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the United States SST will be somewhat better than Mach 2.2. It is also probably that the United States plane will be constructed largely of titanium and steel because of heat problems at high airspeed. (At Mach 3, skin temperatures may reach 450-500 degrees Fahrenheit, and higher at the leading edges.)

Principal problems for Mach 3:

1. Some of the design and construction problems are now "beyond the state of the art," which means that our technical knowledge does not now include answers to all of the problems; we have much to learn.

2. There is now no jet engine in the world with thrust sufficient to propel a practical-size airliner at a speed of Mach 3. Before Mach 3 is attained, a new engine will have to be designed and built. For interim testing, at less than design gross take-off weights, a modification of existing engines may possibly suffice.

3. The "sonic boom" increases intensity as the size and speed of the airplane increases, but is reduced with increased altitude. The sonic boom generated by a large Mach 3 transport would be much more difficult than any we have experienced to date. The problem of the sonic boom may be tempered by operating at subsonic speed until high altitude is reached, but whether this will be sufficient has not been proven.

Who has "ordered" what?

There have been announcements, by various airlines, tending to covey an impression that firm orders have been entered, either for the British-French or the United States plane, or both.

To the best of our knowledge, no United States airline has entered a firm, irrevocable order for any supersonic airplane, either the British-French or the United States plane.

There have been various transactions, in both of the areas, but in most cases they have resembled (at least in many respects) what we would in the United States an option to purchase, with a deposit.

When will we have supersonic air transportation?

The British-French project intends deliveries in 1970-1971. Whether or not this schedule will be realized remains to be seen. It is probably that the United States project will develop an airplane with somewhat later deliveries.