03-01 space transportation

[publius]

This has too much material which might better be in 02-08, & vice versa, & they both have material which should be better included in separate technical supplements. This paper needs to concentrate more on what needs to be moved where when, what kind of capabilities that requires, & what the implications are.

[publius]

Logistics 01 : Space Transportation

0. Summary
Access to cislunar space is clearly a critical issue for lunar settlement. Means of placing the initial settlement are examined, as are possible lunar contributions to broader space activity.

1. Initial Requirements
The first expedition has to be fully described before detailed planning can take place. In general, we anticipate that the original settlement will consist of 60 to 120 members, who will go in parties of two to six, at intervals based on the progress made by preceding parties.
Aside from the settlers themselves, a considerable quantity of supplies will be needed. As the Luna Project aims to achieve the greatest degree of self-sufficiency in the least time, the supplies will be principally tools, especially tools for building tools and machinery, rather than finished products or raw materials. Clearly, considerable supplies will have to be pre-positioned at the settlement site before the first party arrives, in order to permit them to commence work when they arrive.

2. Direct Ascent
The most straightforward approach to achieving Luna is direct ascent, a single maneuver launching from Terra and landing on Luna without orbits, rendezvous, or intermediate stops. Direct ascent was rejected by the Apollo planners because it would have required an enormous rocket known as the Nova, but for the mission profile which does not contemplate a return trip, requirements are less severe.
Current commercial launch vehicles could be used to deliver relatively small lunar payloads, such as orbiters, survey landers, or homing beacons. There are, however, no available rockets capable of delivering the required large payloads, even in the direct-ascent non-return configuration. The Saturn V rocket would be entirely suitable, but it is forty years obsolete ; rather than devote a year to reverse-engineering the unit being used as a lawn ornament at Johnson Space Center, the members of the Project would be well advised to develop a new vehicle for the purpose. If the design could be worked out soon enough, the components could well be fabricated and the assembly tested within the available time.
To illustrate the characteristics of the one-way lunar mission, let us examine a hypothetical rocket of 1000 tonnnes, one third the mass of the Saturn V. Taking the velocity needed for direct ascent as 13.6 km/s, the sum of the escape velocities of Terra and Luna, assuming a constant exhaust velocity of 4000 m/s and an empty mass for the first two stages equal to 0.1 the fuel mass, with three stages of mass ratios 2, 3, and 5 such a rocket could land 24 t on Luna.
This 24 t figure includes all the dead-weight of the final stage, but for vehicles not intended to return, and especially the cargo carriers, the L59 principle can be applied. That is to say, the whole rocket would be designed for a maximum of reusability. For example, a cylindrical outer shell could be made to be rapidly cut into large strips of unequal width, with fittings built in to allow ready assembly into Quonset huts.
Even those parts which could not be reused could be recycled. For example, given a choice between copper and aluminum tubing in the fuel system, the designer would choose copper. The increased mass of the ship would be balanced by less need to carry copper (a substance not known on Luna) as payload, effectively saving the mass of the aluminum. Similarly, fuel tanks might be made of carbon composites.
The short time available for developing such a rocket would require treating the initial cargo launches as test firings. A suitable course might be to fire a specimen of each stage as it became available, with a dummy load, followed by a test launch of the complete rocket including a specimen of the manned lunar stage. All three stages could be fired to put the upper stage in a trajectory to pass behind Luna and return to Terra. In such a test, the guidance system, the restart capability of the upper stage, and the ability of the personnel capsule to withstand emergency reentry following an aborted launch, could all be proven.
Upon successful completion of the true test firings, a series of unmanned operational launches with extensive telemetry (and backups standing by in case of failure) would serve to validate the design for manned flight. The first might be a launch into lunar orbit, to serve as the nucleus of a future lunar orbital station. Subsequent launches would pre-position cargo required by the first party of settlers.

3. Terra Orbit Rendezvous
Using the smaller expendable launch vehicles presently available would require Terra-orbit rendezvous to fuel, and possibly to assemble, the lunar stage. This poses certain difficulties. First, it would drastically increase the number of individual rockets required, raising the overhead expenses of fabrication, &c, and reducing the overall mission mass ratio. Secondly, particularly if cryogenic propellants are to be used for the lunar stage, Terra-orbit rendezvous requires numerous launches, with close coordination, within a short period of time. Considerable difficulty may be expected in this regard, as the problem of access to space today seems to be, not the cost of launches, but their infrequency and their interminable delays. It is reported that the largest number of a single type of expendable rocket launched in one year is 60 units of the Russian Soyuz booster, some years ago. Reckoning three to five boosters per lunar stage, the Terra-orbit rendezvous configuration might require that many in two or three months of the first phase.
This problem could be addressed with a reusable launch vehicle. A man-carrying fully-reusable orbiter, a true space ship, could be built today as a two-stage-to-orbit combination. Without the need to wait for expendable rockets to be produced and tested, a vehicle based on aircraft principles could enable more frequent launches.
The lower stage would be a large delta-wing aeroplane, generally similar to the North American B-70, using ramjets or hybrid engines, and possibly launched from a catapult. The upper stage would be basically a lifting body propellant tank, the compact form providing structural efficiency. The low density of the empty craft contributing to a gentle reentry, the broad, flat ventral surface might well withstand thermal loads without special shielding if made from titanium. Whatever the details, it seems likely that the first such ships would not have a significant payload capacity.
For cargo lifting, the best hope may be a semi-reusable configuration. The current NASA Space Shuttle is semi-reusable, but in effect it is the bottom stage which is discarded, and the Space Shuttle has the infrequent launches and high per-launch costs of typical expendable rockets. The combination of a reusable lower stage and an expendable upper stage, resembling the commercial Pegasus launch vehicle, bears examination. The rapid turnaround possible with an aircraft lower stage should increase the possible launch frequency, which will still be limited by the supply of upper stages and the time required for loading and checkout.
Further advantage might be gained by making the upper stage itself partially reusable. For example, the rocket engine might be built into a spherical heat-resistant shell which would be cast off the rocket, once in orbit, and reenter in the manner of a Vostok capsule. In this way, although the propellant tanks and body of the rocket would be lost, the engine, guidance system, and ancillary equipment could be saved. Different configurations of the disposable upper stage would be produced for different applications. For fuelling the lunar stage, the upper stage could be all tank, and its payload would be the residual fuel in the tank upon reaching orbit.

4. Launch Site
Whatever type of spacecraft is used, in whatever mission configuration, must be launched somewhere, and a reusable system will also require a landing facility. Existing launch complexes which might be large enough for the necessary volume of traffic are owned by governments. This suggests that the Luna Project will need to develop its own launch facilities, at the cost of considerable effort. In compensation, these facilities would remain available for future use.
It seems reasonable that a lunar-launch facility should be located in an area where Luna passes directly overhead, confining possible sites to a certain band of lower latitudes. As the rotation of Terra contributes velocity to the rocket, locations closest to the Equator are most favourable, as the circumference, and hence tangential velocity, is largest there. For direct ascent, this advantage is available once daily, approximately when Luna is rising in the East. Since the atmosphere reduces the effectiveness of rocket engines and resists the passage of air- and spacecraft, a location at high altitude where the air is rarefied seems preferable. On this basis, the high basins near Quito, Ecuador, seem almost ideal.
Aside from the issue of coming to terms with the Ecuadorian government, however, the choice of Quito presents various problems. The first is that of transportation. Large rockets in large quantities demand large-scale transportation, typically by water, and Quito is isolated from the sea and poorly furnished with transport facilities. This objection might be met by assembling the rockets on site using parts brought in separately, including sheets for fabrication into tank sections. Secondly, a large if sparsely populated stretch of South America lies to the east, posing liability problems in case of a failed launch. A third consideration is that reusable stages or sections, especially the very large aeroplane type, might require a water landing. These factors suggest that an island, accessible to large ships and situate where possible crashes would fall in the sea, might be preferable.
The Atlantic and Indian oceans are poorly supplied with remote equatorial islands of sufficient size for a spaceport complex. This leaves the islands of the Pacific. Christmas, in the Gilbert Islands, the largest of all coral atolls, already possesses airfields and a Japanese satellite-tracking station ; the populace and local authorities might be receptive to the establishment of a space port. Alternatively, there are islands vacant due to nuclear testing, which would be suitable for medium-term occupation given suitable facilities. Unfortunately, Johnston, which has been used for rocket launches (and was briefly offered for sale by the U.S. government) lies above 15° North latitude, and Bikini lies above 10°, while the small French test islands lie above 20° South latitude.

5. Lunar Developments
Once established, the lunar settlement will become highly relevant to issues of space transportation. Beyond the settlement effort itself, a collateral expansion of space activity in general is anticipated, as described elsewhere. The lunar settlement will be in an excellent position to contribute materially to this activity.
In the airless lunar environment, space launching "guns" become entirely practicable. The Northrup induction machine, which uses high-frequency three-phase current, is especially suitable because it acts on any object with a conductive surface, rather than requiring the magnetic buckets of some designs. Such a machine, arranged in the East-West direction at 45° North latitude in Mare Imbrium, with the muzzle end elevated and the breech end depressed, should be capable of putting objects into a high-inclination lunar orbit suitable for transferring to terrestrial orbits of various inclinations.
Rocket assemblies could be fabricated on Luna for use in lunar space, between Luna and Terra, and even in terrestrial orbit. The available materials dictate a hybrid design using liquid oxygen, presumably in large excess, and a metal fuel such as aluminum or calcium, probably in the form of sintered powder to reduce heat conductivity and the problem of melting. While the efficiency of such a unit would be low, its very availability would cover a multitude of sins. Mass production of a small range of sizes is likely, to be used in combination as each application requires.

6. Conclusion
Space launch capability for the settlement expedition is certainly the most pressing issue confronting the Luna Project, and also perhaps the worst defined. Mutually compatible solutions must be found to each of the component problems. The choices made will play a major role in shaping the development of the solar system.

revision 0
submitted for comment by publius
2007-196

[publius]

Space Gun

As always, dividing material between "Launch Vehicles" (probably to be renamed "space access systems" or something) & "Space Transportation" is difficult. Here is some material on the Space Gun.

The Space Gun will run approximately horizontally along the ground, with its muzzle pointing east, near the prime meridian on the subterrestrial hemisphere. This is retrograde with respect to the geocentric lunar motion, which gives us a more interesting family of trajectories than a direct orientation. For missions in lunar space, the projectile is put into an elliptical orbit & fires a "kick motor" at aposelene, roughly in the center of the averted hemisphere (180 degrees longitude). This allows it to establish any desired selenocentric orbit, or access the L1 libration point. For other purposes, the projectile is given some velocity greater than the selenocentric escape velocity ; when it passes out of the lunar sphere of influence, it is at roughly the altitude of the Moon but with a lower orbital velocity, & so takes up a highly eccentric direct geocentric orbit. For Earth-orbital missions, it will make an aerobraking pass & then fire its kick motor at its new apogee to establish the desired orbit, with trimming burns or rendezvous with an orbital transfer vehicle to follow. For escape missions, it will fire its kick motor at perigee, to inject into the desired escape hyperbola with a maximum of efficiency. Only rarely would it be desirable either to use the kick motor to reach lunar escape velocity on an "upward" trajectory, or to go into a retrograde geocentric orbit (although an orbit for one of the inner planets would be a possible case).

Therefore, the maximum muzzle velocity which the Space Gun needs to be capable of producing is one which causes the projectile to escape from the lunar sphere of influence, with enough selenocentric excess hyperbolic velocity that it has zero geocentric orbital velocity. As the selenocentric escape velocity is closely 2380 metres per second, & the geocentric lunar orbital velocity averages closely 1030 m/s, the necessary muzzle velocity is approximately 2590 m/s. In many cases the velocity required will be less than this.

The figure of 2950 m/s corresponds to approximately 3.36 megajoules of kinetic energy per kilogramme of mass. This figure is important for several reasons, one being that it permits us to calculate the power needed for the accelerator. At an acceleration of 100 metres per second squared (approximately 10 Earth gravities), it requires 25.9 seconds to achieve the desired velocity. 3.36 MJ/kg divided by 25.9 s gives 130 kilowatts per kilogramme, or (more importantly for our purposes) 130 MW/tonne. This not only sets the delivery rate of the energy-storage system, it also determines our shot rate. Assuming for the sake of argument a nominal projectile mass of 2.5 t, an end-to-end efficiency of 30%, a settlement power generation capacity of 100 kW, & a load factor (amount of power devoted to the gun) of 0.5, we can launch one projectile every 155 hours, or twice in each lunar day. It is fair to say that, by the time the gun is finished, our power generation capacity will be at least in tens megawatts when the Sun is in the sky, meaning that on this same basis we could launch every 90 minutes.

The question of efficiency is important because of the issue of where the lost energy goes. In general, it appears as heat. In the O'Neill "mass driver", the projectile is accelerated by a separate "bucket" which is then braked at the end of the accelerator, releasing the payload packet. This bucket is taken as using superconducting magnets, so the losses in the gun generally do not show up as waste heat in the bucket or payload. The Northrup type of gun, however, works by induction on the projectile body, which is typically a tube of ordinary metal. In this case, an amount of heat equal to the one less than the reciprocal of the propulsive efficiency shows up in this tube, which is ordinarily only a small fraction of the mass of the projectile. If, for example, the propulsive efficiency is 40%, then the tube will generate 5.04 MJ for every kilogramme of projectile mass, & if the casing mass is 10% of the total, the heat burden will be 50 MJ/kg. Obviously this will volatilize practically anything. Northrup's proposed solution to this problem, besides increasing the propulsive efficiency of the gun as far as possible (he reported attaining 50% on a regular basis, & with modern computer analysis techniques we can probably do better), was to use the waste heat to vaporize a liquid coolant, which would escape in rocket fashion, contributing additional thrust. In this case, the waste heat goes back into propulsion, raising the overall propulsive efficiency considerably.

The analysis of this approach, however, is somewhat troublesome. The mass of the projectile is constantly changing as it passes down the bore of the gun. This causes the resistance of the projectile to motion to change, & results in (for a constant acceleration) a change in the rate of energy delivery, both of which change the rate of waste-heat production, & thus the thrust available by conversion. It also means that the driving frequency has to be constantly retuned, which is probably within the capabilities of computer control. The mass-ratio of the projectile considered as a rocket will depend on the molecular weight of the propellant, the lower the better, & on the temperature at which waste heat is developed, the higher the better. The latter consideration calls for making the skin from something with mechanical properties stable at high temperatures, since as soon as the skin heats enough to lose its strength, it will shred & shoot out the barrel, leaving the payload in the lurch ; some dispersion-strengthened system such as titanium/thoria is probably desirable. The efficiency of conversion of waste heat to propulsion energy will depend on several other factors, including the efficiency of the heat-exchanging mechanism. It is, withal, a complicated problem, but we see that considerably economies are possible.

[publius]

OK, I think I have it now. The material regarding Space Gun efficiency should be moved to 02-08, & descriptive material should come in from there. Three lunar-specific topics which should be covered are the lunar rocket, the space gun, and lunar orbit rendezvous / lunar space station. It's also desirable to give consideration to lunar local space transportation, certain aspects of which are the subject of a technical paper I have under consideration.

5. Lunar Contributions

The lunar settlement is concieved not simply as an end in itself, but as the center of further space development. Space access issues contribute significantly to this central role. As the velocity needed to reach Luna from Terra is approximately the terrestrial escape velocity, and the minimum-altitude circular orbital velocity for any body is (by the laws of celestial mechanics) approximately 70% of the escape velocity, significant advantages are available for practically any mission. The velocity difference between the lunar surface and low terrestrial orbit is less than 5.5 km/s, as opposed to 8 km/s from the physically much closer terrestrial surface, and many of the maneuvers involved can employ aerobraking.

Launching a projectile into a retrograde selenocentric escape orbit with a sufficient excess hyperbolic velocity to largely cancel out the geocentric orbital velocity of Luna will put it, upon departing the lunar sphere of influence, into a high-eccentricity geocentric orbit. If the perigee of this orbit is low enough to pass through the terrestrial atmosphere, proper heat-shielding being provided, the resulting friction will lower the apogee, & a relatively small rocket burn will put the projectile into a circular orbit ; by controlling the depth of atmospheric penetration or the number of passes, most desired orbits can be attained. For an initial perigee of 90 km and an apogee of 200 km, the required circularizing impulse is approximately 35 m/s. As Luna is practically at infinite distance, as far as geocentric orbits are concerned, a broad range of orbital inclinations is available at very little additional velocity cost. If the perigee is a little higher, and the rocket burn occurs at this time, the phenomenon of excess hyperbolic velocity can be used to achieve escape trajectories at relatively low cost. If the projectile falling from Luna achieves a perigee velocity of 11 km/s, the additional velocity needed to place it into a Hohmann transfer orbit for Mars is only about 0.6 km/s. These results are achieved with a selenocentric departure velocity no greater than 3 km/s, so the largest required total velocity increment is not more than 3.6 km/s.

An interesting case is that of the geosynchronous orbit, at an altitude of 35 900 km, both because of its economic importance and because attaining it in a two-impulse maneuver from Terra requires more than escape velocity. If we begin with the descent to Terra, as above, followed by aerobraking with an initial perigee of 100 km, the circularizing impulse is approximately 1.5 km/s, for a total velocity increment of about 4.5 km/s. Alternatively, a Hohmann transfer from the geocentric lunar altitude requires a circularizing impulse slightly less than 1.1 km/s, and a selenocentric departure velocity of slightly less than 2.5 km/s, for a total velocity increment of about 3.5 km/s. (Neither of these calculations accounts for the necessary plane change of 23°, which is of the order of 200 m/s if performed at departure from the lunar sphere of influence.) This second technique is not only more economical, but avoids the design & packaging problems associated with aerobraking, and does not require passing through the Van Allen radiation belts, making it feasible for personnel.

[publius]

6. Lunar Operations
The roles of space transportation in the lunar context include not only the movement of imports, immigrants, and visitors from Terra (which has been the emphasis of most of this paper), and exports and return personnel to Terra or other points in the solar system (which has been touched upon in the above section), but also local transportation. The type of rapid transportation between widely-separated points which is the role of terrestrial air transport must, absent a sensible atmosphere, be filled for lunar applications by some form of space transport. At first blush, this seems reasonably straightforward, since the low-altitude selenocentric circular orbital velocity is so small that even the venerable V-2 would have been capable of achieving orbit.

In fact, however, the lack of atmosphere imposes some unexpected limitations for surface-to-surface transport. Firstly, braking must be carried out by rocket thrust, and requires just as much velocity change as the initial takeoff. Secondly, without the ability to perform boost-glide maneuvers, flight must proceed along ballistic trajectories, meaning that, e.g, a range of approximately 5% of the lunar circumference, or 500 km, requires an initial velocity fully half the minimum circular orbital velocity. Put together, these factors dictate that a lunar transport rocket, in order to be able to take off from anywhere on the globe, land anywhere else, and return to its starting point, must have a total velocity potential four times the circular velocity, or 6.75 km/s. A rocket vehicle constructed with the most sophisticated terrestrial aerospace technology, and using hydrogen and oxygen as propellants, could achieve this in a single stage with capacity left over for payload. Even considering the smaller static load and acceleration factors applicable to lunar conditions, and the elimination of aerodynamic design constraints, this level of performance represents a serious challenge for the crude production techniques to be expected for early lunar industry and the lower-performing propellants available from lunar sources.

While it is possible to apply staging techniques, the costs of discarding usable hardware are significant, and there are impact hazards and contamination issues associated with the spent stages and residual propellants. For some purposes,the inconveniences attendent upon a limited degree of staging, typified by "drop tanks", may be acceptable, but the benefit available from this source is limited as well. On the other hand, if a space station were available to serve as an intermediate stopping point, an individual rocket vehicle would require only enough velocity potential to ascend to the station and return ; most of the traffic would be shipments of propellant from Luna City, but the relatively high operating cost is acceptable considering the overall benefits of the system. When we come to consider the location of this station, we realize that low selenocentric orbits are unstable due to the uneven distribution of the lunar mass, while high orbits are made unstable by solar and terrestrial perturbations. Furthermore, because of the long lunar rotational period, it is only once or twice in seven hundred hours that the ground track of an orbital station would pass over a specified point on the surface, except in the special cases of an equatorial or a polar orbit (which pass over any equatorial point during every revolution, or the two poles once each revolution, respectively). Due to the necessity of large plane changes, an orbital station would, in the course of the month, require a constantly-changing velocity increment to reach from an arbitrary point on the surface, or at any one moment would require very different velocities to reach from several different arbitrary points, variations which could amount to a factor of 2 or more. Unless we are willing to accept very limited access windows, this returns us right back to the problem of requiring four times the orbital velocity, which the station was intended to solve.

It would appear that the best position for the lunar station may be at the first Lagrange libration point, known as "L1". This point is more difficult to reach than an orbital station passing overhead, requiring approximately escape velocity (2.4 km/s) rather than low-altitude circular velocity (1.7 km/s). On the other hand, it is reachable at any time, from any point on the surface, with approximately the same velocity increment. A station at this point, or rather in a "halo" orbit centred on the point (in the plane perpendicular to the line of barycentres of the Terra-Luna system), could actually serve several functions. Essentially fixed in the lunar sky, it would be ideally placed for a communications relay to the whole subterrestrial hemisphere ; well away from both the lunar and terrestrial surfaces, rarely in shadow except during an eclipse, it would be well-situated for a power-satellite (despite the beam-spread issues associated with the long transmission distance) ; straddling the boundary between the lunar and terrestrial spheres of influence, it would make an ideal location for the assembly of large, delicate space structures which could not be lifted intact from the lunar surface. In fact a small constellation of satellites is likely here, with the initial power-satellite being a small unit sent from Terra before First Landing to provide nocturnal power (and incidentally serve as a demonstrator for the SPS concept), the rendezvous point being separate from the assembly platform, and so on.

In addition to the considerations listed above which arise from the lack of atmosphere and the low gravity, it becomes apparent that no major obstacle exists to the acceleration of projectiles to high velocities at ground level. This leads us to introduce the concept of the "space gun", an accelerator relying on reaction against the lunar mass itself and thus not tied to the use of rocket power. A gun in the familiar terrestrial sense, using the hot gasses produced by burning a charge of chemical explosive to drive the projectile down a close-fitting barrel, is probably not a reasonable lunar development, but there are other possibilities. Considerable results have been achieved by terrestrial experiments using "light-gas guns", relying on the application of a sudden impulse to a charge of hydrogen gas, propagating a shock wave (which moves faster than the speed of sound) in medium having a much higher speed of sound than air or ordinary propellant gasses. Such a gun poses various problems, not the least its need for make-up hydrogen in substantial quantities, which probably cannot be satisfied from lunar sources. For our purposes, the best gun is probably some form of linear electric motor, a device which can be built principally from lunar materials and uses little more than electrical energy to run.

The position of the gun is an interesting question. It must be convenient to Luna City, and capable of reaching the high-value targets, namely the L1 station and terrestrial orbit, in the most straightforward way. It should probably be laid out horizontally, rather than vertically, for convenience and for reasons of orbital mechanics. A gun directed toward the zenith, at about 45° north latitude, would not be pointing anywhere in particular. A gun directed toward Terra would not have its axis directed through the lunar barycentre, making it very tricky to use for launches into selenocentric orbits, and for escape launches its projectile upon leaving the lunar sphere of influence would be in a geocentric orbit with elements similar to those of Luna herself. A horizontal gun may be arranged with any desired azimuth. It appears that a direct eastward orientation may be best. Owing to the locked lunar rotation, a closed orbit from an eastward launch will be in the opposite sense to the geocentric lunar orbit, and an escape orbit from an eastward launch anywhere between about 75° west and 45° east longitude on the subterrestrial hemisphere will be directly opposed to the geocentric lunar motion, allowing the projectile to enter one of the high-eccentricity orbits identified above as favourable.

The significant disadvantage of the eastward gun is that, if the muzzle is situated near the zero meridian, the crossings of the equator for closed orbits will occur at approximately 90° east and west longitude, and a projectile intended to reach the L1 station will require a substantial plane change (or a long on-orbit wait for the rotation of the system to carry L1 into its orbital path). There are two ways of dealing with this. Firstly, the aposelene kick motor, which is required to provide the additional impetus to send the projectile on to L1 instead of returning to its launch site, may be used for plane change as well ; if the aposelene altitude is high enough, the overall velocity increment can still be reasonably small. Secondly, a second gun may be constructed, oriented along a great circle passing through the mean subterrestrial point, allowing a more direct ascent. As the L1 station grows in importance, not only as a waystation for lunar traffic but also as a rendezvous and assembly point for spacecraft and space platforms, it may be found that the great-circle gun begins to dominate the eastward gun in importance. We can expect an evolutionary development, with a small, short gun being built first, capable of launching small bulk cargoes at relatively high accelerations, followed by larger-bore guns for larger payloads, and then longer low-acceleration guns for more delicate cargoes, and eventually perhaps personnel.

[publius]

0. Prolegomenon

Issues of space access and transportation are key to the entire lunar settlement concept. As a matter of orbital mechanics, rapid and regular communication can be maintained between Terra and Luna as between either and no other substantial body. Furthermore, related considerations make the energy requirement for reaching most geocentric orbits lower from the surface of Luna than from that of Terra, and the lack of a sensible lunar atmosphere considerably eases some of the problems associated with space launch from a planetary body. The conclusion which presents itself is that Terra and Luna form something of a natural economic unit, making Luna not only the most obvious first destination in Man's reach for the stars but also the logical center of early extraterrestrial human activity.