QUESTION:  
What was done to modify the C-141 cargo jet that gives it the 
ability to fly higher than commercial airliners?

ANSWER from Terry Rager on October 24, 1995:
The modifications to the KAO basically have to do with the 
installation of the telescope and the telescope door. These modifications 
actually hurt the climb capability just a little because of some 
increased drag caused by the cavity opening.

The reason the KAO is such a good airplane for the telescope is 
because it can get to higher altitudes faster than other aircraft. If you 
look at the question from Georgia asked me, you see I talked about the 
thickness of the air and how the wing creates lift.

The KAO has a big thick wing. It was designed to create lift 
"quickly."  This capability allows more weight to get off the ground in a 
shorter distance, but also allows the airplane to go higher in a shorter 
time period. The trade off is that the airplane can not go as fast as 
many other commercial airliners because of the frontal resistance of the 
wing to the air. Since the the astronomers are interested in altitude 
(41,000 ft) and the amount of time on an object, they do not care how 
fast we go. Commercial aircraft are trying to get people to a destination 
as quickly as possible, so they go faster and do not have to travel as high.

This is true for aircraft built the same time as the KAO. The 
problem with the aircraft being built now is the engines are so powerful, 
they can now go as high as the KAO sooner than the older planes. So now 
there really is more traffic at the higher altitudes.

To demonstrate the frontal resistance of the wing, try this 
demonstration.  Hold a broom straight out so the brush is horizontal and 
swing it. Then hold the broom out with the brush vertical and swing it. 
The difference in the speed is the resistance from the air.