Why does the shuttle throttle on ascent?

"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...

"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.

"Greg D. Moore (Strider)" wrote in message
ink.net...

"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.

That was my point exactly.
Thanks for the clarification, Greg.

Tim

Absolutely correct!

Bud



On Jan 8, 6:00 pm, "Greg D. Moore \(Strider\)"
wrote:
"Blueskies" wrote in odigy.net...







"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.



- Hide quoted text -- Show quoted text -- Hide quoted text -- Show quoted text -

In article ,
Blueskies wrote:
: 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.

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

When all the forces (engine thrust and air drag) are from your back to
your chest or vice versa, being head-up or head-down is precisely the same
as being head-northward or head-southward while lying on your back on
Earth, i.e. it makes not the slightest difference in what you feel.

The shuttle in ascent is in free fall except for thrust and drag. The
ascent path, and the shuttle's orientation during ascent, are carefully
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.
--
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"Henry Spencer" wrote in message
...
In article ,
Blueskies wrote:
: 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.

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

When all the forces (engine thrust and air drag) are from your back to
your chest or vice versa, being head-up or head-down is precisely the same
as being head-northward or head-southward while lying on your back on
Earth, i.e. it makes not the slightest difference in what you feel.

The shuttle in ascent is in free fall except for thrust and drag. The
ascent path, and the shuttle's orientation during ascent, are carefully
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |


That's interesting, I'd wondered about that.

Is it also the case that the zero-lift trajectory you describe is very
similar to the optimum flight path for orbital insertion? Or is a lot more
fuel used because of it?

In article ,
MichaelJP wrote:
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.

Is it also the case that the zero-lift trajectory you describe is very
similar to the optimum flight path for orbital insertion? Or is a lot more
fuel used because of it?

Yes and no. :-)

If memory serves, the ascent trajectory is pretty close to what a wingless
rocket with similar mass and propulsion characteristics would fly. Flying
even slightly sideways at supersonic speeds is very hard on lightweight
structures; even jet fighters, built for violent maneuvering, can handle
only a very little bit of this. Rockets normally take considerable pains
to fly pretty much(*) straight "into the wind" until clear of most of the
atmosphere. The shuttle trajectory isn't *exactly* what a wingless rocket
would use, because the trajectory that minimizes loads on the orbiter
wings isn't exactly the trajectory that would minimize structural loads in
general -- the wings have priority. But the penalty for this is small.

(* There are minor exceptions, in which lift can be of some use after the
air thins out, plus some complications for air-launched rockets like
Pegasus. But this is still basically correct. )

*However*, there is a more general caveat: even the wingless-rocket
trajectory actually isn't optimal. For one thing, an optimal ascent would
tip over toward the horizontal much more quickly. On Earth, the early
ascent has to be close to vertical, to get the rocket up out of the
atmosphere before the speed builds up too much. For another thing, even
disregarding that, the straight-into-the-wind trajectory isn't exactly
optimal, although it's not too far off.

The only rocket ascent that was ever able to use a truly optimized
trajectory was the Apollo LM ascent stage's departure from the Moon. On
Earth, you inevitably pay some price for the necessities of getting clear
of the atmosphere quickly and pointing straight into the wind while you
do. It's not huge, but it's significant. This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)
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spsystems.net is temporarily off the air; | Henry Spencer
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"Henry Spencer" wrote in message
...
In article ,
MichaelJP wrote:
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.

Is it also the case that the zero-lift trajectory you describe is very
similar to the optimum flight path for orbital insertion? Or is a lot more
fuel used because of it?

Yes and no. :-)

If memory serves, the ascent trajectory is pretty close to what a wingless
rocket with similar mass and propulsion characteristics would fly. Flying
even slightly sideways at supersonic speeds is very hard on lightweight
structures; even jet fighters, built for violent maneuvering, can handle
only a very little bit of this. Rockets normally take considerable pains
to fly pretty much(*) straight "into the wind" until clear of most of the
atmosphere. The shuttle trajectory isn't *exactly* what a wingless rocket
would use, because the trajectory that minimizes loads on the orbiter
wings isn't exactly the trajectory that would minimize structural loads in
general -- the wings have priority. But the penalty for this is small.

(* There are minor exceptions, in which lift can be of some use after the
air thins out, plus some complications for air-launched rockets like
Pegasus. But this is still basically correct. )

*However*, there is a more general caveat: even the wingless-rocket
trajectory actually isn't optimal. For one thing, an optimal ascent would
tip over toward the horizontal much more quickly. On Earth, the early
ascent has to be close to vertical, to get the rocket up out of the
atmosphere before the speed builds up too much. For another thing, even
disregarding that, the straight-into-the-wind trajectory isn't exactly
optimal, although it's not too far off.

The only rocket ascent that was ever able to use a truly optimized
trajectory was the Apollo LM ascent stage's departure from the Moon. On
Earth, you inevitably pay some price for the necessities of getting clear
of the atmosphere quickly and pointing straight into the wind while you
do. It's not huge, but it's significant. This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |


Thanks for the extra detail!

MichaelJP wrote:
This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)

Thanks for the extra detail!



There's another advantage if you're using cryogenic propellants. The
propellants can be kept in insulated tankage within the carrier until
altitude is reached and the transferred into the LV. Since the
temperature is well subzero at altitude, there isn't water vapor around
to form ice on the tankage, so the weight and complexity of insulation
can be done away with.
Assuming you are using a Shuttle-style jettisonable ET, that a built-in
performance boost, as well as a cost savings on the ETs themselves.
Although a completely rreusable LV will have a TPS to take reentry
heating, and therefore will already have exterior insulation, the drop
tank solution makes for far easier design as far as vehicle weight goes.

Pat

In article ,
Pat Flannery wrote:
This is one of the two big technical advantages of air launch...

There's another advantage if you're using cryogenic propellants. The
propellants can be kept in insulated tankage within the carrier until
altitude is reached and the transferred into the LV. Since the
temperature is well subzero at altitude, there isn't water vapor around
to form ice on the tankage, so the weight and complexity of insulation
can be done away with.

You don't really need insulation against ice anyway, unless you've been
stupid enough to put something fragile downstream of the tank surfaces.
Just let it fall off after engine ignition, as the Saturn V did.

The big reason why you might need tank insulation is if the tank holds
LH2, in which case you need to insulate to prevent liquid air from
condensing... and that'll happen even at subzero temperatures, so you
can't get away with leaving it off.

Although a completely rreusable LV will have a TPS to take reentry
heating, and therefore will already have exterior insulation, the drop
tank solution makes for far easier design as far as vehicle weight goes.

The gain is actually rather questionable, after you consider reentry --
the drop tank leaves behind a heavy, dense vehicle that makes a severe
reentry. At reentry time, it's *good* if lots of the volume inside the
TPS is empty tanks. The drop tank does make for far easier design if you
can "throw the TPS problem over the fence" to the materials team...
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