Why does the Shuttle roll during launch?

[Gedd]

I was watching the shuttle launch today and noticed that as soon as it was off the launch pad it was curving and rolling to where it was “on its back.” What’s the reason? Is it more aerodynamic? Is it due to the direction it heads on take off? Will it take a rocket scientist to find the answer? ;)

[Llama Llogophile]

I believe it's due to the direction it needs to fly after takeoff, and the geographical location of the launch site. They generally launch eastward to take advantage of the Earth's rotational velocity, and I believe the roll cancels out a positional necessity for launch. Will try to find a cite later.

[TerpBE]

From http://spacelaunchinfo.com/faq.html#bottom:

The Space Shuttle "rolls over" onto it's back to reach orbit for almost the same reason an airplane turns after leaving the runway. That is, in order to turn into the direction it wishes to go.

While an airplane does not "roll over" like the Space Shuttle, airplanes don't travel above the atmosphere! In going from 0 to 17,500 miles per hour in just 8 minutes, the "vector of thrust" (the direction that the rocket engines push) must go from the rocket engines (at the bottom of the orbiter), through the center of gravity of the entire Space Transportation System (Orbiter, External Tank, and Solid Rocket Boosters). This means that the engines need to be "underneath" the Shuttle as it blasts through the atmosphere, overcoming the Earth's gravity, until it gets to orbit, where gravity doesn't mean as much any more. In the process of "going uphill" (as the astronauts call it), the crew is pretty much "upside-down" in their seats! However, with 3 G's (three gravities) of acceleration of the launch pushing them into their seats they hardly notice the 1 G of force in the "wrong" direction on their way "up" to orbit.

[Francis Vaughan]

The shuttle is designed to fly inverted, hanging under the external tank. Secondly, although everyone sees the shuttle (and indeed all rockets) take off vertically, the vast majority of the flight and energy isn't in gaining height, it is in gaining speed. Orbit is around the center of the earth, and you need to get past the atmosphere, and be travelling in an ellipse around the centre of the earth. You do however need to be travelling fast enough that the ellipse is big enough that it doesn't intersect the surface. Which is bad. So, if you think of the track of the shuttle over the Earth, you realise that it must be travelling on a plane that intersects the center of the Earth. Since it took off in Florida, the easiest orbit that this is true of is a plane that is inclined to the equator by the lattitude of Cape Canaveral. If you want a different inclination you need to add more energy.

So, what does this mean for takeoff? Well the shuttle is going to get high quickly to get past the worst of the atmosphere, and then is going to put most of its effort into getting enough orbital velocity. That means pointing itself to follow the desired orbital track. Since it flys on its back it must roll around to point its back to the right direction, and then lay back as it climbs.

The origional orientation of the pads at the Cape were done so that the Apollo era takeoffs were already pretty much already correct.

[DanBlather]

This means that the engines need to be "underneath" the Shuttle as it blasts through the atmosphere, overcoming the Earth's gravity, until it gets to orbit, where gravity doesn't mean as much any more.What a horrible explanation. Earth's gravity is just slightly less at the altitude the space shuttle orbits. I think what they mean to say is that having the shuttle on the bottom provides an "upwards" effect which is what you want when trying to climb away from the earth.

[DanBlather]

The origional orientation of the pads at the Cape were done so that the Apollo era takeoffs were already pretty much already correct.What does this mean? Aren't they just vertical?

ETA: That looked bad. I'm not doubting you, I just don't understand.

[Duckster]

Considering the Shuttle is already shaking and rattling during launch, the roll merely completes the sequence.

[Stranger On A Train]

Although the article cited by TerpBE gives some of the answer, it doesn't explicitly answer the question of the o.p. The reason the STS (which includes the Shuttle Orbiter, the External Tank and associated flight structure, and the Solid Rocket Boosters) have to roll belly up after launch is because the aerodynamic loads on the Orbiter delta wing are such that if it flew upright it would exceed the structural margins on the wings. (This is actually what tore apart Challenger on her fateful final mission, not the combustion of propellants that leaked from the ET.) The margins are actually quite slim; smaller than would normally be acceptable on a manned spacecraft per NASA-STD-5001 Structural Design and Test Factors of Safety, and similar commercial standards for manned or high reliability vehicles. Because weight is critical (and the wings are all dead weight upon launch all the way to orbit) they don't want to overdesign the structure, and on the way down during re-entry the wing only sees load pushing up from underneath. Making the wings also capable of resisting large aeroloads pushing down from the top (as in prone orientation during launch) would require more dead weight from reinforcement structure that otherwise does nothing useful. Instead, it is easier to flip the Orbiter in supine orientation and just make the crew uncomfortable during the eight minutes or so it takes to put them on a ballistic track to low earth orbit.

For all of the carping by many space enthusiasts about how going back to a capsule-type spacecraft is "going in reverse", the STS is actually a highly compromised design with limited (and not readily scalable) capability, requiring intensive maintenance and inspection, and with numerous inherent design flaws and exceedingly complex solutions to problems that a capsule-type design wouldn't have to cope with. Rather than being the future of spaceflight vehicles, it is a blind alley that demonstrates the folly of delta wing re-entry vehicles with large exposed leading edges and a wide spread of non-functional lifting surface. The single capability that the Shuttle is really optimized for--a polar orbit, once around and return to launch site mission--has never been utilized and given a lack of involvement or interest by the US Air Force in such vehicles post-Challenger, isn't a useful capability in the foreseeable future.

Stranger

[Francis Vaughan]

What does this mean? Aren't they just vertical?

ETA: That looked bad. I'm not doubting you, I just don't understand.

When the vehicle was on the pad it was already rotated to pretty much the right direction. Although it is a cylinder, the capsule has an orientation around its axis. With the crew strapped in it has a top and bottom to the circle. Thus the Saturn was paced on the pad, and the pad structure placed so that this direction was close to that needed. So when it took off, the vehicle nosed over to point the right direction, and didn't need to roll as well (or much) to keep the capsule the right way up. For the shuttle this is the direction its back points.

Remember that roll is defined to be rotation about the longitudinal axis of the craft. When the craft (either Saturn or Shuttle) is travelling vertically roll is still about the long axis. When the pads were reused for the shuttle, the pad orientation was already fixed, so the shuttle with its different design had to roll more. Also, the shuttle was intended for a much wider set of activities, and has to reach orbits over a wide range of inclinations, so there isn't a perfect initial orientation anyway.

[Chronos]

So, if you think of the track of the shuttle over the Earth, you realise that it must be travelling on a plane that intersects the center of the Earth. Since it took off in Florida, the easiest orbit that this is true of is a plane that is inclined to the equator by the lattitude of Cape Canaveral. If you want a different inclination you need to add more energy.
Further, it's extremely hard to go into an orbit with an inclination less than the latitude of your launch site. This is why the space station is in such a highly-inclined orbit: The Russians launch from a higher latitude than we do, and it's easier for the Shuttle to launch into a Russian orbit than it is for the Russian vehicles to launch into a Cape Canaveral orbit.

[DanBlather]

When the vehicle was on the pad it was already rotated to pretty much the right direction. Although it is a cylinder, the capsule has an orientation around its axis. With the crew strapped in it has a top and bottom to the circle. Thus the Saturn was paced on the pad, and the pad structure placed so that this direction was close to that needed. So when it took off, the vehicle nosed over to point the right direction, and didn't need to roll as well (or much) to keep the capsule the right way up. For the shuttle this is the direction its back points.Thx

[robby]

This means that the engines need to be "underneath" the Shuttle as it blasts through the atmosphere, overcoming the Earth's gravity, until it gets to orbit, where gravity doesn't mean as much any more.

What a horrible explanation. Earth's gravity is just slightly less at the altitude the space shuttle orbits.I agree. It's idiotic explanations like this that are responsible for so many of the basic misconceptions people have about basic physics.

When I was teaching this material in a physics class years ago, most of my students (all high school graduates) arrived with the misconception that objects in orbit were "weightless" because they had "escaped the pull of Earth's gravity."

Contrary to the quoted explanation above, I'd say that Earth's gravity matters a whole lot, being the fundamental force keeping objects in orbit.

[Xema]

When I was teaching this material in a physics class years ago, most of my students (all high school graduates) arrived with the misconception that objects in orbit were "weightless" because they had "escaped the pull of Earth's gravity."
I once saw a highschool textbook that had a picture of a Shuttle spacewalk with the caption:
"As you get farther from the center of the Earth, the Earth's gravity decreases, until in outer space, you're weightless."

Sheesh.

[Stranger On A Train]

I agree. It's idiotic explanations like this that are responsible for so many of the basic misconceptions people have about basic physics.While it isn't the best explanation, it is at best unclear rather than outright wrong. It is correct that the force vector needs to go through the c.g. of the vehicle, and given that the SRBs and External Tank are mounted below the Orbiter (the only practical place to do so) this mandates a thrust axis (and thus, the position and orientation of the rocket nozzles) such that the resulting thrust vector is going slightly nose-down with respect to the Orbiter. Some of the earlier concepts had engines mounted directly in parallel with the Orbiter, or even a separating V-shaped flyback booster vehicle such that the axis of thrust would be directly in line with the longitudinal axis of the vehicle; however, this caused a lot of conflicts with size and shape of the Orbiter, whereas the current configuration offered a great deal more flexibility, and thus the design and production facilities for the SRBs and the ET could be accelerated before the detail design of the Orbiter was near complete.

Gravity does of course matter in orbit; it is the velocity vector of the orbiting object, and its inertia directing it along a path that is tangent to the orbit, that negates the downward pull of gravity (from the perspective of the occupants) and causes it to be in freefall; that is, continuously falling around the central body in an ellipse. It is easy to criticize science textbooks and pop-science articles for such semantic inaccuracies as those cited above, but in fact the average student or layman won't put enough thought into understanding how gravity works to discriminate between the astute answer and the sloppy one.

Stranger

[robby]

While it isn't the best explanation, it is at best unclear rather than outright wrong. It is correct that the force vector needs to go through the c.g. of the vehicle, and given that the SRBs and External Tank are mounted below the Orbiter (the only practical place to do so) this mandates a thrust axis (and thus, the position and orientation of the rocket nozzles) such that the resulting thrust vector is going slightly nose-down with respect to the Orbiter. Some of the earlier concepts had engines mounted directly in parallel with the Orbiter, or even a separating V-shaped flyback booster vehicle such that the axis of thrust would be directly in line with the longitudinal axis of the vehicle; however, this caused a lot of conflicts with size and shape of the Orbiter, whereas the current configuration offered a great deal more flexibility, and thus the design and production facilities for the SRBs and the ET could be accelerated before the detail design of the Orbiter was near complete.An interesting edification, but I was actually only addressing the part about "gravity doesn't mean as much any more" in orbit, not the thrust vector of the shuttle during launch.

...It is easy to criticize science textbooks and pop-science articles for such semantic inaccuracies as those cited above, but in fact the average student or layman won't put enough thought into understanding how gravity works to discriminate between the astute answer and the sloppy one.

StrangerThat may be so. However, I don't think that the basic concept that gravity is in fact necessary to keep objects in orbit is all that hard to comprehend, particularly for students who have taken or are taking an introductory physics class. In this way, you can get students to realize that gravity is actually acting as a centripetal force, that it is very present (exerting a force at low Earth orbit of something like 83% of the magnitude experienced on the surface of the Earth), and that the reason that astronauts experience weightlessness is because they are constantly falling, but because of their "sideways" tangential velocity, they never actually hit the surface.

IMHO, any article purporting to explain science to the layman, much less a science textbook, that includes quotes like the one related by Xema indicating that objects in orbit have somehow escaped Earth's gravity (unless the article is speaking in the poetic sense), is a travesty that only perpetuates scientific illiteracy.

[ZipperJJ]

Considering the Shuttle is already shaking and rattling during launch, the roll merely completes the sequence.

I see what you did there.

[what do I type here]

From http://spacelaunchinfo.com/faq.html#bottom:


The Space Shuttle "rolls over" onto it's back


Uuuuuugggghhhhhhhhhhhhhhhhhhhh COME ON. You run a website about space shuttles and astronomy and you should know these grammar rules! Come on!

Modern life is rubbish.

[Francis Vaughan]

Just because I can, I'm going to be really pedantic here.

The shuttle does not roll onto its back. It pitches up, which because it is already pointing vertically, pitches it onto its back. Before it does this, it rolls (i.e. rotates about its longitudinal axis) so that its back is facing the direction it wants to orbit.

If the shuttle was to roll onto its back it would need to be in level flight, which it isn't.

When you watch a shuttle launch you will hear a comment about a "roll maneuver" early in the flight, just after throttle up and max Q. This is the roll that points the back of the shuttle down track. The shuttle is still flying vertically at this point. It isn't until after SRB separation, two minutes in, that the shuttle starts to pitch up to fly upside down. It is then that it starts to really gain speed.

[NinjaChick]

I once saw a highschool textbook that had a picture of a Shuttle spacewalk with the caption:
"As you get farther from the center of the Earth, the Earth's gravity decreases, until in outer space, you're weightless."

Sheesh.
I recently saw a news article about an astronaut on the current crew* who was going to bring a sliver of [allegedly] Mr. Newton's apple tree. Neat thing to do, right?
"I'll take it up and let it float around for a bit, which will confuse Isaac," said the 55-year-old Nasa astronaut, a veteran of two previous shuttle missions and a graduate of the University of Edinburgh.

"While it's up there, it will be experiencing no gravity, so if it had an apple on it, the apple wouldn't fall … Sir Isaac would have loved to see this, assuming he wasn't spacesick, as it would have proved his first law of motion to be correct."
Bolding is mine. Really?
Uuuuuugggghhhhhhhhhhhhhhhhhhhh COME ON. You run a website about space shuttles and astronomy and you should know these grammar rules! Come on!

Modern life is rubbish.
Speaking as someone with two engineers in the immediate family - including a sister who is literally a rocket scientist: a solid grasp on the mechanics of space flight does not, in any way, guarantee a solid grasp on the mechanics of the English language. My sister had me proofread her papers all through her master's thesis for grammar, and probably would still have me proofing reports for her if she weren't working with classified material.

*Which, btw, is Atlantis's very last flight. There are two more planned after this, so assuming nothing goes sour, Discovery and Endeavour will also get one last trip. If you're both a nerd and a total sap like I am, you may find Atlantis's final mission patch (http://www.collectspace.com/review/sts132_patch02.jpg) a bit sweet - off into the sunset.

[andros]

Bolding is mine. Really?

Sure. I think it's safe to assume that Sellers, as a NASA astronaut, is experienced in lay terminology, and to a layman (ie, most all 'Mercuns) "gravity" means "that what makes stuff fall to the kitchen floor, usually butter-side down."

Of course, the tree-chunk will be experiencing plenty of gravitational force, being as how it's in orbit and all. Just not weight, which is how we terrestrials experience and interact with gravity.

I dunno. Is it harmful to use that kind of laic shorthand? I don't think so, but ghod knows there's plenty of GD fodder in that question.

*Which, btw, is Atlantis's very last flight.

I heard something about a remotely possible additional flight for Atlantis, schlepping supplies to the ISS, since it'll be fully prepped and on standby after this mission's conclusion. Anyone know anything about that, or am I smoking the crack?
.

[PlainJain]

This is actually what tore apart Challenger on her fateful final mission, not the combustion of propellants that leaked from the ET.
Could you elaborate on this? I know nothing of these things but I'm sure I've watched a video that stated the crew cabin and the severed left wing emerged from the vapor cloud approximately 1 1/2 seconds after the combustion of propellants that leaked from the ET. This seems to suggest the force of the combustion was responsible for the break up.

Not challenging your assertion, just looking for clarification.

ETA: link (http://www.youtube.com/watch?v=u0V-ZRNzMWc&feature=related)

[Clothahump]

Considering the Shuttle is already shaking and rattling during launch, the roll merely completes the sequence.

For the win! :D

[NinjaChick]

Sure. I think it's safe to assume that Sellers, as a NASA astronaut, is experienced in lay terminology, and to a layman (ie, most all 'Mercuns) "gravity" means "that what makes stuff fall to the kitchen floor, usually butter-side down."

Of course, the tree-chunk will be experiencing plenty of gravitational force, being as how it's in orbit and all. Just not weight, which is how we terrestrials experience and interact with gravity.

I dunno. Is it harmful to use that kind of laic shorthand? I don't think so, but ghod knows there's plenty of GD fodder in that question.



I heard something about a remotely possible additional flight for Atlantis, schlepping supplies to the ISS, since it'll be fully prepped and on standby after this mission's conclusion. Anyone know anything about that, or am I smoking the crack?
.
Eh, dumbing things down generally bugs me - maybe I'm overly optimistic, but I'd like to think that most people are actually reasonably intelligent and have the capacity to understand things - there's no need to dumb things down. But I also acknowledge that there's a big element of picking my battles here.

As for a potential STS-135: There's been some talk.

One side is saying: We're going to have the whole thing close to fully prepped anyway, and the ISS isn't scheduled to be abandoned anytime soon, so why not use that last prepped shuttle to bring up a buttload of supplies? Which makes sense.

There's a number of problems, though, which (IMO) make it extremely unlikely. One big concern is they'd only have a four-man crew because of the way they outfit potential rescue shuttles, and they also wouldn't have time do a complete and total turnaround on Atlantis (and if they did they still probably wouldn't for just one flight), so there would potentially be some significant safety issues. There's also a little problem where Congress has pretty much decided to stop giving the shuttle program money in 2011, and STS-135 would probably run into 2012.

There's also some damn issue with the external tank, where there's basically two and a half-ish tanks: one really old one stuck in a closet somewhere, one fully built but damaged by a hurricane and not yet repaired, and two which are partially built and (apparently) abandoned in the factory (for what reason I cannot possibly imagine). The old one is heavier than they'd want to use, the damaged one is, well, damaged, and it would be costly to complete one of the two in-progress ones. Obviously they'll have to figure something out for the contingency flight, but from what I've heard it's a pretty good example of government "planning".

[MikeS]

Could you elaborate on this? I know nothing of these things but I'm sure I've watched a video that stated the crew cabin and the severed left wing emerged from the vapor cloud approximately 1 1/2 seconds after the combustion of propellants that leaked from the ET. This seems to suggest the force of the combustion was responsible for the break up.I'm not a rocket scientist, and hopefully Stranger will return to elaborate, but: the large cloud that enveloped the orbiter and the boosters was due primarily to the fact that the fuels were incredibly cold (liquid oxygen & hydrogen.) Some of the hydrogen did burn, but not very much, and not with enough force to destroy the orbiter on its own. In other words, if the SRBs and the external tank had failed the way they did on while the orbiter was sitting on the ground, the orbiter probably would have survived largely structurally intact.

Instead, it was the imbalance in the aerodynamic forces that tore the orbiter apart. When the right SRB failed, the force from the rockets was pushing and torquing the orbiter in unexpected directions, and once the orbiter got out of the correct, streamlined orientation, the wind essentially caught it and tore it apart.

[Francis Vaughan]

Rather than mess about - this is the critical part from the Rogers commision report. Note the word "almost" in the second paragraph.

At 73.124 seconds,. a circumferential white vapor pattern was
observed blooming from the side of the External Tank bottom dome.
This was the beginning of the structural failure of hydrogen tank that
culminated in the entire aft dome dropping away. This released
massive amounts of liquid hydrogen from the tank and created a sudden
forward thrust of about 2.8 million pounds, pushing the hydrogen tank
upward into the intertank structure. At about the same time, the
rotating right Solid Rocket Booster impacted the intertank structure
and the lower part of the liquid oxygen tank. These structures failed
at 73.137 seconds as evidenced by the white vapors appearing in the
intertank region.

Within milliseconds there was massive, almost explosive, burning of
the hydrogen streaming from the failed tank bottom and liquid oxygen
breach in the area of the intertank.

At this point in its trajectory, while traveling at a Mach number of
1.92 at an altitude of 46,000 feet, the Challenger was totally
enveloped in the explosive burn. The Challenger's reaction control
system ruptured and a hypergolic burn of its propellants occurred as
it exited the oxygen-hydrogen flames. The reddish brown colors of the
hypergolic fuel burn are visible on the edge of the main fireball.
The Orbiter, under severe aerodynamic loads, broke into several large
sections which emerged from the fireball. Separate sections that can
be identified on film include the main engine/tail section with the
engines still burning, one wing of the Orbiter, and the forward
fuselage trailing a mass of umbilical lines pulled loose from the
payload bay.

[NinjaChick]

I could be totally wrong, but I've heard the details of Challenger summed up into three main points:
1. The SRB leaked, came loose at the bottom strut, and rotated slightly around the top strut. Result: your two very powerful engines are no longer pointing you in the same direction, which is bad, and also...
2. The ET, due to damage inflicted by the SRB, lost it's internal containment, resulting in a very uncontrolled combination of liquid hydrogen and oxygen, which resulted in the entire tank simply disintegrating.
3. The above points were bad and the crew was already doomed, but what actually caused the orbiter to break up was that the SRB and ET problems pushed it into a decidedly aerodynamic attitude (i.e., the shuttle was no longer leading with it's nose and was roughly as aerodynamic as a brick), and doing so while traveling at upwards of a thousand miles per hour after having been severely damaged by everything else caused the orbiter to basically crumble like a cracker.

[Francis Vaughan]

Tp paraphrase the events.

SRB field joint leaks - from an intial spurt of gas at liftoff the pressure inside the SRB eventually pushed enough hot gas past the joint to completely erode the O-ring - so that there was flame shooting out of the joint. So much so that the internal pressure measured inside the SRB was noticably less than it should have been.

Flame plume impinges on the ET and the aft attachment point - where the SRB is attached to the ET. As in the quote above, this flame cut though the ET at its base. The entire ET below the upper attachament points (which are on the intertank seperator between the hydrogen and oxygen tanks, is the liquid hydrogen tank. (Liquid hydrogen has very low density - this is one of its significant disadvantages.) The ET starts to leak lots of LH into the slipstream. Eventually the entire bottom of the tank simply falls off. This occurs pretty much at the same time as the aft attachment point for the SRB fails.

So, the huge amount of liquid hydrogen in the tank now blows back out of the back of the tank slamming the top of the liquid hydrogen tank up into the bottom of the liquid oxygen tank, at the same time the SRB, now only held at the top, pivots around. It hits part of the wing, but most importantly, the pointy end is pushed into the side of the liquid oxygen tank, which ruptures here too. At this point the stack is blown apart by the ferocity of the combustion of the hydrogen and oxygen in the inter-tank space. The reason it isn't called an explosion is that it didn't detonate. It just burnt very fast. Just like it does inside the rocket motor. There wasn't much chance for the oxygen and hydrogen to mix. Most of the hydrogen had blown out of the back of the tank, and the oxygen was at the top of the structure. By the time any mixing and further combustion had occured the orbiter was already long broken up.

But yes, the orbiter pitches back into the hypersonic stream and the wings get instantly ripped off, and the rest of the orbiter is torn into chunks.

[PlainJain]

Thanks all... ignorance fought.