Hydrazine has been the go-to propellant for spacecraft and satellites for more than half a century. While effective and proven, its use comes with many hazards. Researchers are currently developing and testing a new generation of green propellants that will eventually replace hydrazine for space propulsion applications.
HYDRAZINE—THE ORIGINAL MONOPROPELLANT
Hydrazine, chemical composition N2H4, is a colorless liquid used in many propulsion applications. Hydrazine and its derivatives have been associated with the rocket community for over 80 years and used for propulsion, control, and power generation purposes. It continues to be used as a monopropellant (passed over a catalyst, decomposed into hot gaseous products, and exhausted through a nozzle), mainly in satellite control operations and as a bipropellant (generally combined with a hypergolic oxidizer, causing combustion with gaseous products expanded out of the nozzle) for spacecraft propulsion.
Hydrazine was first developed as a hypergolic propellant by the Germans for the Me 163 rocket plane in 1937. They combined “C-Stoff,” a mixture of 30% hydrazine hydrate, 57% methanol, and 13% water, with hydrogen peroxide (H2O2) to achieve a hypergolic reaction. Since then, hydrazine and its derivatives, such as unsymmetrical dimethyl hydrazine (UDMH – H2NN(CH3)2) and monomethyl hydrazine (MMH – CH3(NH) NH2), have found widespread use in both hypergolic systems for high-impulse, propulsion applications. The Viking orbiters of the late 1970s used a hypergolic combination of MMH and nitrogen tetroxide for propulsion. The Mars Phoenix Lander used hydrazine retrorockets for its descent to Mars. The TITAN-II and TITAN-III missile systems used a hypergolic combination of hydrazine and dinitrogen tetroxide for their main propulsion systems on both the first and second stages (Figure 1) .
The first use of hydrazine as a monopropellant was demonstrated by the National Aeronautical and Space Administration (NASA) Jet Propulsion Lab in Pasadena, CA, in 1954. The development of Shell Catalyst 405, which allowed extended reusability and restartability of a hydrazine-based propulsion system, marked a true breakthrough in the use of hydrazine as a monopropellant. As a monopropellant, hydrazine continues to see considerable use in commercial satellites and low-impulse applications, such as the Iridium Satellite Constellation. This application provides altitude control orbit maintenance coverage to satellite phones due to the reliability of its decomposition reaction with a catalyst and its long development and heritage .
Benefits of Hydrazine
Hydrazine has a multitude of benefits as a propellant, especially in altitude maintenance and attitude adjustment on satellites and spacecraft. Its relatively low decomposition temperature and the high-pressure gas generated during decomposition make it ideal as a monopropellant for spacecraft position control. Prior to hydrazine, satellite altitude control was implemented with high-pressure gas (cold nitrogen), which provided a very low impulse and had a number of other problems such as long-term storage and leakage issues through the valves. Hydrazine solved these issues while improving performance; it could be stored as a liquid at low pressure, in a lower-volume tank, and with better sealing in the valves. It also provided higher performance through its decomposition than any cold gas system could achieve . Because hydrazine does not undergo long-term chemical changes in storage (provided that material compatibility is observed), it allows a robust supply of propellant to conduct maneuvers for the satellite’s lifetime. A long development history and flight heritage continue to make low-risk, hydrazine-based propulsion systems attractive for mission designers, despite the hazards associated with the chemical.
Problems and Dangers of Hydrazine
Despite its many benefits for spacecraft propulsion, hydrazine is a dangerous substance, especially during ground operations. It is toxic to humans, which necessitates extreme care in handling during satellite or spacecraft loading operations, thus driving up costs. The safety data sheet for hydrazine lists many potential health effects, such as irreversible eye injury, temporary blindness, allergic reaction with the skin, severe burns and ulceration, and liver kidney damage, chemical pneumonitis, and pulmonary edema if inhaled. It has also proved to be a carcinogen for animals; however, this carcinogenic nature has not been verified in humans. Absorption through the skin may cause death . These toxicities necessitate using Self-Contained Atmospheric Protective Ensemble (SCAPE) suits when handling hydrazine to prevent any possible bodily exposure to the fuel or its fumes (Figure 2).
The second most concerning problem with hydrazine is its strong reducing-agent properties. Upon exposure to air, a porous material, or a small amount of heat, hydrazine can spontaneously decompose. These properties make hydrazine attractive as a monopropellant (low energy required to induce decomposition) but extremely dangerous for humans. Its reaction to air means it must be kept under a nitrogen blanket to prevent exposure to oxygen. It can corrode metals, leading to tank leakage and fuel contamination. Though common metals like 304 stainless steel and aluminum 1100 show good compatibility, care must be taken to ensure that the hydrazine does not come into contact with incompatible materials. Iron, nickel, brass, bronze, and copper all show poor compatibility with hydrazine. Hydrazine can also absorb carbon dioxide from the environment, causing it to become more corrosive and accelerating its decomposition rate .
Due to its long use as a propellant, many effective risk mitigation techniques have been developed to ensure operational safety when dealing with hydrazine; however, the overall cost and difficulty of handling hydrazine have made developing alternative green fuels a priority for the propulsion community.
Green Alternatives to Hydrazine
Although hydrazine is a remarkable chemical, the challenges associated with handling it and the true hazard it poses in the event of human exposure have driven a search for a more benign alternative that meets or exceeds the performance of hydrazine as a monopropellant. The term “green monopropellant” encompasses a reduction in overall handling and safety requirements—low toxicity, noncarcinogenic, noncorrosive, low vapor pressure to reduce exposure routes and flammable headspace, no requirement for SCAPE suits when handling, and transportability by commercial aircraft.
The 2006 European Union (EU) Regulation (EC) No. 1907/2006 requires all chemical substances produced or used by industry to be registered with the European Chemicals Agency (ECHA) . Substances of very high concern (SVHC) are given a sunset date after which the manufacture, import, and use of the substance in the EU will be prohibited unless specific authorization is granted by ECHA. Hydrazine was added to the candidate list of SVHCs in 2011; while no sunset date has been assigned, it is clear to industry that hydrazine will need to be replaced in order to comply with EU regulations.
For all green monopropellants, the strategy for achieving the safety goals is generating ternary ionic solutions based on an oxidizing salt and fuel mixture with water. Common oxidizing salts are hydroxylammonium nitrates (HANs), ammonium dinitramide (ADN), hydrazinium nitroformate (HNF), and ammonium nitrate (AN) .
The European search for a hydrazine replacement began in earnest in 2008 with a project called Green Advanced Space Propulsion (GRASP), a consortium of 12 universities and organizations led by the University of Applied Sciences Wiener Neustadt . The GRASP program identified a number of possible hydrazine replacements, including some already well into their development, such as FLP-106 and LMP-103S. Both of these propellants are based on ADN, a strong oxidizer originally discovered at the Zelinsky Institute of Organic Chemistry in Moscow, USSR, in 1971. ADN remained a state secret until it was rediscovered in 1988 by researchers at Stanford Research Institute in the United States .
FLP-106 and LMP-103S have been in development since the late 1990s by Totalförsvarets forskningsinstitut, the Swedish Defense Research Agency (FLP-106), and commercial firm Ecological Advanced Propulsion Systems (LMP-103S), both collaborating with the Swedish Space Corporation . These propellants, which have demonstrated low toxicity and high performance, are viable candidates for replacing hydrazine for monopropellant space propulsion systems . LMP-103S has been flight tested on the Prisma satellite launched in 2010 and demonstrated 2.3 hours of accumulated firing time through the summer of 2011 .
In 1993, the United States began the Integrated High Payoff Rocket Propulsion Technology Program to drive large improvements in the state of propulsion technology for the U.S. Department of Defense (DoD) and NASA . Goals for improving monopropellant systems included a 30% improvement in density-specific impulse (Isp) by the year 2000 and subsequent increases every 5 years. The result of this density-Isp goal is a propellant based on HAN that was developed by researchers at the U.S. Air Force Research Laboratory at Edwards Air Force Base, CA, with funding from the U.S. Air Force Office of Scientific Research. Designated AF-M315E (also known by its commercial name ASCENT [Advanced Spacecraft Energetic Non-toxic Propellant]), AF-M315E has a density-Isp improvement of 64% over hydrazine, greater than any other hydrazine replacement yet identified.
The physical property comparison between FLP-106, hydrazine, LMP-103S, and AF-M315E can be found in Table 1. A higher density allows more propellant mass to be stored in the same volume of space, allowing more thrust. Isp is analogous to fuel economy for a car—the higher the Isp, the more thrust per pound of fuel used. There have been many studies on the performance of the propellants, and comparisons have been made between each [16–19]. Compositional differences of LMP-103S and FLP-106 are summarized in Table 2.
CHALLENGES FOR GREEN MONOPROPELLANTS
Despite multiple candidates vying to replace hydrazine, there are still challenges associated with green monopropellant use. Ignition is difficult compared to hydrazine. A study by the German Aerospace Center (DLR) examined thermal ignition of ADN-based monopropellants  and found that the water in the ADN-based propellants had to evaporate before decomposition could occur. Two different methods were devised to achieve this objective—one involved thermal ignition with a pilot flame, and the other involved a glow plug insertion. Only the glow plug was found to be successful.
Catalytic ignition of AF-M315E is initiated through a preheated catalyst bed. The catalyst bed requires a large amount of power in order to generate the necessary thermal conditions. Minimizing the necessary heater power while ensuring repeated and consistent ignition over the thruster system’s life is critically important and remains an area of active research.
Although the concept of energetic ionic liquids using HAN and ADN is 20 years old, the practical reality of implementing entire propulsion systems around these new fuels is challenging; however, the difficulties are not insurmountable. LMP-103S has already been demonstrated in operational spacecraft, and AF-M315E is currently getting an on-orbit shakedown as part of NASA’s Green Propellant Infusion Mission (GPIM).
GPIM REACHES ORBIT
Early on the morning of Tuesday, June 25, 2019, the flames from a SpaceX Falcon Heavy rocket lit up the darkness of Kennedy Space Center Launch Complex 39A (Figure 3). The Falcon Heavy hosted the DoD’s Space Test Program-2, a multimanifest mission involving DoD, NASA, and National Oceanic and Atmospheric Administration satellites, along with a number of satellites from educational institutions and the private sector . For the spacecraft propulsion community, NASA’s GPIM satellite was a particularly important and long-awaited component of the Falcon Heavy launch manifest. Funded by NASA’s Technology Demonstration Missions program within the agency’s Space Technology Mission Directorate, the satellite was built by Ball Aerospace & Technologies Corp. based on the company’s BCP-100 bus Evolved Expendable Launch Vehicle Secondary Payload Adapter-class mission. The GPIM satellite will serve as a testbed for the AF-M315E propellant . The satellite’s thrusters were developed by Aerojet Rocketdyne, and the AF-M315E-based propulsion system was codesigned by Ball Aerospace and Aerojet Rocketdyne [23, 24].
Approximately an hour-and-a-half after the successful launch of the Falcon Heavy from Florida with its manifest of 24 satellites, the GPIM satellite deployed from the rocket’s second stage and began to power up in anticipation of commands from its ground controllers. Within a week of its launch, controllers fired the satellite’s five thrusters in order to test the propulsion subsystem and ensure that GPIM was functioning as designed [21, 26].
Over the next 13 months, the GPIM satellite will perform a number of in-orbit maneuvers, including attitude control, spacecraft pointing, thruster performance characterization and mapping, and orbit lowering, to test the efficacy of the AF-M315E-based propulsion system and demonstrate its viability for future government and commercial spaceflight missions .
Future use of AF-M315E will help lower the cost of spacecraft fueling prior to launch since the fuel is nontoxic and will require fewer safety precautions than the highly-toxic hydrazine propellant currently in use. It will also provide greater flexibility for spacecraft designers due to its high-energy density, which will permit smaller quantities to be used for missions or permit missions to last longer . Green monopropellants are currently being considered for many space missions, including Mars ascent vehicles and hoppers, lunar landers, and deep-space microsatellites . Time will tell whether hydrazine joins “C-Stoff” in the space and rocket propulsion history books.
- Schmidt, E. W. “One Hundred Years of Hydrazine Chemistry.” Third Conference on Environmental Chemistry of Hydrazine Fuels, September 1987.
- Wetmore, D., Maj. “Titan II Blasts Its Way Into History.” U.S. Air Force, https://www.af.mil/News/Article-Dis-play/Article/138285/titan-ii-blasts-its-way-into-history/, 24 October 2003.
- Iridium. “Hydrazine – Toxic for Humans, but Satellites Love It.” https://www.iridium.com/blog/2017/06/20/hydrazine-toxic-for-humans-but-satellites-love-it/, 20 June 2017.
- Moynihan, P. I. “Back in the Day… Selected Events in Early Monopropellant Hydrazine Thruster Development,” 10 July 2006.
- Fisher Scientific. “Hydrazine Anhydrous.” Material safety data sheet, https://fscimage.fishersci.com/msds/11040.htm, 29 June 2007.
- Stewart, C., Tech Sgt. “Hydrazine Response Team Hones Skills.” U.S. Air Force, https://www.114fw.ang. af.mil/News/114FW-News/Article/863976/hydrazine-response-team-hones-skills/, 20 June 2014.
- Martin Marietta Corporation. Material Compatibility With Space Storable Propellants Design Handbook, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019028.pdf, March 1972.
- European Parliament and Council. Regulation (EC) No. 1907/2006. “Registration, Evaluation, Authorisation [sic.] and Restriction of Chemicals (REACH),” https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32006R1907, accessed 31 July 2019.
- Wingborg, N., C. Eldsäter, and H. Skifs. “Formulation and Characterization of ADN-Based Liquid Monopropellants.” Proceedings of the 2nd International Conference on Green Propellants for Space Propulsion (ESA SP-557), Chia Laguna (Cagliari), Sardinia, Italy, edited by A. Wilson, 7–8 June 2004.
- University of Applied Sciences Wiener Neustadt. “Final Report Summary – GRASP (Green Advanced Space Propulsion).” https://cordis.europa.eu/project/rcn/89683/reporting/en, accessed 31 July 2019.
- Larsson, A., and N. Wingborg. “Green Propellants Based on Ammonium Dinitramide (ADN), Advances in Spacecraft Technologies.” Edited by Dr. Jason Hall, https://www.intechopen.com/books/advances-in-space-craft-technologies/green-propellants-based-on-ammoni-um-dinitramide-adn-, 2011.
- Anflo, K., T. A. Grönland, and N. Wingborg. “Development and Testing of ADN-Based Monopropellants in Small Rocket Engines.” The 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Huntsville, AL, 16–19 July 2000.
- Wilhelm, M., M. Negri, H. Ciezki, and S. Schlechtriem. “Preliminary Tests on Thermal Ignition of ADN-Based Liquid Monopropellants.” Acta Astronautica, vol. 158, pp. 388–396, https://doi.org/10.1016/j.actaas-tro.2018.05.057, May 2019.
- PRISMA (Prototype Research Instruments and Space Mission Technology Advancement). https://earth.esa.int/web/eoportal/satellite-missions/p/prisma-prototype, accessed 31 July 2019.
- DeGeorge, D., and S. Fletcher. “The Integrated High Payoff Rocket Propulsion Technology Program (IHPRPT) and Tactical Missile Propulsion Status.” Paper presented at the RTO AVT Specialists’ Meeting on Advances in Rocket Performance Life and Disposal, Aalborg, Denmark, 23–26 September 2002, https://www.sto.nato.int/publi-cations/STO%20Meeting%20Proceedings/RTO-MP-091/MP-091-02.pdf, accessed 31 July 2019.
- Negri, M. “Replacement of Hydrazine: Overview and First Results of the H2020 Project Reform.” Paper presented at the 6th European Conference for Aeronautics and Space Sciences (EUCASS), Kraków, Poland, July 2015.
- Wurdak, M., F. Strauss, L. Werling, H. K. Ciezki, D. Greuel, R. Lechler, N. Wingborg, D. Hasan, and C. Scharlemann. “Determination of Fluid Properties of the Green Propellant FLP-106 and Related Material and Component Testing With Regard to Applications in Space Missions.” Paper presented at the Space Propulsion 2012 Conference, Bordeaux, France, May 2012.
- Werling, L. “Experimental Investigations Based on a Demonstrator Unit to Analyze the Combustion Process of a Nitrous Oxide/Ethene Premixed Green Propellant.” Paper presented at the 5th CEAS Air & Space Conference, Delft, Netherlands, September 2015.
- Spores, A. R., R. Masse, S. Kimbrel, and C. McLean “GPIM AF-M315E Propulsion System.” Aerojet Rocketdyne, Redmond, WA, and Ball Aerospace and Technologies Corporation, Boulder, CO, September 2015.
- Wilhelm, M., M. Negri, H. Ciezki, and S. Schlechtriem. “Preliminary Tests on Thermal Ignition of ADN-Based Liquid Monopropellants.” DLR Institute of Space Propulsion, Hardthausen, Germany, September 2015.
- Skelly, C., and K. Fox. “NASA Technology Missions Launch on SpaceX Falcon Heavy.” https://www.nasa.gov/press-release/nasa-technology-missions-launch-on-spacex-falcon-heavy, accessed 1 August 2019.
- U.S. Air Force Office of Scientific Research. “Long Awaited Launch of NASA’s GPIM Mission to Use AFRL-Developed Propellant.” https://www.wpafb.af.mil/News/Article-Display/Article/1891179/long-awaited-launch-of-nasas-gpim-mission-to-use-afrl-developed-propellant/, accessed 1 August 2019.
- NASA. “Green Propellant Infusion Mission.” https://www.nasa.gov/sites/default/files/atoms/files/g-484591_gpim_factsheet.pdf, accessed 1 August 2019.
- Mohon, L. (editor). “Green Propellant Infusion Mission (GPIM) Overview.” https://www.nasa.gov/mis-sion_pages/tdm/green/overview.html, accessed 1 August 2019.
- Thacker, Z., Airman 1st Class. “45th SW Support Falcon Heavy STP-2 Rocket Launch.” U.S. Air Force, https://www.patrick.af.mil/News/Photos/igphoto/2002149671/, accessed 30 July 2019.
- Hall, L. (editor). “Green Propellant Infusion Mission Fires Thrusters for the First Time.” https://www.nasa.gov/directorates/spacetech/home/tdm/gpim_fires_thrust-ers_for_first_time, accessed 1 August 2019.
- Deans, C. M., et. al. “An Evaluation of the Impacts of AF-M315E Propulsion Systems for Varied Mission Applications.” NASA Glenn Research Center, https://ntrs.nasa.gov/archive/nasa/ casi.ntrs.nasa.gov/20150021510.pdf, accessed 31 July 2019.
NICHOLAS KEIM is the Deputy Director of the Johns Hopkins University (JHU) Whiting School of Engineering’s Energetics Research Group (ERG) and principal investigator at the Advanced Engine and Rocket (AERo) Fuels Laboratory. He has worked at the Chemical Propulsion Information Analysis Center and performed original research on rocket propellants and propulsion topics, resulting in the publication of over 40 technical papers. In 2012, he helped establish the AERo Fuels Laboratory, which is dedicated to the scientific investigation of rocket and aerospace fuels for space launch and hypersonic applications. Mr. Keim holds a B.S. in mechanical engineering from JHU and an M.S. in mechanical engineering from Imperial College London.
ALEXANDER BISHOP is an associate staff engineering supporting propellant and fuel-related researcher at the JHU Whiting School of Engineering’s ERG. He serves as the technical representative to the Joint Army Navy NASA Air Force (JANNAF) Interagency Propulsion Committee’s Air Breathing Propulsion, Space Propulsion, and Modeling and Simulation subcommittees. His current fuel research topic areas include RP-2 and methane/LNG for space launch vehicles, green monopropellants for space craft propulsion, and endothermic fuels for hypersonic vehicles. Mr. Bishop holds a chemical engineering degree from the University of Maryland Baltimore County and is pursuing a master’s degree in chemical and biomolecular engineering from JHU.
BENJAMIN HILL-LAM is an associate staff engineer involved in studying hydrocarbon fuels’ thermal stability performance at the JHU Whiting School of Engineering’s ERG. He currently works on research to develop a standardized thermal stability metric utilizing the Compact Rapid Assessment of Fuel Thermal Integrity rig in the Advanced Engine and Rocket Fuels lab. His other research areas include endothermic fuels research for hypersonic vehicles and green propellant materials compatibility. He serves as the Energetic Research Group’s technical representative for the JANNAF Liquid Propulsion and Combustion subcommittees. Mr. Hill-Lam holds a bachelor’s degree in physics from Bowdoin College and is currently pursuing a master’s degree in materials science and engineering at JHU.
BENJAMIN SCHWANTES is the Managing Editor of the JANNAF (Joint Army Navy NASA Air Force) Journal of Propulsion and Energetics published by the ERG at the JHU Whiting School of Engineering. Prior to joining ERG, he served as a research associate at the German Historical Institute in Washington, DC, where he helped to manage a long-term, online research and publication project on immigration and entrepreneurship. He has served as editor and reviewer for various publications and journals and authored articles on entrepreneurship, ethnicity, business, and communication for immigrant entrepreneurship. Dr. Schwantes received his B.A. in history from the University of Pittsburgh and his M.A. and Ph.D. in the history of technology and industrialization from the University of Delaware.