Lunar Missions and CubeSat Propulsion

Lunar Missions and CubeSat Propulsion

Moon is Earth's biggest and only natural satellite and has always drawn humanity’s attention, inspiring artists and scientists alike. It has also been at the center of the Space Race and the interest in it has been renewed with the Artemis program and preparations for construction of the Gateway space station. Lunar missions can take many forms, from deployment directly in Lunar orbit to deployment in orbit of Earth and powered flight to the Moon.

There are three main options for spacecraft to get to Lunar orbit. The most straightforward is to be deployed directly to the Lunar orbit. This is the costliest option, but it also simplifies the overall planning of the mission. It can also simplify the design of the spacecraft as there are way fewer worries about radiation from Van Allen radiation belts and about performing complicated departure and capture maneuvers. But it is important to remember this does not eliminate the need for a propulsion system and all that it entails, since the new ESA guidelines for space debris mitigation are valid even in Lunar orbit, from where the spacecraft nearing the end of their lifespan need to be sent into either heliocentric orbit or on an impact course with the Moon. But while this severely lowers the upside of having your spacecraft delivered all the way to the final destination, it is also one of the fastest ways to achieve the target orbit and make the spacecraft operational. And with the hard upper limit on the lifespan of CubeSats imposed by the new space debris mitigation guidelines, shortening the transfer time is becoming more important.

The second option for achieving Lunar orbit is deployment after trans-lunar injection, TLI. The best way this can be done is after passing through the Van Allen radiation belts to utilize additional shielding that can be provided by the launch vehicle. Radiation can be very dangerous to spacecraft, and it is important to remember that every mission to the Moon should account for it. How much depends on the chosen trajectory and approach to the mission. If your spacecraft is deployed already on approach to the Moon, you have a considerable amount of freedom in selecting your target orbit. The downside is that this style of deployment requires dedicating a large part of your spacecraft to propulsion and fuel. The one exception is if you are already on a ballistic capture trajectory, but you may still want to perform some adjustments to your final orbit. All things considered, this mission profile can usually lead to larger spacecraft, thus increasing the mission costs, especially when coupled with the increased complexity.

The third option is for the spacecraft to be deployed to Earth orbit and then perform its own maneuvers to get to the Moon. This is the most complicated way to approach a mission to the Moon, but it offers by far the cheapest launch. For this approach, it is of utmost importance to optimize maneuvers as these missions will need a large amount of propellant which will likely take up most of the spacecraft. Additionally, there is a need for swift departure from Earth’s orbit, not only to cut down on travel time but also to avoid excessive exposure to radiation in the Van Allen radiation belts.

Propulsion

With these options in mind, we have conducted extensive research into the viability of different maneuvers with different thrust-to-mass ratios. We have considered not only the mass and volume constraints but also the power limitations, including possible battery capacity.

We were investigating both chemical and electric propulsion options. At first glance, electric propulsion systems seem to be the better choice, with their very high specific impulse, but in large part, this is marred by their low thrust. The lower the thrust, the longer each maneuver takes. Some maneuvers, like the departure from the orbit of Earth, can be split into multiple smaller maneuvers, but some, like the Lunar capture maneuver can’t. This then means that such maneuver will need to be performed for longer and thus further away from the optimal location, decreasing its efficiency and increasing the total delta-v needed. This is then followed by the need for not only more propellant but also for more batteries as the electric propulsion has a quite high-power demand. This can be mitigated to a degree by using ballistic capture, but that may not always be an option or it may not be the best option for your spacecraft.

In general, the chemical propulsion has way lower specific impulse, but it offers much higher thrust and the power budget is much smaller. The main downside is the volume of propellant needed for transfer from Earth’s orbit to the Moon. This is due to the low density of the propellant itself, which forces the construction of larger spacecraft than would otherwise be necessary.

Conclusion

We have taken an in-depth look at the available options for making our dreams of affordable missions to the Moon a reality. Depending on the specific approach trajectory to the Moon, the correct engine needs to be carefully selected to avoid problems with performing maneuvers in time and safely achieving orbit. At least for now, it seems that chemical propulsion is the better choice for missions that will be deployed in orbit of Earth, but electric propulsion has rightfully earned its place for tasks where time is not of the essence.

Author: Ondřej Hladík