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The Walter Submarine Turbine Propulsion System

Basic Submarine Tactics in World War II

German submarine tactics in the Second World War were strongly influenced by experiences in the first. Captain (later Grand Admiral) Karl Dönitz, the officer in charge of submarines, and later of the Kriegsmarine itself, had been a U-boat commander during World War I. In the first war, submarines operated alone, mostly attacking targets of opportunity. Dönitz was determined to change this. He instituted a system of radio control from his headquarters in occupied France that allowed him to vector several boats onto the same convoy. Once engaged, the boats operated independently against the convoy, as coordination was much more difficult during the actual battle. The favored option was the night surface attack.

The use of surface tactics once engaged with a convoy recognized a major limit on submarine technology at that time. World War II submarines were essentially surface vessels. They would dive only to escape attack, or in order to make an attack. Because the top submerged speed of a U-boat was about 6~8 knots, while it could make 18 to 20 on the surface, and that top submerged speed was possible for no more than about an hour starting with a fully charged battery, it wasn’t practical to pursue a surface target while submerged. This limited speed also meant that it was harder for a submerged U-boat to maneuver to escape an attacker, and absolutely precluded trying to run away from one.

Allied advances in anti-submarine warfare, particularly the use of airplanes flying from escort carriers, and airborne radar, soon made surface operations extremely hazardous. Yet submarines, at least at first, had no real option but to spend much of their time surfaced. Massive quantities of air were required to run the diesel engines that propelled them on the surface and charged the batteries they used when submerged.

And, while the Germans never learned this until years after the war, Dönitz’s central control system was largely responsible not only for their early successes, but also for their later troubles. For much of the war, the British were decoding German communications almost as quickly as the Germans themselves. Radar equipped aircraft could spot a U-boat much quicker when they already knew where to look, thanks to the boat having to tell headquarters where it was.

Air Independent Propulsion for Submarines

The obvious solution to the known problem was a propulsion system that didn’t need outside air. The most common modern AIP system is nuclear power. This didn’t become practical until the 1950s, though. The basic physics were known by the 1930s, but there were too many problems to be overcome before a practical nuclear powered submarine could be built. Not the least of those problems was size. The main reason nuclear subs are so much larger than diesel subs is the size of the reactor. Could Germany have built nuclear boats for World War II? Possibly, but they also had Hitler to contend with, and Hitler was a short-term thinker when it came to weapons. He wanted what could be built now, and finished within a few months at most, not boats that might take ten or more years to enter the fleet. (Just about everyone but Hitler wanted to wait until about 1945 to start the war, but fortunately he got his way, which meant we were faced with the Germany of 1939, and not the Germany with a powerful surface fleet, aircraft carriers, and hundreds of submarines we would have faced six years later. If they’d waited until 1945, they might just have won.)

An interim solution was the adoption of the snorkel. This was a hollow mast, with a float valve to keep out water, which allowed air to be drawn into the boat while it was submerged. This meant the diesels could be run while submerged, with only the snorkel head exposed. It was still a target, but a much smaller one.

Still, the snorkel was, at best, an interim solution. The submarine had to remain close to the surface, and in snorkeling during the daytime would often be visible from the air. The snorkel head was smaller than a submarine, but it was still big enough to be picked up on radar. Anti-radar coatings were tried, with some success, and radar detectors were built into the heads. While the snorkel somewhat reduced vulnerability, something better was obviously needed.

In the 1930s, Professor Helmuth Walter (not Walther, that’s a firearms manufacturer) began experimenting with hydrogen peroxide as a possible fuel. By the early 1940s, Walter’s research had progressed to the point where he was able to convince the Kriegsmarine to build some prototype submarines. By 1943, a Walter turbine had been used to power an unarmed test U-boat at a speed of 26 knots. This was some 13 knots faster than the fastest conventional submarine of the day could manage submerged, and actually about five knots faster than the most common Allied escorts. (As 21 knots is about the fastest speed at which Asdic can be used, and very few merchant vessels were faster than about 15 knots anyway, it wasn’t thought necessary for escorts to be much faster.)

Designs were drawn up for several types of Walter submarines. In the end, none ever became operational. The design of the big Typ XVIII Walter boat did, however, become the basis for the Typ XXI Electroboot, which was about to be deployed when Germany surrendered.

Hull Design Integral

In addition to the actual Walter turbine, professor Walter designed the boats themselves. He recognized that conventional submarine hull designs were optimized for surface operations, but were terribly inefficient when submerged. His designs removed deck guns and other projections, which caused drag. The hulls were streamlined, becoming more rounded. The conning tower fairwaters and bridges were replaced by designs with a small cockpit and smoothly plated upper surfaces to reduce drag and, with it, the amount of noise the boats generated under water. The results were designs that were actually faster submerged than they were on the surface. They also tended to be larger than conventional designs, and therefore slower to dive, but this disadvantage was compensated for by a design that made it generally unnecessary for the boats to be on the surface except when entering and leaving harbor.

While no Walter boats entered service, the Typ XXI, which employed the hull design of the largest Walter boat, with the huge Perhydrol tanks replaced by extra batteries, and conventionally powered, did enter service right at the end of the war. The only example that was in position to make an attack did not do so, as the cease fire order had just been received, but her commanding officer, Korvettenkapitän Adelbart Schee still made a mock attack, closing to within 1600 yards of HMS Norfolk before slipping away. After Schnee returned to port and surrendered, Norfolk’s commander refused to believe his story until the respective logs were compared and it was shown that boat vessels were, in fact, in the same location at the same time on the day in question.

What Walter envisioned was the transition from a diving torpedo boat to a true submarine. His streamlined hull designs were the basis of most postwar submarines until the teardrop shaped Albacore hull superceded them. The first nuclear submarine, U.S.S. Nautilus, was essentially an enlarged Walter hull with a nuclear powerplant. (Though a close look at both hulls suggests Nautilus may owe more the the streamlined Japanese I-201 design, which was slightly faster than the German boats.)

The Walter Turbine

Walter achieved his remarkable speeds by using Perhydrol, a nearly pure hydrogen peroxide, as an oxydizer. This was run through a catalyzing system, which broke down the hydrogen peroxide (H2O2) into hydrogen and oxygen, in the process producing his pressure steam and oxygen at a very high temperature. The creation of the steam used up both of the hydrogen atoms and one of the oxygen, leaving a free oxygen atom in the mixture. Since the temperature of the gases was hot enough to sustain combustion, diesel fuel was injected, which used up the free oxygen atom as it burned. This increased both the heat and pressure of the steam. The steam was then used to power a turbine, which combined elements of both gas and Parsons (steam) turbine technology.

Unfortunately for the Kriegsmarine—but probably fortunately for the Allies—the Walter system had nearly as many problems as benefits. The Perhydrol fuel was extremely corrosive, requiring the use of special fuel lines. Another problem, which was actually discovered by the Japanese, who used the same fuel in some torpedoes, including a Kaiten prototype, was that the Perhydrol, unlike conventional fuels, required fuel lines with no right angle turns in them. The Japanese discovered that the Perhydrol would sometimes “pile up” in the sharp bends and spontaneously combust, with the obvious disastrous results. While the Japanese obviously didn’t care if a Kaiten pilot died, they wanted it to be when he’d rammed his human torpedo into an enemy ship, not during training, so the diagnosed and fixed the fuel line issue and told the Germans about it.

Another drawback was that the Walter system was very thirsty. One reason the subs were so much bigger was to accommodate the huge fuel tanks needed to give the boats a reasonable range.

The Typ XXVIw Walter Submarine

The Typ XXVIw Walter boat employed in With Honour in Battle was never actually built, though contracts were let and some sections begun. Thus, performance figures cited in the novel are only estimates, and probably a bit overoptimistic at that. For dramatic purposes, some liberties were also taken with the crew. I needed more officers, for one thing, so my U-2317 has seven officers, while a real Typ XXVIw would have had three.

There are also a few other errors, mostly the result of errors in the reference materials available at the time I started writing the book in the 1970s. The descriptions in those old books appear to have combined elements of different Walter designs in their description of this one. For example, information available at that time suggested the 850 ton Walter boat had twin shafts and ten forward-firing torpedo tubes. In fact, the actual design called for a single shaft, and six of the tubes fired toward the stern. Also, unlike most submarines of that day, the attack center was in the control room, or Zentrale. None of these technical differences affect the story, of course, and would probably be noticed only by very serious U-boat scholars, but they were still corrected when the first eBook edition was released, and in the Riverdale paperback.

After World War II, several navies attempted to continue development of the Walter turbine. In the end, they all dropped the design as too dangerous. Most did, however, adopt many of Professor Walter’s ideas when it came to hull design. The development of nuclear reactors small enough to use in a submarine in the 1950s permanently terminated Perhydrol propulsion research except for torpedoes. Most countries have now given up on those as well. A defective hydrogen peroxide fueled torpedo is suspected as the primary cause of the sinking of the Russian submarine Kursk in 2000.

There has been considerable renewed interest in AIP submarines in recent years. Nuclear boats are expensive to build, and many countries are more concerned with coastal defense than operating at great distances from port. Modern designs, however, tend more toward the use of Sterling cycle engines, or hydrogen fuel cells for generating electricity.