Re: Shuttle Aerodynamics

On Fri, 21 Sep 2007 07:51:47 -0500, Craig Fink <WeBeG...@xxxxxxxxx>
wrote:

The SSME are moved independently, it's the flight control portion of the
On-Board flight software and the mixing logic that transforms the error
signals into gimbal movements. This mixing logic is changed during flight
for various reasons. So, with respect to pitch rate errors, they all move
together because the flight control mixing logic commands them to do it
that way.

Or, google groups sci.space.shuttle, there are plenty of good discussions in
the past about this subject.

Or just google the web at large for terms like [space shuttle tvc] or
[shuttle atvc] and you'll find great pages like this:

http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/thvector.html

....that will tell you more stuff than you ever cared to know, like
that there are three different sizes for SSME actuators. Strange that
they don't go ahead and tell you *why* there are three different sized
actuators, but it is easy to deduce that the reason is because the
engine bells experience different aero loads in each axis and that the
center engine has the biggest fight against pitch of all of them.

Here's another good reference that Google coughed up:

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-mps.html

Again, they still leave room for going into greater detail, such as
the history of the design decisions. I've been told by the designers
that they tried to get away with fixed nozzles on the SRBs, but that
there was not enough control authority in the SSMEs to do this. And I
expect that the loads on the stack get improved once you sink the
extra cost and complexity for vectoring the SRBs.

And after deciding to control the SRBs, they found there was so much
flexing in the stack that each SRB needed their own set of RGA
sensors. They couldn't just control off the orbiters RGAs.

One thing that I have not understood is why the SRB actuators were
designed 45degrees off-axis from the SSME actuators. While SSME
nozzles move in Pitch and Yaw, the SRB nozzles move in "Rock" and
"Tilt". My best guess is that this improves robustness in case an
actuator fails.


~ CT

.

Relevant Pages

  • Re: Shuttle Aerodynamics
    ... On-Board flight software and the mixing logic that transforms the error ... that there are three different sizes for SSME actuators. ... extra cost and complexity for vectoring the SRBs. ...
    (sci.space.shuttle)
  • Re: Dear Space Shuttle Launch-Abort Experts
    ... > that attempting to separate the orbiter/ET from the SRBs, ... > Flight Director relies on his flight controllers to make the calls on their ... who in your opinion would have been the first flight ...
    (sci.space.shuttle)
  • Re: 6x as fast or lift 6x as much or ...
    ... or would it have three times the velocity of two SRBs? ... Ignoring such trivia as vehicle redesign, ... increase in flight risk and a stiff increase in flight cost. ...
    (sci.space.shuttle)