Information on German Bomben Torpedos

[Darthtabby]

Does anyone have access to relatively reliable sources on the BT series Bomben Torpedos Germany developed/was developing near the end of the war? I know they've been mentioned a few times in various threads on this forum but the focus has generally been on the aircraft that could carry the weapons rather then the design and capabilities of the weapons themselves. The info I've been able to find elsewhere on the internet is contradictory: I've seen a scan of a page from an unknown source claiming the Bomben Torpedos had a 2800 meter range and were equipped with both magnetic and hydrostatic fusing (the former to detonate the weapon under the target's keel, the latter for if the weapon struck the target's side) but I've also seen it claimed that such a range would be impossible with the attack profile described and that the fusing was changed for production weapons.

About the only thing that seems very clear is that plans for the Me-163 to carry such weapons were probably an April Fool's hoax by a German aviation magazine.

 

[GrauGeist]

The BT (Bombentorpedo) was never used operationally.
It's design was based on kinetic energy, meaning that the aircraft's speed at the time of release was used as it's means of propulsion. In a way, very similar to skip-bombing.
The one exception to the kinetic design, was the BT1000, which was intended to have rocket assist, but never worked properly during testing.

There were seven types in various stages of development:
BT200 - weighed 441 pounds, warhead was 220 pounds. Length was 7.8 feet, diameter was 11.8 inches.

BT400 - weighed 959 pounds, warhead was 440 pounds. Length was 9.6 feet, diameter was 1.2 feet.

BT700A - weighed 1719 pounds, warhead was 727 pounds. Length was 11.4 feet, diameter was 1.3 feet.

BT700B - weighed 1664 pounds, warhead was 705 pounds. Length was 11 feet, diameter was 1.4 feet.

BT1000 - weighed 2601 pounds, warhead was 1565 pounds. Length was 13.9 feet, diameter was 1.5 feet.

BT1400 - weighed 3329 pounds, warhead was 2028 pounds. Length was 14.9 feet, diameter was 2 feet.

BT1850 - weighed 4239 pounds, warhead was 2315 pounds. Length was 15.3 feet, diameter was 2 feet.

In regards to an Me163 delivering any sort of dropped ordnance, not going to happen. It required a detachable dolly to get airborne with zero clearance for anything beneath it's fuselage.
They did have some equipped with R4M launchers under each wing, but their combined weight was much less than that of the BT200 (each rocket weighed 8.5 pounds).
Even the Ju248 and Me263 variants of the Me163 would not have been capable of delivering a bomb or torpedo.

So I suspect that the person was either terribly informed or, as mentioned, was having a bit of April Fool's fun.

 

[Wurger]

With GG above ....

From Fritz Hahn's book - "Deutsche Geheimwaffen 1939-1945" vol.1 Flugzeugbewaffnungen

Torpedo type: length, diameter, torpedo weight / warhead weight

BT 200: 2395 mm, 300 mm, 220/100 kg
BT 400: 2946 mm, 378 mm, 435/200 kg
BT 700A: 3500 mm, 426 mm, 780/330 kg
BT 700B: 3358 mm, 456 mm, 735/320 kg
BT 1000: 4240 mm, 480 mm, 1180/710 kg
BT 1400: 4560 mm, 620 mm, 1510/920 kg
BT 1850: 4690 mm, 620 mm, 1923/1050 kg

 

[Darthtabby]

Thank you for your replies.

I should probably have posted this earlier, but here's the scan of the page/document I mentioned in my original post that claims a 2800 meter range. Unfortunately I do not know the original source of this page/document as the source was not provided on any of the websites I have seen it posted on.

Aside from not being able to determine the source, the problem with the above page/document is that its credibility was called into question in the first place I'm aware of it being posted online, a 2013 discussion on the Secret Projects web forum. One of the respondents pointed out that 2800 meters would not be a realistic range for an attack profile using a 20 degree dive with release at 200 meters altitude. Said poster indicated that the weapon was released in a climb but did not say what range could be achieved using that profile. (The poster also contended that 960km/h was not achieved by the rocket propelled version, but was the maximum speed the weapon could be released at (did the Luftwaffe even have any aircraft that could have made a test drop at that speed?) and that the fusing was altered because the magnetic fuse did not work well and the hydrostatic one was not tested.)

Some webpages about the Bombentorpedos also include an extract from a 1946 US Navy publication about German ordnance but while it provides some interesting insights into the issues involved in developing such a weapon it doesn't provide any info about the weapon's range.

 

[elbmc1969]

There is no way that thing is following a straight line after entering the water.

 

[GrauGeist]

Yeah, some of those numbers are suspect.
960kph is just under 600 miles an hour. No torpedo (or bomb) is going to survive a water drop at that speed.
As for 600mph, that's just above the Me262's max. speed, even *if* it could deliver a bomb/torpedo from it's centerline.
The Ar234 or Hs132 (nearing production at war's end) would have been able to deliver all but the largest BTs, but their max. speeds were close to 500mph.

As far as the 2800 meters is concerned, that has to be an error, as 1.75 miles is a long way for a kinetic weapon to travel. I'm wondering if it was meant to read "280 meters", which would be far more realistic. A release point of 918 feet from a target is close to what the Americans were using in skip-bombing against Japanese shipping in the Pacific.
- their formula was approach at 200mph at 200 feet (or less) and release at 300 yards (900 feet).

 

[elbmc1969]

Given the form of the BT, I have trouble imagining that it couldn't skip any further than the standard US bomb shapes.

 

[GrauGeist]

Given the form of the BT, I have trouble imagining that it couldn't skip any further than the standard US bomb shapes.

The BT was designed to be dropped at a shallow diving angle (about 20°) so it would act like a torpedo.
But in certain circumstances, I imagine it could have been skipped like a conventional bomb.

 

[Darthtabby]

Yeah, some of those numbers are suspect.
960kph is just under 600 miles an hour. No torpedo (or bomb) is going to survive a water drop at that speed.
As for 600mph, that's just above the Me262's max. speed, even *if* it could deliver a bomb/torpedo from it's centerline.
The Ar234 or Hs132 (nearing production at war's end) would have been able to deliver all but the largest BTs, but their max. speeds were close to 500mph.

As far as the 2800 meters is concerned, that has to be an error, as 1.75 miles is a long way for a kinetic weapon to travel. I'm wondering if it was meant to read "280 meters", which would be far more realistic. A release point of 918 feet from a target is close to what the Americans were using in skip-bombing against Japanese shipping in the Pacific.
- their formula was approach at 200mph at 200 feet (or less) and release at 300 yards (900 feet).

I've read that some of the later Japanese and American aerial torpedoes were good for drop speeds up to ~400 knots/740km/h though I think they included drag producing features to slow them down after release (that and I'm once again wondering what torpedo carrying aircraft were actually capable of reaching that kind of speed).

Given the form of the BT, I have trouble imagining that it couldn't skip any further than the standard US bomb shapes.

The US Navy document on German ordnance that I linked to in a previous post states that "The bomb must in no event richochet off the water, not even in flat angles of entry, but must continue without deviation of its path of entry." It goes on to note that with an ogival nose as seen on the illustration of the BT 1400 a bomb will ricochet off the water if it strikes as a flat angle, while with a flat nose "as seen on the BT 700, or better yet by using spoiler plate, this ricochet at flat angles is definitely avoided."

The way some of the description is written makes me wonder if the write was based more on captured developmental documents then an actual example of the weapon.

 

[pbehn]

Thank you for your replies.

I should probably have posted this earlier, but here's the scan of the page/document I mentioned in my original post that claims a 2800 meter range. Unfortunately I do not know the original source of this page/document as the source was not provided on any of the websites I have seen it posted on.

View attachment 640650

Aside from not being able to determine the source, the problem with the above page/document is that its credibility was called into question in the first place I'm aware of it being posted online, a 2013 discussion on the Secret Projects web forum. One of the respondents pointed out that 2800 meters would not be a realistic range for an attack profile using a 20 degree dive with release at 200 meters altitude. Said poster indicated that the weapon was released in a climb but did not say what range could be achieved using that profile. (The poster also contended that 960km/h was not achieved by the rocket propelled version, but was the maximum speed the weapon could be released at (did the Luftwaffe even have any aircraft that could have made a test drop at that speed?) and that the fusing was altered because the magnetic fuse did not work well and the hydrostatic one was not tested.)

Some webpages about the Bombentorpedos also include an extract from a 1946 US Navy publication about German ordnance but while it provides some interesting insights into the issues involved in developing such a weapon it doesn't provide any info about the weapon's range.

The maths and physics seem to be a bit "off" on the schematic drawing. Ignoring the obvious that a bomb or torpedo doesnt travel in a straight line in air or water, if you draw it out with a battleship like the POW it is nonsense. 11 meters is just below the draught of the ship attacking at a 20 degree angle the torpedo enters the water about 15-20 meters from the side. The plane dropping the bomb/torpedo is doing everything that a Kamikaze plane does without actually trying to hit the ship, if the bomb is released at 20 degrees at a height of 200 ft the plane would be very lucky to clear the funnels and aerials of a military ship. At an attack angle of 10 degrees the bomb/ torpedo must travel close to 70 meters under water just with its own kinetic energy, it is fantasy stuff.

 

[Wurger]

 

[elbmc1969]

The US Navy document on German ordnance that I linked to in a previous post states that "The bomb must in no event richochet off the water, not even in flat angles of entry, but must continue without deviation of its path of entry." It goes on to note that with an ogival nose as seen on the illustration of the BT 1400 a bomb will ricochet off the water if it strikes as a flat angle, while with a flat nose "as seen on the BT 700, or better yet by using spoiler plate, this ricochet at flat angles is definitely avoided."

Ah, yes, that's the point of a flat nose on early AP rounds as well. Should have been obvious to me.

Thanks!

 

[elbmc1969]

At an attack angle of 10 degrees the bomb/ torpedo must travel close to 70 meters under water just with its own kinetic energy, it is fantasy stuff.

It's very high density compared to a torpedo (very high density) and very low drag compared to a torpedo. I'd have to see some modelling before I could form an opinion, but it doesn't seem entirely out of the question.

 

[GrauGeist]

It's very high density compared to a torpedo (very high density) and very low drag compared to a torpedo. I'd have to see some modelling before I could form an opinion, but it doesn't seem entirely out of the question.

It was definately a good idea on paper.

Using drop speed as a means of propelling it instead of installing a propulsion unit seems like an attempt at an economical and simple version of a torpedo.

 

[pbehn]

It's very high density compared to a torpedo (very high density) and very low drag compared to a torpedo. I'd have to see some modelling before I could form an opinion, but it doesn't seem entirely out of the question.

So is a bullet, which stops after about 3 to 6 feet in water and is solid steel. Steel is about 8 times more dense than water but water isnt compressible. Air is about 750 times less dense than water and is compressible. Concrete has only twice the density of concrete, which is why you cant dive from huge heights into water, if you get it wrong it hurts or kills, the reason why it hurts or kills is because you slow down so quickly.

 

[cherry blossom]

Torpedo dropping had changed quite a bit by 1944-5. The American Mk13 was typically released at 800 ft and 260 kts Aerial Torpedo Attack with Mk 13 Torpedo and flew forward for 1,000 yds to hit the water ideally 400 yds short of the target. However, in I think a test "On one occasion in early 1945, six torpedoes were dropped from altitudes between 5,000 and 7,000 feet (1,500 to 2,100 m). Five out of the six were observed to make their runs hot, straight and normal." according to World War II Torpedoes of the United States of America - NavWeaps. The same site states that the Japanese 45 cm (17.7") Type 4 (1944) Mark 2 was "A further modification of the Type 91 Mod 3 Strong. This version was strengthened to permit a launch speed of 400 knots." World War II Torpedoes of Japan - NavWeaps.

The Bombentorpedo is likely to be much less delicate than a torpedo and is rather like the Japanese Type 91 shell although that hits the water at a much higher speed. The Bombentorpedo works better the faster the aircraft is flying as it goes further before hitting the water and thus allows the aircraft to release without exposing itself to light AA. For any given distance, the bomb covers the distance faster if dropped at higher speed. The longer the flight, the greater the downward velocity and thus the steeper the angle of fall, which shortens the danger space. However, most shells and possibly the BT tend to curve upwards which would slightly (?)extend the danger space. If the carrier can dive to obtain a higher speed and level out for release, things are better.

Putting in numbers, a BT dropped by an aircraft at 750 km/h in level flight will cover the 2,800 metres in 13.5 seconds neglecting air resistance, which gives a downward velocity of 132 m/s compared to 208 m/s forward. Thus the BT enters the water at steeper than 30 degrees and may need to hit the water perhaps as close as 15 metres from the ship's side. Thus the BT only roughly doubles the target size compared to an impact fused bomb if dropped at 2,800 m.

 

[elbmc1969]

So is a bullet, which stops after about 3 to 6 feet in water and is solid steel. Steel is about 8 times more dense than water but water isnt compressible. Air is about 750 times less dense than water and is compressible.

That may be true, but it is also irrelevant.

The difference is the cube-square law, which is why large-caliber howitzer shells can fly much further than extremely high velocity rifle bullets. The howitzer shell has far more mass behind each unit of frontal area than the bullet, so air resistance slows it much less than it slows the bullet.

The 8" Japanese Type 91 "diving shells" were specifically designed to enter the water, level out a bit, and zip through the side of the ship below the armored belt. Obviously, this doesn't do much good if you have to hit the surface of the water within 6, or even 20, feet of the side of the target. That gives you an effective target size vastly smaller than the above-water target, which is seriously counterproductive.

As you know, fineness ratio matters a lot for air resistance. It also matter underwater. The TBs are far longer and narrower (finer) than any shell or normal bomb. This, too, allows them to travel vastly further underwater.

I don't insist that 280m was perfectly attainable, but there's nothing in a back-of-the envelope analysis that says it's impossible.

So, for a 14" naval shell with optimal shape, you expect it to travel 1400" or 117 ft. (35m) before it loses half of its striking velocity. If the TB depends on its explosive power, not kinetic penetration of the hull, striking velocity makes almost no difference.

We'd expect the lower density of the TB compared to an AP shell to result in a shorter travel, but its greater fineness to result in a longer travel. If doubling the fineness ratio quarters form drag, we expect the TB to travel 140m before losing half its striking velocity. If we're willing to accept 1/10 of the initial striking velocity, the base travel is increased to 200 calibers, double the 1/2 distance. That puts the travel at:

280m

I didn't even pick numbers to come up with this. It was utter dumb luck.

As I said, this is back-of-the envelope garbage. However, it shows that a back-of-the envelope calculation doesn't show that 280m is obviously impossible.

 

[Brooke]

I've read that some of the later Japanese and American aerial torpedoes were good for drop speeds up to ~400 knots/740km/h though I think they included drag producing features to slow them down after release (that and I'm once again wondering what torpedo carrying aircraft were actually capable of reaching that kind of speed).



The US Navy document on German ordnance that I linked to in a previous post states that "The bomb must in no event richochet off the water, not even in flat angles of entry, but must continue without deviation of its path of entry." It goes on to note that with an ogival nose as seen on the illustration of the BT 1400 a bomb will ricochet off the water if it strikes as a flat angle, while with a flat nose "as seen on the BT 700, or better yet by using spoiler plate, this ricochet at flat angles is definitely avoided."

The way some of the description is written makes me wonder if the write was based more on captured developmental documents then an actual example of the weapon.

Bombs are much more streamlined than aircraft, so when released they speed up and go faster than the plane that launched them. This was a BIG problem if the bomb was mounted on the center line of a plane with a propeller on the center line since the bomb would crash into the propeller. A trapeze bomb release was required to move the bomb outside the propeller disk, see: Aircraft Also note the body of the BT was solid metal not a metal shell so the speed-altitude zone will be very different than for a more fragile bomb.

 

[elbmc1969]

Bombs are much more streamlined than aircraft, so when released they speed up and go faster than the plane that launched them. This was a BIG problem if the bomb was mounted on the center line of a plane with a propeller on the center line since the bomb would crash into the propeller. A trapeze bomb release was required to move the bomb outside the propeller disk, see: Aircraft Also note the body of the BT was solid metal not a metal shell so the speed-altitude zone will be very different than for a more fragile bomb.

That's only a problem in a rather steep dive. Fighter-bombers without bomb crutches could easily bomb in 45-degree angles. In a 20 degree dive, the bomb will accelerate "down" at 9m/s, but only accelerates "forward" at 3.4m/s. That's enough to clear the prop disk in essentially every situation.

In addition, the aircraft will pop up a bit as the bomb is released because it's generating the same lift, but it suddenly weighs less. Of course, the pilot can also pull back on the stick a little (or a lot) after release to ensure that the disk is clear.