Immediately after watching The Avengers movie (on opening day; and yes, it’s been quite a while), I set off, tongue firmly in cheek, to reality-check S.H.I.E.L.D’s Helicarrier. Professor Allain’s Wired blog “Could the S.H.I.E.L.D Helicarrier Fly?” arrives at a different conclusion than I did, so I thought it’s finally time to write up my alternate assessment.
Like the professor, I began by looking into Nimitz class aircraft carriers. Built using HSLA-100 steel, those 333 meter long carriers displace about 100,000 long tons. S.H.I.E.L.D, with flight as a design goal, would clearly upgrade to titanium, employ aircraft construction techniques, and use other advanced methods to lighten their 450 m Helicarrier by about half to 55,000 metric tons (t).
Next, could real fans fit in the space allocated and still generate enough thrust to lift 55 kt? Using the Harriers Alpha Jets on deck as measuring sticks shows that each of the four fans is 51 m in diameter giving a total rotor area of 8,200 m². The Helicarrier name invites comparison with helicopters, so I initially checked its fans against the remarkable Russian Mi-26 heavy-lift helicopter. The Mi-26′ has a maximum takeoff weight of 56 t using a rotor 32 m across to provide a lift/area ratio of 0.07 t/m². That herculean helicopter’s lift/area ratio provides only 1% of what S.H.I.E.L.D needs to fly. Helicopter rotors are out.
Fortunately, engineers have already done better, much better; the Rolls-Royce LiftSystem in the F-35B produces far higher lift/area ratios. For instance, the front LiftFan generates 20,000 lb of lift from a 127 cm (50 in.) fan and the whole system generates 19 t (41,900 lb) of lift from 2.51 m² of duct area (LiftFan, jet exhaust, and roll-posts). Its 7.56 t/m² ratio leaps two orders of magnitude past the Mi-26 and provides plenty of lift-off thrust for S.H.I.E.L.D’s headquarters. To sustain a single rotor failure, the Helicarrier engineers must have improved on Rolls-Royce by at least 14% to 8.61 t/m² or more. Nonetheless, the Helicarrier appears to fly within the laws of physics here.
Spinning those fans requires an enormous amount of power and generating it presents an even bigger engineering problem, but it hovers (barely) within the realm of possibility. The F35-B’s thrust ratio (55,000 shp delivering 41,900 lb thrust) implies the Helicarrier carries engine(s) capable of 157 million horsepower (shp) or ~117 gigawatts output (that’s more power than all of the nuclear power plants in the USA combined). Any power source would need to be scaled up, but allocating 20% of the carrier’s gross tonnage to the power plant sets the minimum power density at 10.8 kW/kg; for comparison, here’s a quick rundown of some real-world power densities:
- A Pratt & Whitney F135 jet turbine in the F35B produces 38 kW/kg
- The RS-25D Space Shuttle Main Engine delivers the highest power to weight ratio available today at 1445 kW/kg
- For something closer to home, the LS9 supercharged V8 in a Corvette C6 ZR1 produces 638 HP and weighs 530 lb (dry) for a power to weight ratio of 0.9 kW/kg
- The Indy Car Series 2012- engine weighs just 112.5 kg (248 lb) and puts out 522 kW (700 HP) 4.46 kW/kg or 5× the specific power of a Corvette but just half of what’s needed
- A Nimitz A4W nuclear reactor weighs a lot more but at least it contains enough fuel for two decades; it generates 0.05 kW/kg with older, 18% efficienct steam turbines
- The Los Alamos Heat Pipe reactor (alpha of 0.43) combined with 40%+ efficiency turbines raises its specific power to 0.8 kW/kg
Unlike the Nimitz aircraft carrier, nuclear reactors, even lightweight research reactors, won’t work. S.H.I.E.L.D would need around 530 A4W reactors to generate 117 GW and those would weigh over 2 million metric tons; that wouldn’t just ground the Helicarrier, it would sink it!
Shuttle main engines would weigh 81 t or 0.15% of the total. Using the rest of the weight allocation for cryogenic fuel provides 2 hours of flight time in a tank 111 m long and 11 m in diameter.
Turbines have high enough specific power but need to be scaled up a lot; just ganging together 3,000 P&W F135 turbines would be an engineering and maintenance nightmare! Real-world engines have high enough specific power for the Helicarrier, but scaling them up would be incredibly difficult.
Musings 