Why PW1120 instead of F100 for Super phantom?

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Airman 1st Class
Jul 13, 2020
Both Israel and Boeing Super Phantom project use PW1120 engines. The Jews wanted the Super Phantom to have the same engine as Lavi,But why did Boeing choose PW1120 over F100? If I remember correctly J79,Spey and F100 should have been around the same size.
The F100 was of significantly larger diameter than the J79. The British had to do some significant rework on some of their Phantoms when they fitted them with their RR 201/202 series Spey turbofan engines. The diameter of the Spey was 43", the F100 was 46.5", while the J79 was only 39".
There are two parts to the equation... engine size and air-mass requirement.

The first required a minor modification in the lower aft fuselage of the UK's Spey-engined Phantoms, but the second required the entire intake and ducting to be enlarged in cross-section, which increased drag - reducing max speed and eating up part of the fuel economy savings in going from the J79 to the Spey.

J79: 31.6" body diameter, 39.1" afterburner diameter; 170 lb/sec airflow; SFC* .85/1.97

PW1120: 33" body diameter, 40.2" afterburner diameter; 178 lb/sec airflow; SFC .80/1.86

Spey RB168: 37.5" body diameter, 44" afterburner diameter; 210 lb/sec airflow; SFC .63/1.95

F100: 40" body diameter, 46.5" afterburner diameter; 225 lb/sec airflow; SFC .76/1.94

As you can see, the F101 would require much greater modifications in the intakes and ducting, which would increase drag etc much more.

* SFC = specific fuel consumption, given in lbs fuel used per lb thrust per hour

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