The space tug would fly for several missions between our planet and the moon, serving as a reusable transportation system. To keep it going, fuel will be needed from a future fuel depot in low Earth orbit, and maintenance would be performed by astronauts at the moon and (if it still exists) the International Space Station, which at the moment is expected to be retired in 2024.
Mammarella said the tug’s big advantage comes from using electric propulsion, specifically from a type known as Hall Effect Thrusters. These kinds of thrusters transform neutral atoms into propellant using electric and magnetic fields. The propellant is made up of charged atoms that are called ions. The ions are then boosted to give the spacecraft thrust. Examples of electrically powered spacecraft currently in use include NASA’s Dawn spacecraft (which explored Ceres and Vesta) and Japan’s Hayabusa 2 (which is on its way to asteroid 1999 JU3).
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Compared with traditional chemical propulsion, Mammarella said, the Lunar Space Tug’s electric propulsion would save fuel and also have fewer energy limitations. The tradeoff is that this kind of propulsion generally takes longer to bring spacecraft from place to place, as its thrust is low. Mammarella pointed out there is no particular hurry to move the cargo since the spacecraft is uncrewed, meaning there is no urgency in terms of the consumables humans need, such as oxygen or water.
SLS-Orion, by contrast, “adopts chemical propulsion and the development cost of both the system is very high, mainly because the launch vehicle,” Mammaralla said. “On the other hand, the LST is able to transfer the same habitat module and it is designed considering the adoption and support of existing ground and launch infrastructures. It can be launched using the actual launchers available, reducing the development cost for the new concept. It means transferring an habitat module from Earth to the cislunar station should cost less, even if the transfer duration is much longer.”
The researchers plan a second phase of the project to improve the performance of the space tug, mainly by reducing the transfer time and the fuel performance. The team also plans to do an analysis of the best transfer trajectories, and the best ways for the tug to reach the station.
“Further analyses will consider in detail also fuel and maintenance operations,” Mammaralla said, “and all the technical implication of adopting electric propulsion.”
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