Last Week's Flight Paves The Way For Upcoming Contract
Flight
Editor's Note:
SpaceX founder Elon Musk says the data proves it. Despite a second
stage oscillation that popped up late in last week's test flight of the company's
Falcon I rocket, the mission came close enough to
achieving all its goals to be good enough for government work...
and that's not meant to be a disparaging comment. SpaceX will
go ahead with plans to launch a US Navy satellite later this
year.
Musk fills us in on the details...
Having had several days to examine the data, the second test
launch of Falcon 1 is looking increasingly positive. Post flight
review of telemetry has verified that oscillation of the second
stage late in the mission is the only thing that stopped Falcon 1
from reaching full orbital velocity. The second stage was otherwise
functioning well and even deployed the satellite mass simulator
ring at the end of flight! Actual final velocity was 5.1 km/s or
11,000 mph, whereas 7.5 km/s or 17,000 mph is needed for orbit.
Altitude was confirmed to be 289 km or 180 miles, which is
certainly enough for orbit and is about where the Space Shuttle
enters its initial parking orbit.
This confirms the end of the test phase for Falcon 1 and the
beginning of the operational phase. The next Falcon 1 flight will
carry the TacSat 1 satellite for the US Navy, with a launch window
that begins in September, followed by Razaksat for the Malaysian
Space Agency in November. Beyond that, we have another nine
missions on manifest for F1 and F9. Note, the first F9 mission will
also be a test flight and the three NASA F9/Dragon missions are all
test flights for Dragon.
Telemetry shows that engine shutdown occurred only about a
minute and a half before schedule (roughly T + 7.5 mins), due to
the oscillations causing propellant to slosh away from the sump.
When the liquid level in the tank was low, this effectively starved
the engine of propellant. A disproportionate amount of the velocity
gain occurs in the final few minutes of flight, when the stage is
very light, which is why the velocity difference is greater than
just linearly subtracting 1.5 mins from the burn time..
Except for a few blips here and there, we have now cleaned up
the raw data feed and recovered video and telemetry for the entire
mission well past 2nd stage shutdown. Including all the launch pad
video and ground support equipment data, we have somewhere close to
a terabyte of information to review. This was far too much to send
over the T1 satellite link from Kwaj and had to be brought over in
person after the flight. Given that a number of our engineers have
only just returned from Kwaj, please consider this still a
preliminary analysis:
In a nutshell, the data shows that the increasing oscillation of
the second stage was likely due to the slosh frequency in the
liquid oxygen (LOX) tank coupling with the thrust vector control
(engine steering) system. This started out as a pitch-yaw movement
and then transitioned into a corkscrewing motion. For those that
aren't engineers, imagine holding a bowl of soup and moving it from
side to side with small movements, until the entire soup mass is
shifting dramatically. Our simulations prior to flight had led us
to believe that the control system would be able to damp out slosh,
however we had not accounted for the perturbations of a contact on
the stage during separation, followed by a hard slew to get back on
track.
The nozzle impact during stage separation occurred due to a much
higher than expected vehicle rotation rate of about 2.5 deg/sec vs.
max expected of 0.5 deg/sec. As the 2nd stage nozzle exited the
interstage, the first stage was rotating so fast that it contacted
the niobium nozzle. There was no apparent damage to the nozzle,
which is not a big surprise given that niobium is tough stuff.
The unexpectedly high rotation rate was due to not knowing the
shutdown transient of the 1st stage engine (Merlin) under flight
conditions. The actual shutdown transient had a very high pitch
over force, causing five times the max expected rotation rate.
We definitely intend to have both the diagnosis and cure vetted
by third party experts, however we believe that the slosh issue can
be dealt with in short order by adding baffles to our 2nd stage LOX
tank and adjusting the control logic. Either approach separately
would do the trick (eg. the Atlas-Centaur tank has no baffles), but
we want to ensure that this problem never shows up again. The
Merlin shutdown transient can be addressed by initiating shutdown
at a much lower thrust level, albeit at some risk to engine
reusability. Provided we have a good set of slosh baffles, even
another nozzle impact at stage separation would not pose a
significant flight risk, although obviously we will work hard to
avoid that.
I will be posting another DemoFlight 2 post launch update within
a week, which will include a list of all subsystems color coded for
status: green = good, yellow = cause for concern, red = flight
failure if unchanged, black = untested. Of the hundreds of
subsystems on the rocket, only the 2nd stage LOX tank slosh baffles
are clearly red right now, but that could change with further
analysis. As much as is reasonably possible (subject to ITAR and
proprietary info), SpaceX will provide full disclosure with respect
to the findings of the mission review team.
Musk also pointed out that, in the rocket business, a
less-than-optimal launch does not mean the mission was a
failure.
There seems to be a lot of confusion in the media about what
constitutes a success. The critical distinction is that a test
flight has many gradations of success, whereas an operational
satellite mission does not. Although we did our best at SpaceX to
be clear about last week's launch, including naming it DemoFlight 2
and explicitly not carrying a satellite, a surprising number of
people still evaluated the test launch as though it were an
operational mission.
This is neither fair nor reasonable. Test flights are used to
gather data before flying a "real" satellite and the degree of
success is a function of how much data is gathered. The problem
with our first launch is that, although it taught us a lot about
the first stage, ground support equipment and launch pad, we
learned very little about the second stage, apart from the avionics
bay. However, that first launch was still a partial success,
because of what we learned and, as shown by flight two, that
knowledge was put to good use: there were no flight critical issues
with the first stage on flight two.
The reason that flight two can legitimately be called a near
complete success as a test flight is that we have excellent data
throughout the whole orbit insertion profile, including well past
second stage shutdown, and met all of the primary objectives
established beforehand by our customer (DARPA/AF). This allows us
to wrap up the test phase of the Falcon 1 program and transition to
the operational phase, beginning with the TacSat mission at the end
of summer. Let me be clear here and now that anything less than
orbit for that flight or any Falcon 1 mission with an operational
satellite will unequivocally be considered a failure.
This is not "spin" or some clever marketing trick, nor is this
distinction an invention of SpaceX -- it has existed for decades.
The US Air Force made the same distinction a few years ago with the
demonstration flight of the Delta IV Heavy, which also carried no
primary satellite. Although the Delta IV Heavy fell materially
short of its target velocity and released its secondary satellites
into an abnormally low altitude, causing reentry in less than one
orbit, it was still correctly regarded by Boeing and the Air Force
as a successful test launch, because sufficient data was obtained
to transition to an operational phase.
It is perhaps worth drawing an analogy with more commonplace
consumer products. Before software is released, it is beta tested
in non-critical applications, where bugs are worked out, before
being released for critical applications, although some companies
have been a little loose with this rule. :) Cars go through a
safety and durability testing phase before being released for
production.
Rockets may involve rocket science, but are no different in this
regard.