Several years ago I decided to see if I could figure out how high a rocket would fly based on weight and motor thrust.
After alot of research and trial and error, I came up with a fairly accurate set of math equations that predicted rocket
altitudes very close to the data published by rocket kit manufacturers. Next came the task of writing the computer software
to input the rocket / motor data, process the calculations and then send the data to the display for viewing.
The result of all this effort is RocketFlight.
to access a .zip folder containing a copy of the program, support documentation and data files for all current NAR and TRA
certified motors. Once the file window opens, on the toolbar click "File" then "Extract" to open the Zip Extraction Wizard.
Please be sure to extract the files to a new folder in the root directory on the boot up drive (usually the C drive).
Apart from good sound construction techniques, the 2 most critical aspects of rocketry are thrust/weight ratio and stability.
The first is pretty self explanatory - making sure the motor you have selected is powerful enough to lift the rocket as well as
accelerate it to the minimum speed required to be stable before it leaves the launch rod/rail. The generally accepted
rule of thumb is 5:1 - or 5 units of thrust per 1 unit of mass. Stability is a bit more complicated and is determined by the
relationship between the rockets center of mass and center of aerodynamic pressure. All objects are subjected to the laws
of gravity regardless of size or shape. In the case of rockets, they will balance at a specific point along its length. This
point of balance is called the center of mass. Once set in motion by the motors thrust, another force begins to act upon the
rocket. This force is caused by the airflow around the rocket and across the fins. The total combined force exerted on the
rocket in motion is called aerodynamic pressure. As a rocket moves through the atmosphere the total of all aerodynamic forces
will be focused on one point along the length of the rocket. This focal point is called the center of aerodynamic pressure.
The most basic example of stability is the common arrow. Let's start with a length of wooden dowel. If you throw it like
a dart or an olympic javelin what happens ? The dowel tumbles wildly until it comes to rest on the ground. This is because
the dowels center of mass and center of aerodynamic pressure are in the exact same spot and there is no stabilizing force.
Now add the feathers to the back end and try again. Now the arrow flies true. This is because the center of aerodynamic pressure
has moved to a point behind the center of mass. Rockets will act the same way. As long as the center of pressure is behind
the center of mass it will fly straight and safe. Finding the center of mass is easy. Install the motor you want to use and
balance the rocket from a piece of string or on the edge of a ruler. Mark it with a piece of tape. Calculating the center
of pressure requires a bit of math. In the early 1960's a NASA engineer named James Barrowman developed a set of equations
to calculate the CP using fin shape, location , size as well as the shape and lenth of the nose cone. I simply incorporated
Mr. Barrowman's equations into an XLS spreadsheet. It's pretty accurate - I use it for every rocket I have flown with no
unstable flights. Click Here to download the spreadsheet along with
a diagram and key for all the required measurements. The calculations will provide the CP in inches behind the tip of the
nose cone. Measure back along the rocket to that point and mark it with another piece of tape. If the CP tape mark is behind
CM piece of tape by a distance equal to or greater than the diameter of the body tube, you are good to go !
For me, building the occasional kit is still alot of fun especially when it's a really cool rocket. But for the most part
I still prefer to design and build my rockets from scratch. There are several excellent places where I can buy the things I can't
make myself like airframe and motor mount tubing. Once I get a basic idea for a new design in my head, I use Open Rocket to
"build" it in a virtual environment. Thes allows me to make sure the design will be stable and also lets me simulate flights
with different motors. Critical flight parameters like weight, center of mass and pressure, max velocity and acceleration will
all dictate how I build the rocket. Things like fin size and shape, airframe length and parachute sizing can all be adjusted
and the new stability and performance data will change in real time along with the changes. It's a great program and it's free !
Click Here to get to the SourceForge Home page then click "Download"
in the menu box on the left side of the page.