Nos. 9 and 617 Squadrons' Lancasters were modified to extend their range. All of the aircraft selected for the operation were fitted with powerful Rolls-Royce Merlin 24 engines, which were hurriedly obtained from maintenance units and airfields across the UK. The Lancasters also carried two extra fuel tanks inside their fuselage: a tank usually fitted to Vickers Wellington bombers and a type of drop tank used by Mosquitos. The extra fuel put the aircraft greatly above their authorised maximum take-off weight. To address this, the bombers' forward and mid-upper gun turrets were removed, along with 3,000 rounds of ammunition from the rear turret, the flare chute, the armour plating around the pilot's seat and some of the oxygen and nitrogen bottles. The reduction in armament left the Lancasters very vulnerable to German fighter aircraft, and they would have to fly without escort as no British fighters had sufficient range to reach Tromsø.