Instead, from the very beginning, SpaceX
designed
its
Falcon
rockets
with
commonality in mind. Both of Falcon 9’s
stages are powered by RP1 and liquid
oxygen, so only one type of engine is
required. Both are the same diameter
and are constructed from the same
aluminum-lithium alloy, reducing the
amount of tooling and the number of
processes and resulting in what Musk calls
“huge cost savings.”
No choice was more critical than the
Merlin rocket engines used to power
Falcon 9. SpaceX propulsion chief Tom
Mueller and his team selected an engine
type called the pintle that was pioneered
by Mueller’s former employer, TRW, which
used it for the descent stage of the
Apollo lunar module. Unlike most rocket
engines, in which droplets of fuel and
oxidizer are sprayed into the combustion
chamber through an injector plate
resembling a shower head, the pintle uses
a needle-like injector that’s more like the
nozzle on a garden hose. It’s not only less
expensive to make, Mueller says, but it is
also less susceptible to combustion
instability, a runaway buildup of energy
within the thrust chamber that has vexed
engineers since the dawn of the Space
Age (it added years and many millions of
dollars to development of the giant F-1
engines for the Saturn V moon rocket, for
example). Combustion instability can
make an engine undergo what veterans
dryly call an RUD, for Rapid Unscheduled
Disassembly; civilians would call it blowing
up. Even the Merlin had a couple of RUDs
in the early days of development. “There
are a thousand things that can happen
when you go to light a rocket engine,”
Mueller says, “and only one of them is
good.”
That, of course, was hardly news by the
time SpaceX got started; studies had
shown that over the previous two
decades, the vast majority of rocket
failures were due to engine malfunctions.
And so, before attempting the multi-
engine Falcon 9, Musk began with the
smaller and less expensive Falcon 1,
which uses a single Merlin engine in its first
stage. Test launches of this 70-foot rocket,
beginning in 2006, were SpaceX’s
baptism by fire. Only after three failed
attempts did the Falcon 1 become the
first privately built liquid-fuel vehicle to
reach orbit, in September 2008. Musk and
his team were both elated and sobered.
“We knew it would be hard,” Mueller
says, “but it was harder than we
thought.”
The rest of the aerospace world took
notice. With the early Falcon 1 failures,
says Stern, Musk “showed spine, showed
he would spend his own money, showed
he would stick with it.” And the lessons
learned from Falcon 1 smoothed the
path for Falcon 9, whose successful
maiden launch, in June 2010, impressed
observers accustomed to watching other
would-be rocket startups, from AMROC in
the 1980s to Kistler in the 1990s, fail before
getting anything into space.
The Falcon 9 was designed from the
beginning to be human-rated, meaning
an increased focus on reliability. The
rocket’s avionics and controls are triple-
redundant (as will be some sensors in the
human-rated version of the Atlas V), and
the flight computers, which run on Linux,
will “issue the right commands even if
there’s severe damage to the system,”
Musk says. The choice of nine engines for
the first stage was made with reliability in
mind: From the moment of liftoff, Falcon 9
can suffer an engine shutdown and keep