rossi, Andrea. Method and Apparatus for Carrying Out Nickel and Hydrogen Exothermal 

Reactions. WO 2009125444 October 15, 2009.

9. Defkalion Green Technologies. Products. Defkalion Green Technologies. [Online] [Cited: 

January 28, 2012.] http://www.defkalion-energy.com/products.

10. Ultra Low Momentum Neutron Catalyzed Nuclear Reactions on Metallic Hydride Surfaces. 

Windom, Allan and Larsen, Lewis. 1, 2006, The European Physical Journal C - Particles and 

Fields, Vol. 46, pp. 107-111.

11. D'Angelo, Martin M., et al. N+3 Small Commercial Efficient and Quiet. s.l. : NASA, 2010. 

NASA/CR-2010-216691.
NASA Contract NNL08AA16B – NNL11AA00T – Subsonic Ultra Green Aircraft Research – Phase II

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12. Sehra, Aurn K. and Shin, Jaiwon. Revolutionary Propulsion Systems. s.l. : NASA, 2003. 

NASA/TM—2003-212615.

13. EG&G Services; Parsons, Inc.; Science Applications Intrnational Corporation. Fuel Cell 

Handbook: Fifth Edition. s.l. : U.S. Department of Energy; National Energy Technology 

Laboratory, 2000.
A-1 

Appendix A – Propulsion Concept Information 

In 2011, GE was awarded a contract by Boeing to support phase II of the Subsonic Ultra Green 

Aircraft Research (SUGAR) program. SUGAR phase II is a three year effort funded by NASA under the 

N+3 Subsonic Fixed Wing (SFW) Aircraft program. The program is aimed at identifying transport aircraft 

and propulsion system concepts with the potential to be available in the 2030-2035 timeframe for 

substantial reductions in aircraft fuel burn, emissions, noise and required field length. GE’s involvement 

in the SUGAR program as a subcontractor to Boeing represents a continuation of phase I support, where 

the team of Boeing, GE, Georgia Tech (GT) and Virginia Tech (VT) collaborated to identify and begin 

exploration of several innovative aircraft and propulsion system concepts. GE was specifically 

contracted in phase II to perform propulsion system conceptual design work under three tasks: 1. N+4 

Advanced Vehicle Concept Study Support, 2. Truss Braced Wing Aircraft Support and 3. Hybrid Electric 

Aircraft Support. This N+4 final report details the analysis and results set forth in fulfillment of task 1. 

Task 1 – N+4 Advanced Vehicle Concept Study Support

The N+4 Advanced Vehicle Concept study is focused on aircraft entering service in the 2045 

timeframe. This task provides the airframer and engine maker with an opportunity to make aggressive 

technology assumptions at both the component and architectural level. In June, Georgia Tech hosted a 

workshop with NASA, Boeing, GE and VT to identify candidate technologies. Following a team 

brainstorming session, candidate technologies were ranked for their potential to positively impact the 

NASA SFW metrics. The hybrid fuel cell and gas turbine hybrid electric concepts ranked highly in this 

evaluation, but were not selected for further study due to the extensive coverage offered in the 2035 

timeframe by SUGAR task 2.2. In addition to evolving GE’s 2035 advanced propulsion system offering

for a 2045 entry into service, GE agreed to address concepts incorporating liquefied natural gas, solid 

oxide fuel cells, unducted fans and aft mounted fuselage fan technologies. The novelty and shear 

number of the concepts to be evaluated prompted the team to utilize a design point class analysis for 

each engine concept. For a commercial mission, this level of analysis should be adequate to identify the 

potential of the concepts at a high enough degree of fidelity to understand the merits of a more detailed 

investigation. The results from a few key flight conditions have then been applied to scale a full flight 

envelope of data based on the phase II gFan+, accounting for the unique thermal, transfer, propulsive 

efficiency and weight characteristics associated with the new components and technologies. 
A-2 

N+4 Aircraft Thrust Requirements

Boeing has provided thrust requirements representative of the truss braced wing (TBW) aircraft

concept for use in N+4 propulsion system studies. These requirements are shown in Figure 1. GE has 

provided scaling rules to resize the engine for use as the aircraft evolves, or with other narrowbody class 

aircraft over a reasonable thrust range. 

Figure 1 - TBW Aircraft Thrust Requirements

gFan++ Advanced Turbofan(JP+2045GT+DF) 

The gFan++ advanced turbofan is a direct evolution of the phase II 2035 gFan+. The details of 

the phase 2 gFan+ engine are outlined in the task 2.1 section of the report. The gFan+ fan pressure ratio 

has been adopted as a starting point for the gFan++. The key distinguishing feature of the gFan++ is the 

utilization of 3rd generation CMC technology, enabling a substantial increase in turbine inlet temperature 

while retaining an uncooled high pressure turbine. Because the turbine inlet temperature of the 

uncooled gFan+ is low compared to the state of the art cooled powerplant, the additional firing 

temperature of the gFan++ brings about a marked reduction to specific fuel consumption. It also serves 

to reduce the powerplant size and weight. The efficiency of the high pressure compressor (HPC) and 

high pressure turbine (HPT) have been penalized to approximate the effects of reduced HPC discharge 

blade height and HPT size in the 2045 EIS timeframe. A conceptual layout of the gFan++ is shown in 

Figure 2. 
A-3 

Figure 2- gFan++ (JP+2045GT+DF) Concept Layout

The key characteristics of the engine are summarized with preliminary design margins in Table 1. 

JP+2045GT+DF

Fan Diameter 71.4 In 

Length 127 In 

Propulsion System Weight 6379 Lbm

Performance Thrust (lbf)

SFC 

(lbm/lbf/hr)

SLS 21943 0.214

Takeoff 16592 0.286

Top of Climb 3931 0.453

Cruise 3145 0.442

Table 1 – JP+2045GT+DF Key Characteristics

Advanced nacelle

Slender OD

Unitized composite

Advanced acoustic features

Advanced Composite Fan

1.46 PR, 71.4” fan

Advanced 3-D aero design

Sculpted features, low noise

Thin, durable edges

Integrated thrust reverser/VFN

Highly variable fan nozzle

LP Compressor

1.45 PR

3 Stages Ultra-high PR core compressor

28:1 PR class, 9 stages

Active clearance control

HPT

2-Stage, uncooled

3rd Gen CMC nozzles + blades

Active purge control

Next-gen disk material

LPT

7-Stage

Advanced Tech Loading

GEnx style architecture

CMC+Ti/Al Blades/Vanes

Advanced eTAPS

combustor
A-4 

Liquified Natural Gas Fueled gFan++ Advanced Turbofan(LNG+2045GT+DF) 

 By introducing a series of well understood modifications to the combustor, the gFan++ can be 

converted to run on natural gas. GE has a range of LM gas turbines in service today with the capability 

to run on natural gas or jet fuel. A series of performance deltas were tabulated to account for the 

performance potential of using liquefied natural gas (LNG) and supplied to Boeing as part of the task 1 

effort. These deltas account for the difference in the fuel heating value of natural gas relative to jet fuel 

and also make a first pass at estimating the benefits available to the propulsion system as a result of the 

heat sink capacity of LNG in terms of intercooling and recuperation. In the tank, LNG is stored at 

approximately -260F. It is unknown at the time of this report whether the engine or aircraft would make 

the most effective use this heat sink, so the estimated benefit of utilizing the sink in the engine was 

provided and the option has been left to the airframe for N+4 studies. Table 2 summarizes the key 

characteristics of the LNG fueled gFan++ ducted turbofan without intercooling or recuperation. 

LNG+2045GT+DF

Fan Diameter 71.4 in

Length 127 in

Propulsion System Weight 6379 lbm

Performance Thrust (lbf)

SFC 

(lbm/lbf/hr)

SLS 21943 0.192

Takeoff 16592 0.257

Top of Climb 3931 0.406

Cruise 3145 0.396

Table 2 - LNG+2045GT+DF Key Characteristics

LNG Fueled gFan++ Powerplant with an Unducted Fan Propulsor

(LNG+2045GT+UDF) 

 A promising configuration for the N+4 timeframe is the truss braced wing (TBW.) The additional room 

under the wing due to its high mount location on the fuselage makes it well suited to propeller and/or 

unducted fan (UDF) propulsor concepts. The ultra low pressure ratio of the propeller and UDF

propulsors enable substantial improvements to propulsive efficiency relative to its ducted counterpart. 

A qualitative investigation into the pros and cons of using a propeller versus a counterrotating UDF was 

conducted under N+4 funding. At this time it is thought that an advanced propeller could be swept in 

such a manner as to enable a reasonably high flight mach (0.7) without excessive shock losses. 

However, the propeller leaves a small component of swirl in the exhaust stream that is inherently 

inefficient. To mitigate this effect and achieve a level of propeller efficiency similar to the UDF, the 

diameter of the propeller would have to be substantially larger. A more detailed investigation of this 

trade is worth consideration. The UDF was carried forward in subsequent analysis. The counterrotating 

fans on the UDF run at nearly a constant speed over the course of the mission and necessitate a change 
A-5 

to the gFan++ powerplant architecture. A 2nd spool was introduced to achieve gFan++ overall pressure 

ratio levels and maintain operability at lower power settings. A full quantification of the details of this 

update were beyond the scope of the study. The thermal benefits of running on LNG, coupled with the 

propulsive benefits of the UDF are summarized in Table 3. 

LNG+2045GT+UDF

Fan Diameter ~144 In 

Length ~194.6 In 

Propulsion System Weight 7,662 lbm

Performance Thrust (lbf)

SFC 

(lbm/lbf/hr)

SLS 28769 0.129

Takeoff 20193 0.186

Top of Climb 3931 0.357

Cruise 3145 0.349

Table 3 - LNG+2045GT+UDF Key Characteristics

LNG Fueled gFan++/Solid Oxide Fuel Cell (SOFC) Powerplant with UDF 

Propulsor

 The concept of utilizing solid oxide fuel cells to augment or replace the combustor in a gas generator 

for aviation use was explored in phase I of the SUGAR program. The complexity of an integrated fuel cell

and gas turbine arrangement may place it beyond of the N+3 timeframe. However, with an additional

decade of development time, the team felt this concept warranted additional attention under the N+4 

task. GE utilized internally developed SOFC analytical models to estimate the performance and sizing of 

the fuel cell toward N+4 applications. The model attempts to account for the inlet pressure and 

temperature of the fuel cell, and also adjusts cell efficiency and sizing based on the design current 

density selection. The process of reforming the LNG into hydrogen is assumed to occur on board the 

fuel cell is this time frame, eliminating a potentially heavy component. The SOFC-to-electric motor 

power conditioning unit specific power is assumed equal to that of the fuel cell for this study. A 

superconducting electric motor sits in line with the high speed LPT, and both units provide power to the 

geared UDF. The lapse in thrust with altitude as air density diminishes plays a major role in determining

how effective the SOFC will be in improving performance throughout the mission. For the UDF based 

architecture, the SOFC is sized to provide roughly 40% of the overall fan power electrically at the top of 

climb condition. At takeoff, the power requirement of the propulsor is substantially higher, but the 

electric output of the SOFC only increases slightly with flight condition. As a result, the turbomachinery 

must provide the vast majority of the power through the LPT, and the fuel cells contribution to 

performance is minimalized. The off design operation of the integrated gas turbine and SOFC is a 

complex problem. However, given the large performance potential, GE believes it warrants continued 

investigation. A conceptual layout of the propulsion system is shown below in Figure 3. 
A-6 

Figure 3 - LNG+2045GT+SOFC+UDF Concept Layout

The key characteristics of the propulsion system concept are summarized in Table 4. 

LNG+2045GT+SOFC+UDF

Fan Diameter 144 in

Length 194.6 in

Propulsion System Weight 10162 in

Performance Thrust (lbf)

SFC 

(lbm/lbf/hr)

SLS 26565 0.125

Takeoff 19077 0.176

Top of Climb 3145 0.321

Cruise 2359 0.313

Table 4 - LNG+2045GT+SOFC+UDF Key Characteristics

gFan++/SOFC Powerplant with Wing and Aft Fuselage Mounted Ducted Fans 

for Boundary Layer Ingestion and Wake Propulsion

The remaining technology identified by the SUGAR team for exploration under N+4 funding is 

that of boundary layer ingestion and wake propulsion. In a conventional commercial aircraft 

configuration, the propulsion system is intentionally installed at a distance from the aircraft surfaces, 

such that the aircraft and propulsion system interact with separate airflows. In this environment, the 

net thrust the engine produces is directly proportional to the difference between the velocity of the 

exhaust jet from the engine and the freestream velocity of the air approaching the engine. Similarly, the 
A-7 

drag the airplane produces is directly proportional to the difference between freestream velocity and 

the average velocity of the wake downstream of the aircraft. The concept of boundary layer ingestion 

and wake propulsion is to feed the boundary layer flow from the aircraft into the propulsion system. 

This allows the drag created by the airplane to effectively reduce the freestream velocity of the 

propulsion system, also reducing the exhaust velocity required to produce a given net thrust. Two 

methods of bookkeeping thrust between the A/C and propulsion system were explored. The 

methodology the team agreed to is summarized as option 2 in Table 5. 

Table 5 - BLI/Wake Propulsion Thrust Bookkeeping

 In order to evaluate the impact of the boundary layer on the effective freestream (0’) total pressure 

and velocity, Boeing supplied CFD based boundary layer profiles versus distance from the fuselage. The 

profiles were provided at altitude and takeoff flight conditions. GE post processed these profiles to 

arrive at representative properties for cycle design and performance prediction. 

 In the narrowbody thrust class, adding a third engine to ingest the fuselage boundary layer would 

result in smaller powerplants, leading to inefficiency in core components and forcing a trade between 

powerplant component efficiency and ideal thermal engine efficiency. A synergy may be possible 

between SOFC power production and wake propulsion. The SOFC is designed in this architecture to 

provide electric power to the aft fuselage fan, and to provide gas power to the wing fans. A conceptual 
A-8 

layout of a candidate propulsion system employing boundary layer ingestion and wake propulsion in a 

tube and wing aircraft configuration is shown in Figure 4 and Figure 5. 

Figure 4 - LNG+2045GT+SOFC+DF+BLI (Wing Fans)

Figure 5 - LNG+2045GT+SOFC+DF+BLI (Fuselage Fan)
A-9 

The key characteristics of the propulsion system concept are summarized in Table 6. 

LNG+2045GT+SOFC+DF+BLI

Fan Diameter 59.1, 60.1 in

Length 124.1, 60.3 in

Propulsion System Weight 6467, 3584 lbm

Performance Thrust (lbf)

SFC 

(lbm/lbf/hr)

SLS 19106 0.188

Takeoff 15032 0.243

Top of Climb 3931 0.348

Cruise 3145 0.339

Table 6 - LNG+2045GT+SOFC+DF+BLI Key Characteristics

 Following the conceptual design to varying degrees of the five N+4 engine architectures shared above, 

tabular datasets were developed based on design point level analyses and provided to the Boeing team, 

along with weights, key dimensions and scaling rules. Boeing then installed the propulsion systems on 

the SUGAR High Aircraft (765-095) variants TS1-5 and evaluated the combined aircraft and propulsion 

systems for fuel burn reduction potential. The results to this analysis are shown in Table 7, along with a 

qualitative assessment of the noise and emissions characteristics of the concepts.
A-10

Case 1 2 3 4 5 6 7 

Config. 

Number 765-093 765-094-TS1

765-095-

TS1

765-095-

TS2

765-095-

TS3

765-095-

TS4

765-095-

TS5

Name SUGAR 

Free 

N+4 

Reference

N+4

High Wing 

Reference

SUGAR

Freeze

SUGAR 

Freeze

SUGAR 

Freeze 

SUGAR 

Freeze

Fuel JP JP JP LNG LNG LNG LNG

Engine CFM-56 

JP+

2045GT+ 

DF

JP+

2045GT+ 

DF

LNG+

2045GT+ 

DF

LNG+

2045GT+ 

UDF

LNG+

2045GT+ 

SOFC+

BLI

LNG+

2045GT+ 

SOFC+

UDF

Propulsor Ducted Fan Ducted Fan Ducted Fan Ducted Fan Unducted 

Fan DF + BLI Unducted Fan

Quantitative Scoring Goal

Block Fuel / 

Seat

(900 NMI)

(Base) -53.5% -54.5% -57.2% -62.1% -60.8% -64.1% -60%*

BTU / Seat

(900 NMI) (Base) -53.5% -54.5% -52.0% -57.6% -56.1% -59.8% -60%*

Qualitative Scoring Goal

Noise +3 0 0 0 +1 -2 +1 -71 dB†

LTO NOx 

Emissions +3 0 0 -1 -3 -2 -4 -80%‡

Cruise NOx 

Emissions +3 0 0 -1 -2 -3 -4 -80%*

Color Legend to 

NASA’s Goal Qualitative Ranking System

*

Relative to Baseline SUGAR Free Far From Goal Acoustics Emissions

†

Cum Margin Relative to Stage 4 Does Not Meet Goal Quietest -4 Least 

‡

Relative to CAEP/6 Nearly or Meets Goal 765-094-TS1 0 765-094-TS1

Exceeds Goal Loudest 4 Most

Table 7 - N+4 Performance, Noise and Emissions Summary
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Contractor Report

 4. TITLE AND SUBTITLE

Subsonic Ultra Green Aircraft Research Phase II: N+4 Advanced Concept 

Development

5a. CONTRACT NUMBER

NNL08AA16B

 6. AUTHOR(S)

Bradley, Marty K.; Droney, Christopher K.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center The Boeing Company 

Hampton, Virginia 23681-2199 Boeing Research and Technology

 5301 Bolsa Avenue

 Huntington Beach, CA 92647 

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

 

Langley Technical Monitor: Erik D. Olson

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 05

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

This final report documents the work of the Boeing Subsonic Ultra Green Aircraft Research (SUGAR) team on Task 1 of the 

Phase II effort. The team consisted of Boeing Research and Technology, Boeing Commercial Airplanes, General Electric, and 

Georgia Tech. Using a quantitative workshop process, the following technologies, appropriate to aircraft operational in the 

N+4 2040 timeframe, were identified: Liquefied Natural Gas (LNG), Hydrogen, fuel cell hybrids, battery electric hybrids, 

Low Energy Nuclear (LENR), boundary layer ingestion propulsion (BLI), unducted fans and advanced propellers, and 

combinations. Technology development plans were developed.

15. SUBJECT TERMS

Aircraft; Design; Electric; Emissions; Hybrid; Hydrogen; Noise; Nuclear; Propulsion

18. NUMBER

 OF 

 PAGES

148

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

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c. THIS PAGE

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b. ABSTRACT

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17. LIMITATION OF 

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Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

NNL11AA00T

5f. WORK UNIT NUMBER

561581.02.08.07.42.03 

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/CR-2012-217556

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 05 - 2012