Why does the shuttle throttle on ascent?

In article ,
Morgans wrote:
I had always heard that the fuselage tank was the source of the instability,
with it being so far behind the CG, to give it a dangerously aft CG. Today,
in peacetime, I don't suppose they would ever dream of putting that much
weight that far back, but it was war.

Yep, lots of compromises in some of those wartime designs... Some of the
photo-recon versions of the Spitfire had a tail tank that compromised
stability even more drastically: the aircraft was outright aerodynamically
unstable with that tank full or nearly full. (Naturally, you emptied that
tank *first*... and didn't have much attention to spare for anything else
until it was empty.)
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"Mxsmanic" wrote in message
...
One wonders what sort of emergency would justify running the engine a
bit faster for just ten minutes, and then replacing the entire
aircraft.

Getting away from a bunch of someones shooting at me, for one. If I can't
get away within ten minutes, it's pretty obvious I'm not going to get away.

"Dudley Henriques" wrote in message
...
At 40 gals in the tank, anything involving maneuvering flight was a toss up
for pitch control.

"This is your wingman. You appear to be leaking oil."
"Negative, wingman. The CG just shifted and I **** my pants."

In article ,
Kyle Boatright wrote:
That said, the SSMEs are cranky, marginal engines, and taking *them* up to
120% (as was once intended) is much more iffy than doing the same for
robust engines like the H-1 or F-1.

What makes the SS engines "cranky and marginal" vs the H-1 and/or F-1?

Mostly, they were too much of a leap into the technological unknown at the
time: NASA tried to pioneer bold new technology on what was supposed to
be a long-lived reusable engine, and unsurprisingly, this didn't work too
well. The H-1 and F-1 were much more conservative designs -- notably,
although the F-1 was a lot bigger than anything previously built, the
project tried hard to *avoid* pioneering in any other way -- and although
they did hit surprises and ended up breaking some new ground, they had
much more continuity with previous experience.

(For a while there was some feeling that the SSME's staged-combustion
cycle was just *inherently* troublesome, but that idea sort of collapsed
when it became clear that every major Russian rocket engine since about
1960 had been a staged-combustion design, typically including features
like oxidizer-rich preburners, which even NASA had deemed impractically
difficult...)

Three specific snags also aggravated this problem on the SSME:

(a) Most of the technology development on staged combustion had been done
by Pratt & Whitney, but oddly, the contract for the staged-combustion SSME
went to Rocketdyne instead. So the experienced people were shut out, and
the guys who were actually doing the work were having to come up to speed
on a technology that was new to them.

(b) The SSME program, like the shuttle in general, was starved for money
and opted to cut corners on subsystem testing in particular. The result
was an unusually long and painful development process, with subsystem
problems often not surfacing until whole-engine testing.

(c) Partly as a result of (a) and (b), it didn't become clear until too
late that the main LOX (I think it was) turbopump really needed one more
pump stage. Since a major redesign was politically and financially
impossible at that point, the result was a pump in which each stage was
pushed to the ragged edge of engineering practicality to meet a very
ambitious spec.

The combination of (b) and (c) was particularly nasty, because all too
often, a LOX-pump failure becomes a LOX-pump fire, which destroys the
evidence of what went wrong. Having this happen repeatedly to whole test
engines was just what an already-stressed development program didn't need.
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"Mxsmanic" wrote in message
...
John writes:

Ummm . . . a FW-190 (option: replace FW-190 with opponent on your six
. . . really close . . . perhaps still there and shooting after after
what you have just finished what you used to think was your very best
move . . . in my book that rises to the level of an emergency. At that
point getting the pilot home was goal one.

OK, but if you can only fly at emergency war power for five minutes,
and if it only offers a moderate advantage over normal maximum power,
it seems that there would be relatively few situations in which it
would make a difference. Either you'd be out of luck to begin with
and EWP wouldn't get you out of it, or you wouldn't be in danger and
so you wouldn't need EWP.

If EWP can give you an extra 50 kts, for example, at best you'd have
an advantage of five nautical miles when the engine disintegrates. If
the bad guys are only 30 knots slower, your advantage shrinks further.
And with a blown engine, you'll need to be completely out of danger
after five minutes, or all that effort wouldn't help.

Yes, but that 5 nm may be enough to get out of their range, or get back to
friendly air-cover, etc.

The piont of it being emergency power is you can't always predict when or
how it'll be used.



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"Morgans" wrote in message
...

"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


OK, now I'm trying to figure out how to insert a magical, imaginary conveyor
belt into this scenario. :-)

TP in FL (Go Gators!)

"Tim Rogers" wrote in message ...
:
: "Blueskies" wrote in message
: . net...
:
: "Danny Deger" wrote in message
: ...
: :
: : "Danny Deger" wrote in message
: : ...
: :
: : Why does the shuttle throttle to 3 Gs on ascent?
: :
: :
: : The 3 G throttling is done late in the flight (about 7:30)and has
: nothing to
: : do with dynamic pressure. It was designed in to allow "regular" people
: fly
: : the shuttle.
: :
: :
:
: Yes, the aerodynamic loads are highest early in the flight so the engines
: are throttled then back up. The shuttle rolls
: over on its back to fake the occupants into feeling 3 gs when in fact the
: vehicle is pushing 4 gs....
:
: No.
: The 3 Gs is at the backs of the occupants (and along that same axis for the
: vehicle.) This is the same if they are heads up or heads down.
: If you lie with your back on the floor, you feel the same 1 G if your head
: is facing north or south.
:
: Tim
:
:

Not talking about facing north or south, they're talking about hanging from your feet or standing upright...

Glad you liked the report.
The Mustang is really very easy to fly. It does require constant attention
and doesn't suffer fools gladly however. Like any high performance airplane,
it has to be flown by the book and fooling around on the left side of the
envelope can get you killed in a hurry. Other than that....a piece of cake.
Dudley Henriques


"Pat Flannery" wrote in message
...
I tried to reply to you personally, but the e-mail address doesn't work.


I flew a D, as well as various other prop fighters as a civilian
operator. Never flew ours with external tanks. The airplane is stable on
takeoff if flown correctly and I wouldn't anticipate any specific issues
with the external tanks except the extended run. I believe the only
caution on the external tanks was for high speed buffet above 400 mph.
We had the fuselage tank removed and only flew the Mustang using the 2
mains at 92 gallons each.(90 usable)
If you are interested in a pilot report on what it was like to fly the D,
I did one for the Warbirdalley site some years ago at the following ;
http://www.warbirdalley.com/articles/p51pr.htm#pirep1
Hope you find the report interesting.
Dudley Henriques

Thanks for putting me on to that pilot report, that was very interesting
to read.
The overall feeling I got from it was of an aircraft that takes a lot of
effort to get fully attuned to and in sync with, but is capable of doing
outstanding things once you get enough hours in on it and understand how
it behaves.
I particularly liked your description of starting the Merlin up. :-D
You mentioned you had flown several types of prop driven fighters, what
other ones did you fly?
I once talked to a Navy pilot of a Grumman Bearcat who was very taken by
that aircraft.
About the most fascinating conversation I ever had with a pilot was years
ago after the Soviet Union had just fallen who flew a two-seater TA-4J
Skyhawk to the Minot N.D. airshow. He worked for Navy R&D and had done
three tours in Vietnam with the A-4.
His backseater was in awe of him, and like most really competent combat
pilots, he was one of the easiest-going down-to-earth guys you ever ran
into... no attitude, no mirrored sunglasses, no waxed mustache.
The reason he was there was that the Ukrainian Air Force had sent a pair
of MiG-29's on a U.S. tour, and the Navy wanted to see what they could do
aerobatically and try to get some insight into what they'd do if they came
up against a FA-18, so they had him and his inconspicuous Skyhawk
following them around the airshow circuit and observing them in action.
One thing we both noticed was the effortless way the Fulcrum could ignite
its afterburner stages; this is apparently nearly fully automatic- the
pilot merely advances the throttle, and the aircraft senses airspeed, g
forces, air density, and what type of maneuver is going on and gives the
amount of afterburner required. The MiG was going in and out of various
degrees of afterburner at several points during even fairly simple
aerobatic maneuvers.
He had been in on the abortive A-12 program, and his take on it was that
the Navy screwed over General Dynamics because it wanted to spend money
elsewhere after the end of the cold war; he described flying the simulator
and stated that he thought it was an excellent aircraft, particularly
complementing the pilot's view out of the cockpit, which he said was truly
outstanding.

Pat

"Blueskies" wrote in message
et...

"Tim Rogers" wrote in message
...
:
: "Blueskies" wrote in message
: . net...
:
: "Danny Deger" wrote in message
: ...
: :
: : "Danny Deger" wrote in message
: : ...
: :
: : Why does the shuttle throttle to 3 Gs on ascent?
: :
: :
: : The 3 G throttling is done late in the flight (about 7:30)and has
: nothing to
: : do with dynamic pressure. It was designed in to allow "regular"
people
: fly
: : the shuttle.
: :
: :
:
: Yes, the aerodynamic loads are highest early in the flight so the
engines
: are throttled then back up. The shuttle rolls
: over on its back to fake the occupants into feeling 3 gs when in fact
the
: vehicle is pushing 4 gs....
:
: No.
: The 3 Gs is at the backs of the occupants (and along that same axis for
the
: vehicle.) This is the same if they are heads up or heads down.
: If you lie with your back on the floor, you feel the same 1 G if your
head
: is facing north or south.
:
: Tim
:
:

Not talking about facing north or south, they're talking about hanging
from your feet or standing upright...

Considering the velocity vector is forward, it still doesn't really matter
which way they are.

"Henry Spencer" wrote in message
...
Three specific snags also aggravated this problem on the SSME:

(a) Most of the technology development on staged combustion had been done
by Pratt & Whitney, but oddly, the contract for the staged-combustion SSME
went to Rocketdyne instead. So the experienced people were shut out, and
the guys who were actually doing the work were having to come up to speed
on a technology that was new to them.

(b) The SSME program, like the shuttle in general, was starved for money
and opted to cut corners on subsystem testing in particular. The result
was an unusually long and painful development process, with subsystem
problems often not surfacing until whole-engine testing.

(c) Partly as a result of (a) and (b), it didn't become clear until too
late that the main LOX (I think it was) turbopump really needed one more
pump stage. Since a major redesign was politically and financially
impossible at that point, the result was a pump in which each stage was
pushed to the ragged edge of engineering practicality to meet a very
ambitious spec.

The combination of (b) and (c) was particularly nasty, because all too
often, a LOX-pump failure becomes a LOX-pump fire, which destroys the
evidence of what went wrong. Having this happen repeatedly to whole test
engines was just what an already-stressed development program didn't need.

To be fair, the engine has improved greatly since the first ones were built.

The current Block IIs appear to have incorporated several major changes
improving them, prolonging their cycles between tear-downs and over-all
making them far better than the originals.

(and much of the work was done by Pratt & Whitney.)

(of course I still think some people continue to hold old biases against the
SSME :-)

--
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