In recent years, small satellites have become a popular tool for accessing space. CubeSats in particular offer a standardized platform for ride-along access to space, carrying payloads for space-based science research or technology demonstrations. These systems are typically on the order of less than a cubic meter in volume and weigh only tens of kilograms, and yet their capabilities continue to increase in scope along with the advancement of technologies suited to miniaturized space systems.
One such key technology is the advancement of in-space propulsion. The inclusion of propulsion systems on small satellites adds significant capability to their missions, allowing them maneuverability, momentum control, and orbit adjustment. The development of such a propulsion system requires considerable design effort due to their miniature size, custom architecture, and inclusion of cutting-edge technologies necessary for their success.
Projected growth in rocket launches for space tourism, moon landings, and perhaps travel to Mars has many dreaming of a new era of space exploration. But a NOAA study suggests that a significant boost in spaceflight activity may damage the protective ozone layer on the one planet where we live, thus, creating a detrimental effect on the Earth’s climate. In June 2022, researchers from the Massachusetts Institute of Technology (MIT), the University of Cambridge, and the University College London (UCL) observed that the soot emitted from rocket launches had impacted the atmosphere by generating more heat when compared to other emitting sources. Kerosene-burning rocket engines widely used by the global launch industry emit exhaust containing black carbon, or soot, directly into the stratosphere, where a layer of ozone protects all living things on the Earth from the harmful impacts of ultraviolet radiation, which include skin cancer and weakened immune systems in humans, as well as disruptions to agriculture and ecosystems.
Hydrazine-based propulsion systems are state-of-the-art for various applications ranging from launchers to large and small satellites. They have a long and successful heritage and a great variety of space-qualified, off-the-shelf components. Hydrazine has dominated the space industry as the choice of propellant for over six decades, due to its high-performance characteristics, despite its environmental and health hazards and the challenges faced in its manufacturing, storage, ground handling, and transportation.
However being a toxic compound of nitrogen and hydrogen that is on the EU’s list of substances of high concern. It is also the suspected cause of abnormally high rates of hormonal and blood disorders around the Baikonur rocket launch site in Kazakhstan. Hydrazine was identified as a substance of high concern by the REACh regulation in 2011 there is a threat that these systems might be forbidden in the future.
One aspect of clean space is to use more green / eco-friendly or non-toxic propellants to replace classical toxic hydrazine based systems. The need for nontoxic propellants is increasing not only since classical, hydrazine based propulsion systems are facing legislative regulations but also because non-toxic alternatives can offer significant technical and economical assets. Space agencies are trying to replace the conventional hydrazine rocket fuel, a highly toxic and carcinogenic chemical, with a greener propellant for future missions. These technologies present performance benefits such as reduced launch mass, increased scientific payload mass, and/or extending on-orbit lifetimes. The advantages are further reinforced due to the significant reduction in health risks encountered during launch site and ground handling operations.
Scientists at the German Aerospace Centre (DLR) in Lampoldshausen are working on new, greener fuels that can future-proof space exploration methods and make them environmentally friendly. The efforts are centred around a compound known as ammonium dinitramide (ADN), which when heated decomposes into only nitrogen, oxygen, and water. ‘ADN was an oxidiser salt first found in the Soviet Union, but was rediscovered in Sweden in the 1990s where they had the idea to develop it into a liquid propellant,’ said Dr Michele Negri, leader of a space propulsion project called RHEFORM.
Green Propellant technology
Monopropellant propulsion is a decomposition-based form of chemical propulsion. The stored propellant is heated and flowed over a catalyst bed that triggers the decomposition. The decomposition itself is an exothermic reaction that releases chemical energy, resulting in a high-temperature gaseous medium that may be accelerated out of a nozzle to produce thrust.
As mentioned in NASA’s identified key technologies, it is highly desirable to seek alternatives to hydrazine because it is such a dangerous and volatile chemical. Green monopropellants such as LMP-103S and AF-M315E are hydroxylammonium nitrate-based alternatives. In comparison to Hydrazine, green monopropellants most notable advantages include decreased toxicity and significantly safer storage and handling. Each candidate propulsion technology offers increased performance over hydrazine monopropellant propulsion, reduces personnel and environmental hazards, and simplifies transport and handling for ground operations. In fact, their ‘green’ moniker originates from the fact that they are so much less toxic that they can be “[safely handled] in open containers for unlimited durations.” Other technologies such as Nitrous Oxide Fuel Blend (NOFBX) or hydrogen peroxide either do not improve performance, or do not reduce safety concerns.
The term “ADN Technology” is used for a liquid monopropellant where solid oxidizer (ADN – Ammonium dinitramide salt) is solved in water and then fuel and stabilizer are added. In the combustion chamber the oxidizer and fuel are burned with subsequent high combustion temperatures. ECAPS LMP-103S Technology: LMP-103S is a liquid propellant where the solid oxidizer (ADN) is solved in water and Ethanol is used as fuel. The ADN technology is closely linked to the Swedish company ECAPS that developed the propellant LMP-10S and subsequent thrusters which are for the first time used on a commercial mission.
The problem is that ADN is a salt, so it is solid. While it can be dissolved into other fuels like methanol or ammonia, it takes a high temperature – more than 1500ºC – to ignite it. Hydrazine thrusters do not require pre-heating, if you just open the valves then they start firing. On the other hand, with an ADN thruster if you just open the valve the blend would come out in liquid form. It would not react,’ Dr Negri said. The RHEFORM project looked at the ADN-based propellant LMP-103S used by a Swedish space company called ECAPS, which was a project partner and has already launched 13 propulsion systems based on the compound.
AF-M315E also is expected to improve overall vehicle performance. It boasts a higher density than hydrazine, meaning more of it can be stored in containers of the same volume. In addition, it delivers a higher specific impulse, or thrust delivered per given quantity of fuel, and has a lower freezing point, requiring less spacecraft power to maintain its temperature. It also requires fewer handling restrictions and potentially shorter launch processing times, resulting in lowered costs.
Hydroxyl Ammonium Nitrate fuel/oxidizer blend, known as “AF-M315E.”
Hydroxyl Ammonium Nitrate fuel/oxidizer blend, known as “AF-M315E.” This innovative, low-toxicity propellant, developed by the U.S. AFRL (Air Force Research Laboratory) at Edwards Air Force Base, CA, is a high-performance, green alternative to hydrazine. AF-M315E has significantly reduced toxicity levels compared to hydrazine, making it easier and safer to store and handle. It also requires fewer handling restrictions and potentially shorter launch processing times, resulting in lowered costs.
AF-M315E also is expected to improve overall vehicle performance. It boasts a higher density than hydrazine, meaning more of it can be stored in containers of the same volume. In addition, it delivers a higher specific impulse, or thrust delivered per given quantity of fuel, and has a lower freezing point, requiring less spacecraft power to maintain its temperature.
NASA Green Propellant Infusion Mission
Through the Green Propellant Infusion Mission, or GPIM, NASA is developing a “green” alternative to conventional chemical propulsion systems for next-generation launch vehicles and spacecraft. The GPIM project will demonstrate the practical capabilities of a Hydroxyl Ammonium Nitrate fuel/oxidizer blend, known as AF-M315E or HAN. This innovative, low-toxicity propellant, developed by the U.S. Air Force Research Laboratory at Edwards Air Force Base, California, is a high-performance, green alternative to hydrazine.
NASA’s Green Propellant Infusion Mission (GPIM) has successfully deployed on 24th June 2019. GPIM, a small, box-shaped spacecraft powered by green technology, will test out a low-toxicity propellant in space for the first time, according to NASA. The GPIM payload flew to space aboard a Ball compact small satellite or “smallsat” on a SpaceX Falcon Heavy rocket as part of a technology-testing mission dubbed STP-2.
The Lunar Flashlight mission is a 6U Cubesat that aims to investigate the poles of the Moon for volatiles including water ice. It will ride along with the Artemis-1 mission on the Space Launch System (SLS) as part of the United States’ national effort to reestablish a human presence on the moon. The Lunar Flashlight Propulsion System accounts for approximately one half of the spacecraft. It will be a technology demonstration of green monopropellant propulsion, and will contain all supporting hardware such that the entire subsystem is a functional standalone component. Upon the successful completion of this mission, Lunar Flashlight would become the first CubeSat to reach the Moon and the first CubeSat to achieve orbit around a celestial body other than the Earth. Both of these accomplishments are directly dependent on the contribution of the propulsion system
NASA and Ball Aerospace & Technologies Corp. of Boulder, Colorado, are collaborating on the Green Propellant Infusion Mission, which seeks to improve overall propellant efficiency while reducing the handling concerns associated with the highly toxic fuel, hydrazine. There is only one prior instance of AF-M315E used as an in-space propulsion system: the Green Propellant Infusion Mission (GPIM). This mission was also managed by NASA Marshall, and included engineering efforts by Aerojet Rocketdyne and Ball Aerospace. Its primary objective was the technology demonstration of its AF-M315E propulsion system. This system carried five thrusters for orientation control and orbit maneuvering. It launched on June 25th, 2019 as part of the STP-2 mission on a Falcon Heavy rocket in a Ball Aerospace SmallSat platform. A week later, it reported successful firing of all five of its thrusters as part of system checkouts and an orbit lowering maneuver
Hydrogen peroxide is a chemical compound with the formula (H2O2). In its pure form it is a colourless liquid, slightly more viscous than water. Hydrogen peroxide is a strong oxidizer and is used as a bleaching agent and disinfectant. Concentrated hydrogen peroxide, or ‘high-test peroxide’ (HTP) is used as a rocket propellant since 1934. Currently HTP is being used on the Sojuz Launcher for the first stage gas generator and on the Sojuz capsule for the reaction control thrusters used during re-entry. In the frame of the H2020 project HYPROGEO the manufacturing and transport of 98% was qualified and this propellant blend is now commercially available on the market.
The Indian space rockets are set to go green as ISRO is working to replace its existing hazardous fuel with non-hazardous and environment- friendly fuels for its rockets and satellites, the ISRO Chairman said. He also said that the Indian Space Research Organisation (ISRO) is looking at green propulsion through hydrogen peroxide in its rocket that would take Indians into the space under its ‘Gaganyaan’ mission.
“Since humans will be inside the rocket, we want only non-hazardous fuel and not the hazardous one to power the human space mission’s Gaganyaan rocket,” ISRO chief K Sivan, who is also Secretary in Department of Space, told IANS.
Scientists at the Indian Space Research Organisation (ISRO) have reported progress in the development of an environment-friendly propellant to power satellites and spacecraft. Replying to a question on using a green propellant for its launches, he said, “Green propulsion is essential. We are going to fly Gaganyaan with green propulsion. It is an essential requirement,” he said. He said ISRO has already started the development of green propulsion. “We are going to bring the green propulsion system in our launch vehicles because this is the need of the hour. We want to ensure that all the toxic and hazardous propellants are avoided.
Initial tests by a research team at the Liquid Propulsion Systems Centre (LPSC) here have shown promising results in the formulation and associated tests of a propellant blend based on hydroxylammonium nitrate (HAN). The LPSC team comprising Arpita Dash, B. Radhika and R. Narayan formulated the HAN-based monopropellant and carried out a variety of tests to investigate its characteristics, like thermal and catalytic decomposition and compatibility with different materials. A monopropellant is a chemical propulsion fuel which does not require a separate oxidizer. It is used extensively in satellite thrusters for orbital correction and orientation control. The in-house formulation consists of HAN, ammonium nitrate, methanol and water. While methanol was added to reduce combustion instability, the choice of AN was dictated by its capacity to control the burn rate and lower the freezing point of the propellant.
According to him, ISRO is also looking at rocket engines powered by hydrogen peroxide as a mono-propellant or as a bi-propellant along with ethanol. ISRO is also developing another green fuel – LOX/Methane- liquid oxygen as oxidiser and methane as fuel. “One month back, we tested the LOX/Methane engine and the results were good,” Sivan said. The LOX/Methane propellant has advantages in terms of specific impulse, storability, low toxicity, and cost. According to Sivan, the space agency is looking to replace its existing liquid engine fuels with green fuel-powered ones. ISRO has also developed ISROSENE, which is a rocket-grade version of kerosene as an alternative to conventional hydrazine rocket fuel.
Water propulsion is defined as propulsion that uses water as a stored propellant which is decomposed into gaseous Oxygen and Hydrogen via an electrolyser in orbit. These gases are then exothermic combusted for generation of thrust. It is a semi electric propulsion where propellant is generated over a longer time period with low power and is then exploited during short boost.
However green propellants like AF-M315E and LMP-103S have operational restrictions that do not exist for hydrazine propulsion systems. The main operational restriction for both systems, is that the thruster must be preheated. There is no cold start capability for either type of thruster. This is a significant restriction, since a common contingency requirement for hydrazine thrusters is to be able to detumble a spacecraft when it comes off the launch vehicle, and the thrusters often cannot be preheated to meet this requirement.
With the higher temperatures, materials in the thrusters are likely to be more expensive, driving the cost of the thrusters significantly higher than hydrazine thrusters. Although the hardware is likely more expensive, the savings for ground operations are significant. In addition, when environmental aspects of the complete life-cycle costs are considered, there are additional significant savings, as analysed by Eric H. Cardiff, Henry W. Mulkey, and Caitlin E. Bacha, of NASA Goddard Space Flight Center.
Novel gel propulsion to slash cost of communication satellites
Propelling the largest of those satellites requires costly and highly toxic jet fuel, while launching smaller satellites requires other liquid fuels that could benefit from improvement. Israeli startup NewRocket is developing gel propulsion – a cheaper, more environmentally friendly rocket-engine technology offering the same level of performance and control as toxic “legacy fuel” and a better solution for smaller satellites, too. Prof. Benny Natan at the Technion-Israel Institute of Technology has developed gel technology that fits between the two existing propulsion alternatives: solid fuel, which burns for a single burst to launch rocket ships, and liquid fuel such as hydrazine, typically used to position satellites.
The NewRocket gel is based on standard liquid kerosene airplane fuel. “It’s much more ‘green’ than any other material used today in space. The stable gel provides several attributes that offer very cheap, flexible and powerful propulsion. It will open the door for many applications in space,” says Privman. He estimates that compared to legacy fuel, gel fuel could lower the overall cost of the satellite system and operation by five times. The weight of the gel fuel is similar to liquid fuel but it’s safer to transport
The fast-growing international private communications satellite market is the main focus for NewRocket. “Companies like OneWeb and SpaceX are building smaller, cheaper satellites that orbit Earth 100 times closer than the big communications satellites so they can provide very fast Internet data services to every cellphone on the globe. We are enablers for this industry,” says Privman. He notes that smaller satellites need more powerful alternatives than they have now, and makers of the larger billion-dollar communications satellites also seek safer, greener solutions.
“Current liquid fuels used on satellites are made of very hazardous materials because they need to be very powerful,” Privman says. “Therefore, many precautions need to be taken when handling the fuel. For the last decade, the space industry has been searching for something less toxic.”
Astranis selects ECAPS green propulsion for geostationary communications constellation
Astranis, a startup developing geostationary satellites to offer broadband internet access, announced plans in June 2018 to equip its MicroGEO spacecraft with Bradford of the Netherland’s high performance green ECAPS thrusters. “This is a huge step towards abolishing hydrazine from spacecraft worldwide,” Ian Fichtenbaum, Bradford ECAPS director, told SpaceNews by email. “We think once people realize that there is an operational hydrazine alternative (with better performance than hydrazine) currently being used almost nobody will want to go back to hydrazine.”
Under the agreement, Astranis will purchase eight ECAPS thrusters for each MicroGEO satellite. The initial order covers as many as 12 spacecraft. In addition, the agreement calls for Bradford’s Space Group to supply the Astranis constellation with electrical propulsion feed systems and a set of eight Cosine Sun Sensors for attitude control. Astranis plans to launch dozens of 300-kilogram satellites to provide capacity of up to 10 gigabits per second to underserved communities. In March, the San Francisco-based startup announced it raised $18 million in Series A funding.
“The ECAPS technology is mature and ready to be deployed on a large scale,” John Gedmark, Astranis founder and chief executive, said in a statement. “Their team impressed us with their ability to move quickly and their ability to execute.” The deal marks the introduction of ECAPS, which stands for Ecological Advanced Propulsion Systems, to geostationary orbit, which is dominated by large telecommunications satellites with hydrazine-fueled thrusters. ECAPS thrusters burn LMP-103S, a storable liquid monopropellant that blends ammonium dinitramide with water, ammonia and methanol.
Because ECAPS is less toxic than conventional propellants, Astranis will be able to fuel satellites in its factory rather than at the launch site, Fichtenbaum said. “Hydrazine thrusters cannot be fueled at the factory because of toxicity, handling and transportation [requirements], but the ECAPS propellant can,” he added. The Astranis agreement is one of the largest orders to date for ECAPS. Bradford also supplies ECAPS thrusters for San Francisco-based Planet’s SkySat Earth imaging constellation, Fichtenbaum said. A dozen satellites have used ECAPS propulsion systems in orbit and several more, including imaging satellites and U.S. government spacecraft, slated for launch later this year will employ the technology, according to the Bradford announcement.
Chinese space propulsion startup Space Pioneer has raised $14 million in funding for completing a series of next-generation liquid engines.
Space Pioneer, full name Beijing Tianbing Technology Co., Ltd., secured two previous funding rounds in 2019, one of which also included ZJU Joint Innovation Investment. The company, established in 2015, will use the funds to develop a 30-ton-thrust HCP liquid engine named Tianhuo-3. It aims to fully develop the engine and take it to the test stand this year. Igniter hot fire tests were performed late last year. Tianhuo series engines use a ‘next-generation’ green, ambient temperature propellant. A hot test of the Tianhuo-2 engine was conducted early 2020. A company press release states it aims to develop launch vehicles and space propulsion systems.
Benchmark Space Systems Merges Tech with Tesseract, Signs Green Propulsion Deal with Spaceflight
Benchmark Space Systems, an in-space propulsion systems supplier coming off of a recent capital fundraising round of more than $3 million, has just secured technology partnerships with Tesseract Space and Spaceflight Inc. Under a permanent licensing partnership, Benchmark will integrate Tesseract Space intellectual property, assets, and staff to expand the development, deployment and support of its non-toxic chemical propulsion solutions for global rideshare markets. Benchmark said the move would make the company more agile in meeting market demand for green, launch vehicle-agnostic thrusters and services supporting 1U CubeSats through ESPA-class (1-to-500kg small satellites), large lunar landers, and orbital transfer vehicles (OTVs).
“Getting into orbit quickly, affordably and safely is the top priority for small satellite missions looking to start generating revenue within days, not months of their launch into space. Benchmark Space Systems has the leading-edge non-toxic chemical propulsion and support solutions to make that happen,” said Tesseract Co-Founder Erik Franks, who will now join Benchmark’s leadership team. “Rideshare and OTV competition is a huge catalyst for propulsion market growth, and Benchmark is well ahead of the curve with required green solutions that far exceed the performance of electric propulsion.”
“By exclusively adding Benchmark’s reliable, green in-space propulsion systems to our portfolio of powered Sherpa vehicles, we can cost-effectively deliver our customers’ spacecraft to optimal orbits with greater precision, and then rapidly deorbit. That’s good for everyone – including the environment,” said Curt Blake, Spaceflight president and CEO. Benchmark’s new Halcyon propulsion system will debut aboard two small satellites scheduled to launch aboard a SpaceX Falcon 9 in December. Benchmark will also demonstrate its Starling thruster (formerly DFAST) aboard a 3U small satellite set to liftoff with Firefly Aerospace on the inaugural Alpha flight in October.
Phase Four wins DARPA contract to test novel propellant, reported in Feb 2022
Phase Four won a Defense Advanced Research Projects Agency contract to demonstrate the California startup’s radio frequency thruster technology with a new propellant.
Phase Four proved last year that its xenon-fueled Maxwell Block 1 engine performed as designed in orbit. Maxwell Block 1 is currently providing thrust for six small satellites. Another four are scheduled for launch by the end of the year.
Since the company was founded in 2015, Phase Four has tested thrusters with a number of different propellants including air, water, iodine and Advanced Spacecraft Energetic Non-Toxic, or ASCENT, a green propellant developed by the U.S. Air Force Research Laboratory.
With each new propellant, Phase Four conducts tests to compare its performance to RF thrusters with xenon propellant. Then, the company modifies “both the thruster and the feed system to optimize for performance,” Wallace said. “There are some tweaks for each propellant and certainly some unique characteristics depending on the type of propellant.” (Phase Four thrusters are designed to run on gas, liquid and solid fuels.)
Each propellant offers different benefits for satellite operators. Iodine, for example, sells for about one-tenth the price of xenon. “For customers who are intent on optimizing electric-propulsion performance, iodine basically provides the same performance as xenon at a much lower price point,” Jason Wallace, Phase Four vice president of advanced development said. In addition, Phase Four is investigating propellants that could be harvested in orbit, on planetary bodies or collected from other rocket engines.
With ASCENT, Phase Four seeks to show space vehicles could possess multiple propulsion modes, akin to gears on a car. Chemical propulsion would offer high thrust for rapid maneuvers like orbit changes and collision avoidance. Electric propulsion, meanwhile, would provide lower thrust but higher efficiency for orbit transfers that require significant changes in velocity and long duration station-keeping and phasing.
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