Hello, and nice to meet you all,

In 11 years this is my very first thread or post in the Military and Space forum. So, please forgive me for my lack of knowledge.

I have been seeing pictures on the landing page here of modified 707 frames. I have noticed (and always wondered) about the new engines on them. Not the old smaller and narrower bare aluminum engines of the passenger version but large engines on what “appears” to be the same airframe (despite some obvious modifications like the nose cone)

What are the new engines? They look like 4 767 engines (similar in size visually anyway) and, wouldn’t those engines provide MUCH more thrust than the original passenger version? If so, what is the impact of those more modern engines?
Is it that additional thrust required for higher MTOW? Or is it just that the 1960’s engines were just too old and beyond maintaining? What are (if any) benefits to performance do they provide? Are they faster at cruising? Etc, etc.

Thanks in advance for what might be one of the most ignorant of questions asked here, and to anybody here in the service: Thank You for everything you do!

R

I'm going to assume you are referring to KC-135R Stratotankers. In that case they are pretty much the same engines that 737NG's have, CFM56 engines. The thrust is very similar to the older engines like the J-Stars use but the fuel burn is MUCH lower. They also don't have thrust reverse.

Andre3K wrote:

I'm going to assume you are referring to KC-135R Stratotankers. In that case they are pretty much the same engines that 737NG's have, CFM56 engines. The thrust is very similar to the older engines like the J-Stars use but the fuel burn is MUCH lower. They also don't have thrust reverse.

Yes, thank you, it was the KC-135R

Interesting that a 707-like frame can just have 4 737 engines attached. Are all of these being replaced by the KC 767 refueling aircraft?

Were the J-Star engines turbofans or real “jets”- the scalloped tubes protruding from the back of those engines always piqued my curiosity. The VC-10 was my very first airplane flight, then 707’s. Makes one feel old

It will be a sad day when one has zero chance of spotting a 707, in any form.

Thanks for answering and happy New Year

The TF33 is an early low bypass turbofan. https://en.wikipedia.org/wiki/Pratt_%26_Whitney_TF33

The scallops you mention are sound reducing modifications from memory. Given the first jet engines were very very very noisy.

You might also want to look up the DC-8 which went through the same modification where the original engines were replaced by CFM-56s.

EDIT:

Hah, just remembered what they were called. Hush kits. https://en.wikipedia.org/wiki/Hush_kit

Some E-3s, and all the E-6s, were delivered from the factory with CFM56s. IIRC at least one commercial 707 flew test flights with CFM56s before being delivered with JT3Ds.

So could one say that the scallops on the 787 type engines are a modern day take on the 1960’s “Hush Kits”?!?!

VC10er wrote:

So could one say that the scallops on the 787 type engines are a modern day take on the 1960’s “Hush Kits”?!?!

That's exactly what they were for. Reducing noise. Only instead of being an after market thing like hush kits they're baked into the original product.

Hush kits were really a one off for those original turbojets and turbofans to meet noise restrictions without having to pay for entirely new engines and the effort that entails. Any engines developed since the introduction of these noise restrictions had noise reducing technology and designs built into the base design.

If you look you will see more recent engine and nacelle designs since the 787 don't use those chevrons as they've been able to integrate even better stuff into the engines without the drag penalty of the chevrons.

Thanks, I had wondered why they have disappeared. I must say I did love (or do love) the look of the chevrons. They are not much, but they are something new that imho, look very cool.
Now that their function can be done without the chevrons- will future 787’s come fitted with engines without the chevrons?

VC10er wrote:

Thanks, I had wondered why they have disappeared. I must say I did love (or do love) the look of the chevrons. They are not much, but they are something new that imho, look very cool.
Now that their function can be done without the chevrons- will future 787’s come fitted with engines without the chevrons?

As far as I know, no. The engines for the B787 are designed with those as part of the system. Any changes would probably require some sort of recertification for that part as well as new engines. So not worth it in the grand scheme of things.

The KC-135 and the 720 airframes and basic systems have greater commonality with each other than with the larger 707-100

All US and NATO E-3s (from the 707-320B) came with TF-33s (the test EC-137s, which were the first two had JT3Ds for testing). French, British, and Saudi E-3s were delivered with CFM56s.

This question may be too nebulous and difficult to answer because of the great amount of variety of modifications that exist, but what are the effects to aerodynamics when there are such significant changes to the airframe, like the crazy large noses, the “awax” (?) radar disks atop of a 707 etc? Is more thrust required, additional fuel tanks and/or new wings, etc?

I would assume the drag created in many cases is significant?

AWACS, not awax. https://en.wikipedia.org/wiki/Airborne_ ... rol_System

As for the various attachments and pokey bits. I'm not aerodynamic engineer so take this with a grain of salt.

But basically yes, such changes do affect how the aircraft flies. The engineers will do their best to minimize any extra drag. So you will see anything poking out all curved and made as aerodynamic as possible. Look at how the spinning radome on the E-3 is designed even though the actual transmitting bit is basically a flat bar. But it still adds drag and adds to the empty weight of the aircraft.

As for wing or engine changes? Not that I've ever heard of. The engine changes on the 720 based airframes was more around fuel efficiency than any thrust increase. And changing the wing is an even larger project so best to avoid that if possible. The closest I can think of is the P-8 where Boeing added raked wingtips to the wing. But that's just adding something, not changing the base design of the wing.

Any aerodynamic engineers. Please feel free to point out where I'm wrong.

Along with the drag, which can be compensated with more thrust and fuel tanks, the more critical aerodynamic issue to consider is whether the added "stuff" changes the aerodynamic CG of the aircraft causing inherent instability. When this happens, additional fins and stabilizers have to be added to counter the instability.

bt

VC10er wrote:

Hello, and nice to meet you all,
In 11 years this is my very first thread or post in the Military and Space forum. So, please forgive me for my lack of knowledge.

At least you have admitted your ignorance up front; in contrast to some of the people answering here who need to take a good look at themselves and what they have written

I have been seeing pictures on the landing page here of modified 707 frames. I have noticed (and always wondered) about the new engines on them. What are the new engines?

Buried in the detail of various answers are two related but separate issues.
Whilst USAF operate various modified 707 frames, the majority of their fleet are KC-135s, which strictly speaking are NOT Boeing 707s at all.
They are, in vaguely civil parlance Boeing 717s. Not to be confused with the more recent DC-9 look-a-like.
This isn't just a change of name, although plenty here from both civil and military sides no doubt believe it to be the case. No Sir! The Model 717/KC-135 is similar in appearance to the 707, but structurally quite different from the civilian airliner, with a narrower and somewhat shorter fuselage. The fuselage diameter for the production KC-135 was 144 inches and Boeing originally hoped to build the 707 fuselage with that width. By the time the Boeing company committed to production, the decision had been made to design the production model 707 as a six-abreast design, with a larger 148 inches (3.76 m) diameter fuselage. The military readers here will argue that 4 inches is nothing (some wives might disagree!), but in civil terms, the difference between five-abreast, and six-abreast, is everything

That aside, original power was four JT-3C engines (or to keep the locals here happy, J57s) producing a bare 10,000 lb thrust, rising to 13,000 lb for up to three minutes of water-injection. (Remember that 10,000 lb for later!)

Engine Refit program #1
In the 1980s the first modification program retrofitted 157 Air Force Reserve (AFRES) and Air National Guard (ANG) tankers with the Pratt & Whitney TF-33-PW-102 turbofan engines from 707 airliners retired in the late 1970s and early 1980s. The modified tanker, designated the KC-135E, was 14% more fuel-efficient than the KC-135A and could offload 20% more fuel on long-duration flights. One poster has already referred to TF33s, but for the benefit of the OP, these are simply JT-3Ds as should be screamingly obvious by now.

But this isn't quite what the OP was asking'
Engine Refit program #2
The second modification program retrofitted 500 aircraft with new CFM International CFM56 (military designation: F108) high-bypass turbofan engines producing approximately 22,500 lbf (100 kN) of thrust, nearly a 100% increase compared to the original J-57 engine. That's not quite the same as a straight swap as somebody put it.

And remember, these much more powerful engines are situated on a 717 frame, which is shorter and narrower than any B707.

... wouldn’t those engines provide MUCH more thrust than the original passenger version? If so, what is the impact of those more modern engines?
Is it that additional thrust required for higher MTOW? Or is it just that the 1960’s engines were just too old and beyond maintaining? What are (if any) benefits to performance do they provide? Are they faster at cruising? Etc, etc.

Yeah, most of those things, plus one aspect you forgot....
The modified tanker, designated KC-135R (or occasionally KC-135T) can offload up to 50% more fuel (on a long-duration sortie), is 25% more fuel-efficient, and costs 25% less to operate than with the previous engines. It is also significantly quieter than the KC-135A, with noise levels at takeoff reduced from 126 to 99 decibels.Yes, even military bases might have nearby resident populations who would appreciate a quieter life. Of course if that base also hosts a squadron of F/A-18s, it's a moot point, but there are many others that are surrounded by civilian dwellings.

Finally; although most KC-135Rs were converted directly from KC-135A dinosaurs, some frames started life as KC-135A with JT-3C turbojets, were upgraded to KC-135E (JT-3D turbofans) and then upgraded a second time to CFM56 high bypass turbofans. The CFM56 still offered an considerable improvement such as the KC-135R's operational range is 60% greater than the KC-135E for comparable fuel offloads.[/quote]

I acknowledge this concentrates entirely on the Boeing 717/KC-135 series, which is the majority of USAF strength.
Regarding the Boeing 707 related models in service - that's another story.

Something else with the E6 Mercury, inside the flightdeck it resembles a Boeing 737NG quite a lot:

https://www.military.com/equipment/e-6-mercury

So not just newer engines, but the flight deck is considerably more modern than B707 aircraft of a bygone era. The newer engines are also as said before not just quieter, but a lot more efficient so the plane becomes a lot more useful.

The E-8 JSTARS are 707 derived aircraft and have been planned to be re-engined with JT8Ds. Looks like only a test frame flew with the new engine though before the project got cancelled.

ADent wrote:

The E-8 JSTARS are 707 derived aircraft and have been planned to be re-engined with JT8Ds. Looks like only a test frame flew with the new engine though before the project got cancelled.

Mostly because the airframes are beat like hell.

Every JSTARS out there was converted from used airframes, which were purchased from a variety of sources, some of which had questionable pasts. Many were bought from small air freight outfits, who weren't that great at maintaining their jets to begin with.

Some of the airframes are ex-RCAF aircraft, and include one which suffered a significant landing gear collapse towards the end of it's service in the RCAF.

cpd wrote:

Something else with the E6 Mercury, inside the flightdeck it resembles a Boeing 737NG quite a lot:

https://www.military.com/equipment/e-6-mercury

So not just newer engines, but the flight deck is considerably more modern than B707 aircraft of a bygone era. The newer engines are also as said before not just quieter, but a lot more efficient so the plane becomes a lot more useful.

The E-6As were delivered by Boeing with classic 707 style flight decks (steam guages). Under the Multi-function Display System ( MDS) upgrade, the original 707 flight deck front panels and avionics were replaced with 737NG instrument panels adapted for 4 engines.

http://www.navair.navy.mil/index.cfm?fu ... ry&id=2020

Beautiful aircraft

Hard to believe that cockpit in a 707 !