James May's big transport challenge gives you the opportunity to compare different modes of transport. We have extended the range of vehicles to include more mundane forms of transport such as bus and train. It would have been nice to include others - such as ships and aeroplanes - but we had to draw the line somewhere. We had to perform many calculations to arrive at the figures and graphs that you see on this site and this section will hopefully give you some insight into how we arrived at our figures.
There are ten different forms of transport described here and true journey comparisons are very challenging because of the wide range of fuel types, routes and modes of operation.
For example, aircraft fuel consumption is normally measured in gallons per hour, cars in miles per gallon and electric trains' energy consumption in kilowatt hours per passenger kilometre. In addition to this, craft such as the space plane (which don't yet exist in a commercial form) may use two different types of fuel; one within the Earth's atmosphere and one beyond. The route from London to Paris by ekranoplan is likely to be very different to that by train given that the ekranoplan would have to go by water. In looking at routes, we allowed road vehicles such as the solar powered car to cross the channel by ferry but not the jet pack pilot which is perhaps a little unfair.
In assessing the energy used we also have to consider the number of occupants a vehicle might contain. Trains and buses usually travel with a number of passengers, whereas cars often travel with a single occupant.
This presented two issues: which vehicles should we treat as having multiple occupants and, if so, at what proportion of their capacity should we assess them? We decided that in the case of trains, buses, space planes, airships and ekranoplans we would treat them as "public" and assume an appropriate level of occupancy. There is a case for making the ekranoplan personal, however, we decided that, given its origins as a troop carrier, we'd make it public.
Generally, you can't book a ticket to travel by airship at the moment so we modelled our energy use and performance characteristics on the Skyship 600 originally designed by Airship Industries. You can find out more about this airship from this Wikipedia entry.
Strictly speaking it's not correct to say that the solar car doesn't require fuel; it's using nuclear energy from the Sun. But that energy is freely available rather than being extracted from a fossil fuel such as petrol. As I've said above we did allow the solar car to travel across the channel, we did this simply because that's the way (or one of the ways) that you'd undertake the journey.
Again, rocket packs are not really readily available, although there are some interesting developments in this area. We based these figures on the original Bell Rocket Belt rather than the British design that James demonstrated simply because there were figures available. Both are fuelled using hydrogen peroxide and have similar characteristics. The huge fuel consumption is caused by the fact that hydrogen peroxide carries its own oxygen, rather than using oxygen from the surrounding air. Interestingly the main thrust (no pun itended) of development is around jet packs using fossil fuels (Jet A) so that it doesn't have to carry its own oxygen. This will require a jet engine of some sort, and the weight penalty that this will incur, but could increase the range and flight duration considerably.
The Aerocar presented some interesting issues. For many of the tasks you could simply use it as a car although given how it drives you might choose not to. As a plane we assumed you'd have to go to an airport and incur some lost time converting to a plane. Of course it is 1950s technology so perhaps very unfair to compare it with the twenty-first century technology of the space plane.
This is probably the oddest comparison that we made and yet it's an option that many of us face. In assessing the energy used we have made many assumptions, unlike the other vehicles the energy cost depends mostly on how long you make the connection rather than the journey distance. We tried to allow not only for the energy cost at each end of the connection but also for the cost of relaying the information around the world.
Ekranoplan or WIG craft
We assumed that this could travel down the Thames and the Seine, the reality is that the odd low bridge and other traffic on the water could make this quite difficult. Travelling over water seems the only realistic option as the ekranoplan is so difficult to control. Again, the ekranoplan is not currently a transport option and there are many designs, look at the list on The WIG Page. We assumed a vehicle similar to the one that James drove, which is relatively small.
We based our calculations on electric trains; something like 60% of UK trains are electric. Electricity is derived from a variety of fuel sources and it's not really sensible to give a miles per gallon equivalent for trains. Train usage levels vary wildly across routes and times of day so evaluating energy use per passenger mile requires an averaged figure. Interestingly, commercial aeroplanes tend to fly with a very high level of occupancy which means that from an environmental viewpoint they can be competitive with trains. Our numbers are based on Association of Train Operating Companies (ATOC) figures.
We based the fuel consumption figure on an average UK occupancy of nine passengers and assumed an average fuel consumption of 7 miles per gallon, giving 63 passenger miles per gallon. Again, with buses there are a range of different types and it would be possible to come up with quite a range of figures for occupancy and fuel consumption these figures are simply a rounded average from a relatively small sample of UK, US and European figures.
The robot car technology isn't quite there yet we assumed about 40 miles per gallon but I expect that robots could be made to drive more efficiently than humans and, of course, car fuel consumptions will probably improve given the current pressures to emit less CO2.
The space plane was a difficult one. We based our figures on the Skylon simply because James talks about HOTOL in the web interview and Skylon is a derivative of HOTOL. Skylon uses hydrogen and oxygen from the atmosphere initially, then switches to its own oxygen supply once in space. We took the easy way out and just quote average fuel consumption figures for the flight. This again highlights the problem with trying to make comparisons because the weight and volume of the oxygen dominate the calculation – it's interesting to think that nearly all of the other vehicles get their oxygen for free.
Distance, speed, time and energy
We calculated distances:
- by water for the ekranoplan;
- in a straight line for the flying machines with maybe a five mile trip to the airfield;
- by road and rail for the cars (and bus) and train respectively.
We assumed that each vehicle travelled, on average, at its cruising speed (rather than its maximum) for any journey and we assumed a constant fuel consumption based on the average fuel consumption figures quoted.
The energy calculations are based on the volume of fuel consumed for the entire journey multiplied by the energy released per unit volume.
All of the calculations are quite crude for the reasons I've already given, but nevertheless give a good enough estimate for the comparisons that we make here.
There are a few things to take away from having looked at these journeys. Firstly, that journey and fuel use comparison calculations are far from straightforward. In addition to the above, in comparing the vehicles we have taken no account of the energy used to manufacture them or comparisons of their useful life which could be very significant in an overall energy evaluation.
It's interesting to note that, although the accuracy of our calculations is limited, it is still possible to make useful comparisons of journey times and energy consumptions. It is often the case that quite crude calculations can be used to evaluate an idea; the important thing is to know for the accuracy of a calculation as well as the final figure.