# Why does the Shuttle roll during launch?

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?

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.

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.

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.

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

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

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

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

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.

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.

Thx

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.

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.

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

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.

That 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.

I see what you did there.

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.

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.

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?

Bolding is mine. Really?

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 a bit sweet - off into the sunset.

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?
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