How is a rocket stabilized during the initial, slow speed, portion of launch?

Question

Let's say we're at T+0, just as the rocket starts to move, what keeps it upright as it clears the tower and gains speed?

Answer 1

You can recreate the problem by placing a pencil point-first on your finger. Try to keep the pencil/rocket upright by moving your hand back and forth. If you managed it for more than a few seconds, congratulations! You're doing better than Proton 535-43 did.

In the very early stages of flight (before aerodynamics has any major effect) the rocket can be described as an inverted pendulum, just like the pencil. In order to keep itself stable the rocket must ensure that the thrust vector from its engines passes directly through its center of gravity.

From http://exploration.grc.nasa.gov/education/rocket/gimbaled.html

Most modern rockets gimbal their engines to direct the thrust, but it's not the only way to achieve thrust vectoring. Here are a few more:

See Source for more info. (Note: Parts of the site seem to be identical to Rocket Propulsion Elements, George P. Sutton, Oscar Biblarz.)

One of the simplest ways of solving the inverted pendulum problem is with a proportional-integral-derivative controller (PID controller). I'll let Wikipedia explain:

A proportional-integral-derivative controller (PID controller) is a control loop feedback mechanism (controller) commonly used in industrial control systems. A PID controller continuously calculates an "error value" as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error over time by adjustment of a control variable, such as the position of a control valve, a damper, or the power supplied to a heating element, to a new value determined by a weighted sum:

$$u(t) = K_pe(t) + K_i\int^t_0e(\tau)d\tau+K_d\frac{de}{dt}$$

where $K_p$, $K_i$, and $K_d$, all non-negative, denote the coefficients for the proportional, integral, and derivative terms, respectively (sometimes denoted $P$, $I$, and $D$).

Control Solutions, Inc. has a very good (and quite easy to follow) explanation on their website of the bare basics of a PID controller.

Answer 2

The other answers here are correct: gimbaling or other active correction measures are used.

While most launchers do try and maintain a vertical flight off the pad, the Antares rocket is known for the intentional "Baumgartner Maneuver" it does on takeoff, deliberately gimbaling the engine to maneuver away from the tower in the first seconds of flight, as you can see here.

Answer 3

Most rockets gimbal their engines actively to maintain stability. Shifting the axis of thrust slightly works just fine to keep it upright.

Answer 4

Comparing a rocket to an pencil on your finger is the pendulum rocket fallacy. When you balance the pencil, the direction of the normal force depends on the direction of gravity, the pencil's attitude, and your finger's position, but is generally up. As the pencil tips, it's unstable because the disturbing torque increases. The torque from a gimballed rocket engine does not depend on the attitude or gravity, since the angle of the thrust is in relation to rocket itself, not the ground. The only thing that matters is the actuation angle of the gimbal, so there is no runaway. It is no more difficult to stabilize than if the engine were at the top—if anything it's easier since the actuation is further from the center of mass, which is closer to the nose. Robert Goddard famously made this misjudgment when he pioneered rocketry with early rocket prototypes that placed the motor at the top. It had no effect on stability and with no active control or aerodynamic stability by design it careened into the ground. It's not an inverted pendulum because gravity acts on all parts of it equally and there is no constrained pivot. It's just a mass in free fall with a input torque that can be imagined to act at all points simultaneously, if you disregard flex in the structure.