View Single Post
Old 23 July 2015, 12:52 AM
jimmy101_again jimmy101_again is offline
Join Date: 29 December 2005
Location: Greenwood, IN
Posts: 6,906

Fins sometimes work but they have restrictions, in particular they only work when the air velocity is high enough. For many missiles there is an early phase when they are moving to slow for fins to have any effect. Rocket propelled grenades are often fin stabilized but they require a launch rail or tube to keep them going straight for the first couple feet until they get moving fast enough for fins to have an effect. If you launch an RPG without a launch tube you have zero idea where it is going and its flight will not be "ballistic".

Stability can also be obtained by having most of the weight in the nose. That is basically how an arrow works. With most rockets though the center of mass (weight) is coincident with the center of thrust (aka center of drag) at least at the time of launch when it is carrying a full load of fuel. So for many rockets/missiles having the weight forward isn't possible.

Spinning is of course how most rifles work. Indeed that is what the term "rifle" basically means, threads (rifling) in the barrel spin the projectile and that removes tumbling vie gyroscopic forces (at the expense of not actually aligning the front of the shell with the air flow over the shell, arrows are better at actually pointing into the wind than are bullets). On a spherical round the spin can be in any direction and as long as every round spins in the same direction then the trajectory will be reasonably predictable. Baseballs are thrown with spin in different directions and any spin makes the pitch more controllable and more predictable. A "knuckle ball" is a pitch purposely thrown with as little spin as possible, the result is that the trajectory is hard to predict (it is not sufficiently "ballistic") and batters don't know where to swing the bat because they can't predict where the ball is going go.

Big rockets, as in ships into space, are a special case where "ballistics" is really really misleading. This type of rocket, dynamically, is like trying to balance a pencil on its point... it is impossible. All rockets of this type have one or two types of steering. The first is gyroscopes that can twist the rocket about its center of mass and correct the direction it is pointing. This approach only works with relatively light rockets and the gyro need to have a mass that is significant relative to the rocket's mass, which is a huge problem with space craft since the fuel cost to lift the massive gyro becomes prohibitive. That leaves the true technological advance that the rocket scientists of the 40's, 50's and 60's had to figure out -- you must be able to gimbal (steer) the rocket engines so you can point the thrust in different directions allowing the engines themselves to keep the rocket pointed in the right direction (keep the pencil balanced on its point). The direction of pointing is often determined by comparison with a gyro, a gyro that is way to light to actually turn the rocket itself, or by reference to ground tracking data. Spin won't work since it is too hard to get the rocket spinning before lift off. Fins don't work because at launch the rocket is moving to slowly for aerodynamics to have much effect. Having the weight forward doesn't work because most of the weight is the fuel, which is in the bottom of the rocket.

So all the heavy lifters, from Gemini to Apollo (Saturn V) to the Space Shuttle to ICBMs have steerable engines. Below are some videos showing the Shuttle's gimbaled engines exercising right before lift off. The gimbal engines perform two tricks; they keep the "pencil balanced on its point" and they adjust the flight path to follow the precomputed trajectory. "Ballistics" is acquired by actively correcting the flight path, not by an intrinsic property of the rocket. These rockets intrinsically have no capability to follow a ballistic trajectory. (We've all seen the rocket failure videos from the 50's and 60's, many of those failure are failures of the steering system and the rocket behaves like all unsteered rockets will behave, they pivot, corkscrew, turn 180 degrees and generally do not behave "ballisticlly".)

The shuttle, at about 27 seconds you can see the nozzles get moved by the engine gimbals in preparation for lift off.

In the video below, at about 10 seconds the gimbals are tested, if they don't work the launch will be aborted because no gimbals means dead astronauts about 3 seconds after liftoff because a rocket being propelled is aerodynamically unstable and not ballistic.

A rough idea just how far the Shuttle's engine's gimbals can swing the engine, it is a pretty long way considering how much thrust is going out the nozzle at launch (this is a test without the engine running);

The massive Saturn 5 engines of the Apollo program were gimbaled, as were the engines for the 2nd and 3rd stages.

"Cruise missiles" use gyros, fins and vectored engine thrust to keep pointing in the right direction, all actively controlled to follow a particular trajectory. They are not ballistic and without active guidance would be hard pressed to hit a particular continent let alone a particular building.

That's probably more about missile stability than most care to hear.
Reply With Quote