Personal jetpacks using ultra-high power density compression-ignition low SFC engine technology

Personal jetpacks using ultra-high power density compression-ignition low SFC engine technology

Christophe Pochari

Man has dreamed of micro-air vehicles that are highly compact, essentially fitting around a single person, a flying backpack, that enables him reach the most challenging and confined geographies that even helicopters cannot tackle. Conventional aircraft have large footprints and exposed rotors, even recent EVTOL designs all feature not only large physical footprints but exposed rotors which risk sustaining foreign object damage in highly confined environments. While the jetpack has tremendous application in a wide range of end uses, such as surveying, surveillance, law enforcement, defense, etc, the Jetpack is not simply a tactical or utilitarian technology, it is rather an attempt to attain complete freedom of mobility, to unleash man from his bondage to the earth.

Jetpack technology has been historically handicapped by poor propulsion efficiency. While Martin Jetpack had solved this by using ducted fans, the Martin Jetpack suffered from poor prime mover power density. While the propulsion efficiency was stellar, the engine used had high SFC and poor power-weight. Few people realize modern diesel engine technology can achieve enormous power density. Decades of NASA research on compound cycle diesel engines originally studied for rotorcraft propulsion achieved power densities of 1.66 lb/hp with SFCs of 0.32 using advanced turbo-compounding. The research was abandoned as increasing turbine inlet temperatures permitted by the advancements in single crystal alloy turbine blades overshadowed the best efforts of reciprocating propulsion. Fast-forward 40 years since the 1980s NASA/Garrett turbo-compounding program and we find ourselves needing high power density low-SFC propulsion more than ever for advanced micro-air vehicles to achieve desired endurance requirements. Turboshaft propulsion cannot be scaled down efficiently below 500 hp within an acceptable SFC range. More recently, SMA, a subsidiary of Safran, designed a 530 lb direct-injection compression ignition engine producing up to 800 hp. This does not include potential increases through turbo-compounding. NASA’s original estimate was 1.6 hp/lb.

The second major enabling technology are light-weight ballistic low-altitude parachute systems. This technology has come a long-way and is poised to make Jetpacks as safe as conventional air vehicles.

Micro tactical air vehicle with 2 hour endurance 750 lb gw

Propulsive system: High speed CFRP ducted fan 215 mm (20,000 rpm) 232 lbf/ft2 38.5 lbf/lb 2.5 lbf/hp 3x

Prime mover: 240 kW 2 stroke diesel engine, 0.35 lbs/hp-hr (2 hp/lb, currently achieved is 1.5 hp/lb by SMA)

Transmission system: Epicyclic titanium gears with CFRP casing

Weight breakdown:

Ducted fan shroud and blades: 21 lbs

Fuel tank: 15 lbs

Fuel: Exo-Tetrahydrodicyclopentadiene 0.976 g/cm3 42 MJ/kg

CFRP frame: 30 lbs

Seat 6 lbs

Belt and shafts: 10 lbs

Radiator: 7 lbs

Coolant: 4 lbs

Electronic flight controls: 5 lbs

Powerplant 340 hp: 170 lbs

Ballastic Parachute: 35 lbs

Fuel load: 178 lbs

Empty weight: 298 lbs

loaded weight: 476 lbs

Payload: 274 lbs

*Note: picture contains an turbine engine for reference, a compound cycle diesel engine has similar volumetric density.

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