Why does the shuttle throttle on ascent?

"John T" wrote in message
m...
"Ron Natalie" wrote in message
m

I don't know about G's but the shuttle adjust the engine thrust
up and down at various times in the launch based on the dynamic
pressures involved.

"'Go' for throttle up."

My generation's equivalent of "Where were you when Kennedy got shot?"

Even after many successful launches, always worry when I hear those words...

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

As the fuel is burned/jettisoned the engines have a smaller vehicle mass
to accelerate. So to maintain an acceptable acceleration level, you
throttle back engines.

Also, as you leave the atmopsphere, there is less and less air resistance,
so you also need less power to maintain the desired acceleration level.

And early in the ascent, they throttle back to ensure the shuttle's air
speed isn't too great as it travels through the still dense atmosphere
since it would create too much aerodynamic drag.

"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

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

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

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

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

Stubby wrote:

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.
Shouldn't that be the other way around? the gold would weigh less as you
approached the equator due to the spinning of the Earth causing
centrifugal force on it.

Pat

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

"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