April 2020 - Opening the High Frontier

Return to the Moon Space Tug

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.

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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.

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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.

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Return to the Moon Lunar Landers

Return to the Moon Lunar Landers (low lunar orbit)

The initial Return to the Moon lunar landers that will operate from the low lunar orbit space station will consist of 3 different vehicles, a 2-person crewed vehicle that will look like the ascent stage of the lunar lander used in the Apollo program but with landing gear legs added,

a cargo lander that will look like the descent stage of the Apollo lunar lander that will be capable of landing 7,000 kg payloads on the Moon, and a tanker based on the cargo lander but with oversized propellant tanks.

The tanker will be able to land 7,000 kg of propellant that will be used to refuel the crewed lander, the cargo lander, and still have enough propellant leftover that it will be able to return to the Lunar Orbiting Space Station in low lunar orbit. After returning to the lunar orbiting space station, all three landers will be refueled and prepared for another mission. These landers will use the same storable propellants that were used for the Apollo lunar lander and that are used in the service module of the Orion spacecraft.

A typical landing using the three Return to the Moon lunar landers will start with the landing of the tanker at a landing site near the lunar South Pole. Once the tanker is successfully on the lunar surface, the cargo lander will land nearby. After that, the crewed vehicle will land. It is assumed that the cargo lander will carry a pressurized rover and two small propellant tank trailers for the first landing. The rover will use the trailers to refuel both the cargo lander and the crewed lander for their return trips to low lunar orbit with propellant from the tanker. Once that is done, the crew will use the rover to explore the lunar South Pole area for easily accessible ice deposits that are near an area in permanent sunlight that is large enough for building a lunar base.

For missions requiring only an unmanned rover, the tanker with a partial propellant load can land a 4,700 kg rover on the surface of the Moon and still have enough propellant on board to return to the low lunar orbit space station.

Another way to use this system will be to remove the landing gear from the crewed lander, attach it to the top of the cargo lander, and operate the combination like the original Apollo Lunar Module. Assuming the cargo lander is not damaged when the crewed lander takes off, it can be refueled and sent back to the lunar orbiting space station at a later date.

A second way to use the crewed lander will be as a sub-orbital hopper for sending astronauts to other parts of the Moon on exploration missions from the Moon base. For this type of mission, the lander will use 1/4^th of its propellant to head to a site of interest, 1/4^th to land, and the remaining half to return to the Moon base.

The crewed lander will have a space station departure mass of 4,700 kg and will use 2,300 kg of propellant to land on the Moon. It will require an additional 2,300 kg of propellant from the tanker to return to the low lunar orbit space station. The cargo lander and tanker will both have a space station departure mass of 18,000 kg and will use 8,500 kg of propellant to land on the Moon when fully loaded. They will need 2,300 kg of propellant each for the return to the low lunar orbit space station (no payload). The total amount of propellant required for these three vehicles to land on the Moon and return to the low lunar orbit space station is 26,200 kg. The amount of propellant required for the tanker to land a 4,700 kg unmanned rover on the Moon and return is 10,800 kg.

The advantages of the three Return to the Moon lunar landers are many; the landers are reusable, they build on our experience with the Apollo Lunar Module, they use existing propulsion systems that use storable propellants, they are small enough that they can be sent to the Lunar Orbiting Space Station with existing launch vehicles, and their propellant needs are manageable. Propellant resupply will be discussed in more detail in the article Return to the Moon Space Tug.

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Lunar Surface Exploration

All the articles about the Return to the Moon program show the first mission staying on the lunar surface for 2 weeks. If we are to make effective use of this time on the Moon, the astronauts will need some type of habitat to stay in. Camping out for 2 weeks in a crewed lander that lacks an airlock is not an option. This habitat can be a small inflatable structure, a structure that is designed to fit on top of a cargo lander,

or some kind of pressurized rover that can also be landed by the cargo lander. All of these solutions have been studied many times since the 1960s and the majority of them are within the payload capacity of the proposed cargo lander. One pressurized rover concept that is particularly well suited for this mission is the Space Exploration Vehicle with wheeled chassis. This vehicle is designed to support 2 astronauts for 2 weeks on the lunar surface. Prototypes of this vehicle have already been built and tested on Earth. If the SEV can be ready in time to meet the 2024 deadline, we will not only be returning to the Moon, we will be returning in style. The SEV with chassis and supplies has a mass of approximately 5,000 kg.

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Gateway Station Lunar Lander (L2 Halo orbit)

By comparison, a lunar lander designed to operate from the Gateway Space Station in an L2 Halo orbit is a much larger and more expensive vehicle. Lockheed Martin has proposed a single-stage reusable lander for this that carries a crew of four, a payload of 1,000 kg, has a surface stay time of 2 weeks, and uses a LOX/LH2 propulsion system. According to Lockheed Martin, this lander has a departure mass of 62,000 kg when it leaves the Gateway Space Station, and a dry mass of 22,000 kg when it returns at the end of the mission. The total amount of propellant required for each landing is 39,000 kg.

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Storable versus Cryogenic Propellants

Before the Moon Base is built and cryogenic propellant production using lunar water begins, all the propellants used by the Return to the Moon program will need to be brought up from Earth. The problem with using cryogenic propellants for the initial phases of this program will be the need to develop a cryogenic propellant storage system that can keep the propellants cold and recompress the boil-off gasses. Developing and testing such a system and getting it in place in lunar orbit in time for a 2024 lunar landing adds additional risk to not being able to meet the deadline. If the propellant losses due to boil-off are considered part of the cost of using cryogens then it will be necessary to send enough extra propellant with every shipment to account for this. Unfortunately, if a mission is delayed, the amount of propellant remaining at the lunar space station for the landing could become an issue depending on how long the delay is. Using storable propellants for the first landing, the lunar exploration phase, and the lunar base construction phase of the program eliminates these issues and provides a backup system for when the transition to cryogenic propellants begins in the future.

Ad Astra! (To the Stars)

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Return to the Moon Lunar Station

Return to the Moon Lunar Station

After the launch vehicle and upper stage for the Return to the Moon program are selected, the next step in building an affordable Earth-Moon space transportation system will be to build a Return to the Moon Lunar Station in low lunar orbit. Like the currently planned Gateway Space Station, this Return to the Moon Lunar Station will need the same first two modules, the Power and Propulsion Element (PPE) and the Minimum Habitation Module (MHM). In addition, the Return to the Moon Lunar Station will also need the already planned Gateway Logistics Module. The Gateway Logistics Module is the module that will refuel the lunar landers for going to the Moon and the Orion spacecraft for its return trip to Earth. So other than including the Gateway Logistics Module from the beginning and placing the station in a low lunar orbit, the Return to the Moon Lunar Station will be made from the same components that are already being designed and built for the Gateway Space Station.

The Orbit

As currently planned, the Gateway Space Station is to be placed in a halo orbit around the Earth-Moon L2 Lagrange point. The primary reason for choosing this location is because it takes less velocity change to go from the Lunar Transfer Orbit to the L2 halo orbit than it takes to go to Low Lunar Orbit. The velocity change required to go from Lunar Transfer Orbit to the L2 Halo orbit is 420 m/s. The velocity change required to go from Luna Transfer Orbit to Low Lunar Orbit is 820 m/s. This reduction in velocity makes it possible for the Orion spacecraft to go to the L2 halo orbit and return to the Earth without refueling. The disadvantage to this is that it takes 730 m/s of velocity change to go from the L2 Halo orbit to low lunar orbit. This increase in velocity is added to the lunar lander both when it goes down to the Moon and when it returns to the L2 Halo orbit. The additional propellant required to do this dramatically increases the size, complexity, and cost of the lunar lander. By comparison, lunar landers designed to operate from low lunar orbit are a fraction of the size and cost. These differences will be discussed in more detail in “Return to the Moon Lunar Landers.”

Summary

Selecting the most cost effective orbit for the lunar orbiting space station will be the second most important decision for the Return to the Moon program due to its impact on the size and cost of the lunar lander, the size and cost of the launch vehicle needed to send the lunar lander out to the Moon, and the amount of propellant that will need to be sent to the Moon for its on-going operation. The total difference in cost between the lunar landers designed for these two orbits is huge. The selected space station orbit will also have a significant impact on the cost of lifting lunar water, lunar propellant, lunar oxygen, lunar food, and lunar regolith for shielding, to the station once the lunar base starts to produce these items in quantity. As stated before, designing for low cost is crucial if the Return to the Moon program is to become a Return to Stay program. It will also have a very large impact on the cost of going to Mars once we are ready to do that.

Ad Astra! (To the Stars)

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Return to the Moon Launch Vehicle

Return to the Moon Launch Vehicle

As stated in the previous post, this new series of articles is about how to Return to the Moon by 2024 using existing launch vehicles that are lower in cost than the currently planned Space Launch System.

Of all the flight vehicles that will be required for the Return to the Moon program, the Earth to orbit launch vehicle and the upper stage for accelerating from Low Earth Orbit to Lunar Transfer Orbit, are the largest and most expensive. It is the cost of these two vehicles that will determine the overall cost of the entire program. The reason for this is simple, the apparent velocity required to go from the surface of the Earth to a low Earth parking orbit is 9,000 m/s. The velocity change required for going from the parking orbit to a Lunar Transfer Orbit is 3,200 m/s. The velocity change required to go from the Lunar Transfer Orbit to a Low Lunar Orbit is 820 m/s. Landing on the Moon will require an additional 1,600 m/s to 2,000 m/s depending on how much hover time is required to find a suitable landing spot. So of all the velocity change required for going to the Moon, the launch vehicle and upper stage will supply over 80% of the total. This is why the cost of these two vehicles will determine the overall cost of the program. This is also why making this part of the transportation system affordable is so important if the Return to the Moon program is to be a Return to Stay program.

Falcon Heavy w/ Delta upper stage & Orion spacecraft

The initial Return to the Moon launch vehicle will be the flight-proven Falcon Heavy. When flown as an expendable launch vehicle, the Falcon Heavy can launch 63,800 kg into a due east low Earth orbit. To send a crew to a low lunar orbit space station, the Falcon Heavy will need to launch an Orion spacecraft with a Delta IV Heavy upper stage into Earth orbit. The upper stage of the Delta IV Heavy launch vehicle has a total stage mass of 30,700 kg, a propellant mass of 27,220 kg, and a specific impulse of 462 seconds. The Orion spacecraft with Launch Escape Systems has a launch mass of 33,450 kg. The total of these two items comes out to 64,150 kg. The Launch Escape System on the Orion spacecraft, which has a mass of 7,585 kg, is jettisoned at the same time the core stage of the Falcon Heavy drops away. This reduces the amount of payload going to low Earth orbit to 56,565 kg, well within the maximum payload capacity of the Falcon Heavy. Once the orbiting spacecraft is in the proper position relative to the Moon, the upper stage will then accelerate the Orion spacecraft to Lunar Transfer Orbit velocity and the Orion spacecraft will arrive at the low lunar orbit space station with approximately 300 m/s worth of propellant left in its tanks. As a result, the Orion spacecraft will need to take on approximately 4,500 kg of propellant from the lunar orbiting space station for the return to Earth.

Falcon Heavy w/ Delta upper stage & cargo module

This same launch system is also capable of sending a gross payload of approximately 13,000 kg to the lunar orbiting space station when a payload module is substituted for the Orion crew module. This should be sufficient to deliver the components of the lunar orbiting space station to low lunar orbit as well as the lunar landers.

Falcon Heavy w/ Atlas upper stage & cargo module

Another way to send cargo to the lunar orbiting space station is to launch a 9,000 kg cargo module with an Orion service module on an Atlas V upper stage. The total launch mass for this, including the payload fairing, is approximately 47,400 kg, which is well within the payload capacity of the Falcon Heavy when both side boosters are recovered.

In-Space Refueling

The main issue that some people have with this transportation concept is that the Orion spacecraft does not carry enough propellant for returning to Earth from low lunar orbit without refueling. They consider this to be too risky. Yet one of the main reasons for the Return to the Moon program is to develop a local source of propellant that can be used for crewed expeditions to Mars. This requires in-space refueling. The lunar orbiting space station and lunar landers also require in-space refueling. It is also appropriate to point out that the International Space Station is already being refueled on a regular basis. So why not refuel the Orion spacecraft in lunar orbit?

Summary

The two main advantages of using this system are lower cost and the fact that the Falcon Heavy, Delta IV Heavy upper stage, Atlas V upper stage, and Orion service module, are all existing flight-proven vehicles. Using existing flight-proven vehicles will eliminate any chance of not meeting the 2024 deadline due to development delays.

Ad Astra! (To the Stars)

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