Why does the shuttle throttle on ascent?

"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
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Thanks for the extra detail!

Henry Spencer wrote:
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. |

Henry,

During the few test shots of Trident missles that I have seen, I always
thought that the angle at which the vehicle flew was remarkable; like
nothing I had ever seen, save for the occassional errant Estes rocket.
The angle seemed close to 45 degrees, although I had no way of really
knowing, almost immediately after emerging from the water and ignition.

I wonder about the considerations that went into choosing the approach
of dispensing of the climb above the densest portion of the atmosphere
before beginning the trip downrange.

Take care all

John

"Pat Flannery" wrote in message
...


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.

Comments?

I'll take that any day of the week over the Bf-109, where you're main
fuselage tank goes under the pilot's seat, or the Me-163 where you're
sitting squeezed in between two tanks of hydrogen peroxide at your sides.

I always think flying an Me-163 in combat must have been one of the most
crazy experiences in wartime aviation, firstly you have all the explosive
fuel around you, secondly you are shortly to be boosted at tremendous climb
rates into the middle of a heavily armed B-17 formation, thirdly if you
survive all that and manage to get a shot in before the couple of minutes
before the motor dies, you have to glide back like a brick to a tiny
airfield and land on a skid!

On Wed, 10 Jan 2007 19:45:06 -0600, Henry Spencer wrote
(in article ):

(The other is also related to thinner air: rocket engines are more efficient

with less back pressure.

My undergraduate propulsion prof would be gagging at your use of the
term "back pressure" Henry. He used to almost spit and fume when
someone let it slip. :-p

--
Herb Schaltegger
"You can run on for a long time . . . sooner or later, God'll cut you
down." - Johnny Cash
http://www.angryherb.net

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

John wrote:

During the few test shots of Trident missles that I have seen, I always
thought that the angle at which the vehicle flew was remarkable; like
nothing I had ever seen, save for the occassional errant Estes rocket.
The angle seemed close to 45 degrees, although I had no way of really
knowing, almost immediately after emerging from the water and ignition.


Here's a photo of one doing that right after launch:
http://encyclopedia.quickseek.com/im...sile_image.jpg
One thing Trident has is a extensible nose aerospike that sets up a
shockwave ahead of it for drag reduction during ascent.
I wonder if that influenced the ascent trajectory?

Pat

MichaelJP wrote:
I always think flying an Me-163 in combat must have been one of the most
crazy experiences in wartime aviation, firstly you have all the explosive
fuel around you, secondly you are shortly to be boosted at tremendous climb
rates into the middle of a heavily armed B-17 formation, thirdly if you
survive all that and manage to get a shot in before the couple of minutes
before the motor dies, you have to glide back like a brick to a tiny
airfield and land on a skid!


As a glider it was superb, thanks to Lippisch's background as a glider
designer.
Although the pilots tended to dive away at high speed to escape enemy
fighters once their fuel was gone (and to get back to base ASAP for the
same reason), it had a really good gliding performance, and the pilots
who flew it said its handling qualities were superior to any other
German aircraft.
It's only drawback in gliding flight was that it was _too_ good at it -
once it got down in ground effect near landing, it had a tendency to
just float along above the ground till speed bled off and it would
settle down. Even the addition of underwing extensible spoilers didn't
completely solve the problem, and a lot of pilots were injured or killed
by the aircraft remaining stubbornly airborne down the whole length of
the landing field (they landed on grass generally) and not touching down
till it arrived on the rough ground outside the field's boundaries.

Pat

In article ,
Herb Schaltegger wrote:
(also related to thinner air: rocket engines are more efficient
with less back pressure.

My undergraduate propulsion prof would be gagging at your use of the
term "back pressure" Henry...

This is the difference between someone whose idea of an unsophisticated
audience is upper-year engineering students, and someone who's actually
had practice writing for, and talking to, non-captive audiences. :-)

Is "back pressure" strictly correct? Arguably not, although the issue is
more complicated than it looks (for one thing, ambient pressure at the
nozzle exit isn't necessarily the same as ambient pressure elsewhere on
the engine, which in turn isn't necessarily the same as ambient pressure
on the vehicle -- rocket exhausts can be powerful ejector pumps). But it
*is* what you say if you want to give the right general impression to an
audience that doesn't care to hear the rigorous details.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

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...
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article . com,
John wrote:
During the few test shots of Trident missles that I have seen, I always
thought that the angle at which the vehicle flew was remarkable; like
nothing I had ever seen, save for the occassional errant Estes rocket.
The angle seemed close to 45 degrees, although I had no way of really
knowing, almost immediately after emerging from the water and ignition.

I wonder, though, if it actually ascends in that direction, or if that's
just a transient error -- perhaps something to do with the dynamics of
breaking the surface -- that the guidance system sorts out a second or
two later.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |