<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I have no insight as to why all-solid EELV was not selected, but from running rough calc, the all-solids EELV are very inefficient from the Isp perspective. Second, the solids produce very high vibrations which I'd assume would drastically affect how NRO payloads are designed.</DIV></p><p>I do have some insight, which I don't think I shuld talk about in a public forum. Solids have lower ISP than do liquids, but they have much better mass fraction and lower costs. Solids produce some vibration, but I would dispute the characterizatin of "very high". Pressure oscillations are typically a few psi, and in the case of the shuttle SRBs only about 1 psi ( about 2 psi peak to peak). </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> I know from talking to the payload community that, going from a hypergolic upper stage to a cryo upper stage is a welcome move as it significantly reduces the payload vibration level. I can just imagine what a solid upper stage will do.</DIV></p><p>There are few solid final stages, the Star series being a bit of an exception. Generally a liquid final stage is preferable because the thrust-time curve can be tailored for the mission. Even with what I call an all-solid EELV design the final stage would have been liquid -- sorry if I mislead you on that. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> There are two key parameters here: 1) the forcing function of T.O. and, 2) the structural response of the vehicle.As far as the forcing function goes, extrapolating from the 4-segment + predicted pressure oscillation for the 5-segment by ATK, is probably best one can do. Of course I'd assume this "assumption" will be updated when a 5-segment solid is available for ground testing.I am sure that you know this T.O. issue is an "integrated" problem in that, the combined structural response (Ares I + Orion) is the only way to solve this problem. This involves 1) "de-tune" the structural resonance frequency away from the T.O. forcing function frequency and 2) create a "reactive force" to counter the forcing function.</DIV></p><p>Correct. There is not much that can be done with the forcing function at the level that the thrust oscillation is being predicted. What one does in general is design the grain features to avoid "organ pipes" and have solid particles in the combustion gasses. The grain design is, I think, pretty well established and the thrust oscillation seen in the 4-segment design is quite low. The issue of solids in the gas is taken care of in large rocket motors by the aluminum that constitutes a significant portion of the fuel. Their primary purpose is to provide high flame temperature, but they also serve to scatter sound waves and thereby dampen acoustics.</p><p>I agree that the problem is an "integrated" issue. I also have enough experience with the analytical methods and difficulties involved in using them to know that predictions at this stage regarding not only the forcing function, but also the coupling and transmission through the structure are formidable. Generally in building a model (often using NASTRAN) you have a resonable idea of stiffnesses of the structural members, but little idea of the damping coefficients or of the more complex vibrational modes. Damping coefficients are very important in a problem such as this. To even begin to get a handle on the necessary parameters you really need a thorough modal survey, and there has been no opportunity for such and activity as yet. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> Personally I think ATK should be doing more in reducing the forcing function but that's another story.The strcutural response of the Ares I + Orion is being done by FEM analysis modeling the stiffness of each section of vehicle. I am not aware the "non-conservative" assumptions but I am sure they can always be questioned. Right now, it looks like there are a few viable options to minimize the T.O. effect, based on analysis.Most of the LV you mentioned above do not align payload axially with the solid booster, with the exception of Pegasus and ICBMs. The nature of payloads of latter are not in the same sensitivity class as the former, therefore high vibration from the solid may not be an issue. The shuttle, for example, has 2 SRBs but the T.O. becomes a shear load to the Orbiter, so the T.O. effect is minimized. <br />Posted by propforce</DIV></p><p>There is really not much that ATK can do, as noted above. Dynamic FEM analysis is notoriously inaccurate for problems such as this without the benefit of some experimental data input, in particular a modal analysis. There are assumptions made in modeling the stiffness, generally pretty good for metals that have been characterized, and less so for composite structures. But the major uncertainties lie with the joints, and associated coupling and damping. Those are critical and not readily modeled from first principles, hence the need for the empirical data that is not yet available. Alignment or not of the solids is a matter of how the loads are coupled to the vehicle structure and the payload. If all joints were perfectly tight and damping is neglected then it does not matter if the loads are transmitted throught tension/compression or shear. Of course they joints are not perfectly tight, and the nature of the joints is critical to the accuracy of the analysis -- again whether or not the load path involves shear.</p><p>The bottom line is that the issue is far more comples than what one would gather from what is posted in public forums, where not only the discussion of basic engineering methodology and uncertainties is watered down, but also the nature of the concern and the mission requirements are garbled.</p><p>I can't speak to how NRO payloads are designed,or their specific concerns. But I do know that they have regularly taken successful rides on solids. I don't think they had any aversion to the solids on the Titan IV B. </p><p>I am afraid that I will have to bow out of many of these discussions. I developed a potential conflict, future but near term, a few minutes ago. Probably not a real problem, but better to err on the side of conservatism in this case. </p><p><br /><br /> </p> <div class="Discussion_UserSignature"> </div>
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