The Urgent Need for Greener Propulsion

The global space economy is projected to exceed $1 trillion by 2040, with launch rates accelerating dramatically thanks to reusable rockets and small satellite constellations. Yet each launch of a conventional rocket leaves a complex environmental footprint. Traditional liquid propellants—such as hydrazine, unsymmetrical dimethylhydrazine (UDMH), and refined kerosene (RP-1)—are effective but come with steep costs: toxic handling procedures, carcinogenic exhaust in the upper atmosphere, and persistent groundwater contamination at launch sites. Even solid boosters, used on vehicles like the Space Shuttle and Ariane 5, release chlorine-based compounds that deplete stratospheric ozone.

In response, the space industry is undergoing a shift comparable to the automotive transition from leaded fuel to electric powertrains. The development of environmentally sustainable propellants for future rocket engines is no longer a niche academic pursuit—it is a strategic priority for agencies such as NASA, ESA, and JAXA, as well as for commercial players like SpaceX, Rocket Lab, and Blue Origin. This article explores the principal challenges of conventional propellants, the most promising green alternatives under development, and the technical, regulatory, and economic factors that will determine their adoption.

Why Conventional Propellants Are Under Scrutiny

Toxic legacy: Hydrazine and its derivatives

Hydrazine (N₂H₄) and its methylated variants have been the workhorse monopropellants for satellite thrusters and upper-stage engines for decades. They are hypergolic—igniting on contact with an oxidizer—which eliminates the need for an ignition system. But hydrazine is also a known carcinogen and highly corrosive. A single leak can force an entire launch facility into hazardous-materials lockdown. The 2019 incident at Cape Canaveral, where a hydrazine leak grounded a GPS satellite mission for weeks, highlighted the operational risks. Disposal of expired hydrazine propellant is expensive and poses long-term environmental liability.

Upper-atmosphere emissions from kerosene

RP-1, a highly refined kerosene used in the Falcon 9 and Atlas V first stages, produces CO₂, soot (black carbon), and nitrogen oxides (NOx) during combustion. While the total mass emitted per launch is small compared to global aviation, the plume is released at altitudes where the radiative forcing effect of black carbon is amplified. A 2021 study in Earth’s Future estimated that the black carbon from rocket exhaust heats the stratosphere by trapping outgoing radiation. As launch cadence grows—SpaceX alone has launched over 4,000 Starlink satellites with more than 100 flights per year—the cumulative impact becomes non‑negligible.

Solid boosters and ozone depletion

Solid rocket motors burn ammonium perchlorate aluminum powder and a rubber binder, producing alumina particles, HCl, and chlorine gas. The chlorine species—even at low concentrations—actively catalyze ozone destruction in the stratosphere. A 2020 NASA report noted that the global fleet of solid boosters could contribute up to 0.1% of annual stratospheric ozone loss. Although this seems small, the effect is concentrated over polar regions, where launch corridors often lie.

Economic and regulatory pressure

Beyond environmental ethics, regulations are tightening. The European Union is moving to restrict hydrazine under its REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) program. In the United States, the EPA is evaluating perchlorate discharges from solid rocket tests. Launch providers that cannot demonstrate lower-toxicity propulsion may face expensive permits, insurance premiums, or launch bans in environmentally sensitive areas.

Leading Candidates for Green Propellants

Green monopropellants: ADN and HAN‑based blends

The most mature replacement for hydrazine in satellite and upper‑stage thrusters is ammonium dinitramide (ADN). ADN‑based monopropellants—such as LMP‑103S (used on the Swedish Smart-1 lunar mission) and FLP-106—are ionic liquids that decompose exothermically in the presence of a catalyst. Their specific impulse is 10–15% higher than hydrazine, and their density is similar, while their toxicity is comparable to that of household ammonia. They do not require the same extreme protective gear for ground handling.

Hydroxylammonium nitrate (HAN) is another ionic liquid candidate, often blended with water and fuels like methanol. The US Air Force and NASA have demonstrated HAN thrusters on small satellites, achieving smooth ignition and high combustion efficiency. The primary challenges remain thermal management (HAN decomposes exothermically at temperatures above 150°C) and catalyst degradation over extended burns.

Cryogenic clean combustion: Liquid hydrogen and oxygen

The ultimate zero‑emission rocket propellant pair—liquid hydrogen (LH₂) and liquid oxygen (LOX)—produces only water vapor. Every major hydrogen‑oxygen engine, from the Space Shuttle Main Engine to the RL‑10, demonstrates that high performance is achievable (Isp ~450 s in vacuum). However, hydrogen faces enormous infrastructure hurdles: it is extremely low‑density (70 kg/m³), requiring large, heavily insulated tanks; it embrittles certain metals; and its production is still predominantly from natural gas (grey hydrogen). Blue Origin’s BE‑3U and the European Prometheus engine are pushing hydrogen‑oxygen propulsion toward reusability and lower cost. Scaling green hydrogen production—via electrolysis powered by renewable energy—will make hydrogen truly sustainable.

Bio‑derived and drop‑in fuels

Biofuels offer a near‑term solution for kerosene‑based launch systems. Synthetic iso‑paraffins from camelina or waste oils can be refined to meet RP‑1 specifications. Relativity Space has tested a 3D‑printed rocket engine using a bio‑derived fuel. The advantage is that existing RP‑1 engines require minimal modification; the lifecycle carbon footprint is reduced if the biomass was grown sustainably. The drawback is that biofuels still produce black carbon and NOx at altitude, so they are a stepping‑stone rather than an ultimate solution.

Liquid methane: A bridge between kerosene and hydrogen

Liquid methane (LNG) has emerged as a compelling middle ground. It is denser than hydrogen, less sooty than kerosene, and can be produced from captured CO₂ and renewable energy (e‑methane). SpaceX’s Raptor engine, Blue Origin’s BE‑4, and Relativity’s Aeon R all burn methane with LOX. Methane’s combustion produces primarily CO₂ and water, but at significantly lower black carbon emissions than kerosene. Moreover, methane’s intermediate storage temperature (‑162°C) makes it easier to handle than hydrogen. The main challenge for full sustainability is sourcing green methane at scale—which requires CO₂ capture and renewable hydrogen electrolysis—a loop that is not yet economically viable for high launch cadences.

Technical Barriers and Breakthroughs

Catalyst lifetime and chamber erosion

Green monopropellants like ADN and HAN operate at higher combustion temperatures (1,500–1,800°C) than hydrazine (800–900°C), placing extreme demands on catalyst and injector materials. Traditional iridium‑based catalysts used for hydrazine degrade rapidly in the hot, oxidative environment. Researchers at the German Aerospace Center (DLR) are testing ruthenium and perovskite catalysts that can survive thousands of restarts. Similarly, for methane engines, the issue of soot deposition on injector faces—even at low levels—must be managed for high reuse rates. SpaceX has mitigated this with fine‑atomization injectors and fuel‑rich pre‑burners.

Storage and handling infrastructure

Transitioning to green propellants often requires new ground support equipment. ADN and HAN are not hypergolic (they require a catalytic bed or electrical ignition), so the entire ignition sequence must be redesigned. Hydrogen demands cryogenic storage, transfer lines, and venting systems that are currently only available at a handful of launch pads. The investment required to retrofit a facility like Cape Canaveral to handle hydrogen for every launch is enormous; however, economies of scale could bring costs down as the industry moves toward complete cryogenic architectures.

Performance trade‑offs

No green alternative matches every metric of the incumbent. Hydrazine has a specific impulse around 230 s in a typical monopropellant thruster; ADN‑based fuels reach 250–260 s. For upper stages, where every second counts, the small improvement is welcome, but for first stages, the thrust‑to‑weight ratio often favors kerosene or methane over hydrogen because of tankage mass. Designers must consider the whole system: a denser fuel may allow a smaller, lighter stage even if the Isp is slightly lower.

Current Implementations and Flight Heritage

Europe leads with ADN

The European Space Agency (ESA) has flown LMP‑103S on the Prisma formation‑flying mission, the Small GEO satellite platform, and the Hera asteroid mission. In 2023, a demonstration flight of the FLP‑106 thruster on a CubeSat validated the technology for low‑cost small satellites. ESA’s GRASP (Green Advanced Space Propulsion) program aims to qualify a new generation of ADN thrusters in the 1–200 N range by 2026.

NASA and the Green Propellant Infusion Mission (GPIM)

NASA launched GPIM in 2019 aboard a SpaceX Falcon Heavy. The satellite carried a set of ADN thrusters developed by Aerojet Rocketdyne. Over 18 months, the thrusters performed 38 burns, demonstrating stable operation, high impulse bits, and no catalyst degradation. GPIM proved that a green monopropellant is flight‑ready for Earth‑orbiting missions and can even be used for deep‑space trajectory correction.

Methane engines in flight

SpaceX’s Starship/Super Heavy is the most ambitious methane‑powered vehicle, with 33 Raptor engines on the first stage and 6 on the upper stage. The vehicle completed its first integrated orbital test flight in April 2023. Blue Origin’s New Glenn and United Launch Alliance’s Vulcan (with BE‑4 engines) are both set to fly in 2024–2025. These vehicles represent a significant shift away from kerosene, and their large production runs will drive down methane engine costs and improve reliability.

Regulatory, Economic, and Collaborative Pathways

The role of environmental standards

National and international bodies are beginning to codify green metrics for propulsion. The International Organization for Standardization (ISO) is developing a standard for “greenhouse gas intensity of launch operations.” The FAA’s Office of Commercial Space Transportation is evaluating a voluntary sustainability rating system. Propellants with lower toxicity and lower stratospheric impact will likely be incentivized through reduced licensing fees and preferred launch window allocations near populated areas.

Cost competitiveness

Currently, green monopropellants like ADN are about five times more expensive per kilogram than hydrazine. However, when the total cost of ownership (handling suits, detoxification, insurance, disposal) is factored in, the gap narrows to 30–50%. With increased production volume—forecast to surge as the small satellite market grows—the price is expected to fall below hydrazine by 2030. Hydrogen remains more expensive than kerosene, but if green hydrogen becomes abundant, its cost could drop by 60% by 2040, according to IRENA (International Renewable Energy Agency).

International collaboration

No single organization can solve every technical challenge. The International Astronautical Congress has established a working group on green propulsion to share catalyst data, test results, and safety protocols. NASA, ESA, and JAXA are jointly funding a study on in‑space refueling with methane and hydrogen, which would further incentivize the use of sustainable fluids. Private‑public partnerships, such as the UK Space Agency’s “Green Propulsion Demonstrator” program, are accelerating the qualification of ADN and HAN thrusters for commercial use.

Future Outlook: Beyond 2030

Full‑scale hydrogen‑oxygen first stages

If reusability and green hydrogen production continue to mature, fully reusable hydrogen‑powered launchers could become economically viable. The ESA’s Phoenix concept (a reusable hydrogen first stage) and China’s Long March 9 (now designed with methane and hydrogen variants) both target the 2030s for first flight. Hydrogen’s high specific impulse allows for smaller upper stages and more aggressive reuse profiles.

Hybrid and electric‑augmented propulsion

For in‑space propulsion, green monopropellants will increasingly be paired with electric thrusters. A hybrid system might use ADN for high‑thrust burns (orbit insertion, collision avoidance) and Hall‑effect thrusters for station‑keeping, minimizing the total environmental footprint. NASA’s next‑generation ion thrusters, like the NASA‑457M, operate on inert xenon but are now being tested with iodine, which is non‑toxic and storable at low pressure.

Lifecycle assessment as a standard design criterion

The industry is moving toward full lifecycle modeling. Future propulsion engineers will not only ask “does this propellant burn cleanly?” but also “how much energy does it take to produce it? Where does the carbon come from? What happens to the manufacturing waste?” The European Green Deal and similar policies will likely require launch providers to publish the environmental cost per kilogram delivered to orbit. Sustainable propellants will be those that score well on this comprehensive metric, not only on exhaust composition.

Conclusion: A Shared Responsibility

The development of environmentally sustainable propellants is not merely an engineering problem—it is a defining challenge for an industry that aspires to open the space frontier while protecting the only planet we have. From the ban on hydrazine in Europe to the rise of methane engines and the growing flight heritage of ADN monopropellants, the trajectory is clear. The next decade will see a dramatic reduction in the use of toxic and polluting chemicals on launch pads and in orbit. But success depends on continued investment in catalyst research, production scaling, and regulatory frameworks that reward sustainability.

As launch costs plummet and access to space becomes routine, the environmental footprint of each mission becomes a strategic differentiator. Companies and agencies that invest now in clean propulsion will not only meet regulatory requirements but will also earn the trust of a public increasingly concerned about the health of the atmosphere and oceans. The transition to green propellants is not just possible—it is inevitable. The question is how quickly we choose to accelerate it.

For further reading on the chemistry of green monopropellants, see the DLR Institute of Space Propulsion. Data on the atmospheric effects of rocket exhaust can be found in the 2021 Earth’s Future study. The operational results of the Green Propellant Infusion Mission are detailed on NASA’s GPIM page.