F-105 Thunderchief

[noxioux]

I'm wondering if anyone out there has either drawings or pictures of the internal intake ducting on an F-105.  There are plenty of references to the rest of the plane, but I have not been able to find anything on this particular part.

I know that the two sides were separated from each other, and there was a hydraulic ram in between them (to push an internal bomb out of the slipstream).  But I can't find any references.  It is looking like I may be forced to fake it, but I hate to do that with this particular project.

[Jeffry Fontaine]

A couple of links for the F-105 that should be saved to your favorites folder:

• Dave Gurtner's F-105 called Cold War Thuds -- The Republic F-105 Thunderchief in Europe1961-1968
• Rick Versteeg's Thud Ridge Web Page

Both of these are "keepers" and containe a lot of good reference images on both sites.  Dave Gurtner provides a good description of the weapon device on one of the supplemental pages on the F-105.  Here is that portion of the text describing that particular feature from F-105D/F Specifications:

"Up to 8,000 pounds of nuclear ordnance could be hauled internally and externally:  a B28 or a B43 "special device" or "shape" (generally with a retarding drogue 'chute) in the 20' long internal bomb bay; or on the centerline pylon attached to the bomb bay doors (B57 or B61, aerodynamic profile); or on the inboard wing pylons (B61, aerodynamic profile).  Vertically above and extending into the internal bomb bay was the THUD's unique bomb "displacing gear" or "ram" -- essentially a large-bore vertical piston powered by compressed air -- to push the weapon bolted to it into the rock-hard 1200 MPH air stream during the bomb run.  Reportedly, the ram could also "displace" the shape through battle-damaged, "frozen" bomb bay doors if necessary!  Early in the design of the Thud, both Republic and the Air Force were concerned about potential problems of delivering the special devices at supersonic speeds.  Earlier USAF experience with Boeing B-47 StratoJets at high subsonic Mach numbers resulted in some released bombs striking the bomb bay interior due to high turbulence and the dynamic pressures in the bomb bay at those speeds.  Because of this, the F-105's unique weapon delivery system was based on ejecting the weapon out of the bomb bay with the large air-piston ram.  Using 250 ±25 PSI compressed air and a 30-inch stroke, the ram imparted a full velocity of 32 fps to the "shape," allowing the weapon to punch through the adverse air flow in the bomb bay and under the aircraft.  As the aircraft was completing its design, however, a special weapon with a "blunt shape" casing was especially tailored for use in the Thud.  Early drop tests with the prototype F-105 (YF-105A 54-0098) showed this shape almost totally stable over a wide range of release angles and velocities -- even in adverse air flow and high dynamic pressure -- and the air-powered weapon ejection ram was not needed.

THUD nuclear strike configuration while on "Victor Alert" status at Bitburg generally consisted of the "special device" shear-bolted to the bottom of the bomb bay ram, a centerline-mounted 650 gallon external fuel tank mounted on the closed bomb bay doors, and two 450 gallon underwing tanks -- all these external tanks were to be emptied ASAP in the mission and then jettisoned for a "clean wing" configuration for the close-in, down low, Mach 1.2 dash to the target ("toss bomb" or "over-the-shoulder" deliveries were contemplated, and the low nose-on RCS added stealthy assurrance to the mission)."

In the reference books I have on the F-105, the device that kicks the weapon out of the bomb bay is not very special in appearance or shape.  Just a round thing in the top of the bomb bay centered over what would be the suspension lugs of the  weapon.

[noxioux]

I knew about the weapon bay and the ram before from talking to the guy I'm building this thing for.  And I have some fuzzy details on the intake ducting, just not enough to build it.

In practice, they almost never used the weapon bay to carry anything but an extra fuel tank.  The ram was there to push out a bomb/missile, because otherwise, the airflow would prevent the weapon from dropping out of the bay, period.

The weapon bay on this Trumpeter kit is pretty well detailed, too.  It is an option, and they provide an extra set of doors if you want to display it open.  This one will be open, with the spare fuel tank inside.

[elmayerle]

I don't know the exact details, but you can figure that the inlet ducts curve in after passing the main gear wells simply because there are auxiliary inlets there - with blocking doors that won't open them 'til the gear is down, preventing a rather embarrassing problem an early prototype had.

[noxioux]

Yeah, I've combed the web and I can't find anything really helpful.  So I'm just going to have to fake it.  I should be able to get it pretty close, there's only so many ways you can do these things.  I do know that there was a divider between the intakes, so that narrows things down a little.  I should be able to do this with like 3 or 4 pieces of styrene and a little putty.  I'll also use a piece of sprue in the ram's position for something to hook onto.  I will post pics when I figure out how it's going to go down.

Thanks for looking, guys!

[elmayerle]

Do you ahve a good location for the front face of the engine?  That will establish where you're going with the intake duct and hte final diameter of the intake.  You have, as I described earlier, the start points for each inlet duct and some idea of where they start to converge.  You can figure that the two ducts will merge into a single duct the diamter of the engine front face with some room in front of the engine face to allow the flow to stabilize.  That's the best guide I can give, at the moment, for "faking it".

HTH,
Evan

[noxioux]

Thanks Evan.  Yeah, with the trumpeter kit, you have a great idea of where the face of the engine goes, because it comes with this wonderfully detailed engine that's completely hidden when you have a finished model.  The ducts must've gone level for a short distance, then curved down and converged at the engine face, so they could clear the weapon bay.  I was going to just run them both straight to the engine face.  But having the ducts converge at the engine face is probably a very sensible engineering kind of thing to do.  Basically what I'll end up doing is splitting the open space between the inlets and the engine, and then just using styrene and putty to smooth it all out.  As long as it looks convincing while peering down the intakes, I'll be happy.

This was easy when I did the F-16, because it wasn't hard at all to find pics of the engine bay sans engine.  If I could find a shot like that of the thud, I'd be set.

Well, time to go hack up some perfectly good styrene. . .  

[elmayerle]

You can figure the inlets will come together and then expand into a bellmouth that'd match the engine face wth some length for flow straightening.  SOmehwat the reverse of the B-2's ducting where one inlet gets split to feed two engines.

Editted Afterthought:  They won't be completely accurate, but you might see what you can find on the inlet ducts of other aircraft of similar configuration (Hunter, RF-84F) to serve as a guide

[noxioux]

I did find a picture of the engine bay without the engine installed, and it does give a better idea of how the inlets come together.  It's tough to make out at first, but you can see a bare-metal area, with the inside corners of the two inlets where they come together.  It's not a real clear picture, but it's enough for me to make reasonably correct inlets.

From talking to the guy I'm building this for, they had to crawl down into the inlets to do the visual inspection of the turbines.  He said there was barely enough room to crawl down inside, and turn around to come back.  He's about 6'3".  Combined with that picture, that gives me a little better idea of how to approach this.

I found the pic here: cybermodeler thud page.  And there's a bunch of other pretty helpful images on that site.

[Overkiller]

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[tigercat2]

In one of my F-105 books, there is discussion of doing just what you are talking about.  Rebpublic had a several plans:  One was  to rebuild about 300 surviving Thuds to an F-105-35-RE configuration, which would include many upgrades.

Also Republic would have been very willing to build severa hundred new Thuds, and looked at an F-105H model.   The engine they were looking at, IIRC, was the J-75B-24 engine, with over 30,000 pounds of thrust.  A hopped up TF-30, similar to the F-111F engine might have worked, but probably was not available in the 1968 time frame.

Its too bad that no new Thuds were built, but MacNamara's preference was for the F-4.


Wes W.

[Archibald]

Now that's a scenario I like! TF-41 would be good (albeit it is less powerfull than the J-75 it replace). Later, F-100 is the obvious choice.
The two seater is a good idea, too (less workload for the pilot of course).
Pave tack with GBU-10 maybe... HARM missiles (obviously).
We can imagine more F-105s if the F-111 fail in the late 60's (nearly real world). Maybe GB and French variants to replace the TSR-2 and Mirage IV ?
Whatif F-105s were used in GW.1 ?  

[upnorth]

With an engine change, I'd also switch that odd four part tailpipe/airbrake contraption for a simplified tail pipe with the airbrakes relocated elsewhere on the airframe.

[elmayerle]

Now that's a scenario I like! TF-41 would be good (albeit it is less powerfull than the J-75 it replace). Later, F-100 is the obvious choice.

Actually, the test afterburning TF41s demonstrated over 26,000 lbt. in full afterburner and would be closer in performance to the J75 than then-available variants of the TF30.  *chuckle* Of course, if our northern neighbors had stayed the course, the obvious re-engining choice would be the Iroquois (hmm, anyone for replacing the cancelled Arrow with Iroquois-powered F-106 varaints?) as Republic studied.

Regarding a blown canopy covering both cockpits, it would be feasible but it would take considerable redesign of the cockpit structure to incorporate it.  It works for the F-105C/E layout but less well for the F-105F's layout.

[Zen]

I understand the Thud had a loooonggg take of run, so maybe blown wings?