Our Research Focus: 3D-printed Catalysts for Rocket Fuel

Could HTP thrusters finally take off? The New Zealand research partnership looking at an alternative to the dangers posed by hydrazine

THERE’S a particularly great scene in The Martian (2015), directed by Ridley Scott. Stranded alone on Mars, and left with scant rations, astronaut Mark Watney (played by Matt Damon) devises a method to grow potatoes inside the Martian base. To water his crops, he takes a leftover canister of hydrazine propellant and directs small drops of it onto an iridium catalyst, causing the fuel to spontaneously decompose into nitrogen and hydrogen. All that’s left to do is carefully channel the hydrogen into a small burner and voilà – water. What a fantastic demonstration of chemical engineering in action!

No wonder then that this simple catalytic reaction, turning hydrazine into hot expanding gases, has been used to move things around in space since the 1960s. The downside? Hydrazine is incredibly toxic and a suspected carcinogen, necessitating the highest level of personal protective equipment during refuelling operations (think full hazmat suits). In recent times, these health hazards have led to rising costs, threats of an outright ban in Europe, and subsequently, a renewed search for reduced-toxicity propellants, also known as “green” propellants.

One particularly attractive green propellant is concentrated hydrogen peroxide, or high-test peroxide (HTP). Although the thrust output of 98% HTP is approximately 20% lower than that of hydrazine for the same mass flow, the fuel is relatively benign, and has a higher density, making it cost-effective in small-scale and/or volume-constrained systems. Possible applications are the propulsion systems of rocket-powered aircraft and satellites, such as those being developed by Dawn Aerospace here in New Zealand.

One particularly attractive green propellant is concentrated hydrogen peroxide, or high-test peroxide (HTP)...the fuel is relatively benign, and has a higher density, making it cost-effective in small-scale and/or volume-constrained systems

As with hydrazine thrusters, a catalyst is needed to decompose the liquid propellant into gaseous compounds (steam and oxygen). HTP thruster technology has remained largely unchanged since the 1960s (due to the fact that, for many years, it was simply abandoned in favour of hydrazine). The traditional catalyst is silver, coated onto a fine stainless-steel mesh and stacked into tightly packed layers. However, silver melts at 960°C, whereas the products of 98% HTP decomposition can exceed 1,000°C. The only practical way to lower the temperature is to dilute the HTP with water, which is the reason why many rocket engines in the 1960s would reduce the HTP concentration to 90% or lower. The inert water reduces the reaction efficiency and thus equals dead weight, which no one wants to carry into space. High feed pressures are also needed to flow the propellant gases through the fine mesh support. Higher pressure equals thicker storage tanks, which equals more unwanted weight. All of this to say: without significant innovation in the catalyst space, HTP thrusters are unlikely to ever “take off”.

In 2020, our research team based at the University of Canterbury in Christchurch and Callaghan Innovation in Wellington, New Zealand, set out to tackle the peroxide problem. The first issue to solve was that of the catalyst support. It is well known that for high temperature catalytic reactions, a large surface area and tortuous flow paths are needed to overcome mass-transfer limitations. However, there are only limited ways to generate such structures using traditional manufacturing techniques. Packed beds are one example, consisting of random arrangements of catalyst particles inside a fixed reactor volume. The mesh catalyst used for HTP decomposition is another example. With these types of structures, as you increase the geometric surface area, the void fraction decreases. In other words, there is an inherent trade-off between the conversion efficiency and the pressure drop.

In stark contrast to subtractive manufacturing, additive manufacturing (or 3D-printing) grants the user almost complete design freedom, and the technology now encompasses a wide range of technical engineering materials such as metals and ceramics. Therefore, it is possible to generate catalyst structures that enhance mass transfer while simultaneously minimising pressure drop compared to traditional designs. But how do we identify these structures, and what do they look like?

Our research team is focused on a particular class of structures known as triply periodic minimal surfaces (TPMS). TPMS have several desirable properties, such as a tortuous and ordered pore network, but crucially, the surface area can be varied independently of the void fraction. By measuring pressure drop and heat transfer rates, we assessed a variety of TPMS unit cell geometries and identified that the gyroid, discovered by American physicist Alan Schoen, was the best candidate for use in HTP thrusters. This is perhaps unsurprising; the gyroid can be found in nature (most notably in butterfly wings), and nature as we know tends towards “optimal” structures. Our data shows that at a fixed flow rate and pressure drop, the gyroid has mass transfer rates at least three times improved over that of a packed bed of spheres.

We opted for ceramic catalyst supports due to their excellent thermal and chemical stability, suitable for high-temperature and corrosive environments. However, 3D printing of a famously brittle material can prove challenging. On top of that, how do you then ensure that the ceramic has a porous microstructure with high specific surface area? We needed to be competitive with commercial catalyst supports, which often have pore surface areas totalling hundreds of square metres per gram of material.

Our approach uses a technique known as digital light processing (DLP). First, fine ceramic powders are mixed into a vat of photosensitive resin at a high solid loading. Then, the virtual catalyst model is sliced into hundreds of layers by a computer. An LED screen projects images of these slices onto the resin, causing it to polymerise and bind the ceramic particles in place. This process is repeated iteratively, layer by layer, until the “green” body is formed, part binder and part ceramic. Subsequently, the green body undergoes careful thermal treatment at elevated temperatures over several days to remove the binder and facilitate the sintering and densification of the ceramic monolith. Careful optimisation of the sintering temperature ensures that the parts reach their desired level of porosity. In this way, we can generate entirely novel, monolithic, ceramic catalyst supports with a gyroid morphology.

We initially tested magnesium oxide as the catalyst active phase, but ultimately found superior activity with platinum, which has a high melting point relative to silver. Despite its significantly higher cost – about 40 times that of silver – the porous nature of the 3D-printed support means we can get away with using much less catalyst, as little as 0.5% of the total catalyst mass, by distributing small nanoparticles of platinum over all that additional surface area. Since only the atoms exposed to the surface participate in the reaction, this method is more economical than electroplating – the process used to coat mesh catalysts, which inevitably buries the vast majority of the silver atoms beneath other silver atoms.

Putting it into practice

So, to briefly summarise: better mass transport, lower pressure drop, and higher thrust output – at least, that’s what the theory suggests. Now, for the experimental validation. The only way to truly assess these catalysts is by installing them into a real HTP thruster and measuring their performance. Not only that, HTP testing also indicates whether the catalyst can survive such extreme environments for prolonged periods of time, as is required for most space missions. Catalysts inside a monopropellant thruster are regularly exposed to high temperatures, oxidising atmospheres, rapid thermal cycling, and thermal gradients exceeding hundreds of degrees Celsius per centimetre.

With invaluable guidance from Dawn Aerospace (designing rockets isn’t exactly second nature to chemical engineers) we fabricated a small test platform roughly equivalent to a 10 N thruster, flowing just 10–20 grams of HTP per second. Even at this scale the results are impressive to behold; within just a few millimetres of the catalyst bed inlet the liquid propellant vaporises, accelerating by nearly 5,000 times and vanishing into a roaring plume of superheated gases. We measured the performance of our catalysts according to the exhaust temperature they reached at a given flow rate (or theoretical thrust output), comparing them to “traditional” platinum catalysts supported on spherical particles and honeycomb structures. The gyroid catalysts achieved thermal efficiencies of >90% with a lower pressure drop than the spherical catalyst, whereas the honeycomb catalysts only achieved a maximum thermal efficiency of 22% under the same conditions. And crucially, when removed from the thruster after testing, the printed supports were fully intact.

Numerical modelling supports these results. It shows that using current printer technology, we can produce catalysts that have a lower pressure drop than any honeycomb catalyst for a certain size of thruster. If volume is a constraint, then spherical catalysts still have us beat. Ceramic feedstocks for DLP printers cannot currently compete with commercially produced pellet-based catalysts when it comes to total surface area. However, these highly porous pellets are prone to fracturing, and if thruster size is sacrificed somewhat, 3D-printed designs like the gyroid once again offer a significantly reduced pressure drop. Finally, relative to silver screen catalysts, they can be used with higher concentrations of HTP, which improves overall weight and fuel efficiency.

The University of Canterbury, Callaghan Innovation, and Dawn Aerospace plan on continuing their collaboration to develop 3D-printed catalyst structures for aerospace applications. The learnings from this niche application of reaction engineering will be applicable to other industrial scale applications, especially when a highly endo- or exothermic reaction is involved.

If The Martian’s Mark Watney had access to high-test peroxide, instead of hydrazine, he could have produced the heat, oxygen, and water needed to support his life on the red planet by simply decomposing the HTP directly

So, if The Martian’s Mark Watney had access to high-test peroxide, instead of hydrazine, he could have produced the heat, oxygen, and water needed to support his life on the red planet by simply decomposing the HTP directly – all with the help of a custom-designed 3D printed catalyst. Not to mention it would have been far safer for his health!

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