While the goal of colonization is to achieve some high degree of self-sufficiency, eliminating a reliance on supply from Earth or elsewhere, a robust routine transportation system will still be necessary to support these new lunar and planetary branches of civilization. When supplying early outposts a transportation strategy based on transporting single-use landing craft whole makes sense and, as we have discussed previously, economizing on this through the use of ‘rough’ lander systems for cargo is key to the telerobotic pre-settlement strategy. But as the settlement approaches the status of a permanent colony and more routinely deals in human traffic than cargo, a different strategy for transit will be necessary that can employ local resources and reusable vehicles in transit. Routine transorbital transit to these destinations in the solar system will become based on reusable vessels that carry only what is necessary for that transorbital journey, optimizing passenger comfort. On arrival, they will be serviced by reusable surface shuttle vehicles optimized for the task of transferring people and cargo between orbit and surface and relying on locally produced fuels and service components. As traffic volumes increase, orbital way-stations –space stations dedicated to the task of ‘buffering’ transit– may be employed to allow long and short haul transit to function with some independence of one another so as to improve their efficiency, support a larger number of incoming and outgoing transit ‘windows’ for different destinations in the solar system, and provide contingencies for minor transit delays or emergencies. In this article we will examine some of the possible surface shuttle designs likely to be developed during the Avalon development phase.
Different environments will demand different shuttle designs. One of the key reasons for employing an approach based on the use of local shuttle vehicles is their specialization to a local environment, which would preclude production of a single vehicle system suited to all destinations in the solar system. However, some standardization in the handling of cargo across the solar system is likely. The key difference would be the presence or lack of an atmosphere demanding a vehicle of more aerodynamic design, but even variations in gravitation will impact vehicle design, demanding more subtle variation in systems even operating in a vacuum. Further specialization in design is likely for the support of cargo transit –more commonly automated in operation– and passengers as well as for support of very specific forms of cargo containerization or packaging. For the sake of brevity, we will concern ourselves primarily with the differences in design between shuttles for the Earth’s Moon and the planet Mars; the key destinations in the Avalon phase.
Lunar shuttle vehicles would tend to enjoy a greater simplicity of design by virtue of operating in a vacuum where aerodynamics are not a concern. Open space frame structures hosting retrofit components would be the norm. Though aerodynamics are no issue, one key concern in lunar lander design has been stability relative to center of gravity. Landing vertically, a low center of gravity near the main propulsion tends to be optimal.
For many decades there has been a quiet conflict among space engineers about the placement of engines and the relative merits and limitations of ‘outrigger’ engine arrays. Many engineers consider a central-bottom position for engine arrays –as seen in the original Apollo LEM– safer than outrigger arrangements where engines are placed at the sides or ends of a structure. In the event of an engine failure with outriggers, it’s argued that stability is quickly compromised and even when multiple engines are used in each outrigger module, control systems must very quickly detect a failure and compensate to prevent a radical shift in vehicle attitude for which there may not be sufficient time to respond in the midst of a descent. The counter-argument is that reliance on a single central engine module –often with a single engine– incurs an equally critical failure risk negating any apparent safety advantage. Employing multiple smaller engines and more modern automated control systems, an outrigger system has a potentially higher level of redundancy coupled to a lower center of gravity offering potentially better flight stability. Furthermore, outriggers have already proven themselves functional with surface probes such as the Viking Lander. Be that as it may, outrigger designs remain popular –even among NASA designers– because of a much lower vehicle profile providing easier surface access, much easier transfer of cargo and passengers, easier servicing of systems modules, and less surface disturbance on landing resulting in less kick-up of dust and debris. Though current new non-reusable lunar lander designs, based on the model of the Apollo LEM, are still based on central-bottom engine placement, for many years NASA designers have been proposing reusable landers based on outrigger arrangements. These designs, frequently based on ‘table lander’ configurations (as we described in the earlier article on telerobotic Soft and Rough Lander Systems) and commonly featured in illustrations by NASA artists, are typical of these outrigger designs.
Particularly interesting is the reusable ‘lunar commuter concept, developed by NASA in the 90s and frequently illustrated by artist Pat Rawlings, that employs a standardized cylindrical habitat module as the basis of a 24hr transit from Earth to the Moon via transfer whole between launch vehicle, transorbital vessel, surface shuttle lander, and finally a robotic carriage allowing it to function as a rover. (though not illustrated, presumably some re-entry vehicle would also allow for the transfer of this same module from Earth orbit to the Earth’s surface)
Clearly, the virtues of this outrigger configuration are many from a logistics standpoint, greatly facilitating the use of automation in payload handling and safety and comfort for passenger transit. The transportation of bulkier payloads, such as robots and vehicles, is also greatly enhanced when a vehicle can be deployed simply by lowering it a few feet to the ground. In this author’s opinion, many of the problems assumed associated with the outrigger concept may relate to the over-elaborateness of proposed designs, their non-axisymmetric geometries, and insufficient redundancy in propulsion units. Thus we propose a much simpler vehicle concept based on a pallet like chassis structure of uniform symmetric geometry.
The Pallet Lifter
The Pallet Lifter shuttle vehicle would be based on an octagonal shaped space frame chassis with a primary engine module and attitude control module held within the volume of the chassis at four of its eight corners and a vertical landing leg module at the four alternating corners surrounding a square payload space. Payloads are attached to the underside of the pallet while simple spherical fuel tanks are attached in a radial arrangement to the top. Additional systems are attached within the central volume and along the sides of the octagon. The vertical landing legs are designed to completely extend from either side of the pallet, functioning as a vertical lift on the surface and retracting completely out of the way of the payload-side face of the pallet on orbit. This arrangement would allow for payloads of variable height and allow the unloaded landed vehicle to lower itself completely to the surface for easier access to its fuel tanks, which would be swapped out as modules for quick refueling, or to completely lower itself onto a flat bed carriage for transport whole to a service hanger/bunker. This vertical landing pylon arrangement would also be ideally suited to incorporation of powered wheeled bogies, allowing the larger forms of the vehicle complete self-mobility on the surface and potential for use with self-mobile habitats.
Using a standard modular plug-in grid, the Pallet Lifter would support an endless variety of containerized, strapped-in, and frame supported payloads as well as being outfit for passenger transit in much the same way as the proposed NASA lunar commuter, using specialized cradle attachments and added systems to support whatever passenger module configuration is used. TransHab style hull modules, based on an external support truss, would be one possibility as well as larger rounded box and short vertical cylinder shapes. These would be particularly useful where a bottom and side hatch arrangement is used to accommodate more convenient docking alignments. On flight, deployable radiator and solar panels in a radial array would extend from the sides of the ocatagonal chassis. Just as with cargo transfer, the lander could lower these passenger modules onto flat bed carriages, allowing them to function as temporary rover vehicles allowing transfer to a habitat in shirt-sleeve comfort. Of course cargo and passenger vehicles would not necessarily be interchangeable due to the added cost of man-rating hardware. This may lead to some divergence and specialization between vehicle design over time.
Such an adaptive vehicle platform could be scaled freely and reconfigured for many more specialized uses. Landing legs could be made fixed-position to support permanently attached payload structures. The pallet chassis could be thickened to allow modest sized permanent passenger cabins while leaving the underside free for cargo. Entire self-mobile habitats could be derived from this platform. Another variation of the chassis design could also be used as the basis of a flying low-gravity surface transit vehicle or ‘hopper transport’ carrying cargo and/or passengers like a helicopter between points on the lunar surface. It is likely this simple basic vehicle form may find countless uses and see a long history across the Avalon phase.
Providing surface shuttle transit for a planet with an atmosphere, like Mars, is a much more complex challenge than the Moon as both aerodynamic effects and a higher gravity must be accounted for. A key issue is how to cope with the high thermal energy and plasmas produced by air resistance at the hypersonic entry velocities from orbit. Aerodynamic entry shielding of some kind is a necessity but its design becomes complicated by the fact that the placement of this shielding typically conflicts with the placement of rocket thrusters and landing gear it must somehow help protect and must also account for the comfort of passengers subject to the g-forces of deceleration. This is a problem that has vexed designers of reusable shuttle vehicles for both Earth and Mars for a long time and, on Earth, most ‘solutions’ have been rather elaborate and thus difficult and expensive to develop and implement –the NASA Space Shuttle being the key example of this. Essentially, the atmospheric shuttle vehicle is a kind of oxymoron –a vehicle that, ideally, must fly up and forward in launch then, on return, fly backwards at hypersonic speeds and land upright!
Planetary probes –being single-use vehicles– have generally employed the approach of disposable bottom-mounted aerodynamic faring shells that are jettisoned in the final stage of a descent. This approach will quite likely be used with many kinds of cargo delivery vehicles in the telerobotic settlement phases of Avalon colonization. But it’s not well suited to routine transportation with reusable vehicles, such jettisoned components representing a materials waste and a hazard to settlements as they grow in size and volume of traffic.
The simplest approach to this problem is a vehicle employing a blunt sphere-conic aeroshield able to quickly and safely flip completely over for different phases of flight. One can imagine such a vehicle as consisting of a shallow sphere-cone (a cone with a spherical apex) shape with engines and deployable landing gear behind it. It is launched nose-up then when reentering the atmosphere it turns tail-forward for de-orbit burn, flips nose forward for re-entry, then flips nose-up again for rocket propelled vertical landing. For an automated cargo vessel such maneuvers would not be particularly problematic and a variety of Mars shuttle design concepts have been based on this simple model. But it’s not well suited to passenger travel since, during the aerodynamic deceleration phase, passenger seats would be facing backward or need some mechanisms by which the seats or an entire passenger cabin can rotate, greatly increasing mass, while the rotation maneuvers would generate especially stomach-churning g-forces causing disorientation.
A re-entry vehicle shape commonly considered more appropriate to passenger transit employs the ‘biconic’ form with a heat-shielded longitudinal side and engine and landing gear placement in the tail end. This is sometimes further enhanced by a flattened or elliptical profile and the addition of delta wing surfaces. This form is typified by such vehicles as the somewhat mysterious McDonald Douglas AMaRV vehicle (for which no photos or illustrations apparently exist but whose performance is considered legendary), the later similarly-shaped DC-X, and the more recent JAXA experimental VTOL landing vehicle. Launching vertically, a landing vehicle with this shape would, after de-orbit, position to a sideways or ‘glide entry’ orientation then swing to a vertical tail-down descent. With this approach passenger couches would be placed in a head-to-nose orientation so passengers are facing backwards into aerodynamic deceleration vector then are shifted to a vertical seating position during vertical landing. This has long been a promising configuration for reusable shuttle vehicles, hampered by the conflict over a presumed superiority of ‘spaceplane’ vehicles with a runway landing. Many Mars mission concepts have featured this kind of vehicle and it features in many visualization illustrations, as this NASA concept shown here and illustrated by Pat Rawlings;
The compromises here, however, are in a much more complicated flight dynamics requiring more active control and more aerodynamic stress on the structure requiring a heavier and more complex structural design –especially if any sort of wing surface is added. Center of gravity for the vehicle is high demanding a large radius for landing gear to prevent toppling and a high narrow stacking configuration makes access to the ground from passenger cabins more complicated. (in the above illustration we see a rather elaborate self-mobile passenger transfer and cargo crane robot to accommodate this) However, many recent biconic entry vehicle designs –particularly for use as space station crew escape vehicles or new Apollo-like crew vehicles feature a very squat compact form of this biconic shape, as shown here with the ESA Viking crew capsule and a wingless variant of the Russian Klipper crew vehicle concept;
These much simpler shapes seem to offer an interesting prospect in the context of our previously suggested Pallet Lifter design. Thus we arrive the Octoconic Shuttle
Employing a very squat biconic aerodynamic faring, the Octoconic Shuttle would modify this basic form by blending/merging it with an octagonal profile surrounding, at its base, a similar chassis frame structure as employed by the Pallet Lifter and with the same configuration of four perimeter engine modules alternating with four vertical landing piers around a central, but in this case recessed, modular payload pallet. Eight pneumatically actuated ‘body flaps’ would extend from the bottom edge of the faring. Taller than the Pallet Lifter, the vehicle would employ the additional volume inside the aerodynamic faring primarily for tanks accommodating the higher fuel volume needed for SSTO and VTOL landing in the Martian environment while also shifting the center of gravity of the vehicle lower on descent.
Functioning like other biconic shuttle vehicle concepts, the Octoconic Shuttle would launch and land vertically and re-enter in a sideways or glide profile, using its eight flaps and independently controlled engine modules for dynamic flight control. When landing, the eight flaps are moved to a 90 degree position and the usual Pallet Lifter style vertical landing legs are extended and, upon landing, could be used as a lift and mobility system as with the Pallet Lifter, allowing for automated transfer of cargo containers/pallets using flat-bed carriage vehicles. A secondary integral lift system would be use to separate the base vehicle chassis from its faring shell, allowing side access to fuel tanks.
The manned version of the vehicle would employ a passenger module –most-likely of vertical cylinder form– extending partly outside and inside the primary vehicle chassis and supporting bottom and side docking, with only the bottom hatch exposed during flight. Deployable solar and radiator panel arrays –similar to those of the manned Pallet Lifter– would extend from the under-edge sides of the chassis in flight. There would be no windows employed in the faring shell of the vehicle and few, if any, on the bottom of the passenger module, though a periscope for landing views may be employed in addition to a large array of digital video cameras. Upon landing, the vehicle could lower the passenger module to a flat bed carriage as with the Pallet Lifter or lower it to the ground independent of the chassis as a habitat module. Exploration versions of the vehicle –not designed for reusability– could even deploy the passenger module as a completely independent and self-mobile hybrid habitat/vehicle.
In a later article we will discuss the possibility of adapting Space Elevator and other tether transport systems for lunar and extra-terrestrial planetary applications, the employ of these technologies potentially obsolescing the need for rocket transport between orbit and surface, though these technologies may not find universal application. By the time of Solaria, these may be joined by such technologies as laser/molecular conveyance and many other transportation concepts we can scarcely imagine at present. However, we can anticipate a trend in specialization in transportation concepts as a result of improved local infrastructures and the progressive reduction of cargo to raw materials with the trend toward progressively immediate manufacture on-demand thanks to the increased ease and automation of production based on nanotechnology. A greater focus on passenger transit as the primary role of vehicle transport across the solar system is likely.
- Life In Avalon
- Telerobotic Outpost
- Excavated Settlement
- Excavated Colonies
- Surface Transit Waystation
- Mass Launcher System
- Lunar/Planetary Space Elevator Systems
- Avalon Supporting Technologies