Altitude Simulation
Tools
To estimate the expected
peak altitude of my rockets I use two different computer programs.
One is wRASP and one
is
RockSim. I find them both useful since each one has advantages
and disadvantages.
wRASP is very simple, quick and easy to use. (It's also free!)
I use it when all I want to know is the altitude or an optimum ejection
delay time.
RockSim is a much more complete design tool that not only predicts
altitudes, but also calculates center of pressure (CP), center of gravity
(CG), and many other useful pieces of information. It's available
from Apogee
for about $95. (By the way, I would highly recommend that all
rockets be simulated before first flight! This allows the liftoff
acceleration, launcher departure speed, peak altitude, ejection delay, and
CP to CG relationship to be checked prior to putting the flight hardware
and the people on the ground at risk.)
To predict how high a
rocket will go we need to know a few key parameters. Rocket
diameter, weight, motor thrust profile, and launch environment all affect
peak altitude. Another really important factor is the aerodynamic
drag of the rocket. This effect is described by a number called the
Coefficient of Drag and is commonly referred to as CD. I
have found that for all my rockets, if I simply assume CD =
0.45, then wRASP will give an estimated altitude that is very close to
what I measure during the actual flight. In reality the coefficient
of drag is not just a constant since it increases as the rocket approaches
supersonic speed. However, most of my flights are subsonic and
approximating CD with a fixed value of 0.45 yields very good
results.
To see how well wRASP
calculates the expected peak altitude, I compared the calculation to the
actual altitude for several of my rockets on a variety of motors.
The results are shown in the table below. All of these calculations
used a fixed value for CD of 0.45. The error was generally less
than 5%, but in one case was as large as 16.2%. All of the flights
listed in the table are ones which were observed to be "straight up" from
the ground. (Obviously one reason a rocket would not achieve the
predicted altitude is if it weather cocks and does not go straight up.)
The flights in the table are also ones which I had weighed the rocket just
before launch to get an accurate value for the liftoff weight. The
environmental conditions were also recorded for these flights at the time
of launch.
