The Spitfire Mk.IX carried 85 gallons of fuel internally in its two tanks behind the instrument panel. The upper tank held 48 gallons and the lower tank 37. With this the aircraft could fly for just over an hour and depending on conditions, have a range of some 430 miles.
Thus not a lot…
For extending the range when operations moved into mainland Europe drop tanks were thus essential. The Spitfire carried a unique slipper tank under the fuselage which provided an additional 45 gallons of fuel. (Other marques such as the Mustang carried under wing tanks, more on this later).
Our Mk.IX is thus fitted with a release mechanism which is situated to the right front of the seat on Frame 9. (the thingy with the red handle shown below:)
The mechanism includes a lever by which fuel flow from the external tank is controlled. While the lever is in the open position to the rear, it locks the red jettison handle into place. This can only be pulled when the lever is in the forward (closed) position, to prevent the fuel from the internal tanks from gushing out once the tank is released.
It would seem there were also bigger tanks available. A quote from Commander Crosley on the slipper tank fitted to Seafires indicates that these could carry 90 gallons, so were considerably bigger:
“Amongst the piles of ammunition, wireless sets, pilot’s seats, propeller spinners, tail steering arms, wing mats, sweating bodies and noise, we could see several huge slipper-shaped petrol tanks. Some of these were being offered up to the underside of the Spitfire’s fuselage — where a bomb might ordinarily be — and the fuel lines were being connected by an invisible, sliding fit. There was only a half-inch gap between the underside of the fuselage and the top surface of the tank, which was all of six feet long by two feet wide. We found that it could hold 90 gallons of 100 octane fuel. This was more than the Spitfire carried in its internal tanks. A closer study of the jettison arrangements showed that a Bowden cable release in the cockpit let go the lifting ring — stressed to three tons breaking strain — in the top surface of the tank. The tank then slid backwards onto two lugs sticking out two inches from the fuselage underside. The nose of the tank then dropped and the airflow forced it downwards and clear of the fuselage underside. The slightest skid, we thought, and the whole thing would come clear of the two lugs, slide back and hit the tail. However , the Spits would now have a range of 400 miles and would allow a fly-off to Malta well before we got to ‘bomb alley’.”
For some other aircraft types such as the Mustang and Typhoon, the British invented paper tanks. Many hundreds of thousands of these were manufactured.
The following is a quote from Warbirdnews.com:
“Faced with wartime metal shortages, and a need to extend the range of fighter craft heading to Europe, the British came up with an ingenious design for auxiliary drop tanks, making them out of resorcinol glue-impregnated kraft paper, which while having excellent tolerance characteristics for extreme heat and cold necessary for operation on an aircraft as well as being waterproof, the glue would slowly dissolve from the solvent effects of the fuel contained within the tank, developing leaks within a few hours of being loaded with fuel, making them a strictly a one-time use item, filled right before takeoff.
The tanks were not considered robust enough to land a plane with them attached, so if a mission was scrubbed, pilots were required to drop their sometimes still-full tanks at a specified drop point, usually the airfield’s dump where the tanks would be jettisoned, though surprisingly we could not locate any cases where this caused a conflagration.”
The coming update of DCS World Spitfire expected this month will incorporate slipper tanks.