~Stephanie Osborn
http://www.stephanie-osborn.com
* * *
A Few Observations on
Reusable Space Hardware,
or Why the Space Shuttle was
an engineering masterpiece and a logistical nightmare.
by Larry Bauer
The National Space
Transportation System, or what people commonly refer to as the Space Shuttle, was
composed of four parts: the Shuttle itself, the three Shuttle main engines, the
External Tank, and two Solid Rocket Boosters. In theory everything but the tank
and its fuel were recoverable.
I was reminded of this when I
saw an article noting the sixth successful landing of a SpaceX Falcon 9 rocket
after the successful launch of the JCSAT-16 commercial communications
satellite.
From a purely economical
aspect it all comes down to what is the cheapest means to launch a payload into
a desired orbit. On the face of it reusing hardware only makes sense. However
there are a good many factors that mitigate against such as assumption.
With NSTS we salvaged
everything except the ET. The Shuttle and engines landed and the SRB shells
parachuted back into the sea for recovery. In theory maximum reuse of critical
hardware. But let's look first at those boosters. They were made in sections
far from the launch site, built and filled in sections so they could be
transported by barge to Kennedy Space Center. From a logistics standpoint it
would have made much more sense to build them as a single tube and fill them
with solid propellant right there at the launch site. However that was not an
option. The state of Florida would allow launches from KSC, but they refused
permission to build the SRBs there. The solid propellant is nasty stuff and the
process of filling the boosters violated too many state pollution restrictions.
So the SRBs were built and filled originally by Thiokol of Brigham City, Utah,
later bought out by ATK.
I will note that the recovery
and remanufacture of the boosters was very cost- and labor-intensive, and there
was always a debate over whether a cheaper throw-away design might have been
more cost effective. I will also point out that a design incorporating a single
continuous tube would have made the failure that caused the Challenger disaster
impossible. Which does not mean something equally tragic might not have
happened, but you cannot have a joint failure if there are no joints.
The high-performance Space
Shuttle Main Engines, known as SSMEs or the Aerojet Rocketdyne RS-25, are an
example where reuse of hardware only makes good sense. These are the pinnacle
of the state of the art for liquid fuel rocket engines. With the help of the
solid boosters, these engines, sucking immense quantities of liquid hydrogen and
oxygen from the External Tank, could lift a combined vehicle and payload weight
of roughly 2060 tons. Bringing these highly intricate and fine-tuned engines
back for reuse only made sense.
And that does lead me to the
subject of a rather controversial opinion of mine. As magnificent an
engineering achievement as the NSTS was — and I spent the majority of my career
at NASA doing ground support to on-orbit experiment operations so I have a
great fondness for the beastie — the reason why it was a huge logistics failure
rests in the numbers. The shuttle itself weighed in at 2030 tons. Its payload
was 30 tons. The NSTS was a true heavy lift rocket, but most of what should
have been useful payload mass to orbit was spent instead on creating the ability
to land sort of like an airplane. A requirement imposed, by the way, by the Air
Force — who withdrew from the project between the time the design was firmed up
and the first launch. I will also observe that every astronaut pilot I've ever
spoken with all described the shuttle as “that flying brick.” [I can confirm
that astronaut description. —Steph]
And all of the above is
explanation as to why we don't see any shuttle-like designs these days. It is ever
so much more efficient to make as much of the upmass be useful payload as
possible, with the crew compartment just sufficiently robust to carry the
astronauts up and get them back to Earth safely.
* * *
Thanks much for that information, Larry! It squares pretty nicely with what I know of the various programs, as well.
BOOK RECOMMENDATION:
A New American Space Plan, by Travis S. Taylor with Stephanie Osborn, available in print and ebook, discusses the history of space exploration, where we are, how we got there, and where we ought to be and be going. You can find a lot more detail on the Space Shuttle, how it worked, the main engines and how the whole system was designed, right in this book. There's also a good bit about the recent efforts to develop commercial space launch systems and why the epithet "commercial" is often a misnomer.
http://www.stephanie-osborn.com
