5. External Transport System
DaedalusaL4 will require external transportation for three main purposes. Firstly, materials and people will have to be launched from the surface of Earth, achieve orbit and be transferred to the settlement. Secondly, for DL4 to fulfil its mining and satellite production capabilities, transfer of materials and crew must be available between the settlement and Earth and Solar orbits. Finally, minerals must be transferred from lunar and asteroid mining facilities to the station. Such transportation must be catered for economically, safely, and, since space travel will become part of daily life, easily.
During construction of DaedalusaL4 in low Earth orbit a large amount of material and equipment will be required from the Earth. These materials have been kept to a minimum and have been designed to be as low in mass a possible. However, as DL4 will be one of the first settlements of its kind, the first stages of fabrication must unfortunately be terrestrially based (see 3 Construction). Additionally, as we are basing the settlement on reasonable extrapolations of current technology, the transport mechanism during this fledgling period must also be achievable in the near-term. Hence, at least in this stage of DL4’s life chemical rocketry is required.
In order to reduce costs wherever possible, the best rockets are obviously reusable ones. Out of many bright ideas to achieve this two current methods, that will undoubtedly multiply in the near future, seem to stand out. These are the Roton Rotary Rocket and the Kistler Reusable Rocket. The single stage Roton Rocket takes off vertically while its 7m diameter engine, consisting of 96 combustion chambers into which oxidiser and fuel are fed, spins at 720rpm providing sufficient centrifugal force for combustion pressure.
Having reached and deployed its payload in low Earth orbit it then enters the atmosphere base first while deploying helicopter blades. These blades, passively spinning at first then later actively thanks to tiny rockets, allow a steady decent rate for the vehicle while it is stabilised by side thrusters (ref. 92). The second choice is the more plausible for heavier payloads. The Kistler Reusable Rocket is a two stage vehicle using kerosene and liquid oxygen Russian engines. After lift off, using three NK-33 engines the first stage separates and returns to the launch site on parachutes. Meanwhile the second stage’s motors fire, carrying it to the deployment orbit. Here, after releasing its payload, the vehicle pitches and re-enters the atmosphere nose first. It then lands slowly, thanks to parachutes, and softly, thanks to airbags (ref. 94). It was considered that these vehicles could additionally carry the construction crews to orbit, however a purpose built astronaut carrier would better ensure suitable crew transport at this stage of DaedalusaL4’s life. While other concepts like the X-34, a crew version of the X-33 model utilising a linear aerospike engine that automatically adjusts to changing atmospheric pressure (ref. 93), are still on the drawing board we would suggest the use of a modified X-38. While this vehicle is only being designed for nine hours autonomous life support as a four person crew return vehicle, slight modifications could enable it to carry seven people for longer periods and allow it to function as a launch vehicle. Such a launch capability would probably be achieved thanks to a co-operative effort, where an X-38 would be strapped to an Ariane 5 booster for launch to Earth orbit (ref. 70).
Once construction is complete in low Earth orbit, the settlement’s central sphere and mirror must be transferred to the Lagrange libration point 4. This will be achieved, most likely, by using a solar sail for the mirror and a system of tethers for the central sphere. The solar sail will be of reflective configuration, providing greater thrust over its heat producing, light absorbing counterpart. Either using a low aperture space-based laser or photons from the sun, a solar sail generates thrust when light bounces off the sail (ref. 69). This small acceleration is proportional to the force transmitted and unfortunately inversely proportional to the payload’s mass. However, as the mirror will have been constructed of paper-thin sodium foils, and not yet have received its jacket of PV cells, its mass should still be reasonably low allowing for its transfer in this manner. The space tether infrastructure suggested for use in transferring the sphere to L4 is detailed later. Despite the hopefully successful movement and final construction of the settlement it will still be missing one final, vital element – people.
The use of the X-38 for all passenger services would be extremely costly, given a required passenger list in excess of 8,000 (initial population), and the range required of a craft to effectively reach lunar orbit. Therefore, as we would hope that, by this stage of DL4 ‘normalising’, significant advances in space plane technology would have taken place, a ramjet/rocket engine space plane was chosen as our main passenger transport. This system (under development by Space Access amongst others) involves the acceleration of the multiple stage vehicle to Mach 6 on air breathing engines before switching to chemical rocketry for the last leap out of the atmosphere. Once in orbit smaller stages are released from the first deploying the payload and eventually flying back to the launch strip along with the first stage. The system offers the advantages of horizontal takeoff as well as total reusability. While it does not escape the need for carried oxygen, this being required for the final burst, the current designs for a vehicle that can accelerate beyond escape velocity using only atmospheric oxidiser – a scramjet – were ruled out. Once the domain of the scramjet is entered, the air no longer needs to be compressed before combustion as this is catered for by the velocity of the craft (above Mach 6 = six times the speed of sound). However this is difficult to achieve as the air must additionally be slowed in order to allow time to fully burn the propellants. Hence an extremely aerodynamic craft, that slows the incoming air and compresses it without overheating, is required and this was considered too long-term and finely balanced for the large number of people (not to mention their possessions) needing to be shipped to DL4. However, should these engines become a tested reality before DaedalusaL4 then they would most certainly be reconsidered for ferrying services. Meanwhile the ejector ramjet, used by the space plane, works on a system where air entering the engine is initially compressed before mixing with the fuel, where it is burnt to produce expanding combustion products and generate thrust.
While it is plausible that the second stages of a space plane could travel to the L4 libration point, as this is where residents must be delivered, this would reduce the amount of ‘cabin’ area available due to the increased space required for propellants. Therefore, it is suggested that, during construction of the sphere and deployment of the mirror in low Earth orbit, a space tether infrastructure be laid down. Tethers would consist of multiple microscopic fibres made of carbon atoms assembled into ‘buckytubes’ just a few nanometres thick. Once grown into long intertwined and threaded ropes these will hopefully create tethers 600 times stronger than current materials can provide. Once in orbit these operate on the principle that objects farther from the Earth’s centre must maintain slightly slower horizontal velocities than closer objects. Hence, when a tether, more than a few hundred kilometres long, connects two objects in different orbits the tether is kept in tension. Then, using the conservation of angular momentum, the tether can be used as a giant slingshot to transfer momentum between bodies and swing satellites to different orbits. This power can even be supplemented by designing the tether out of conductive materials that then interact with the Earth’s magnetic field to generate electricity and contribute lift. Meanwhile the tether’s own orbit, affected slightly by the transfer of momentum, may be maintained by solar produced electricity running through the rope and pushing against the Earth’s magnetic field. The suggested system for DaedalusaL4 involves the provision of two principle tethers, one in low Earth orbit (LEO) and the other in a high Earth elliptical orbit (HEO). The LEO tether would pick up the payload, in this case one of the passenger carrying smaller stages of the space plane or the central sphere, and orbit round the Earth. The cartwheeling tether, once in line with the HEO tether, would transfer the payload to the second tether. This would then, once in a suitable position, slingshot the payload towards L4 (ref. 69, 95), where once within suitable range of the libration point, the passenger carrier or central sphere could initialise electric engines, such as Hall thrusters, to decelerate and bring it safely to the required position (see fig. 5.1e).
At the time of DaedalusaL4’s construction it is assumed that no major orbital solar-power stations will be in operation. Hence during low Earth orbit construction, the use of magnetohydrodynamic thrust lightcraft was ruled out. However when certain terrestrial materials during DL4’s lifetime are required the use of such lightcraft for the initial stage of transport is a definite possibility as it would use the infrastructure that the settlement had put in place and hence reduce launch costs. The system works on the basis of microwave power beamed to an ascending lightcraft from an orbital solar-power station. The microwave beam is focused by the vehicle’s body to produce an air spike, culminating in a point of explosively heated air. This deflects air from the front of the craft forcing it round in a shock wave. As the deflected air reaches the rim of the vehicle it encounters electrodes which ionise the air and generate thrust. Once the lightcraft reaches a certain altitude the vehicle would unfortunately have to supply, from a stored supply, hydrogen to replace the thinning atmosphere. However, by using atmospheric propellants and beamed power for the majority of the journey, the system still reduces the amount of propellants required significantly, and once in low Earth orbit could be transferred to the tether infrastructure for its continued voyage to DL4.
This transport will be necessary to move satellites from DL4 to other orbits. This will be catered for through the solar orbit transfer vehicle, which utilises a large mirror to focus solar radiation onto graphite. Once heated to 2,100ºC this in turn vaporises stored liquid hydrogen propellant to generate thrust which can then gently adjust the orbit of satellites, transferring between low Earth orbit and Geosynchronous orbit in eight weeks (ref. 96). Around DL4 this system can be supplemented with Hall thrusters. Like ion drives these generate ions, however radial magnetic fields (produced by the Hall effect) replace grids to accelerate the ions. This set-up leads to an increased impulse that is attractive for near-Earth transport where fast changes of velocity may be required. Another system can be implemented for mini- and micro-satellites. Using microwave power to heat a propellant, an extremely energy-efficient thruster can be designed. This can produce three to five times as much thrust as an ion drive on the same power levels. It offers the additional advantage that practically any propellant can be used, whereas ion drives are restricted to a limited range of substances.
Unfortunately these systems can only cope with the transfer of satellites and unless incorporated into special craft designs will not be suitable for crew transfers to other stations or satellites. Hence, it is expected that special craft will be designed for this use. Meanwhile the X-38 seems the most plausible option requiring even fewer adjustments for this task. However transporting crews to and from the Moon and more specifically to and from asteroid mining sites, will definitely require specialist craft. On the reasonably temporary mining sites of asteroids it is not expected that there will be sufficient time to construct a significant launch pad, however this may be possible on the Moon, enabling a suitable craft to be constructed there, probably based on an X-38 with an appropriately designer lunar booster.
When considering the methods by which DaedalusaL4 will be constructed, one of the most important considerations is the transport of building materials from the Moon and asteroids to the construction site. There are several ways in which this could be accomplished but all methods can be divided into two main categories: rockets and projectile launchers. In the rockets category, the main contenders are various conventional chemical or nuclear powered rockets. The projectile launcher would either be a mass driver or a chemical cannon.
The concept of using chemical rockets at this stage of DL4’s life was abandoned because of the small payload they can carry and the high operating costs. The Moon also has sufficient levels of gravity to retain an atmosphere, and chemical rocket launches could create a significant atmosphere if they were to persist for any length of time.
This leaves us with a choice between a mass driver and a chemical cannon. A chemical cannon would be an enormous construction project even if prefabricated parts were to be used. There are also significant logistical problems and the issue of radiation contamination. As such the mass driver is the preferred method of transporting materials for construction. A mass driver has several clear advantages over other forms of transportation (ref. 59).
1. It is a simple automatic device to operate.
2. It could catapult thousands of tonnes per month whereas it would take many rockets of excessive size to achieve this.
3. It is cost effective to operate and does not utilise costly or environmentally damaging fuel propellants.
One of the defining characteristics of mass drivers is their length, even relatively short designs are required to be several kilometres long. The choice of length for our mass driver was a thorny one. An exceptionally long mass driver, say 68 kilometres requires an acceleration of only 4.5g, whereas a shorter mass driver of 10 kilometres needs an acceleration of approximately 30g (ref. 60). Therefore the acceleration that materials in the mass driver can withstand must be balanced with economical length of the mass driver and the escape velocity required to transport masses from the Moon and asteroids to our mass catcher at L4.
The length of a mass driver may be approximately calculated from the simple formula below:
where = length of mass driver, = velocity of mass packet and = acceleration.
While variable for our temporary asteroid mass drivers – depending on the mass and density of the asteroid – the required escape velocity of a lunar mass driver is related primarily to the Moon’s own escape velocity but is also affected by the Earth’s gravitational pull.
lunar escape velocity
to escape velocity
+ 73.144 m/s
required escape velocity
The maximum acceleration is dependent on the types of materials to be launched from the device and their ability to withstand high ‘g’ forces. Taking a figure of 19-20 g as acceptable a figure of roughly 16 kilometres was arrived at for the length of the lunar mass driver. The exact figure for acceleration is 186.4 m/sec2 or 19.02g. Putting this into the length equation we get the following result.
length of mass driver =
length = 15,998.374 metres or 15.99 kilometres
The principles behind a lunar mass driver are very straightforward. Put simply, a mass driver works by two electromagnets attracting each other and hence causing acceleration. One coil is larger than the other and the smaller coil passes through the centre of the larger. The larger coil is the drive coil while the smaller coil is referred to as the bucket coil. The mass driver consists of a tunnel of numerous drive coils accelerating a bucket coil. Inside the bucket coil is a container or ‘bucket’ within which the material to be catapulted is transported.
The drive coils are not always turned on but rather are activated in sequence as the payload travels along the track. Each drive coil only activates when the bucket is close enough to feel the attractive pull significantly (in order to save power) and turns off when the bucket coil reaches the centre of the drive coil to avoid decelerating the payload. As such, each drive coil only gets a short pulse of current while the bucket requires a continuous current. A working prototype of such a device was constructed and successfully tested by the Space Studies Institute (SSI).
The mass to be transported is carried in the container along the 16 kilometre track. The bucket is accelerated through 186.396 m/sec2 or 19 g by a linear electric motor. Following this, the buckets then enter a section of track where all oscillations and vibrations are reduced in amplitude enough to allow the launching of the mass with very great precision.
During acceleration the payload is held tightly in the bucket, however when lunar escape velocity has been reached and any trajectory corrections have been made, the payload will be released from the bucket and escapes the lunar surface. At this point the bucket is magnetically decelerated and thus becomes separated from the payload (which is not decelerated). The bucket then enters a section of rack, where it is further decelerated by a linear synchronous motor. It is then returned to the loading area via a parallel track.
It is extremely important for all payloads leaving the driver to have exactly the same velocity so that they all go to the same point in orbit and the collector can be of reasonable size. Despite our best efforts there will, from time to time, be some variation in trajectory and as such it is desirable to have some means of correcting a trajectory once the package has been set in motion. Trajectory determination can be easily achieved by means of optical laser ranging, where striking one side of the payload with a low powered laser or particle beam brings about trajectory correction. This would remove a thin layer of the payloads outer surface creating an action-reaction impulse sufficient to prevent the payload from missing the orbital catcher.
The minerals mined on the moon are processed and packaged in thin fibreglass bags which are easily manufactured using lunar material. The bag itself is designed to conform to the shape of the bucket so that the bucket will assume the stresses during acceleration. By the time the payloads clear the Moon’s gravity, the velocity will have been greatly reduced and they will be travelling more slowly. It is at this point that the mass catcher intercepts the payloads. The payload’s momentum carries it through the funnel-shaped catcher into a collector bag. When this bag is full, it is removed and replaced with an empty one.
The mass catcher could easily by located at the L4 point. This location has several clear advantages over the more established mass catcher location of L2. Obviously, if our catcher were to be located a L2, we would then have to transport the building materials to the L4 construction site by conventional means. This not only increases costs but also increases considerably the amount of time to get the material from the lunar surface to the construction site. Due to the fact that the L4 and L5 libration points are significantly more stable than L2, the issue of maintaining position is not nearly as important.
The catching device itself is fully automated. The catcher is in the form of a large but light net which is manipulated by three cables to position the net anywhere within an equilateral triangle.