The US Marine Corps had initially required the Grumman A-6 Intruder to meet an STOL requirement to take off in 1500 feet over a 50 foot obstacle. To accomplish this, the first seven A-6 Intruders had tilting tailpipes in which the exhaust of the twin J52 engines was vectored downward 23 degrees on takeoff and landing. There was a knurled knob on the outboard throttle that the pilot used to activate the feature. During testing, the Navy evaluation team set out to see if the feature worked as advertised and found the tilting tailpipes only marginally effective in meeting the USMC requirements- in fact, in many configurations the approach speeds were actually lower than they had been in propeller-driven aircraft.
The Navy team recommended deleting the feature at a cost savings of $25,000 per aircraft. After some back and forth arguing between the Marines and the Navy, the tailpipes and the associated actuation mechanisms were deleted beginning with aircraft number 8.
Source: Intruder: The Operational History of Grumman's A-6 by Mark Morgan and Rick Morgan. Schiffer Publishing, 2004, p16-17.