Here is the continuation of the technical article we began in KTB #168, but had to miss in KTB #169 last month for space limitations

The exhaust from the turbine was mainly saturated steam and carbon dioxide. Much of the steam was turned back into water, or condensed, in a device called, of all things, a condenser. The condenser converted the turbine exhaust steam into water by further extracting energy (heat) out of it and dumping this non-recoverable excess heat overboard (as mandated by the Second Law of Thermodynamics). The liquid “condensate” was recycled back into the combustion chamber as feed water. The condenser also separated the cooled gases (mostly carbon dioxide) from the water and discharged the residual carbon dioxide overboard where it was readily dissolved in the seawater.

We’re almost finished with the explanation of how the Walter engine worked, only one more component to go: The exhaust gas compressors. These were needed at the outlet of the condenser to pump the remaining exhaust gases over the side. Otherwise the turbine and condenser machinery would have to work against the back pressure of the seawater at the operating depth. Without this little pump, the back pressure would have severely limited the submarine’s operating depth.

All of this machinery was located at the rear of the submarine in its own compartment, unmanned, and sealed off from the rest of the U-boat by a pressure bulkhead and a waterproof hatch. This could almost be an admission by the designers of the inherent dangers posed by this new propulsion concept. Because the compartment containing the Walter machinery was the aftmost compartment, it precluded the installation of any aft torpedo tubes. In addition, the temperature in and around the turbine would get very hot and there were always gases present. Evidently, non-combustible gases were allowed to accumulate in this compartment, as during one submerged test run a fire broke out but was quickly extinguished due to the lack of oxygen in the compartment.

But overall, the result was a closed-cycle propulsion system more compact and having less weight than the conventional diesel electric engine combination of a similar power level. Additionally, the bipropellant system releases nearly double the power of the monopropellant approach although at an increase in complexity and risk of danger.

Let’s look at a couple of examples. The best examples of the bipropellant system are the seven Walter U-boats that actually hit the water and began testing (the four Walter test U-boats and three of the XVIIB U-boats). Typically, the turbines in these three hundred ton U-boats developed 2,500 horsepower each and drove the submarine at speeds up to 25 knots but for only about 4 to 6 hours (enough to prosecute an attack or two & speed away). Twin 2,500 horsepower turbines were installed in two of the test U-boats (which would have only slightly increased their speed over the other two test U-boats, as would have been demonstrated had testing been allowed to continue unhampered).

The effort of the German engineers did not stop with these small boats. Plans were developed for turbine propulsion plants to be installed in larger ocean going U-boats capable of delivering 7,500 horsepower and providing sprint speeds in the mid-20 knot range (a feat not surpassed until USS NAUTILAS got underway on nuclear power). One can take a journey into the “what-if” zone at the twilight of the War by reading about the exploits of a fictional Type XXVI in McDaniel’s book With Honour In Battle.

Two more examples:

For a few years after the war, Britain continued development work on the use of H2O2 for submarine propulsion plants by operating U-1407 (a new German Type XVIIB) as HMS METEORITE until 1949. Based on the successful test program with HMS METEORITE, they built two unarmed experimental boats with improved Walter power plants: HMS EXPLORER (1956) and HMS EXCALIBUR (1958). While both boats achieved their stated design goals for high underwater speed (one even reached 26 knots), the highly concentrated hydrogen peroxide created such a safety hazard that the two boats became know as HMS Exploder and HMS Excruciator. Both were decommissioned in the 1960s.

The ME 163b Komet interceptor rocket plane is an example of a bipropellant application using the self-igniting catalyst process. Both hydrogen peroxide and a catalyst / fuel solution were pumped into the rocket’s combustion chamber. The solution contained a mixture of hydrazine hydrate (catalyst), methyl alcohol (fuel), and water (remember C-Stoff?) and was self-igniting when it made contact with the H2O2 in the combustion chamber. Thrust was achieved in the combustion chamber with the hot exhaust gases exiting through a nozzle.

A Few Words About Catalysts

There are three methods to bring about controlled decomposition of H2O2 with the subsequent release of heat and that good old working fluid, steam.

• Thermal heating
• Direct injection of a self-igniting liquid
• Use of a catalyst

As mentioned earlier, you could attempt to boil H2O2 (thermal heating) but that’s not a very controlled way to release its energy, no catalyst was needed, and thermal heating wasn’t used in submarines. So we’ll move on.

Decomposition by the second method used a liquid catalyst and sprayed it, along with the liquid H2O2, into a decomposition (or combustion) chamber where a violent reaction occurred as the H2O2 explosively dissociated delivering its energy to its surroundings. German engineers made use of this technique in the catapult that launched the V-1 cruise missile and drove the turbine of the A-4 rocket’s fuel pump. Here H2O2 acted as a monopropellant producing the hot gases by reacting with the liquid catalyst. There were two main problems with this arrangement: (1) the complexity of having to pump and control two liquids and (2) the products of the reaction contained small particles which did not always pass harmlessly through the sets of high-speed turbine blades. The propulsion system in the ME 163b Komet also used this method in a bipropellant, fuel-burning, process.

The third method (used in Walter U-boats) incorporated a solid catalyst. To make this hard catalyst, highly porous porcelain stones were soaked in a calcium permanganate solution, dried, and then soaked in a mixture of calcium permanganate and potassium chromate and finally dried again. The decomposition chamber, the Disintegrator, was separate from the main combustion chamber. It was the job of the catalyst to quickly decompose the H2O2 and raise the temperature of the products to the ignition point of the fuel. Obviously, this heat was also sufficient to vaporize all of the water released during the decomposition. Remember, it was the job of the combustion chamber to burn the fuel oil (using the released oxygen) and make more steam from the feed water (in what we are calling the bipropellant system).

Other catalytic agents (all producing vigorous decomposition) include platinum, silver, and lead.

Actually, just about anything will act as a catalyst at these high concentrations. Hence, if H2O2 spills on dirt or just about anything else, consider it to have just been poured over a catalyst, usually causing a fire, at the very least. That’s because the heat of dissociation raises the temperature of the “dirt” in the substance you poured it over to the ignition point (spontaneous combustion) and then the free oxygen feeds the fire. So here we have an example of the unintentional decomposition involving an impurity.

If you can get enough water on it, you can cool things down, put out the fire, and lower the concentration of H2O2 to a safer level. It looks like absolute clinical cleanliness is rule numero uno for the safe use of H2O2.

One more word about catalysts: Let’s go back to its pharmaceutical use because now we know how H2O2 heals. We know the answer to why it bubbles when poured over a cut or scratch? What’s it doing? It’s releasing nice healing oxygen as it is being decomposed by your blood!

Advantages and Disadvantages

We’ll Do Disadvantages First:

The Walter U-boats were very complex and expensive to build and operate. The “fuel” was vastly more expensive than diesel oil (about 75 times as much per pound to produce) and it was consumed in much higher quantities (almost 9 times as much H2O2 was “burned” than diesel fuel). From the British experience, submarines with this type of propulsion were more expensive to operate than submarines powered by nuclear reactors.

The way it was implemented for submarine propulsion resulted in low overall efficiency. The very high consumption rate of H2O2 resulted in rather poor mileage. In operation, in a 7,500 horsepower Walter engine, 52 gallons of H2O2 and 7 gallons of diesel fuel, combined with 240 gallons of feed water were consumed every minute. And performance became worse as the depth increased due to the need to pump the residue overboard against the ambient seawater pressure (back pressure). Apparently, there was also a problem with the piping-- there couldn’t be any sharp 90° bends in the pipes lest the H2O2 back up and, you guessed it, explode.

Metering and controlling the exact rate of flow of H2O2 into the catalytic chamber, the exact rate of flow of diesel fuel and feed water into the combustion chamber, and the flow of compensating seawater to replace the reactants proved too much for the German engineers. They were never able to reliably control their new chemical engine.

Once the supply of H2O2 carried on board was depleted, there was no way, at sea, to obtain any more hydrogen peroxide (unlike a diesel electric system where the exhausted storage batteries can be recharged numerous times).

The Walter U-boats had to compete for the limited supply of hydrogen peroxide with other high priority weapon systems demanded by the German military. During the course of the war, Germany was never able to ramp up production, distribution, or storage of this limited resource. There just wasn’t enough industrial capacity to supply the demand (the Navy alone projected a monthly need of up to 7,000 tons by 1945).

Material selections were rather tricky. Aluminum was about the only good material for long term storage but some stainless steels could be used in valves, pumps, and piping systems. Most other metals acted as catalysts. In addition to outright explosions or fires, H2O2 had adverse effects on non-compatible materials (especially various plastics and elastomers), causing hardening, swelling, blistering, deteriorations, pitting, and dissolving.

Under certain (most) conditions it can be quite unstable, especially in the presence of impurities. We know what happens when H2O2 leaks and spills onto dirt or oil. Leaks, either of H2O2 out of the system or of just about anything into the H2O2 storage or piping system, don’t have to result in a dramatic event (like a fire or explosion). Such leaks can deteriorate the strength to below the 80% minimum required for system operation. Some forms of degradation can usually be detected because the storage container will get warm due to the decomposition reactions.

All in all, it turned out the use of hydrogen peroxide was a rather inflammatory method of propulsion.

There Were Some Advantages:

The main advantages to the use of hydrogen peroxide were the independence from atmospheric air as a source of oxygen and the ability to develop a very compact engine with a high power-to-weight ratio (but this was not enough to overcome the very high fuel consumption rate). And since no nitrogen was involved anywhere in the process, no noxious nitric oxide pollutants were produced (a benefit of combustion in the absence of atmospheric air). In the monopropellant case, there were no pollutants produced at all. The products of the reaction were just water and oxygen.

The reaction that liberated the sought after oxygen was exothermic. It also liberated lots of useful heat and steam (more bang for the buck, so to speak).

It could be stored at ambient pressure while on the submarine and, because it was not stored under high pressure, it did not require a strong pressure vessel for general (non-shipboard) surface handling and storage. As dangerous as it was, at least it didn’t freeze valves or boil away as liquid O2 did when it was used as an oxidizer.

Because of its high underwater speed potential, it was thought that a new way to achieve depth control would be possible. Instead of using buoyancy (emptying and filling ballast tanks) to control the depth, they tried dynamic control (like in an aircraft). There were no hydroplanes at the forward end of the boat. The horizontal stern planes were thought to be able to change the pitch sufficiently to allow the engine to drive the submarine to a new depth regardless of its buoyancy at that new depth. It worked fine in the wind tunnel. But, as can be expected, dynamic control couldn’t be achieved at slow speeds (about 5 knots or less). Nor, of course, could dynamic control compensate for the sudden weight changes resulting from firing a torpedo. So, maybe we shouldn’t list dynamic control as an advantage, it was more like wishful thinking.

To the Germans, the four Walter test U-boats did demonstrate the potential of using H2O2 in a submarine propulsion system. Nearly advertised submerged speeds were obtained for short periods of time (U-792 achieved 24.78 knots). There were no serious accidents, no “show stopper” breakdowns, and no one was killed during the short test program. There were no major catastrophic explosions associated with the numerous breakdowns, which testifies to the overall robustness of the system. And all this despite the numerous and war-related problems encountered. It also provided important experience in the operation of steam (gas) turbine propulsion in submarines, which was so important to the next step in submarine propulsion.

And the Answer is …

So, after all this, are we any closer to answering the question posed at the beginning of this paper? What kind of “fuel” is hydrogen peroxide? It sure was a powerful and potentially useful fluid, although it wasn’t ready for “prime time” in submarines and got up-staged by nuclear power. We could take a vote to see how often it was used as a “fuel” and how often it was used as an “oxidizer.” It may have started out its career in submarine propulsion as a “fuel” but I would vote that it spent most of its short life as an “oxidizer.” I’ll stick with my original opinion.

A Few Final Words

One question usually asked is how well the Walter propulsion plant might have held up against a concentrated depth charge attack (and even during development of the machinery there were depth charge tests in an effort to answer this question). But the real question is not so much the Walter submarines' vulnerability to depth charges, but rather their ability to evade the recently introduced acoustic homing torpedoes. It can be appreciated that these U-boats could have easily run away from a depth charge attack. But could they have run away form this new type of torpedo – a torpedo capable of operating in all three dimensions – a torpedo driven to seek out its target using its own onboard sonar? Up to that time all previous torpedoes were restricted to operating at a single depth, near the keel of a ship sailing on the surface.

So let’s ask the question: Could an alerted Walter U-boat have evaded a salvo of either (or both) the MK 24 or MK 32 acoustic torpedoes? Each of these new torpedoes had a speed of only 12 knots but deployed in salvos, would they have been effective against the Walter U-boats? It has been well documented that the MK 24 was certainly very effective against the slower conventional U-boats. The “Mine MK 24 (FIDO)” was a passive homing torpedo that listened for the submarine’s propeller noises and had a range of 4,000 yards (10 minutes). The term “Mine” was a ruse. The MK 32 Torpedo was an active, echo ranging torpedo with a range of 9,600 yards (24 minutes). Of the 10 MK 32 Torpedoes built, none saw action during the war.

Even if these new Walter U-boats had been available in meaningful numbers beginning in 1944, wouldn’t they have still suffered from some of the same “old” operational problems plaguing the Type VII and IX U-boats at that time? These problems included:

1. Ultra and the reading of German Navy (U-boat) code (Enigma codebreaking) combined with good intelligence and operational research analysis.
2. The advanced state of Allied Anti-Submarine Warfare (ASW) at the time (i.e., aircraft with centimeter wavelength radar and the Leigh Light to illuminate the target at night, High Frequency Direction Finders (HFDF or “huff duff”), and homing torpedoes).
3. Large numbers of merchant ships per convoy.
4. The logistics and nearly unlimited resources brought to the war by the Allies.
5. Round-the-clock bombing and the chewing up of real estate as the Allied armies advanced toward Berlin.
6. And the U-boats continued to maintain radio contact to coordinate attacks.

One last word

With so much ASW capability so heavily concentrated in these years (the early to mid 1900s), it should be noted this was the only time in the long history of submarine development where the advantage did not belong to the submarine.

Great article CHARLIE – thanks!