I remember reading, somewhere back in the fading mists of time, that one of the benefits of the "double delta" configuration was its ability to handle a broad range of CG locations. However, the shape dictated an extensive lengthwise distribution of tankage to fit a reasonable fuel load into that slender profile, requiring an elaborate automatic sequencing of tank selection and constant pilot attention to monitor the system. IIRC, the engine was essentially a license built, but modified and upgraded, J79. (or was that the Viggen? I forget.) In any case it was a downright thirsty SOB.I suspect one reason may have been fuel and C/G management, as the fuel tanks were distributed along the fuselage and the aircraft may have had a fairly limited c/g range.
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