Return to the Moon Space Tug
The idea of an unmanned, solar electric powered, reusable space tug for hauling payloads around the solar system like an ocean-going tug that moves barges and oil drilling platforms around the world, has been around for a long time. What has given this idea staying power over the years is its potential to reduce cost by significantly reducing the amount of propellant required to move a payload to its desired destination. Less propellant means less cost. In the case of a Return to the Moon space tug, it will eliminate the need to launch a large chemical powered upper stage into Earth orbit every time we need to send a payload to the Moon. The disadvantage of the solar electric space tug concept is that it is slow. What takes 3 to 5 days with a chemical powered upper stage takes 8 to 10 months with a solar electric space tug. This is too slow for people, but perfect for moving non-time sensitive payloads like air, food, water, propellant, spare parts, habitat modules, lunar landers, pressurized rovers for exploring the Moon, propellant making equipment and so on. In short, a Return to the Moon space tug will be perfect for moving just about everything except people and it will do that for a fraction of the cost of a chemical powered upper stage.
So where do we get one?
It turns out we already have one. The Power and Propulsion Element (PPE) for the planned Gateway Space Station was originally designed as the solar electric propulsion system for the Asteroid Redirect Mission. Its job was to go out and capture a 20-ton asteroid and bring it back to lunar orbit. When the Asteroid Redirect Mission was canceled, this power and propulsion module was repurposed to provide power and propulsion for the Gateway Space Station in its L2 Halo orbit. This same power and propulsion module will also be perfect as an unmanned space tug for hauling cargo from Earth orbit out to lunar orbit. Considering the amount of equipment, supplies, and propellant, that will need to be hauled out to the Moon for building a lunar base, we will need a small fleet of these space tugs and they will save the Return to the Moon program huge amounts of money.
For example, a fully loaded Pressurized Cargo Module from a Cygnus cargo spacecraft has a mass of 5,300 kg when it arrives at the International Space Station (1,800 kg for the cargo module and 3,500 kg of cargo). If storable propellant is being shipped to the lunar orbit space station that will be 5,000 kg of propellant and 300 kg for the tanks that hold it. When either of these payload modules are attached to a PPE space tug, it then becomes possible to move that cargo out to lunar orbit for a small fraction of the cost of a chemical powered upper stage. The 4,000 kg space tug loaded with 3,000 kg of propellant and 5,300 kg of payload, will take approximately 300 days to go from the International Space Station to low lunar orbit. The advantage of this is that the space tug will use only 1,800 kg of propellant to do this. That is a huge saving. Flying without payload, the space tug will then be able to return to the International Space Station in approximately 127 days using an additional 750 kg of propellant. That comes out to .48 kg of propellant for every kilogram of payload delivered to lunar orbit. Using an expendable chemical powered upper stage like the ones described in Return to the Moon Launch Vehicles, will require 3.4 kg of propellant and hardware for every kilogram of payload delivered. That is 7 times as much mass that needs to be lifted to Earth orbit and then thrown away in order to deliver a payload to lunar orbit. Based on this difference, the reduced cost of using a solar electric space tug should be obvious.
Moon Base. Building a base on the Moon for mining lunar water is a huge logistics problem. It will require a steady stream of equipment and supplies to get it going. It will also need a way of delivering the water and propellant it produces to where it is needed. All of this movement of goods needs to be done as cost-effectively as possible. This is what the Return to the Moon space tug is for. To use this type of vehicle for hauling cargo out to a lunar orbiting space station on a regularly scheduled basis will require a small fleet of space tugs flying all the time. Assuming a round trip travel time of 450 days, 5 tugs will have 90 days between flights, 10 tugs will have 45 days, and 15 tugs will have 30 days. Larger payloads will require multiple space tugs to make the journey in the same amount of time. This is the supply pipeline that will allow us to build a lunar base and start mining the Moon for water so we can go to Mars. This is the pipeline that will make the Return to the Moon program sustainable so it can be a Return to Stay program.
Having a fleet of space tugs is not necessary for meeting the 2024 deadline for returning to the Moon. Yet since we already have a space tug it makes sense to start using this technology as soon as possible to reduce the cost of hauling payloads to the Moon. Savings, that can be used to help build the lunar base and to make improvements to other parts of the transportation system to further reduce transportation costs. As previously stated, Returning to the Moon to Stay means designing for minimum transportation cost. Not paying attention to this will make the Return to the Moon program into another government boondoggle that will not last.
Propellant. The ion propulsion system on the space tug uses xenon for propellant. This is a somewhat rare and expensive gas. As the number of space tugs increases, obtaining sufficient quantities of xenon for them could become an issue. Therefore, sometime in the future, it will be necessary to change the propulsion systems on the space tugs to one that uses argon for propellant. Since argon is the third-most abundant gas in the Earth’s atmosphere, supply will not be an issue. One possible propulsion system for this change is called VASIMR, which stands for Variable Specific Impulse Magnetoplasma Rocket. This propulsion system has the additional advantage of having almost twice the specific impulse of the current ion propulsion system which will further reduce the cost of transportation. It is also important to keep in mind how valuable these space tugs will be for moving a space station, landers, and return propellant to Mars orbit prior to the first human landing on Mars.
It is time to step into the universe of unlimited possibilities and create the most wondrous future imaginable.
It is time to Open the High Frontier.
Note: For those of you who are new to this website, this site is about making spaceflight affordable to everyone so we can finally start building the most incredible civilization the world has ever seen. A civilization that will include planets, asteroids, moons, space-based industries, and space colonies throughout the solar system. For those of you who are fans of space exploration but are new to the subject, the best way to read this site is to start at the beginning and read through to the most current article. If you are already familiar with the subject then feel free to jump around.
Index of Articles
- Opening the High Frontier
- Skyhook, a Journey to Orbit and Beyond
- In the Beginning . . .
- Why do Rockets Cost so Much?
- Combination Launch Systems
- It’s All About Speed!
- Visions of the Future
- The Call of an Unlimited Future
- Combination Launch Systems, part 2
- Outward Bound: Beyond Low Earth Orbit
- and someday . . . Starships!
- Mars: how to get there
- Outpost Space Stations
- Dreams of Space
- The Moon or Mars?
- Skyhooks and Space Elevators
- Stratolaunch and the X-15
- Starship Congress
- Making Spaceflight Affordable
- How a Combination Launch System Works
- Starship Conference 2017
- New Worlds Conference 2017
- Opening the High Frontier
- Building a Spacefaring Civilization
- Space Exploration and the Future
- Guiding Vision
- Return to the Moon to Stay
- Return to the Moon Launch Vehicle
- Return to the Moon Lunar Station
- Return to the Moon Lunar Landers
- Return to the Moon Space Tug
- Skyhooks, Space Elevators and Space Exploration
- Vertical Accelerator Launch Tower
- Combination launch system
- Opening the High Frontier (the Book)