The Airline Metro System

When I was travelling in Chile a short while ago I took a flight from the capital Santiago de Chile to the city of Calama in the Atacama dessert. What was interesting about this flight, was that on its way to Calama the airplane landed for a short stop in Copiapó. Immediately after leaving the runway the doors opened, a couple of people got off and were immediately replaced by others already waiting on the tarmac. I had never seen this metro-style system of operating an airline before and was surprised how efficient this system was being implemented. I was also struck by the albeit ludicrous idea of operating an air-bus (no pun intended) style fixed travel route between major European cities, say London-Paris-Madrid-Rome-Vienna-Berlin-London, with people hopping on and off at their pleasure. How cool would that be?

I understand that the fixed costs of this system would be relatively high, and making any money on the tight margins that airliners are operating on would be incredibly tough. However, research is currently ongoing to realise a similar system for long distance travel. One possibility is exploiting the concept of air-to-air refuelling that has been used by the military and the Air Force One for many years. A collaborative European study Research on a Cruiser-Enabled Air Transport Environment (Recreate) has been running simulations at the National Aerospace Laboratory (NLR) in Amsterdam since 2011. The aim of these simulations is to investigate the technical challenges and potential savings of refuelling airliners in midair.

Leading Boeing 707 refuelling a trailing 747 using a rearward extended boom

This may sound like a fanciful notion but given that airlines have to cut the 2005 carbon emissions in half by 2050 it well worth looking into these radical ideas. In fact, preliminary results of the study show that fuel burn could be reduced by 11% to 23% if airliners could be refuelled by tanker planes. Passenger safety being paramount in civil aircraft the military concepts currently in use will have to be adapted to meet the required reliability standards. In military environments the tanker flies ahead of the aircraft and supplies fuel through a boom from above. To reduce the likelihood of collisions a forward extending boom refuelling from the bottom is the solution preferred by the researchers. In this manner the civil aircraft does not fly in the wake of the tanker, which could affect turbulence and passenger comfort. Furthermore, the responsibility and training remains with the tanker pilots who have better visibility of the refuelling process when flying from the rear.

The researchers also intend to take the concept one step further by exchanging cargo and passengers in midair, thus getting closer to the idea of an airline metro system. This research envisions a new type of large cruising airliner that is fed by much smaller feeder planes. In this scenario, the larger cruisers fly fixed routes over large distances, while the smaller feeders exchange passengers, crew and cargo with the cruiser in midair. One major challenge with the scheme is that the cruiser aircraft will require an incredible durable engine with low fuel consumption. Such a system does not seem to be economically feasible using current chemically fuelled jet engines. The greater amounts of fuel to be stored has to be offset by a larger engine and airframe, which naturally increases the loads on components in turn requiring thicker sections and structures. Thus, with current gas-fuelled engines you are very much caught in the downward payload spiral that is so frustrating in rocketry.

But what if the cruisers are propelled by nuclear engines? Well the efficiency of the system improves significantly. In fact the efficiency gains are so great that a large cruiser could fly continuously for a whole year just on a few litres of gasoline. Powered by nuclear fusion a cruiser could stay airborne for months, and passengers could hop on and off a continuously airborne global fleet of international airlines.

And it turns out that in October 2014 Lockheed Martin’s Skunk Works announced that they could have a prototype fusion reactor ready within five years and a working production engine within ten.  The obvious “buts” are that that a fusion process requires temperatures in the millions of degrees in order to separate ions from electrons which creates hot plasma in the process. In fusion the danger is not a nuclear fallout as is the case in fission. The problem with fission engines is that they require shielding to protect passengers and also carry the dangers of spreading radioactive material in the event of a crash. In a fusion engine the difficulty is in stabilising the plasma and safely containing it in the reactor to guarantee the fusion of ions. The Skunk Works are currently working on an eloctro-magnetic suspender system to guarantee a stable reaction. Furthermore, neutrons that are emitted in the fusion process can damage the materials in the containing structure and turn them radioactive. Thus materials that minimise this radioactivity are needed. Finally, the fusion reactors need to be miniaturised from the scale of family houses to something more akin of an SUV. In that event fusion reactors will also become an interesting propulsion method for spaceships and other spacecraft that have limited space for power generation.

While this is all science fiction for now it presents an interesting option for facilitating a global metro-style airline system. And how cool would that be?