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Most spacecraft propulsion concepts can be classified into two categories: chemical and electric. Chemical propulsion relies on chemical reactions and can produce high thrust, but requires a large amount of fuel. Electric propulsion uses electromagnetic fields to accelerate ionized gases and is very fuel-efficient, but produces small amounts of thrust and thus long trip times. Researchers have been developing propulsion systems with both chemical and electric modes available.
MULTIMODE in-space propulsion is the integration of two or more propulsive modes into a single spacecraft propulsion system. In a multimode propulsion system, the key attribute is shared propellant between the different propulsive modes. The multimode propulsion concept is in contrast with ‘hybrid propulsion’ wherein two or more propulsive modes are available on a spacecraft but do not share propellant. The literature on multimode propulsion also uses the terms ‘dual mode’ and ‘bimodal’ propulsion, which inherently means a multimode system with two possible propulsive modes.
In the case of hybrid propulsion, they are separate and independent systems. Sharing propellant between different propulsive modes in multimode propulsion has significant benefits in terms of enabling new missions and, perhaps more importantly, in situ mission adaptability. Maximum mission adaptability and flexibility is attained by multimode systems because they do not require pre-allocation of propellant to a specific mode of operation. Multimode systems that are entirely chemical, entirely electric, and a combination of chemical and electric propulsion have been investigated.
Fig. shows seven different propulsion system combinations commonly employed on a spacecraft. System 1 consists of completely separate chemical monopropellant (hydrazine) and electric Hall-effect thruster (HET) (xenon) propulsion, and is classified as hybrid propulsion because there is no shared propellant between the two propulsive modes. Hybrid propulsion is quite common on contemporary spacecraft. Deep Space One and DAWN planetary spacecraft used xenon gridded ion engines plus hydrazine monopropellants for roll control. Lockheed Martin A2100 spacecraft use xenon XR-5 HETs, hydrazine monopropellants, and hydrazine MR-510 arcjets. Space Systems Loral has a similar system with HETs. Boeing uses a similar system with the xenon ion propulsion system (XIPS) gridded ion engines.
System 2 in Fig. is an illustration of an all-chemical partially multimode propulsion system. It consists of monopropellant and bipropellant chemical rocket engines. A common hydrazine (N2H4) fuel tank feeds both engines, while the nitrogen tetroxide (N2O4, NTO) serves as oxidizer in the bipropellant engine. This system is only partially multimode because fuel and oxidizer must be pre-allocated for specific mission requirements. BepiColombo mission, a 4100 kg spacecraft launched in 2018 to study Mercury, uses this type of dual mode propulsion system consisting of 22 N NTO/hydrazine bipropellant thrusters for Mercury orbit insertion and 5 N hydrazine monopropellant thrusters for attitude control
Systems 3 and 4 in Fig. illustrate two different all-electric multimode systems. These systems are adjustable between a high specific impulse mode and what is often referred to as a high thrust-to-power (and correspondingly lower specific impulse) mode. System 3 is a xenon HET system and system 4 is an electrospray propulsion system. Additionally, dual mode ion thrusters and hybrid Hall-ion thrusters have also been investigated.
Systems 5, 6, and 7 in Fig. are multimode propulsion systems that integrate chemical and electric propulsion. Systems 5 and 6 are examples of separate chemical and electric propulsion thrusters that share a common propellant, while system 7 illustrates a multimode system with shared propellant and a common thruster.
Multimode propulsion advantages
Sharing propellant between different propulsive modes in multimode propulsion has significant benefits in terms of enabling new missions not achievable by chemical or electric propulsion alone, many different types of maneuvers. More importantly, it enables in situ mission adaptability allowing significant mission changes on-orbit.
Maximum mission adaptability and flexibility is attained by multimode systems because they do not require pre-allocation of propellant to a specific mode of operation. Multimode propulsion has the potential to provide unprecedented flexibility and adaptability to spacecraft as a direct result of shared propellant, and can provide mass savings for certain missions. These benefits can apply regardless of spacecraft size. Additional mass savings may be realized by sharing thruster hardware between modes, especially for small satellites
Multimodal propulsion application areas
Multimode propulsion is the integration of two or more propulsive modes with shared propellant into a single spacecraft propulsion system.
Deep Space Missions
Multimode propulsion is emerging as an enabling technology that promises enhanced capabilities for spacecraft and space missions, and can therefore play an important role in the future of in-space propulsion. Lunar and subsequent inner solar system industrial growth becomes feasible with lunar oxygen use in the recommended, multimodal, space nuclear propulsion and power system that provides the high specific impulse, system flexibility, life, and reliability needed to propel and power early spacecraft and outposts as well as future vehicles in more demanding mission architectures.
Warship
The Office of Naval Research (ONR) planned a novel craft envisioned to have three modes of operation: Fuel-efficient, good sea keeping mode for open ocean transits, High-speed, shallow water mode, Amphibious mode to enable “feet dry on the beach” capability The disparate operating modes impose discrete requirements on the various elements of the ship propulsion system, opening a wide range of potential solutions.
Satellite Constellations
With the advent of microelectromechanical systems (MEMS), reduced satellite size and mass has led to lower launch costs, enabling the use of satellite constellations and space-based educational platforms . Although the benefit of this phase of satellite development is clear through their contribution to human welfare and the global economy, the increased quantity of orbital vehicles inevitably leads to more debris. Furthermore, most methods with which to de-orbit satellites have not scaled down with the same efficiency as other subcomponents of the spacecraft bus.
Conventionally, Δv is achieved using chemical or cold gas thrusters providing a relatively high thrust and low specific impulse (Isp) compared with their electric counterparts. Devices of the latter type produce higher specific impulse through the addition of electrical energy to heat up the propellant, accelerate ions from plasma or a liquid, or some combination of both. The use of electric propulsion also has limitations; for example, a three-unit CubeSat measuring 10 cm×10 cm×34 cm is typically restricted 10 W of power production for finite periods of time assuming a body-mounted solar panel configuration.
The use of multimodal space propulsion has the benefit of increasing satellite mission flexibility with the ultimate goal of decreasing mission development costs. Researchers have explored the development of a dual-mode thruster for small spacecraft. By combining an electrospray within the confines of a cold gas nozzle, the fabricated and tested propulsive device can operate either in a high thrust or high specific impulse mode.
Unmanned aerial vehicle
NASA’s Langley Research Center has developed an unmanned aerial vehicle concept for long-range, distributed aerial presence and delivery applications. This aircraft concept is capable of delivering multiple independent payloads over a range of more than 60 miles. The aircraft concept consists of two sub-aircraft: a “mother” vehicle and several “children” vehicles. The mother and children vehicles may operate both independently of one another and as a composite assembly. When not assembled, the mother and children vehicles rely on embedded ducted rotors for vertical lift and general propulsion.
Basic operation is as follows:
The assembled vehicle takes off either vertically under the power of the ducted propellers in a vertical takeoff and landing (VTOL) operation or conventionally utilizing the propulsion system(s) of the mother (and potentially children) vehicles.
If performing VTOL operations, the vehicle transitions to forward flight after achieving a safe height.
When at the primary rendezvous point, the assembled vehicle transitions to a hover position. Once in a hover, the children vehicles undock from the assembled vehicle.
The children vehicles fly to independent locations for delivery, data collection, search-and-rescue operations, or science mission tasks. They then return to the primary rendezvous point.
The vehicles dock to form the assembled vehicle.
The vehicle transitions to forward flight and flies back to the takeoff location.
DARPA launched LongShot program in 2021 to develop and flight demonstrate a weapon system using multi-mode propulsion. According to DARPA, the agency seeks to develop an air vehicle that can be deployed from existing fighter jets or bombers and can carry air-to-air missiles (AAMs) to effectively engage multiple adversary air threats at longer ranges.
Aurora Propulsion Technologies and Aliena releases a multi-modal all-electric propulsion system for advanced in-orbit mobility, reported in Feb 2022
Aurora Propulsion Technologies (“Aurora”) and Aliena Pte Ltd (“Aliena”) have released a multi-modal all-electric satellite propulsion system catered for emerging small satellite missions.
The joint-product, a Multi-modal Electric Propulsion Engine (MEPE), is designed to be a turn-key solution for in-orbit propulsion, and features Aliena’s MUSIC Hall thrusters, and Aurora’s ARM Resistojets. MEPE brings along with it, elegance in design and a compact form-factor for simplicity of integration and smoothening of regulatory protocols for the planning and execution of operations from assembly, launch, to in-orbit deployment. A shared propellant source and fluidic regulation system feeds inert propellant to both thruster units, providing volume savings as well as plug-and-play capabilities for dual-mode propulsion.
Conventionally, small satellite operators are faced with the prospects of making compromises in the selection of their propulsion systems for their missions, given that smaller platforms do not have the capacity to carry onboard multiple propulsion systems. This results in either longer times for operations to be performed in-orbit, or the requirements to carry onboard larger quantities of propellant for the planned mission duration. With the release of MEPE, satellite operators now have access to high thrust resistojets and high specific impulse hall thrusters, giving multi-mission flexibility for the most sophisticated of operations to be performed in a timely and efficient manner in space.
MEPE will provide unprecedented agility for microsatellite missions, and come with an attractive price point and accelerated delivery lead times, catering specifically to emerging small satellite constellations. With this, satellite operators can focus on what matters most to them – running their operations in space in the most effective manner. Additionally, MEPE leverages on the engineering expertise of 2 international teams, providing absolute scrutiny over design and quality control over the systems to be delivered to customers in space.
Aurora, a Finland based company, offers a unique portfolio of small satellite propulsion devices, including Resistojet thrusters, plasma brakes and the upcoming E-sail for deep space missions. Auroras Resistojets offer a compelling thrust to power ratio, with up to 5mN of thrust, excellent control and quick response times. This makes the thrusters optimal for spacecraft attitude control, collision avoidance manoeuvres and for example docking operations in space. The Plasma Brakes enable deorbiting satellites at the end of their useful lifetime ensuring future generations a clean space. In addition to the products Aurora also offers manufacturing services of space grade high precision mechanical components as well as services in space craft and mission design from its home base in Finland.
Aliena is a Singapore based space tech company that focuses on the design and development of advanced low power Hall thrusters that would enable their microsatellite imaging constellation to fly at lower altitudes, resulting in high-resolution images from space, allowing for advanced analytics to empower business decisions on Earth. Their propulsion product line includes the in-house developed MUlti-Staged Ignition Compact (MUSIC) Hall thrusters in various neutralization modes that can be employed in a modular fashion to meet the power and thrust requirements of various customers and different platforms. MUSIC can be operated in “self-ignition” mode without the need of a hot cathode, giving it the ability to ignite on demand, or with a hollow cathode for a boost in thrust and specific impulse. The “self-ignition” mode was first deployed on the smallest satellite (3U) to carry a Hall thruster in 2022, and demonstrates the future of mobility through Hall thruster technology.
The companies are currently taking orders for MEPE, and the 1st in-orbit deployment of the system has been scheduled in 2023.
Multi-Mode propulsion enables new mission capabilities for the commercial in-space mobility market, reported in June 2022
Phase Four, the creator of the radio-frequency thruster (RF Thruster) for satellite propulsion, announced today that it has signed a memorandum of understanding with Impulse Space, a Space 2.0 pioneer providing agile, economical last-mile space payload delivery capabilities. The companies seek to explore and collaborate on opportunities to develop and market multi-mode propulsion capabilities that may be employed by future commercial space vehicles. The companies will evaluate the performance of Phase Four’s propellant-agnostic RF Thruster on green propellants of interest.
“Multi-mode propulsion systems are compelling because they allow a space vehicle to have two vastly different thrust and efficiency operating points, but leverage a single propellant supply, saving mass and cost,” said Phase Four CEO, Beau Jarvis. “We are excited to work with the Impulse Space team to bring this novel propulsion capability to market and support the Impulse mission of developing vehicles that are capable of delivering multiple payloads to unique orbits from a single launch.”
Phase Four’s Maxwell Block 1 engine gained flight heritage in 2021 and is currently operating on several commercial small satellites. The company has begun production of its Block 2 Maxwell engine. Block 2 utilizes the company’s second generation RF Thruster and offers significant performance improvement over the Block 1 engine. Phase Four is also working to adapt its RF Thruster to operate on nontraditional propellants that are less expensive and more readily obtained than purified noble gasses like xenon and krypton. The company recently won a Space Force Pitch Day award to test on green propellant.
“The space vehicles in development by Impulse Space address missions for all orbits and will benefit considerably by having the added capability of the Phase Four’s Maxwell engine,” said Tom Mueller, CEO of Impulse Space. “As we focus on offering the broadest breadth of missions for our customers, we are eager to extend the efficiency of propulsion systems to include Phase Four’s products, ultimately enhancing those missions.”
Phase Four will perform exploratory performance testing with its RF Thruster operating on propellants of interest to Impulse Space. The cooperative effort may also include an in-space multi-mode capability demonstration with an Impulse Space vehicle.
References and Resources also include:
https://www.sciencedirect.com/science/article/pii/S0376042120300397
http://spaceref.com/news/viewpr.html?pid=60414
