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Why did the USAF not upgrade the E3 from TF33 engines to the CFM-56 like they did the KC135? From the Wikipedia page on the E3, the latter variants sold overseas had the CFM-56 from the factory and had a considerable range advantage, so I'm assuming it would not take a great deal of engineering to retrofit the older aircraft, but maybe I'm wrong.

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As mentioned in another answer to your question, the mission requirements for the two planes are different. The maximum takeoff weight of the KC-135 puts a limit not only on its range but also on the amount of fuel that it can carry to fuel other aircraft. The newer engines greatly increased this capability. According to a Boeing Frontiers article in July 2005 about the KC-135:

The re-engining program has increased the aircraft's fuel transfer capability by 50 percent and reduced fuel requirements by 25 percent.

For the E-3 the newer engines would mostly provide increased range and reduced fuel consumption. While this would be desirable, apparently the existing performance was considered adequate for the typical E-3 mission. Interestingly in the Wikipedia article on the E-3 it states:

As the test beds did not need the same 14-hour endurance demanded of the production aircraft, the EC-137s retained the Pratt & Whitney JT3D commercial engines, and a later reduction in the endurance requirement led to retention of the JT3D engines in production.

EC-137 being the development designation of the E-3, TF33 was the military designation of the JT3D.

The CFM-56 does provide quite a bit of extra range. Again from Wikipedia:

USAF and NATO E-3s have an unrefueled range of 7,400 km (4,600 mi) or 8 hours of flying.The newer E-3 versions bought by France, Saudi Arabia, and the UK are equipped with newer CFM56-2 turbofan engines, and these can fly for about 11 hours or more than 9,250 km (5,750 mi).

Apparently the U.S. felt that the existing range supplemented with aerial refueling was adequate, at least as in comparison to the additional cost and disruption of a re-engining program.

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    $\begingroup$ I think you hit on a key point with, "supplemented with aerial refueling." The E3 can refuel midair, while the KC135 cannot. $\endgroup$
    – RetiredATC
    Sep 21, 2023 at 21:35
  • $\begingroup$ This answer is a good example of the delicate maths involved in operating the aircraft: how minor savings and improvements here and there scale up to massive improvements in certain areas of performance. $\endgroup$
    – Jpe61
    Sep 22, 2023 at 11:04
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I do not have access to material that would definitely prove the following in the specific case of the E-3 Sentry, but:

As I understand the original USAF versions were delivered with T33 and the UK, France and other overseas versions had the CFM-56 from factory. Upgrades within the fleet always lead to temporary, possibly even long term complexity. Having the same Aircraft, the E-3 in this case, run two different engine options will present challenges in logistics, maintenance and will also require extra some training.

Those and other possible considerations along with the bottom line, the overall cost of the upgrade, will be taken into account and weighed against the achieved performance gains of fuel consumption, thrust etc. when making the decision whether the upgrade is justified.

It is most likely that in the case of E-3 the overall equation did not justify the upgrade. In the case of the KC-135 the parameters of the equation would have been very different. The mission profile is, if not completely, at least substantially different from that of the E-3 which flies extended missions at stable altitude and mass, whereas the KC-135 flies much shorter missions within a much wider mass range.

The performance envelopes of jet engines of varying generations will be very different. The TF33 was probably deemed efficient enough in the use case of E-3, whereas the upgrade to CFM-56 offered large enough benefits across the varying loads required for the KC-135 operations.

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    $\begingroup$ To your point, from a Boeing Frontiers July 2005 article about the KC-135 re-engining, "The re-engining program has increased the aircraft's fuel transfer capability by 50 percent and reduced fuel requirements by 25 percent." A very different situation from the E-3 which would have just benefited from additional range and reduced fuel consumption. Which was likely desirable, but as you indicated the existing performance may have been adequate for the typical E-3 mission so the cost justification may not have been there. $\endgroup$ Sep 21, 2023 at 13:00
  • $\begingroup$ @StevePemberton that would make an excellent answer! The extra range and endurance would not make much sense in the case of E-3 because of crew fatigue considerations for example. $\endgroup$
    – Jpe61
    Sep 21, 2023 at 13:08
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At least, in the case of the USAF, this would be redundant, as the E-3 Sentry is going to be replaced by the Boeing E-7 AWACS aircraft. Spending a great deal of money to re-engine the E-3 fleet then is just wasteful.

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    $\begingroup$ That's the situation now, but the KC135 started getting CFM-56 engines back in the 80's. My question asks why it was never done for the E3. $\endgroup$
    – RetiredATC
    Sep 21, 2023 at 2:20
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    $\begingroup$ Besides the reasons given in this answer, there would have also been fairly large fixed costs in a re-engineering program which would be spread over all of the aircraft. In the case of the KC-135 a total of 420 aircraft were re-engined. In the case of the E-3 it would have been around 60. Maybe that wouldn't have been a major factor, but it was possibly part of the equation. $\endgroup$ Sep 21, 2023 at 13:11
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    $\begingroup$ @RetiredATC I suspect, because, at the time, the Air Force felt that the JT-3D engines powering the E-3 did not merit change out as opposed to the far more inefficient J-57 engines, which powered the original KC 135s $\endgroup$ Sep 21, 2023 at 20:59

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