Falcon first stage finished

wrote:

On Mar 21, 8:52 pm, Brett Buck wrote:
On 3/21/07 8:15 AM, in article
,

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.

Here are a few interesting numbers. From SpaceX, the second stage
starts at 0.65 Gs and builds to 4.5 Gs, at starts at t=165 sec.
Ignoring the fairing, and assuming uniform burn, we get the
accelerations below. From timing 5 oscillations at 4:20, 4:40, 4:50,
and 5:00, I get the T5 numbers. This gives the oscillation period in
seconds, and in Hz.

Time sec M Acc(g) T5(sec) osc(sec) osc(Hz)
2:45 165 1 0.65
4:20 260 0.804 0.808 6.6 1.32 0.757
4:40 280 0.762 0.852 5.64 1.128 0.886
4:50 290 0.742 0.875 5.5 1.1 0.909
5:00 300 0.721 0.900 5.4 1.08 0.925

One interesting thing is that the oscillation frequency is going up
faster, or at least as fast, than the G force. Offhand I would think
slosh would only go up as sqrt(acc), but that's assuming it's only at
the surface of the fluid and not throughout the volume (which is
decreasing, clearly). It also does not fit a bending model where the
stiffness is constant and the mass is decreasing as the acceleration -
in this case you would also expect a sqrt(acc) as well.

Anyone have any ideas for a model that might fit this data??

Thanks for the nice data, I would also think the magnitude of the
oscillations is important. The Radius of the circle the engines moved in,
it grew at a very fast. A surface wave can only get so large.

Sloshing vortex? In a bucket of water the surface wave starts first, then
the fluid begins to move, with a hole in the bottom the bucket, a strong
vortex will appear. Or in a 2 liter half full bottle of water, upside down,
move it in a circle to get the vortex going. I've used this technique to
empty water out of 5 gallon jugs, to stop the glug, glug, glug of each gulp
of air. With each glug, the fluid stops moving, reverses direction to let
air in, and the bottle empties very slowly. Give it a couple of good
circles, surface wave, motion, low pressure at the hole in the bottom gets
the vortex started. The vortex gets much stronger as the jug empties. It
helps to empty the bottle when small, giving a clear path for air to enter,
and slows the rate after the vortex grows very large.

The vortex can be along the centerline with no C.G. shift, or offset from
the centerline with a C.G. shift. As the vortex gets stronger, the hole or
tub of Helium increases in size, so would the C.G. shift. I have no idea
what the frequency increase of a moving vortex in a liquid with positive
jerk would be in response to a rotating force (the engine moving in a
circle).

So, maybe the engines were chasing the vortex around and reinforcing it.

Brett Buck wrote:

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.

A vortex would cause a roll anomaly also, conservation of momentum
(angular). The Helium gas roll thruster would have seen the roll moment
caused by a growing vortex and counteracted it. To keep the roll rate at
zero. As more and more liquid began to move in the vortex, I would imagine
the roll thruster had to supply a larger and larger moment to counteract
it. Roll moment supplied by the thruster is normally zero, with a growing
vortex it would be diverging towards the limit.

Brett Buck wrote:

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.

A vortex would cause a roll anomaly also, conservation of momentum
(angular). The Helium gas roll thruster would have seen the roll moment
caused by a growing vortex and counteracted it. To keep the roll rate at
zero. As more and more liquid began to move in the vortex, I would imagine
the roll thruster had to supply a larger and larger moment to counteract
it. Roll moment supplied by the thruster is normally zero, with a growing
vortex it would be diverging towards the limit.

Or, the main engine is supplying the roll moment? Then viscous effects on
the tank wall would be the roll anomaly? Kind of an interesting problem if
it's a vortex.

Brett Buck wrote:

On 3/22/07 7:29 AM, in article
. net, "Craig Fink"
wrote:
Round and round the fluid went,
larger and larger circle. Stop moving the engine. Maybe, it did get bent,
asymmetrical thrust changing control frequencies and cross-coupling. But,
it sure recovered fast (very tight) from the separation attitude
transient, one quick movement of the nozzle and back on attitude.

I agree, it was clearly stable right off the booster, and it went
unstable
later. The mostly likely controls effect of a bent nozzle is a thrust
vector misalignment, and that would be trimmed out quickly by the
autopilot, or it would diverge in very short order.

I agree, a misalignment quickly integrated out by a little re-trimming. A
weird shaped dent could cause a rolling moment, but it would not change
over time and it would be trimmed out by the Helium roll thruster. A
constant usage rate.


If you just "stop moving the engine" it goes ass-over-teakettle almost
immediately, as it will diverge rapidly if the thrust vector doesn't end
up going exactly through the CG.

I agree, not literally, if it were literally just stopped and not through
the CG, ops...

But, this is equivalent to a pilot induced oscillation, what the pilot is
doing is just making the oscillation worst. The harder he works at fixing
it, the bigger it gets. A pilot in this situation is well advised to simply
let go of the stick. Stop doing what he's doing and if the vehicle is
stable, the oscillation will dampen out and the vehicle returns to it's
trimmed out attitude.

Not the case with a rocket out of the in a vacuum, but it still can be
thought of that way. Stop rotating the engine in a circle and worry about
just trimming the average attitude to whatever the guidance command is.
Equivalent to letting go of the stick. Not exactly the right thing to do
but better, it would stop the engine induced part.



To see how bad the later instability is, look how much the gimbals
moved
to control the separation transient (just a little bit), and compare that
to even the early stages of the divergence -much more.

I agree, it was a really big attitude dispersion at separation. The required
engine movement (integrated over time) was tiny compared to the size of the
divergent engine movements of the anomaly.



I still think it's slosh. Either way, auto-gains in there flight control
would have fixed it.

Maybe, but maybe not. If it went unstable from gain changes due to
changes in mass properties, a *programmed* (open-loop) gain change would
certainly fix it. Truly adaptive control (actively determining the system
poles from observer performance and altering the system to maintain them,
closed loop) is a dirty word and has had a very poor history.

Yeah, it may. But, at some point in time someone will figure out a good
reasonable auto-gain strategy. If not actively determining system poles,
some other technique. Tables seem to work great with known systems, I still
think some other better technique will come along sometime. I'm always
optimistic about man's ingenuity to come up with better things over time.

But if it went unstable due to varying slosh characteristics, or more
likely not understood slosh characteristics, then there's no reason to
think that you would know how to correctly change the gains/filters to
overcome it. Bear in mind that even if everything else stays the same
except the propellant running out, the slosh frequencies change
DRASTICALLY during the burn. If for no other reason, the acceleration is
going up, and that directly affects the slosh frequency.

A sloshing vortex probably fits in this category. If it is a vortex slosh, a
simple algorithm to spiral the engines out to some radius, then spiral back
in to the centerline (trimmed position) would kill the vortex. The spiral
being in the opposite direction from what happened in this case which
reinforced the vortex.

Of course it could be a plain old structural bending thing, too. The
less
fuel the less rigidity in the tank and the more the weight gets to the
ends of the spring, er, tank.

The rigidity of the tank is probably supplied more by the Helium pressure
that the hydrostatic fluid pressure. But I imagine they may have let the
pressure drop as it burned towards MECO.


BTW, it occurred to one of my buddies earlier that there is a good
chance the roll RCS fuel was depleted trying to remove the oscillation
coupled from the other axes going unstable.

I agree, the roll control Helium is limited and used to pressurize the tank.
Also, the control authority of the Helium roll thruster is probably very
limited. Only able to handle relatively small rolling moments. It could
have been out of Helium when it rolled out of control at the very end, or
out of roll control authority, helium engine supplying as much roll moment
as it can.

"Damon Hill" wrote in message
31...
"MichaelJP" wrote in
:

0214 GMT (10:14 p.m. EDT Tues.)

Musk says 90 percent of the Falcon 1 rocket's technical challenges
were proven out with this launch. He doesn't foresee needing another
test flight before launching the first operational mission in late
summer carrying the U.S. military's TacSat 2 spacecraft.

I'm surprised at this - would the underwriters insure that payload on an
unproven booster?

What insurance? It's a government payload, and they'll make the
call on whether they want to fly on the third attempt or wait for another
validation flight.

Lots of things were happening on this flight and we need to wait for a
full analysis of what actually happened, and what the necessary fixes
will be. I'm happy that it got as far as it did, exercising all of the
hardware and flight modes. That's how problems get found; the launch
business is not for the timid.

--Damon

Fair enough, didn't know there was no insurance.

You're right about it not being for the timid! I'm amazed that they would
consider launching such a valuable payload without at least one successful
test.

"zoltan" wrote in message
ps.com...
The instability can be a result of too low servo rates, or time delays
in the control loop. The use of high level languages and real time
operating systems can easily eat up the CPU time and result in
unstable control loops. The time delay in the pneumatic system may
also be the culprit for the oscillations.

I hope there is enough data to determine why the roll took off.

One would hope so. Any respectable test program should include enough
telemetry data so you can analyze failures and fix them.

As someone else pointed out, this could also be caused by too few anti-slosh
baffles in the tanks.

Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)

"Craig Fink" wrote in message
link.net...
My guess as to what happened,

Have your balancing stick handy, helps with the words.

Yeah, a modeling problem. There are three modes to balancing a stick.
Finger
moving in and out, pitch, using pitch attitude, rate, acceleration,
jerk...
and left and right, yaw, again attitude, rate, acc... and a third mode
roll
(but not the roll one normally thinks of, rolling the stick between your
fingers). The roll in terms of moving your finger in a big circle to
balance the stick. The stick isn't rolling but is going in a big circle,
finger moving in pitch and yaw.

This seems to be the one that failed, the engine was moving in circle that
spiraled out. Looking at the gimbal motion in Polar Coordinates, the
circle
has a radius R and an angular rate w. The cause of this circular motion
was
most likely slosh, a wave at the surface of the liquid in the tank moving
around the wall of the tank. Like moving a bucket of water in a circle,
one
side of the bucket has the crest, the other side has the trough.

The average C.G. is still along the centerline of the vehicle, but the
instantaneous C.G. is moving in a circle with some radius R and the same
angular rate w as the engine gimbal's angular rate. If the angular
position
of the gimbal is behind the angular position of the C.G., the wave is
flowing down hill and getting larger. The instantaneous C.G. shift R will
grow. If the angular position of the gimbal is ahead of the angular
position of the C.G. then the wave is trying to flow up hill, energy is
taken away and the instantaneous C.G. shift R will be reduced.

The normal pitch and yaw should only be used with respect to the average
C.G. and not used to chase transients around circlecaused by the slosh
instantaneous C.G. moving in a circle.

It wouldn't be too hard to add the a slosh roll control model to the
pitch/yaw control of the engine. Or, they could just add some more slosh
baffles.

Just a guess.

It's been a long time since my dynamics and control classes, but this theory
sounds reasonable. Now if Space-X would publish their telemetry data...
;-)

Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)

Damon Hill wrote:

Lots of things were happening on this flight and we need to wait for a
full analysis of what actually happened, and what the necessary fixes
will be. *I'm happy that it got as far as it did, exercising all of the
hardware and flight modes. *That's how problems get found; the launch
business is not for the timid.

Everyone who does something new, even if it has been done before by someone
else, has to go through a learning curve. By doing it, yes mistakes are
made, but much is learned from each mistake. The learning curve is very
steep initially, then begins to flatten out. SpaceX's learning curve will
flatten out soon.

To me SpaceX is taking the right approach and is much further along the
learning curve than a company like Kistler. Kistler seems to have a large
list of "big" names, has learned how to burn money, is building a wonder
vehicle (expensive), and will bleed much more money when they finally get
on the learning curve and have something goes wrong.

SpaceX is almost to Orbit in the real world, not the paper world. And the
experience and improvements of Falcon 1 keeps the learning curve going in
the right direction with Falcon 9.

Jeff Findley wrote:


"Craig Fink" wrote in message
link.net...
My guess as to what happened,

Have your balancing stick handy, helps with the words.

Yeah, a modeling problem. There are three modes to balancing a stick.
Finger
moving in and out, pitch, using pitch attitude, rate, acceleration,
jerk...
and left and right, yaw, again attitude, rate, acc... and a third mode
roll
(but not the roll one normally thinks of, rolling the stick between your
fingers). The roll in terms of moving your finger in a big circle to
balance the stick. The stick isn't rolling but is going in a big circle,
finger moving in pitch and yaw.

This seems to be the one that failed, the engine was moving in circle
that spiraled out. Looking at the gimbal motion in Polar Coordinates, the
circle
has a radius R and an angular rate w. The cause of this circular motion
was
most likely slosh, a wave at the surface of the liquid in the tank moving
around the wall of the tank. Like moving a bucket of water in a circle,
one
side of the bucket has the crest, the other side has the trough.

The average C.G. is still along the centerline of the vehicle, but the
instantaneous C.G. is moving in a circle with some radius R and the same
angular rate w as the engine gimbal's angular rate. If the angular
position
of the gimbal is behind the angular position of the C.G., the wave is
flowing down hill and getting larger. The instantaneous C.G. shift R will
grow. If the angular position of the gimbal is ahead of the angular
position of the C.G. then the wave is trying to flow up hill, energy is
taken away and the instantaneous C.G. shift R will be reduced.

The normal pitch and yaw should only be used with respect to the average
C.G. and not used to chase transients around circlecaused by the slosh
instantaneous C.G. moving in a circle.

It wouldn't be too hard to add the a slosh roll control model to the
pitch/yaw control of the engine. Or, they could just add some more slosh
baffles.

Just a guess.

It's been a long time since my dynamics and control classes, but this
theory
sounds reasonable. Now if Space-X would publish their telemetry data...
;-)


SpaceX with their data would already know if it was a vortex by now. The
signature in the data would be obvious (i would think the roll anomaly
would be telling). It would be nice if they published their telemetry data
on their web site to see what the actual interaction was, and if it fits a
vortex. The experience they are going through is kind of like a blast from
the past. What was going on in the late 50s and 60s in NASA heyday, but
with a modern twist, much more knowledge about the subject.

Statics, Dynamics and Control are always fun subject to explore.

"Craig Fink" wrote in message
thlink.net...
Everyone who does something new, even if it has been done before by
someone
else, has to go through a learning curve. By doing it, yes mistakes are
made, but much is learned from each mistake. The learning curve is very
steep initially, then begins to flatten out. SpaceX's learning curve will
flatten out soon.

To me SpaceX is taking the right approach and is much further along the
learning curve than a company like Kistler. Kistler seems to have a large
list of "big" names, has learned how to burn money, is building a wonder
vehicle (expensive), and will bleed much more money when they finally get
on the learning curve and have something goes wrong.

SpaceX is almost to Orbit in the real world, not the paper world. And the
experience and improvements of Falcon 1 keeps the learning curve going in
the right direction with Falcon 9.

This from the same guy who keeps posting about "fluid variable intakes" on
hypersonic air breathing engines as a propulsion system for a launch
vehicle?

I'm reminded of the "jokes" about what and airplane would look like if you
had different aerospace engineers design an airplane. The propulsion guy
would design a plane that was all engine, the aerodynamics engineer would
design a plane that was all wing, the structural engineer would design a
plane with wings that had a rectangular cross section, the dynamics and
control engineer would design a plane that was all control surfaces... I
swear one of my aerospace engineering professors had view graphs for this
series of jokes.

Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)