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CEV Design Q&A
ugordan
post Jan 19 2008, 05:34 PM
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From what I've read on nasaspaceflight.com this issue is a very serious one and untrivial to resolve. Without any dampening applied, the crew would experience longitudinal G force oscillations far beyond anything experienced till now and far above NASA recommendations on human safety. The vibrations would adversely affect both the Orion spacecraft and the cryogenic Ares I second stage, too. Any dampening mechanism would likely add significant weight penalties, even more so if you were to put it between the SRB and second stage (instead of between 2nd stage and Orion) to protect that stage as well.

I never could get my head around the reasoning of making a 5-segment SRB the base of a man-rated vehicle when SRB's are known to provide a very bumpy ride and cannot be shut down. SRB vibrations in itself have been a known issue ever since first solid boosters were developed (there are apparently two vibration modes, each due to different factors) so it's a bit funny no one figured out until now they might be problematic for an inline configuration, especially with the uprated booster. Just like (in retrospect) the shuttle payload configuration looks like a wrong design decision, this too looks to me like a bad choice.

The vibration problem should not affect the Ares V because the vibrations are dampened by "tying" two SRBs through the intertank structure, just as the Shuttle currently works.


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nprev
post Jan 19 2008, 06:04 PM
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QUOTE (ugordan @ Jan 19 2008, 09:34 AM) *
SRB vibrations in itself have been a known issue ever since first solid boosters were developed (there are apparently two vibration modes, each due to different factors) so it's a bit funny no one figured out until now they might be problematic for an inline configuration, especially with the uprated booster.


I'm not too surprised, frankly; projectitis is endemic in government nowadays, and large issues seem to be discovered later rather then sooner. mad.gif Still, despite that cheap shot on my part, at least they realized this seven years before the planned first launch. Based on the problem description, I speculate that they might be looking at an exhaust bell redesign of some sort to minimize the vibes, but this might be too simplistic from what you were saying, Gordan.

In analogy to aircraft, perhaps a stability augmentation system (SAS) would be a good answer, using vanes in the exhaust flow to disrupt the resonance. The problematic flight phase is just the first few minutes of flight; might be worth sacrificing some thrust for vibe dampening.


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dvandorn
post Jan 19 2008, 06:27 PM
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Problem is, the CEV has been redesigned and made smaller once already, based on performance projections for the Stick. We're now at the raw edge of the Stick's current ability to orbit Orion as currently designed. Any reduction in performance will result in a further redesign that will severely limit the CEV's ability to perform its design mission.

I think we may be looking at the potential for changing over to an Atlas V or a Delta IV (in their Heavy configurations) as the first stage for Orion. Any such decision has to be made *very* soon, though.

-the other Doug


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nprev
post Jan 19 2008, 06:42 PM
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QUOTE (dvandorn @ Jan 19 2008, 10:27 AM) *
I think we may be looking at the potential for changing over to an Atlas V or a Delta IV (in their Heavy configurations) as the first stage for Orion. Any such decision has to be made *very* soon, though.

-the other Doug


I like the Delta IV Heavy idea; in fact, I wonder if it would be feasible to cluster additional cores (up to six) for incrementally greater boost capability. Hell, this might make Orion more achievable in the short term, esp. because the IV Heavy is going to be used extensively over the next few years; lots of shake-down (sorry! tongue.gif )/testing will already be in place.


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ugordan
post Jan 19 2008, 06:52 PM
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I don't think the problem can be adressed by changing engine bells, it has to do with oscillations in booster casing volume, pressure and hence burn speed. It's a process that feeds itself and is a natural characteristic of all solid boosters. An analogy might be pogo in liquid fuelled engines, even though the mechanisms at work are different.

I agree with oDoug in that it would be wise to bite the bullet and consider abandoning the Stick altogether. I'm really pessimistic that will actually happen. Rather, billions of dollars might be spent on fixing this issue that came up in the first place because they wanted to use as much Shuttle technology as possible, but that's going too much into politics here.

There are obvious problems with switching to the Atlas V - it uses Russian engines so it's probably a big no-no. The Delta IV is more expensive and the Heavy variant produces a rather large fireball during ignition which, although it doesn't pose any significant problems to the payload, does look rather discomforting. There are also things like Ares V using components built for Ares I which then can be scratched off the funding for V, i.e. if you were to cancel the Ares I, Ares V development costs would be seen to rise. It looks like it's going to be a pretty expensive booster even now, I hear.


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nprev
post Jan 19 2008, 07:13 PM
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QUOTE (ugordan @ Jan 19 2008, 10:52 AM) *
I don't think the problem can be adressed by changing engine bells, it has to do with oscillations in booster casing volume, pressure and hence burn speed. It's a process that feeds itself and is a natural characteristic of all solid boosters. An analogy might be pogo in liquid fuelled engines, even though the mechanisms at work are different.


Argh. If so, there's no conceivable active control system to counteract that. Despite the fireballs, I'm gonna back D4Hx as the best solution at this point (limited as the data is, of course...)


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Jim from NSF.com
post Jan 20 2008, 02:00 PM
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QUOTE (nprev @ Jan 19 2008, 01:42 PM) *
esp. because the IV Heavy is going to be used extensively over the next few years; lots of shake-down (sorry! tongue.gif )/testing will already be in place.


Not really, only 3 flights in the next ten years
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Greg Hullender
post Jan 20 2008, 04:50 PM
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Great opportunity for those SpaceX guys to get their Falcon 9 Heavy ready.

--Greg
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