*Actively seeking criticism*

         Analysis of air launched lunar passenger spacecraft 
               using beamed-power electric propulsion
               
                            Simon Rowland
                            simon@eagle.ca
                              15/10/97
         
                                    Introduction
         
             While large-scale lunar tourism is an enticing industry
with a demonstrated demand, previous passenger transportation
architectures have proven unsatisfactory in keeping cost and margins
at manageable levels. Typically, reusable Single Stage To Orbit
vehicles, which must be refuelled with terrestrial hydrogen and
lunar oxygen, have severe weight and performance constraints, in
addition to the need to refuel with large amounts of hydrogen both
in LEO and on the lunar surface with very large associated launch
costs. A conservatively-designed spacecraft, burning CH4 instead of
LH2, could make a high-speed trip if it uses a beamed-power,
externally ionised, applied field electrothermal/electromagnetic
hybrid engine of about 2,000 seconds of specific impulse. It is
staged from what is essentially a winged, flyback drop tank using a
combination of afterburning turbojets and subsonic combustion
ramjets. The spacecraft maintains a lunar oxygen refuelling station
at the L1 balance point between the Earth and the Moon. Flight time
for an Earth-Moon return mission is about 30 hours. For a 200
passenger spacecraft, total on-runway GLOW is about 830 tonnes,
which requires large but reasonable landing gear at ground pressures
typical of large airports. For an initial investment of about $16
billion, 1,000,000 passengers per year are moved at a cost of $4,500
per passenger.

                                  Mission Profile
         
              The mission begins with the spacecraft mated to the carrier
aircraft, at an airport terminal. While passengers board and inspection is
completed, jet fuel and about 323 tonnes of liquid methane and oxygen are pumped
into the aircraft's tanks. The spacecraft carries about 51 tonnes of fuel
internally, including over 23 tonnes of fuel for the electrical propulsion
system. 
         
              Using its six turbojet engines, the 580-tonne loaded aircraft taxis
to the runway, lights its afterburners, and takes off. After climbing a short
distance, the afterburners are disengaged and the aircraft flies to the nearest
ocean or unpopulated area at about Mach 0.9.
         
              Upon reaching the designated area, the aircraft lights its
afterburners and throttles up its engines, accelerating beyond the sound barrier.
Shortly after, the aircraft's large ramjets engage and eventually take over,
bringing the ensemble to Mach 5 and high altitude.
         
              At this point, the spacecraft's exposed methane/oxygen engines
ignite, using the rear section of the aircraft's body as an extension of the
spacecraft's plug nozzle. The aircraft's ramjet is run slightly longer until
shutoff. The fight profile runs through the atmosphere at high speed until the
tanks are approaching empty, to increase lift and reduce thrust-to-weight
requirements.
         
              After about 5.3 km/s of DV, the aircraft's onboard fuel tanks are
exhausted and the spacecraft's engines are throttled down to idle. The aircraft's
payload bay doors open and, while in free-fall and far above the atmosphere, the
127-tonne spacecraft separates using vectored thrust. The aircraft reenters the
atmosphere at about Mach 22 and returns to the launch site, at speed where
practical.
         
              The spacecraft immediately reignites its engines with the slushed
onboard fuels, and performs a burn of 1.32 km/s  to bring it into a 250 km
circular orbit.
         
              While the passengers get a good look at the Earth, the spacecraft
then deploys a large rectenna and radiator array, which are used to power and
cool the magnetoplasmadynamic engine. The electric engine performs a 400 m/s burn
to raise the spacecraft's apogee to 1,000 km.
         
              The spacecraft is now just below the lower Van Allen belt.
Consuming the last of its onboard oxygen, a subset of the spacecraft's chemical
engines reignite, consuming the last of the onboard oxygen. The burn increases
the spacecraft's velocity by 1,080 m/s, raising the spacecraft's apogee further,
quickly taking the spacecraft through the radiation belt. The electrical
propulsion system then completes the manoeuvre, thrusting for 45% of an orbit as
the spacecraft approaches its apogee creasing velocity by another 940 m/s, for an
apogee and circularisation at 6,000 km. The 4 tonnes of methane for the lunar
landing and ascent remain, which is allowed to melt from its slushed form. At his
point, the spacecraft weighs 65 tonnes.
         
              Over the next 15 hours, the engine performs 7.2 km/s of velocity
change, bringing the spacecraft to L1 to load 11 tonnes of lunar oxygen, then to
low lunar orbit and finally performing the first 200 m/s of the lunar descent
before furling the rectenna and radiators.
         
              With its supply of lunar fuel, the spacecraft's chemical engines
fire, slowing the 61 tonnes of spacecraft by 1.5 km/s to a landing on the lunar
surface, over half of which falls into the "dead man's curve". On the lunar
surface, lunar oxygen for the spacecraft's ascent is loaded, in addition to fuel
for the next flight's landing. Busses dock with the spacecraft to ferry
passengers to the Luna City Hotel, and the spacecraft is inspected in the few
hours spent on the surface. The crew is exchanged, and a group of returning
tourists board the spacecraft.
         
              The spacecraft again uses lunar oxygen and onboard methane to
perform the first 1.5 km/s of lunar orbital insertion. From there, the electric
engine is used to complete the 1.7 km/s of DV required. Should the rectenna fail
to deploy, or the electrical propulsion system be otherwise disabled, fuel
reserved for the Earth landing and the lunar oxygen cargo can be consumed to
complete orbital insertion before being repaired and refuelled on orbit.
         
              The magnetoplasmadynamic engine provides 3.7 km/s of DV the
spacecraft to the L1 fuel depot, for the off-loading of the next flight's descent
oxygen, and continues on to transearth insertion. This impulse return the
spacecraft to Earth's vicinity in 11 hours.
         
              The spacecraft's trajectory takes it on a direct reentry into
Earth's atmosphere at over 14 km/s. The spacecraft then lands vertically at the
airport of origin, generating a few times greater noise than an afterburning
military fighter.


                              A Supersonic Launch Pad
         
         Much of the difficulties of getting passengers to the Moon is that
of getting them first into orbit. Even with ultra-high performance chemical
engines, such as LH2/LOX, fuel and engines consume all but 7% of liftoff mass. It
is difficult and expensive to build a spacecraft with structure, fuel tanks,
landing gear, heatshield, and margin lightweight enough to leave a useful
payload. Margin on SSTO concepts are generally inadequate for low-cost operation
should any surprises arise.
         
         The main reason for this is the energy needed to reach orbit; a change
in velocity of at least 9.4 km/s is needed to reach orbit and land, making
provisions for gravity losses, aerodynamic drag, steering, deorbit and a vertical
landing, in addition to 7.8 km/s of orbital velocity. With turbojet and ramjet
engines aboard the carrier aircraft bringing the spacecraft to Mach 5 at
altitude, however, the velocity remaining is reduced to only 6.7 km/s. Because
high altitude reduces back pressure on the engines, the specific impulse is near
its maximum for the entire rocket flight‹and the fraction remaining for structure
and payload rises from 7% to 19% of fuelled spacecraft weight.
         
         By using the spacecraft's engines while still mated to the aircraft,
orbital performance can be increased still. The aircraft can be used as a drop
tank, supplying fuel and oxidiser for most of the ascent. In fact, the tanks
onboard the aircraft can be arranged in front of and behind the spacecraft in
such a way to give the spacecraft's otherwise high-drag shape streamlining for
the component of ascent where drag aerodynamic heating are most significant.
         
         Landing gear, typically 3% of the supported weight, need only be used on
the spacecraft's vertical landing, where total weight is about 40 tonnes, rather
than supporting the weight of the spacecraft at light-off of over 140 tonnes. The
admittedly impressive landing gear required to support the whole 830 tonne stack,
of course, remains with the aircraft.
         
         These factors grant great freedom to the designers of spacecraft. The
vehicle can be built cheaply with inexpensive, easy-to-machine materials.
Margins, which today are so scant that moderate winds risk destroying a launcher,
can be generous enough to permit all-weather operation. The payload can be
heavier, and the fuel can be a cheaper, denser, and easier to handle
low-performance combination.
         
         Development costs are generally the killer for SSTO economics.
Gram-shaving, which drives up costs tremendously, is required for a useful
payload. Ultra-high performance causes great stress on a lightweight spacecraft
and its engines, which require costly overhauls or significant technology
development. Further, total dry weight, which is a major factor in cost, is far
high for SSTO scenarios. Basically, if you throw weight at a problem, development
costs go into free-fall, and this scenario does exactly that. Two relatively easy
vehicle development projects, a new ramjet engine, and a new MPD engine are much
less costly combined than the very difficult development of a reusable SSTO of
the required performance, generally set at $10 billion.
         
         Operations are greatly simplified with an aircraft first stage. With a
runway loading no higher than that of a Boeing 747-400, and afterburners to
optionally assist with takeoff, the carrier aircraft could launch the spacecraft
from any major airport. The aircraft, with a wingspan and length comparable to
that of a medium-sized commercial airliner, can board at existing airport
terminals with little adaptation, avoiding the capital cost of dedicated
spaceports. With world market demands permitting combined flight rates both to
orbit and to the Moon as high as tens of millions of passengers per year, several
percent of current air traffic, the use of existing infrastructure is essential.
         
         Noise would be much greater than todays's quiet airliners, but far
within practical limits if airports and runways were chosen carefully. Near
populated areas, the booster aircraft's six military turbojets will be the
largest source of noise. Compared to a vertical take-off rocket, of course, the
aircraft's noise is trivial, and when afterburners, ramjets and rockets engage,
the aircraft is at altitude and has flown as much as many hundreds of kilometres
away from the population centre. The spacecraft's vertical landing would generate
a noise levels a few times higher than an afterburning fighter aircraft, although
this is over a large airport, and not near residential areas. There may be
regulatory issues involving Phase III noise level limits in commercial aviation
which require attention.
         
         The aircraft offers self-ferry, flyback and fetch capabilities which
also simplify operations. The aircraft can perform subsonic, nonstop flight to
nearly any point on Earth, or antipodal flights at high Mach numbers with several
refuelling stops. Further, the carrier aircraft can ferry the spacecraft, fuelled
or not, thousands of kilometres from airport to airport at Mach 0.9, using six
Pratt & Whitney F100-PW-229A turbojets, or up to Mach 5 using afterburners and/or
ramjets. This fetch capability allows greater flexibility with landing sites,
reducing the wait for an acceptable site to rotate under the spacecraft's orbit
should the landing site targeted at TEI be unacceptable due to weather,
contingency, etc.
         
         With air-launching the lunar spacecraft, the problems of SSTO which
contribute to cost are solved. Weight and performance requirements are greatly
reduced, the spacecraft can be designed for vacuum use rather than suffer
compromises, operations and noise issues are simplified, and development costs
are reduced, giving the air-launched spacecraft a distinct cost advantage.
         
         
                            High-Isp Electric Propulsion
         
             To avoid the expense of refuelling from Earth, and keep takeoff mass
low enough for the carrier aircraft to use existing runways, the spacecraft uses
a magnetoplasmadynamic (MPD) engine. A simple device, the MPD engine uses a
conventional convergant-divergant design, with a microwave emitter at the throat
which ionises the working fluid, the basic design of a microwave plasma-jet
engine. A magnetic field is then used to accelerate the plasma to between 104 and
105 kilometres per second. Both the microwave ionisation and the applied field
serve to create a spiral path of propellant inside the engine, which keeps it
away from the both engine walls and thus the limits of materials.
         
         An array of approximately twenty 7.5 MW engines is baselined, with about
70 MW of mission-average energy requirements beamed from solar power facilities
on the Lunar surface. The engine reclaims much of the waste heat energy either
through thermodynamic generators, or by passing propellant through the engine's
components, which is then added to the exhaust stream to increase thrust. Ammonia
serves as the fuel, and the waste-heat reclamation leads to a total efficiency of
75%. The array develops a steady-state thrust of 10 kN, and a specific impulse of
2,000 seconds, driven mostly by power beaming requirements and low fuel cost.
         
         
         Graph of runway take-off mass (tonnes) with MPD specific impulse (sec)
         
         
         On the return trip, where thrust requirements are much lower, the
engine is operated in a low-thrust mode. This may entail reducing or eliminating
the bypass hydrogen, to reduce cryogenic storage requirements, or operating in
either a quasi-steady or pulsed mode, to reduce thermal fatigue and corrosion.
         
         A MPD engine would require significant development, as engines do not
yet exist at the laboratory level which are ready for orbital applications, with
a technological readiness on NASA's scale of about 3. Also, the largest pulsed
engines developed to date run in the range of 1 MW, while currently developed
steady state engines are considerably less powerful. A major cost would involve
constructing a testing facility which can maintain a high vacuum while a high
mass-flow rate engine is operating. Two experts in the field independently quoted
a value of $100 million to develop and test a MPD engine in the thrust range
required, with a production cost of order $1 million per engine. A value of $500
million is assumed for the purposes of this study.
         
         The electrical power is then beamed to a deployable rectenna onboard the
spacecraft at the typical microwave frequency of 60 GHz. This is driven by
avoiding windows in Earth's atmosphere with regards to power spill over, and
other frequencies, such as 120 GHz, 200 GHz, and >300 GHz are not transmitted
through the atmosphere, and would reduce antenna requirements, although limits to
power beaming frequency exist.
         
         It is assumed that inefficiency in power beaming is reduced by 33% over
the previous lowest value, leading to an overall efficiency of 88%. At that
value, assuming a large enough dish on the Moon to keep beam spread low, 150 MW
of electrical generating capacity are required for 50.4% of the 36-hour flight,
as the spacecraft spends some time on the ground and coasting.
         
         Alternately, the beamed power could be focused into a wave guide
apparatus, which would direct the microwave beam to the engine directly, without
the intermediary step of conversion into electricity. This would require a
cyclotron resonance accelerator engine rather than a magnetoplasmadynamic engine,
which is simpler but is at a less advanced state of development. This
architecture, however may reduce area and weight requirements for the power
receiving and conditioning apparatus considerably.
         
         Requirements for engine thrust and power beaming capacity are reduced by
the nearly constant-thrust nature of the spacecraft's flight. While keeping
engine and rectenna mass to a minimum, this flight profile also reduces floating
debris. Eating and growing accustomed to the space environment is made easier for
passengers by about 2% of Earth's gravity, while giving the passengers much of
the weightless experience. Further, the slight acceleration creates atmospheric
convection and helps to separate gaseous and liquid propellants. Passengers will
likely be more prepared for the return journey, where much of the flight is spent
weightless due to decreased mass and velocity requirements.
         
         Use of a high-isp engine allows very high transfer speeds between the
Earth and Moon at a small cost in fuel. The spacecraft travels to the Moon in
only 16 hours, using 7.2 km/s of DV, and returns to Earth in 11 hours, also using
3.7 km/s of DV. This increases the number of flights over which the spacecraft
are amortised greatly over multiple-day transfer times. In addition to reducing
consumable requirements, it further allows much more spartan conditions ‹
amenities such as private rooms, showers, and exercise facilities are not
required for such short flights.
         
         The high-isp engine also reduces the costly lunar landings and ascent by
firing for 30 minutes at the beginning and end of each, respectively. This both
runway takeoff mass and cost by about 20%, and reduces lunar oxygen requirements
by about 60%.
         
         
                       Next Service Centre: 58,000 kilometres
         
         In 1772, the French mathematician Joseph Louis Lagrange discovered
that when two large bodies orbit each other, five gravitational balance points,
or "Lagrange points", are  created. Bean-shaped "halo orbits" exist around these
points, which are more stable than the points themselves. Of these five, the
point of most interest is L1, an orbit which lies approximately 58,000 km above
the centre of the Moon's nearside.
         
         The site will be exploited as a lunar oxygen storage depot. Passenger
spacecraft bound for the Moon will receive enough lunar oxygen to land. On the
lunar surface, the spacecraft is loaded with lunar oxygen both for ascent and for
the next spacecraft's landing, the latter of which is deposited at the L1 base
before the spacecraft continues on to Earth.
         
         The gravitational tug of the Moon and planets cause objects at L1 to
gradually drift away from the exact balance point. This, in addition to the solar
wind, helps to keep the space around the station relatively free of orbital
debris. Maintaining position in a halo orbit around L1 requires a station keeping
DV of approximately 120 m/s per year, comparable to station keeping requirements
in Low Lunar Orbit (LLO). However, a free body in LLO will crash into the far
side of the Moon in a matter of months, rather than slowly drift away but likely
remain somewhere in cislunar space.
         
         An orbiting facility at L1 offers many amenities to cislunar space.
Planetary spacecraft ‹ and those in low lunar orbit ‹ may be occulted by the
Moon, ruling out line of sight power beaming from a station on the lunar surface.
However, a space station far above the Moon can relay power to spacecraft
circling the Moon for over 50% of their orbit.
         
         Power can also be beamed from between points on the lunar surface. Two
solar power stations at 90°E and 90°W longitude could supply energy to the lunar
nearside for nearly the entire local night. Large power storage facilities could
also supply power to the entire nearside during eclipses. This avoids the need
for extensive ground power transmission infrastructure, or expensive
decentralisation.
         
         When energy is beamed between two points, modulating the energy allows
the transmission of data along with the energy. Power requirements for spacecraft
or ground stations in the range of hundreds of megawatts allow truly huge volumes
of data to be piggybacked on the power beam. A station can act as a
communications relay station, servicing bases, expeditions and personnel on the
lunar surface. Question-response transmission delays would be less than a second,
compared to the over 5 seconds experienced using Earth as a relay station. The
location of the station, visible to the entire nearside, also lends it to
communications broadcasting in addition to point-to-point communications.
         
         Many have suggested using a Lagrange point for interplanetary staging.
One analysis shows that, using a Moon-Earth gravity assist, DV required for
insertion into Mars transfer orbit is reduced from 3.65 km/s to about 1.14 km/s.
As a returning craft experiences similar savings by entering the Earth-Moon
system at L1 over LEO, return-trip savings are about 5.0 km/s. With L1 being used
as a hub for lunar passenger spacecraft, transportation to Earth or to quarantine
facilities on the Moon(1) is addressed.

         
                               Service Module Design
         
             The spacecraft is designed mainly to fit inside an aircraft's
payload fairing. The spacecraft is shaped like an ice cream cone, with the
narrowest half of the cone removed. The separate and modular service module
contains fuel tanks, furled rectennas and radiators, a set of CH4/LOX engines,
and the magnetoplasmadynamic engine array, and weighs 14.7 tonnes dry.
         
         Passenger Section @120 kg per passenger      24,000
         Tankage                                       2,270
              LOX Tankage for 46.5 tonnes slushed              940
              CH4 Tankage for 17.4 tonnes slushed              800
              MPD fuel Tankage for 23 tonnes @1.5 g/cc         530
         Engines                                       3,560
              MPD Engines, 10 MW, 500 N, 100 kg each         2,000
              CH4/LOX Engines T/W 80, 1.2 MN                 1,560
         Structure                                     7,600
              Structure Group @5%                            1,850
              TPS @14% Braked Mass                           5,000
              Gear @2% Supported Mass                          750
         ---------------------------------------------------------
         Margin @5.7%                                  2,280
         Total                                        40,000
         
         Liquid methane was chosen as the fuel for a variety of its
characteristics. At 440 kg/m3, methane is more than five times denser than
hydrogen at only 79 kg/m3, with a specific impulse only slightly smaller, of 385
seconds. Further, the requirement of removal, disassembly and cleaning can be
replaced with a warm nitrogen purge, such as in kerosene engines where fuel is
even suspected in having entered the oxidiser injector.
         
         Boiling and melting points of the propellants are also a major factor. 
Oxygen is a liquid between 90.18 K and 50.35 K, and methane between 111.5 K and
90.5 K; at a storage temperature of about 90 K, little insulation is required
between the tanks, and problems resulting when one fuel can readily freeze
another are avoided. Even with slushed oxygen at earth launch, the insulation
needs only to maintain reasonable melt rates for the period between fuelling and
lightoff, which is always less than 3 hours, where the oxygen can be warmed and
the methane cooled by as much as 10% as acceptable limits. Handling is much less
of an issue above liquid nitrogen temperatures (77 K) than at liquid hydrogen
temperatures (20 K), as material embrittlement, insulation costs, and atmospheric
condensation are greatly reduced. In fact, a self-serve liquefied natural gas
(methane) automobile filling station has operated safely for more than a year in
New Mexico, serving an estimated 3,000 vehicles to date.
         
         As the two 56.2 m3 of propellants are stored at nearly the same
temperature and pressure, a bulkhead is required to separate the fuels, but
provide little structural support. To accommodate melting slushes and changing
relative quantities of the two fuels, either a shared floating bulkhead or a
pleated diaphragm of thin aluminium are under consideration. The pleats would be
in concentric circles, adding about 25% to the area of a flat dividing structure,
but allowing irregular tank shapes and requiring no seals. A floating tank
bulkhead separating the methane and oxygen is more capable and involves less
mass, although it is more complex and requires a straight section of tankage for
the wall to travel along. While leakage is not expected, significant levels of
propellant communication between the two tank sections would be acceptable with a
non-hypergolic fuel combination such as CH4/LOX.
         
         The lunar passenger service module uses a set of about eight CH4/LOX
engines based on RL-10 technology, each with a thrust of about 150 kN, for a
total thrust of 1200 kN. This is scaled-up from the RL-10A-4-1's 98 kN. At an
assumed thrust to weight ratio of 80:1, the engine system would weigh 1,530 kg.
         
         The RL-10 engine family uses a low-cost, low-performance approach. The
RL-10A-3-3A, for example, uses a stainless steel bell, a chamber pressure of only
50 psi, and an oxidiser turbopump running at only 13,000 RPMs. RL-10 engines cost
about $1 million each. The Black Horse study states that a methane-burning
version of the RL-10 engine would cost roughly $30 million to develop over a
period of three years. A unit cost similar to the hydrogen-burning versions of
the engine could be expected.
         
         For orbital passenger flights, stock RL-10A-4-1 or RL-10B-2 engines may
be used primarily due to the development lag time of CH4/LOX engines, but also
the fact using a LH2/LOX based propulsion module reduces weight significantly. In
fact, a passenger spacecraft stack using a LH2/LOX based service module, without
the electrical propulsion systems and with larger weight margins, would weigh
about 250 tonnes. This would allow it to make an orbital ascent from the back of
the AN-225 aircraft with no significant development projects beyond that of the
passenger module, and a service module which uses all off-the-shelf parts.
         
         The pair of rectennas deploy from the service module, each supplying
energy to a power bus running half of the electrical propulsion system. The
service module also houses three nonrechargeable lithium sulphur dioxide
batteries, each of which provide power to set of the chemical engine and
housekeeping systems. Costly turboalternators are thus not required, although
these batteries must be replaced before takeoff on each flight. A specific weight
of 300 watt-hours per kilogram is well within that available off the shelf,
meaning 93 kg of batteries per kilowatt of continuous power. The spacecraft's
primary and backup systems can choose between any of these three power busses.
         
         
         				Passenger Module Design
         
          The a 12 metre diameter sphere houses 200 passengers with over 4.5
cubic metres of interior space per passenger, with a total mass assumed of 24
tonnes. The mass budget per passenger is 120 kg. 
         
         The spherical passenger section is mated to the service module on
tracks, which allow the sphere to rotate 180°. This allows the passengers decks
to point "down" while the spacecraft is lying prone in the carrier aircraft,
during rocket ascent and cislunar transfer, and during reentry. As the structural
loads between the two spacecraft elements are almost exclusively compressive, the
sphere's freedom to pitch does not come at a great cost in complexity or mass.
Further, this allows the windowless bottom half of the sphere to receive the
thermal flux of reentry.
         
         For ease of operations on Earth, the passenger section is nominally
pressurised to 14.7 psi. With vacuum structural margins of 50%, including
structural properties of thermal protection materials, internal structures, and
outfitting, the passenger section can withstand a pressures differential of 22.5
psi before yielding. As the highest stresses on the spacecraft come from
atmospheric flight, the pressure differential is relatively small, and margins
are very high. Margins for tankage and structure are high to make a durable,
reliable spacecraft able to operate in a wide range of weather conditions.
         
         Each of the four passenger decks provide 50 passengers with the basic
amenities needed to enjoy a flight lasting a maximum of 16 hours, including a
galley, provisions for two stewardesses, and a pair of washrooms. Pressure
hatches, open during the lunar cruise, allow communication between different
passenger decks and the observation deck at the top of the spacecraft.
         
         The spacecraft provides over 4.5 cubic metres of interior volume per
passenger. While this may seem crowded, one's personal space is that within a one
metre radius, while the spacecraft provides roughly 1.5 metres between passengers
in every direction. Clustering near windows and at the weightless amusement area
introduces expected and escapable crowding, while reducing the passenger density
elsewhere in the spacecraft, which may prove to make crowding much more
comfortable for the passengers. Further, evidence shows that crowding serves
primarily to enhance whatever emotional state is extant when crowding occurs,
rather than necessarily generating negative emotions.
         
         The seats are contoured to the neutral buoyancy position, with
passengers lying on their backs. During flight, the seats are essentially part of
the floor, and have little utility beyond providing a home base and a place to
strap down for sleep. Approximately 350 cubic centimetres of personal storage
space is provided in cavities beneath the knee and head rests.
         
         Each deck, in addition to its many windows, includes a standardised
hatch used for routine ingress and egress. Emergency egress on the ground is
augmented by several windows serving also as emergency escape hatches. A
combination of exploding bolts and a hand lever release the window, which falls
from the spacecraft and deploys a Kevlar cable reaching to the ground. Passengers
attach their standard Navy sling to a friction device on the cable, which
maintains a constant rate of descent as they slide down the cable to the ground. 
The falling window also deploys a sheet of thermal insulation one metre wide,
which prevents passenger from coming into contact with the spacecraft's skin.
         
         
                                  Aircraft Design
         
             The carrier aircraft is designed to bring the 127 tonne loaded
spacecraft to altitude, and feed the spacecraft's engines some 323 tonnes of
CH4/LOX. The aircraft's structural and jet fuel component is assumed to be 40% of
the payload mass, or 180 tonnes.
         
         With a maximum payload of 450 tonnes, the aircraft is capable of using
the onboard turbojets to make subsonic domestic flights. If the 383 m3 CH4/LOX
tank system were modified to accept jet fuel, payload to the aircraft's maximum
range (of perhaps 15,000 km) is 155 tonnes, at a fuel consumption per kilometre
significantly higher than that of a conventional subsonic freighter. If a
supersonic flight mode with ramjets is used, range and specific fuel consumption
change by a factor of three to four, although a top speed of over Mach 5 can be
sustained. Both maximum fuel storage and payload volume are greater than double
that of a 747-400 freighter.
         
         The aircraft uses six F100-PW-229A turbojets, the newest version of the
F-15/F-16 engine now in testing. At a weight 1,860 kg, each develop 98 kN of
thrust, increased to 156 kN with the use of afterburners. The engine allows
throttling down to about 82%, and has thrust-vectoring capability.
         
         Weight will be mostly in the body of the aircraft, leading to increased
structural mass requirements over an aircraft with weight distributed more evenly
between wings and body.
         
         Assuming off the shelf 52 inch diameter tires, about 140 main wheels
will be needed, although only 72 wheels are needed for 79 inch (2 metre) tires
given a 830 tonne takeoff mass. This assumes a tire pressure of 110 psi, which is
in accordance with 120 psi maximums for major civil airfields, and 200 psi for
major military airfields, but not for 50-90 psi for minor airfields using tar
macadam construction. The landing gear would use a multi-bogey layout, with 8
tires on a set of 9 trucks. The aircraft require brakes less complex, massive and
expensive than those typical of large transport aircraft to stop while fully
loaded using 2 metre wheels, even from quite high speeds. With the less massive
payload of ferry, orbital tourism or terrestrial alternate applications, the
tires can be inflated to a lower pressure to extend lifespan, or operate from
small airfields with ground pressure a linear function of gross weight.
         
         Housing gear which could amount to 9 trucks of 12 cubic metres each on
an aircraft the size of a 737 could be an issue. To avoid cost and time
requirements of metal castings for the landing gear, regular aluminium or
graphite/epoxy composite construction is likely, with an cautiously assumed gear
mass of 5% the supported weight.

         Average temperature at an altitude of 100,000 feet and a cruising speed
of Mach 5 is about 460°C, and peak temperatures of under 760°C are expected. 
This allows very inexpensive chrom-moly steel, such as that used on inexpensive
bicycle frames, to be used for most of the aircraft's skin and structure, while a
high-temperature alloy such as Inconel must be used for the rear underbody and
nacelle leading edges. At altitudes of about 110,000 feet, chrom-moly steel can
be used on stagnation points such as the nose and leading edges of wings with a
temperature margin of about 50°C. Thermal protection requirements for a Mach 22
reentry will probably blow steel out of the water, alas.
         
         Medium-strength steels such as chrom-moly, with a yield stress of only
70 kpsi, can be worked into structural elements with little difficulty and by
conventional large fabricators for around $5 per kilogram. Low alloy steel and
aluminium for slightly higher temperature tolerance and cryogenic tankage,
respectively, cost about $20 per kilogram at about twice the strength of the
aforementioned medium steel. High-temperature nickel alloys such as inconel,
however, are relatively expensive, with strength properties similar to chrom-moly
steel.
         
         As passengers are contained within the independently-powered spacecraft,
operating and emergency power requirements for the carrier aircraft are at a
minimum. A set of rechargeable NiH2 batteries provide engine start energy for the
first two engines, which are equipped with alternators to power the aircraft's
systems, recharge the starter batteries completely before takeoff, and to start
the remaining engines. Emergency power is also supplied by the NiH2 battery,
rather than a jet-fuel turbine or monopropellant engine, which are expensive and
not required. When in hypersonic flight, the two turbojet engines equipped with
alternators are allowed to idle or windmill, generating electrical power.
         
         The aircraft is calculated statistically to mass 44% of gross weight, or
about 225 tonnes. Engines, landing gear, and inexpensive construction drive a
relative initial mass of 1.2, which compares favourably with other large
aircraft, such as 0.93 for military cargo or bomber aircraft, or 1.02 for jet
transports. It scales on a per-pound basis at an exponent of -0.07, the same as
military cargo aircraft or bombers and much less quickly than most other types of
aircraft. Various assumptions change this weight generally between 175 to 350
tonnes. The fact that much of this gross weight is payload and dense fuels, and
that the total aircraft volume is very small may make this estimate conservative,
although it is quite elastic to assumptions.
         
         At Mach 5, the aircraft's drag coefficient is only 59% of profile drag
below the aircraft's critical Mach number.
         
         
                                 Radiation Exposure
         
             Radiation exposure, including radiation dosages from passage through
the Van Allen belts, are not unacceptably large. The main mechanism behind low
overall dosages is the rapid transit through the lower Van Allen belt augmented
by the chemical engines. The upper Van Allen belt can be passed through at an
even higher rate, and its radiation is relatively easily shielded.
         
         The inner Van Allen belt has by far the most intense radiation, at an
average of a couple mSv per hour for a transfer of about 4 hours. This assumes an
average of 2.5 g/cm2 of shielding, which is considered reasonable due to the
heatshield, pressure vessel, and exponentially more effective shielding through
the higher shielding densities from the service module.
         
         The spacecraft will insert itself upon a Hohmann transfer from 1,300 km
to 8,000 km, which puts the spacecraft on a high-speed trajectory through the
most intense areas of the radiation belt. The chemical engines provide a 660 m/s
burn, and the electrical engine continues the manoeuvre, attaining the final
transfer velocity in 5.7 hours, most of which is spent above the altitude of
about 6,000 km.
         
         I need low-thrust orbital time equations for this. :(
         
         At lower altitudes, the belt is made up of highly energised protons,
which are difficult to shield against, with an increasing proportion of
high-energy electrons as altitude increases into the upper Van Allen belt. These
doses are above 1.2 mSv per hour between altitudes of about 1,300 km and 7,000
km, above 4 mSv per hour from 1,600 km to 4,000 km, and peaking at 13 mSv per
hour around 2,500 km altitude.
         
         At altitudes above 15,000 km, radiation levels of about 0.2 mSv are
expected at the area densities envisioned, consisting manly of bremsstrahlung
X-rays from interactions between the radiation and shielding.
         
         Total radiation dose breaks down as 5-10 mSv from the passage through
the inner Van Allen Belt, 1-2 for the outer radiation belt, ____ mSv of radiation
from space, and a few mSv are incurred passing through the belts at high speed on
the return flight.
         
         The standard annual radiation dose is 2.7 mSv(2) as an average for North
America, while a upper gastrointestinal x-ray gives a dose of 5.4 mSv. Exposure
from a transatlantic Concorde flight is about 0.05 mSv. The regulatory limit for
a member of the public to be exposed to from terrestrial nuclear industries is 5
mSv.
         
         Several sources show the rate of fatal cancer induced by radiation to be
roughly 1-2 deaths per one hundred thousand for radiation doses estimated,
comparable to the average of 2.0 annual deaths(3)  per 100,000 passengers in
commercial air travel over 15 years previous to 1992, and 14.0 annual deaths per
100,000 in agriculture. The noted British physician Sir Edward Pochin's
estimations put the risk inherent in the Van Allen belts' radiation dose over a
return trip to be the same as smoking 3-5 packs of cigarettes, though as this
uses the linear theory of radiation exposure effects, may be significantly
inflated.
         
         
         Impact on GLOW and DV requirements of orbital insertion beyond a 300 km
orbit
         
        The graph above shows the additional mass required to deposit the
spacecraft at various orbits beyond a typical 300 km low earth orbit. One may
note that the high-isp engine is capable of accelerating the spacecraft by
approximately 12 m/s per minute to increase and circularise the orbit. Over half
of a 120-minute orbit, about 600 m/s of DV can be imparted, 
         
         
         Chemical DV requirements by final altitude, starting at 850 and 1,000 km
         
        As the graph above shows, the DV required for the low specific
impulse chemical engines is greatly affected by the altitude of the orbit from
which manoeuvre starts. For every additional kilometre of altitude performed by
the magnetoplasmadynamic engine, the final altitude achieved by the chemical and
electrical engines increases by nearly four kilometres. At a 6,000 km final
altitude baselined, switching over to chemical engines at 1,000 km altitude
decreases DV requirements by 172 m/s over a 850 km starting orbit, and gives a
larger reduction in radiation exposure per unit of velocity change.
         
(2) The mSv is the SI unit for radiation, 1 millisievert being equal to 0.1 REM
(3) Including fatalities from acts of terrorism

         
                            Alternate Vehicle Functions
         
             Many alternate mission scenarios for the vehicles exist, which
spread development and production set-up costs over a larger number of vehicles,
introduce economies of scale, and generate extra operations revenues. To
facilitate these revenue-generating missions, the vehicles should be designed
with an adaptability and modularity in mind.
         
         
         Air Cargo
         
         The carrier aircraft is a very versatile aircraft, capable of performing
a wide range of functions. Its very large payload, high speed, and the express
cargo industry's very rapid growth make for enticing possibilities of perhaps a
dozen aircraft sales for high-volume express cargo routes.
         
         Cargo transport is an important possible application of the aircraft. In
the past, high-speed reusable spacecraft and rocketplanes have met serious
consideration by overnight delivery services for very-high-speed parcel delivery.
Turbojet engines generate noise within acceptable limits, and a top speed of more
than Mach 5, the aircraft is almost ideally suited to high-speed freight
operations.
         
         Boeing has conducted studies on the future markets for cargo transport.
Boeing predicts long-term growth in freight traffic will average 6.6% per year.
Express traffic, however, is expected to average 18% annual growth through 2015.
Long-distance Asian routes are experiencing the largest growth rates, with market
demands favouring wider-bodied aircraft over these routes.

         Some 404 freighters with 50 tonnes or more payload will be added to
world fleets by 2017, 289 of which will be added between 2007 and 2017. While
many of the express aircraft are small or medium size aircraft, there is clearly
a great demand for large, new cargo aircraft as airports and trade routes are
inundated with increasing trade. Only 37% of these new aircraft, however, will be
used for international express in 2015.
         
             As air traffic steadily increases, airports are put under greater
and greater strain to accommodate large numbers of aircraft, including
freighters. Combining many regular flights using smaller freight aircraft with a
single larger aircraft also saves in operation costs, due to a higher flight
rate, lower crew and maintenance payroll, and somewhat lower fuel costs. With a
New York to Los Angeles payload of several hundred tonnes, provided the routes
exist, as many as ten large freighters could be amalgamated into a single flight
using the carrier aircraft. Using an aircraft which costs three times as much,
although replaces many smaller aircraft, has even greater amortisation economics
if the severalfold-increased flight rate is considered. Extensive use in subsonic
flight mode may, however, demand significant modification and redesign to the
aircraft's aerodynamics and engines, driving the unit price up significantly.
         
         The main issue concerning such large daily payloads is the uncertain
existence of total intercity traffic rates justifying such a tonnage capacity.
However, the aircraft's many daily flights could be distributed over several
cities, reducing the city-to-city traffic rates replaced from roughly ten large
freighters between two cities to two or three large freighters between many
closer cities. Concerns about market demands considered, it may be that flight
economics favour the use of a large, high flight rate aircraft to replace
multiple routes between many cities each. Further study, however, is required to
determine if this is the case.
         
         
         Supersonic Passenger Air Transport.
         
         Supersonic air transport has been long considered as a possibility. The
lack of widespread supersonic transit is the result of regulatory issues
concerning sonic booms, and the availability of operational aircraft. For the
long-distance intercontinental routes, which Pacific Rim countries will cause to
show the largest growth in traffic over the next decades, high-speed aircraft
have a large market of dozens of aircraft.
         
         Mass requirements for passenger flight allow the ferrying of much more
than 500 passengers for extended distances at a speed of about Mach 5. Outfitting
the spacecraft's payload bay to seat large numbers of passengers would be a
significant task, although the systems used are off-the-shelf and not exceedingly
expensive.
         
         Air travel is expected to grow at 4.9% per year over the coming twenty
years, declining to an average of 4.3% over the second half of the study period.
By 2016, the proportion of large aeroplanes is  expected to decline from 8.4% to
7.4% of the world fleet, although world fleet size will more than double. Total
demand over between 2007 and 2017 for new 747-class or larger aircraft is
expected to be 620. Aircraft larger than the 747, mostly during the second decade
of the study period, will come into demand:
         
         
                 "The market for aeroplanes larger than today's 747-400 becomes
significant during the second decade of this forecast. Only by then will traffic
volumes support an aeroplane larger than the current 747, for most major
intercontinental routes will have daily service, and airport capacity problems
will be more severe. The projected requirement for aeroplanes of 500 seats or
greater is estimated at 480 jets over the study period."
         
         
         Impact of sonic boom will be significant for both express cargo and
passenger flights. Although minimising sonic boom would be a major design factor
in the aircraft, routes over populated regions for much of the flight are
unlikely to use hypersonic aircraft. In addition to the small seating capacity
and high costs, one of the major factors preventing the widespread application of
the Concorde in North America was the sonic booms it generates. Nevertheless, a
greatly inferior high-speed passenger aircraft such as the Concorde sold 16
units; a larger, faster, and cheaper to operate aircraft could have the potential
to capture much of the market for large passenger aircraft for busy routes over
ocean or unpopulated regions.
         
         
         Parabolic Flights
         
         A significant market exists for balloons and sounding rockets for
high-altitude research. With a peak altitude of perhaps 60 kilometres yielding
free-fall conditions for over six minutes at a time, and a very large payload,
the aircraft may capture some of this market. Other scientific applications, such
as chasing solar eclipses and lofting astronomy equipment above much of the
atmosphere will likely also benefit from the aircraft's unique features.
         
         Flights reaching altitudes of perhaps 60 km and extended free-fall would
not only capture the tourist market for parabolic flights, but also for astronaut
training programs and movie production. Companies such as Interglobal Space Lines
are already profitably providing this service aboard conventional subsonic
aircraft.
         
         
         Orbital Tourism
         
         Despite the large appeal of lunar tourism, orbital tourism is
significantly cheaper and easier ‹ and will probably be done first. In fact, due
to the reduced performance requirements, first-generation spacecraft can be used
while the development of more capable versions of the aircraft and spacecraft,
able to carry the greater payload for lunar tourism, complete development and
testing.
         
         In recent years, several very reputable surveys of demand for space
tourism have been conducted. At a cost of $10,000, the number of people in the
world interested in an orbital flight lasting several hours is estimated to be on
the order of 100 million.
         
         Spacecraft cost and mass for use only in low earth orbit are
significantly reduced. The electric propulsion system, including fuel for the
lunar flight, can be omitted, the reduced mass allowing 4.9 km/s of velocity
change using tanks identical to the lunar spacecraft. This also allows the heavy
carrier aircraft to separate over twice as soon, reducing total fuel requirements
and cost. Even forgoing expensive slushing, this leaves enough fuel to land and
perform large orbit changes, including docking with an orbital hotel. The
on-runway takeoff weight is about 370 tonnes, the aircraft is able to operate any
medium-sized airfield, without the need for afterburners at takeoff.
         
         A more near-term option would be to replace modified, methane-burning
RL-10 engines with the stock, hydrogen-burning RL-10B-2 engines in the modified
service module. This removes a lag time of several years for development of the
CH4/LOX engine. Further, a hydrogen-oxygen service module using all off-the-shelf
parts, launched from AN-225's built-in mating pylons, can take a stock passenger
module on an orbital tourist flight.
         
         The AN-225, being by far the world's largest aircraft at 600 tonnes of
maximum take-off weight, requires a takeoff runway 3.5 km long at its full fuel
and cargo weight. However, retrofitting the aircraft's six turbofan engines with
more reliable and better performing models, such as the GE-80, would reduce this
considerably. Regardless, due to the near-term, low flight rate nature of this
option, the foibles of the AN-225 are not more than inconveniences.
         
         All considered, the annual flight rate for an orbital ticket costing
less than $2,500 could be expected to be at least 10 million passengers per year.
Assuming a flight duration and turnaround time of 12 hours, this corresponds with
some 250 spacecraft operating from dozens of airports worldwide. Earlier in the
development timeline, 5-10 spacecraft could service a smaller market at a much
higher ticket price, providing many billions in profit annually for an initial
investment of less than one billion dollars per operational spacecraft.
         
         
         Cargo Launch to Low Earth Orbit
         
         The carrier aircraft, with a payload of some 560 tonnes to Mach 5.0 and
high altitude, can also be used to launch upper stage spacecraft. Despite payload
fairing constraints, the price tag of under $100,000 for moderate flight rates is
attractive. If equipped with a large rocket engine, fuelled by the 350 tonnes of
CH4/LOX usually used by the passenger spacecraft, the carrier aircraft could act
as a first stage for a cluster of Centaur upper stage and nearly 100 tonnes of
orbital payload.
         
         Without the passenger compartment or electric propulsion apparatus, a
cargo version of the passenger spacecraft could enter production relatively early
in the overall development cycle. An early model might carry 70 tonnes of payload
to orbit, with a cost of production of the first few units being 15% of the
development cost. One might assume a downtime of 120 days per year, a flight rate
of only 1 per day, a lifetime of 5 years, and similar assumptions for the
prototype carrier aircraft, and a substantially reduced payload over the full
orbital model. This works out to about $6 per pound to orbit for a 60-tonne
payload. As production of lunar spacecraft drove down costs, an evolved model
might carry 80 tonnes to orbit at a cost based on lunar passenger spacecraft, of
about $500,000. This equates to a cost of less than $3 per pound to orbit,
through sharing the development cost with lunar passenger spacecraft and making
several orbital flights daily.
         
         In the nearer-term, the 225-tonne LH2/LOX service module mentioned to
propel a passenger module into orbit from the back of the AN-225, without the
passenger spacecraft, could place 25 tonnes into orbit at an amortisation and
fuel cost per flight considerably less than $1 million at a few flights per week.
This equates to $18/lb to LEO ‹ a value which is insensitive to flight rates, as
the hardware involved would be diverted for the flight from the possible
passenger business.
         
         
         Orbital Ferry and Service Vehicle
         
         The ability to land on Earth periodically for servicing and refuelling
would make the spacecraft much more economical in the near-term than dedicated
space-only orbital servicing vehicles. Also, the off-the-shelf nature of the
spacecraft reduces capital costs. That said, however, it may reduce costs for
long-term, large-scale orbital operations if massive items such as the
heatshield, landing gear, chemical engines, tankage, and fuel could be omitted,
and even the passenger compartment for some applications. This, however, is a
separate development effort, although made much less costly by having an
atmospheric, passenger model to start from.
         
         As designed, a spacecraft could ferry cargo and passengers between
destinations in Earth orbit, Lunar orbit, and at Lagrange points. Satellites
stranded in a useless orbit could be repaired, returned to Earth, or captured and
redeployed in another orbit. Routine servicing of microgravity manufacturing or
scientific satellites would be economical, and allow the construction of
lower-cost spacecraft.
         
         With an adaptor affixed to structural hard points, the spacecraft could
mate with an object as large as a solar power satellite or small space habitat
and perform station keeping manoeuvres. As the mass of the spacecraft is trivial
compared to the facility's weight, this is quite cost-effective compared to
dedicated hardware and slightly smaller refuelling flights.
         
         Mars Cyclers are large spacecraft in permanent orbits between Earth and
Mars, at each end using gravity assists to return. Without refuelling, the
spacecraft could transfer some 2,400 passengers between LEO or L1 and cycling
space habitat as it approaches, including 200 passengers carried each to and from
Earth's surface. As cycling hotels are one of the most economical methods of
transferring large numbers of passengers between Earth and Mars, this is a clear
market for the spacecraft.
         
         
         Booster Stage for Interplanetary Missions
         
         The service module of the passenger spacecraft can be used as a booster
stage for interplanetary or very massive spacecraft with a specific impulse of
2,000 seconds. If the orbital cargo launcher discussed above retained an
electrical propulsion system, the vehicle could accelerate a 32 tonne payload to
trans-Mars insertion, separate, and return to the airport of origin. The vehicle
can also carry a maximum of about 16 tonnes to 6.2 km/s before returning, or 44
tonnes to 1 km/s.
         
          Alternately, the spacecraft could mate with and accelerate a
previously-orbited payload of 260 tonnes to trans-Mars insertion, decelerate, and
return to Earth. Similar masses can be moved between earth orbit, lunar orbit,
and near-earth asteroids without requiring the payloads to withstand aerocapture
manoeuvres. Alternately, 33 tonnes could be inexpensively accelerated to a
maximum of 11 km/s before the spacecraft separating and returning. Assuming the
market supports a modified spacecraft launching a payload weekly, the mission
would cost a few million dollars.
         
         Staging reusable spacecraft is another attractive option. Using two
identical spacecraft, a 112 tonne object in earth orbit can be carried to 13
km/s, and 610 tonnes to Mars, or 930 tonnes to L1, with both spacecraft returning
to Earth. The use of space-only versions of the spacecraft have a minor impact on
performance, although almost arbitrarily large payloads and velocities are
possible if additional fuel is orbited with another spacecraft. With staged or
additional-fuel options, however, the final distance at fuel exhaustion becomes
an issue for power beaming. Multiple passes, including using interplanetary
gravity assists, may be required for the largest additional fuel supplies,
although the use of multiple spacecraft in parallel reduces these problems
significantly.

         If the spacecraft is not reclaimed and reentry structures are removed,
the modified spacecraft would weigh about 7 tonnes. In this scenario, the
spacecraft can accelerate a 10 tonne payload to nominal payload to over 30 km/s,
or a nominal payload to a whopping 45 km/s. Even at these speeds, the engine can
exhaust the total onboard fuel supply before power beaming costs and ranges
become a very serious issue. As the mass accelerated increase, however, the
advantages evaporate; 300 tonnes can be put on a Mars insertion orbit, an
increase of only 60 tonnes by using a disposable spacecraft. These high
velocities, however, are enough to conduct useful studies of the Oort cloud and
outer planets, especially if combined with Jovian or Solar gravity assists. A
cost less than one hundred million is expected for launch and acceleration of the
booster spacecraft and payload at the production rates expected for the service
module through the passenger transportation architecture.
         
         
                             Economics and Flight Rates
         
             A cost model with about 200 equations was developed for both lunar
and orbital passenger services, although it does not include the cost of capital.
At a flight rate of 1,000,000 passengers per year, a direct cost per passenger of
$4,500 is expected for a ticket to the Moon. This does not include indirect costs
of sales and customer support, administration and overhead which significantly
add to and often double ticket price.
         
         Development and production costs for both aircraft and spacecraft incur
a capital cost of $16 billion, which excludes port facilities on the Moon, a
lunar hotel, some minor modifications to several airport terminals around the
globe, or passenger training facilities.
         
         Cost Model Assumptions
         
         Fuel costs are 14% -- a significant factor -- of total costs, at
$128,000 per flight. Terrestrial oxygen costs $0.05 per kilogram, manufactured
on-site, and the world average trading price for liquefied natural gas is about
$0.135 per kilogram. A value of $0.20 per kilogram of methane ($3.84 per MMBTU)
is assumed to include some purification processes, for a propellant total of
$28,800 per flight. Ammonia for the electrical propulsion system costs
approximately $0.20 per kilogram, or $6,200 for 31 tonnes, although prices may be
increased by seeding the fuel with a fraction of a percent of other chemicals.
Lunar oxygen constitutes most of the fuel cost at $78,300 per flight. Roughly
59,000 tonnes of lunar oxygen are produced per year, at about the cost of
terrestrial aluminium production, or $3 per pound. Jet fuel prices at major US
airports have fluctuated between about $0.14 and $0.19 per litre in the last six
months; a typical value of $0.16 per litre ($0.61 per gallon) is assumed for a
total aircraft propellant cost of $14,700.
         
         Vehicle amortisation costs, due to the high flight rate, compose only
15% of costs, with overall spacecraft amortisation costs at $88,300 per flight.
Spacecraft development costs are assumed to be $2 billion, a very generous figure
for a low-performance structure with high margins. Total aircraft costs of
$44,000 per flight are shown. The carrier aircraft's airframe development is
assumed to cost $5 billion, also for a heavy structure with large margins and
off-the-shelf technology. Henry Spencer suggested values of one billion and a few
billion for the spacecraft and aircraft airframes, respectively, and stated that
"done right", the values of $2 billion and $5 billion are definitely reasonable.
         
         In addition to the above airframe development costs, the amortisation
cost per flight includes a firm estimate for development of a CH4/LOX version of
the RL-10 engine at $30 million. Also, a 10 MW magnetoplasmadynamics engine are
assumed to cost $500 million to develop. Two estimates of the development costs
for the electrical propulsion system, which is far below limits of specific
impulse and uses well-understood devices and principles, were placed at the order
of $100 million. Main costs for the electrical engine development include some
fundamental research in electrical propulsion, and ground test facilities for a
high mass-flow engine which requires a high quality vacuum.
         
         A large ramjet, a simple device, is also required for a development
budget of $500 million. The Mach 5 engine envisioned is significantly below the
limits of materials technology, and has been used on the highly successful SR-71,
a French fighter aircraft flown as early as 1949, and on several military
missiles. However, in addition to the requirement of a supersonic wind tunnel for
testing, few ramjet engineers are currently available, driving up costs.
         
         Production costs of the four required aircraft are calculated to be $410
million. This assumes a learning curve of 80%, which is consistent with observed
curves of 75-85%, although such rates are conservative for the low production
rates envisioned. The ratio of development costs to theoretical first unit costs
suggested by "Space Mission Analysis and Design" are between 2 and 3 for
conventional spacecraft, and a value of 3.0 is chosen for both the aircraft and
spacecraft through use of easy-to-machine alloys and the relatively costly
development of a ramjet engine, and a policy of extensive (and possibly
destructive) flight testing and large margins reduce unit testing and fabrication
costs. Also, the estimate may further be conservative in that the aircraft is
large and much of the dry mass is composed of tankage, further reducing unit
fabrication costs by mass and development cost.
         
         The $410 million average production cost assumes 35 additional aircraft
are produced as operational prototypes and external sales for cargo, passenger,
and scientific applications. If the military transport market raises this value
to 100 external sales, average production costs drop to $274 million, while if
only 5 external sales take place, this rises to $755 million per aircraft.
         
         Spacecraft production costs of $171 million are assumed, including $1.9
million per MPD engine and $1.3 million per chemical engine, for each of 24
vehicles. This is calculated by a learning curve of 90%, which is generally
suggested for production runs of 10-50 spacecraft, and a ratio of development
cost to theoretical first unit production cost of 3.0. It is assumed that one
additional spacecraft airframe is produced for alternate missions, which impacts
airframe unit cost by $2 million. Should the aircraft cost $2 billion per unit
and each spacecraft cost $1 billion to produce, cost per ticket rises by $2,445.
         
         Power costs is a significant fraction of total costs, at 19% with a cost
per flight of $170,000. As the electrical propulsion system requires 149 MW of
energy to 46% of the 24 spacecraft, a fairly constant average of 1.65 GW of power
generation capacity is required on the Moon, at an assumed cost of $0.05 per KWH.
This is the amortisation cost of a lunar solar array, and is not included in the
stated capital costs. Power beaming equipment is only 0.07% of total costs due to
amortisation over many flights. If power costs were instead $0.10 per KWH, and
inefficiency was double, ticket cost would rise by $1,423. At $0.20 per KWH,
costs are increased by $3,693. However, as power costs are a direct function of
specific impulse, these figures for increased energy costs are inflated by as
much as 50% due to the fact of a new, lower optimal specific impulse.
         
         Miscellaneous costs of 23%, or $200,000 per mission, are that of the
Japanese studies on space tourism using a single stage to orbit passenger
spacecraft. However, this includes handling of liquid hydrogen and extensive
infrastructure development at airports, and as neither are required, this figure
may be conservative. Miscellaneous costs for an aircraft flight is generally
$20,000. A miscellaneous cost not included in the above figure is about 0.15% of
the cost per flight for a transportation node at the L1 balance point, and the
small cost of replacing the spacecraft's nonrechargeable main batteries.
         
         Crew and inspection salaries compose 19% of total costs, at $166,700.
This includes burdened pay rates for 2 pilots, 8 stewardesses at a ratio of 1:25,
and 2 ground control crew available per spacecraft at any time. Aircraft crew
costs are assumed to be $1,000 per block-hour, and combined with the spacecraft
crew costs, amount to under $12,000 per flight. The aircraft is assumed to
require 75 maintainence man-hours per flight hour (MMH/FH), and the spacecraft
500 MMH per flight, both with a SOC of 5. The spacecraft maintainence man-hours
is the overwhelming factor in costs. While appropriate values for the spacecraft
are unclear, the aircraft estimate appears to be conservative when compared to
25-50 MMH/FH for bombers, 20-40 for military transports, 15-20 for fighter
aircraft, and 5-15 for commercial transports, although certain special-purpose
aircraft have MMH/FH values over 100. This consists of a maintainence time of
2.35 hours for the aircraft and 8 hours for the aircraft per mission cycle,
separate to the loading and fuelling time, and a downtime of 60 days of the year
for both vehicles.
         
         Using equations for civil aviation, maintainence materials cost for the
aircraft and spacecraft are calculated to compose 11% of total costs, at $95,200
per flight. This consists of $9,100 for aircraft materials and $86,100 for
spacecraft materials. The equations are based on principally on engine and
airframe cost, and block time.
         
         
         Projected Demand and Revenues
         
         One detailed study conducted of orbital tourism demand shows that at a
cost per passenger ticket of $60,000, a global flight rate of about 1 million
passengers per year can be expected. This study includes a substantial margin
from the market surveys, where respondents claim to be willing to spend many
times more at comparable flight rates. Therefore, while severalfold reductions in
prices will occur in the future as competition forces large profit margins down,
the common knowledge of this fact can be kept to a minimum, and significant
amounts of margin already exist in the demand estimates. Tickets will be sold
through an auction, or some similar method which reduces demand to the available
flight rate through raising prices.
         
         Costs at this flight rate are $922,000 per flight, or $4,500 per
passenger. As a result, $54.4 billion of annual surplus is available for further
space development. Some of this can be spent on increasing the fleet size to a
maximum of about 240 spacecraft, carrying 10 million passengers per year at a
price of about $3,500 per passenger, at a capital cost of roughly 150 billion,
excluding 20 GW of lunar power generation. Also, market surveys states that an
orbital hotel would increase demand for tourism to low earth orbit by a factor of
about 15, coupled with the availability of lunar resources to leverage
construction, making it another attractive investment option for a revenue
surplus.
         
         The large possible profit margins allow smaller, precursor services to
pay the lunar architecture's development costs. For example, two aircraft and
five spacecraft could be built several years early for low-performance, orbital
operations, without requiring the magnetoplasmadynamic engine or ramjet
development projects to be complete, or requiring any space infrastructure. The
production costs for these early-model vehicles would be less than $5 billion,
however they could transfer 375,000 passengers annually, at a market bearing
price per ticket of at least $20,000, generating $6.75 billion of profit per year
and funding much of the capital costs of the lunar vehicles.
         



                               Selected Bibliography
         
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