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| Fig. 1:The U.S.S. Nimitz, a Nimitz-class aircraft carrier. (Source: Wikimedia Commons) |
Nuclear fuels can provide truly staggering amounts of energy. For example, the range of a present-day U.S. nuclear powered navel vessels is best measured not in nautical miles, but in years or even decades. [1,2] So, in terms of number of miles, a nuclear submarine or aircraft carrier has an effectively unlimited range. But the tactical merit of an aircraft carrier is not just in sailing the high seas, but in its ability to launch aircraft; the useful range of an aircraft carrier is thus not limited by its nuclear fuel, but by its jet fuel.
U.S. nuclear aircraft carriers, such as the Nimitz class carrier (Fig. 1), can have a jet fuel capacity of around three million gallons, and routinely carry 50+ fighter jets. [2] Compared to the 12 gallon or so tank found in an economy car, this is a fantastic amount of fuel. But clearly, an F-14 fighter jet is significantly thirstier than a Ford Focus; so in terms of aircraft and aircraft carriers, then, how much fuel is three millions gallons? To start off with, typical jet fuel (JP-5) has a density of about 0.8 kilograms per liter (kg/L), so our typical carrier can hold about [3]
of jet fuel. For a fighter jet, I will use the F-14A Tomcat, a carrier-capable aircraft. The Tomcat has a maximum fuel capacity of about 9000+2000 liters (internal + external fuel tanks), or about [4]
so with all 50+ F-14s full of fuel, this is about 4.4×105 kg of fuel. Dividing the fuel capacity of the carrier by the fuel capacity of the fleet of fighter jets, we get that a typical aircraft carrier can refuel every fighter jet about 20 times before depleting its supply of jet fuel.
An alternate calculation can be done to see how many aircraft-hours an aircraft carrier can support, that is, the number of aircraft in the air times the number of hours of fuel available. This just requires a knowledge of the fuel consumption of fighter aircraft. The Tomcat has a one-way range of about 3100 km, fully loaded with fuel. [4] At its maximum cruising speed of 1019 km/h, it will run through all of its fuel in about three hours. From the fuel capacity above, this means it uses fuel at a rate of roughly 2900 kg/h. Dividing this into the capacity of our typical carrier, we get that our aircraft carrier can support roughly 3100 aircraft-hours; it could support all 50+ aircraft simultaneously for roughly two and a half days, or about 4 aircraft simultaneously in the air for a month.
Although these calculations are somewhat contrived, the point is that the jet fuel capacity of a nuclear aircraft carrier is actually not that large. Enough fuel to keep all the planes in the sky for a few days before refueling the carrier is nothing compared to the decades that the nuclear reactors can go between refueling.
Although refueling a carrier with jet fuel is significantly less arduous than refueling the nuclear reactors, it is nonetheless logistically and tactically cumbersome. Since the nuclear energy onboard seems limitless, a way of converting nuclear energy into jet fuel could be a "game changing" innovation. [5] As a quick sanity check, let's see if the nuclear powerplants can actually produce power on the scale required for powering aircraft. Jet fuel has a density of around 46 megajoules per kilogram, and a Nimitz class U.S. aircraft carrier's propulsion system is rated at about 200 megawatts, or 2×108 joules/second. So, in a single day, the nuclear reactors could produce energy equivalent to
or about 125,000 gallons of jet fuel per day (filling up the three million gallon tank in about 24 days). So, although an aircraft carrier does not appear to produce enough power to supply all 50+ aircraft running continuously, it is enough power to keep a few aircraft in the air, continuously. But this, of course, assumes that it is possible to turn nuclear energy into jet fuel with 100% efficiency--clearly, there will be some losses in the process.
A recent article looks at the feasibility of using seawater and nuclear power to synthesize jet fuel. [5] The entire process could take place at sea (e.g., on an aircraft carrier or a dedicated fuel-producing ship). In such a process, carbon (in the form of CO2) is removed from the ocean, and hydrogen is produced by electrolysis. From here, jet fuel can be synthesized through a Fischer-Tropsch process. (The Fischer-Tropsch process is very old, having been used by the Germans in World War II to make liquid fuels from coal.) [6] It is claimed that, with 200 megawatts of continuous power and a supply of seawater, 82,000 gallons of jet fuel could be created per day; from above, this translates to an efficiency of about 66%. At an estimated cost--assuming efficient carbon-capture from seawater--of $3-$6/gal, this is somewhat higher than existing jet fuel costs. However, the associated logistics of getting fuel to an aircraft carrier are circumvented, making this technology very appealing from a tactical standpoint.
Turning seawater into jet fuel is an incredibly roundabout way of avoiding refueling an aircraft carrier. And as a means to avoid dependence on foreign oil, it seems likewise roundabout--the U.S. has an ample supply of natural gas, which can also be used to produce jet fuel via the Fischer-Tropsch process. That said, the ability to turn seawater directly into jet fuel, with no dependence on foreign oil and in a nearly carbon-neutral fashion, is nonetheless intriguing. With the energy coming from the effectively limitless supply of a nuclear fuel, who cares if you burn a little extra uranium to power your jets?
© Brannon Klopfer. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Chockie and K. Bjorkelo, "Effective Maintenance Practices to Manage System Aging," Proc. Reliability and Maintainability Symposium (RAMS) 1992 (IEEE,1992), p. 166.
[2] John Birkler et al., The U.S. Aircraft Carrier Industrial Base: Force Structure, Cost, Schedule, and Technology Issues for CVN 77, (Rand Publishing, 1998).
[3] D. M. Korres et al., "Aviation Fuel JP-5 and Biodiesel on a Diesel Engine," Fuel 87, 70 (2006).
[4] J. Winchester, Jet Fighters: Inside & Out, (Rosen Publishing Group, 2011).
[5] H. D. Willauer et al., "The Feasibility and Current Estimated Capital Costs of Producing Jet Fuel at Sea Using Carbon Dioxide and Hydrogen," J. Renew. Sustain. Energy 4, 033111 (2012).
[6] E. Corporan et al., "Emissions Characteristics of a Turbine Engine and Research Combustor Burning a Fischer-Tropsch Jet Fuel," Energy Fuels 21, 2615 (2007).
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