Why does the shuttle throttle on ascent?

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

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

"Danny Deger" wrote in message
...

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


The answer is:

So Navy Fighter pilots can fly the shuttle :-)

Ah, is this so that they can let the AF pilots catch up?

Danny Deger

P.S. I was an Air Force figher pilot.

I was wondering why you of all people was asking this. :-)

"Henry Spencer" wrote

This is partly for the benefit of the passengers -- it's thought that a
healthy adult can be expected to take about 3G without testing -- and
partly to limit structural loads. (The Saturn V first stage shut down
its center engine early for the latter reason.)

The center engine shutting down is a fact that I had never heard, before
now.

Amazing what you can learn, hanging out here, if you take the time to listen
to ones with information to give. (unlike some we know)
--
Jim in NC

John T wrote:
"'Go' for throttle up."

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


I'm old enough to answer both those questions.

Pat

BTW "go at throttle up" is, as I understood, is simply a radio call to
confirm communication with the vehicle, same as the "roll program" call. Is
that correct?
Jim
"Morgans" wrote in message
...

"Henry Spencer" wrote

This is partly for the benefit of the passengers -- it's thought that a
healthy adult can be expected to take about 3G without testing -- and
partly to limit structural loads. (The Saturn V first stage shut down
its center engine early for the latter reason.)

The center engine shutting down is a fact that I had never heard, before
now.

Amazing what you can learn, hanging out here, if you take the time to
listen to ones with information to give. (unlike some we know)
--
Jim in NC

Thanks Danny, I had a good belly laugh from that one.
Jim
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

"Jim" wrote

BTW "go at throttle up" is, as I understood, is simply a radio call to
confirm communication with the vehicle, same as the "roll program" call.
Is that correct?

I'm no authority, but that is my understanding.

It is a confirmation to those along for the ride, that all functions are
within parameters, (thus the "go")as monitored from the ground, and no
action needs to be taken as the engines throttle back up.

It also probably serves as a reminder that the next jolt they feel is
expected, and to not be surprised at it. g

Kinda' like when you reduce power on final, and the passenger, if not told
to expect it, is rather "concerned" that they are about to crash. g
--
Jim in NC

SSME's are "rated" at a certain thrust. However, the thrust rating has a
minimum, average and maximum. The "rated" thrust is the average. Therfore,
the engines can be run to an amount greater than "rated" thrust. I am not an
engineer or NASA personel, but I have read that during the nominal acent the
engines are throttled from 64% to 104% but can be throttled to 109% of
rated thrust if necessary.
Jim
"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.

"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