Operation Downfall - The Invasion of Japan

[Slamet]

@Slamet: The 20mm gun was smaller and lighter than the Bofors. You could put a lot of them in places where a 40mm won´t fit. And if director controlled they were even more accurate. Furthermore you don´t need to destroy the kamikaze ouright, a small change in the flightpath might result in a near miss or glancing blow.

Hmm, that and the fact they weren't electronically controlled. Makes sense if all the ships electricity goes phut.

The Japanese had plans to disperse the Kamikaze's around roads and open fields and anywhere they could take off from (since landing was not a problem). The CAP and anti air operations would have slaughtered them, but when you are talking 500 planes per attack and they expected to use them all in the first 3-4 days after the invasion, if only 10% got to their targets that is 50 planes per assault. And there would have been 8-10 assaults. It would have been very bloody to say the least. Anyone trying to claim that thousands on our side and 10's of thousands on their side would have died are fools. Frankly I would think around 100,000 casualties on our side and around 1 million on theirs.

If Chilperic puts in the fact that the US does fighter sweeps before the invasion, this could be a factor why those sweeps caused so little attrition to their air forces. Add the above factor and the factor that most Japanese had also built dummy bases aircraft, dummy airfields. AFAIK the US knew of only 125 airfields and airstrips - the occupational forces revealed that the Japanese actually had 325 (95 concealed, add 76 dummy airstrips) as of July 13th 1945.

And also, the Japanese air doctrine for the Ketsu-go (Decisive Operation) called the Japanese to fight as close to the mainland as possible. They could anticipate the feints - at least AFAIK.

 

[CalBear]

The Kamakazis were a threat, and would have been a servere one to the troop transports the first couple days. After that, it would be a very different matter. The U.S. would have had in the area of 500 fighters dedicated to CAP at all times, plus the fighters on ground support missions that could have been diverted if things got bad enough (for that matter the Dive Bombers and torpedo planes could be used against most of the kamakazi aircraft; the Turkey & SBD both had twin forward firing .50's and the SB2C had a pair of forward fixed 20mm). There would also be a lot of counter air going on, with the solid nose B-25 & B-26 medium bombers plus the heavies. (I've always wondered how effective one of the solid noses would have been as a kamakazi interceptor. 300-350 knots speed and up to 16 forward firing fifty cals.)

 

[Matt Wiser]

The British Pacific Fleet by this time was fully integrated into the U.S. Pacific Fleet. In the Okinawa Campaign, they were designated TF 57. For OLYMPIC, they would be TF 37, in Halsey's Third Fleet. All of their carriers took Kamikaze hits, some more than once, and just shrugged off the damage, put out the fires, shoved the wreckage over the side, and continued ops. Most of their aircraft were American: Corsairs, Avengers, and Hellcats, though some U.K.-made Fireflys and Seafires were on their flight decks.

One problem the Japanese would have had in their Kamikaze attacks would have been coordination; Army and Navy air units hardly talked to each other, and even units of the same service sometimes didn't. There was a serious shortage of radios on Kyushu, so land lines would have to be used, or couriers (likely on bikes). Coordinating the various Army and Navy Kamikaze and attack squadrons would've been difficult. Coordinating them with the Shinyo suicide boats, midget subs, and Kaiten would have been impossible.

There were no B-26s in the PTO: other than two groups that fought in New Guinea and the Aleutians in 1942-43, all B-26 combat groups were either in the MTO or the ETO. The two groups that were in the Pacific converted to B-25 gunship models in 1944 and flew to the end. The B-25s and newly arriving A-26s (two A-20 groups had already transitioned) would have been tasked with counter-airfield and antishipping missions, and the B-29s would have been directed to put lots (and I do mean lots) of 500-lb. bombs on airfields as soon as they were discovered. Some airfields were well within range of NGFS, so in between bombardment of the landing beaches and their exits, you can bet 14-inch and 16-inch shells would have fallen on some of these fields.

 

[Slamet]

One problem the Japanese would have had in their Kamikaze attacks would have been coordination; Army and Navy air units hardly talked to each other, and even units of the same service sometimes didn't. There was a serious shortage of radios on Kyushu, so land lines would have to be used, or couriers (likely on bikes).

Land lines? Telephones? There is several advantages to that - there were no communication intercepts. The US could neither jam nor monitor the land lines and the system itself presented few good targets for aerial attack. Correct me if I'm wrong.

 

[Matt Wiser]

The Kyushu phone system was actually in a shambles. And that was before Kagoshima City and Miyazaki had been bombed in June and July of '45. Kinda hard to place a phone call when the telephone exchange either ate a bunch of 500-pounders or got burned to the ground in a fire raid (which happened in Kagoshima City). The Japanese were short of communications gear of all sorts, even field phones for use at battalion and regimental levels. Not to mention that phone lines follow highways, and FEAF was busy interdicting the Kyushu road net beginning in June, so if a telephone pole eats a bomb or rocket meant for a truck, well....and the Japanese electronics industry was in bad shape (along with Japan's other war industries) by that time. While coordination of kamikaze attacks would have been very difficult, it wouldn't have been completely impossible. But it would've been close to it.

 

[burmafrd]

Still. 3000+ kamikaze's in 2-3 days would have caused a lot of damage even if 80% of them did not get through- which is about as good a job as our guys could have done. 20% is 600 planes trying to hit ships that would be at anchor- not moving since they were to target the transports and support ships on the beaches.

 

[Matt Wiser]

The hit rate for Okinawa's suiciders was 1 in 12, I believe. The Japanese estimated that with their pilots in Kyushu being so green (one week's training), that 1 in 20 would hit.

Actually, some 300 JNAF aircraft were tasked with going after the carriers and fire-support groups. More would likely have been so tasked, especially as X-Day drew near (X-2 onwards). This total includes ordinary bomber and torpedo planes, not suiciders. The latter would probably have been used to divert CAP away from carrier and fire-support groups, enabling the bombers and torpedo planes an easier shot at their targets.

One thing about Typhoon Louise that hit Okinawa on 9 Oct 45: the damage to aircraft, invasion craft and shore installations would likely have caused a delay estimated at 1-2 weeks. A DOD staff study in 1985 for the 40th Anniversary of the end of WW II came to this conclusion.

 

[burmafrd]

However at Okinawa the suicide planes as I recall never really got a chance at anchored ships. A sitting target is a lot easier to hit.

 

[sloreck]

Don't forget the P-61. With radar & the ability to orbit and detect at night they would be especially effective against dawn take-offs, I am assuming given the poor level of skill that taking off in the dark to arrive over a TF at dawn is not in the cards. They also had significant loiter time flying from Okinawa to Kyushu. By this time the Navy also had several units of radar equipped night fighters.

Having said all that, being off the beaches of Kyushu for the first few days would not be a happy place for a ship. I would assume that after the initial assault on the first day an effort would be made to land supplies & reinforcements during dark hours so as to be away when the first kamikazes came over in the am - however I have not studied the plans well enough to say that.

 

[Matt Wiser]

There were three P-61 units in the area on 15 Aug 45: two on Okinawa and a third on Iwo Jima. Both were squadrons, not Groups. The Navy had two night-dedicated carriers with air groups specializing in night work that would have been very busy in the run-up to X-Day. The Japanese intended to start their suicide attacks when the invasion force was still a day away from land-these are the troopships and LSTs, mind you, not the fire-support groups and minesweepers-who would've been there for several days prior. And the Japanese for the first time planned night kamikaze raids-which on paper might have worked well, but how it would've done in practice is an open question. The main intent of the night raids was to light up ships for midget subs and Kaiten suicide torpedoes by their AAA firing.

 

[CalBear]

There were three P-61 units in the area on 15 Aug 45: two on Okinawa and a third on Iwo Jima. Both were squadrons, not Groups. The Navy had two night-dedicated carriers with air groups specializing in night work that would have been very busy in the run-up to X-Day. The Japanese intended to start their suicide attacks when the invasion force was still a day away from land-these are the troopships and LSTs, mind you, not the fire-support groups and minesweepers-who would've been there for several days prior. And the Japanese for the first time planned night kamikaze raids-which on paper might have worked well, but how it would've done in practice is an open question. The main intent of the night raids was to light up ships for midget subs and Kaiten suicide torpedoes by their AAA firing.

With the American usage of radar directed guns, all the way down to the 40mm quads IIRC, a night attack by pilots who could barely fly when they could see would have quite the bloodletting. As far as the minis and the Kaitens, they were, in their own very special way, worse than the regular kamakazi in that thay were nothing but the regular fleet wanting the get in on the fight to the death game (Yamato's attack mission at Okinawa being another fine example).

 

[Slamet]

Sooo, is Chilpheric going to update this TL or what? It's fast becoming a discussion about Downfall, not about the TL.

 

[Matt Wiser]

The record of the Kaitens is actually pathetic; only two known sinkings, and two damaged, out of several dozen launched. That doesn't include Kaiten pilots killed on subs that were sunk before their Kaitens could be launched. One of the two sinkings was a fleet oiler at Ulithi Atoll at anchor, while the other one was a DE escorting a Guam-Leyte convoy that saw the Kaiten periscope, thought it was a sub, and rammed it. The ramming set off the 3,000 pound warhead of the Kaiten, and U.S.S. Underhill was blown to pieces from the funnel forward. The stern lasted long enough for the remaining crew to abandon ship and the stern was then sunk by gunfire. IIRC this was two days before the sinking of the U.S.S. Indianapolis by the I-58.

 

[Chilperic]

Sooo, is Chilpheric going to update this TL or what? It's fast becoming a discussion about Downfall, not about the TL.

Yeah, I'll continue, could do with some more opinions on the TL though.

 

[Chilperic]

X+1

November 2 (X+1) - Early in the morning, American forces enter Miyazaki and begin to face their first serious resistance in the area. Many civilians became franc-tireurs and fought against the American invaders. Nevertheless, American forces managed to overwhelm much of the garrison by the end of the day and most regular forces had been killed. American forces had penetrated as far west as Saito by the end of the day.

American forces on Kushikino made little progress. Still hounded by the Japanese, they remained on the beaches, unable to break through. The Japanese were unable to push them off the beaches, and a stalemate settled on the area.

A naval bombardment begun at Shibushi, with the intention of softening up the defences ready for an assault. US forces in Ariake Bay were beginning to improve on their situation due to Japanese forces having to be moved east to face the Americans at Miyazaki.

 

[Slamet]

X+1

November 2 (X+1) - Early in the morning, American forces enter Miyazaki and begin to face their first serious resistance in the area. Many civilians became franc-tireurs and fought against the American invaders. Nevertheless, American forces managed to overwhelm much of the garrison by the end of the day and most regular forces had been killed. American forces had penetrated as far west as Saito by the end of the day.

American forces on Kushikino made little progress. Still hounded by the Japanese, they remained on the beaches, unable to break through. The Japanese were unable to push them off the beaches, and a stalemate settled on the area.

A naval bombardment begun at Shibushi, with the intention of softening up the defences ready for an assault. US forces in Ariake Bay were beginning to improve on their situation due to Japanese forces having to be moved east to face the Americans at Miyazaki.

What about the kamikazes? Have they run out? I mean, there's 12,000 planes.

 

[Chilperic]

What about the kamikazes? Have they run out? I mean, there's 12,000 planes.

I'm going to write that up in the next update.

 

[pacifichistorian]

Introduction

Meanwhile, the US had attempted to test the atom bomb in July but the detonation was too weak and the test was considered a failure. The scientists on the program told Truman that they would have to fix the problem, and this would be done by February 1946. So the Allies prepared for Operation 'Downfall', the invasion of Japan.

ASBs. The Trinity Test was for a plutonium implosion bomb. The "Thin Man" uranium bomb would've worked just fine, & still been used. This is the same ignorant mistake of Burning Mountain...

Perhaps some technical snafu occurs

You might argue for a "misfire" of "Thin Man" on Tainan that takes out the whole island, 509h & all. I don't know if Parsons decided to "live" the bomb in flight based on an earlier OTL t/o wreck, so maybe it doesn't happen, Parsons doesn't feel the need, & BANG. This, plus a Trinity fizzle, leaves Gerow in disgrace (he loses further funding for A-bombs) & Truman, Marshall, LeMay et al. peeved at having to invade... Maybe then (unfortunately for you...) they decide to let the sub blockade, & LeMay's firebombing, continue a few months longer before launching Coronet. Or maybe Truman feels the pressure to end it within 6mo.

That was fall/winter of 46. The Japanese distributed the "reserves" for 46 in 45 and delayed starvation until the USA could help and was willing to.

No, the entire railraod net was still intact. And would have been the next target.

IIRC, there were 4-5 critical ry bridges/tunnels in Japan. Had they been destroyed, Japan would've been divided into a bunch of mutually-unsupportable zones, & food/fuel could not have been moved between them.

the US Navy which suffers the loss of 23 ships including 2 carriers

23 ships? I doubt it. 2 CVs? Not a chance. JAFs were lucky to reach CVs, let alone attack them, never mind sink them. In all the kamikaze attacks, all of one CV, St. Lo (CVE-63), was sunk.

Absolutely. Even before the Lexington was lost the USN held damage control to near religious status (afterwards... deified is just a slight overstatement). IIRC it was the first navy to establish a specific training course for Damage Control officers and to train units of sailors on the rather special skill set needed. The USN also, more than most, recognized carriers as being a floating collection of combustible items.

IIRC, it was around the time of Lex's loss OTL, USN began training every sailor in rudimentary damage control, unlike previously. Also, for all their fearsome rep, kamikaze were actually less effective than bombs would've been, 'cause the aircraft structures were quite light & they broke up on impact, where a bomb would penetrate & explode in the vitals...

proximity fuse made a fairly effective weapon, the 5"/45 into a real killer

It was a bit slow-firing to be maximally effective, & the 40mm didn't have the punch to stop a determined flyer. It took until the intro of the full-auto 3" (76mm/70cal?) to get something that could stop kamikaze on its own.

Are you sure about 40mm VT-shells?

Pretty sure. IIRC, they were first used in ETO against troop concentrations.

 

[Astrodragon]

IIRC, there were 4-5 critical ry bridges/tunnels in Japan. Had they been destroyed, Japan would've been divided into a bunch of mutually-unsupportable zones, & food/fuel could not have been moved between them.

IIRC, the British were sending 617 squadron plus a couple of others with Tallboys and Grand slams specifically to take out bridges connecting to the mainland before the invasion.
Asuuming a reasonable supply of bombs (and remember, by then these guys were bringing back most of the bombs for each mission, as they didnt need them, they were that accurate!), bye-bye all the bridges and tunnels they can fly over...

 

[Michele]

IIRC, the British were sending 617 squadron plus a couple of others with Tallboys and Grand slams specifically to take out bridges connecting to the mainland before the invasion.
Asuuming a reasonable supply of bombs (and remember, by then these guys were bringing back most of the bombs for each mission, as they didnt need them, they were that accurate!), bye-bye all the bridges and tunnels they can fly over...

nice point, and I think you remember correctly.