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| Science Forum Index » Space Forum » Ares I-X flight stage separation successful???... |
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| David Spain... |
 Posted: Sat Oct 31, 2009 10:40 am |
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Jeff Findley wrote:
[quote]There were eight separation motors and four tumble motors on the base of the
SRB. As far as I know, the upper stage didn't have tumble motors, only roll
control motors (which performed well). The intent was to tumble the SRB
*after* a clean separation so it would encounter more air drag. The
ultimate goal was to insure that it did *not* recontact the upper stage.
Obviously something went wrong with that on this flight.
Jeff
[/quote]
Aha, *this* is that other thread I was thinking of!
FWIW, NASA animations for this flight don't show the upper stage tumbling
after separation, which makes me think that was not an expected outcome.
Dave |
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| kT... |
| Posted: Sat Oct 31, 2009 1:22 pm |
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Dave wrote:
[quote]On Oct 31, 11:40 am, David Spain <nos... at (no spam) 127.0.0.1> wrote:
FWIW, NASA animations for this flight don't show the upper stage tumbling
after separation, which makes me think that was not an expected outcome.
Dave
I wouldn't read too much into those animations. Those animations are
released for PR purposes. Needless to say, they aren't based on CFD
analysis.
Bob Ess stated:
"We went back and looked at all the flight models and dispersion cases
that we ran, and we found thousands of them that matched what we saw"
http://www.spaceflightnow.com/ares1x/091030recovery/
"Ess described the upper stage simulator as "inherently unstable"
because its center-of-gravity is located near the back end of the
vehicle."
[/quote]
All upper stages are like that. It has to do with wanting to locate your
center of pressure on the stack behind the center of gravity of the
entire stack while in the atmosphere (i.e. - at launch) in order to
increase aerodynamic stability and control authority (which also affects
fuel efficiency) and wanting to keep the center of gravity behind the
center of pressure at high altitudes nearing vacuum, and in vacuum
itself, in order to increase the control authority and the reduce fuel
consumption via the reduced control forces necessary to control the
vehicle, which more or less in the end reduces to cosine losses by
thrust vector control. Less deviation means better fuel efficiency.
Now I have actually done a study where in certain booster assisted SSTO
core vehicle configurations I reversed this at launch, and just bit the
aerodynamic losses with increased control authority, in order to analyze
the increased forces on the stack and in thrust vector control, in order
to see just what the structural mass and fuel hits were.
This is also highly relevant to exactly when and where the staging
events occur, which in turn depends intimately on the fuel efficiency
and burn characteristics of the booster and core stages. Apparently the
CxP people didn't think this through, or if they did, the just concocted
the tumble event to cover that fact up since the upper stage was indeed
unpowered. Even with power that's a lot of aerodynamic force with very
little time to deal with it. They'll need massive attitude control.
The SRBs stage low in the atmosphere, but in the shuttle the SRB is
ground started so the shuttle just continues on with no staging event.
You would think these people would have learned something from that.
> Dave |
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| Pat Flannery... |
| Posted: Sat Oct 31, 2009 1:26 pm |
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Dave wrote:
[quote]"Ess described the upper stage simulator as "inherently unstable"
because its center-of-gravity is located near the back end of the
vehicle."
[/quote]
Would that be the case in the actual Ares 1 also?
because if it isn't, then the whole CG of Ares 1-X was in a different
position from the actual vehicle.
The weight of the Orion should have counterbalanced the weight of the
engine at the rear of the upper stage.
Pat |
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| kT... |
| Posted: Sat Oct 31, 2009 3:10 pm |
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Dave wrote:
[quote]On Oct 31, 2:22 pm, kT <cos... at (no spam) lifeform.org> wrote:
All upper stages are like that. It has to do with wanting to locate your
center of pressure on the stack behind the center of gravity of the
entire stack while in the atmosphere (i.e. - at launch) in order to
increase aerodynamic stability and control authority (which also affects
fuel efficiency) and wanting to keep the center of gravity behind the
center of pressure at high altitudes nearing vacuum, and in vacuum
itself, in order to increase the control authority and the reduce fuel
consumption via the reduced control forces necessary to control the
vehicle, which more or less in the end reduces to cosine losses by
thrust vector control. Less deviation means better fuel efficiency.
I doubt that. Upper stages are driven by mass consideration rather
than aerodynamic stability considerations.
[/quote]
Sure they are, with the mass as far back near the engine as possible so
that the stack can be controlled with the minimum of control forces.
Most upper stages try to get out of the atmosphere before they separate
and ignite, but Ares I staging, particularly with the four segment
booster, was still in a regime of relatively strong aerodynamic forces.
[quote]With the full stack, you
may have aerodynamic stability considerations. But once separation
occurs and the upper stage is off, you're generally well over 100kft,
well past Max-Q, and aero forces are becoming minimal...
[/quote]
Are you kidding? Minimal at 100kft is still quite relevant. Large, in
fact. Things don't start to really calm out until well above that level.
It's debatable if even moving to a 5 segment SRB would remediate that.
I propose you run a quick Orbiter space flight simulator program for us
and let us know what you find.
[quote]And control forces don't significantly affect fuel efficiency. Drag
is a small part of a LV's delta-V budget anyway, and any off-nominal
AOA is a small part of THAT small part. Gravity drag is a more
significant loss by comparison.
[/quote]
Right, but drag is directly countered by control forces, and the more
you have to counter aerodynamic pressure, drag and lift, the greater
your fuel consumption. Cylindrical stacks are flying through the
atmosphere basically behave like a very inefficient boat hull.
Even the OSP ran into these kinds of problems.
> Dave |
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