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The United States Department of Defense is pioneering a transformative new approach to satellite propulsion with the DARPA OTTER program, which seeks to unlock the future of space operations through innovative “air-breathing” electric propulsion systems. These technologies are specifically designed to support operations in Very Low Earth Orbit (VLEO) – altitudes as low as 90 to 450 kilometers – where traditional propulsion systems face extreme challenges due to the rapid decay of altitude caused by atmospheric drag and Earth’s gravity.
Overcoming VLEO Challenges
VLEO presents unique operational hurdles for satellites. Unlike higher orbits like Low Earth Orbit (LEO), objects in VLEO experience substantial atmospheric drag, which means their altitude decays quickly without continuous propulsion. This creates a need for constant thrust, making conventional satellite propulsion methods impractical due to fuel limitations. Traditional propulsion systems rely on finite propellants, which are eventually exhausted, leading to satellite deorbiting. The OTTER program seeks to overcome this limitation by utilizing a virtually unlimited source of propellant: the ambient air in VLEO.
“Air-Breathing” Electric Propulsion Systems
At the core of the OTTER program is the development of “air-breathing” electric propulsion (EP) systems that can harvest low-density air from the surrounding environment in VLEO. Once this ambient air is captured, it can be ionized and accelerated to produce thrust, providing a continuous and sustainable propulsion method. The primary innovation here is that this system uses the environment’s natural resources, eliminating the need for traditional fuel sources and enabling satellites to operate for much longer periods without running out of propellant.
The harvesting of low-density air, combined with the ability to ionize and accelerate this air, is a game changer for VLEO missions. It allows for continuous, sustainable propulsion without the typical fuel limitations of standard propulsion systems.
One of the most exciting technologies being developed under the OTTER program is Phase Four’s radio-frequency thruster (RFT). This unique thruster is propellant-agnostic, meaning it can function effectively with non-traditional propellants, including the ambient air found in VLEO. Traditional propulsion systems often degrade when exposed to non-ideal propellants, but the RFT is designed to withstand these conditions, making it ideal for long-duration missions in space. With the ability to operate on low-density air, this technology promises to significantly extend satellite operational lifespans in challenging environments like VLEO.
Advanced Propulsion Inlet Technologies
Another key component of the Otter program is the development of inlet technologies. These are crucial for efficiently collecting and funneling ambient air into the RFT system for ionization and propulsion. The inlets must be designed to capture air at extremely low densities while maintaining high efficiency to ensure that the system can produce sufficient thrust without excessive energy consumption.
In addition, these inlets will be tested and optimized for operation at the high speeds and extreme temperatures typical in VLEO. They must be able to withstand the harsh conditions in orbit while maintaining optimal performance, making this technology a cornerstone of the Otter program.
The RFT is designed to be mass-manufacturable and capable of highly efficient operation, which is crucial for small satellite missions. This makes it an ideal solution for low-cost, sustainable propulsion for space applications where traditional propulsion systems might not be effective due to limited resources.
Phase Four’s RFT technology is part of a larger initiative to develop propulsion systems that are more cost-effective and adaptable to a variety of propellants, making it suitable for VLEO environments. With a focus on mass-manufacturability and miniaturization, Phase Four is driving the future of satellite propulsion. The company’s innovative approach to space propulsion was demonstrated through its previous collaboration with DARPA, which developed a proof-of-concept for novel low-cost propellants.
DARPA Awards Driving Innovation
The OTTER program is part of DARPA’s broader effort to advance space propulsion technologies and ensure the U.S. maintains technological supremacy in space. Phase Four, a leading provider of next generation electric propulsion (EP) solutions for satellites, was awarded a $14.9M contract by the Defense Advanced Research Projects Agency (DARPA) to deliver an “air-breathing” EP system to enable extended satellite operations in Very Low Earth Orbit (VLEO), at altitudes as low as 90-450km. DARPA’s Otter program will leverage Phase Four’s innovative core technology, the radio frequency thruster (RFT), which is inherently propellant agnostic compared to legacy electric propulsion systems which rapidly degrade when exposed to non-traditional propellants. This feature uniquely enables the RFT to operate on the low-density air harvested in the VLEO environment. The Phase Four RFT represents a revolutionary new architecture that realizes lower cost, mass-manufacturability, miniaturized power electronics, and propellant agnosticism over incumbent technologies, without compromising performance.
The Otter program will build upon a previous effort with DARPA that developed a proof-of-concept thruster prototype for novel, low-cost propellants. Phase Four’s Maxwell Block 1 engine gained flight heritage in 2021 and is currently operating on several commercial small satellites. The company recently introduced its second generation RF Thruster, which revealed significant performance improvements over the first generation thruster. Phase Four has demonstrated the ability of its RF Thruster to operate on a variety of nontraditional propellants, including water, oxygen-nitrogen mixtures and isopropyl alcohol. The company is currently performing thrust measurements with its Maxwell engine adapted to operate on iodine on an Air Force Research Laboratory (AFRL) sponsored program, and recently won a Space Force Pitch Day award to test on green propellant.
Real-World Testing: The “Orbiting Wind Tunnel”
To prove the viability of these air-breathing propulsion systems, the OTTER program will culminate in an ambitious spaceflight demonstration that will act as an “orbiting wind tunnel”. This will involve the long-duration testing of propulsion systems in actual space conditions, enabling engineers to collect crucial on-orbit data about performance, durability, and efficiency. By comparing the results from this demonstration with ground-based simulations, DARPA aims to validate the technology’s ability to harvest ambient air and provide sufficient thrust for sustained operations in VLEO.
This innovative demonstration, which will last for over a year, is a critical step in characterizing the performance of these propulsion systems and refining them for future satellite missions. The data gathered from the demonstration will be invaluable for improving satellite designs, enhancing mission success, and enabling longer-term operations without the need for constant resupply of propellant.
In addition to Phase Four’s contributions, other DARPA awards are supporting various aspects of space exploration and propulsion, including research into new thruster technologies and sustainable energy systems. These efforts focus on making space operations more efficient, affordable, and reliable, while also paving the way for future deep-space exploration.
In addition to Phase Four, other contractors are also working on technologies under the DARPA Otter program to enable extended satellite operations in Very Low Earth Orbit (VLEO). One of the key players is Redwire, which has been selected to integrate “air-breathing” electric propulsion systems into its modular satellite design, SabreSat. This system aims to operate efficiently at altitudes between 90 and 250 kilometers, potentially extending to 450 kilometers, offering a new operational space beyond traditional orbits like Low Earth Orbit (LEO) and Geostationary Orbit (GEO).
Other contractors, including Electric Propulsion Laboratory and Phase Four, are focusing on developing propulsion systems capable of operating in the unique conditions of VLEO, where atmospheric drag and degradation from atomic oxygen are significant challenges. These technologies aim to support long-duration missions and rapid re-entry capabilities, providing new flexibility for satellite operations, especially in contested or congested space environments.
The technologies being developed under these initiatives are critical for enhancing the agility and resilience of satellite constellations, particularly for defense and national security applications. The closer proximity of VLEO satellites to Earth also introduces new opportunities for overcoming interception risks in sensitive communication channels. This innovation is particularly relevant in an era where the space domain is becoming increasingly contested and requires advanced capabilities to secure critical operational environments.
Redwire Awarded DARPA Prime Contract for SabreSat Spacecraft in Very Low-Earth Orbit
Redwire Corporation has secured a prime contract from the Defense Advanced Research Projects Agency (DARPA) to develop an air-breathing satellite under the Otter program, utilizing its SabreSat Very Low-Earth Orbit (VLEO) platform. This mission aims to demonstrate novel electric propulsion technologies capable of sustaining satellite operations in VLEO, a region below traditional low-Earth orbit that offers unique advantages for national security applications.
As the mission systems integrator, Redwire will develop the SabreSat spacecraft bus and integrate key technologies essential for sustained operation in this challenging environment. The SabreSat platform is designed as a modular orbital drone capable of supporting various mission profiles, including high-resolution intelligence, surveillance, and reconnaissance (ISR); resilient communications; navigation; and Earth science. The mission will leverage proprietary digital engineering tools, utilizing high-fidelity physics and atmospheric models to address the challenges of operating in VLEO, such as atmospheric drag and material degradation due to atomic oxygen exposure.
Operating in VLEO provides strategic benefits, including enhanced imaging capabilities due to closer proximity to Earth, reduced signal latency, and lower orbital congestion. Additionally, satellites in this orbit experience natural deorbiting within a short timeframe, mitigating the long-term accumulation of space debris. The SabreSat platform is designed to maximize these benefits, offering improved endurance and cost-effectiveness for future space operations.
This contract highlights Redwire’s commitment to advancing VLEO capabilities and supporting critical defense initiatives. By integrating advanced propulsion technologies and digital engineering practices, the company aims to demonstrate the feasibility of sustained operations in VLEO, potentially shaping the next generation of space-based intelligence and defense systems.
Impact on Future Space Missions
The OTTER program represents a significant leap forward in space propulsion technology. It will pave the way for satellites to operate in VLEO for extended periods, which is increasingly important as the space environment becomes more congested and contested. As traditional orbits become overcrowded with satellites, the ability to operate in lower altitudes will provide new opportunities for secure, sustainable, and resilient space missions. This will be particularly beneficial for national security, commercial enterprises, and scientific research, as it reduces dependence on costly fuel and provides a reliable way to maintain satellite position in space.
Cybersecurity Considerations in VLEO
From a cybersecurity perspective, the OTTER program’s focus on VLEO presents several unique risks. The proximity of SabreSat to Earth means that a compromised propulsion system could lead to swift mission termination due to rapid atmospheric reentry. Unlike higher altitudes, where satellites can maintain a certain level of stability, the low altitudes of VLEO provide little margin for error. If the propulsion system is compromised or disabled, the satellite could quickly lose altitude and reenter the atmosphere, resulting in mission failure.
Moreover, the reliance on relay satellites for communication is another vulnerability. Given the low altitude of SabreSat, it’s likely that signals will need to be bounced off satellites in Geostationary Orbit (GEO) for extended communication capabilities. If the relay system is targeted by a cyberattack, adversaries could intercept, manipulate, or block the communication between SabreSat and its ground stations. This could have devastating consequences, especially if the attackers gain control over the relay satellite system, potentially redirecting or blocking communications.
Increased Surface Area for Cyber Threats
Another cybersecurity challenge stems from the development process itself. Since the propulsion system is being developed by different companies, there is a larger “surface area” for potential cyber attacks. With data flowing between organizations, there are more opportunities for cybercriminals to exploit vulnerabilities. Each organization may have different levels of security, and a targeted attack on the less-secure party could allow adversaries to gain access to more sensitive systems.
The Potential for “Man-in-the-Middle” Attacks
One last concern for cybersecurity in VLEO is the possibility of man-in-the-middle (MITM) attacks. While difficult to execute, the low altitude of VLEO satellites means they could potentially intercept or block communications between higher-orbiting satellites. In a MITM attack, an adversary could insert themselves between two communicating parties, intercepting and possibly altering the information being exchanged. Though this kind of attack is unlikely, it remains a theoretical risk, especially if nation-state actors pursue the creation of their own VLEO constellations for military or strategic purposes.
The Road Ahead: Pioneering a New Era of Satellite Operations
As Redwire and its collaborators advance the OTTER program, the impact on future space missions is expected to be transformative. Operating in Very Low Earth Orbit (VLEO) using air-breathing electric propulsion systems represents a groundbreaking shift in satellite technology. This innovation promises to significantly reduce operational costs, extend mission lifespans, and enhance the flexibility of satellite deployments.
These propulsion systems, which utilize ambient atmospheric air instead of finite traditional propellants, are a paradigm shift toward sustainable and cost-effective space missions. The data gathered through OTTER will validate these propulsion technologies, offering critical insights for designing future VLEO satellite configurations. Additionally, the program will refine modeling and simulation tools, strengthening the foundation for next-generation spacecraft engineering.
As operations in VLEO expand, so do the associated cybersecurity risks. The proximity to Earth makes VLEO missions more vulnerable to hacking attempts that could disrupt propulsion systems or communication links. The reduced margin for error due to rapid orbital decay in VLEO underscores the need for robust cybersecurity measures. Redwire and its partners must prioritize developing and implementing resilient security protocols to safeguard mission success.
Shaping the Future of Space Exploration
The OTTER program’s successful implementation will significantly bolster the U.S. national security infrastructure, providing sustained, reliable operations in low Earth orbit. This milestone will not only strengthen the United States’ position as a leader in space exploration but also enhance its defense capabilities.
As the program progresses, it will serve as a testbed for cutting-edge propulsion technologies while addressing the cybersecurity challenges of the modern space era. The success of SabreSat and its pioneering propulsion systems will mark a defining moment in satellite operations, setting the stage for a future where space missions are more efficient, sustainable, and resilient than ever before.
