On Fri, 22 Oct 2004 at 23:50:36 in message
. net, C Kingsbury
wrote:
One interesting result of this is that many more modern passenger jets
consume less fuel per passenger-mile than high-speed trains, because the
trains carry a lot more dead weight per passenger. It is thus quite likely
that the BOS-LGA shuttle operates at a higher fuel efficiency than the
high-speed Amtrak train making the same trip.

What matters is drag. On the level a train may be heavy but its friction
is very low.

Rolling friction is roughly 10lb per ton or a bit less. So a train on
level ground has a 'Lift' to drag ratio of 224. Air drag is a tiny part
of a train's drag but let's say assume it easily achieves 200 to 1. For
a 747-400 let's assume that it has a lift drag of 20 to 1. In cruise at
say 705,000 lb. the 747 drag is therefore around 35,250 lb.. A train
with about the same drag could therefore weigh around 3,250 tons. They
both need a lot of energy to climb and the weight of train may be
against it but they don't have to climb above 30,000 ft! :-)

Let me know the AUW weight of the high speed Amtrak train and I can do
it bit better.

Incidentally the drag of an aircraft flying at its maximum Lift/Drag
ratio is the same at all heights. But at altitude it flies much faster
for the same drag.

So some questions for you. Do modern jet airliners use less fuel per
mile by flying higher or do they just get there quicker? Or perhaps the
engine efficiency is much higher at altitude? Better specific thrust
perhaps?
--
David CL Francis