Ares I Thrust Oscillation meetings conclude with encouraging data, changes

[Herb Schaltegger]

Are you an engineer?  If so, you should know better.

No, I'm a maintenance man by trade, and an inventor with 3 U.S. Patents.  And I'm not afraid to make mistakes.  Is that your point?

That is my point.

Combustion physics and supersonic/hypersonic fluid dynamics, combined with structural and vibrational analysis of an integrated vehicle, are not subjects that are necessarily intuitive or amenable to first-order guesstimates about what "might" work.

[kyle_baron]

Clever idea.   It might work if the frequency of the forcing function is close to the frequency of the stack movement.  But, I don't think they are.  I think TO frequency is much higher than any frequency you could rock the stack back and forth at.

Danny Deger

Yes, you're probably correct.  The maximum forcing function is 12hz according to this link:
http://www.nasa.gov/pdf/221186main_toft_checkpoint_report.pdf.
However, that doesn't mean that rocking the SRB back and forth could create a stable (sine function) combustion flow, and eliminate the vorticies in the 1st place.  I'm looking at the structural Response Chart, and wondering if those two vehicle structural modes (peaks) would be flattened out.

[Danny Dot]

Clever idea.   It might work if the frequency of the forcing function is close to the frequency of the stack movement.  But, I don't think they are.  I think TO frequency is much higher than any frequency you could rock the stack back and forth at.

Danny Deger

Yes, you're probably correct.  The maximum forcing function is 12hz according to this link:
http://www.nasa.gov/pdf/221186main_toft_checkpoint_report.pdf.
However, that doesn't mean that rocking the SRB back and forth could create a stable (sine function) combustion flow, and eliminate the vorticies in the 1st place.  I'm looking at the structural Response Chart, and wondering if those two vehicle structural modes (peaks) would be flattened out.

I don't think the flow of gasses in the SRB would be effected at all by rocking the stack back and forth.

Danny Deger

[kyle_baron]

I don't think the flow of gasses in the SRB would be effected at all by rock the stack back and forth.

Danny Deger

I did make a mistake in saying there's high pressure in the SRB.  The pressure inside the SRB is only 1-2 psi as shown in the Motor Test pressure graph, along the left hand side.  You'll need a magnifying glass to see that.  If that's the case, then the propellent (and combustion) must be burning extremely fast, when traveling through the exit nozzle (I think).  Now, with only 1-2 psi in the SRB, do you still think the flow of gases wouldn't be effected, by rocking the stack?

[AnalogMan]

I don't think the flow of gasses in the SRB would be effected at all by rock the stack back and forth.

Danny Deger

I did make a mistake in saying there's high pressure in the SRB.  As Dr. Rocket over at Space.com pointed out, the pressure inside the SRB is only 1-2 psi as shown in the Motor Test pressure graph, along the left hand side.  You'll need a magnifying glass to see that.  If that's the case, then the propellent (and combustion) must be burning extremely fast, when traveling through the exit nozzle (I think).  Now, with only 1-2 psi in the SRB, do you still think the flow of gases wouldn't be effected, by rocking the stack?

The pressure inside the RSRM is much higher than this.  See attached graph. (Note: ETM-3 is a full sized Engineering Test Motor that used 5 Shuttle RSRM segments, but is not to the same design as Ares-I will use.)

[kyle_baron]

The pressure inside the RSRM is much higher than this.  See attached graph. (Note: ETM-3 is a full sized Engineering Test Motor that used 5 Shuttle RSRM segments, but is not to the same design as Ares-I will use.)

The Nasa graph data used 1 or 2 psi in the SRB motor, to predict the 5-6g occilations in the structural response.  Not the real psi in the SRB.  Thanks for clearing that up.

[phred]

I'm not quite clear on this system.  It's a passive oscillator system?  Does that mean that it's not driven by a linear motor?  Does anyone know how this works, or a link to a discription?

[nooneofconsequence]

It's not a passive oscillator system - it's a chaotic oscillator (non linear feedback mechanisms that are highly leveraged). Potentially each time is different then prior times, because it depends on too many variables, and the randomizing effects aren't strong enough to interfere with the resonant coupling effects.

One idea does occur - has anyone tried using catalytic and dampening agent injection into the "castles" of the SRB? If you enhance/reduce the combustion rate reactions, then you can destabilize the coupling that causes the TO in the first place.

You'd monitor the pressure in each segment in the booster, and when the pressure would surge in a segment you'd inject a dampening agent, while when it dropped in another you'd enhance combustion.

It might be you'd only need one agent, and very little reaction mass to moderate TO. You might even be able to enhance the performance by increasing specific impulse by creating a more uniform combustion pattern (assuming you could get away with using only a catalyst).

You might also create intentional pressure wave patterns that worked with the FEM of the booster casing and whistle for optimum efficiency.

This technique has been used in other systems in dealing with chaotic oscillations - including nuclear reactions (e.g. moderation of neutron flux). Of course, that's on microsecond levels as opposed to the millisecond ones here, but it does apply.

[phred]

In the article sited at the beginning of this thread, it's mentioned that a "passive rather than active" system is now recommended.  I'm just wondering what is the difference.

It's not a passive oscillator system - it's a chaotic oscillator (non linear feedback mechanisms that are highly leveraged). Potentially each time is different then prior times, because it depends on too many variables, and the randomizing effects aren't strong enough to interfere with the resonant coupling effects.

[renclod]

Two Radical Ideas:

1.  Having a slightly oversized LOX tank, to act as a damper, by allowing a sloshing back and forth of propellent, on the 2nd stage, right beneath Orion.  I visulaize a 2-3 ft. empty space, with in the LOX tank.
.. .. ..

http://blogs.nasa.gov/cm/blog/Constellation/posts/post_1239311627391.html

>>> LOX damper :
Engineers are also evaluating a concept called a LOX damper, which uses the fundamental physical properties of liquids to leverage the kinetic energy in the movement of the existing liquid oxygen in the upper stage tank to dampen out vibrations. The devices, installed within the liquid oxygen tank, can engage the mass of the liquid propellant to generate momentum in the fluid itself to counter the vehicle acoustic response and disrupt oscillation. Engineers are evaluating the effectiveness and applicability of this design. <<<

"Liquid oxygen has a sufficient paramagnetic susceptibility that a strong magnetic field gradient can lift it in the earth's gravitational field."
(20020039337_2002064440.pdf @ ntrs)

If you place a coil just above the top surface of the LOX in a tank, and inject pulsing current, the magnetic field gradient will produce a LOX column to rise and, guess what, oscillate. Theoretically this could dampen out the oscillations of the stack forced by T.O.

Ain't science/engineering fascinating.

[JIS]

Sloshing of LOX in the tank is not a good idea. Introducing Helium bubbles into the bottom of LOX tank would be probably easier to do and small amount of bubbles at the bottom of tank at the right time can make significant difference to the vehicle oscillation response. 

I think that current models simply neglect liquid propellants effect because it's too hard to model. Fortunatelly, it can be trialed on the planned test stand. 

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080036835_2008036024.pdf   

It should be relativelly easy to add this system as the last stand in case of troubles. Far easier than making changes to the Orion or Cx architecture.

The trick would be to introduce bubbles at the moment of developing oscillation. This would shift vehicle response and suppress oscillation. As we know that thrust oscillation will be an issue for very limited amout of time (only few seconds) only small amount of gas and tank volume would be required. Perhaps gas in the LOX feeding line propagating upwards in 2-3g environent would be enough. Similar system works fine for liquid engine POGO suppression for decoupling pressure variations at engine chamber inlet from engine thrust oscillation.

That remains to be seen at the test stand.

[wannamoonbase]

renclod, intriguing to be sure but how many flight tests are required to certify magnetic control of cryogenic liquids?

Very cool concept but not likely.

Also, how much weight is required to include the magnetic induction coil and what happens to the LOx tank weight if you use it as a damper?  You would need to have increase the tank size because empty space in the tank is needed to provide room for the LOx to move.

[JIS]

You would need to have increase the tank size because empty space in the tank is needed to provide room for the LOx to move.

There is always some empty room and few mT of liquid doens't need to move a lot. Perhaps few mils or less.

I would imagine that as active mass actuators using usefull LOX mass instead of dead mass.

The same goes with my bubbles idea. Volume of bubbles doesn't need to be big. The best place for them would be at the bottom of the tank for few seconds to act as conventional pulsation dampener on the pump or suspension in the car. 
It changes response of the vehicle in case the oscillation starts to develop.

The problem with current passive mitigation options is that they are  tuned before the launch and the response can't be changed intentionally during the flight.

[Herb Schaltegger]

The problem with current passive mitigation options is that they are  tuned before the launch and the response can't be changed intentionally during the flight.

No, face it:  the real problem is that they need to exist at all.  All that mass detracting from the payload and margins, AND a whole slew of new independent failure modes (and we're all still waiting see some FMEA documents on this kludge). 

[kyle_baron]

Two Radical Ideas:

1.  Having a slightly oversized LOX tank, to act as a damper, by allowing a sloshing back and forth of propellent, on the 2nd stage, right beneath Orion.  I visulaize a 2-3 ft. empty space, with in the LOX tank.
.. .. ..

http://blogs.nasa.gov/cm/blog/Constellation/posts/post_1239311627391.html

>>> LOX damper :
Engineers are also evaluating a concept called a LOX damper, which uses the fundamental physical properties of liquids to leverage the kinetic energy in the movement of the existing liquid oxygen in the upper stage tank to dampen out vibrations. The devices, installed within the liquid oxygen tank, can engage the mass of the liquid propellant to generate momentum in the fluid itself to counter the vehicle acoustic response and disrupt oscillation. Engineers are evaluating the effectiveness and applicability of this design. <<<

"Liquid oxygen has a sufficient paramagnetic susceptibility that a strong magnetic field gradient can lift it in the earth's gravitational field."
(20020039337_2002064440.pdf @ ntrs)

If you place a coil just above the top surface of the LOX in a tank, and inject pulsing current, the magnetic field gradient will produce a LOX column to rise and, guess what, oscillate. Theoretically this could dampen out the oscillations of the stack forced by T.O.

Ain't science/engineering fascinating.

It may be, that the damper might act like a bladder, that you would find in a well pump water tank.  None the less, very cool!

[renclod]

About LOX damper...

HELP requested !

Please find in attachment a clip from Brian Muirhead's oral presentation at the Project Management Challenge 2009 (a few weeks ago).

I cannot understand the critical words (English is not my native language). His description is met with laughs in the audience, so it must be some tricky wording that escapes me.

Please provide transcript !

Other details: link to the paper (not related to thrust oscillation):
http://pmchallenge.gsfc.nasa.gov/docs/2009/presentations/Muirhead.Brian.pdf

link to podcast (55 MB):
http://pmchallenge.gsfc.nasa.gov/podcasts/2009/BrianMuirhead.mp3

Mr. Muirhead is talking about t.o. to the end, past 55 min.

Clip in attachment :

[strangequark]

About LOX damper...

HELP requested !

Please find in attachment a clip from Brian Muirhead's oral presentation at the Project Management Challenge 2009 (a few weeks ago).

I cannot understand the critical words (English is not my native language). His description is met with laughs in the audience, so it must be some tricky wording that escapes me.

Please provide transcript !

Other details: link to the paper (not related to thrust oscillation):
http://pmchallenge.gsfc.nasa.gov/docs/2009/presentations/Muirhead.Brian.pdf

link to podcast (55 MB):
http://pmchallenge.gsfc.nasa.gov/podcasts/2009/BrianMuirhead.mp3

Mr. Muirhead is talking about t.o. to the end, past 55 min.

Clip in attachment :

There's nothing I can find that is terribly tricky in the wording. Though, the first thing that comes to mind when he mentions that is propellant slosh, which is generally viewed as a problem. That could be the source of the unintended humor.

[renclod]

strangequark ,

"per-phood deck-ing" ?!

Please read a few posts above; this is about a method to engage the LOX in vibrating, and I don't think that by "fundamental physical properties" they mean sloshing... really now...

[AnalogMan]

strangequark ,

"per-phood deck-ing" ?!

Please read a few posts above; this is about a method to engage the LOX in vibrating, and I don't think that by "fundamental physical properties" they mean sloshing... really now...

Your quoted words sound like "a fluid damping" system to my ears.

[yinzer]

strangequark ,

"per-phood deck-ing" ?!

Please read a few posts above; this is about a method to engage the LOX in vibrating, and I don't think that by "fundamental physical properties" they mean sloshing... really now...

The "fundamental physical property" could easily be viscosity.  The phrase in question is probably "fluid damping".  They probably plan to have a weighted baffle attached to the tank by a spring, tuned such that  the SRB resonance frequency causes the baffle to vibrate, dumping energy into the LOX.

It's a clever idea, if they can make it work.  Given that the thrust oscillation is expected to become most serious at the very end of powered flight, taking a "wait and see" approach seems wise.

The more interesting bit happens a few minutes earlier, when he talks about how they waived the requirement for LIDS to take the loads caused by a J-2X gimbal hard-over at the end of the TLI burn, because that failure mode was "not credible".

Now, I can think of three launch vehicle failures involving gimbal hard-overs off the top of my head - Titan IV K-17 in 1993, Ariane V 501, and one of the Athenas.

Maybe Ares I isn't vulnerable to these failures, but it'd be interesting to know why.