Re: Falcon first stage finished

On 3/21/07 8:15 AM, in article lti20350uc42aqsocfnod8bmjm2cimlaa0@xxxxxxx,
"Fred J. McCall" <fmccall@xxxxxxxxxxxxx> wrote:

"Jeff Findley" <jeff.findley@xxxxxxxxxxxxxx> wrote:

:
:"Fred J. McCall" <fmccall@xxxxxxxxxxxxx> wrote in message
:news:moh2035cphi9r47vnis9h148ebmnsubii9@xxxxxxxxxx
:> Of course, we're all just guessing based on one optical video...
:
:True. Hopefully they've got some good 2nd stage telemetry which will point
:them to the root cause. But this wouldn't be the first time that the
:simulations all looked o.k. but something unforeseen turned up in the flight
:test. It's the unknowns that get you.

Yep. This is why I lean toward it being an actual aerodynamic
instability that is increasing during the flight rather than a pure
G&C 'lag' issue. The latter sort of problem DOES tend to show up
during simulation, while finding the former relies on the fidelity of
the aero model and flow fields that you have for your vehicle (which
you have to fly to get and prove out).

No aerodynamics involved, it's far too high. After watching it several
times, I would have to guess that it's a fuel swirl/slosh issue.

It was clearly stable at separation and not later. This is not all that
unexpected. The stability margins typically go down as the fuel runs out,
because the inertia goes down, and the gimbal torque/angle ratio goes up
(since the CG moves forward as the stage gets lighter). Both of these raise
the effective system gain, and anything like this will certainly go unstable
if the gain goes up sufficiently. Of course you design it for adequate
margin at the worst-case condition, but if you miss something, like the fuel
slosh "swirl" mode that couples the axes together, you may go unstable. I
would guess that's what happened here.

I would disagree that this couldn't or even wouldn't likely have been
found in analysis. It's a well-understood problem and while it's non-trivial
to come up with a good linear stability model, it's certainly well within
the state of the art. What I suspect, and once again, not to cast any
aspersions on anyone, is that all they did was a bunch of time-domain
simulations. This is very common these days, and very appealing to newbies
since you have enough computing power available to do thousands and
thousands of them in a Monte Carlo procedure. But that's also how you get
caught on stuff like this, as many people have found (the hard way) before.
That's why they call them "start-ups", not an unexpected situation. I have
access to some great reports from the late 50's/early 60's about stuff like
this, let's say they aren't the first people to run into problems like this!
All "not for public discussion" of course.

It's also not all that trivial to solve. It's not insoluble but from
experience it's not as easy as Mr. Musk seems to want to imply. I wouldn't
be surprised if they end up needing more tank baffles. Just dropping the
gain might be possible, but if you didn't see the problem the first time,
you won't know how much to drop it. The first step needs to be to modify the
analysis so it accurately predicts the problem.

I have no idea why they are referring to it as a roll control anomaly. It
was quite obviously going unstable in pitch/yaw long before roll gave up the
ghost.

Brett

.

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