15 mins read 21 Aug 2020

Renewable Rocket Fuels – Going Green and Into Space

Access to space is becoming cheaper, and more frequent with reusability - and the upcoming launches of satellite constellations likely to leave more rocket emissions in the atmosphere. Is it time we start to consider green fuel options and are these possible?

Credit: Asgardia Space.

One of the key elements of making space more accessible to a larger portion of the population, be that through global space industries directly accessing it or the applications of data, services and products for the wider economy – is the ability to reuse rockets.

In 2019, SpaceX Founder Elon Musk called the reusability of rockets the “holy grail” – highlighting how it would reduce the costs and affordability of space travel and exploration, likening it to the reusability of terrestrial Jumbo Jets – passenger transportation systems that we use over and over again.

Space agencies, rocket builders and launch service providers around the world have since started developing and implementing innovative solutions that factor in multiple usages of rockets and their components. 

Musk’s SpaceX, a leader in this field, has even been reusing components of the Falcon 9 launch system since 2015, showcasing images of what is often described as ‘text-book landings’ of the lower stages of giant Falcon 9 rockets, as they touch down on floating barges out at sea.

The historic return of the Falcon Heavy booster from the SpaceX test mission signified an important turning point in the reusability of rockets. Credit: SpaceX.

Whilst reusability of rockets benefits science, exploration and human spaceflight – one of the greatest drivers for stakeholders in the global launch segment is the scale and demand for in-orbit assets by industry and economy, fuelled in tandem by the plummeting costs and size of satellites (e.g. CubeSats), instrumentation, and even ride-sharing platform services.

However, one consideration has been overlooked. With more rockets climbing into the sky – the impact of rocket emissions is now starting to become a topic of discussion – and in particular, how the output from rockets can cause anomalies in the higher levels of the atmosphere.

Atmospheric Impact of Rockets

In 2018, the Scientific Assessment of Ozone Depletion report (a study sponsored by the World Meteorological Organisation, the United Nations, NOAA, NASA and the EU) found that the increasing number of rocket launches (and thus increasing volume of atmospheric rocket emissions) does impact the atmosphere, in particular the more sensitive region above the Ozone layer.

One example of this impact is the accumulation of black carbon particles (soot – usually resulting from kerosene-based systems) that remain at altitudes between 30 – 50 km, which are then carried into global circulation patterns.

Another example is larger and heavier particles of aluminium – one of the usual components found in the propellant of solid rocket boosters – which tend to also be caught up in global circulation. Interestingly, the northern hemisphere exhibits higher concentrations of this material due to most launch facilities operating north of the Equator. But as nations south of the equator start to consider their own launch capabilities in practice or planning (e.g. New Zealand’s Rocket Lab or upcoming Australian launch facilities), these values are expected to change  .

The issue of atmospheric impact arises with particles of soot and aluminium absorbing and reflecting incoming solar radiation in the stratosphere, thus changing the temperature and radiation levels at both these altitudes and the Earth’s surface (and the troposphere between). And it is these changes that can cause harm to the Ozone layer (e.g. a warmer stratosphere can increase the rate of chemical reactions that reduce ozone).

International legislation (such as the Montreal Treaty) designed to reduce damaging effects to the Ozone layer does not specifically address emissions from sources such as rockets and aircraft whose particles and pollutants are emitted directly into the stratosphere, as opposed to from the ground where they need to climb above the troposphere. In fact, several environmental studies have concluded that rocket emissions impact the stratosphere in a way that no other industrial activity does – a reality that is the cost of placing payloads into orbit using chemical propulsion. 

Earth's Gravitational Prison

A 2-dimensional representation of the gravitational field, curvature of space-time, that are caused by the masses of the Earth and Moon. Credit: BBC.

Earth’s gravity is a deep well. It requires a lot of force to lift off it and then to continue to climb out of it to reach orbit. The notion is fairly simple, and likely a familiar understanding that we have all experienced (at much slower speeds) in our everyday lives. Newton's third law states that for every action, there is an opposite and equal reaction.

When rocket fuel enters the combustion chambers of the vehicle, it’s ignited which causes it to expand with lots of energy. The walls of the combustion chamber are designed to withstand this enormous force, and include a small section for the gases and energy to escape, at the base of the chamber. By allowing this downward escape of energy, the rocket then counters and moves upwards. Action-Reaction.

To reach orbit, the escaping gases and energy need to be moving at 11 km/s (or in everyday terms roughly 39,000 km/h), from the base of the combustion chamber. To push out even further and higher from Earth’s surface requires even more energy, though once in orbit the air resistance is minimised and gravity is countered with velocity.

Currently, chemical-based propellants are used because they provide the highest energy density – that is, they supply the most amount of energy stored in the minimum amount of storage space. For example, refined kerosene (known as RP-1) combined with liquid oxygen (known as LOX) is a very energy ready and dense fuel combination that when ignited produces a high flux of energy output.

So to make rockets more directly green - engineers, chemists, rocket scientists, environmentalists and more have to consider materials that are high in energy density, small in volume/mass (as adding weight to the rocket means it needs more fuel) and low in environmental impact across all parts of Earth’s atmosphere.

Indirectly, the production of these fuels can also assist with countering net carbon footprints.

Renewable Rocket Fuel

The now retired Space Shuttle (Atlantis). The Shuttle used aluminium-based solid propellant in the two side boosters, and a mix of liquid hydrogen/oxygen from its three main engines. Credit: NASA.

Whilst the wonderful vertical take-off and landing engines we see in science fiction technology (like fusion drives) are still a long way off, and considerable amounts of experimentation over the last 60 years have outlined the best energy density fuels that are used in rockets today, it seems that propellants used to leave Earth’s gravity might not be fully green with zero environmental impact for some time.

But that does not mean they cannot be carbon neutral.

Prof. Wojciech Lipiński, Leader of the Australian National University (ANU) Solar Thermal Group, recently outlined new methods of creating carbon neutral fuels for the aerospace and aviation industries, designed to offset carbon/climate impacts. 

Prof. Lipiński describes carbon neutral fuel as synthetic fuels produced from solar energy, water and renewable carbon sources (such as biomass or air captured carbon dioxide), which would enable sustainable aerospace transportation, compatible with existing infrastructure. 

“Australia has great potential for the development of domestic aerospace and renewable fuel industries. This is linked to the country's geographical location and local climate conditions. Australia will always depend on air and sea transportation to stay connected with the world.”

“Our conditions are geographically favourable for mission launches, and there’s growing domestic space ambition to work with foreign space industry partners. Australia’s renewable energy resources, in particular solar and wind, are unparalleled at the global scale but not effectively used yet. Thus, growing the sustainable aerospace fuel sector can improve national energy security, economic prosperity and, in the context of renewable fuel exports, protect the global environment” said Prof. Lipiński.

Producing fuel sources from a renewable source origin can reduce carbon footprints of propulsion systems through the production pipeline. Credit: Focus Malaysia.

Synthesising fuel from renewable sources of energy to use in rockets can be achieved through a number of different methods. One example is the production of high volumes of hydrogen and oxygen (which form hydrolox fuel sources for rockets) by using solar power to carry out large-scale electrolysis (the splitting of water into its hydrogen and oxygen components through a photochemical process).

Given Australia’s large surface area and favourable climate (for solar and wind renewable energy production), Prof. Lipiński thinks we could utilise our geographic potential to be a global production leader for renewable fuels. 

“Solar energy harvested from a square of 600 km x 600 km somewhere in the Australian outback could cover the entire global energy demand in all forms. This requires conversion technologies that go beyond electricity to create energy carriers such as chemical fuels,” he said.

Carbon-neutral fuels could be used in rocket fuel production and could indirectly reduce the net climate impact of rockets, but their entire production pipeline would need to be considered to be truly ‘carbon-neutral’. 

“In the case of carbon-neutral hydrocarbon fuels, one also has to consider the development of sustainable technologies for water processing and atmospheric carbon fixation. For truly carbon-neutral fuels, all processes in the production chain must be driven by renewable energy,” he said. 

Prof. Lipiński also believes that the sustainable fuel industry could provide downstream benefits for a country like Australia, given its capacity to produce renewable energy. 

“Sustainable fuels produced domestically can grow into a substantial industrial sector.  This can bring other positive effects such as demand for advanced engineering education programs in Australia. These can lead to attractive employment opportunities for specialised technologists and will prevent future brain-drain.”

“Australia has a long coastline and vast land areas away from agricultural and populated land where such technologies can be deployed at an unparalleled scale. Environmental impacts have, however, to be considered at any stage of such development to avoid future degradation of local land and water resources.” said Prof. Lipiński.

Overall Impacts

Aircraft contrails in the atmosphere. Credit: Carbon Brief.

We’re now at a point in history - a place where we have a very thorough understanding of human impacts on Earth’s climate on the one hand, and the opportunity for thousands of rocket launches per year on the other - to start asking questions about our overall impact to Earth’s systems through the continual migration of our species beyond the confines of our protective bubble.

Prof. Anna Moore, Director of the ANU Institute for Space, believes it is now that the long-term benefits of sustainable practices - even for rockets - can be seeded and built into the processes of emerging industries. 

“Australia is in a unique position as one of the newest players in the global space industry. Now that we are part of the space race, we cannot afford to ignore sustainability.  Growth and sustainability are compatible, and we can ensure we are building a sustainable culture in our industries and emerging start-ups. Ultimately, this will be a competitive edge and will provide real benefits for Australia, and the world,” she said.  

Whilst per rocket launch there is a minimal deleterious addition to the global climate, this small addition has a big impact on different regions of our atmosphere. This includes a range of emissions that are considered pollutants, ozone-depleting substances, and greenhouse gases.

For example solid rocket propellants can produce aluminium oxide, hydrogen chloride, nitrogen oxide, soot and carbon dioxide as emissions – all of which can impact the atmosphere. As do kerosene and hypergolic fuel engines, which output CO2, nitrous oxides, sulfur compounds and water vapour as well.

Some engines however, are considered cleaner, for example hydrolox engines, which combust hydrogen and oxygen, produce mostly water vapour and small amounts of nitrogen oxides. Water vapour however is still a very powerful greenhouse gas.

So the question then becomes - is there enough of the bad stuff going into the atmosphere to cause damage? Yes and no. When compared to passenger airline data from 2018 the spaceflight industry produced approximately 23,000 tonnes of CO2, about 40,000 times less than the global airline industry (which accounts for roughly 2% of climate impacts) for the same year – so not a significant impact.

Jets however, don’t fly through multiple layers of the atmosphere, and so their effects are constrained to lower altitudes – whereas rockets produce pollutants which do have a direct impact at higher atmospheric levels, even if this impact is minimal. It is for this reason that rocket launches are usually planned to be spaced apart – to allow the ozone layer an opportunity to regenerate.

Do these impacts mean we will need to cease all space launches? No.

The higher benefit of satellite based assets for nearly all aspects of our lives (e.g. Earth Observation for weather and disaster management, or GPS positioning) outweighs the detrimental effects of rocket fuel aerosols on our atmospheric layers.

But that does not mean we can’t be smarter and more sustainable about the atmospheric effects of rocket launches, the exact area of research that Vivienne Wells – a fourth-year Engineering Research and Development Honours Student from ANU is studying. 

“My project aims to produce renewable energy by transforming the methane produced during human activity into synthesis gas, a mixture of hydrogen and carbon monoxide, which can then be used to make liquid hydrocarbons, such as rocket fuel.” 

“This reaction takes place at high temperatures, which can be achieved through concentrated solar power, and can use biogas from a range of different sources, such as human waste or agricultural bi-products. This technology still has a number of techno-economic challenges that need to be solved, but we hope that the use of the newly developed catalysts will be a significant improvement in this process,” she said.

Along with studying at ANU where she is interested in solar thermal energy and fuel production, signal processing and machine learning, Vivienne is also the ACT representative for the Australian Youth Aerospace Association, and also works as a project officer with the ANU InSpace team.  

“The next decades are poised to see massive changes in the way we access space. With increased satellite launch numbers due to the potential of constellations of low-earth orbit (LEO) satellites and the potential of commercial space and long distance earth-earth travel, the number of rockets that we use is going to increase greatly.”

“Current levels of rocket use mean that emissions are small in comparison to other forms of transportation and human activity. However, with the predicted increase in rocket use, their individual environmental impact needs to be decreased to ensure that rocket activity doesn’t have a significant environmental impact,” said Vivienne.

Another area of opportunity for development of synthetic fuels, through carbon neutral processes is the capture of methane as a bi-product of human and agricultural activities, in a robust carbon recycling cycle that can output sustainable fuels that can be used for aviation or aerospace purposes. 

“By using methane captured from bi-products of human activity, the fuels being produced recycle carbon through the carbon cycle in a sustainable manner. The consumption of hydrocarbon fuels leads to emission of carbon dioxide into the atmosphere. Some of this carbon dioxide is then used by plants to produce carbohydrates through the process of photosynthesis. This plant matter is then used for feedstocks for animals or humans and becomes organic waste which can be fermented to produce biogas – a mixture of carbon dioxide and methane. This biogas can then be converted to fuels to power human activity, and the cycle starts again.” she said.  

“Whilst the level of biogas production through human activity isn’t enough to power all our activity, it does have the potential to make certain industries which rely on combustion propulsion, such as aviation and space have sustainable fuels”

Capturing methane as a bi-product of human activity and utilising this inside the carbon cycle to lower overall footprint. Credit: Vivienne Wells (supplied).

Innovation to the rescue

The Australian space industry, after lagging years behind the rest of the world when it came to launching capabilities is now starting to gain ground. A number of different launch facilities (like Equatorial Launch Australia, and Southern Launch Australia) and rocket building companies (like Gilmour Space Technology) progressing their objectives for sovereign launch capability.

Up and coming STEM talents, like Vivienne – who have grown up in a world where climate change has been of the highest importance to them – are tackling new problems with their eyes freshly viewing the opportunities that space capabilities bring to economies and societies, whilst continually assessing how to make practices more sustainable for our shared future.

We’ve built parts of our civilisation to the point where we need to use space applications and services to meet the needs of our everyday lives – integrating our technology and efficiency with the data that rains down from above us.

Rockets will not stop launching, and the energy density of rocket fuels means we will need to expend a lot of fuel to lift out of Earth’s gravitational well. The side effect of this is the cost of emissions from rockets into the atmosphere.

As with everything that requires progression and evolution, we now need to be sustainably smarter about our access to space. It is within these pioneering days of scalability and commercialisation that we should reflect upon our current practices to consider our place and impact on Earth’s overall systems and continually look for improvement.

And that’s the thing about space. Once you get there you look back down and see one thing, as a lifetime of realisation comes pouring in, like a cascading waterfall.

Spaceship Earth.

There’s only one of them.


 SpaceX Rocket launch video credit: Jesse Watson/Vimeo