STS - Then and now...... (Long article on Shuttle)
- From: eugene@xxxxxxxxxxxxx
- Date: 1 Aug 2005 11:31:03 -0700
I post this article from Flight International of September 1976. It is
sad to see that so very few of the aspirations have actually been
achieved.
SPACE SHUTTLE DEBUT.
Flight International 25 September 1976.
The first Orbiter vehicle, main constituent of America's Space Shuttle,
was
rolled out at Palmdale in California last week. The Shuttle is a
second-
generation launch vehicle, and from 1980 will carry into orbit most US
and
European payloads.
DAVID BAKER and MICHAEL WILSON here present a users' guide to the
Shuttle and its growing range of "add-on" equipment.
NASA has coined a new term, the Space Transportation System or STS,
to more accurately reflect the capabilities of the composite launch
vehicle
based on the Space Shuttle as a kind of multi-purpose first stage. It
comprises the Shuttle itself, Europe's Spacelab manned orbital
laboratory-
cum-observatory, and a series of upper stages to boost satellites into
higher orbits and start deep-space or interplanetary probes on their
journeys. There is also a scheme for an unmanned, heavy-lift vehicle in
which the payload, taking the place of the winged Orbiter, is thrust
into
space by the Shuttle's main engines and boosters. The STS is therefore
a
building-block launch system. and will replace three of the four
expendable
rockets - Delta, Atlas-Centaur and Titan-Centaur - which comprise the
current ffeet of Nasa/DoD launch vehicles. The smallest of the quartet
is
the Scout, and this is likely to co-exist inde;finitely with the
Shuttle.
When Nasa's search for a major project to succeed Apollo began to
crystallise earlier this decade into a recoverable launch vehicle, the
assumption was that it would replace all current US expendable rockets.
Indeed, this was how the venture was justified to the US Government.
Production rates of the two large rockets, Atlas-Centaur and
Titan-Centaur,
have now slowed, and their launch costs are likely to be considerably
greater than those anticipated for the Shuttle. There will thus be
little or no
incentive for the commercial or scientific communities to use them when
Shuttle operations get under way.
But the smaller Delta remains attractive, and the Shuttle may be
economic
only if two or more payloads can be carried on one flight. This may be
difficult or impossible if the desired orbital inclinations are widely
different, a
situation which is more likely to affect the scientific-satellite
users, with
their multitude of orbits, than the commercial customers requiring
generally
synchronous paths. The former group can often wait for a Shuttle flight
going their way, however, while the latter, geared to maintaining a
public or
commercial service, may be able to "double-up" with another Shuttle
customer. So, provided Nasa can organise its route network and
schedules
suitably, there is likely to be little call even for the Delta after
1980.
Certainly, the continued use of expendable rockets would embarrass
Nasa,
though the administration would rather phase them out through the
greater
appeal of the Shuttle than by placing an embargo on their use.
Meanwhile
a new expendable, Europe's Ariane, will become operational at the same
time as the STS to ensure an independent European launch capability.
How far this will compete with the STS will depend on pricing policies.
In order to keep costs down, the STS will be managed and operated like
an
airline, with scheduled flights into space leaving once a week from one
or
other of the two US launch sites. Nasa's chief job now is to market the
Shuttle to potential users throughout the world in order to achieve the
highest utilisation, particularly in the early years. To this end the
agency
several years ago prepared a traffic forecast or mission model covering
the
first 12 years of operation, 1980-1991. This is continuously revised
(the
latest one covers the period 1979-1992), since the costs of STS
operation
will be very sensitive to its utilisation. Space budgets in the US have
fallen
in recent years, and it seems likely that the Shuttle will be initially
under-
utilised despite Nasa's endeavours.
Based on forecasts of eventual utilisation, Nasa plans to acquire five
Orbiters (the delta-winged Orbiter, flown by a crew of four, can be
regarded
as the key module of the STS). Nasa has ordered one Orbiter, is
negotiating for the second, and has signified its intention to buy a
third.
There is some doubt about who pays for the fourth and fifth Orbiters;
the
Shuttle is a Nasa programme, but the DoD will fly one-quarter of the
missions during the first 14 years.
Shuttle and its ancillaries
The basic shuttle comprises the first two stages of a launch vehicle to
which may be added one or more third stages to accommodate
synchronous-orbit and planetary payloads. The first stage is powered by
three liquid-propellant engines, supplemented by two Solid Rocket
Boosters (SRBs), which provide the Orbiter with 99 per cent of the
impulse
required to impart low Earth orbit velocity. Fuel for the Orbiter's
liquid-
propellant engines is carried in the external tank, mounted under the
vehicle's belly. Shortly after the tank has been jettisoned, two
Orbital
Manoeuvring System (OMS) engines at the rear of the Orbiter ignite
briefly
to impart the, additional impulse needed for a 50 n.m. x100 n.m.
ellipse.
This is transformed into a 100 n.m. circular orbit by re-igniting the
OMS
propulsion unit at first apogee.
The Shuttle's load-carrying capacity reflects a preference for this
two-stage
configuration, which will be used when the altitude required is less
than
650 n.m. for a 10,000lb payload. The maximum lifting capacity of
65000lb
limits altitude to a 220 n.m. circular orbit. If payload/ height
requirements
fall outside these limits a third stage is carried in the cargo bay.
The
Shuttle then becomes a platform from whicha supplementary expendable
propulsion system known as the Interim Upper Stage (IUS) and its
payload
can be separated and put into its own trajectory. The IUS is a DoD-
sponsored vehicle to deliver at least 4,500lb to synchronous orbit and
several such stages can be assembled to cater for high-energy planetary
or heavy synchronous-orbit payloads which exceed this limit.
In this three-stage configuration the Shuttle promises to accommodate
all
anticipated Earth-orbit and interplanetary unmanned payloads for the
remainder of the century.
IUS costs
The IUS will cost the Dod about $1 million a unit, but launch costs to
the
user will work out at about $5 million, including development
amortisation
and launch services. In preparation for introducing the IUS in 1980,
the
USAF's Space and Missile Systems Organisation (SAMSO) proposes a
definition phase from September 1976 to February 1978 (mainly to settle
the payload rrquirement), followed by full-scale development From
February
1978 to June 1980. The two-stage solid-propellant IUS will employ low-
cost, off-the-shelf equipment and SAMSO anticipates placing a
production
order for about 270 units in 1980. The USAF intends to retain its Titan
III
launch vehicle as an insurance against Shuttle non-availability or
technical
failure, and requires IUS to be compatible with the Titan family so
that it
could replace the existing Burner II upper stage if necessary.
The IUS will be uneconomic for small synchronous-orbit satellites, and
for
payloads in the 2,000lb class (equivalent to the lifting capacity of a
Delta
3914) Nasa hopes to introduce a cheap, spin-stabilised, small boost
stage.
Definition begins next February, with detailed design getting under way
by
the end of 1977, leading to first flight in December 1979 on one of the
six
Shuttle development missions. If development of this supplementary
propulsion system is approved, Nasa will have two third-stage boost
elements (albeit expendable ones) tailored to synchronous-orbit
payloads.
Planners consider it likely that by the mid-1980s some customers will
be
calling for payloads to be retrieved from synchronous orbit for
refurbishment
and subsequent re-use. With this requirement in mind, Nasa proposes to
develop a Space Tug for service In 1985-87. The current basic design is
a
single-stage liquid oxygen/liquid hydrogen-powered vehicle 30ft long,
15ft in
diameter and with a propellant capacity of 50000lb. Carried to a low
Earth
orbit by the Shuttle, the Tug would boost its payload to synchronous
orbit
following separation from the Orbiter cargo bay, just like the IUS and
the
small boost stage. After circularising its orbit and placing the
payload on
station the rocket would return to a low Earth orbit for retrieval by
the
Shuttle. The Tug will be lifed for up to 100 flights and will be able
to carry
8000lb to synchronous height, retrieve 3400lb or deliver and retrieve
2,400lb. Its synchronous-orbit delivery capability is twice that of the
expendable IUS, and retrieval and delivery/ retrieval (or so-called
round-trip)
missions would provide a new level of flexibility for payload planners.
Emphasising simplicity and reliability, the Tug will be powered by the
RL-
10 rocket motor designed for the Centaur upper stage. With a minimum
development time of six years a decision to proceed will have to be
taken
within the next three years if it is to be introduced by 1986.
Although designed primarily for synchronous-orbit activities, the Tug's
generous performance opens up the possibility of larger versions
capable of
supporting construction of the space station that will supercede
Spacelab.
A two-stage development of the Tug would deliver 34,500lb to
synchronous
orbit or perform a round trip with 8,800lb. Another version, the
Aeromanoeuvring Tug designed to use Earth's atmosphere for braking,
would deliver 12,800lb to synchronous orbit or take 6,700lb on a
round-trip
fight. It would "skip" through the outer layers of the atmosphere to
lower
apogee and reduce the propellant required to set up a low orbit after
returning from synchronous altitude. Yet another scheme, the two-stage
Growth Tug, would require an improved Shuttle capability but promises
to
deliver 48,700lb to synchronous orbit or send 12,600lb on a round trip.
A
follow-on Tug, called the Orbital Transport Vehicle (OTV), would be
able to
deliver 45,000lb to synchronous orbit and, following refuelling from an
orbital propellant dump, could return with the same payload mass.
Spacelab is the manned laboratory which, carried aboard the Shuttle
Orbiter, will enable experimenters to carry out a wide variety of
scientific,
technological and biological activities for civil and military
customers.
Though considerably smaller than America's Skylab, it will - like the
Shuttle itself - be reusable and, with specially designed modular
experiments and equipment, be capable of rapid turnround on the ground
to
suit it for different missions.
It comprises a pressurised habitable laboratory, known as a module, in
which up to four payload specialists or experiment operators may work,
and a number of unpressurised pallets on which are mounted those
experiments or payloads calling for direct exposure to space. The
configuration can be tailored to the needs of the flight. Some missions
may
call simply for a module, others for a combination of module and
pallets,
and yet others will require any number of pallets up to five, but no
module.
The basic module itself is a cylinder 4.5m in diameter and 2.7m long,
but
two such units can be joined together, increasing the length to about
5.93m to provide additional space for experiments.
Fokker-VFW of Germany was signed up as prime contractor on September
30, 1975, for the six-year design and development task. The initial
contract
calls for the delivery to Nasa free of charge of a single flight unit,
two
engineering models and three sets of ground-support equipment and
initial
spares. Nasa and ESA are negotiating a second Spacelab, but in this
case the US agency would pay the cost. At one time Germany was
considering a purpose-built Spacelab to fly its own equipment, but this
scheme has been abandoned owing to the high cost of the unit and the
sharply increased levels of national expenditure required for regular
Shuttle
launches and the experiments needed to justify the acquisition. There
is a
possibility that Europe may buy a Spacelab of its own, though similar
arguments appear to make it unlikely.
In February 1975 Austria, though not an ESA member, officially joined
the
programme, relieving Germany of 0.8 per cent of its agreed commitment.
Programme costs at mid-1976 prices for the single flight vehicle were
estimated at 396 million accounting units, $515 million.
The engineering models are to be delivered in August 1978. followed by
the
flight unit in August of the following year. Spacelab will go into
orbit for the
first time during the fifth Shuttle development flight, in 1980, and
its first
operation mission is scheduled for 1981. Germany is the main supporter,
with 53 per cent of the programme, and Hawker Siddeley Dynamics,
building the pallets, accounts for Britain's 6.5 per cent. The design
lifetime
is ten years or 50 flights, and a European will be one of the four
payload
experimenters on the initial flight.
In terms of economy, Spacelab should be one of the most efficient
payloads, since it makes use of a relatively large proportion of the
facilities
offered by the Shuttle. Maximum allowable Spacelab weight is 32,000lb,
limited by the Orbiter's design landing weight. Between 12,000lb and
20,000lb of this figure, depending on the Spacelab configuration
chosen,
can be assigned to it.
Spacelab will be the platform for the Western world's manned space
activities up to about 1985, when the United States plans to bring
along a
"Mk 2" Spacelab. This at present appears likely to be an all-American
design master-minded by the DoD, in the absence of a more pronounced
European commitment in the present Spacelab.
Predicting the market.
The Space Transportation System will be capable of accommodating a
wide range of missions. The Shuttle, spin-stabilised small boost stage
and
IUS offer a payload/ height performance better than that of current
expendable launch vehicles.
For the first three years of Shuttle operations (1980-82) payload
planners
will be cramped by the limited orbital capability from Kennedy. Not
until the
Vandenberg facility becomes operational will missions be possible with
orbital inclinations greater than 55 degrees. Nevertheless, the initial
performance envelope will include low Earth orbit, synchronous orbit
and
escape trajectories. With this restriction in mind, Nasa has prepared a
preliminary schedule of missions, or traffic model, on the assumption
that
US Government, commercial and foreign users will call for a maximum of
60 flights a year by 1985.
A single launch-pad at Kennedy will be available in 1979, followed by a
second in June 1982; the first Vandenberg pad will not be ready before
early 1983, with the second reaching completion by December 1986.
Mission opportunities will be tailored to this schedule and to the
Orbiter
procurement plan.
On present planning the Department of Defence will fly 27 per cent of
all
Shuttle flights proposed by the traffic model. Of the 578 fights
envisaged in
the 1979-1992 period, 70 per cent will be launched from Kennedy, 39 per
cent will require seven-dlay Spacelab operations, and 6 per cent call
for 30-
day missions with the manned laboratory. About 34 per cent of the
fights
will involve the use of an IUS or Space Tug for synchronous-orbit
delivery or
delivery/retrieval missions. Spacelab and IUS/Tug missions account for
79
per cent of all launches tabulated in the 14-year traffic model,
leaving 21
per cent for solo Shuttle missions or low-orbit delivery and retrieval
work.
The new traffic prediction shows a build-up to 60 flights a year from
1985 to
1991, with 40 launches a gear from Kennedy and 20 a year from
Vandenberg, and replaces the original 1973 traffic model, which
envisaged
a total of 725 missions.
Cost to the customer of a Shuttle launch is expected to be $20 million
at
1976 prices, with Interim Upper Stages charged at 1.8 times the nominal
$5 million rate for those users who want a launch at less than three
years'
notice. It is unlikely, however, that foreign participation will
justify specific
Shuttle flights, and most STS missions will accommodate a mixture of US
and foreign payloads. In this case the cost to the user will be based
on the
proportional weight and volume requirements of the payload.
Benefits to payload designers
Introduction of the Space Transportation System in 1980 will greatly
influence payload planners. To begin with the Shuttle will provide a
comparatively comfortable environment for the payload, permitting
designers many new freedoms. Payloads of up to 65,000lb can be carried
to orbit within a volume of 10,500 cu ft, and satellites weighing up to
32,000lb can be returned.
It is impossible to predict with accuracy the impact of the Shuttle's
more
favourable acceleration, temperature and vibration environment on
payload
design, but several technology studies have indicated cost-savings of
up to
25 per cent. Nasa has already discovered that costs arising from the
use of
an expendable rocket with expensive excess lifting ability can be more
than offset by a cheaper payload-development phase. The time-consuming
process of designing to stringent weight, volume and reliability
constraints
imposed by expendable rockets can add significantly to the cost of
payload development. The Shuttle also provides an "intact abort
capability"
which permits the Orbiter to return with its payload to a safe landing
if the
fight is terminated early after a mechanical or "software" failure. All
of these
features will serve to reduce space transportation costs beyond the
current
level.
Organisations providing commercial services such as weather forecasting
or communication are at present required to have back-up satellites in
orbit
ready to take over in the event of the primary spacecraft becoming
unserviceable. The Shuttle will remove the need for orbital spares by
quickly flying modularised repair packages to ailing or dead satellites
in
low orbit. A similar service for synchronous satellites will of course
have to
await the introduction of the Space Tug.
On a typical repair mission, part of a scheduled Orbiter flight would
be
dedicated to carrying a Tug and an equipment dispenser. The combination
would fly under remote control from low Earth orbit to a synchronous
satellite. Once on station the dispenser would dock with the satellite,
remove the faulty module and replace it with a new unit. The Tug, still
attached to the dispenser, would then return to the Shuttle for capture
and
return to Earth.
There could foreseeably be some opposition from the satellite builders,
whose production rates might fall if the replacement market became
slack
as a result of extensive satellite refurbishment. But this objection
might be
offset by an increase in the market for satellites as launch costs
fall.
Again, refurbishment of a satellite will become progressively more
uneconomic as it ages.
Development of a standardised modular satellite would be essential for
the
effetive use of orbital-repair techniques, as would be equipment and
experiment packages tailored to specific missions. Low-Earth-orbit
satellites could also be built to a common design, with a rotary
equipment
or experiment dispenser carried within this Orbiter cargo bay. The
modularised approach to satellite engineering would, again, reduce
payload
design costs by permitting a potential user to procure a standard,
proven
spaceframe and simply install his own experiments, sensors and other
equipment.
Modular sensors on an Earth-resources satellite could. for instance,
permit
adjustments to be made to the choice of spectral surveillance bands
from
time to time, so increasing the useful life of the satellite and
circumventing
the need for a costly replacement every few years. This approach would
permit the more immediate application of even minor technological
improvements. At present, advances in technology have to accumulate
over a number of years before they justify a costly new project.
Flight operations
Shuttle launches and recoveries will be confined to the Kennedy Space
Centre in Florida and Vandenberg AFB in California. By US regulation,
no
part of a launch vehicle's ground-track may intersect a major land-mass
during the ascent to Earth orbit, and this limitation will be
particularly
important in the case of the Shuttle. The Eastern seaboard of the
United
States is so shaped that Shuttle operations from Kennedy will be
limited to
orbits not-exceeding an inclination of 55 degrees. Missions with
orbital
inclinations greater than this will have to use the USAF launch site at
Vandenberg.
Three elements of the Shuttle will return to Earth through the
atmosphere
during the Orbiter's ascent. The two boosters separate at an altitude
of
about 25 n.m., with the Shuttle 24 n.m. down-range from the launch site
and moving at 4,600ft/sec. The resulting trajectory carries them to a
height
of 54 n.m. before they fall back towards the sea 210 n.m. from the
launch
site. Their descent through the atmosphere is slowed by a single drogue
and three main parachutes, which lower the 147ft long units into the
sea at
85 ft/sec. The external tank, the only expendable part of the Shuttle
vehicle. separates at an altitude. of 70 n.m. and breaks up in the
atmosphere 2,200 n.m. down-range of the launch site. Boosters from
Kennedy fall into the Atlantic, those from Vandenberg into the Pacific.
Each booster is designed for 20 flights and each recovery unit
(attachment
points, transponders and locator beacon) is expected to survive ten
cycles.
Shuttle launch costs are assessed on these utilisation rates and on the
assumption that each Orbiter can fly at least 100 missions, with the
main
engines being replaced after 55 launches. Nasa assumes that the
thermal-
insulation tiles will survive 100 flights, with the reinforced
carbon-carbon
segments (fitted to 3.5 per cent of the exterior surface area, mainly
the
wing leading edges) replaced after 60 flights. External tank elements
will
be shipped from the Martin Marietta plant to Kennedy or Vandenberg as
required. The tank costs about $2 million a unit at 1971 prices, or 22
per
cent of the total Shuttle launch cost, with the boosters contributing
$3.3
million per flight, 31 per cent of each launch.
Shuttle services from the Kennedy Space Centre with Orbiter 102 (the
second vehicle) will use the converted Saturn V launch pad (LC-39A)
from
the first flight in April 1979 until the second pad (LC-39B) becomes
available in June 1982. Nasa expects to introduce Orbiter 101,
refurbished
from its drop-test configuration, in March 1981. It will be followed by
Orbiter
103 in March 1982. Operations from Vandenberg will begin in March 1983
with delivery of Orbiter 104, followed a year later by Orbiter 105.
Tentative
plans envisage the introduction of a second Shuttle launch pad at
Vandenberg by December 1986.
Orbiter landings at Vandenberg call for a 7,000ft extension to an
existing
8,000ft runway, but emergency landings can be made on the Edwards AFB
runway 150 miles east of the launch site. Edwards was ruled out as the
prime west-coast landing site because it would require a 747 to return
the
Orbiters to Vandenberg--with attendant delays in turnround time and
expense--and an emergency landing following an aborted launch would
necessitate an approach over Los Angeles. Emergency landing sites will
be provided at Guam and Hawaii to accommodate east or west-coast
flights aborted at an early stage.
Nasa expects to provide opportunities for non-astronauts to fly in the
Orbiter. The two pilots responsible for flying the Orbiter and
controlling it in
orbit will be supplemented by up to four payload specialists and a
mission
specialist. The latter will oversee the programme on each flight and
ensure
that each experiment operates for its allotted period. Payload
specialists
will be recruited from the scientific community and will require only a
minimum of pre-flight training since they will not be involved in
piloting or
managing Shuttle systems. Flight crews will be recruited from the
astronaut corps.
Initial flight tests
An important part of the Shuttle flight-qualification programme
involves
Orbiter 101, the initial vehicle, in a series of air-launched approach
and
landing tests. It will be carried to 24,000ft atop a modified Boeing
747 and
released for a glide return to the Dryden Flight Research Centre at
Edwards AFB, California. Test objectives are verification of Orbiter
airworthiness and equipment operation; verification of the glide
approach
and landing techniques; qualification of the automatic landing system;
and
verificaticun of Orbiter performance and handling qualities over the
weight
and centre-of-gravity envelopes. The tests will also qualify the
Shuttle
carrier aircraft for ferrying Orbiters between the manufacturer's plant
and
the launch sites.
Orbiter 101 will fly without main ascent engines, orbital manoeuvring
engines or reaction-control equipment. Nor will it carry fuel-cell
cryogenic
tanks, cargo-bay payload, radiators, star-trackers or S-band and
rendezvous radar. No water, waste-management equipment or food will be
carried, and the thermal-protection tiles will be simulated by plastic
plates
to duplicate the mass characteristics of an operational Orbiter. The
leading
edges of the wings will be clad with glass-fibre and an instrumentation
boom will be mounted on the nose. Fuel-cell reactants will be high-
pressure gaseous hydrogen and oxygen (operational flights will use
liquid
hydrogen and oxygen), and simulated rocket nozzles will ensure
realistic
airflow characteristics across the boat-tail rear end. Standard
aircraft-type
ejection seats will be fitted for these drop tests, with blow-out
panels in the
roof of the flight deck providing an escape path if the vehicle has to
be
abandoned in the air.
The first of a number of captive flights will take place in February
next year,
with a gradual progression from simple take-off and landing to
long-duration
handling trials of the 747/0rbiter combination. The Orbiter will be
unmanned
for the first 15 flights, but by May 1977 the first of six
"captive-active" tests
will provide an opportunity for manning, powering up the equipment and
testing the flying controls while remaining anchored to the 747. During
these tests the crew will be able to check their procedures in
preparation
for the first of eight manned drop-tests, which Nasa hopes to begin
next
year. A typical flight profile begins with a climb to 24,000ft,
followed by a
turn on to the desired heading relative to the runway selected for the
landing. When the checks are completed the combination resumes its
climb, to 28,000ft. From this altitude, about 31 n.m. from the runway,
the
carrier aircraft pitches 9 degrees nose-down and releases the Orbiter
at
260kt, 22,000ft. Carried on a single attachment point under the nose
and
two latches under the rear fuselage, the Orbiter pitches up 6 degrees
relative to the 747 and flies off the carrier aircraft. After
separation the two
vehicles bank in opposite directions to avoid a collision, and the
Orbiter
glides back to the Dryden Research Centre.
On early flights a tailcone will shroud the simulated main engines, but
on
later drop-tests the cone will be jettisoned so that the flow across
the inert
rocket engines of a Shuttle returning from space may be simulated. The
tailcone is carried at all times during mated flight to prevent
turbulence set
up by the aerodynamically dirty boat-tail from impinging on the 747's
tail.
The Orbiter will normally be released about 55min after take-off and
will
take about 5min to reach the ground.
Follow-on Shuttle
Believing that requirements may emerge for payloads in excess of
65,000lb
to be launched on single flights, Nasa proposes a Heavy Lift Launch
Vehicle (HLLV) using standard Shuttle elements. In this configuration
the
Orbiter would be replaced by a cylindrical payload shroud mounted on
top
of a normal Orbiter boat-tail structure carrying the three
main-propulsion
and the two orbit-insertion engines. The flight profile would closely
follow
that of a conventional manned Shuttle, with all propulsive elements
being
retained, but the automated control mode would call for a substantially
revised guidance system. Based on present manned Shuttle performance,
payloads of up to 230,000lb could be delivered into a due-east 200 n.m.
orbit. Excluding development, the HLLV would thus offer a launch cost
of
$65/lb of payload at 1976 prices.
Future American space-transportation systems will build on
Shuttle-derived
technology. This includes high-pressure/high-energy rocket engines, re-
usable thermal insulation, large-diameter/high-thrust solid-propellant
rockets, composite materials for structures, and better performance
simulation and environmental-prediction techniques.
Decision on a go-ahead with any one of several proposed Shuttle
follow-on
configurations obviously depends on the traffic achieved during the
early
1980s. If Nasa is correct in its assumptions about initial traffic
demands.
the present Orbiter will need to be replaced bv a second-generation re-
usable transporter by the early 1990s.
-- End --
All OCR errors mine!
Eugene Griessel
.
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