Paper Rocket Backgrounder
Rocket stability is an important issue for rocket scientists. The success of a space launch depends upon “pinpoint” accuracy. If a future Ares I rocket arrives in space in the wrong orbit, it may not have enough fuel or supplies to make rendezvousing with the International Space Station possible. The crew would have to return to Earth and “chalk off” a failed mission.
Stability means making sure the rocket follows a smooth path in flight. If it wobbles, the ride will be rough and extra fuel will be burned to get back on course. If it tumbles, it’s time to push the destruct button! An unstable rocket is dangerous.
Fortunately, it is relatively easy to ensure stability if two things are kept in mind. These two things are center of mass and center of pressure.
Center of mass (COM) is easy to demonstrate. It is the balance point of a rocket. Think of it like balancing a meter stick on an outstretched finger. If the stick rests horizontally, the COM is directly over your finger. If the COM is to the right of your finger, the stick will tip to the right. If to the left of your finger, the stick will tip to the left.
An object, tossed into the air, rotates around its COM. Rockets also try to rotate around their COM while in flight. If this rotation is allowed to happen, the rocket becomes unstable. This is where Center of pressure (COP) comes to the rescue.
COP is a balance point too but it has to do with the pressure exerted on the rocket surface by air molecules striking the surface as the rocket flies through the air. Like COM, there is a midpoint for the flight pressure on the rocket body. This is the COP. For a stable rocket, the COP is located to the rear of the rockets and the COM is to the front. To understand why the rocket is stable, let’s take a look at a couple of devices that also depend upon the placement of COM and COP.
A weather vane pivots on a vertical axle when the wind blows. One end of the vane is pointed and the other end has a broad surface. When the wind blows, the broad end of the vane catches more air (more air pressure) and is blown down wind. The narrow end of the vane has less pressure exerted on it and points into the wind.
One end of an arrow is long, narrow, and pointed while the other end has large feathers (or modern plastic fins). In flight, greater air pressure is exerted on the feathers than on the narrow end. This keeps the arrow from tumbling around its COM and on course to its target.
In both examples, there was more surface area on one side of the COM than the other. Both devices were stable. Stability of a rocket is the same thing.