ID of WW2 aircraft guns & ammunition holing a Thai railway bridge

I've been looking at a railway bridge here in northwest Thailand which apparently has three shell holes from WW2.

I've completed a first draft for a webpage on the subject here: San Khayom Bridge.

But the presentation is weak because I can't really say with any assurance what produced the damage. Since Thailand was a member of the Axis during WWII, I think I can reasonably assume that the two smaller holes probably came from an Allied aircraft strafing Thai railway rolling stock as it entered the bridge --- that is, if the projectiles, that hit the bridge, were 50BMG which "spread" on hitting structural steel, growing from .50 caliber to 7/8 and 1 inch. A possible source of the larger hole with diameter over 2 inches, though, is less clear.

So I'm looking for assistance / direction in interpreting the damage as pictured on the webpage: identifying ammunition and associated guns used, and types of aircraft associated with those guns --- and comments on the assumptions I've already made on the webpage, including my suggested scenario for Hole Nos. 1 and 2 of an aircraft pulling out of a dive while firing.

I thank you for any assistance.

 

My compliments on an exacting and comprehensive report.

The main fighter armament on American planes was of course the 50 cal which shot AP, Incendiary and HE rounds. The Brits had some Hurricanes and maybe a few Spitfires in the area, the Hurricanes could be armed with 40mm cannon but maybe not in this area. But 20mm would be more likely on later marks of those two. The Mosquito could carry quad 20's in the nose and even a 60mm semi-auto but the likelihood would be low. I do not know if any Bell Air Cobras (P 39), US or British were in that theatre but they mounted a 37mm cannon ( T -9) in the nose. It had a claimed penetration of 2CM of steel at 450 meters.

Gaines

 

[SIZE=10pt]I thank you. As far as I can tell, there were Mosquitoes present with 211 Squadron, May 1945-end of war: they replaced Beaufighters that 211 was reported to have started over Thailand in Mar 1944. I've not found any mention of P-39s over Thailand, which is unfortunate as a 37mm cannon might be an easy explanation for the two inch + hole. [/SIZE]

[SIZE=10pt]I've folded comments from various forums plus additional information I've generated (photos and measurements) into a major rework of the San Khayom Bridge webpage for possibly teasing out further comments. With regard to bridge engineering questions, I'm in contact with a fellow familiar with riveting methods who has promised comment. I'll also try to query additional forums, including the Int'l Ammunition Assoc.[/SIZE]

 

islandee, I remain immensely impressed with quality and thoroughness of your research shown in your webpage. As one who spent much of his life as a college professor I was exposed to myriad types of research and the quality of your work would easily qualify.

I did have a thought. It might have been possible that a Beaufighter or Mosquito could have been field modified to specifically attack such bridges though bombs or rockets seem simpler. . . Still creativity exist in war in fertile minds.

My choice would have been a Hawker Hurricane Mk 11D with 40mm's. It seems likely that some could have been in that theatre as Hurricanes in their new role as ground attack planes would have been useful.

Kindly,

Gaines

Gaines

 

Thank you for the compliment. My engineering background, I guess.

 

The prime Beaufighter-flying candidate just now, RAF 211 Squadron, had those aircraft replaced with Mosquitoes in May 1945, but didn't complete retraining by war's end; so none of their Mosquitoes flew over Thailand during the war. I've not as yet found mention of Hurricanes in this area, other than in Mar 1942, flying as bomber escort.

Approximately in accord with your speculation, certain Beaufighters, Mk Xs, were fitted with RP3s; however they were factory-installed. Their three inch diameter (as in RP3), or 760mm, was too large to have made the 600mm hole at Point G.

Source for the above: RAF Squadrons: 211 Squadron.

But the aircraft still sported its four Hispano 20mm cannon. I've queried another forum if a 20mm shell, loaded with HE ammunition, and detonating on impact, might have been able to produce the hole at Point G without creating further damage. Key to that scenario: the explosive release of gas expanding within the relatively unconfined space of an open built-up structural steel member, and that within an open structural steel truss, might not have otherwise damaged the structure.

Regarding your comment about in-the-field innovation, I came across this example for a different aircraft:

The 25th bombed and strafed enemy troops, concentrations, supply dumps, bridges and enemy communication lines for twelve consecutive days, but failed to slow the Japanese advance on Fort Hertz. B-25 heavy bombers were needed to halt the enemy's drive, but none were available. Lt. Col. John E. Barr, the executive officer for the 51st Fighter Group, modified a P-40 to carry 1,000 pound bombs, and by May 1943, had halted the Japanese offensive.

Source: Osan Air Base: 25th Fighter Squadron.

 

My guess from the size of the dents is 20mm...the penetration puts any rifle calibre out of the picture IMO. I would dismiss the mosquito, if only because one would see much more hit damage around each dent/hit...I think a 50cal could leave these marks, but again, the fire rate would suggest more hits...Are there any bombs/arty rounds with bearings?

 

Small point, the nose guns in the Mosquito, when mounted, were four .303s. Four 20mm cannon or one Molins automatic 6-pounder/57mm could be mounted in the forward part of the bomb bay.

 

After some interruptions, I'm back to the shell holes in the San Khayom bridge. I apologize for the length of this; but I don't know how to shorten it further.

With regard to the hole at Point A, I'm trying to visualize the actions of the aircraft that produced the hole. I'd like to include a sketch on my webpage of the possible / probable path of the aircraft during its attack.

The consensus is that the hole is apparently from a 20mm projectile; and in the Southeast Asia Theater both the P-38 and the Bristol Beaufighter carried HS.404 cannon which used that ammunition. I'll use the P-38 as a subject here simply because I can find more information about it (the Beaufighter might be the better candidate because, beyond its harmonization range, it would have offered a dispersion pattern with its four cannon: such a pattern might more easily have included Point F, the other 20mm impact point on the bridge. Does anyone know of an on-line presentation, including sketches, perhaps a manual, for harmonizing the guns on a Beaufighter?).

So, scenario: a P-38 pilot happened upon a "target of opportunity", a train traveling south out of Lamphun and approaching San Khayom bridge. The pilot reduced altitude so as to fire on the train. Coming into range, he fired for a certain period; then he broke contact and climbed away from the train and the bridge.

Angle of attack: The exit angle of Hole A measures about 12° to the horizontal. In the sketch below, I assume the bullet path was almost unaffected in passing through the comparatively thin bridge plate (projectile diameter was twice the thickness of the target plate). On that basis, I assume the pilot dropped to an altitude matching the 12° allowing an angle of attack of 12°.

A P-38's single 20mm cannon was centrally mounted in the fuselage and its operation was basically point (the plane)-and-shoot. The range of the cannon was limited primarily by the pilot's visual acuity and his properly adjusting for bullet drop (excluding weather, aircraft performance, pilot skills, whatever).

I assume an attack speed of 350 mph: a P-38's cruising speed was 275 mph and max speed was 414 mph. A velocity diagram would look like this for a 12° dive at 350 mph:



The important information here is that, flying at a down-angle of 12° and at a speed of 350mph, the aircraft would have been dropping 107 feet per second.

The idealized flight path would have looked like this:



Of course, the pilot would have disengaged at some point before his aircraft flew into the target.

Questions.

Do these assumptions and the scenario seem plausible? If not, I'd appreciate being corrected.

If the scenario is not too far off:

At what range would the pilot likely have started firing? (I read that one B-25 gunner had his guns harmonized for 1000 yards: he cautioned that this was useful only for stationary targets)
And at what altitude would the pilot have had to disengage (how much altitude would be burned up after the pilot tried to gain altitude; and how much spare altitude should he have allowed)?
Tying these down would also establish how long the pilot might have fired.

Alternate scenario: With the above presented, there is a piece of data which could contradict this scenario, and I don't have the background to interpret it: the "floor" of Hole A is at an angle of about 7° to the horizontal. If 7° (not the 12° used above) were the angle at which the projectile entered the plate, and the exit angle was, as actually measured, 12°, then passing through the plate would have bent the projectile's course by 12° - 7° = 5° (see cross-section).

If the angle of attack were a very lean 7°, then at a range of 1000 yards, the gun would have been only 366 feet off the ground --- with bullet drop compensated for by either, an extra 24 more feet for a total of 390 feet, or a slight tilt upward in angle of attack. And at 500 yards 183 feet (compensated for 4 feet of bullet drop). On the other hand, the aircraft's altitude loss while changing from a down-angle of 7° to actually moving upward would have been less than recovering from a down-angle of 12°. But operating that close to the ground at 350 mph or more seems unlikely (which is why I didn't include it in my main presentation above); though I am willing to be corrected.

Are there any comments about this alternative?

 

Glad you asked...
I think it can be assumed it was an aircraft as it is unlikely that a water craft or foot soldier would be firing a 20mm and the angle probably discounts this...i say probably becasue rounds can ricochet wildly sometimes and give the weirdest angles.
I would balk at this being a "target of opportunity" - Aircraft were regularly sent to attack spotted trains...unlikey that the pilot just happened to see the train as it was crossing an open bridge, more likey the aircraft was waiting.
And the approach speed seems a little high for me too...pilots would slow to what they felt comfortable with - not what was in the book - many ways to reduce speed - including speed breaks...i would slow as much as possible if i wasnt seeing any return fire.

 

Watercraft: no, I agree --- the stream was not navigable. Foot soldier: possible, but improbable. A Type 97 20x125mm automatic cannon, an anti-tank rifle, and a two man weapon by weight was suggested: a very long range shot might have involved enough bullet drop to produce the possible 7° impact angle. Or the weapon might have been fired from a railcar ahead of the locomotive. However, with further digging, those scenarios were dropped.

Target of opportunity. I concede on a search of the USAAF Chronology that "targets of opportunity" were reported very few times by the Tenth and Fourteenth Air Forces. On the other hand, they were regularly mentioned in reports from the European Theater.

The bridge would not have been the target of opportunity: if it were designated a target, bombs would have been used: bullets / projectiles don't bring down structural steel bridges (however, there is no evidence of bombs having been used on this bridge). The Eastern Air Command was actively targeting bridges for bombing in the north of Thailand in order to disrupt an IJA supply line using the railway from Bangkok to Lampang and then connecting vehicle roads north and west into Burma. Bridges specifically targeted for bombing (and successfully bombed) were the Mae Mo (Mae Chang), the Mae Ta (Kaeng Luang), and the Ban Dara. The San Khayom Bridge (the subject here) was north of Lampang and therefore not part of the IJA supply line feeding Burma.

I'm not familiar with trains having been assigned as targets. I don't think either the Tenth Air Force, based in India, or the Fourteenth, based in China, were in a position to post an aircraft above a railway in Thailand just to await a train. Aircraft just didn't have enough fuel to "hang out", waiting for a train. And on the ground, while there may have been train schedules, they were impossible to adhere to if for no more reason than the constant need to stop and refuel with locally cut wood of low heat content, typically piled at every station. Aircraft had to actively search out targets before they ran short of fuel and had to turn back. Pilots had orders to return to base without bombs, and with minimal gun ammunition: not only was there a danger at base of a landing accident setting off munitions carried on an aircraft, there was also the weight of unexpended munitions and the fuel expended to bring it back. The train was the target of opportunity, attacked after the primary assignment of the flight had been completed (whatever that might have been). The projectile holing the bridge member was probably in the extreme of the dispersal pattern for the aircraft's weaponry targeting the train.

The approach speed is definitely high: I agree. Too high, particularly since there were minimal defenses on the ground: Allied intel regularly noted mostly empty AA emplacements at Thai airstrips, and enemy aircraft were rarely sighted. The only major confrontation in the north of Thailand between Axis and Allied aircraft was over Lampang in Nov 1944 and five RTAF aircraft were downed; they were all older aircraft, hand-me-downs from the IJAAF after it had received newer aircraft. So from, say, late 1943 when the Allies were sufficiently re-equipped to the end of the war, the air over Thailand was under the control of the Allies (which is not to say that there were no Allied aircraft lost over Thailand. Three were lost during the Flying Tiger period through mid-1942. Thereafter, one was downed during the Nov 1944 action, two were knocked down by lucky ground shots, one was hit by lightning, at least one just disappeared, and recently discovered wreckage of another WWII-era aircraft remains unidentified).

 

I'm in the process of writing up what all I have to date on the two 20mm shell holes in the San Khayom bridge. I still have to deal with the 57mm shell hole (Point G).

I've not found as yet any aircraft, either Allied or IJAAF, operating over Thailand that used a 57mm cannon. On the ground, the Japanese had a Type 97 57mm Tank Gun, used in a Type 97 Medium Tank "Chi-Ha". The gun would have been firing at the RAF Beaufighter or USAAF P-38 which was putting the 20mm holes in Points A and F of the bridge. The scenario in itself is not farfetched: 170 km SE at the Kaeng Luang Bridge, an enterprising Thai gunner using an anti-tank gun shot down a B-25. But at the San Khayom bridge, no Allied plane was downed though it might have been targeted.

I've found no record of tanks with 57mm guns or independently-mounted (jury-rigged) 57mm tank guns having been assigned or used at Lamphun; however, I see two possibilities by which such a tank or tank gun might have been present to defend the Lamphun bridge (plus a longer bridge just 6 km north):

1. One of the main IJA supply routes for Burma followed the railroad from Bangkok to Lampang, 80 km south of San Khayom, where goods were transferred to road vehicles for transport to Kengtung, on to Mandalay, and then in 1944 to Imphal/Kohima, etc. There is also a story that an alternate supply route continued north by rail to Chiang Mai where goods were transferred for transport north into Burma. In both cases, rail transport came under heavy attack by Allied aircraft starting in early 1944 (there doesn't seem to have been much recorded of attacks on road convoys). I'm guessing that those Allied air attacks might have damaged one or more tanks or their carriers sufficiently that they were abandoned along the way. Afterwards, enterprising Thais or Japanese might have salvaged a tank, or a tank gun, for anti-aircraft defense at Lamphun.
2. In preparation for defending Thailand very near the end of the war, various IJA units were assigned to the general Chiang Mai area to meet any Allied ground attack from the north. IJA units included elements of the 4th and the 56th Divisions as part of the 15th Army which was itself headquartered in Lampang. All were essentially in position by June 1945. These units might somehow have acquired some Type 97 Medium Tanks, or at the least a Type 97 57mm Tank Gun, which found its way to the Lamphun area.

While this is wholly speculative, I do have the hole in the bridge which records the diameter of the projectile, its angle of impact, and its bearing. If I could get a set of ballistics curves for the tank gun, I might be able to estimate the location of the gun, if it had been fired from the ground, and check that ground for any possible evidence (that's where the metal detector will be a necessity).

So I ask, does anyone have or know how to get ballistics curves for this Type 97, 57mm tank gun?

I thank you.

 

Are you sure that the two are related to the same event? A bridge in WW2 can be a constant target...

 

[SIZE=10.5pt]It is only my assumption that all the shell holes are related. --- this is only one of many possible scenarios (all of which I of course can't even envision)[/SIZE] and I will emphasize that in a revision to the discussion about Hole G.

[SIZE=10.5pt]However, I do think it unusual that one bridge should display shell holes from up to three separate attacks. Bridges were not targets for strafing; or they should not have been. Strafing does not destroy bridges; evidence of such strafing implies attacks on targets passing over (or under) a bridge. The holes, such as at Points A and F in the San Khayom bridge, should have been secondary to an attack or attacks on a train or trains using the bridge; call the shell holes "collateral" damage. I have not seen shell holes in any other bridge in northwest Thailand; nor has a local RR buff mentioned any other. [/SIZE]

[SIZE=10.5pt]A few RR bridges in the north [/SIZE]deemed critical to supply of IJA's Burma Army [SIZE=10.5pt]were repeatedly targeted by Allied bombing, and were either brought down, or sufficiently damaged, to require replacement after the war (one span on the [/SIZE]Ban Chang[SIZE=10.5pt], the entire [/SIZE]Kaeng Luang[SIZE=10.5pt], and the entire Ban Dara, which I've not yet written up). That action may have lost evidence of additional strafing. One highway bridge in Chiang Mai was said to have had [/SIZE]one[SIZE=10.5pt] shell hole; however, it was replaced in the 1960s as a part of a roads improvement program, so that information is also lost. As with the San Khayom bridge, I think it can be assumed that the shell hole in the [/SIZE]highway[SIZE=10.5pt] bridge evidenced an attack on something on the bridge, not the bridge itself.[/SIZE]

[SIZE=10.5pt]The structure at San Khayom was one of six through-bridges between Lampang and Chiang Mai; in addition, the longest (and highest) tunnel in the railway system is in this same stretch. Had this section of rail line between Chiang Mai and Lampang been deemed critical to destroy by Allied planners, I would guess that the tunnel would have been the most likely prime target. Located in a mountain range, it was very isolated, it was very exposed, and there was no effective paralleling land route. Closure of the tunnel would have ended any substantive military function in the three provinces bordering the northwest corner of Thailand. Destroying the San Khayom bridge would not have been nearly as effective: it is, I believe, the shortest of those six bridges; it is in flat valley land; temporary repairs or a simple road detour would have been relatively easy to effect. The flow of goods to Chiang Mai could have been restored with relative ease.[/SIZE]

 

Your logic is sound...however, from the terrain it is possible that the allies did not want the bridge brought down...because,of thick jungle only a few open areas are available for an attack...it could be that they saw this bridge as a 'killing' bridge...allowing an attack. Without the train, supplies would go by road, even morgdifficlut to attack if surrounded by high jungle...just a thought...playing devils advocate more than anything...remember the soldiers addage....assumption is the mother of all fu@k-ups....

 

I catch your drift: a chance to shoot fish in a barrel.

 

[SIZE=10pt]I've attempted to finalize my webpages on this subject, but am of course still interested in feedback. There are some topics which I leave open: some seem more effort than is warranted; others can have no answers; a metal detector is on order as a long shot. [/SIZE]

[SIZE=10pt]I appreciate the forum's comments.[/SIZE]

[SIZE=10pt]My treatment of this subject has grown to 11 webpages (which start here)[/SIZE][SIZE=10pt], and offers more detail than the average reader probably cares to wade through. As a result, I've started the subject off with a single page condensation --- it's a long page, but just one.[/SIZE]

 

Good on you mate.

 

I think this effort of yours should be highlighted as a poster case for how to research and present such a topic. It also illustrates just how much can be learned (especially if one follows the farious forum threads) by looking at a relativly small event in detail. Overall a very impressive study. Well Done.