Flight, 30 Oct 1959:
Largest of the new Pratt & Whitney fans is the TF75 (commercial designation JT4D), which is a straight conversion of the J75/JT4A, following the formula proven with the J57/JT3. The company have no contract for this work and are unlikely to take it to the hardware stage without military backing or assurance of substantial civil sales. The simplest conversion is the addition of the front fan of the JT3D-1 on to a basic J75 or JT4A-9. This is rendered possible since the hub diameter of the two compressors is essentially the same. Compared with the JT4A-9 this somewhat compromised turbofan will have 8 per cent better s.f.c. It is designated JT4D-3 and would be rated at 22,500 lb. The optimum JT4D-1 would have a new ad hoc fan of appreciably greater diameter. The sea-level rating would be 25,000 lb, and cruise s.f.c. would be approximately 15 per cent better than that of the straight turbojet.
The military TF75-P-1, corresponding to the JT4D-1, is the engine specified by Douglas for the larger of their two CX military freight aircraft, in which four of these big engines would be installed in twin pods. In their submission to the U.S.A.F. for this aircraft last August, Douglas said: "The turbofan power system was chosen for the future cargo airplane because it provided the most favourable balance between speed, productivity and direct operating cost for the design cargo/range mission. A truboprop propulsion system would have provided more range or more cargo and better take-off performance. The turbofan provided greater speed, and takeoff performance comparable to current jet aircraft.
