Kanmon Undersea Railway Tunnel & the Invasion of Japan

[Mil-tech Bard]

The vulnerable section was the Moji part of the tunnel on land and submerging under the straits. The rail tunnel there was parallel to the shore with a bright contrast for radar night/bad weather bombing.

In order of vulnerability, there were the following infrastructure targets that the destruction of any one of which would have closed the Kanmon Undersea Railway Tunnel --

1. The Electrical Powerplant supplying the Kanmon Undersea Railway Tunnel -- a soft target
2. The Moji-Dari rail marshaling yard -- a soft target
3. The Shimonoseki rail marshaling yard -- a soft target
4. The Moji tunnel entrance and above water tunnel structure - a semi-hard target via soft ground near the tunnel
5. The Shimonoseki tunnel entrance structure and above water tunnel structure -- a hard target
6. The Shimonoseki Rail viaduct -- a hard target and the likely target of the Javaman.

None of this extended target complex was hit by a single bomb, boat bomb or otherwise.

[ChristopherPerrien]

x2 post

[phylo_roadking]

1. The Electrical Powerplant supplying the Kanmon Undersea Railway Tunnel -- a soft target

You'd need to know more about the set up of the japanese electrical grid; did this powerplant solely serve the tunnel...and was it linked to the grid? If so, it would have been possible to restore power by rerouting/switching current.

2. The Moji-Dari rail marshaling yard -- a soft target
3. The Shimonoseki rail marshaling yard -- a soft target

Rail lines are notoriously easy to clear/repair/re-lay if you have enough manual labour to hand

None of this extended target complex was hit by a single bomb, boat bomb or otherwise.

I think the lesson here is actually -

After the Javaman program was cancelled, the Kanmon tunnel was
assigned to the 20th Air Force bombers, which firebombed both Moji and
Shimonoseki, which were either end of the electric railway tunnels.

Firebombing was ineffective in destroying the rail lines, tunnel
entrances or the electrical power plant supplying the tunnel electric
engines.

After August 7, 1945, the Kanmon Railway tunnel was reassigned to the
FEAF, which made one attempt to bomb the tunnel that was thwarted by
bad weather.

...It was attempted - but none of this extended target complex was hit by a single bomb, boat bomb or ordnance capable of destroying it.

[phylo_roadking]

Christopher...

Look at it more carefully - over 2/3s of the underwater length is marked as "invulnerable". The last third is marked as "vulnerable" only because of what's over the tunnel - sandy clay.

...my point being that sitting ON granite, that part of the tunnel is actually equally invulnerable to the "earthquake bomb"/undermining action of Tallboy.

[ChristopherPerrien]

Consider 300 B-29, with 20x1000-lb bombs each (6000 ea. 1000-lb bombs), and a .0002 hit percentage.

Multiple .0002 X 6000 = 1.2 hits.

No such attack was made.

Please note the target vulnerability assessment bar under the tunnel depth profile in middle and upper right.

Look at it more carefully - over 2/3s of the underwater length is marked as "invulnerable". The last third is marked as "vulnerable" only because of what's over the tunnel - sandy clay.

It's an assessment based only on known soil and rock types the tunnel went through - doesn't say anything about proximity of impact, height to be delivered at etc.,...

And of course - any of the eastern part of the tunnel destroyed by bombing on land - could be far more easily cleared/repaired than any part of the unserwater section The effort might not actually "destroy" the target, just incapacitate it for a time.

Are you kidding Phylo? Or can you just not admit when you are wrong?

It is obvious that this target profile says half of this tunnel was vulnerable to mere TALL-BOYS. It is fairly obvious, the Grand Slam being close to twice the weight of a Tall-Boy(22000/12000) and a fair bit more than twice as BAD , IMHO, would have rendered even the "invulnerable half ", vulnerable.

And again you continue to discount the effect of these earth-quake bombs, and their ability to collapse underground structures WITHOUT A DIRECT HIT. Perhaps you know more about bomb effects than Barne Wallis did, of bombs he designed himself, and why he might have been wrong in naming/labelling the tall-boy and grand -slam, "earth-quake bombs" and how they worked in the war with their much greater percentages of HE as opposed to just plain "iron bombs"; However I have my doubts.

On a side note , I will at least admit that I wrong in my suspicion that a type of earth-quake bomb was not considered for this target by the USAAF, obviously it was. However it only re-affirms my earlier post that Grand Slam bombs would have been fairly a 100% effective means of destroying this target if they had been employed. And we now know that tallboys could have done the job too, for all intents and purposes.

Also yes , Tallboys did destroy a tunnel at Samur, not in Italy, not Grand-slams,, still that hardly discounts anything I mentioned earlier, as even my minor error of recollection there, only proves the plausibility in this case even further. A half sized bomb did that job.

I'll leave it at this. Good Day.

[phylo_roadking]

Christopher - time you went and did a bit of research...

And again you continue to discount the effect of these earth-quake bombs, and their ability to collapse underground structures WITHOUT A DIRECT HIT.

No I'm not discounting it - actually, you are. As you're continuing to ignore HOW they worked. If they were droppd into CLAY they excavated a cavity for the structure to DROP into.

Kindly note the section of the underwater part of the tunnel that is marked "vulnerable" to Tallboys...but sits ON GRANITE. Please illustrate how a bomb penetrates and excavates a cavity in granite...

...remembering that these were ordnance that had a reputation for breaking apart?

It is fairly obvious, the Grand Slam being close to twice the weight of a Tall-Boy(22000/12000) and a fair bit more than twice as BAD , IMHO, would have rendered even the "invulnerable half ", vulnerable.

It is fairly obvious that the U.S. didn't have Grand Slams - and as even the OP states it would have been more than difficult to deploy them even if they had.

Also yes , Tallboys did destroy a tunnel at Samur, not in Italy, not Grand-slams,, still that hardly discounts anything I mentioned earlier, as even my minor error of recollection there, only proves the plausibility in this case even further.

And we now know that tallboys could have done the job too, for all intents and purposes.

Re my research comment above -

NINETEEN Tallboy-equiped Lancasters of 617 Sqn attacked the rail tunnel at Saumur...and only ONE had the required effect, by actually entering the tunnel and bringing it down from within. In other words...didn't you wonder what happened to the efficacy of the other 18?

Here's what...

From http://www.loire-gites.com/

3.jpg (80.74 KiB) Viewed 1653 times

They actually have to be delivered in the right place... The lowest was actually delivered from 8,200 feet, the highest from 10,500 feet...the bombers having to drop them from a high enough altitude that they're not brought down from shrapnel from their own bombs!

[ChristopherPerrien]

[Mil-tech Bard]

The US modified a B-29-75-BW S/N 44-70060 to carry TWO Grand Slams externally. Thus its bomb bays could be used for either normal bomb storage or bomb bay fuel tanks. Bomb bay fuel tanks was most likely, because two external Grand Slams plus bomb bays full of bombs resulted in VERY short range. Plus, the internal bomb payload would have had much different bomb trails resulting in much different bomb trajectories than the Grand Slam.

The B-29 was also capable of carrying the Grand Slam, after modifications, internally. This left the B-29's so modified with much shorter ranges due to partially open bomb bays after drop, as I mentioned previously.

The Grand Slams planned to be used in the invasion of Japan were the Tarzon guided bombs, a Grand Slam modified with the guidance of the radio controlled VB-3 Razon bomb.

However, it would not have been available until the winter or spring of 1946.

The alternate VHB design, the B-32, could carry two Grand Slams internally without modification and was just coming into squadron service with the 5th Air Force at War's end.

AFAIK, there were no Grand Slams in theater for General Kenney's B-32's to use

[Mil-tech Bard]

Japan does not have a national power grid, for reasons of history and bureaucracy, see --

http://en.wikipedia.org/wiki/Utility_frequency

However, as of the turn of the 21st century, places that now use the 50 Hz frequency tend to use 220-240 V, and those that now use 60 Hz tend to use 100-120 V. Both frequencies coexist today (Japan uses both) with no great technical reason to prefer one over the other[1] and no apparent desire for complete worldwide standardization.

and


In Japan, the western part of the country (Kyoto and west) uses 60 Hz and the eastern part (Tokyo and east) uses 50 Hz. This originates in the first purchases of generators from AEG in 1895, installed for Tokyo, and General Electric in 1896, installed in Osaka. The boundary between the two regions contains four back-to-back HVDC substations which convert the frequency; these are Shin Shinano, Sakuma Dam, Minami-Fukumitsu, and the Higashi-Shimizu Frequency Converter.

The Japanese electrical system had and has two grids, both of which relied to a great extent on small hydroelectric plants, however, the dry season economy and spot electrical users like the Kanmon tunnel relied upon thermal/coal plants for their primary supplies.

See pages 21-22 pf the pdf document at this link http://www.comw.org/pda/fulltext/griffith.pdf

"The survey team did offer one criticism of the targeting work during the war. While there was sufficient information on the production of electricity and the location and size of the facilities, the team felt that there had been a lack of analysis on the relative importance of the installations. They based this observation on the fact that there had been little consideration given to bombing the thermal plants in Japan. In contrast to the hydroelectric plants, the thermal facilities were few in number and vulnerable to air attack. While the thermal plants only accounted for 17 percent of the overall generating capacity, during the dry season the Japanese relied on these facilities for 30 percent of the power generated. The USSBS report also noted that not every part of Japan was the same regarding the use of power—some portions were highly dependent on steam-generated electricity and would have been more affected by the bombing of these stations than other areas.80"

79. USSBS, Electric Power Industry of Japan, 5–6, 17.
80. Ibid., 30–32, 35, 38. In fact, most of this information was known, at least in some parts of the US intelligence structure. A report by the Office of Strategic Services in May 1944 highlighted the very ideas that the USSBS brought up. Whether this report was ever seen by any air planners in time to influence operations is unknown. See Office of Strategic Services, Research and Analysis Branch, Summary of Strategic Information: Far East Axis Economy Japan’s

Given that the OSS was trying to use the Javaman to destroy the Kanmon Undersea Railway Tunnel complex, there are a lot of additional shades of meaning to note 80 above

1. The Electrical Powerplant supplying the Kanmon Undersea Railway Tunnel -- a soft target

You'd need to know more about the set up of the japanese electrical grid; did this powerplant solely serve the tunnel...and was it linked to the grid? If so, it would have been possible to restore power by rerouting/switching current.

2. The Moji-Dari rail marshaling yard -- a soft target
3. The Shimonoseki rail marshaling yard -- a soft target

Rail lines are notoriously easy to clear/repair/re-lay if you have enough manual labour to hand

None of this extended target complex was hit by a single bomb, boat bomb or otherwise.

I think the lesson here is actually -

After the Javaman program was cancelled, the Kanmon tunnel was
assigned to the 20th Air Force bombers, which firebombed both Moji and
Shimonoseki, which were either end of the electric railway tunnels.

Firebombing was ineffective in destroying the rail lines, tunnel
entrances or the electrical power plant supplying the tunnel electric
engines.

After August 7, 1945, the Kanmon Railway tunnel was reassigned to the
FEAF, which made one attempt to bomb the tunnel that was thwarted by
bad weather.

...It was attempted - but none of this extended target complex was hit by a single bomb, boat bomb or ordnance capable of destroying it.

[phylo_roadking]

The Japanese electrical system had and has two grids, both of which relied to a great extent on small hydroelectric plants, however, the dry season economy and spot electrical users like the Kanmon tunnel relied upon thermal/coal plants for their primary supplies.

That's my point; primary supply...but was the tunnel ALSO linked to the Tokyo area power supply?

The Grand Slams planned to be used in the invasion of Japan were the Tarzon guided bombs, a Grand Slam modified with the guidance of the radio controlled VB-3 Razon bomb.

Tarzon...the ASM-A-1 Tarzon...was a derivative of Tallboy, not Grand Slam.

However, it would not have been available until the winter or spring of 1946.

Design work wasn't begun until February 1945...and when used, in Korea, was dropped (sic) because of lack of success - only six of twenty-eight used hit their targets, and there were a number of "interesting" accidents!

[ROLAND1369]

Re Mil-tech Bards comment on 18 Feb 2013 17:13 the B32 was not capable of carring 2 grand slams. It could carry only 20,000 lbs of bombs and had a two piece bomb bay which precluded eventhe 12000 lb tallboy as an internal load. I believe the only aircraft cabable of carring the Grandslam internally was the postwar B 36.

[ROLAND1369]

I forgot to add that the B 36 could, on shortrange mission carry 3-22000 lb grand slams, all internally.

[Mil-tech Bard]

Yeah, I saw that after further searches.

When you DAFS you need to make sure the link is a good one. :roll:

Re Mil-tech Bards comment on 18 Feb 2013 17:13 the B32 was not capable of carring 2 grand slams. It could carry only 20,000 lbs of bombs and had a two piece bomb bay which precluded eventhe 12000 lb tallboy as an internal load. I believe the only aircraft cabable of carring the Grandslam internally was the postwar B 36.

[Mil-tech Bard]

I found the Japanese standard 3' 6" narrow gage track the Kanmon Undersea Railway Tunnel used permits an axle load of only 15.5 tonnes.

[See - Akira Saito (June 2002), "Why Did Japan Choose the 3'6" Narrow Gauge?", Japan Railway & Transport Review (EJRCF) (31): 33–38, http://www.jrtr.net/jrtr31/pdf/f33_sai.pdf]

The 3' 6" narrow gage track is dirivative of Norwegian track gage supplied by the British in the late 19th century and adopted by the Japanese for reason of British financing.

In addition, the eletrical rail engines in the Kanmon Undersea Railway Tunnel (KURT) used 1500 volt DC. Given the very limited transmission range of DC power, an air attack destroying the KURT railway marshaling yards would take out the step down transformers converting the 60 hertz AC power to 1500 Volt DC the trains ran on. And how ever swiftly the Japanese could rebuild tracks and re-cable overhead power lines from bomb damage to the electrical train lines & yards, the replacement times for those AC to DC step down transformers were measured in weeks and months.

[Mil-tech Bard]

This is the technical information on the standard WW2 Japanese freight car rolling stock --

Standard freight cars built in the wartime
were the high-sided six-wheel open
wagon of the Toki-900 type. It was designed
to implement a bogie-class payload
with minimum construction material
while reducing overall length. In addition,
a special measure called 'extra-tonnage"
was introduced for existing freight cars,
allowing temporary overloading of the
original payload.

Source --

Japan Railway & Transport Review No. 9 (pp.32–35)
Japanese Railway History 8
Wartime Railways and Transport Policies
Yasuo Wakuda

http://www.jrtr.net/jrtr09/history.html
http://www.jrtr.net/jrtr09/pdf/history.pdf