How does the roll maneuver allow more mass to be lifted into orbit?

Question

I read recently that the roll maneuver that rockets perform shortly after clearing the tower to become tilted serves a few purposes (Shuttle and Apollo Saturn V alike). I understand most of these reasons like generating lateral velocity, getting a better view, etc.

One thing mentioned was that it allowed the vehicle to lift more mass. All things being equal, I fail to see how tilt can increase the ability to increase the potential energy of the payload.

Can anyone explain how tilt of a rocket allows it to lift more mass into orbit?

Answer 1

First, let's get terminology straight: "Tilt maneuver", or "Gravity turn", sometimes also called "Pitch maneuver". It was called "Roll Program" in case of Space Shuttles, because it was connected with a roll, necessary for technical reasons but not contributing to flight efficiency directly.

All rockets (and all flying bodies on Earth for that matter) are subject to gravity drag. That's a jargon name for pull of gravitational force that makes everything fall to Earth if not suspended somehow.

So, a rocket just to hover above launchpad, without rising, each second must burn enough fuel to grant it 9.8m/s^2 acceleration - just to break even with Earth acceleration. That's all a wasted fuel that doesn't contribute to accelerating the rocket or lifting it, it just prevents it from falling.

Understandably, we want to minimize this waste - and the only way to do this is to minimize the time between launch and the moment we reach orbital speed, at which gravitational drag is reduced to zero.

The launch trajectory is a balance between horizontal acceleration which serves exactly that, and vertical, which is to reduce atmospheric drag by lifting the rocket into increasingly thinner atmosphere and eventually out of it entirely. So the thrust is divided between the horizontal factor (reducing gravitational drag) and vertical (reducing atmospheric drag). This is why the rocket is tilted, to start gaining horizontal speed immediately, and this tilt increases as atmospheric drag becomes weaker - the higher the horizontal speed, the lower the gravitational drag as well.

Optimizing this means we can reach orbit using less fuel than we'd use if we took an unoptimal trajectory (e.g. launch vertically up, and only later start accelerating horizontally, while gravitational drag costs us a fortune in fuel), and of course - instead of actually burning less fuel we just increase the payload - a rocket flying on optimal trajectory can take a larger payload than one with the same amount of fuel not flying optimally.

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In case of Space Shuttles, the Tilt Maneuver was called "Roll Program". That is because the maneuver of pitching the shuttle to the right inclination was connected with it rolling to position "shuttle below, tank and boosters above". That roll was necessitated by structural and other technical requirements - it was a quirk of the space shuttle construction, the irregular shape, utilizing its wings etc. It's not needed in great most launch vehicles.

Even in the shuttle it wasn't exactly necessary - it was dictated by the launchpad and crawlerway location, as rotating the shuttle on the ground, to launch in proper roll orientation would be too cumbersome - so it was done only after launch. But despite the name, the "roll" part of the "roll program" was merely a technical quirk of this particular construction. Tilt into proper inclination was the general "rocket science" part though, contributing to craft efficiency.

Answer 2

One thing mentioned was that it allowed the vehicle to lift more mass. All things being equal, I fail to see how tilt can increase the ability to increase the potential energy of the payload.

All things weren't equal. The Shuttle was not an axially symmetric vehicle.

The Shuttle roll program was performed starting about ten seconds after launch and lasted for only eight seconds or so. It was called the roll program because the primary effect was to roll the vehicle by about 180 degrees. Without this maneuver, the layout of the launch pad would have dictated a heads-up orientation of the Orbiter.

The Shuttle stack comprised a number of parts, including the Orbiter itself, the external tank, and the solid rocket boosters. Flying heads sideways would not have made any sense at all. The stresses on the connectors between those components would have torn the vehicle apart. Flying in a heads-up configuration also would not have made much sense. This would have resulted in tensile stresses between the Orbiter and the external tank. The heads-down configuration that was flow resulted in compressive stresses.

While flying heads-up might have been controllable, that the forces would have been tensile rather than compressive would have mandated a significant reduction in payload mass lest the vehicle tear itself apart at maximum dynamic pressure (max Q). This was part of what the cited wikipedia article meant by "Increasing the mass that can be carried into orbit."

Another aspect of the roll program was to orient the vehicle so that it was on the correct heading. A third aspect was to start the vehicle pitching downward. To attain orbit, a vehicle has to gain altitude and a huge amount of horizontal velocity. A vehicle that is launched vertically from the ground has to pitch downward throughout the course of the launch to eventually achieve this mostly horizontal course eight to ten minutes after liftoff. Ideally, this is done so as to constantly keep the vehicle on a zero angle of attack throughout the launch. Doing so is called a gravity turn.

No real spacecraft performs a true gravity turn. The vehicle has to make it through the thickest part of the atmosphere first. Minimizing stresses at maximum dynamic pressure plays a key role here. The short Shuttle roll program set the stage for when max Q occurred. In addition to the stresses on the connectors and structure, it was very important to minimize stress on the Orbiter's wings, particularly at max Q. The wings were more of a hindrance than an aid during launch. The Shuttle flew at a negative angle of attack to keep the stresses on the wings down to a minimum.

The Shuttle roll program, short as it was, set the stage for the rest of the flight. Getting that just right did indeed affect how much mass could be carried into orbit.

Answer 3

Almost all the launch vehicles lift off vertically and are designed to reach orbital speed, altitude and orientation as the upper stage completes its injection burn.

Consider a launch vehicle lifting off vertically- The vehicle accelerates to overcome two forces- earth's gravity and the atmospheric drag.

Image from rocketmime.com

If the launch vehicle goes vertically up the whole way, it will reach the needed altitude, but will fall down as it won't enter into an orbit around the earth. However, if the vehicle is launched horizontally, the time spent in the atmosphere will make the fuel requirement prohibitive.

An ideal trajectory will reach the required position with minimum expenditure of fuel and minimum load on the rocket structure (which saves weight). This is usually achieved through the use of 'gravity turn' or zero-lift turn', which uses the gravity (which is perpendicular to the launch vehicle's longitudinal axis initially) to turn the velocity vector as it ascents toward orbit, instead of using the vehicles on-board propellant.

Basically, as soon as the vehicle clears the launch platform, a pitchover maneuver is executed (usually by gimbaling the rocket engines), directing some of the thrust to one side and creating a net torque on the vehicle.

Image from spaceflightsystems.grc.nasa.gov

Now, a small part of the gravitational force is directed perpendicular to the longitudinal axis. This is the beginning of the gravity turn. After the pitchover is complete, the engines are reset to point straight down the axis of the rocket again. From this point until orbit injection, the transverse gravity component continues to grow (as gravity is basically turning the rocket about its lateral axis) and causes the vehicle's velocity vector to rotate toward the horizon as it ascends (by turning the rocket nose towards ground).

"Gravity turn - phase 2" by AndrewBuck - Own work. Licensed under CC BY-SA 3.0 via Commons.

For a properly executed gravity turn, this gravity is the only force acting on the rocket that acts to turn the rocket, thus saving fuel (there may be some corrections due to wind, however, these are negligible). Also, the angle of attack is reduced to (near) zero, thus reducing transverse aerodynamic forces and enabling a lighter vehicle.

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The exact initial pitchover angle depends on the specific launch vehicle, time of launch, orbital destination etc.

For Saturn V, this maneuver is initiated 000:00:13 seconds after launch, after the rocket has lifted off some 450 ft,

After clearing the tower, a tilt and roll maneuver is initiated to achieve the flight attitude and proper orientation for the selected flight azimuth. Launch azimuth is 90 degrees; flight azimuth may vary between 72 and 108 degrees, depending upon time and date of launch. From the end of the tilt maneuver to tilt-arrest, the vehicle flies a pitch program (biased for winds of the launch month) to provide a near zero-lift (gravity-tum) trajectory.

In the case of the space shuttle, the maneuver is initiated before the spacecraft experiences the maximum dynamic pressure (termed "max q").

The roll maneuver is performed shortly before max q is reached because this "heads-down" orientation helps alleviate the stresses that the dynamic pressure loads cause on the vehicle's structure.

The second factor we need to consider is that for each mission, the Shuttle must launch at a certain azimuth angle in order to be inserted into the correct orbital plane. Since the launch pad (and therefore the Shuttle) sits in a fixed position, the Shuttle must perform a roll maneuver during ascent in order to orient itself to achieve the desired launch azimuth angle.

Answer 4

You might be getting confused by the terms used since they don't always mean the same things. There are basically 2 different manoeuvres

1. Roll program as in Roll to align azimuth: Here the rocket rolls to align the axis around which it pitches orthogonal to its desired orbital plane (It's more complicated but this is the gist of it). This means that its gravity turn can be achieved with only pitch, this means it is a lot easier to calculate, especially in older flight computers. It also lessens the risk of gimbal lock.
2. Pitch program : Here the rocket (that is aligned with its desired plane) pitches to start a gravity turn (most of the time a gravity turn like trajectory). In a pure gravity turn the rocket keeps it angle of attack 0 and rotates as the gravity accelerates the spaceship towards the ground, if the rocket is not pointing straight up this will result in its compound velocity pointing a little more down. By continually following its velocity vector the rocket performs a "turn". The rotation is thus not caused by gravity, but by the rocket following its speed vector.

In the case of the space shuttle these manoeuvres are combined into one, called the roll program. The gravity turn is the source of the performance gains. You can read more about it on wikipedia