forever? Not Exactly.
Shuttle is an evolving system. In the current investment climate NASA is
the only remaining player capable of developing a reusable spacecraft.
by Dale M. Gray
US has announced plans to continue to upgrade and fly the Shuttle for the next
20 years. This is not to say they
are expecting the Shuttle to be flying for the next two decades.
NASA has learned not to throw away functioning systems like Apollo until
new systems come on-line. The
Shuttle has to continue to fly to assure NASA's access to space and to the
International Space Station.
philosophy comes with a heavy fiscal cost.
Each Shuttle flight costs about $400 million. This cost has not dropped much in the past few years.
However, the amount of mass that the Shuttle can put into orbit has just about
doubled since it was introduced. This
is due to evolution of subsystems such as Main Engine upgrades and three
generations of lighter External Tanks. New
flight software and hardware have been incorporated into the Shuttles as they
are periodically upgraded.
Shuttle is an evolving technology platform.
Subsystems have been radically redesigned, even though the outward
appearance of the Shuttle is little changed.
However, there are physical and financial limits to technological
evolution of subsystems without periodic major changes to the overarching
design. We have seen this in the
evolution of airplanes from canvas to steel, from bi-plane to monoplane, from
propeller to jets. NASA recognizes
that it is rapidly approaching subsystem design limits. Improvements can still
be made, but they become more expensive and harder to implement as the system
approaches the design limits.
One of the ways NASA sought to create a new generation of launch systems was the X-33. The program sought to rethink the approach to space flight and achieve true reusability. Three main infrastructure changes were sought through the program. These changes included:
new metallic thermal protection,
new high-powered linear aerospike engines,
an advanced composite liquid hydrogen fuel tank.
managers and designers sought a quantum leap in technology just as the Shuttle
was a giant leap beyond Apollo technology.
However, the three advanced technology tracks gave three tracks for
failure while dividing the resources of the project three ways. The program
managed to produce thermal protection on schedule, but the aerospike engines
proved more difficult to create and test than anticipated. This hurdle was
eventually overcome, but left the managerial infrastructure with few resources
to deal with the delamination of the composite hydrogen tank.
A substitute aluminum-lithium tank was proposed and some work done, but
time and money had run out. American
resolve had been removed from the program and it was allowed to expire.
the X-33 a failure? Hardly,
the metallic thermal protection system will either be used or will be the
baseline for future thermal protection systems on reusable spacecraft.
The linear aerospike engines are only a few tests away from flight
readiness. We now know a lot more
about how composite structures respond to contact with corrosive liquid
should be remembered that the original development of the Shuttle lagged almost
two years behind schedule and was filled with difficulties and engineering
compromises. However, NASA had to
press on because it had no alternative method of sending astronauts to space --
having abandoned Apollo. Few
remember that one of the first Shuttle missions was to correct the orbit of the
Saturn V-launched Skylab. By the time Columbia
finally launched, Skylab had reentered.
Shuttle succeeded in the quantum jump in technology because it had the
full national will to overcome whatever problems developed.
Lockheed Martin, by comparison, had a limited budget and limited time
frame. When its resources ran out,
there was no ultra deep pocket available to spend through the problem and no
national will to push through a solution.
is currently approaching a Shuttle replacement with the Space Launch Initiative.
The program seeks build on existing infrastructures by evolving systems
and subsystems in a generational approach.
Changes will be implemented one at a time to reduce the risk and reduce
the cost. The SLI also seeks to avoid competition with private American space
launch system development by seeking NASA unique missions.
Because the "investment bubble" for speculative technical
investments has popped, there is little money available to fund private efforts.
Such events usually leave the investment field sterile for about a
decade. This leaves NASA the only
major player in developing a reusable spacecraft.
If they can get a Shuttle replacement in the air in the next 10 years,
they will have made it easier for follow-on private efforts to improve upon
will the replacement look like? I
suspect we will see a marriage between Shuttle and X-33 technology.
The tile and the wings will be gone, but Shuttle avionics and other
infrastructure will remain. The
engines will either be evolved Shuttle Main Engines or Linear Aerospike --
depending on the engineering approach used.
One thing you can count on at this point is that advanced composites will
not be used on the liquid hydrogen fuel tank. The lives saved by avoiding this
material will be entirely due to the success of the X-33 program.
Dale M. Gray
- TDF 2/2001 - 28/04/2001]