Douglas Low-drag External Fuel Tanks

11 October 2017: A3J-1/RA-5C tank drawing and discussion updated.

13 September 2016: For the earlier 150 and 300-gallon tanks, see http://tailspintopics.blogspot.com/2016/09/things-under-wings-post-war-external.html

30 July 2016: I added some discussion about the 400-gallon tank.

27 July 2012: I added a few more example pictures of the different sizes of standard Douglas-design tanks and an evaluation of the accuracy of the tanks in some 1/72-scale kits here: http://tailspintopics.blogspot.com/2012/07/douglas-low-drag-external-fuel-tanks.html

7 August 2011 Update: Gerry Whiteside provided some first-hand information on the fin attachment arrangement, which I have incorporated.

3 August 2011 Update: I made some changes to the 150-gallon tank based on a drawing provided to me by Craig Kaston. It corrects the number and location of frames and the 14-inch lugs, as well as adding the 30-inch lugs. Note that on 2 August, I also added some Don Hinton pictures of the 300-gallon tank..

In the early 1950s, Douglas created a series of low-drag external fuel tanks and bombs. The latter became the  Mk 8X series. The tanks came in three standard sizes: 150, 300, and 400-gallons. The number and orientation of the fins varied depending on the airplane and pylon station. The 150-gallon tanks, after appearing initially on the A4D-1 and early F4Ds, appear to have had limited operational use although the Douglas AD Skyraider and Grumman F9F-8 Cougar were qualified to carry them and the AD-5W often carried one of them. The 400-gallon tank was primarily carried by later models of the A4D Skyhawk and was also the basis for the Douglas D-704 in-flight fueling store, which was often referred to by the Douglas design number, and carried by the AD tankers for maximum giveaway.

While the bombs became standard across the board, the 300-gallon fuel tanks were primarily utilized by Douglas aircraft: the AD Skyraider, A2D Skyshark, F3D Skyknight, F4D Skyray, and A4D Skyhawk. The other Navy aircraft that deployed in the 1950s like the F3H Demon, FJ-3/4 Furies, and F7U Cutlass carried custom low-drag tanks. Later fighters also had to be equipped with unique tanks to maximize the amount of fuel that they could carry: the F4H/F-4 wound up with two, a huge 600-gallon one on the center-line station and/or 370-gallon tanks on the outboard wing stations. However, the Douglas 300-gallon tank configuration did become standard on the Grumman A-6, Vought A-7, and Lockheed S-3.

The number and orientation of the fins could be easily changed to the configuration required for a particular airplane's requirements. I had guessed that the aft end of the tank would have to be rotated to mount fins at 45 degrees. Gerry Whiteside provided the following:

I was in A-7's (VA-82) and we used the same tanks. The longitudinal slots (at 0, 90, 180 and 270 degrees on the A-7 tanks) are the mounting points for the fins and the vertical slots (at 45, 135, 225 and 315 degrees) are for access to the mounting nuts for the bolts that attached the fins. For the A-7 there was a different fin configuration for the outboard stations (1 and 8) versus the inboard stations (3 and 6); (stations 2 and 7 were dry and for nukes).

Every once in a while, somebody asks about these or compares/contrasts the tanks available from different kit manufacturers. I don't have much but here is a comparison of the 150 and 300-gallon tanks. I drew the 150-gallon tank from a Douglas engineering reference drawing for the A4D; the 300-gallon one was done by a Vought predesign engineer once upon a time. The shapes can be considered to be close to the actual tanks. I was particularly surprised and pleased to see that the fin on the Douglas drawing was exactly the same as the one on the Vought drawing.

I did have Navy documentation for the 300 and 400-gallon tanks:

Unfortunately, the fin isn't shown for the 400-gallon tank but the span shown suggests that it used the same fin as the other two tanks.

30 July 2016 Update: I'm not convinced that the shape of the 400-gallon tank shown in the drawing above is exactly representative of the fielded tank. For sure, both the A4D and the A3J appear to have carried a tank with a less pointed nose, probably ending at the station 0 shown on the drawing.

The A4D tank (the aft end was also bobbed to allow access to the hell hole in the aft fuselage):

The A3J tank:

The A3J 400-gallon tank appears to taper more going aft than the standard tank drawing. The following is based on a pretty good North American general arrangement drawing.

 Note that the nose of the tank in the drawing doesn't appear to bulge as much as the one in the picture above or taper as much. Here is another perspective:

However, the 400-gallon tank was rarely seen on the RA-5C since it had more internal fuel than the A3J-1.

2 August 2011 Update: Don Hinton photos of the 300-gallon tank on the F4D at the National Museum of Naval Aviation at Pensacola.

Note that because of parallax, etc. it wouldn't be wise to try to use this to establish the shape of the tank.

The tip of the tank is a little bulbous and definitely not pointed.

Note the 30-inch lugs, sway braces, and connections to the pylon.

Note the vertical and longitudinal slots on the aft end of the tank for installing different fin configurations as described by Gerry Whiteside above.

The silver rectangle on the top of the tank is a data plate.

When a tank required the "X" orientation of the fins, as on the S-3 shown here (from a crop of a Christopher Ishmael picture found HERE), the aft end of the tank was rotated 45 degrees.

As a bonus, here is the Navy documentation for the D-704 store:

 For more on the inflight refueling store, see http://tailspintopics.blogspot.com/2015/06/things-under-wings-inflight-refueling.html

Getting It Right

This is another in a series of examples of how hard it is for model kit manufacturers to produce one with no errors of shape or detail.

The Mk 4 atomic bomb was the improved production version of the Mk 3 Fat Man that was dropped on Nagasaki. According to Chuck Hansen's book, U.S. Nuclear Weapons: The Secret History, the first one was delivered to the the national stockpile on 19 March 1949. A total of 550 were built between March 1949 and May 1951. All were retired by May 1953, replaced in part by the very similar but lighter Mk 6.

Because of the security imposed on these weapons, documentation is scarce. The 12 Squared Mk 4 kit (see Here) was based on the following drawing, which turns out to be for a 1/2 size ballistic test specimen. What looks like a gun-type mechanism (the Mk 4 actually had a spherical implosion device, accounting for its very low aspect ratio) was actually a pipe on which was mounted two 700-lb lead weights. The afterbody was a straight taper with four simple fins mounted on the back.

Bill Maloney (web page) photographed a full size Mk 4 model with this fin configuration that is in the Diefenbunker Cold War Museum about 20 miles west of Ottawa, Canada. It was built by Russ Gray in 2003 for a documentary film. Note that the afterbody is more bulbous than the drawing above.

I don't know whether the original Mk 4 had fins like the one above. The more familiar configuration has wedge-like fins.

 

 I don't know if there are any other Mk 4 shapes on display. However, there are a few Mk 6 shapes in museums, including the National Museum of the US Air Force at Dayton, Ohio and the Museum of Aviation at Warner Robins AFB, Georgia. The Mk 6 appears to be externally identical to the Mk 4 except for circumferential stiffeners around the mid-body resulting from a change to a lightweight casing that reduced the bomb's weight by 2,000 lbs.

I wasn't happy with the first drawing that I had made of the Mk 4 for an illustration of the AJ Savage in my book Strike from the Sea, so I revisited the available material to see if I could improve it. Once again, I proved to myself that working solely from pictures leads to error. Fortunately, Bruce Radebaugh responded to my request for someone to measure the Mk 6 at Warner Robins, so I'm fairly confident that the size of the fins on my Mk 4 drawing is now about right and the body shape is close:

Bruce also took several pictures.

Although the fin looks flat on one side in the picture above, it's an optical illusion caused by the downward slant of the back half of the top of the fin. It is symmetrical.

The Mk 6 shape in the Warner Robins museum appears to be a mockup. Compare, for example, its boat tail:

With the one on the Mk 6 in the Air Force museum:

I can't say which is correct; the one in the Air Force museum is likely a training aid with details not representative of the real thing.

However, the Smithsonian Air and Space Museum reportedly was given a Mk 3 that was later determined to have not been fully disarmed, so anything's possible...

S2F One More Time

I decided to double check my assertion that the vertical tail did not change between the S2F-1/2 (S-2A/B) and the S2F-3 (S-2D). With respect to the size of the vertical tail, there's no difference shown on the Grumman factory drawings between the S2F-1/2 and the -3 or any mention in Grumman documents of a change. The mean aerodynamic chord is identical and the tip of the tail is at the same fuselage station of 157 inches. There is a difference in height between the top of the tail and the ground, but it can be accounted for by the -3 forward fuselage stretch between the main landing gear and the nose gear, which would result in a more nose down sit since the length of the nose gear did not change. Here is an overlay of the respective SAC side views:

 Note the difference in the "sit" in additional to the exact overlay of the vertical tails..

Since it's not unknown for draftsmen to be a bit lazy (note the inaccurate depiction of the aft end of the S2F-1 engine nacelle) and perhaps not to have bothered to update the S2F-3 SAC drawing for a change in the tail, I compared a couple of pictures taken from about the same distance and angle.

They look identical to me...

AJ Savage Notes

Updated 20 August 2022

This is a placeholder for a more detailed discussion of the North American AJ Savage program.*

North American won a Navy competition for a carrier-based atomic bomber in 1946. It was big enough to carry the projected 10,000-lb Mk 4 atomic bomb.The bomb bay was clearly sized in cross section by its 60-inch diameter. I have wondered why it was so much longer. My best guess is that in was sized in length by 2,000-lb bombs.

Note that the AJ-1 had the short canopy with the third crewman's seat on the left side of the fuselage across from the entry door. The AJ-1s originally had dihedral in the horizontal tail and a very large rudder. After a fatal crash in flight test, the stabilizer dihedral was eliminated, the rudder reduced in size, and the vertical fin increased in height and chord. That left the shorter canopy and the different nose landing gear door arrangement as the only external configuration differences between the AJ-1 and -2.

Both the AJ-1 and the AJ-2 had a small fairing for an antenna ahead of and just to the left of the nose gear door. When equipped with the ASB-1 bombing system, they also had a periscope fairing on the right side of the nose (the fairing was jettisoned before a bombing run):

Note that this illustration above depicts the two-piece AJ-2 nose landing gear door and the periscope dome after the fairing was dropped.

In response to a question about the tip tanks, they originally had a clear tip for left and right position lights. (Note that the wings are folded in this picture so the tanks are almost upside down...)

Subsequently, the inboard half of the tip was either painted (in some cases it looks like Corrogard) or otherwise made opaque.

Tip tank location (these are AJ-1s that have been retrofitted with the AJ-2 empennage):

*The best 1/72 kit by far is now the Roden AJ-1:  http://tailspintopics.blogspot.com/2022/08/roden-172-north-american-aj-1-savage.html

Although my CD produced for Nostalgic Plastic's sales of the Anigrand AJ-2 (now sold out as well as out of production) is no longer commercially available, for the time being this blog entry only contains material not covered in the CD or errors in the material that is included. For example, I didn't know at the time that the AJ-2 vertical fin (retrofitted on the AJ-1 along with the rest of the empennage changes) was 12 inches taller than the AJ-1's. However, the kit's vertical fin is the correct height. Since I provided the material on the CD to Nostalgic, I have also photographed the interior of the AJ-2 at the National Museum of Naval Aviation and therefore have identified some minor corrections to the kit's interior. See http://tailspintopics.blogspot.com/2013/04/aj-2-savage-cockpit.html.

I built an AJ-1 model from the Airmodel "kit," working on it intermittently for more than a decade.

 

The landing gear and propeller blades were provided by bits and pieces sold by Roberts Model once upon a time. A replacement canopy was vacuformed from a master created from the one in the Mach II AJ Savage kit, which is another huge time waster. The Mk 4 and cart shown above was from 12 Squared and is out of production. However, Spectre Resins produces a Mk 6 which is basically the same except that it had reinforcing ridges around its mid section. No cart, though.

The great Gordon Stevens (Rareplanes, among other brands) also produced a vacuform AJ kit which was very good for the type and could be built as either the AJ-2 or -2P. It included decals. However, it is long out of production and the AnigrandRoden kits are more accurate in a few areas like the propeller blades.

More later...

Model Kit Errors

Whenever a new kit is released, or even before if photos of test shots are available, errors and omission in shape or detail are immediately identified by the cognoscenti, often accompanied by expressions of wonderment and/or rude comments as to how the kit manufacturer could have gotten it wrong.

I spent a lot of time reviewing the Kinetic 1/48 S-2E and detailing the differences between the early and later Trackers and derivatives of the S-2. (See Here.) It was an airplane that I was very familiar with. I first sat in one in 1956.

I got to fly one in 1993, courtesy of Conair, who converted surplus S-2s to firebombers, including a version powered by P&W PT-6 turboprop engines, the Turbo Firecat.

So you can imagine my chagrin when Alan Weber recently asked me "I was wondering if you had any idea of when and why the big side front windows were enlarged on the S-2." and I had to say "Huh?" I had not noticed that.

It turns out that the S2F-1 (S-2A) and -2 (S-2C) and the TF-1 (C-1A) were originally delivered with cockpit side windows that did not extend beyond the contour of the fuselage immediately aft of the window:

It's not always easy to tell from photographs taken from the side that the window is bulged out. Based on this picture taken by Grumman, which is probably of a new airplane, the change appears to have occurred late in the production of the S2F-1, after the change to the gray/white color scheme.

Here is a closeup of the bulged window:

The new side window was retrofitted to the earlier airplanes, including the C-1, and was almost certainly standard on the S2F-3 (S-2D) production.

My guess is that its primary purpose was to provide the pilot and copilot with more elbow room as shown here on an S-2E (note the shadow of the bulge).

Other likely reasons for the bulge were better downward visibility and more clearance when using a camera to photograph foreign ships and submarines.

The WF-1 (E-1B) appears to have been produced with a bulged window. (Another E-1B feature common with the S-2D and subsequent production is that the control column was between the pilot and the instrument panel, whereas the C-1A COD version of the early S-2s had the control wheel extending out of the instrument panel.)

With respect to the Kinetic kit, Alan reports: "The more I work with them, the more it seems to me that the Kinetic windows are bulged greater than the early S2F-1 windows but are way less bulged than the later windows on the S2F-3 and other earlier models that operated later on. If a person wanted a more accurate window for a blue painted S2F-1 or -2, I would sand down the existing bulge and then polish the window out. On my tanker I reworked the sides of the fuselage behind the bulged windows and flattened them some (the fuselage sides). Then I took a hard look at the side windows and reshaped them to a somewhat closer curve than what Kinetic tried to do. On the later windows that have the pronounced bulge, the bulge extends the farthest out in the bottom half of the window. Kinetic's is centered and it gives the whole window an angled down appearance from the front which is weird looking. The windows now are still way too flat, but at least they are closer in shape than they were to start with. They did polish out nicely with not a whole lot of work."

One of the reasons that kit manufacturers don't get everything right is that it's not easy, particularly when the airplane configuration changes over time...