Ares I Thrust Oscillation meetings conclude with encouraging data, changes

[kyle_baron]

The resonant frequency of the SRB is not from the plate steel casing, it is from acoustic effects in the cavity at the SRB's core. The SRB's case must presumably have a different resonant frequency in order not to rip itself apart on every Shuttle flight.

Resonance may originate in the SRB, but requires additional vibrations from the 2nd stage to complete the natural frequency of the stack.

edit:  Changed the word help, to additional vibrations.

From what I understand of damping panels used for domestic acoustics (ie music) foam on the upper stage tanks will do exactly nothing to vibrations within several octaves of 12Hz.
cheers, Martin

That's yet to be determined, by a full mock-up test flight.

[kyle_baron]

Because of the difference in the fully fueled mass of the SRB (~590,000 kg) verses the unfueled mass of the Ares-1X second stage mass simulator, which is around 190,000 kg, plus the internal structural bracing found in the Ares-1X second stage that is not found in the Shuttle SRBs.  These structures are nothing more or less than spring/damper systems tied to a supporting structure with a certian stiffness tied to a vibrational forcing generator called a rocket.  So once you know the stage's strucure stiffness, mass, bending modes and force function, the structural resonant frequencies fall out naturally for each element in the system. 

So, it's the materials of the rocket:  steel, aluminum + polyurathane insulation, carbon fiber (all of which react differently to vibrations)  AND the internal structure of the stages (along with the propellent), that affect the resonant frequencies.

[clongton]

Resonant oscillations are not the same thing as vibrations. You need to select your terms more carefully in order to better understand what the Thrust Oscillation is actually doing.

Definitions:
In physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies).

Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.

So damping has to do with affecting the system's resonant frequencies, which is not related to "vibrations". Notice that the term harmonic oscillator does not refer to sound, as in "harmony", but rather to the synchronizing of two or more separate oscillatory systems.

[rdale]

NASA and contractor engineers have developed multiple options for "de-tuning" the Ares I rocket to prevent any problematic thrust oscillations from originating in its solid-rocket main stage to sync up with the natural resonance of the rest of the vehicle.

http://wiki.nasa.gov/cm/blog/Constellation/posts/post_1261434125038.html

[sdsds]

NASA and contractor engineers have developed multiple options for "de-tuning" the Ares I rocket to prevent any problematic thrust oscillations from originating in its solid-rocket main stage to sync up with the natural resonance of the rest of the vehicle.

http://wiki.nasa.gov/cm/blog/Constellation/posts/post_1261434125038.html

Fascinating.

"The team will 'scar,' or prepare, the upper stage design to accommodate the addition of this [C-Spring isolator and LOX damper] mitigation hardware at a later time, if desirable."

What is the meaning of, "if desirable" in that sentence? 

[Antares]

It would mean "if trade studies support it" IMEO.

[sdsds]

It would mean "if trade studies support it" IMEO.

Oh.  You don't think they mean, "We'll add the mitigation hardware once we've actually seen a vehicle shake apart in flight?" 

Perhaps they mean, "We won't be adding this until we find out if the design can carry that much added mass."

[kyle_baron]

NASA and contractor engineers have developed multiple options for "de-tuning" the Ares I rocket to prevent any problematic thrust oscillations from originating in its solid-rocket main stage to sync up with the natural resonance of the rest of the vehicle.

http://wiki.nasa.gov/cm/blog/Constellation/posts/post_1261434125038.html

C-Spring Isolators:
http://blogs.nasa.gov/cm/blog/Constellation/posts/post_1244149653134.html

Is NASA going to use both (dual) isolators, or just the top (of the upper stage) isolators?  It wasn't really clear, in the article.

I hope that it is only at the top of the 2nd stage, the bottom of the 2nd stage, looks like an obvious bending point!

[Comga]

The link on C-Spring isolators is interesting, but mainly because each advantage is just less of a disadvantage.

Steel C-Springs were too heavy, but titanium C-springs weight less.  They still weigh more than no springs.

The C-springs can be used to meet the new criterion on longitudinal vibration, but that limit has been increased from the historic limit of 0.25 g peak to 0.75 g. 

Can someone explain how a full ring of C-springs allows axial compliance without making the stack more susceptible to bending?

[jarmumd]

Can someone explain how a full ring of C-springs allows axial compliance without making the stack more susceptible to bending?

It will make it more susceptible to bending, but not by that much.  The spring stiffness for the C-springs is still very high, it is more that it is a different stiffness than the surrounding structure than just a softer spring.  So far the bending analysis has been done and the forces are acceptable.  There are other cards to pull in case those forces are deemed to high (like tuning the trajectories for the day of launch)

[kyle_baron]

NASA and contractor engineers have developed multiple options for "de-tuning" the Ares I rocket to prevent any problematic thrust oscillations from originating in its solid-rocket main stage to sync up with the natural resonance of the rest of the vehicle.

http://wiki.nasa.gov/cm/blog/Constellation/posts/post_1261434125038.html

Fascinating.

"The team will 'scar,' or prepare, the upper stage design to accommodate the addition of this [C-Spring isolator and LOX damper] mitigation hardware at a later time, if desirable."

What is the meaning of, "if desirable" in that sentence? 

So far, thrust oscillation hasn't shown up when tested on the Shuttle, or Ares-1X.  So, IMO, "if desirable" means, if it shows up.

[kyle_baron]

C-Spring Isolators:
http://blogs.nasa.gov/cm/blog/Constellation/posts/post_1244149653134.html

Is NASA going to use both (dual) isolators, or just the top (of the upper stage) isolators?  It wasn't really clear, in the article.

I hope that it is only at the top of the 2nd stage, the bottom of the 2nd stage, looks like an obvious bending point!

After rereading the article, it would appear the the location is at the top of the 2nd stage ("upper plane" and "module"-singular, not dual):

upper plane C-Spring isolator module

[khallow]

So far, thrust oscillation hasn't shown up when tested on the Shuttle, or Ares-1X.  So, IMO, "if desirable" means, if it shows up.

There was TO in both the Shuttle and the Ares 1-X. For the Ares 1-X test, it is claimed that the TO peaked at a significantly lower value than predicted. "Peak pressure" was 1/3 to 1/2 predicted values.

[MP99]

So far, thrust oscillation hasn't shown up when tested on the Shuttle, or Ares-1X.  So, IMO, "if desirable" means, if it shows up.

There was TO in both the Shuttle and the Ares 1-X. For the Ares 1-X test, it is claimed that the TO peaked at a significantly lower value than predicted. "Peak pressure" was 1/3 to 1/2 predicted values.

OK, 1/3 of predicted.

But why don't they list the absolute size of the oscillation?

cheers, Martin

[Integrator]

So far, thrust oscillation hasn't shown up when tested on the Shuttle, or Ares-1X.  So, IMO, "if desirable" means, if it shows up.

There was TO in both the Shuttle and the Ares 1-X. For the Ares 1-X test, it is claimed that the TO peaked at a significantly lower value than predicted. "Peak pressure" was 1/3 to 1/2 predicted values.

OK, 1/3 of predicted.

But why don't they list the absolute size of the oscillation?

cheers, Martin

Because that would be SBU, which is why the axes on the data plots are not marked either.

The lurking monster in this issue is a possible closed loop interaction between the TO forcing function and the structural dynamics of the vehicle.  Ares-1X upper stage simulator tuned to completely different resonant frequencies because it was basically a stiff steel pipe so there was no interaction. This is how you detune, by designing away from the sensitive frequencies from the beginning.  Mitigations they are baselining now are band-aids applied to the vehicle because it is too late to change the structural resonances of Orion. 

Of course, the worst case scenario would be if vehicle oscillations ever somehow feedback into the booster to amplify the internal acoustics that create the thrust oscillation.  This would be a solid rocket version of pogo.  We need to prevent this situation from ever happening.  It is completely avoidable.

[MP99]

So far, thrust oscillation hasn't shown up when tested on the Shuttle, or Ares-1X.  So, IMO, "if desirable" means, if it shows up.

There was TO in both the Shuttle and the Ares 1-X. For the Ares 1-X test, it is claimed that the TO peaked at a significantly lower value than predicted. "Peak pressure" was 1/3 to 1/2 predicted values.

OK, 1/3 of predicted.

But why don't they list the absolute size of the oscillation?

cheers, Martin

Because that would be SBU, which is why the axes on the data plots are not marked either.

So, do they expect it to remain SBU after the first crewed flight?

The lurking monster in this issue is a possible closed loop interaction between the TO forcing function and the structural dynamics of the vehicle.  Ares-1X upper stage simulator tuned to completely different resonant frequencies because it was basically a stiff steel pipe so there was no interaction. This is how you detune, by designing away from the sensitive frequencies from the beginning.  Mitigations they are baselining now are band-aids applied to the vehicle because it is too late to change the structural resonances of Orion. 

Of course, the worst case scenario would be if vehicle oscillations ever somehow feedback into the booster to amplify the internal acoustics that create the thrust oscillation.  This would be a solid rocket version of pogo.  We need to prevent this situation from ever happening.  It is completely avoidable.

I guess that's SBU, too?

cheers, Martin

[clongton]

NASA and contractor engineers have developed multiple options for "de-tuning" the Ares I rocket to prevent any problematic thrust oscillations from originating in its solid-rocket main stage to sync up with the natural resonance of the rest of the vehicle.

http://wiki.nasa.gov/cm/blog/Constellation/posts/post_1261434125038.html

Fascinating.

"The team will 'scar,' or prepare, the upper stage design to accommodate the addition of this [C-Spring isolator and LOX damper] mitigation hardware at a later time, if desirable."

What is the meaning of, "if desirable" in that sentence? 

So far, thrust oscillation hasn't shown up when tested on the Shuttle, or Ares-1X.  So, IMO, "if desirable" means, if it shows up.

It HAS shown up and been quantified.
The problem though is not with Shuttle, nor with Ares-IX, but with the 5-segment Ares-I. It has completely different burn characteristics than either of them. Measuring TO on Shuttle and AIX was done - not to see if it exists (it DOES) - but to validate the models created to measure it, because those same models predict TO at deadly levels for the last few seconds of the 5-seg burn.

[yg1968]

Why are you going back to the Ares I will "shake its passengers to death" arguments despite all the evidence pointing that this would not be the case.

[kyle_baron]

Why are you going back to the Ares I will "shake its passengers to death" arguments despite all the evidence pointing that this would not be the case.

That's exactly my point.  I realize that T.O. will always be part of a SRM.  However, the "shake its passengers to death" requires additional inputs (similar frequencies) from the rest of the rocket.  IMO, the similar frequencies will not happen, because the stages and interfaces, have different materials, all vibrating at different frequencies.

[Integrator]

That's exactly my point.  I realize that T.O. will always be part of a SRM.  However, the "shake its passengers to death" requires additional inputs (similar frequencies) from the rest of the rocket.  IMO, the similar frequencies will not happen, because the stages and interfaces, have different materials, all vibrating at different frequencies.

It will not happen because we will not allow it to happen. You can't sit back and think 'oh, it will never happen' because then it surely does.  I intend to make sure it does not happen by putting this specific environment (and several others of similar concern) in the SRD of this new vehicle.