EDITOR NOTE – It is always a pleasure receiving an article by CHARLES GUNDERSEN, as they are always well researched, well written and very interesting.

It seems as though we always hear hydrogen peroxide (H2O2) referred to as a fuel, but I always thought it was an oxidizer. So I decided to look into this issue a little further and see if I could figure out what it really is. The reason I had thought of it as only an oxidizer was the ease at which it is reduced to produce water and that strong oxidizing agent, oxygen.

My daughter pleaded with me not to title this paper What Kind of Fuel Am I? But I still think that title is somewhat appropriate as it asks the pertinent question: Is hydrogen peroxide a fuel or an oxidizer? This issue will be the theme of this paper. I will also give just a few examples of its use in Germany before, and during, World War II (as Germany was an early proponent of its use).

What’s In A Name?

Considering all the different names for hydrogen peroxide, this paper could almost be titled What’s in a Name? Many of these other names for H2O2 were uncovered while searching for the true meaning of H2O2. For example, one name used in Germany during WWII (intended to confuse Allied intelligence) was “Ingolin,” named after the eldest son, Ingol, of Professor Hellmuth Walter, the engineer who pioneered the use of H2O2 for submarine propulsion applications. And since its specific gravity is greater than that of water (upwards to 1.37) some people referred to it as “heavy water,” which truly must have radiated confusion.

Because it is composed of hydrogen and oxygen (just like water except with a little more oxygen) we, too, can play the name game with oxygenated water.

It was also known as Perhydrol in the British military when in a nearly pure form (or as highly concentrated as can be manufactured) and when stabilizers were added. And, of course, for those involved with the US Navy’s MK 16 Torpedo program, it was called Navol. And we can’t forget High-Test Peroxide, HTP, another military name used by the British.

Actually there were lots of German military code names for hydrogen peroxide, among them:

T-Stoff (in formulations termed TN, TSS, or TS), which translates to T-Substance. For aircraft or missile uses.
Aurol. For ship propulsion uses.
Ingolin. For torpedo and artillery applications, as already mentioned.
Thymiol
Renal
Subsidol

Then they got really carried away and started renaming all sorts of related solutions (i.e., liquid catalysts and solutions of fuel and catalysts). Let’s start with Z-Stoff, an aqueous solution of sodium or calcium permanganate and used as a catalyst. Then there was B-Stoff, hydrazine hydrate, another catalyst. Both B-Stoff and Z-Stoff were used as catalysts for T-Stoff. Please note that C-Stoff is a mixture of methyl alcohol (M-Stoff) and hydrazine hydrate (B-Stoff). Are we getting all this? OK, just one more. The Germans called the liquid catalyst and fuel combination Helmann after you-know-who’s second son.

What Is It?

We are talking about hydrogen peroxide, H2O2, a peroxide of hydrogen (as in one hydrogen molecule per oxygen molecule), rather than its more familiar oxide ‘cousin’, dihydrogen oxide, better to call that one just plain old water (H2O).

Normally the substance comes in a variety of concentrations (or strengths) of H2O2 from as low as 3% H2O2 for general household and pharmaceutical use to as high as 85% as used by German engineers for propulsion. Scientists in the U.S. were getting concentrations as high as 90% by 1944. Whatever the concentration of the H2O2 substance, what’s not hydrogen peroxide is just plain water. So how could this seemingly benign substance be a propellant? How could this contain the potential to drive submarines and propel rockets? How could this be so dangerous that the stuff was stored outside the “people tank” (pressure hull) of Walter submarines?

The short answer is, in two words, Violently Unstable when provoked. When boiling is attempted, detonation usually results. That’s not to say it’s always unstable. Removing impurities and adding stabilizers definitely prolongs its shelf life.

But why, you ask, is it so unstable? To slightly paraphrase a well-known political expression: “It’s the ‘energy’, stupid!” The chemical energy required to maintain a hydrogen peroxide molecule (energy required to bond the 2 oxygen atoms to the 2 hydrogen atoms) is more than the energy to maintain a simple water molecule (and that extra, now free, oxygen atom).  When H2O2 molecules are decomposed (broken apart), this difference in chemical energy (let’s call it excess energy) is liberated as heat.  When requested, H2O2 molecules gladly (sometimes even explosively) give up their extra oxygen atoms in order to get to a lower and more stable energy state. Just about anything can be used to “request” the release of this excess energy. Dirt and other impurities are particularly good at it, and are the enemies of stable H2O2.

Ironically, the more concentrated (pure) the solution, the more stable it becomes for handling and storage purposes (due to being rather free of impurities). This is why high concentrations of liquid H2O2 can be made safely (i.e., the product becomes progressively more free of impurities as its concentration is increased). The stability for safe manufacturing and handling is more dependent upon its purity than any other factor. But true purity is hardly ever reached, so stabilizers are added to combine with the impure decomposition agents. The trick is to make a high strength aqueous solution of H2O2, which can be handled and stored but with a reasonably low rate of decomposition.

Why Are We Interested In It?

It’s a propellant … But what makes H2O2 a good propellant?

To be a submarine, and survive, in those wartime days you really wanted high underwater speed to prosecute an attack and then to flee the scene after releasing your torpedoes to avoid the wrath of the escorting destroyers. You could get by with a somewhat slower speed for the long transits to the operating areas or while searching for, and following, a convoy. But to gain an attacking position (especially if attempted underwater) speed is important. In this game, speed does kill!

The need for high underwater speed requires a high power propulsion plant. And this was only attainable, at that time, with an engine (or turbine) fueled with some kind of fossil fuel. Most engines we are familiar with (like the ones in our garage) accomplish useful work through the combustion of the fossil fuel using oxygen from the air. But in order for a submarine to burn a fossil fuel there had better be available a lot of air, or oxygen (which was tried with a diesel engine, but that’s another story). So when underwater, where air is so limited, how can combustion be achieved without atmospheric air?

Before nuclear power, there was a quest among submarine designers to find some way to free the submarine from its dependency on surfacing to grab some atmospheric oxygen. And one way was to make oxygen on board from some substance that wants to readily give up its oxygen like, you guessed it, hydrogen peroxide; highly pure and highly concentrated.

You might ask, just how much oxygen there is in H2O2? It turns out the oxygen available for combustion in pure H2O2 is only about 47% by weight (the rest stays locked up in the water molecule). But that’s not a problem because it also contains nice hot steam upon chemical release of that oxygen, due to the high heat associated with the decomposition process. What more could you ask for, superheated steam and useful oxygen at temperatures reaching 500 °C (932 °F). This heat flashes any water into steam (with its consequent increase in volume whether or not there is room for this volume expansion). But this expansion can be put to good use by spinning a turbine connected to a drive shaft, reduction gears, and propeller.

In the 1930s Professor Hellmuth Walter recognized that this rapid dissociation of H2O2 and consequent release of stored energy would make an ideal power source for a closed-cycle chemical engine, an engine able to operate without using air to support combustion. He discovered that besides being independent of atmospheric air for its oxygen, this kind of engine developed a high power-to-weight ratio that resulted in a very compact power unit. Unfortunately, it was soon realized that this was offset by the high consumption rate of H2O2 at high power levels (requiring the storage of a large quantity of H2O2 or a reduction in operating range). And Walter wanted a high power engine (high power density) to attain high underwater speed, rather than a high energy engine (high energy density) to have long endurance. For example, a high power engine in an automobile will allow it to go fast, but a large gasoline tank (containing a large amount of stored energy) will allow it to go a long distance.

Walter was originally thinking of H2O2 as a means to provide a supply of oxygen for a conventional combustion engine (i.e., a diesel engine), but he quickly dropped that idea when he realized just how much heat was released and the potential of using that heat in a new and revolutionary submarine propulsion system.

It’s A Fuel

Or rather, it’s a monopropellant or sometimes it’s called a monofuel and at other times it’s called a primary fuel. Here again, too many names!

Here’s how this cycle works. Lots of energy is released during the dissociation of H2O2 into hot water vapor and oxygen. Well, actually, the “hot water vapor” is superheated steam under considerable natural pressure. If you started with 80% concentrated H2O2, you would wind up with, after the reaction, 80% superheated steam and 20% oxygen gas at a temperature approaching 500 °C (932 °F). This will definitely spin your turbine. Most of the energy is expended in the steam turbine to produce useful work (the rotation of the propeller shaft). Or as we like to say, “work is accomplished by the expansion of the hot gases against the blades of the turbine.”

As far as we are concerned here, the primary example of this mode of propulsion was the small experimental submarine V-80 built by Walter in 1940. The H2O2 was pumped from flexible bags under the pressure hull into the catalytic reaction chamber (more about catalysts later) where the reaction took placed. The resulting energetic gases directly drove the high-speed turbine. With its energy spent, the turbine exhaust gases (steam and oxygen) were dumped overboard.

It should be noted the pressure of the exhaust gases had to be greater than the ambient seawater pressure at the depth of the submarine or the back pressure would not have allowed its discharge. Hence, with no compression of the exhaust gases, this boat was very limited in depth. The deeper it went, the slower it went. So due to the back pressure, the depth of the submarine during high speed runs was very shallow. But on one of those test runs in April 1940, a speed slightly in excess of 28 knots was reached.

One other aspect of this open-cycle engine was the visible bubble trail of gaseous oxygen it left behind, which was not a problem for an experimental submarine. But this relatively simple system with its modest energy output did successfully demonstrate the potential of high submerged speed.

In the V-80, the liquid H2O2, by itself, provided the raw material used in the engine (i.e., it became the turbine’s working fluid). It behaved almost like a fuel. Except with the burning of a fuel, we usually visualize combustion. Here we should probably visualize “disintegration” (as that is how the German engineers visualized what we would call dissociation).

In addition to the Rocket Guy and the experimental Rocket Propulsion Society, here are two more examples:

Hydrogen peroxide was used as a monopropellant in an auxiliary power source on board the German Army’s A-4 (or V-2) rocket (an early ICBM). When decomposed by a liquid catalyst (Z-Stoff), creating the high temperature steam / oxygen gas, it provided the energy to spin a turbine that drove the rocket’s fuel pump (a turbo-pump which forced the rocket’s main propellants of liquid oxygen and ethyl alcohol / water into the combustion chamber). The hydrogen peroxide can be considered a “fuel” for this auxiliary system. However, H2O2 did not power the rocket; the rocket’s main thrust was produced in the combustion chamber with probably some additional thrust produced in the exhaust nozzle if any unburned fuel and oxidizer made it that far.

The catapult launching mechanism (a piston in a gun tube) for the Luftwaffe’s V-1 flying bomb (an early cruise missile) was powered by H2O2 acting as a monopropellant (again being decomposed by a liquid catalyst). The V-1 had to be launched at a high speed in order to start its ram jet propulsion system, and thus a catapult was required to give the “buzz bomb” a very energetic boost. There’s nothing like a little H2O2 dissociation to give a missile a good “kick in the pants.”

It’s An Oxidizer

This is the bipropellant case where H2O2 is used as an oxidizer (and maybe a fuel too). Apparently only the bigger projects used H2O2 as an oxidizer (or secondary fuel). In this case all that hot oxygen is put to good use in the burning of diesel fuel or methyl alcohol (after all, it’s hot enough to initiate and sustain combustion). Burning this fuel results in a high pressure mixture, which is way too hot to handle. So more water is admitted lowering the temperature to something more manageable, but still around 550 °C (1022 °F), and creating even more steam and increasing the pressure further (i.e., increasing the mass of the working fluid). All this happens in the combustion chamber, and from there the hot gases are used to spin a more powerful turbine.

One unique design feature of the Walter submarine’s hull involved the method of storing the liquid H2O2. Unlike diesel fuel oil, the H2O2 could not come in contact with seawater, which imposed a unique challenge for the submarine designers but produced one of the most novel features of the hull design of Walter U-boats. Ever wonder what was in the lower portion of the figure “8” hulls on Walter submarines? That’s right; collapsible polyvinyl-chloride rubber bags filled with H2O2.

This separate compartment (below the pressure hull) was not a pressure hull in itself but a free flooding space (unlike the Type XXI where both halves were pressure hulls). Stored in a free flooding compartment this way, the ambient seawater pressure forced the H2O2 up into the Walter engine. As the liquid H2O2 was consumed, the bags collapsed & volume was replaced by seawater.

In this bipropellant system, decomposition also took place in the presence of a catalyst, in a device called a “Disintegrator” (probably named after some Romulan weapon). The hot steam & oxygen were then piped into the combustion chamber, ready for more action.

In this chamber, the diesel fuel was burned using the newly produced oxygen from the decomposition of the H2O2. Flame temperatures in the combustion chamber reached 2000 ºC (3,632 ºF), much too hot to be of any use in a propulsion system. At the bottom of the combustion chamber, water was admitted and immediately flashed to steam. Creating steam from this additional water brought the temperature down to an acceptable 550 ºC (1,022 ºF). One important advantage of the bipropellant system was the greater volume of steam produced and therefore a greater power output was achieved due to the addition of this extra water.

Before useful work could be obtained from these hot gases, there was one more step. The gases had to pass through a dust remover (a gas chamber in which the contaminating dust particles were removed). If these dust particles (traveling along with the working fluid out of the combustion chamber) were allowed to strike the turbine blades, excessive abrasion would occur. The dust particles spun around in the small chamber and were forced to the edges by centripetal acceleration (or centrifugal force) where they exited the chamber.

As in the monopropellant case, useful work was extracted from the exhaust gas mixture by allowing the gases to spin a high-speed turbine, which drove a propeller through a set of reduction gears, decreasing the high turbine speed to something the seawater could tolerate without cavitating the propeller.

Part 2 [KTB 170]