In MMD 971215 Mike Knudsen recommends using relays to drive heavy
solenoids.

A few years ago I would have agreed.  I believe in relays and galvanic
isolation.  But modern power transistors are so good and inexpensive
that I have changed my mind.  There are power MOSFETs that will switch
20 amperes at several hundred volts, for a $ or so.

The catch diode matters whether you are using relays or transistors.
If there is nothing to catch the peak voltage at switch-off, the relay
contacts will have a very short life, and the transistor, an even
shorter one.

Jeffrey Borinsky

 [ The vibrating interruptor contacts of the instrument are made of
 [ 1/4-inch diameter tungsten; at night a beautiful shower of sparks
 [ gushes from the active solenoid assembly.  Without a protective
 [ diode, this enery would soon burn the points of a relay, or
 [ destroy a transistor.  -- Robbie