The Airborne Metro

1. The concept
This vast concept envisages an era when climate changing emissions, ground congestion and noise pollution will no longer be acceptable at their presently forecast levels. It seeks to sustain the apparently unlimited demands of people for travel without the huge penalties that these are beginning to impose.
The concept depends upon the feasibility of very large flight vehicles, perhaps carrying over 3000 passengers and for these eventually to be nuclear powered. These large "Air Cruisers" would stay aloft on a semi-permanent basis receiving their loads of passengers, freight and supplies in mid-air.
The concept does not require every airport or airline to be compatible with the cruiser concept but most of the large hubs would need to be capable of working in this way.
Starting from the top level of the concept we could have the Airborne Metro system populated entirely by these long-range cruisers. They would travel on predictable, efficient looping paths that take each individual cruiser near to several major population hubs. Their flight paths would also be arranged to intersect with those of other cruisers so that links could be made to anywhere on the global cruiser network by changing from one cruiser to another.
Cruiser tracks would be followed by a considerable number of similar cruisers each following the other at intervals of perhaps an hour. They would be controlled from the ground and have defined track and altitude instructions together with emergency diversion paths. The North Atlantic route might have 20 or 30 cruisers in its looping path from Europe over New York, Washington and Chicago to Los Angeles and back. Reverse tracks would also be flown to provide more convenience for passenger routing.
At the level of the air-cruiser the system has many attributes of a metro system. It works to fixed routes, it has interchange locations and passengers can usually choose between several routes for the most popular journeys.
Interchanges present specific challenges requiring two cruisers to fly with precision and to dock together whilst airborne for the exchange of passengers.
Feeding the cruisers with passengers and freight would require a fleet of locally based aircraft probably of a standard specification and carry 100-200 passengers. These aircraft would be equipped with docking facilities allowing them to link with the cruisers. The internal arrangements of the aircraft would also reflect this special to role design. For example the loading of personal baggage might need to be streamlined to allow hold bags to be pax-loaded into a designated cargo pod for transfer to the correct cruiser flight.
These feeder aircraft could take off from conventional airports, with or without ground power assist schemes1, and would then fly an intersect course to permit docking with the cruiser and the transfer of passengers. The height, location and duration of these manoeuvres would need to be optimised to give the least impact on journey times and greatest economy. Preliminary calculations suggest that a round trip feeder link from ground base to ground base might last about two hours. This would imply that the passenger takes about one hour from the departure lounge at ground base to being installed in the cruiser. Feeder aircraft would fly relatively fixed routes and be to a "frills-free" concept of mass transportation since the average occupancy time would be something under an hour.
Airport congestion could be materially reduced by such a concept. Airports would have very many fewer VLCT movements and would, instead, be focussed on the rapid throughput of passengers onto and from feeder airlines. This would allow the airport to be re-designed to permit, for example, 4 runway operation and materially reduce airport holding populations. Passengers would have to take responsibility for part of this transaction and ensure that they boarded the correct feeder for their destination. Booking "hard" tickets for a particular flight might be phased out in favour of a controlled board-when-you-come approach. Improvements in the power and reach of computer processes by the time of introduction would no doubt allow this to be adequately controlled with least passenger inconvenience and waiting. This would to a major extent compensate for the additional travel time induced by the feeder/cruiser link. Passengers arriving at the airport would be able to register for a seat in the next cruiser available and thereby spend less time at the airport

2. Delivering benefits
The main benefits foreseen for the concept are:

  • Overall carbon fuel economy and in climate impact.
  • Reduction in airport congestion.
  • Reduction in noise at airports.

The reduction in carbon fuel consumption requires the issue of airborne nuclear engines to be solved. This is a major challenge having issues of size/weight, containment, safety and energy conversion to overcome. Solving these and other challenges of the concept would require a sustained and protracted research programme. Such programmes would build on the work done to date (e.g. the ANP of the mid-50’s that proved nuclear propulsion feasible). Nuclear engines favour use in very large aircraft increasing their weight only as the square root of the size difference. If such a concept is realisable in practice then a nuclear cruiser fleet would contribute virtually nothing to global warming. By 2050 aviation carbon emissions might contribute a global annual saving of 0.23 - 1.45 GtC2.Taking a 20-year horizon and valuing carbon consumption saved at €23 per tonne then the potential worth of the concept could be of the order of 300 € Billion. Against this would need to be set the R&D costs of providing it and the costs of deploying the technology.
The design and construction of the very large high-speed cruiser aircraft would be a substantial challenge and no data exist on such enormous aircraft. However, theoretically such an aircraft is possible even if it needed a wingspan of more than 160 metres. There have been arguments that maintain that since the weight of the wing structure increases to the power 3 on size there is an upper ceiling weight that cannot be exceeded. However studies have been done3 that suggest that even aircraft with wingspans exceeding 130 m are progressively more economical in terms of cost per seat mile.
The benefits of reduced airport congestion would require populations of people to be reduced. Passengers would be required in the airport for less time. The technologies to achieve this along with the no-frills concept of the feeder liner are already well advanced. It would require an approach based more upon ground metro travel than on lingering beliefs in air travel being something special. Security would take into account the passenger and their luggage on some form of real-time scanning whilst the passengers continue down a moving belt.
Self-loading of cargo could be done with present technologies.
Noise at airports would be reduced as a consequence of having fewer, perhaps no,VLCT4 s flying. Smaller feeder aircraft would be designed with noise reduction technologies, shielded multiple engines, and lower takeoff speeds.
One of the greatest technological challenges would, of course, be the concept of docking. This would need to take place at relatively high speed - probably around M0.7 and would need to be fully and securely automated.
For the feeder/cruiser link and assuming that the cruiser was some kind of BWB a/c a docking on top of the cruiser would seem practical. Research would be needed to ascertain the flow filed around the cruiser and how these might be managed to allow docking. The docking mechanism and the flight characteristics of the docked pair would also need extensive research.
Once docked the passengers would need to join the cruiser through some sort of port leading down from the feeder to the cruiser on stairway.

3. Policy issues
The most important political issue with the concept would be the safety and security of the cruiser. No doubt there will be strong voices expressing opposition to the concept but challenges of similar magnitude have been overcome before. But optimism about the future should not overlook the real challenges of making a nuclear cruiser a practical, safe and secure prospect.
The cruiser is not intended to land frequently. Its enormous size would allow landing only on a relatively few service bases and landing would need to be a considered step. This would prompt designers to allow for a range of in-flight incidents to be dealt with during normal flight or to provide sufficient margins for failed systems not to be replaced immediately.

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