Hydrogen aircraft propulsion
The 21st century demands an updated and improved aviation propulsion technology. For over 70 years, kerosene has proven itself safe, reliable, efficient, and most importantly, energy-dense.
Hydrocarbons, being abundant, easy to store, and easily combusted in gas turbines, have resulted in the tremendous aviation industry we have today.
Current hydrocarbon aircraft can suffice, providing sufficient payload and range for most missions, only in special occasions is more capability required. Pochari Hydrogen believes the long-range light business jet is an aircraft category that requires a more powerful and modern propulsion system.
We are rapidly approaching a turning point in aviation history. Efforts to improve aviation propulsion are reaching a wall. Efforts to squeeze every last few percentages points of efficiency in turbines are proving increasingly difficult. The gas turbine’s maximum practically attainable efficiency is around 40%, going above this number is nearly impossible. Efforts to improve aerodynamics have also reached the point of diminishing returns. Reducing airframe weight through the use of composites, mainly carbon fiber, has yielded very minimal reductions in airframe mass, in the order of 5-10%. Thus so far we can state that aviation propulsion and airframe design has reached its peak in terms of technological maturity, there is almost nothing left to do to improve aircraft capability, at least nothing currently considered in the aviation industry.
In summary, we have reached the point of diminishing returns.
If little to nothing remains to improve in the powerplant, airframe aerodynamics, or even the fundamentally design itself, we must begin identifying a superior fuel.
Quite surprisingly, there remains a mysterious, obscure, and powerful propellent, the most powerful of any fuel known to man, that attracts little to no attention in aviation circles, this fuel is nothing other than liquid hydrogen as proposed by G. Daniel Brewer in the late 1970s in Burbank, California, and revived by Christophe Pochari in 2018.
Liquid hydrogen is the most powerful fuel available on earth, with gravimetric energy density unmatched by any other fuel, hydrogen’s only downside compared to hydrocarbon is its low volumetric density, but this proves not to be the least bit of a limitation for subsonic flight. For subsonic flight at high altitude, the dominant form of drag is skin friction, a 20-foot diameter airliner fuselage section (Boeing 787) with a total wetted area of 9500 square feet is only subjected to 4400 lbf of drag with an air density of 3.5 Psi and a cruise speed of 560 mph. (http://www.lissys.demon.co.uk). This small amount of drag present in subsonic flight allows us to store ample quantities of hydrogen while adding minimal drag, provided we choose a configuration with the lowest possible surface/volume ratio.
The turbine powerplant itself remains nearly identical, having only slightly modified combustors designed to take advantage of hydrogen’s much higher flame speed. “Hydrogen is a beautiful fuel for turbines” -Willis Hawkins.
Vaporized or liquid hydrogen is delivered from the LH2 tank to the turbine with aluminum piping from dual high-speed centrifugal liquid hydrogen pumps directly attached to the rear of the tank. The boil-off rate in cruise is nearly half of the cruise fuel flow, the difference is provided by vaporizing liquid hydrogen in the fuel supply line or combusting it liquid.
An electric heater maintains a certain vaporization rate to maintain 20 psi in the tank on the ground, and 15 psi during cruise, if the pressure exceeds this, the tank is vented.
Liquid hydrogen can also be combusted directly in the turbine just as gaseous hydrogen can. During takeoff, when the fuel flow required is at a much higher rate than the natural boil-off, the electric heater is turned up to increase the vaporization rate to maintain pressure, shortly after, when fuel flow is reduced to cruise level, the heat is turned down. Turbine compressor bleed air is cooled by the -423 degree LH2, through a light-weight aluminum heat exchanger. Thisallow for higher turbine inlet temperature, and reduced compressor bleed air mass flow, reducing compressor work require, this contributes to a 5% reduction in SFC (Brewer, G. D, 1991)
The total ratio of SFC is reduced by 2.93x over the 2.8x difference from the calorific value difference alone. (Brewer, G. D, 1991)
The liquid tanks are integral with the fuselage. The fuselage material is made of carbon fiber, the tank material is aluminum-lithium, fastened directly to the carbon fiber fuselage with adhesive. 12.7mm of insulation is placed between the carbon fiber skin and aluminum tank wall. Foam insulation is blown inside the frame and between the frame-wall seams where the vacuum panels end. Tank boil-off is 5% per hour on the ground. A specially designed fuel truck connects a flexible hose to the tank when the aircraft is on the ground to vent the boil-off, keeping tank pressure at 21 psi. Hydrogen aircraft are fueled just before take-off, to minimize losses.
The higher energy density of hydrogen allows for a dramatic increase in aircraft range, exactly what business aircraft need. With our hydrogen propulsion system, a 35,000 lb GW aircraft can fly up to 9000 miles non-stop with a 2700 lb payload, whereas the comparable class kerosene-fueled aircraft can only fly 4700 miles, if it were to attempt to fly the equivalent kerosene-fueled aircraft, the fuel load would exceed the useful load by 160%.
Willis Hawkins (C-130 designer) and a strong proponent of hydrogen aircraft, argued nearly a doubling of range is easily achieved with liquid hydrogen propulsion, this turns out to be exactly correct.
There are two fundamental ways to design a hydrogen-powered aircraft, depending on the incentives and aircraft capability required.
The first way is to essential reconfigure existing aircraft types to hydrogen propulsion, by incorporating an in-fuselage tank. This can be done by simply stretching the fuselage, without needing a design an entirely new clean-sheet aircraft. This design may not necessary increase range or payload, since the tank capacity may be limited by the fuselage volume. This type of hydrogen aircraft design would fly approximately the same distance and carry the same payload as kerosene aircraft. The rational to develop this type of hydrogen aircraft would mainly be emission and environmental impact reduction. The only emission from hydrogen combustion in properly designed gas turbine engines is minimal nitrogen oxides, less than 50 ppm. No CO2, CO, PM or any other emission is produced.
The second way to design a hydrogen aircraft is to tailor the design around hydrogen’s improved range capability. This is the option that makes the most sense, as it takes advantage of hydrogen’s superiority as a propellent, allowing the designer to create an aircraft specifically tailored to utilize hydrogen’s enormous performance benefits. Our design is what we term a “flying rocket” which refers to the hydrogen tanking up 85% of the of the aircraft’s volume. This aircraft dubbed the “flying rocket” is a light-jet, under 35,000 lbs gross weight, designed to fly almost 9000 miles (17 hours of flight time) non-stop while carrying a 2500 lb payload, which translates into 6 persons, two pilots, and ample luggage capacity. This ultra-long-range in a relatively small aircraft can only be accomplished with hydrogen, no hydrocarbon fuel possesses the gravimetric energy density. Hydrogen’s outstanding properties as a propellent allow for the creation of an entirely new class of aircraft, that is the small ultra-long-range jet, previously impossible with hydrocarbon fuel.
Hydrogen is a far superior, for more powerful propellant for all aircraft alike, but it improves the capability of smaller aircraft even more. Since lift to drag ratios decrease with size, and specific fuel consumption increases, smaller aircraft gain more from high energy density of the fuel, these two factors have the most influence on how much an aircraft improves with hydrogen. As G. D Brewer said, “the more energy required to perform the mission, the greater the advantage to be gained by using a high energy fuel”
Six factors that determine the performance increase with hydrogen.
#1 L/D ratio
#2 Powerplant SFC
#3 Desired endurance
#4 EW/GW ratio
#5 Tank parasitic drag penalty
#6 Tank surface/volume ratio (Blended wing body aircraft (BWB) or flying wings unsuited for hydrogen propulsion, since multiple small tanks are required)
Hydrogen aircraft propulsion FAQ
#1 Why has no one done this before?
For three reasons.
#1 kerosene remained extremely cheap up until the 21st century, discouraging the development of alternatives, that is hydrogen, despite the performance gains attainable. The price of kerosene still remains under $2/gal, thus we are strongly incentivized to use hydrogen from low-cost energy sources, mainly hydropower, with the lowest levelized cost of $0.85 cents/kwh. Liquefaction adds $1/kg, electrolyzer CAPEX is $0.50/kg, in total we can confidently state hydrogen can be produced for $2/kg for from clean sources, and $1/kg from waste or coal gasification.
#2 The aerospace industry is highly conservative, and unwilling to experiment with unproven technology, capital investment requirements are often very high, and with certification costs, it may take 10 or more years to fully recuperate the development costs, disincentivizing the development of new propulsion technology, especially “unproven” ones.
#3 Most experts in the aerospace industry are not even aware of this obscure and forgotten propulsion technology, G. Daniel Brewer’s book remains in complete obscurity. Prior to Christophe Pochari’s efforts to revive Brewer’s ideas, virtually no attention was paid to hydrogen. Quoting Allan Epstein from Pratt and Whitney “no one has been able to store hydrogen at less than 10% hydrogen by weight, the rest is the tank” His comment summarizes the stance of the current aviation establishment, and indicates the broader industry being almost completely unaware of G. Daniel Brewer’s work and research, of which is freely available on the internet.
#2 Is it safe?
Safety concerns are likely to arise as an argument against a hydrogen propulsion system. Hydrogen will combust at a significantly higher rate than Kerosene. An Ignition source is most likely to occur as a result of a hard impact from a crash. The minimum ignition of energy (MIE) of hydrogen and kerosene is well below even the lowest level impact force that would cause an ignition. In other words, a violent impact, lightning strike, engine failure, foreign object damage, drone strike, or an onboard fire, will ignite kerosene just as easily as liquid hydrogen. A sufficiently violent impact will cause a fracture in the fuel tank regardless of whether it is hydrogen or kerosene. The only major difference will be the manner in which hydrogen will detonate compared to kerosene. Kerosene will detonate, and then slowly burn in a deflagrative manner. Hydrogen will almost immediately detonate completely leaving no residual fuel to burn. G. Daniel Brewer was firm in his belief that hydrogen-fueled aircraft would be significantly SAFER than kerosene-fueled aircraft, thanks to the high flame speed of hydrogen, long periods of deflagration upon impact inherent to jet fuel is no longer an issue with hydrogen, allowing more time for passengers to escape.
#3 Is it really possible to double the range?
This all boils down to how smart the designer is.
If the designer chooses a tank with a fuselage integral tank with a low surface/volume ratio, then yes the designer can realize tremendous increases in range, doubling is realistic with the same payload as the max range under kerosene.
#4 How do we plan on actually building a jet, is that something only large corporations can do?
Our company is a design and innovation company, led by its founder Christophe Pochari, who is solely credited with reviving and bringing back Daniel Brewer’s work to the mainstream of aerospace propulsion. Our company will use established aerospace manufacturers to carry out the manufacturing and assembly in China and India, Pochari Hydrogen provides only the basic design and idea, albeit a revolutionary and game-changing idea. Our main goal is to promote this technology and idea, and find people with the necessary manufacturing capability.
Computational fluid dynamics is done with Patrick Hanley’s Stallion 3D, Patrick Hanley is a follower of Christophe Pochari on Twitter. Composite fuselage design and fabrication is done in China. Powerplants, small turbofans, are built in house using turbomachinery parts from Shandong Yili Power technology LTD. The turbofan is based off the proven all centrifugal compressor PW100 turboprop architecture, converted to a turbofan.
#5 What are the main differences between hydrogen and jet-fueled aircraft?
The powerplant differences are minimal.
The combustors are slightly different for hydrogen fuel, the principle difference is they are significantly shortened. Nothing in the powerplant itself changes except the fuel delivery system changes. Liquid hydrogen is pumped from the tank with two high-speed centrifugal pump, then the liquid hydrogen flows through the supply line to the engine vaporizing along the way and subsequently raising the gas pressure until reaching 150 psi where it is injected in the combustor. A separation between the low-pressure liquid tank and the higher pressure gaseous hydrogen is required, dual centrifugal pumps provide this separation.
The liquid hydrogen pump is placed right outside the rear of the tank towards the end of the aircraft, the pump is mounted directly to the tank, the second pump adjacent to the first pump, the higher pressure liquid is then routed to the engine in small diameter aluminum piping, serving essentially as a makeshift heat exchanger to vaporize until sufficient pressure is reached. Only a certain margin above the combustion chamber pressure is required, this is in the range of 100-200 psi.
#2 Airframe and aircraft architecture.
The main difference with the airframe and fuselage is the integration of the liquid hydrogen tank. The fuselage is designed with an optimal surface/volume ratio, enabling sufficient volume to store the liquid hydrogen necessary to perform ultra-long-range missions.
The basic aircraft architecture remains identical.
The architecture is a pressurized tubular fuselage with a low wing monoplane configuration with a T-tail with rear-mounted engines. The center of gravity is not altered, in fact the elevator authority is increased due to the longer fuselage. The only possible disadvantage to hydrogen aircraft is reduced agility due to the larger fuselage, which makes the aircraft “fatter” resulting in more drag, which impairs agility, business jets are not fighter jets, smooth flight characteristics are desired, so this is not an issue.
#3 Fuel containment and tank design
Tank design is arguably most critical element of a hydrogen aircraft.
We want to maintain the lowest pressure possible in the tank, to reduce stresses on the tank wall skin, but we also need to achieve sufficiently high pressure to overcome the pressure inside the combustor through vaporizing liquid hydrogen pumped from the tank. To reduce tank wall stress, the fuselage-tank section is pressurized to 11 psi, while maintaining 21 psi in the tank. In the absence of pressurization, significantly more stress is imposed on the thinner less strong aluminum tank wall. By pressurizing the tank-fuselage section, we transfer the majority of the pressure differential stresses to the stronger carbon fiber skin.
The fuselage-tank section is comprised of a 3mm thick outer skin, made of Toray T1100 carbon fiber, a spacing to separate the insulation from the skin and the tank.
The carbon fiber is sheltered from the low-temperature tank and insulation by a layer of air. The monocoque structure comprised of the outer carbon fiber skin and aluminum tank results in an extremely rigid fuselage-tank section.
The tank is fastened to the outer skin with high strength epoxy, no mechanical fasteners are used. Stringers are bonded to the carbon fiber skin, then to the aluminum tanking, forming a fully integral tank structure.
Carbon fiber, although much stronger than aluminum, is not attractive for continuous use at cryogenic temperatures, carbon fiber becomes brittle at low temperature, microcracks can also develop due to the differential in the coefficient of thermal expansion of the individual fibers. In addition, most resins used to bond composites become brittle at low temperature. Thus metal is the most attractive, among the different alloys available, aluminum-lithium provides the highest specific strength, or commonly referred to as strength-weight ratio. Aluminum-lithium is higher even than titanium, the second most attractive option for tank wall material. Aluminum lithium 2195 actually becomes stronger at cryogenic temperatures.
The insulation is placed directly on the outside of the aluminum tank. Vacuum panels insulation, comprised of a membrane wall, used to prevent air from entering the panel, a panel of a rigid, highly-porous material, such as fumed silica, aerogel, perlite or glass fiber, to support the membrane walls against atmospheric pressure once the air is evacuated. These panels manufactured by NanoPore Incorporated, Albuquerque, NM, provide the lowest thermal conductivity with the lowest weight. The thermal conductivity, when accounting for the extremely low surface temperature when the tank is full of liquid hydrogen on the ground, can be as low 0.0025 W/m-K. These vacuum panels provide lower thermal conductivity to mass ratios than S-180 spray-foam insulation. The vacuum panels are specially formed to take the shape of the tank, and fit between the frames/stringers. Foam is sprayed around the seams and inside the frame volume, minimizing thermal bridging.
In summary, the tank is actually very simple and based on mature and proven technology and structural configurations. The insulation is light-weight and provides manageable boil-off when the aircraft is not in flight. The weight of the entire fuselage-tank section weighs 1.18 lbs/ft3. This translates into 2700 lbs for a 4700 kg tank, providing nearly 17 hours of flight time, or 8700 miles. The additional drag (Flat plate drag based solely on wetted area) from the 1200 square foot wetted area is 0.46 lbf/ft2, resulting in 550 lbs of additional thrust required, resulting in a fuel burn of 58 kg/hr, 22% higher than without the hydrogen tank, this reduces the L/D from 17 to 13.2, approximately the same that G. D Brewer found. This calculation is very easy to perform, simply by taking the total zero-lift drag and dividing by the wetted area, then multiplying by the wetted area increase.
main focus is the development of an ultra-long-range light business
jet, but many other types of aircraft would benefit tremendously from
hydrogen propulsion, mainly narrow-body airliners and
In the case of a narrow-body airliner, such as an A319, we could extend the range with an acceptable payload from 3300 miles to over 7600 miles, allowing small narrow-body aircraft to fly much more profitable transpacific routes. This would give airlines the option to purchase a more affordable and smaller aircraft, that can land and operate from smaller airports with lower landing fees. Another benefit of narrow-body aircraft is it’s much easier to fully occupy the aircraft in a shorter period, allowing for higher and more profitable utilization. In summary, this narrow-body would give airlines a viable alternative to wide-bodies for transpacific flights.
Helicopters could be designed to fly longer range offshore oil and gas missions, or fly similar distance with more payload.
The main application of civilian helicopters is off-shore oil/gas, which currently suffers from depressed activity, when this improves, Pochari Hydrogen will begin designing a hydrogen-powered 10,000 lb class rotorcraft for long-range missions. Currently, the longest range civilian rotorcraft is the Leonardo AW139, capable of flying 750 miles with a sufficient payload to carry 15 oil and gas workers, we can improve this to 1400 miles.
The third type of aircraft would be a supersonic business or passenger jet.
This type of aircraft was demonstrated to the most attractive application of liquid hydrogen by Daniel Brewer (1975), but more recent analysis from computational fluid dynamics, and better understanding of wave drag, may result in this not being entirely correct.
The reason is simple, at supersonic speeds, a turbofan engine cannot function, a turbojet or very low-bypass turbofan is required, typically these engines have much higher specific fuel consumption, typically over 1 lb-lbf-hr, instead of 0.70 lb-lbf-hr for subsonic turbofans. As a result, the higher fuel consumption adds up the course of the flight, resulting in the fuel load fraction being much higher, incentivizing to a greater degree the use of a higher energy fuel.
But there’s a caveat, as we previously illustrated, in subsonic flight, the airliner fuselage, 20 feet in diameter, with a wetted area of 9500 square feet, is only subjected to 4400 lbf, but in supersonic flight, this is increased tremendously, to 80,000 lbf (Bjorn, Fehrm, 2018).
The source of drag changes from subsonic to supersonic, skin friction is no longer the dominant form of drag, rather, wave drag appears, in great magnitude. This necessitates a much higher fineness ratio, forcing us to reduce the fuselage diameter, and elongate the fuselage, the goal is minimizing the frontal area as much as possible to minimize wave drag, the theoretically lowest wave drag shape is a Sears/Haack body. The “area rule” “Whitcomb rule” is also critical for minimizing wave drag, but since this forces the designer to reduce the thickness of the fuselage to accommodate the increasing frontal area of the wing, this means our hydrogen tank is forced to be smaller.
In supersonic flight, the ideal fineness ratio is over 20, for subsonic, it’s only 4. (Roskam, Jan, 2017). The low ideal fineness ratio allows for very low surface/volume ratio LH2 tanks, reducing weight and wetted area penalty.
The total drag penalty in supersonic flight is much higher, and not fully compensated by the additional fuel consumption. As a result, supersonic aircraft powered by hydrogen will provide a smaller advantage over kerosene.
The fourth applications is defense-related, mainly long-endurance fixed-wing UAVs, such as Predator drone type aircraft, an increase in the flight-endurance of 100% could be more than attainable. The second defense application would long-range bombers and strategic air-lifters, allowing for more payload and flight endurance in the case of bombers.
These defense aircraft are obviously out of the development scope of small civilian aircraft manufacturers, but nonetheless remains very promising business opportunities. Daniel Brewer (1991) argued liquid hydrogen was extremely promising for military aircraft.
35,000 lb GW multi-purpose ultra-long range light jet with liquid hydrogen propulsion
GW: 35,000 lbs
EW: 19,300 lbs
EW/GW fraction: 55%
Max L/D cruise: 13.2, 22% lower than Jet Fuel equavelant.
Overall length: 76′
Cabin length: 12′
Cabin height: 5′
Cabin volume: 400 ft3
Wing loading: 74 lbs/ft2
Airfoil: Supercritical (NACA)
Useful load: 15,700 lbs
Fuel burn: 263 kg LH2/hr
Cruising altitude: 51,000 feet
LH2 capacity: 4700 kg
LH2 tank weight: 1.2 lbs/ft3: 13,180 lbs system weight total
Range: 17.8 hours, 7800 nmi, 9000 miles
Cruise speed: 440 kts
Additional surface wetted: 1237 ft2, 55% wetted/volume
Drag force FL370: 58 kg/hr (0.46 lbf/ft2/S wt)
Drag force FL450: 47 kg/hr (0.36 lbf/ft2/S wet)
Payload at max range: 2,500 lbs
Passenger capacity: 6
Powerplant: 2x 5500 lbf (take-off) turbofan
Powerplant model: In-house centrifugal compressor single spool, single crystal HP turbine blades (architecture based on PW100 core)
Take-off SFC: 0.407 lb/lbf-hr
Cruise SFC: 0.685 lb/lbf-hr
LH2 Tank material: Aluminum-Lithium 2195
Fuselage material: Toray T1100 Carbon fiber
Tank-fuselage pressurization: 4-7 psi at 51,000 feet.
LH2 tank Insulation: 0.5” 0.0025 W/m-K Nanopore vacuum insulated panels with JFOAM™ S-180 foam filler between frames
Tank boil-off rate on ground: 4.8%/hr
LH2 boil-off on ground: 214 kg/hr
Boil percentage of cruise fuel flow: 55%
Max fuel flow at take-off: 700 kg/hr
Total tank heat flux at ambient temperature: 19,000 BTU/hr
Operating cost: $1600/hr
Runway takeoff distance: 4000 ft
LH2 system weight breakdown in Lbs and percentage
For a 35,000 lb business jet. JET FUEL aircraft L/D: 17 LH2: 13.2. SFC cruise: 0.685 lbs/hr, EW/GW: 55%