Booster engines

From:         bob_wohlsen@qm.sri.com (wohlsen)
Organization: sri international
Date:         04 Jan 95 00:40:58 
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In the '60s, Hawker Siddeley produced a stretched version of the 3-engined
Trident that employed a 4th "boost" engine for take-offs.  Was this the
only example of a boost engine being used on a commercial jet airliner?  

Given the excess thrust available in twin jet airliners when both engines
are running (to meet emergency single engine take-off requirements I
assume), has any manufacturer considered employing a boost engine instead
and using smaller main engines?  Perhaps the boost engine might take the
form of an oversized APU with a special high-power mode designed for
intermittent usage, i.e. take-off and emergency backup. (No, I don't mean
afterburners.)

Bob Wohlsen