The first Air Force air-refueling tankers were KB-29 and KB-50s converted from bombers but by the 1950s the Boeing KC-97 became the standard air refueling tanker- this choice, however, was not without its difficulties particularly in refueling the B-47 Stratojet- the KC-97 wasn't able to reach the higher altitudes where the B-47 was most efficient and the B-47 had to descend to lower altitudes where its fuel burn was higher in order to link up with the KC-97. Furthermore, even with the KC-97's throttles "firewalled", the Stratojet had a tendency to stall during the link up and as a result, the refueling operation would have to carried out with the KC-97 in a shallow dive in order to gain speed above the stall speed of the Stratojet.
In 1953, the USAF trialled two B-47Bs outfitted with the British probe-and-drogue system developed by Flight Refueling, Ltd. One aircraft designated YB-47F and nicknamed "Pa" was fitted with nose mounted probe and the other aircraft, designated KB-47G and nicknamed "Ma", reeled out the hose-mounted drogue from its bomb bay. On 1 September 1953 the pair made the world's first all-jet air refueling.
Despite the higher speeds and ease of rendezvous, the fuel transfer rate with the system wasn't fast enough and in certain conditions was actually *less* than the burn rate of the six J47 engines of the receiver Stratojet. Only the flying boom used on the KC-97 was capable of the fuel transfer rates needed to give the Stratojets "legs" to reach distant targets. The interim solution was to fit the KB-50 and KC-97 tankers with auxillary jet engines to provide extra speed during refueling but the long term solution would arrive in the form of the Boeing KC-135 Stratotanker that had the speed and altitude performance to match SAC's bomber fleet.
Source: Boeing's B-47 Stratojet by Alwyn T. Lloyd. Speciality Press, 2005, p202.
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