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 with shared propellant. 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 mean 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.
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.
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.
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