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Name: Daniel
Status: Student
Grade: 9-12
Location: WI
Country: United States
Date: Winter 2009-2010


Question:
If I were to build a rocket for a class project would the size of the fins of the rocket have to be built to a certain ratio to the length and girth of the rocket?



Replies:
Hi Daniel,

For your model rocket, naturally there will not be any fancy motor that swivels, or guidance system, so the simplest way to keep it on course is to use tail fins (many "real" rockets have no fins at all). Unfortunately there is no magic formula to determine size and shape of the fins. The best size and shape is dictated by the characteristics of the rocket itself. The optimum size of fin is that which keeps the rocket stable in flight, but is not so large to add excess drag. This is very much a trial and error process, but it is better to make the fins a little larger than necessary, since adding a little more drag is always better than fins that are too small and cause unstable flight. In any case, the fins must be located as far to the rear as possible, in order to ensure the center of pressure of the fins is as far behind the rocket's center of gravity as possible. This will help minimize the fin area needed for stable flight.

Regards,

Bob Wilson


Daniel

It depends on how big your rocket is and what you intend it to do. That is, do you intend it to enter earth orbit, or just go high into atmosphere and fall back.

The calculation for rocket fins involve the following parameters: Mass of the rocket Speed of the rocket The desired aerodynamic forces on the fins of the rocket to steer it.

Looking at the space shuttle, On launch the fuel in the large liquid fuel tank strapped to the belly of the shuttle is burned And the assembly (shuttle, fuel tank and two solid fuel rockets strapped to the side) is steered by the gimbaled engines at the back of the shuttle. My guess is that during the launch phase the aerodynamic surfaces (the vertical rudder and ailerons on the trailing edges of the wings) of the shuttle are locked to neutral because the very high speeds of the launch would put such high forces on the aerodynamic surfaces that they would break off.

The aerodynamic surfaces of the shuttle are used during return flight which is a much slower speed.

Here is a great description of the flight profile of the space shuttle:

http://en.wikipedia.org/wiki/Space_Shuttle#Mission_profile

From this article I find that shuttle speed at end of the launch speed profile is 7.68 kilometers per second 27,650 km/h (17,180 mph), roughly equivalent to Mach 23 at sea level.

When the approach and aerodynamic landing phase begins, the orbiter is at a 3,000 m (9,800 ft) altitude, 12 km (7.5 mi) from the runway. The pilots apply aerodynamic braking to help slow down the vehicle. The orbiter's speed is reduced from 682 to 346 km/h (424 to 215 mph), approximately, at touch-down (compared to 260 km/h (160 mph) for a jet airliner).

Enjoy reading the rest of the article.

Sincere regards,
Mike Stewart


No, Daniel, that ratio varies widely in model rockets. Some kits are long and thin like spears, others short and fat.

It is somewhat easier to get a long-and-thin rocket to go straight. I made a model rocket from a plastic Easter-egg once, about as short-and-fat as it gets, and I had to add nose weight inside to get it to be aerodynamically stable. The fat middle of my egg probably shadowed much of the trailing fin area from the airflow.

What is important is to get the Center-Of-Drag (COD) to the rear of the Center-Of-Mass (COM). To do that, it is usually important to make a design in which the fins are mounted as far back as they can go on the body, then make them as big as needed to get C.O.D. behind C.O.M.

And if you build the whole thing and this design method fails and it is unstable, add some weight (up to 1/2 oz.) in the nose cone or otherwise close to the front end. Then your rocket does not go quite as high, but at least it goes straight up instead of looping or crazy.

There are about two ways to estimate whether you have this stable situation:

1) swing it on a string, and see if it spontaneously points forwards all the way around the circle. Tie the string snugly around the midsection, slide it back-and-forth until the rocket hangs level, then swing it around you in a big circle. Probably need 4-6 feet of string. The most stable rockets will point forwards quickly at low speeds. Unstable rockets fly sideways to the wind or tumble randomly. Hanging on a thread several feet in front of a big fan (20" box-fan comes to my mind) might also work, and good weather-vane action would be more self-evident to a student.

2) sketch-on-paper: draw the side-view outline of the rocket on a piece of paper, to scale. perhaps use paper with 1/4" squares, measurements scaled down perhaps 2:1, or a computer printout of the kit-manufacturer's side-view picture of the rocket, scaled so the printed picture is 1/2 the size of the real rocket. On this figure you mark the C.G. (Center of Gravity, synonymous, in this case, with my "C.O.M.") and the C.O.D. You can find the C.O.M. on a finished rocket by hanging it from a short string as above, or by finding the almost-balancing-point on the edge of a ruler or side of your finger. and you estimate the C.O.D. as the center of area of the figure on the paper. The squares on the paper really help here. Draw a vertical line through the rocket so that, of the squares inside the rocket outline, half are to the left, and half are to the right of the line. This is a more intellectual process, though, and can be inaccurate.

Loading your finished rocket with an engine in the rear end always moves the C.O.M. rearwards, and this is the real flight-condition which must be stable. As the fuel burns away, the rear looses mass and the rocket may get more stable.

Be aware of the "weathercocking" phenomenon. If you launch in a sideways wind, any good stable rocket will turn into the wind as it launches. After all, the wind it experiences is the combination of its upwards speed on the launch rod and the sideways wind speed. Being a weather vane, it turns to point into that perceived net wind. Then it traces a straight line diagonally upwards, instead of going straight up.

That is about what I know about it.
I would encourage you to mess around with it.

Jim Swenson


Hi Daniel, there are tons of 'how to design rockets' web sites on the web. I suggest you start with some of these sites to understand the fundamentals of rocket design. The short answer is that fin length is by far not the most important consideration, but the weight, shape, and position of the fins do affect the balance of mass on the rocket, and that balance is very important.

Hope this helps,
Burr Zimmerman



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