The SSME throttle-up?

When the shuttle lifts off just before the SRB's kick-in the ME's are
at full power, which is 100% with the throttles wide open, and yet a
little over 2 minutes after lift off the SSME's go to throttle up and
the ME's increase thrust to 106%. If the ME's are already wide open
at 100%, where does the extra 6% come from?


Christopher
+++++++++++++++++++++++++
"Kites rise highest against
the wind - not with it."
Winston Churchill

In article ,
(Christopher) wrote:

When the shuttle lifts off just before the SRB's kick-in the ME's are
at full power, which is 100% with the throttles wide open, and yet a
little over 2 minutes after lift off the SSME's go to throttle up and
the ME's increase thrust to 106%. If the ME's are already wide open
at 100%, where does the extra 6% come from?


Christopher
+++++++++++++++++++++++++
"Kites rise highest against
the wind - not with it."
Winston Churchill

Actually, no. At launch, SSMEs are typically at 104% of rated power.
At about 1 minute into launch (may be a bit earlier or later, depending
on launch weight, trajectory, etc.) the SSMEs throttle down to reduce
acceleration while passing through the regime of maximum dynamic
pressure ("MaxQ") (which is defined as q = 1/2*rho*V^2 where rho is
atmospheric density and V is velocity).

After MaxQ has passed, the engines throttle back up to 104%.

See http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml
for a very good explanation.

--
Herb Schaltegger, B.S., J.D.
Reformed Aerospace Engineer
"Heisenberg might have been here."
~ Anonymous

Actually, no. At launch, SSMEs are typically at 104% of rated power.
At about 1 minute into launch (may be a bit earlier or later, depending
on launch weight, trajectory, etc.) the SSMEs throttle down to reduce
acceleration while passing through the regime of maximum dynamic
pressure ("MaxQ") (which is defined as q = 1/2*rho*V^2 where rho is
atmospheric density and V is velocity).

After MaxQ has passed, the engines throttle back up to 104%.

See http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml
for a very good explanation.

Is the throttle up/down done manually or automatically?

In article , Dick Justice wrote:

Is the throttle up/down done manually or automatically?

Automatically, by the computers. The crew monitors and confirms it occurs
as expected.

And to add, full power is 104%; they are capable of going to max power
which is 109% but never done except in certain abort modes.

There's been only one in-flight launch abort so far (STS 51-F, Challenger,
1985) but I don't think they went to 109% on that one... I recall they just
simply inhibited (prevented the computers from allowing) automatic engine
shutdown for the other two engines to prevent faulty sensors from causing
real problems -- they didn't want the second engine's sensors to fail too
early which would have caused the ET to land in Europe or Saudi Arabia --
and accepted a lowered initial orbit due to being 119 feet per second (fps)
short of the target orbital velocity. They made up this shortfall by doing
an OMS-1 burn (gained an extra 194 fps) after main engines shutdown and ET
separation, to achieve desired orbit.

Pretty much the entire mission is done by computer control; only very few
phases are done manually by the crew such as the last two minutes for the
landing and performing a rendezvous with another object (eg space station,
satellite, etc). Even then, the computer is constantly calculating
trajectory and making advisory cues to help out the crew achieve their
goals while the crew manually gives input to fine-tune things. At least one
flight crew has done the entire (re-)entry/TAEM/approach/landing phase
manually as a DTO (designated test objective) -- perhaps Engle and Truly on
STS-2?

-Dan

So all that is part of an auto sequence then?

Two supplementary questions.

If they are at 104 percent of rated power, then why is the rated power not
called 100 percent? What I mean is, what is the point in pretending the
engines can gove more than their rated power? Seems daft to me, almost like
a marketing claim, Our watts in our amplifier sound louder than normal ones
etc...

and, I understand that some engines can be designed to work better in the
upper atmosphere, and some to work better at the ground and up to a point.

Given that is so, how are the SMEs designed? Is it just a compromise, or do
mixtures or pressures change as height is gained to keep things efficient?

Brian

--
Brian Gaff....
graphics are great, but the blind can't hear them
Email:
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"Brian Gaff" wrote in message
...
So all that is part of an auto sequence then?

Two supplementary questions.

If they are at 104 percent of rated power, then why is the rated power not
called 100 percent? What I mean is, what is the point in pretending the
engines can gove more than their rated power? Seems daft to me, almost
like
a marketing claim, Our watts in our amplifier sound louder than normal
ones
etc...

That's a good question and I think part of what Christopher was asking.

It has more to do with what the original baseline was. The original engines
were designed to put out X amount of thrust.

Upgrades permitted a higher amount of thrust. In order to make
documentation easier, etc, they kept the 100% = to X amount of thrust and
simply went over that. So when they throttle down, they know, "ok, throttle
to 60% power" and don't have to keep track of "is that 60% of the new
engines, or the old, or what?"

So, the current engines provide 4% more thrust than the original design. If
they developed yet another design that could put out say 200% of the
original rated thrust, you'd see engines at 200% power.



and, I understand that some engines can be designed to work better in the
upper atmosphere, and some to work better at the ground and up to a point.

Correct, mostly the nozzle shape I believe.

Given that is so, how are the SMEs designed? Is it just a compromise, or
do
mixtures or pressures change as height is gained to keep things efficient?

It's a compromise mostly, though I think they compromise more towards upper
atmosphere than sea-level since the SRB's dominate the thrust at sea level.



Brian

--
Brian Gaff....
graphics are great, but the blind can't hear them
Email:

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Brian Gaff spewed out:
So all that is part of an auto sequence then?

Two supplementary questions.

If they are at 104 percent of rated power, then why is the rated power not
called 100 percent? What I mean is, what is the point in pretending the
engines can gove more than their rated power? Seems daft to me, almost like
a marketing claim, Our watts in our amplifier sound louder than normal ones
etc...

That's a great question and one I've wondered about many times. If they can
push out 109% (as mentioned by Dan Foster) then _that_ should be the nominal
100% mark. What's now known as 94% would become 86% and 104% would become 95%.
At full power what's now known as 109% would simply become 100%.

My guess is that the percentages are rated against the original specifications.
If continuous improvements are made that can squeeze out more performance then
the 100% mark would keep changing. That would make it difficult to compare
performance numbers over the life of the program. There are also probably too
many rules-of-thumb and years of experience behind them to change it now.

By the way, mine goes up to 11.

--
bp
Proud Member of the Human O-Ring Society Since 2003

From Christopher ):
When the shuttle lifts off just before the SRB's kick-in the ME's are
at full power, which is 100% with the throttles wide open, and yet a
little over 2 minutes after lift off the SSME's go to throttle up and
the ME's increase thrust to 106%. If the ME's are already wide open
at 100%, where does the extra 6% come from?

I find this topic interesting also...

Obviously, 100% isn't the maximum capability for the SSMEs. There are
even abort modes where 109% throttles get commanded. So even 106%
isn't the "widest" of the "wide open".

While the "%" terminology certainly leads to confusion, one way to
make sense of it is that as some point in time for the program, 100%
was the maximum rated thrust. While this thrust level has remained as
the benchmark, over time the performance has increased.

One analogy would be to think of a car engine's redline. If when the
car is first released, say that a redline of 5000rpm gets tagged as
100%. Then through subsequent improvements over the years, it is
determined that the engine can be reliably pushed to a redline of
5300rpm. The new redline gets termed 106%.

What puzzles me is the reason why engines only run at 100% at lift off
instead of blasting off at 106%. The best guess I could think of is
that the deflector shields and other pad constraints have never been
rated to anything above this.


~ CT

Bruce Palmer wrote:

By the way, mine goes up to 11.

Your wife must be very happy. ;-)

"Bruce Palmer" wrote in message
. net...
Brian Gaff spewed out:
So all that is part of an auto sequence then?

Two supplementary questions.

If they are at 104 percent of rated power, then why is the rated power
not
called 100 percent? What I mean is, what is the point in pretending the
engines can gove more than their rated power? Seems daft to me, almost
like
a marketing claim, Our watts in our amplifier sound louder than normal
ones
etc...

That's a great question and one I've wondered about many times. If they
can
push out 109% (as mentioned by Dan Foster) then _that_ should be the
nominal
100% mark. What's now known as 94% would become 86% and 104% would become
95%.
At full power what's now known as 109% would simply become 100%.

But it's not that linear.

If you simply went by a 9% "shift" then what's known now as 9% (which
technically the SSME can't achieve for other reasons) would then become 0%.
0% would become -9%. Which obviously makes no sense.

No, you have to figure out what a 9% increase at each previous % level is.
(so 1% becomes 1.9%, not 10%).

And now you're redoing a LOT of things.



My guess is that the percentages are rated against the original
specifications.
If continuous improvements are made that can squeeze out more
performance then
the 100% mark would keep changing. That would make it difficult to
compare
performance numbers over the life of the program. There are also probably
too
many rules-of-thumb and years of experience behind them to change it now.

That's it.



By the way, mine goes up to 11.

--
bp
Proud Member of the Human O-Ring Society Since 2003