Author Topic: Ares I Thrust Oscillation meetings conclude with encouraging data, changes  (Read 150756 times)

Offline Kaputnik

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Worth remembering that many people were opposed to Ares-I development even when it didn't suffer TO problems. Chiefly because it could be seen as a waste of time, money, and effort, achieving a goal that can be done with an EELV (or Jupiter). Blah blah....
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Offline JIS

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It shouldn't be overlooked that some of the engineers are still arguing for active TMA. If they're in favor of making a system more complicated, then the confidence level is clearly marginal, but this is the most favorable article I've seen in a while.


TO is not solved and it still stay as an issue. I think that the best mitigation is active TMA indeed.

If the active TMA can suppress thrust induced oscillations by itself then no mods on Orion or interstage would be needed.

It can be scaled back whenever needed and should be robust enough even if few elements fail. It could be reusable (recoverable) and used for Ares V as well (TO could be an issue on crewed Ares V).
   
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Offline Jim

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and used for Ares V as well (TO could be an issue on crewed Ares V).
 

It wouldn't be an issue for Ares V because the SRB attachment to the core.  Much like the shuttle doesn't have TO issues

Offline jarmumd

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and used for Ares V as well (TO could be an issue on crewed Ares V).
 

It wouldn't be an issue for Ares V because the SRB attachment to the core.  Much like the shuttle doesn't have TO issues

This is a bit of a derailment, so I'll try not to make a big deal about it here, but I have to disagree with that statement completely.  TO is a system resonance.  The shuttle is a completely different resonant system than either Ares V (or Direct).  There is no proof that I have seen (i.e. a detailed Finite Element Model) which says that either Ares V (or Direct) will not have a resonant axial mode around 12 Hz.  I know it is a stretch, and I will admit I think it is highly unlikely, but TO could certainly be an issue for ANY in-line LV.

And although the shuttle has no TO issues, it was my impression that the crew have felt TO before (they have felt stud hang ups - which are more benign IMO).

Offline Lars_J

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Worth remembering that many people were opposed to Ares-I development even when it didn't suffer TO problems. Chiefly because it could be seen as a waste of time, money, and effort, achieving a goal that can be done with an EELV (or Jupiter). Blah blah....
Not to mention that launching humans on solid rocket boosters is not (and never was) such a good idea. A single SRB as a first stage does not improve that idea.

Noone would design the Ares I they way it is with a clean-sheet design.
« Last Edit: 12/10/2008 04:26 PM by Lars_J »

Offline Tim S

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and used for Ares V as well (TO could be an issue on crewed Ares V).
 

It wouldn't be an issue for Ares V because the SRB attachment to the core.  Much like the shuttle doesn't have TO issues

This is a bit of a derailment, so I'll try not to make a big deal about it here, but I have to disagree with that statement completely.  TO is a system resonance.  The shuttle is a completely different resonant system than either Ares V (or Direct).  There is no proof that I have seen (i.e. a detailed Finite Element Model) which says that either Ares V (or Direct) will not have a resonant axial mode around 12 Hz.  I know it is a stretch, and I will admit I think it is highly unlikely, but TO could certainly be an issue for ANY in-line LV.

And although the shuttle has no TO issues, it was my impression that the crew have felt TO before (they have felt stud hang ups - which are more benign IMO).

It's possible, yes, but the thrust structure of Ares V is not unlike the Shuttle ET. It would work along similar lines.

Offline Yegor

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Ares I won't survive the first 100 days of the new administration, so no need to comment.

Is it just an opinion or an insider's info?


Offline kraisee

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The key with the strap-on SRB's is that any TO effects which do occur are easy to mitigate using existing and well-proven techniques.

Every strap-on SRB in use, including the very large Shuttle and Titan SRB's, has been attached to the booster using a joint which is specifically designed to either safely absorb the forces of the SRB's oscillations, or to change the frequencies so that the TO frequency doesn't coincide with the resonant frequency of the vehicle any longer.

This approach has been proven on more than a dozen different US designs over the last 30+ years, it is well understood, efficient and is proven.   Because of those three things those mitigation options are today relatively inexpensive to implement and do not negatively affect the schedule as design/development work can be reduced.

The Ares-I doesn't have that option because both the Upper Stage and the Interstage must be hard-bolted straight onto the top of the SRB, and that location is proving to be an extremely tricky one to incorporate any such mitigation options into the design.   This means that new techniques are required to fix the TO problems on Ares-I and that adds major development costs to a program already tight on budget.   The heavier budget impact, combined with the longer DDT&E schedule only means that the programs implementation ultimately gets delayed even further.

Result:   Higher cost.   Longer delays.   Lower performance.

Way to go.

Ross.
« Last Edit: 12/10/2008 06:25 PM by kraisee »
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Offline bad_astra

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Good article. Don't expect to be hearing much from the EELV folks on this thread.

two tons mass dampers and what do you get, lot less payload and deeper in debt..


there.
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Offline renclod

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two tons mass dampers and what do you get,
lot less payload and deeper in debt..


one two three

my name is ares one - you all know me
the solid rocket booster from e.es.em.dee
gonna need your vote in the next election
can I get a 'what what’ from the engineering section


Offline blazotron

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It's definitely surprising me, and I'm having a hard processing it. Resonant burning occurs every flight, correct?

Why every flight ? Here are my attempts at defining variation factors:

1/ Slag accumulation arround the submerged nozzle

The SRM nozzle is submerged, it protrudes inside the casing (for thrust vectoring purposes.) Hot molten aluminium oxides in liquid phase, a.k.a. slag, get trapped in the volume defined by the nozzle and the casing. This is not necessarily deterministic, not to the split second and not to the fraction of an inch.

The cavity arround the nozzle is playing a role in the start of major pressure oscillations, in other words a role in the precise frequency, intensity, timing of TO.

2/ Burn rate variations, due to minute batch variations, the day of flight/test temperature of the solid propellant, and a zillion other factors.

It is the same thing as with POGO. Probabilistic. Different every flight, every test. 4-seg has probabilistic TO, 5-seg the same. IMO of course.



I think these reasons have a lot to do with the non-deterministic nature of TO.  In the grand scheme of things, every parameter of importance has a probabilistic distribution of values (this is the basis of 6 sigma design).  So you might end up on some flights with a burn rate on the low side that when combined with a slightly high payload weight because the astros ate a larger than normal breakfast, and a structure a little stiffer than normal because the machine tools used to make it were a little dull, and a million other factors, makes one particular flight have detrimental TO problems, while another one not.  This is the same thing Antares and I were discussing on this thread (my words about it are in the last major paragraph of my long post):
http://forum.nasaspaceflight.com/index.php?topic=15198.msg341877#msg341877

But there is a similar sounding but subtly and importantly different reason as well (also mentioned in the above post) that TO can be non-deterministic.  Some of the systems involved in generating TO are chaotic to a greater or lesser extent.  Turbulence, including the vortex shedding in SRMs, is an extremely complex and chaotic phenomenon.  The nature of chaotic systems implies that the most minute of changes in starting configurations can lead to dramatically different outcomes.  In a chaotic system, you don't necessarily need to have differences in mean properties (as above) at all.  Even if the average burn rate for the fuel in two SRMs is the same, each SRM has little parts of the fuel grain that have slightly higher or lower burn rates, and small bumps on the surface of the fuel, and a million other factors like this.  These are the types of starting condition variations that are enough to change the course of a chaotic system.  (As an aside, the dirty little secret of non-deterministic systems is that they are, in the end, deterministic.  Physics is physics, and if you know the starting conditions and the laws that govern the process, you can predict the outcome.  However, in order to calculate the outcomes of such systems, you would have to know so many initial parameters in such detail (the location of every particle in the SRM grain, for instance) that the process is effectively non-deterministic).  In one case, you might end up with a vortex being generated at just the right time and strength to start the pressure waves in the engine amplifying.  After a while of this, the process grows strong enough that you end up with pronounced TO issues.  In another case, you might never end up with vortices that line up in just the right way to initiate the process.  This is the same as POGO.  A POGO prone engine is dynamically unstable, but you still have to have random noise or vibration line up in the right way to get the process of amplification started.  It might happen earlier or later in flight, or you might get lucky and it gets delayed long enough that you don't see it at all before staging. 


Offline blazotron

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and used for Ares V as well (TO could be an issue on crewed Ares V).
 

It wouldn't be an issue for Ares V because the SRB attachment to the core.  Much like the shuttle doesn't have TO issues

With all due respect, I take issue with this and similar comments by other posters.  It borders on arrogant to assert this without any kind of analysis to back it up.  Yes, the structures are quite different, and the similarity to the STS configuration tends to suggest vibrational modes closer to STS than Ares I, but this is absolutely no guarantee that you wouldn't still end up with TO issues.  Like jarmumd, Jim, and others, if I had to guess, I would say that Ares V and Jupiter would be better off in this area, but guessing is no substitute for engineering analysis.  And blasting away at NASA's programmatic and engineering decisions because you _think_ another solution would have less TO issues doesn't give sufficient credit to the enormous complexities that arise once the glossy surface of any paper rocket is pierced with detailed design and analysis.  If anything, the side mounted configurations have _much more_ complex vibrational modes, with lots of important higher order modes and non-longitudinal modes that can still couple to the longitudinal TO, and you may very well end up with one that lines up with TO frequencies.  This stuff is difficult to rule in or out even with advanced computational capability, as the non-deterministic nature of the TO ought to show.  Further, we only need to look at how long it took for TO to start being recognized as a problem on the structurally comparatively simpler Ares I design to realize that it isn't easy to dismiss it.

...
Every strap-on SRB in use, including the very large Shuttle and Titan SRB's, has been attached to the booster using a joint which is specifically designed to either safely absorb the forces of the SRB's oscillations, or to change the frequencies so that the TO frequency doesn't coincide with the resonant frequency of the vehicle any longer.
...
The Ares-I doesn't have that option because both the Upper Stage and the Interstage must be hard-bolted straight onto the top of the SRB, and that location is proving to be an extremely tricky one to incorporate any such mitigation options into the design.   This means that new techniques are required to fix the TO problems on Ares-I and that adds major development costs to a program already tight on budget.   The heavier budget impact, combined with the longer DDT&E schedule only means that the programs implementation ultimately gets delayed even further.

Result:   Higher cost.   Longer delays.   Lower performance.

Way to go.

Ross.

My second gripe is with this post.  You admire that the SRB's in other systems are attached using structures that reduce TO effects by being compliant, while you claim that doing exactly the same thing in Ares I is "tricky" leading to "higher cost, longer delays, and lower performance."  The only difference is where the structures are (between side mounted SRMs and the core or in line between stages).  If the STS thrust structures are really as carefully tuned as you claim (which I doubt--I bet that the lack of TO issues is rather a result of the lack of available structural vibrational modes of the appropriate frequencies to couple with the TO) then they are heavier than necessary as well--adding cost and reducing performance (and presumably increasing delays as they were designed).

My point in this post is that engineering is messy.  Compromise is always necessary.  Things are never as simple as they look at first.  You never end up with the best solution--only one that is good enough.

The path to the finish on a project as complex as this will always go through some dead ends before the best solution is determined (where best means the most reasonable that can be come up with for reasonable money and time).  One difference between this project and, for instance, Apollo, is that you are getting to see all of the dirty behind the scenes engineering, and not just the final product pop out at the end.  If this forum had been around during the SSME development, I am sure there would have been no shortage of people claiming that such a high pressure, high performance engine was impossible to make reliable because of the many early problems with turbopumps in particular.  It is easy to be an armchair engineer about something (and I am not referring to DIRECT, as I admire the dedication and work put in there) and come up with solutions that seem obviously better (even if you ARE an engineer) without having to deal with all the nitty-gritty details and with the benefits of hindsight. 

I'm sure that the Ares I config is not the best we could have done, but given the many, many constraints that were juggled, not the least of which is that we have to have funding to build a rocket and so you have to keep people in congress happy and structure the program in a bite-sized way so that you never come across a funding bridge too far (as I believe the Ares I/V development program is designed to do), I think it is a reasonable compromise made by a management whose job is to make difficult compromises with limited information. 

Would I like a better rocket? Yes.  Am I a devoted Ares I fanboy? No.  But I just want to get something flying so we can get on with space exploration, and I believe Ares I is headed in that direction.

[edited for typos]
« Last Edit: 12/10/2008 11:56 PM by blazotron »

Offline robertross

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I tell you Blazotron...I really like your posts. I get a good 'feel' from the technical side of things in that you explain it well (at least to me). The little things, like dull tool cutters, is something I never fully took to heart as an engineer-wannabe, that I now see your point about 1:3 & 3 (or 6) sigma situations.

Obviously a good number of us realize we are getting never before behind-the-scenes opportunities as to the developmental phases of this new rocket (Ares-I). And we certainly do a fair bit of criticizing for armchair engineers (or wannabes, as it were). But again, this is a give-and-take, and a 'learn as you go' system. Relying on the information presented to us is only bringing rise to the answers or decisions we present on this forum. It is FANTASTIC that we can follow along as they make these designs & decisions. But as non-engineers (those among us) you have to expect a certain level of naiivitae when it comes to understanding the whole system. Guys (and gals) like you out there help us make better and more informed decisions. We appreciate all the feedback we get (I know I do), and from there we learn, and grow, and become better observers (or critics)  :)

EDITED for typos
« Last Edit: 12/11/2008 12:31 AM by robertross »
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Offline RedSky

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So does anyone know about TO effects on Ariane 5... since it is somewhat similar to the Direct configuration:  2 SRBs attached to a central in-line core.  Or are its SRB's less subject to TO?

Offline blazotron

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I tell you Blazotron...I really like your posts. I get a good 'feel' from the technical side of things in that you explain it well (at least to me). The little things like dull tool cutters is something I never fully took to heart as an engineer-wannabe, that I now see your point about 1:3 & 3 (or 6) sigma situations.

Obviously a good number of us realize we are getting never before behind-the-scenes opportunities as to the develomental phases of this new rocket (Ares-I). And we certainly do a fair bit of criticizing for armchair engineers (or wannabes as it were). But again, this is a give-and-take, and a learn as you go system. Relying on the information presented to us is only bringing rise to the answers or decisions we present on this forum. It is FANTASTIC that we can follow along as they make these designs & decisions. But as non-engineers (those among us) you have to expect a certain level of naiivitae when it somes to understanding the whole system. Guys (and gals) like you out there help us make better and more informed decisions. We appreciate all the feedback we get (I know I do), and from there we learn and grow and become better observers ( or critics)  :)

Thanks for the compliments.  I enjoy helping people understand things when I can, as I enjoy learning from others.

At the risk of going OT, I'm sorry if I can across as though I believed no one appreciated the all-access pass they are getting, as it were.  I know most, if not all, do, and I am glad for that.  It truly is a wonderful learning experience for people to see how the gears turn.  There should be no shame in being ignorant about something: even engineers don't know all the details about projects in other areas.  And believe me, no one understands the entire system completely.  I also know that people sometimes just want to talk and vent about projects that are going ways they wouldn't have sent them, had it been their decision, especially in a place like this where we all want spaceflight to work well (I am certainly guilty as well).  There is room for discussion and criticism as well, especially when the design is truly flawed (and there are some arguments that it is in this case).  But the danger is in making too strong judgments and pronunciations about others' work based on information that is incomplete at best, even though it can be very detailed.  I don't claim to know where the boundary lies, but sometimes I get frustrated because I don't believe that the engineers, and yes, even the administrators, get the benefit of the doubt they deserve.  Maybe I'm just being too sensitive...

[edited for typos]
« Last Edit: 12/11/2008 12:25 AM by blazotron »

Offline blazotron

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So does anyone know about TO effects on Ariane 5... since it is somewhat similar to the Direct configuration:  2 SRBs attached to a central in-line core.  Or are its SRB's less subject to TO?


If you can get to it, take a look at this paper:
AIAA-2006-4418
Thrust oscillations in reduced Scale Solid Rocket Motors, A new configurations for the MPS of Ariane 5
M. Prévost and J. Godon, ONERA, Mauzac, France; A. Le Quellec, ENSIETA Engineer, Brest, Mauzac, France
42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit
July 9-12, Sacremento, CA.

The justification of the paper is that there are TO effects on the rocket, and the paper describes tests of different grain configurations to try to reduce the TO issues at the source. 

Offline Herb Schaltegger

Further, we only need to look at how long it took for TO to start being recognized as a problem on the structurally comparatively simpler Ares I design to realize that it isn't easy to dismiss it.

Quibble: what do you say to those who have posted here that TO was recognized as a potential show-stopper as soon as the VSE concepts were announced and had their concerns basically hand-waved away until it became clear how much of a Rube Goldberg solution might be required to mitigate it?

Let's face it: Ares I with all the TO mitigation strategies that have been considered (plus all the weight scrubs it has forced onto Orion) is NOT anything close to what an objective rocket designer would have started from.  It has turned to be neither "simple" nor "soon."
Ad astra per aspirin ...

Offline jarmumd

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Quibble: what do you say to those who have posted here that TO was recognized as a potential show-stopper as soon as the VSE concepts were announced and had their concerns basically hand-waved away until it became clear how much of a Rube Goldberg solution might be required to mitigate it?

Is this true though? Because although it is certainly possible that the original concept may have had TO issues, it is my opinion that the original concept (4seg SRB with SSME) would have worked.  Of course if another engine could have substituted for the SSME.  Remember, TO is a structural resonance issue, until you have detailed FEM's of the LV, you won't know if TO will cause resonance.  It is my personal opinion that as soon at they strayed from 4-seg, they should have gone back to square one, but I would hesitate to simply say "an SRB first stage would NEVER work" - I believe in it's original conception it would have.

-Marc

Offline Chris-A

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...
My point in this post is that engineering is messy.  Compromise is always necessary.  Things are never as simple as they look at first. You never end up with the best solution--only one that is good enough.
....

That may be a good quote.

Offline blazotron

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Further, we only need to look at how long it took for TO to start being recognized as a problem on the structurally comparatively simpler Ares I design to realize that it isn't easy to dismiss it.

Quibble: what do you say to those who have posted here that TO was recognized as a potential show-stopper as soon as the VSE concepts were announced and had their concerns basically hand-waved away until it became clear how much of a Rube Goldberg solution might be required to mitigate it?

I don't have any way to verify one way or the other, nor do I have any inside information.  But it would not surprise me if this were true.  From a somewhat cynical perspective, let me present the following possibility: someone's prediction is always right when there are enough someones making enough predictions.  For example, we are all familiar with the famous cases of the engineers that warned of disaster in the Columbia case but were dismissed in part because the prevailing wisdom was that the foam was not particularly dangerous (among other reasons).  These engineers turned out to be right, and looked very insightful (and might very well be--that is not my point here). 

What is never considered is how many other potential problems were identified at some point along the way with someone willing to warn of disaster that never happened.  Engineers in the trenches often cover themselves by being more cautious about their predictions than a program can stand (we have to accept some risk if we want to actually fly, after all).  But engineering management has to make difficult decisions on imperfect information and balance competing pressures to decide which really are serious and which are low probability or unrealistic risks.  Unfortunately, sometimes, as in the case of Columbia, they are wrong. 

I don't mean to knock my own profession of engineering by the comparison, but you never hear about the predictions that psychics get wrong--only the ones they get right.  The wrong ones are just forgotten.  I feel like this may be the same effect that is at least partially at work here. 

I don't want to imply that the people who claim to have warned of TO at the beginning of the program didn't have good engineering reason to do so, and may in fact be incredibly foresightful and insightful.  But I am sure that there are many other things that have been warned about that have not come true (some that come to mind are roll control authority, dynamic stability of the rocket, etc.), and you have to be careful to keep this in perspective. 

Quote
Let's face it: Ares I with all the TO mitigation strategies that have been considered (plus all the weight scrubs it has forced onto Orion) is NOT anything close to what an objective rocket designer would have started from.  It has turned to be neither "simple" nor "soon."

I think that is fair.  However, as I have said, I believe there is a lot more behind the scenes at play here than is often given credit, not the least of which is how to sell the larger Ares program to Congress in a manageable way that will provide incremental progress to keep the money coming.  In the end, isn't it better to proceed with a rocket that has money and political will to build it than a better rocket that doesn't?  Let me also give an example of non-political "nitty gritty" details that are so easily ignored but directly contribute to the design of the rocket:

(from http://www.safesimplesoon.com/mythbusters2.htm)
Quote
In ESAS and until last Spring, the Ares—I injected the Orion into a 30x160nmi transfer orbit and the Orion then circularized itself, to avoid the complexity of deorbiting the large upperstage. Working with Constellation and CEV project teams, the program elected to change to a -30x100nmi orbit to move the ocean impact of the CLV upperstage to the Indian Ocean from the South Pacific to stay away from populated islands.

It's details like where the spent stage will land that can shape the rocket design, but drive the vehicle away from what the objective rocket designer would consider ideal. 

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