Why does the Shuttle roll

180 degrees shortly after liftoff?

Thanks!

Tad Danley
k3td at arrl dot net

wrote:
180 degrees shortly after liftoff?

Thanks!

Tad Danley
k3td at arrl dot net

because the launch towers were built for Apollo and modified for STS.
When the shuttle is on the launch pad the angle it creates wrt the
Equator (and therefore the orbital inclination it would have if it did
not do the roll program) would not be the desired one for either the
normal 38.5 shuttle orbital track nor the ISS track.

Why do it right after the stack clears the launch tower? Takes a lot
less less energy to move it then when the speed is very low, and the
distance between its original flight path and the corrected one is a
hell of a lot smaller than if it tried to maneuver to a new inclination
later in the ascent.

--
Terrell Miller


"Every gardener knows nature's random cruelty"
-Paul Simon George Harrison

In article ,
says...
180 degrees shortly after liftoff?

It doesn't roll exactly 180 degrees. It rolls until, as the stack tips
onto its trajectory, the orbiter is "heads-down", under the stack. For
example, if the orbiter is going to climb into an orbit 46 degrees to
the equator, it will roll until a line through the orbiter and external
tank centerlines is 46 degrees north of east. Then, as the stack
ascends and moves from vertical to lateral flight, the orbiter will
"hang" underneath the ET.

I can't tell you exactly what that centerline-line compass orientation
is on the pad, and I suspect it *may* be different depending on whether
they launch from 39A or 39B. But the stack rolls from that starting
orientation to the trajectory's target orientation, so the amount of
roll can vary from launch to launch. (This whole discussion disregards
the use of "dogleg" trajectories on some missions, where the beginning
of powered flight follows one trajectory and, at a certain point, shifts
to achieve a slightly different orbital inclination.)

Now, the *real* question may well be "Why do they roll until the orbiter
is heads-down?" I'm not completely positive about this -- it may be a
center-of-mass issue, the angle at which the shuttle's main engines have
to fire to thrust through the stack's center of mass may make the heads-
down orientation advantageous from a performance standpoint. However,
all previous American spacecraft flew into orbit in a heads-down
position, as well. I know that, in the cases of Mercury, Gemini and
Apollo, they did it so that the pilot(s) had a good view of the horizon
and landmarks to use in case they needed to manually guide the rocket,
or determine attitude change rates when weighing abort options. So,
there may be similar operational reasons for the shuttle to fly heads-
down -- I'm actually not sure. (It could also be one of the ways in
which they deal with the lift generated by the orbiter's wing during
ascent. A lot of the shuttle's powered flight regime is based on
managing the lift from the wing and the stresses it puts on the stack.)

On some of the more recent shuttle flights, the entire stack has rotated
back to a heads-up position well after SRB sep, near the end of powered
flight. I believe that had more to do with dynamics of ET sep than any
other reason, but would appreciate it if anyone has more solid data on
this.

--

"The problem isn't that there are so | Doug Van Dorn
many fools; it's that lightning isn't |
distributed right." -Mark Twain

Doug... wrote in
:

In article ,
says...
180 degrees shortly after liftoff?

It doesn't roll exactly 180 degrees. It rolls until, as the stack tips
onto its trajectory, the orbiter is "heads-down", under the stack. For
example, if the orbiter is going to climb into an orbit 46 degrees to
the equator, it will roll until a line through the orbiter and external
tank centerlines is 46 degrees north of east.

Umm, not quite. The inclination of the orbit and the direction (azimuth) of
the launch are two different things, and they're only equal to each other
for a launch site on the equator. Another example is that to reach a 46
degree inclination orbit from a launch site at 46 degrees latitude, one
must launch due east. KSC is at 28.45 degrees latitude, so the launch
azimuth required to reach (say) a 51.6 degree inclination orbit is rather
less than 51.6 degrees north of east.

The relationship between launch site latitude (L), launch azimuth (AZ), and
orbit inclination (i) is given by:

sin(AZ) = cos(i)/cos(L)

....except that AZ is actually defined clockwise from north, not
anticlockwise from east.

I can't tell you exactly what that centerline-line compass orientation
is on the pad, and I suspect it *may* be different depending on whether
they launch from 39A or 39B.

For both pads, the shuttle's tail points pretty much due south. For
example:

http://terraserver.microsoft.com/ima...672&Y=3956&W=3


Now, the *real* question may well be "Why do they roll until the orbiter
is heads-down?" I'm not completely positive about this --

(It could also be one of the ways in
which they deal with the lift generated by the orbiter's wing during
ascent. A lot of the shuttle's powered flight regime is based on
managing the lift from the wing and the stresses it puts on the stack.)

That's part of the reason. The stack must maintain negative angle of attack
(alpha) during ascent to prevent excessive lift (which would overstress the
wings and ET attach points). A heads-down stack can maintain negative alpha
with a more vertical trajectory that gets out of the atmosphere quicker,
while heads-up requires a more depressed trajectory that stays in the
atmosphere longer (more drag losses).

Another part is that RTLS aborts are a lot less tricky starting from heads-
down: you *really* want to be heads-up for the ET sep, since it takes place
a lot lower in the atmosphere than a nominal ET sep, so lift from the wings
will help prevent re-contact with the tank.

Interestingly, part of the attraction of the 5-segment SRB upgrade was that
it added enough performance to allow "ATO-off-the-pad" capability for a
single engine failure throughout ascent, thereby eliminating the need for
RTLS altogether. NASA was strongly considering switching to "heads-up-off-
the-pad" for 5-segment SRB flights.

On some of the more recent shuttle flights, the entire stack has rotated
back to a heads-up position well after SRB sep, near the end of powered
flight. I believe that had more to do with dynamics of ET sep than any
other reason, but would appreciate it if anyone has more solid data on
this.

It was actually for comm reasons. Heads-down was better for the Bermuda
tracking station, heads-up is better for TDRS. Rolling heads-up enabled
NASA to close the Bermuda station. It occurs late enough in ascent that
aerodynamics are not a factor and the performance impact is minor.


--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
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wrote:

180 degrees shortly after liftoff?

http://www.geocities.com/CapeCanaver...faq-a.htm#roll

--
Nothing to be done.

My question is the corollary to why the Saturn V also rolled. I know about
the guidance issue of arranging for pitch motions to be in the intended
orbital plane. But LC39 was built for the big Saturns. Presuming it was
possible to site the pads so that the vehicles were already rolled into the
proper attitude (or at least close enough to minimize the amount of roll
needed) at launch, what overriding concern prompted orienting the pads as
they were?

My theory is that it has to do with the sightlines from the launch control
center and with the optimum orientation of the access arms.

I figure you want the side hatch of the CM to be on the centerline (i.e.,
overhead to the crew) so that you maintain left-right symmetry in the CM.
That dictates an optimal position for a crew access arm: you want that
hatch to face the LUT. That means placing the LUT east of the vehicle, so
that your head is east and your feet are west as you sit in the couch.

But then the vehicle itself would obscure the view of the LUT from the
firing room. For that sightline you want the vehicle to be north of the
tower or vice versa so that you can see both at the same time.

Anybody know the real reason?

--
|
The universe is not required to conform | Jay Windley
to the expectations of the ignorant. | webmaster @ clavius.org

In article ,
says...
My question is the corollary to why the Saturn V also rolled. I know about
the guidance issue of arranging for pitch motions to be in the intended
orbital plane. But LC39 was built for the big Saturns. Presuming it was
possible to site the pads so that the vehicles were already rolled into the
proper attitude (or at least close enough to minimize the amount of roll
needed) at launch, what overriding concern prompted orienting the pads as
they were?

My theory is that it has to do with the sightlines from the launch control
center and with the optimum orientation of the access arms.

I figure you want the side hatch of the CM to be on the centerline (i.e.,
overhead to the crew) so that you maintain left-right symmetry in the CM.
That dictates an optimal position for a crew access arm: you want that
hatch to face the LUT. That means placing the LUT east of the vehicle, so
that your head is east and your feet are west as you sit in the couch.

But then the vehicle itself would obscure the view of the LUT from the
firing room. For that sightline you want the vehicle to be north of the
tower or vice versa so that you can see both at the same time.

Anybody know the real reason?

I don't have definitive facts, but I can speculate on a couple of
points. First, unlike the shuttle stack, the Saturn V could achieve its
trajectory from nearly any roll position. The roll attitude used was
designed to place the horizon within sight of the commander's window, so
that he could literally fly the Saturn into orbit by hand if the IU
pooped out. As I said earlier, that was a main reason for the heads-
down position during Apollo-Saturn powered flight.

Also, since you need the controllability necessary to achieve the proper
trajectory, that controllability implies the ability to move from any
launch roll attitude to whatever flight roll attitude you want -- so why
worry about whether your on-pad attitude is anywhere close to your
flight attitude? Rolling the vehicle isn't really any more difficult or
dangerous than pitching it onto its flight path, so why not position
your pad structures in the most logical places for your on-the-ground
activities, and just roll to your preferred attitude once in flight?

However, I think there is yet another factor, here. I know that in
early rocket development, roll programs were included very early in a
flight to "test out" the gimbaling of the engines. Starting and
stopping a simple roll while the rocket is climbing vertically exercises
the gimbals such that any problems that might develop become obvious
very early on (and the behaviors they cause are more easily observed and
filmed -- the roll stripes on the body of the rocket were designed to
allow photogrammetric analysis of roll motions, after all). For von
Braun's people, it was they way you launched a rocket...

I speculate strongly that the Saturn V performed its roll program just
after clearing the tower for the dual purposes of placing the CSM in the
heads-down position as it pitched over, and following the hallowed
Germanic routine of checking the engine gimbaling early in flight.

--

"The problem isn't that there are so | Doug Van Dorn
many fools; it's that lightning isn't |
distributed right." -Mark Twain

In article ,
Jay Windley wrote:
My question is the corollary to why the Saturn V also rolled. I know about
the guidance issue of arranging for pitch motions to be in the intended
orbital plane. But LC39 was built for the big Saturns. Presuming it was
possible to site the pads so that the vehicles were already rolled into the
proper attitude (or at least close enough to minimize the amount of roll
needed) at launch, what overriding concern prompted orienting the pads as
they were?

The choice was fairly arbitrary. The launch azimuth (and orbital
inclination) for a lunar mission were not fixed and constant, but depended
(a little) on the position of the Moon and (a lot) on exactly when in the
launch window you launched. Furthermore, LC-39 was not built specifically
for lunar missions: when it was designed, the Saturns were going to be
NASA's heavy lifters for all sorts of missions for decades to come. So
the LC-39 designers simply oriented the pads to the compass directions.

That said, the Saturn V actually *was* oriented to minimize the roll
required. Had the launch azimuth been exactly 90deg (due east) -- it
never was on a manned flight (don't have numbers for Apollos 4 and 6),
although Apollo 17 came close -- no roll maneuver would have been
required.

That dictates an optimal position for a crew access arm: you want that
hatch to face the LUT...

The access arm doesn't go straight from the tower to the rocket, it goes
to the *side* of the rocket (as seen from the tower), because it retracts
by swinging away, not by telescoping. And the white room was in fact on
the east side, i.e. the crew's heads were already facing *approximately*
in the direction of flight.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

"Henry Spencer" wrote in message
...
|
| The choice was fairly arbitrary.

That's the answer I was looking for. I wondered, as I said, about
sightlines and so forth, and was considering the geography of the site with
respect to the turning radius of the crawler-transporter and so forth, but
as usual I'm overthinking it.

| The launch azimuth (and orbital inclination) for a lunar mission
| were not fixed and constant...

I think that's the clincher. With the wide variety of launch azimuths any
launch site may be called upon to support, there's no way you can orient the
site to support all of them. So don't bother to make it a design
requirement. If you're resigned to having to roll, it doesn't really matter
how extensive the roll is. You only get some of the advantage if you
eliminate the roll, not merely minimize it.

So then other siting concerns (where to put support equipment, range safety,
etc.) dictate the orientation of the site. Or in this case they don't. :-)

| That dictates an optimal position for a crew access arm: you want that
| hatch to face the LUT...
|
| The access arm doesn't go straight from the tower to the rocket, it goes
| to the *side* of the rocket (as seen from the tower)

You know, the minute I wrote that I wondered if I knew for sure that was the
case. Good to know -- thanks.

--
|
The universe is not required to conform | Jay Windley
to the expectations of the ignorant. | webmaster @ clavius.org

Doug... wrote:
However, I think there is yet another factor, here. I know that in
early rocket development, roll programs were included very early in a
flight to "test out" the gimbaling of the engines.


Hmm... A diversion that may or may not apply:

The Polaris missiles didn't have slip rings on their gimbals, they had
'twist cables'. Pre-launch the guidance system was aligned to the
target, (so the pitch maneuvers would be in the plane defined by the
submarine, the target, and the center of the geoid). Post launch they
rolled to a) align the body axes with the platform axes, and b)
minimize the amount of twist in the cable bundles.

Poseidon birds had slip rings, but they still rolled to align the
axes. (The lack of computing capability in both birds may have made
this desireable.)

Tridents don't correct for roll, so you won't see this roll in launch
films. (The roll isn't obvious in all launch films of Polaris and
Posiedon.)

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL