ESA develops logistics vehicle for cis-lunar outpost
A dramatic re-write of the assembly schedule for the Deep Space Gateway at the beginning of 2017 has put severe mass restrictions on the first component of the cis-lunar outpost. To resolve the problem, the international team of experts considered various solutions, including in-flight refueling. According to European engineers, a newly designed robotic vehicle could save the day by taking some of the tasks of the original Power and Propulsion Element.
Initial concept of the ESPRIT module, as of September 2017.
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Known specifications of the ESPRIT module (a.k.a. LCUB) as of middle of 2017:
The project status in the first half of 2017
The most obvious solution to the lack of propellant on the Power and Propulsion Bus, PPB, of the cis-lunar station would be in-flight refueling, because it could dramatically extend the service life and capabilities of the future outpost.
In-flight refueling has been used very successfully on the Soviet Salyut and Mir space stations and on the Russian segment of the International Space Station, ISS. However the latest proposal to employ the same procedure on the Deep Space Gateway could be problematic. The new outpost's reliance on electric thrusters powered by xenon means that the transfer of such a propellant from a tanker to the station would have to be a rather slow affair to prevent the liquefied xenon from heating up. In fact, it could take up to several months to pump a few hundreds kilograms of xenon. Otherwise, the tanks and propellant lines would have to be equipped with a special refrigeration system, hugely taxing the already mass-constrained system.
As an alternative to refueling the PPB, engineers working for the European Space Agency, ESA, proposed to split the module into the two independent sections. According to one of such scenarios, the first vehicle would be the propulsion module based on NASA's spacecraft previously planned for the Asteroid Retrieval Mission, ARM. It would be upgraded with the European electric engine known as ESA Hall Effect Auxiliary Thruster or eHEAT and loaded with a maximum of xenon propellant reaching 2.2 tons.
At the same time, additional tasks previously assigned to the PPB module would be "off-loaded" to the newly proposed Logistics Communication and Utilization Bay, LCUB. It could take over a Canadian-built robotic arm and a small airlock for scientific experiments, as well as a communications gear. Additionally, it could also feature an observation window in its airlock, which would be accessible by the inhabitants of the Deep Space Gateway via a standard docking port. This component would be built light enough to be launched on a standard rocket such as Europe's prospective Ariane-6. It would lift off from Earth around nine months after the EM-2 mission deploying the "abbreviated" PPB bus.
A space tug would have to be developed to boost LCUB from its initial orbit and dock it to the Deep Space Gateway in the cis-lunar space. Ultimately, the LCUB would end up permanently attached to the cis-lunar station between the PPB module and the habitation module, which would arrive with the Orion crew vehicle during its EM-3 mission.
The LCUB module could still serve as a tanker carrying up to 1,100 kilograms of propellant. It could employ traditional refueling methods, if they are proven workable for the transfer of propellant. Alternatively, it could feature replaceable "plug-in" propellant tanks to avoid pumping the capricious xenon between modules.
However, if the refueling was deemed unnecessary after all, the LCUB could feature a larger airlock, which could also double as a pressurized cargo compartment on the way from Earth to the cis-lunar space. Measuring three meters in length and two meters in diameter, this version of the LCUB module could provide up to eight cubic meters of pressurized volume and accommodate up to 1,070 kilograms of supplies.
Engineers at Airbus Defense and Space, Thales Alenia Space and OHB System working on the issue for ESA have also conceived a number of other arrangements and architectures aimed at splitting various components of the original PPB module.
The idea to split the Power and Propulsion Bus was presented at the May 2017 meeting in Montreal and the partners agreed to weigh pros and cons of the various alternatives this option could provide.
Around middle of 2017, the concept of the LCUB module was apparently re-christened ESPRIT, for European System Providing Refueling Infrastructure and Telecommunications. As its name implied, the spacecraft would carry xenon and hydrazine tanks, which most likely would be directly plugged into the propulsion system of the power and propulsion module. Still, the capability to refuel tanks on a docked spacecraft could also be tested on ESPRIT, because it could be later needed for the Deep Space Transport, DST, designed to take astronauts toward Mars.
ESPRIT would also carry a communications antenna specifically designed to maintain contact with assets on the lunar surface and a separate S-band radio for between-spacecraft communications. Finally, a high-data rate channel for communications with the Earth would also be installed, featuring a capability for using the ESPRIT module as a relay station between ground control and hardware on the lunar surface.
On its side facing the power and propulsion module, ESPRIT would have an "active" docking port allowing pumping propellant through it. On the opposite end, facing the habitation module, ESPRIT would have a "passive" port. In addition, the module could get a third docking port for temporary storage of cargo, but without capability to refuel it. Engineers also considered using the exterior of the module to install a special pallet for payloads, as well as cameras and other sensors.
In order to maneuver the module to the cis-lunar station, ESPRIT would be launched along with a space tug based on the Japanese HTV vehicle or provided by one of the private contractors. Depending on the chosen arrangement, the liftoff mass of the entire stack was estimated to be between 6,000 and 7,400 kilograms, requiring either the Atlas-5 or the Ariane-64 rocket respectively.
The ESPRIT module could fly before or after the launch of the cis-lunar habitation module on the Orion spacecraft on the Exploration Mission-3, EM-3, in 2023 or 2024. If ESPRIT follows the EM-3 mission, Orion would be used to undock the hab from the power and propulsion module in the lunar orbit, let ESPRIT dock in its place and after discarding ESPRIT's space tug, Orion would re-dock the hab to ESPRIT...
By April 2018, the brand new Falcon Heavy rocket became another potential carrier for delivering the ESPRIT module to the near-lunar station in 2024, at the earliest. Launching ESPRIT on the SpaceX' rocket would enable the module to carry more propellant, with the module reaching a total mass of between five and six tons. In addition to supporting its host station, European engineers also considered the available propellant cache aboard ESPRIT for refueling visiting lunar lander or its reusable ascent stage.
In parallel, the Japanese space agency, JAXA, studied a possible experimental optical communications system, which could be installed on the ESPRIT. Finally, the module could also carry power systems supplied by NASA and special connectors for robotic systems provided by the Canadian space agency, CSA.
By that time, engineers still considered hexagonal and cubical architectures for the future module. Its length was expected to reach 3.91 meters along the main axis of the LOP-G complex.
Beginning in May 2018, two parallel developments, known as Phase A and B1 were scheduled to begin, further advancing the design of the ESPRIT module. They would bring the project to its major milestone, known as System Requirements Review, SRR, which could come as early as June 2019. The partners planned that further development of the ESPRIT module would be conducted under contracts with the industry awarded on a competitive basis...
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The LCUB module would be equipped with a pair of International Berthing and Docking Mechanisms, IBDM, enabling its docking with the PPB module on one end and the Hab module on the other. Credit: QineticQ via Claude Mourier
The LCUB vehicle could be launched on a standard rocket, such as Ariane-64. Credit: ESA
As of 2017, the Atlas rocket was also considered as a potential launcher for the LCUB/ESPRIT element. Credit: ULA
By April 2018, Falcon Heavy was on the list of potential carriers of the ESPRIT module. Credit: SpaceX
One of two proposed designs of the ESPRIT module as of April 2018. Credit: NASA