Subsequent test flights demonstrated the FICON concept was indeed "tactically sound", but its operational implementation was difficult. A total of 10 GRB-36Ds and 25 RF-84Ks were built and saw limited service in 1955–56. Hookups with the carrier aircraft were challenging for the experienced test pilots under ideal conditions. In combat or in adverse weather, and by less-experienced pilots, they proved difficult, and several RF-84Ks were damaged attempting it. In addition, the RF-84 dramatically reduced the bomber's ground clearance: with 450 gallon (1,700 liter) external tanks on the fighter, the FICON combination cleared by a mere six inches (15 cm). These adversities, combined with the advent of the Lockheed U-2 and the passing of the B-36 into obsolescence, resulted in cancellation of the project in 1956, with the last FICON flight taking place on 27 April.