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starship-design: Re: Beam properties, control, etc.

Kelly replied to Rex:

>>1. Depth of focus
>>Assume the beam from a 2.31E6-km-diameter microwave antenna is
>>focused to a sail (beam-spot) diameter of 100 km at a distance of
>>1 lt-yr.  At the distance from the focus that the beam cross-sec-
>>tion has grown from the area at the focus by 10 percent (power per
>>unit area reduced by about 10 percent), say, the radius of the
>>beam has grown by 100 km * [sqrt(1.1) - 1]/2, or about 2.4 km.
>>The distance from the focus for that growth is (2.4/2.31E6) *
>>1 lt-yr, or approximately 1E-6 lt-yr.
>>So, the sail/ship has to stay within about 1.E-6 lt-yr of the
>>focus to keep the power from dropping off by more than 10 percent.
>I read that as about 10,000,000 kilometers, or 32 light secounds.  Not a
>lot of slack over interstellar distences.  A sligh sail problem could have
>you well out of that pretty quickly.

True, it would be a tight schedule: One minute too late at a certain place
and you have missed the beam for good.
Sailing on a beam means inherently that every second of the trip is planned,
slight deviations are allowed, but will be risky. A sail may sound as an
easy "engine", but you better have a good captain/navigator.