Why does the shuttle throttle on ascent?

Yes, and it is also why the shedding foam can only do serious damage within
the lower atmosphere, as the drag cannot decelerate the chunks enough to
strike with enough force to do harm at that altitude.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Dudley Henriques" wrote in message
...

"Danny Deger" wrote in message
...
Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger

As the shuttle ascends, the dynamic pressure (in slugs/sq.in) increases as
the square of the velocity. The shuttle is throttled back to 65% thrust to
avoid over stress at a speed computed under what is called critical q.
As the shuttle ascends, it is ascending into lower air density (which of
course transfers into a lowering dynamic pressure). The throttling back
takes the shuttle through a transition area during the ascent that ends as
the lowering air density meets the parameters that allow throttle up. This
I believe occurs at about 35 to 37K.
The point where lowering air density meets the ability to throttle up
again is the max q for the shuttle. (Max q meaning maximum dynamic
pressure)
After reaching max q, the shuttle is go for throttle up as the increasing
velocity past max q will never exceed the structural limitations of the
shuttle due to it's entrance into lower air density that doesn't have the
ability to overstress the structure.
Hope this helps a bit.
Dudley Henriques

I don't think this is entirely true. I have some old stuff rattling around
in my mind from the 80s which put the rating at least twice this though of
course, only in the thrust direction. After all, the instantaneous
pressures on change can be rather less than subtle.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


wrote in message
ps.com...

Danny Deger wrote:
Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger

Because structurally the shuttle vehicle is only stressed for 3 g's. If
they didn't throttle back in the late stages of the ascent when the
fuel is almost gone and the vehicle is light, it would exceed this
level of acceleration. Since the engines are liquid fueled, they can
hold full thrust until the fuel is gone.

Bud

But the actual thrust abilities of the smes was increased over the life of
the Shuttle, I'm sure I read that.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Morgans" wrote in message
...

"Richard Riley" wrote in message
...
On Sat, 6 Jan 2007 20:36:52 -0600, "Danny Deger"
wrote:

Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger


Heck, I still want to know how to go to 103% throttle.

Rather like the old prop jobs (DC-3 comes to mind as an example) that had
a wire across the throttle travel, which serves as a stop for full
throttle during normal operations. If it was needed for an emergency,
like an engine failure on takeoff, you can push through and past the wire
for extra emergency power. (100% plus power)

Of course, on some engines, that was grounds for grounding the aircraft to
inspect the engine, to see if it was damaged from exceeding 100% power.
--
Jim in NC

Richard Riley wrote:
Heck, I still want to know how to go to 103% throttle.


Because the designers worked for "Spinal Tap".
Seriously, because of improvements in its abilities as it went from the
original design specifications to actual production, it now is capable
of exceeding its originally planned maximum thrust rating by a few percent.
Where the hell did my smileys go? My new Thunderbird mail program
seems to keep them hidden.

Pat

Note. Slugs are a mass measurement whereas pounds are a force. There
is not a simple ratio because it depends on how strong gravity is. Of
course the shuttle has accelerations that are quite different than we
encounter on the face of the Earth!

There was a famous story of gold shippers that moved quantities of gold
from San Francisco to Anchorage in the 1800s. Of course they carefully
measured the gold before and after, presumably using a spring scale
rather than a balance. The bankers concluded a little bit of gold was
being lost from every shipment. After a lot of finger-pointing, they
identified the difference in gravity as the source of the difference.


Dudley Henriques wrote:
Bear with me David. I've been away from this stuff for a long while :-))


Rho for SSL is 0.002378 slugs/cu ft. Mass in slugs is the weight in
lbs/32.2
For high performance flight test dealing with uncompressible airflow; for
density (slugs ft3) for mass airflow (slugs/sec)


"David Kazdan" wrote in message
t...
Dudley:

Isn't dynamic pressure expressed in pressure units, pounds per square
inch (or SI, Pascals: newtons/sq. meter)? Mass density, which you mention
later, is in slugs/cu.in or cu.ft (or SI, Kg/cu.meter).

Thanks for all your good posts, I've learned a lot from them over the
years.

David

Dudley Henriques wrote:
"Danny Deger" wrote in message
...
Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger
As the shuttle ascends, the dynamic pressure (in slugs/sq.in) increases
as the square of the velocity. The shuttle is throttled back to 65%
thrust to avoid over stress at a speed computed under what is called
critical q.
As the shuttle ascends, it is ascending into lower air density (which of
course transfers into a lowering dynamic pressure). The throttling back
takes the shuttle through a transition area during the ascent that ends
as the lowering air density meets the parameters that allow throttle up.
This I believe occurs at about 35 to 37K.
The point where lowering air density meets the ability to throttle up
again is the max q for the shuttle. (Max q meaning maximum dynamic
pressure)
After reaching max q, the shuttle is go for throttle up as the increasing
velocity past max q will never exceed the structural limitations of the
shuttle due to it's entrance into lower air density that doesn't have the
ability to overstress the structure.
Hope this helps a bit.
Dudley Henriques

"Brian Gaff" wrote

Yes, and it is also why the shedding foam can only do serious damage
within the lower atmosphere, as the drag cannot decelerate the chunks
enough to strike with enough force to do harm at that altitude.

Sorry, but you got that one wrong.

The foam does the damage because of the high speed that it has when it hits
the shuttle.

If there was no drag, the foam would not hit with any force; it would be
going the same speed as the shuttle.

When a chunk of foam falls off, it is the drag of the stationary atmosphere
slowing the foam so effectively and rapidly, that causes the relative
closing speeds of the now nearly stationary foam hitting the speeding
shuttle.
--
Jim in NC

"Brian Gaff" wrote in message
news
I don't think this is entirely true. I have some old stuff rattling around
in my mind from the 80s which put the rating at least twice this though of
course, only in the thrust direction. After all, the instantaneous
pressures on change can be rather less than subtle.

I agree. I recall that the dynamic air pressure on the shuttle is the
limiting factor, and that just happens to come out to the 3 G's that the
shuttle experiences.

Also, the attachment of the shuttle to the tank is a limiting factor.
--
Jim in NC

Morgans writes:

"Brian Gaff" wrote

Yes, and it is also why the shedding foam can only do serious damage
within the lower atmosphere, as the drag cannot decelerate the chunks
enough to strike with enough force to do harm at that altitude.

Sorry, but you got that one wrong.

The foam does the damage because of the high speed that it has when it hits
the shuttle.

If there was no drag, the foam would not hit with any force; it would be
going the same speed as the shuttle.

When a chunk of foam falls off, it is the drag of the stationary atmosphere
slowing the foam so effectively and rapidly, that causes the relative
closing speeds of the now nearly stationary foam hitting the speeding
shuttle.

That's what he said.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Stubby:

Right, that was my point...

David


Stubby wrote:
Note. Slugs are a mass measurement whereas pounds are a force. There
is not a simple ratio because it depends on how strong gravity is. Of
course the shuttle has accelerations that are quite different than we
encounter on the face of the Earth!

There was a famous story of gold shippers that moved quantities of gold
from San Francisco to Anchorage in the 1800s. Of course they carefully
measured the gold before and after, presumably using a spring scale
rather than a balance. The bankers concluded a little bit of gold was
being lost from every shipment. After a lot of finger-pointing, they
identified the difference in gravity as the source of the difference.


Dudley Henriques wrote:
Bear with me David. I've been away from this stuff for a long while :-))


Rho for SSL is 0.002378 slugs/cu ft. Mass in slugs is the weight in
lbs/32.2
For high performance flight test dealing with uncompressible airflow;
for density (slugs ft3) for mass airflow (slugs/sec)


"David Kazdan" wrote in message
t...
Dudley:

Isn't dynamic pressure expressed in pressure units, pounds per
square inch (or SI, Pascals: newtons/sq. meter)? Mass density, which
you mention later, is in slugs/cu.in or cu.ft (or SI, Kg/cu.meter).

Thanks for all your good posts, I've learned a lot from them over
the years.

David

Dudley Henriques wrote:
"Danny Deger" wrote in message
...
Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger
As the shuttle ascends, the dynamic pressure (in slugs/sq.in)
increases as the square of the velocity. The shuttle is throttled
back to 65% thrust to avoid over stress at a speed computed under
what is called critical q.
As the shuttle ascends, it is ascending into lower air density
(which of course transfers into a lowering dynamic pressure). The
throttling back takes the shuttle through a transition area during
the ascent that ends as the lowering air density meets the
parameters that allow throttle up. This I believe occurs at about 35
to 37K.
The point where lowering air density meets the ability to throttle
up again is the max q for the shuttle. (Max q meaning maximum
dynamic pressure)
After reaching max q, the shuttle is go for throttle up as the
increasing velocity past max q will never exceed the structural
limitations of the shuttle due to it's entrance into lower air
density that doesn't have the ability to overstress the structure.
Hope this helps a bit.
Dudley Henriques

Yes, and it is also why the shedding foam can only do serious damage within
the lower atmosphere, as the drag cannot decelerate the chunks enough to
strike with enough force to do harm at that altitude.

Uh... even with no atmosphere, the rocket is accelerating wrt the
detached foam. I'm not convinced this is insignificant.

Jose
--
He who laughs, lasts.
for Email, make the obvious change in the address.