Rocket Stability and Flight Control Systems

Building an efficient rocket engine is only part of the problem. The rocket must also be stable in flight. A stable rocket is one that flies in a smooth, uniform direction. An unstable rocket flies along an erratic path, sometimes tumbling or changing direction. Unstable rockets are dangerous because it's not possible to predict where they’ll go – they may even turn upside down and suddenly head directly back to the launch pad.

What Makes a Rocket Stable or Unstable?

All matter has a point inside called the center of mass or “CM," regardless of its size, mass or shape. The center of mass is the exact spot where all the mass of that object is perfectly balanced.

You can easily find the center of mass of an object -- such as a ruler -- by balancing it on your finger. If the material used to make the ruler is of uniform thickness and density, the center of mass should be at the halfway point between one end of the stick and the other. The CM would no longer be in the middle if a heavy nail was driven into one of its ends. The balance point would be nearer the end with the nail.

CM is important in rocket flight because an unstable rocket tumbles around this point. In fact, any object in flight tends to tumble. If you throw a stick, it will tumble end over end. Throw a ball and it spins in flight. The act of spinning or tumbling stabilizes an object in flight.

A Frisbee will go where you want it go to only if you throw it with a deliberate spin. Try throwing a Frisbee without spinning it and you'll find that it flies in an erratic path and falls far short of its mark if you can even throw it at all. 

Roll, Pitch and Yaw

Spinning or tumbling takes place around one or more of three axes in flight: roll, pitch and yaw.

The point where all three of these axes intersect is the center of mass.

The pitch and yaw axes are the most important in rocket flight because any movement in either of these two directions can cause the rocket to go off course. The roll axis is the least important because movement along this axis will not affect the flight path.

In fact, a rolling motion will help stabilize the rocket the same way a properly passed football is stabilized by rolling or spiraling it in flight. Although a poorly passed football may still fly to its mark even if it tumbles rather than rolls, a rocket will not. The action-reaction energy of a football pass is completely expended by the thrower the moment the ball leaves his hand. With rockets, thrust from the engine is still produced while the rocket is in flight. Unstable motions about the pitch and yaw axes will cause the rocket to leave the planned course. A control system is needed to prevent or at least minimize unstable motions.

The Center of Pressure

Another important center that affects a rocket's flight is its center of pressure or “CP.” The center of pressure exists only when air is flowing past the moving rocket. This flowing air, rubbing and pushing against the outer surface of the rocket, can cause it to begin moving around one of its three axes.

Think of a weather vane, an arrow-like stick mounted on a rooftop and used for telling wind direction. The arrow is attached to a vertical rod that acts as a pivot point. The arrow is balanced so the center of mass is right at the pivot point. When the wind blows, the arrow turns and the head of the arrow points into the on-coming wind. The tail of the arrow points in the downwind direction.

A weather vane arrow points into the wind because the tail of the arrow has a much larger surface area than the arrowhead. The flowing air imparts a greater force to the tail than the head so the tail is pushed away. There is a point on the arrow where the surface area is the same on one side as the other. This spot is called the center of pressure. The center of pressure is not in the same place as the center of mass.

If it were, then neither end of the arrow would be favored by the wind. The arrow would not point. The center of pressure is between the center of mass and the tail end of the arrow. This means that the tail end has more surface area than the head end.

The center of pressure in a rocket must be located toward the tail. The center of mass must be located toward the nose. If they are in the same place or very near each other, the rocket will be unstable in flight. It will try to rotate about the center of mass in the pitch and yaw axes, producing a dangerous situation.

Control Systems

Making a rocket stable requires some form of control system. Control systems for rockets keep a rocket stable in flight and steer it. Small rockets usually require only a stabilizing control system. Large rockets, such as the ones that launch satellites into orbit, require a system that not only stabilizes the rocket but also enables it to change course while in flight.

Controls on rockets can be either active or passive. Passive controls are fixed devices that keep rockets stabilized by their very presence on the rocket's exterior. Active controls can be moved while the rocket is in flight to stabilize and steer the craft.

Passive Controls

The simplest of all passive controls is a stick. Chinese fire arrows were simple rockets mounted on the ends of sticks that kept the center of pressure behind the center of mass. Fire arrows were notoriously inaccurate in spite of this. Air had to be flowing past the rocket before the center of pressure could take effect. While still on the ground and immobile, the arrow might lurch and fire the wrong way. 

The accuracy of fire arrows was improved considerably years later by mounting them in a trough aimed in the proper direction. The trough guided the arrow until it was moving fast enough to become stable on its own.

Another important improvement in rocketry came when sticks were replaced by clusters of lightweight fins mounted around the lower end near the nozzle.

Fins could be made out of lightweight materials and be streamlined in shape. They gave rockets a dartlike appearance. The large surface area of the fins easily kept the center of pressure behind the center of mass. Some experimenters even bent the lower tips of the fins in a pinwheel fashion to promote rapid spinning in flight. With these "spin fins," rockets become much more stable, but this design produced more drag and limited the rocket's range.

Active Controls

The weight of the rocket is a critical factor in performance and range. The original fire arrow stick added too much dead weight to the rocket and therefore limited its range considerably. With the beginning of modern rocketry in the 20th century, new ways were sought to improve rocket stability and at the same time reduce overall rocket weight. The answer was the development of active controls.

Active control systems included vanes, movable fins, canards, gimbaled nozzles, vernier rockets, fuel injection and attitude-control rockets. 

Tilting fins and canards are quite similar to each other in appearance -- the only real difference is their location on the rocket. Canards are mounted on the front end while tilting fins are at the rear. In flight, the fins and canards tilt like rudders to deflect the air flow and cause the rocket to change course. Motion sensors on the rocket detect unplanned directional changes, and corrections can be made by slightly tilting the fins and canards. The advantage of these two devices is their size and weight. They are smaller and lighter and produce less drag than large fins.

Other active control systems can eliminate fins and canards altogether. Course changes can be made in flight by tilting the angle at which the exhaust gas leaves the rocket’s engine. Several techniques can be used for changing exhaust direction. Vanes are small finlike devices placed inside the exhaust of the rocket engine. Tilting the vanes deflects the exhaust, and by action-reaction the rocket responds by pointing the opposite way. 

Another method for changing the exhaust direction is to gimbal the nozzle. A gimbaled nozzle is one that is able to sway while exhaust gases are passing through it. By tilting the engine nozzle in the proper direction, the rocket responds by changing course.

Vernier rockets can also be used to change direction. These are small rockets mounted on the outside of the large engine. They fire when needed, producing the desired course change.

In space, only spinning the rocket along the roll axis or using active controls involving the engine exhaust can stabilize the rocket or change its direction. Fins and canards have nothing to work upon without air. Science fiction movies showing rockets in space with wings and fins are long on fiction and short on science. The most common kinds of active controls used in space are attitude-control rockets. Small clusters of engines are mounted all around the vehicle. By firing the right combination of these small rockets, the vehicle can be turned in any direction. As soon as they are aimed properly, the main engines fire, sending the rocket off in the new direction. 

The Mass of the Rocket

The mass of a rocket is another important factor affecting its performance. It can make the difference between a successful flight and wallowing around on the launch pad. The rocket engine must produce a thrust that is greater than the total mass of the vehicle before the rocket can leave the ground. A rocket with a lot of unnecessary mass will not be as efficient as one that is trimmed to just the bare essentials. The total mass of the vehicle should be distributed following this general formula for an ideal rocket: 

  • Ninety-one percent of the total mass should be propellants.
  • Three percent should be tanks, engines and fins.
  • Payload can account for 6 percent. Payloads may be satellites, astronauts or spacecraft that will travel to other planets or moons.

In determining the effectiveness of a rocket design, rocketeers speak in terms of mass fraction or “MF.” The mass of the rocket’s propellants divided by the total mass of the rocket gives mass fraction: MF = (Mass of Propellants)/(Total Mass)

Ideally, the mass fraction of a rocket is 0.91. One might think that an MF of 1.0 is perfect, but then the entire rocket would be nothing more than a lump of propellants that would ignite into a fireball. The larger the MF number, the less payload the rocket can carry. The smaller the MF number, the less its range becomes. An MF number of 0.91 is a good balance between payload-carrying capability and range.

The Space Shuttle has an MF of approximately 0.82. The MF varies between the different orbiters in the Space Shuttle fleet and with the different payload weights of each mission.

Rockets that are large enough to carry spacecraft into space have serious weight problems. A great deal of propellant is needed for them to reach space and find proper orbital velocities. Therefore, the tanks, engines and associated hardware become larger. Up to a point, bigger rockets fly farther than smaller rockets, but when they become too large their structures weigh them down too much. The mass fraction is reduced to an impossible number.

A solution to this problem can be credited to 16th-century fireworks maker Johann Schmidlap. He attached small rockets to the top of big ones. When the large rocket was exhausted, the rocket casing was dropped behind and the remaining rocket fired. Much higher altitudes were achieved. These rockets used by Schmidlap were called step rockets.

Today, this technique of building a rocket is called staging. Thanks to staging, it has become possible not only to reach outer space but the moon and other planets, too. The Space Shuttle follows the step rocket principle by dropping off its solid rocket boosters and external tank when they’re exhausted of propellants.