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SSX: New- technology spacecraft
From National Space Society BBS (412)366-5208
============================================================
Space Ship Experimental
The Case For SSX
Rockets have been our only way into space for over thirty years. They've
done the job, but recent events have made it all too clear that rockets as we
know them have problems. Current rockets are hideously expensive, costing
thousands of dollars per pound of payload. They're unreliable, with the loss
of one mission in twenty par for the course. They're inconvenient as hell,
with launches having to be booked years in advance.
These problems are not inherent to rockets. Most of our current space
vehicles were originally designed over thirty years ago as ballistic missiles.
Their designs reflect both the limits of fifties technology and their original
military missions. Fifties missile design habits persist to this day, despite
changing missions and huge advances in the available technology. Many of our
current problems with rockets are a direct result of approaching space vehicle
design and operations as if nothing has changed in the last thirty years.
Thirty years of progress in rocket engines and lightweight structures,
combined with a fresh approach to the problem, can give us space vehicles far
cheaper, safer, and more flexible than we're used to. One of our best shots
at doing this soon is a project called Space Ship Experimental - SSX.
We don't need ten or fifteen years of research before we can build SSX.
All the technology and much of the actual hardware is available off-the-shelf.
For about a billion dollars over four years we can build and fly SSX
prototypes. If SSX works even half as well as predicted, SSX-type vehicles
will be a revolutionary improvement in our access to space.
Those are pretty strong statements to take on faith. What makes SSX so
much better than our current rockets?
The SSX Concept
The most important difference between SSX and our current rockets isn't
SSX's unconventional design, but rather the goals of the design: SSX is a
fully reuseable, "savable", Single-Stage-To-Orbit (SSTO) vehicle, designed
from the ground up to be operated more like an advanced aircraft than like a
traditional rocket, with quick turnarounds between missions by a ground
support crew of a few dozen. SSX's advantages in cost, reliability, and
flexibility stem from these characteristics. SSX achieves these
characteristics with a remarkably simple, uncluttered design.
SSX Described
SSX is a wingless SSTO rocket that takes off and lands vertically. The
shape is a blunt-nosed cone about fifty feet high, with a wide, slightly
rounded base just under thirty feet across. The base is covered with heat-
shield material; a ring of twenty or so rocket vents pierces the heat shield
about one-third of the way in from the edge. These rocket vents combine with
the vehicle's base to form an "aerospike" engine, a distinctly non-traditional
type of rocket motor. Unlike a traditional rocket motor with its bell-shaped
expansion nozzle, an "aerospike" engine lets its exhaust gases expand against
the aft surface of the vehicle. Aerospike has several advantages for SSX: An
aerospike motor compensates for altitude automatically, maintaining high
efficiency from sea level on up to vacuum, unlike a conventional rocket motor.
Another advantage is that since the aft surface of SSX has to stand the hot
rocket exhaust gases anyway, it can double as a reentry heat shield. Finally,
aerospikes can match a conventional engine's efficiency at much lower
operating pressure, allowing lighter, cheaper, more reliable fuel pumps and
combustion chambers.
Aerospike engines have been built and tested on the ground, but have never
flown in a full-size vehicle. A key factor in how well SSX works is how close
actual aerospike efficiency comes to the theoretical predictions.
SSX stands on four retractable legs, rather like the Apollo Lunar Module.
The bottom three-quarters of the vehicle is fuel tank and engines, with cargo
and crew carried in the nose. The whole thing looks more like a fifty foot
tall egg with legs than it does any traditional rocket.
SSX takes off vertically from a fairly simple launch pad. The pad will
support its launch weight of about 250 tons (about nine-tenths of this is
fuel) over a flame trench that channels away the rocket exhaust safely. No
complex gantry or multiple umbilical connections are needed, as SSX's avionics
are self-contained with multiple redundancy and built-in self-test features,
modeled more on airliner electronics than on current throwaway rocket
controls.
SSX is like an airliner another way: It is "savable". This means that
SSX is designed so that, like an airliner, if it loses an engine, it can abort
the mission and land safely, even at the most vulnerable moment in a flight,
taking off with full fuel tanks. Current rockets that lose an engine on
takeoff tend to become smoking holes in the ground.
SSX lands vertically under rocket power after doing a largely unpowered
aerodynamic reentry. With most of its fuel gone, SSX is light enough that it
can use the same steering technique as the Apollo reentry capsules, "gliding"
on its broad flat base and controlling direction by tilting the whole vehicle
slightly on its axis. Apollo was able to achieve pinpoint accurate
splashdowns this way. By the time SSX has descended to airliner altitudes, it
will have slowed to a few hundred miles per hour on air drag alone. Lighting
the engines for the last few miles of controlled descent will require very
little fuel and only a small part of the engines' takeoff power rating.
Vertical landings on rocket power were extensively tested and proven in the
Apollo Lunar landers; SSX will be able to land on any hard flat surface a
hundred yards across.
Single Stage To Orbit (SSTO)
Multi-stage rockets have two advantages over an equivalent SSTO. First,
for a given payload and destination, a multi-stage rocket can be made
significantly smaller than a single-stager. Breaking the rocket up into
stages and dropping each stage as it runs out of fuel will reduce weight and
increase performance considerably for the later part of the flight.
The size advantage of multiple stages is greatest for missions where
rocket motor performance is stretched to the limit. This was the case in the
fifties; the single-stage equivalent of an early three-stage rocket would
have been five or six times larger at takeoff. The size advantage decreases
as motor efficiency improves and as structure weights drop, however. Our best
rocket motors are about 50% more efficient than the best of thirty years ago,
and we've made huge advances in lightweight high-strength structures. SSX
takeoff mass will be less than twice that of an equivalent multi-stage
expendable booster.
One of the most pernicious holdovers of the fifties "missile mentality" is
the assumption that the best rocket for a given job is the smallest one
possible. Military missiles may need to be as compact and portable as
possible, but space launchers don't, and trading away simplicity and
reliability for minimum takeoff weight in a launcher makes no sense at all.
Most of the extra takeoff mass of an SSTO is fuel, and fuel is cheap.
An SSTO is simpler to operate, and much simpler to make reuseable.
Multiple stages pretty much have to be expendable. Making dropped stages
recoverable negates much of the size advantage, due to the extra weight of the
recovery provisions. Nobody has yet come out ahead of the game trying to
recover and reuse a dropped rocket stage. Even in an expendable, the weight
and complexity of the extra engines and stage interconnection/release hardware
hurts cost and reliability.
Dropping stages on the way up also severely limits where you can launch
from and in what direction. You end up confined to sites with a lot of wide
open space downrange, like Kennedy and Vandenberg. An SSTO can fly from
anywhere the locals don't mind the noise too much, and it can launch in any
direction without worrying about flying over populated areas, since it isn't
dropping junk all over the landscape.
Multi-stagers' second advantage is that traditional rocket motor nozzles
have to be sized for optimum performance at some particular altitude, and lose
considerable efficiency elsewhere. Multiple stages allow matching each
stage's motor to its operating altitude. An SSTO rocket needs either a motor
with high enough efficiency that the losses can be tolerated, different motors
for different altitudes, or a motor that compensates for altitude. SSX takes
the last approach; a major advantage of the "aerospike" engine is that it
automatically compensates for altitude, operating efficiently all the way from
sea level to vacuum.
Reusability and Savability
The obvious advantage of reuseability is cost. No more throwing away
expensive aerospace hardware after every mission. Imagine the cost of airline
travel if 747's were scrapped after one flight! Reuseability alone is not
enough, though. Airfares would still be outlandish if a 747 took months of
work by thousands of mechanics between flights. Savability requires simple
rugged systems with multiple redundancy; practical reuseability requires
design margins large enough that equipment won't need to be overhauled after
every flight. Combine these qualities with extensive built-in self-test
capability, in a basically simple system like SSX, and you reduce maintenance
requirements a lot. SSX is designed to be readied for flight by a few dozen
technicians in a week or so, cutting personnel costs to a fraction of what we
pay for the standing armies that operate our current launchers.
A less obvious advantage of reuseability plus savability is reliability.
Expendables get minimal flight testing during development because each flight
means using up a vehicle. Some bugs won't be found until later, and the ones
that do show up during test are tough to diagnose because as often as not, all
that's left is a tape full of telemetry data and a smoking hole in the ground.
A savable reuseable like SSX can have all the flight testing it needs,
gradually working up from short hops with minimum fuel through suborbital
flights and finally orbital missions. This conservative incremental flight
test schedule, combined with savable design, means that bugs generally won't
prevent a safe landing followed by hands-on bug fixing. SSX will enter
operations far more thoroughly tested than current rockets.
Even after an expendable's design bugs are swatted, every one launched is
fresh from the factory, with no test flights to catch any flaws that got past
QC. Operational SSX-type vehicles will be more reliable because the
individual vehicles can be thoroughly flight tested out of the factory as well
as after repairs. This will also save on personnel costs; a large part of the
standing armies that fly current rockets spend their days triple-checking and
documenting every last step of the process, trying to make up for the inherent
unreliability of their vehicles, or failing that to at least have some record
of what went wrong.
Ballistic Versus Winged
SSX is a ballistic vehicle; it has no wings and relies almost entirely on
rocket thrust to fly. SSX's reentry "crossrange" - the distance it can
depart from its previous orbital path before landing - will be less than that
of a winged vehicle like Shuttle, but still several hundred miles. Combined
with SSX's small landing field requirements, this crossrange should be
adequate to allow safe emergency landings. There are a lot of parking lots
out there...
Ballistic flight allows SSX to use a very simple shape, easy to design and
build light and strong, with very simple predictable aerodynamics. SSX should
be much cheaper and quicker to develop than an equivalent winged vehicle. The
lessened atmospheric maneuverability is a small price to pay.
A ballistic lands and takes off vertically, giving SSX a unique
capability: Park one SSX in low Earth orbit, refuel it with 20 or so SSX fuel
tanker flights, and the refueled SSX can make a round trip from Earth orbit to
the Lunar surface and back, carrying cargo in both directions. A refueled SSX
can also function as an orbital transfer vehicle, carrying payloads to and
from geosyncronous orbit.
Rocket Versus Airbreathing
Rockets have some of the same advantages over airbreathing launchers as
ballistics do over winged vehicles. Rockets take less time and money to
develop, and rockets can operate beyond low orbit. Again, the lower
atmospheric performance is a small price to pay.
SSX and NASP
SSX is a useful hedge against delay or poor performance of NASP, and if
both succeed, SSX's deep space capability will be a useful complement to NASP.
SSX can be ready a lot sooner at lower cost than NASP, since SSX is much less
complex and requires less advanced technology. SSX won't necessarily be at a
fatal economic disadvantage even after NASP is a roaring success, since SSX's
greater fuel consumption may well be offset by the higher initial cost of an
NASP-type vehicle. We should develop both; each has a role to fill.
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