Section 4.9: Phase 5A - Lunar Development (page 2)
Lunar Surface Transport
These projects include transportation systems which operate entirely on or near the Lunar Surface, and those which are based on the surface but transport to orbit or farther destinations.
- Surface Vehicles
This begins with rovers for science and prospecting purposes remote-controlled from Earth. They progress to mining and construction vehicles which are still remote-controlled or smart enough to self-operate. Once people start to be located on the surface regularly, we add unpressurized and pressurized crew transport, and eventually mobile habitation for extended stays. Although solar power is widely available on the Lunar surface, it is unwieldy for larger vehicles. Mobile transport for longer distances will therefore likely include a network of charging stations, with sufficient solar panels stowed on a vehicle for backup power. The Apollo lunar rovers traveled a total of 90 km without roads, but for longer distances construction may be needed to overcome obstacles, or a different approach than wheels. Possibilities include legged systems or cable ways to overcome difficult terrain. Lunar dust management is likely needed to prevent wear and accumulation around mechanical parts and to prevent entry into crew areas. Long distance point-to-point travel may be more efficiently accomplished by ballistic transport. This would require an ample supply of propellants. Anything beyond existing planetary rover experience will require advance R&D.
- Lunar Catapults
These systems are designed to accelerate bulk materials to orbital velocity. They are not intended for people or sensitive cargo, so they can use high acceleration. This lets them be physically compact. By sending many smaller payloads, the energy needed for each one is smaller. The payloads are gathered into larger loads for a cargo tug. Payloads may include raw unprocessed rock, or finished materials like metals or propellants. The two basic design approaches, centrifugal and linear, both use electric power. Each is best suited to different cargo volumes. Both are capable of delivering many times their own mass to orbit, and greatly reduce propellant needs. A skyhook, discussed above, can also reduce the propellant needed to lift cargo from the Moon. It has lower g-forces, so can also be used for people and sensitive items. But a full one-gee Lunar skyhook is 500 km long. This is a much larger project to build than the catapults, which are about 100-200 m in length. How to sequence or combine these alternatives will require more analysis.
The basic idea is an electric motor driving a rotor with long and short arms that are balanced. See for example Inertia II, but mounted horizontal on the Moon to minimize support structure. Solar panels produce power during daytime to gradually accelerate the rotor until the tip of the long arm is moving at slightly above Lunar orbit velocity. The payload is released at a slight angle above horizontal and coasts to a collection point in low Lunar orbit. At the same time, a counterweight is released from the short arm, which hits the ground somewhere behind the centrifuge. The reason for the counterweight is an unbalanced rotor would produce large forces on the centrifuge structure that could damage it. Whether the payload is sintered blocks of unprocessed surface material (regolith), or first separated to, for example, metals, is yet to be determined. Since there is no atmosphere to cause drag, spinning up the rotor can be done slowly, which keeps input power low. Rotor size can also be physically small, so this approach is suited to smaller delivery volumes and early use with less equipment to get started.
Delivery Orbit - Any object thrown into an orbit from the surface will intersect the launch point one orbit later. By slightly tilting the rotor, the part of the orbit behind the catapult will be below the surface. This ensures a failed delivery will impact the Moon a safe away, where the path first goes underground. The high point of the orbit would be about 100 km above the surface, enough to clear the Lunar terrain and be stable for a time against orbit changes. Normal delivery uses either a propulsion unit on the payload, or an orbiting device to capture it and raise the orbit. One device in orbit is probably less expensive than many on every payload, but this is yet to be determined. One approach for the device is a small skyhook with a payload catcher hanging down a number of kilometers from the main core. The offset allows the catapult to throw slightly slower than orbit velocity to match trajectory with the catcher. Missed catches will then land somewhere else on the Moon instead of coming back to the launch site and doing damage. Transferring accumulated payloads to the upper end of the skyhook raises their orbit, where they can then be picked up by an orbital tug.
The catcher can be a bag or net, which is positioned with cables to meet an incoming payload, or is large enough to account for payload dispersion. Positioning needs a way to see the incoming payloads, such as radar or lidar. The velocity difference between the catcher and payload is fairly low to start with, since it represents the difference between near-orbit and orbit. This difference can be reduced by proper design. The payloads is likely moving slower than the catcher, so it faces forward and scoops them up, adding to their velocity. The capture and altitude raising slows the orbiting platform, so it needs propulsion to maintain orbit. This can use electric propulsion, which is ten times as efficient as launching from the Moon with chemical propulsion. The vast majority of the work to reach orbit is supplied by the catapult, so the electric propulsion can be relatively small.
The on-board approach uses a small solid or cold gas engine on the payload. The design can draw from Rocket Assisted Projectiles developed for artillery, which experience similar high accelerations. Various Propellant Combinations can be derived from lunar materials. This avoids having to supply them from elsewhere. The payload itself has to survive high acceleration. So it can also be the structure for an engine and tank. A control system is needed so the engine points in the right direction and fires at the correct time. The propulsion and control system add complexity and cost to the payload, and if either does not operate correctly, would send the payload in an unpredictable direction. For these reasons it is probably not the best approach.
Cargo Tug Requirements - If the payloads have their own propulsion, they will end up in similar orbits, but somewhat spread out. A collection vehicle will chase them down and gather them into larger cargo loads. If an orbiting platform catches the payloads, they will already be gathered at one place. In either case an electric tug then hauls large loads to a high orbit, such as Earth-Moon L2, a stable point on the far side of the Moon. This location is in sunlight all the time, so you have more energy to turn your raw materials into useful products.
Electric propulsion is efficient, but slow. So the tug capacity needs to be matched to how much material is launched from the surface in the time it makes one round trip. The delta-V from Low Lunar Orbit to EML-2, using low thrust propulsion, is about 920 m/s. An electric tug delivering 200 tons with a 400 kw plasma thruster producing 11.4 N thrust at 50 km/s exhaust velocity will consume 1.86% of start mass delivering the payload. The solar array and propulsion unit mass is about 4 tons. Therefore propellant use is 3788 kg. Flow rate is 19.7 kg/day, so the burn takes 192 days. The return trip is much faster, since the tug will be 50 times lighter. So the round-trip in total is about 200 days. If the catapult is delivering 1 ton/day to orbit, then the flow is matched.
Small Centrifuge Design - The theoretical orbit velocity at the surface is 1680 m/s. However, the Moon is not a perfect sphere and has mass concentrations, so launch velocities of about 1750 m/s are needed to clear obstructions. Selecting a high point to launch from helps avoid hitting anything, and allows tilting the catapult a little. This both provides a safe trajectory in case of a failed capture, and a safe distance downrange for the counterweight to impact. Assuming a 50 meter radius on the long arm, and a tip velocity of 1750 m/s, the total stress along the arm is 156 g-km. High-strength carbon fiber has an ultimate strength of up to 386 g-km, so we are within available material properties. Centrifugal forces are higher at the hub than the tip. That's because the tip only supports the payload, and the hub supports the payload + arm mass. Therefore the arm will be thicker at the hub to handle the increased forces. We don't design at ultimate strength, but instead work at a factor of safety below it. A typical value is 2.4 for rotating aerospace machinery. This reduces our working strength to 386 g-km/2.4 = 161 g-km. The theoretical arm taper in area is then e156/161 = 2.635. In practice it will be somewhat higher due to non-structural overhead. An arm taper of 3-4:1 in area is a reasonable design solution. The circumference of rotation is 2π x 50m = 314 meters. A tip velocity of 1750 m/s then implies 5.57 rotations per second or 334 rpm. This is well within the capacity of a mechanical arm driven by an electric motor.
The short arm has a more slowly moving tip and therefore lower stress. The counterweight needs to be heavier than the payload to balance the forces on the rotor and anchoring structure. The result is the short arm will be the same mass as the long arm, but shorter and thicker. Since the counterweight is released much slower than orbit velocity, it will impact the ground some distance behind the centrifuge. It may self-destruct on impact, but there is no shortage of loose rock on the surface to replace it.
We assumed 1 ton/day of bulk material delivered to orbit. Since low Lunar orbits take about 108 minutes, we can launch 13.3 times a day to have the payloads arrive at a common collection point. Therefore each payload would be 75 kg. The kinetic energy of the payload is then 115 MJ, and the catapult will need 35 kW of average power at 50% efficiency. The other 50% goes into accelerating the counterweight and is wasted. 35 kW is half the output of one of the Space Station's four main solar array wings and about twice that on a large communications satellite. So the power needed is within existing space power levels. The Tesla Model S base model uses a 270 kW electric motor, so a 35 kW motor can be quite small and light. Since you are in Lunar night half the time, you need two catapults to produce the needed launch rate. It is hard to load payloads unless the centrifuge is stopped. So the flywheel energy in one catapult's arms just after launch can be transferred to the other catapult, rather than just dissipating it by braking. Regenerative braking, as this is called, is also common in electric cars. This makes the system more efficient. It is possible enough energy storage can be provided to last the Lunar night, or use a different power source that doesn't depend on sunlight. In that case only a single catapult is needed, but we will leave that choice to more detailed design.
System Mass - Modern space solar arrays plus mounting and tracking on the Lunar surface would produce ~100 W/kg. 70 kW for two catapults would then require 700 kg of arrays. The catapult arms have a mass of 6-8 times the payload, so a complete device may be ~20 times the cargo mass, or 1500 kg/catapult. We will need a mining robot to gather raw materials, and equipment to weigh and package the payloads. A total mission mass allowance is then ~10 tons delivered to the Lunar surface. Since we deliver 1 ton per day, and nominal operating life for space hardware is 15 years, we deliver a total of 5,500 tons, or 550 times the system mass in payload. Since the solar arrays are a small part of the total mass, we can increase the mass return by using more of them. For example, if we use 11 times as much power, this raises the system mass to 17 tons, but total delivery is 60.500 tons. The mass return ratio then is 3559 to 1. If you are launching more than once per orbit, you would need multiple collection platforms. Adding collection platforms and more power is a straightforward growth path from an early system.
Large Centrifuge Design - This version scales up the payload volume, lowers the g-forces, and changes some other assumptions for larger scale space industry. The follows the same type of calculations as the smaller version. We increase the delivery rate to 27.4 tons per day, or 10,000 tons/year. By Earth standards, this is still a small mining operation, equal to two dump trucks of gravel per day, but 10 kT is about the total mass launched from Earth through 2014, and supplying that every year is a significant scale-up. We increase the arm radius to 1000 meters, which lowers the tip acceleration to 3062 m/s2 or 312.3 gees. This length arm would not be a single piece, but assembled as a truss structure to keep it from bending when stationary, with tension cables to take up the load when rotating. The end result is similar to the top portion of a tower crane (Figure 4.12-5), but without the vertical tower, and mounted a local mountain on the Moon.
At 1750 m/s launch velocity, the rotation rate is 16.7 rpm, which is slow for an electric motor. So a belt or gear type mechanism is used to convert the motor speed. The total stress along the arm is still 156 g-km, so it has the same arm taper and mass ratio as the smaller centrifuge. Given 13.3 launches per day, set by the period of the orbital platform, each payload is 2,060 kg. Kinetic energy of the payload is 3.155 GJ, and therefore takes 485.6 kW of average power. Adding the counterweight and system losses increases this to 1.62 MW system power. Using solar panels, this would have a mass of about 16 tons. At this scale, it is reasonable to use local thermal storage or a nuclear reactor to provide power across the Lunar night, rather than building two large centrifuges to make up for lack of sunlight half the time. It is also possible to use local flywheel energy storage between throws to conserve energy, or design a horizontal elevator type loading device so the centrifuge does not have to stop between throws. Another design option is to use a fixed counterweight set between the loaded and unloaded payload, and using strong anchors and bearings to withstand the unbalanced loads. 16 tons of total carbon fiber centrifuge structure is still a reasonable amount to deliver from orbit. If local basalt fiber production is available this can save delivery from orbit, but will be more massive due to lower strength. All of these choices will require more analysis to find an optimal design.
Bulk Lunar soil has a packed density of about 1850 kg/m3, so we need a little more than one cubic meter container size in that case. If cast into a single block the density is 2700-3500 kg/m3, and the payload will be 0.59-0.76 m3. If cast as a 1m H x 1m D cylinder, the self-loading at peak rotation is 8.04 MPa (1166 psi). This is below the compressive strength of most rock, and a container would not be necessary in this case. A thin shell or fiber wrap is probably a good idea in case of fracture. For payloads other than cast rock or solid metal, a container made of native iron would be about 100 kg, and less if made of stronger and lighter materials. The containers are therefore 5% of less of the delivered payload.
Besides the bare rotating arm and power supply, there will need to be anchor structures, robots to collect the raw materials and put it in containers, the fine tuning magnets downrange, and communications and other support equipment. The total for this is unknown until more design work is done, but we can roughly estimate 50 tons in total for delivered equipment. Since the total mass delivered during a 15 year operating life is 150,000 tons, the mass return ratio is on the order of 3000:1.
Linear Accelerator Approach -
These are known as Mass Drivers or Coilguns(Figure 4.12-6), and use a series of stationary coils as sequential electromagnets to accelerate the payload. A Mass Driver recirculates moving coils which push the payload, while a coilgun uses an integral coil that stays with the payload. Both are variants of electromagnetic accelerators (see Section 2.2). The high acceleration over a short distance requires high peak power. Since the acceleration is completed quickly, many smaller payloads can be launched in series. This approach is more suited to high volume delivery, where the large power supply is not a penalty.
Linear Accelerator Design - [TBD]
Payload Containers - Bulk Lunar rock will probably not hold together under the high g-forces from either catapult type, and propellant fluids definitely need a container. There are several options for handling this. For early delivery, pre-made containers can be delivered by landers, although this is less efficient than using locally made ones. Bulk rock can be heated in a solar furnace, then sintered or cast into a single piece. This can be load-tested before the catapult throws it. If metals are being produced, they can be used for containers or vessels. If fibers are being produced, they can be used as reinforcement or bags to hold the payloads. Either container type can be of materials that are needed in orbit, so their mass is not wasted overhead. The centrifuge can supply structural support to the payload until release, but the dynamic loads during release will require careful attention. Larger radius or longer catapults will have lower g-forces, but be physically larger. All of these alternatives need R&D to determine which is best.
- Metal Containers:
We will use local metallic iron as an example for metal containers. Iron gives the opportunity to fine tune the trajectory with magnetic coils after the catapult releases the payload. Together with the catcher mechanically positioning itself during the 30-40 minute ballistic trajectory of the cargo, that should ensure a high percentage of catches. Lunar rock has a mineral density of about 2.7 to 3.5 g/cc, depending on composition (Kiefer, 2012). We will assume an average of 3.0. Our centrifuge concept has a 75 kg payload, so we need a 25 liter container. A cylinder 32 cm tall and 32 cm in diameter has about this volume. It is supported by a payload holder on the end of the arm of matching size, with a bottom that opens to release the payload. The cylinder is in compression with a total load of 75 kg x 61,250 m/s2 = 4.6 MN. If completely self-supporting, with a 250 MPa working stress for native iron, the container needs 184 cm2 of shell cross-section at the base and a wall thickness there of 1.83 cm. The thickness can taper towards the top, where it has less payload to support. At the density of 7.8 g/cc for iron, the shell mass would theoretically be 23 kg, which is about 1/3 of the total payload. This can be reduced by choosing a better shape for the container and supporting the loads with the holder. It can also be lowered by using choosing a different metal with a better strength to density ratio (specific strength).
- Basalt Fiber Containers:
Basalt fiber has about 20 times the specific strength (strength/density ratio) of native iron. To save container mass and deliver more of other materials, we can substitute part or all of the container with fiber instead of metal. Fiber-wound pressure vessels are a common design on Earth. These use a metal liner to prevent leaks, wrapped with high strength fibers to withstand high pressure. Fiber-reinforced metals embed the fibers in a bulk metal matrix, and pure fiber can be woven into a high strength fabric. Which option is best depends on what the payloads are, and the complexity of producing the containers. Ideally no container would be required, but that would limit the payload to cast Lunar rock. which has a relatively low strength. This limits the g-forces and increases the catapult size.
Lunar Orbit Services
Satellite Maintenance and Refueling Support - Most existing satellites are in the Low or High Orbit regions, because most of civilization is on Earth and that is what their services support. This situation should continue for some time. This project covers satellite support from the Lunar region to other regions, as well as internally within the region. With a few notable exceptions (the ISS and Hubble Space Telescope), most space projects have been single-use, because maintenance and refueling were too difficult or expensive. However replacing entire satellites when they break or run out of fuel is also very expensive, due to the high cost of new hardware and transport to space. So a maintenance and refueling capability is desirable. Raw materials from the Moon and Near Earth asteroids, combined with Lunar region energy, can support this capability, along with projects in the other regions and support from Earth. Support products include propellant supply, both for chemical and electric propulsion. Propellants are used to refuel satellites, remove orbital debris, and either transport maintenance equipment the satellites or bring the satellites to a maintenance location. They also include materials supply, including radiation shielding for people and equipment to support maintenance and bulk material for radiation belt depletion.
Lunar Surface Services
The various projects identified above will not be isolated from each other, and the Lunar region is not isolated from other regions and civilization on Earth. So we want to link these projects to each other and to other program phases. Concept exploration is the first step in design, so the projects themselves are not fully defined. Integrating them into a larger program is even farther from complete. Rather than attempt a complete plan, for now we will just note individual relationships between them:
Lunar basalt is widely available on the Moon. A catapult can deliver this in bulk to orbit, where a processing plant can melt it and produce high strength basalt fiber. This can be used to build a skyhook, which makes it easier for people and delicate cargo to reach the surface. Alternately, carbonaceous-type asteroids can supply carbon, which can also make high strength fiber. Carbon fiber is somewhat stronger, but basalt needs fewer processing steps. Delivery of the raw materials to the processing location varies in difficulty. Which approach is preferred in a particular circumstance is undetermined. High strength fibers can also be used as reinforcing for pressure vessels, and mineral fibers in general can be used as insulation.