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In the history of spaceflight, almost all spacecraft have been manufactured and assembled on the ground, then integrated into a launch vehicle for delivery into orbit. This approach imposes significant limitations on the size, volume, and design of payloads that can be accommodated within the fairing of a single launch vehicle. In particular, fairing diameter limitations restrict the size and number of instruments that can be fielded in orbit for science and national security missions. Current manufacturing and technological limitations are evident in the construction of antennas and mirrors that have to be deployed from a single launch with a single satellite.
To overcome this challenge two technologies are being pursued and On-orbit assembly that refers to aggregation onto a platform of ready-made structures (that were manufactured either on the ground or on-orbit) and On-orbit manufacturing, which is the fabrication of structures (including 3D printing techniques),
On-orbit assembly can be defined as the aggregation onto an orbiting platform of ready-made structures that are manufactured on the ground (they could also be manufactured on-orbit for On-orbit manufacturing). Also known as In Space assembly ISA and defined as the assembly activities completed in the target orbit and extraterrestrial space (such as: Low Earth Orbit, Geostationary Orbit, Cis-Lunar Space, Mars Orbit, Mars Surface, Lunar Space and interstellar space), which is to assemble modules in space in order to form a larger functional element or to recombine one or more structures after separation.
In this process, these modules can be combined using their own power and propellant, or they can be assembled from separate spacecraft. According to the complexity of the assembly, the lowest complexity ISA task just involves mating between elements, i.e. two or more independent spacecraft are assembled into a larger space structure and assembled in space with the lowest complexity.
Nowadays, in-space propulsion is either chemical with storable propellant or electric with solar power. Solar energy does not work much beyond Mars and only in line-of-sight with the Sun. Chemical sources don’t work for very long as their energy density is too low and their weight is prohibitive on long missions.
Tomorrow, the chemical propulsion will use cryogenic propellant and electric propulsion will handle large power input. However, these two technologies are getting close to their physical limits, beyond which performance increase is impossible. To pass by these limits and enable the space logistics to a new league, a new type of propulsion shall be introduced: nuclear-based propulsion systems.
Nuclear propulsion would replace a similar thrust process in existing satellites and ships derived from chemical engines – but is able to go further, faster and with less fuel onboard. Militaries are also looking towards Nuclear thermal propulsion as it will enable maneuverability by providing much higher thrust and twice the propellant efficiency of legacy chemical systems.
While most of previous studies in Europe on nuclear propulsion were terminated without sequel, today’s technologies are making nuclear propulsion a plausible alternative to conventional propulsion systems. Nuclear propulsion can be multiple times more efficient than the most efficient chemical propulsion, or exceed the electric power limited by solar energy, thus enabling exploration where no other technology can go.
ESA in space propulsion studies
The ESA Future Space Transportation Systems department (STS-F) initiated two feasibility studies this year. These studies will identify key technologies to mature and intermediate steps to take to develop nuclear propulsion in Europe. Of course, the sine qua non conditions to use nuclear energy for propulsion is the robust implementation of the safety requirements from the early phases of the design. Following recommendations established in 2009 from the International Atomic Energy Agency (AIEA) and United Nation, the safety aspect is at the centre of the development of nuclear technologies for space applications.
Pulled by the ambitious ESA programmes such as Voyage2050, TerraNova, EL3, etc., as well as potential demanding private missions, the high engine efficiency and power generation of Nuclear Electric Propulsion (NEP) would enable exploration and in-space logistics in Earth Orbit and beyond on a scale that neither chemical nor electrical propulsion could ever provide. The ultimate raison d’être of NEP is to explore beyond Mars orbit where solar power is limited. To develop a tug for long term and distant exploration, a stepwise implementation could be considered with a subscale tug to in-orbit demonstrating the added value of NEP with less demanding missions. In a technology push approach, and in the anticipation for the increasing need of demanding in-space transportation, this study is a first step toward an in-orbit demonstrator of nuclear electric propulsion systems.
The main advantage over chemical reaction is the efficiency of the engines. The advantage over solar electric power input is the larger power output and independence of exposure to direct sunlight, especially enabler for transporting heavy cargo with long time constraints (Moon night management for example) and for exploration beyond Mars orbit. In addition, NEP could have strong synergies with other space application. For instance, nuclear power could be used on the Moon or Mars surface to power future habitats or robotic exploration of the solar system, or in space for other purpose than propulsion. On the propulsion side, the development of high-power electric thrusters for NEP could also be used with non-nuclear power input.
Project RocketRoll, or “pReliminary eurOpean reCKon on nuclEar elecTric pROpuLsion for space appLications”, will explore the advantages of using a Nuclear Electric Propulsion (NEP) tug over classical propulsion systems for the demanding missions expected for the future of space logistics and exploration. It will identify main safety features for the design and highlight contingency and mitigation measures. Moreover, the study will determine what are the existing and missing key elements (technologies, modelling capabilities, testing facilities, etc.) in Europe that shall be matured for a nuclear electric tug operational after 2035.
Pulled by the ambitious ESA programmes such as Voyage2050, TerraNova, EL3, etc., as well as potential demanding private missions, the high engine efficiency and power generation of Nuclear Electric Propulsion (NEP) would enable exploration and in-space logistics in Earth Orbit and beyond on a scale that neither chemical nor electrical propulsion could ever provide. The ultimate raison d’être of NEP is to explore beyond Mars orbit where solar power is limited. To develop a tug for long term and distant exploration, a stepwise implementation could be considered with a subscale tug to in-orbit demonstrating the added value of NEP with less demanding missions. In a technology push approach, and in the anticipation for the increasing need of demanding in-space transportation, this study is a first step toward an in-orbit demonstrator of nuclear electric propulsion systems.
The main advantage over chemical reaction is the efficiency of the engines. The advantage over solar electric power input is the larger power output and independence of exposure to direct sunlight, especially enabler for transporting heavy cargo with long time constraints (Moon night management for example) and for exploration beyond Mars orbit. In addition, NEP could have strong synergies with other space application. For instance, nuclear power could be used on the Moon or Mars surface to power future habitats or robotic exploration of the solar system, or in space for other purpose than propulsion. On the propulsion side, the development of high-power electric thrusters for NEP could also be used with non-nuclear power input.
Project RocketRoll, or “pReliminary eurOpean reCKon on nuclEar elecTric pROpuLsion for space appLications”, will explore the advantages of using a Nuclear Electric Propulsion (NEP) tug over classical propulsion systems for the demanding missions expected for the future of space logistics and exploration. It will identify main safety features for the design and highlight contingency and mitigation measures. Moreover, the study will determine what are the existing and missing key elements (technologies, modelling capabilities, testing facilities, etc.) in Europe that shall be matured for a nuclear electric tug operational after 2035.
Today the ambition to power an upper stage with nuclear propulsion ruled out, as the risk to have any Earth contamination of radioactive material would be too high. However, the idea of “assembling” (or at least, starting) a nuclear engine in orbit, for in-space transportation only, is resurfacing. In practice, Nuclear Thermal Propulsion (NTP) is using fission reaction to heat up, and thus accelerate, a coolant (liquid hydrogen) with high velocity. The main advantage over chemical reaction is that the heating process does not need any oxidiser, thus boosting the thrust efficiency in maintaining a lower density of the exhaust gas. For a 10-ton thrust class engine, the engine efficiency is expected to be 2-3 times larger than highly efficient cryogenic chemical propulsion.
Project ALUMNI, or “preliminAry eLements on nUclear therMal propulsioN for space applIcations” (ALUMNI), will explore the advantages of using a Nuclear Thermal Propulsion (NTP) over classical propulsion systems and highlight its potential profitability. More importantly, it will assess the key elements (technologies, modelling capabilities, testing facilities, etc.) and safety design features to make NTP possible.
References and Resources also include:
https://commercialisation.esa.int/2022/09/space-transportation-and-nuclear-propulsion/
