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Issue
98, September 1998
Smart
Rockets - Data Acquisition in Model Rocketry
by
Tom Consi & Jim Bales
Start
• Introduction to Model
Rocketry
• Rocket Science 101
• Control & Data Logging •The
Accelerometer •
Software • Power
• System Construction •
Launch Control Box
• Results
• Future Developments
• Software
& Sources
ROCKET SCIENCE 101
Newton’s First Law,
F = ma, contains all the physics you need
to predict the accelerations that the system will record.
All we need is the mass of the rocket (m) and the
applied force (F), right?
The force exerted by a rocket
engine is not constant as you see in the thrust versus
time curve for an Estes C6-5 engine (the dashed line in
Figure 1). We assume the rocket moves straight up while
the thrust is applied, so the total force is the engine
thrust minus the gravitational force (mg, g = 9.8 m/s2).
If the mass of the rocket
were fixed, then the acceleration is the dashed line scaled
by the mass. However, as the propellant burns, the mass
of the rocket decreases.
How important is this effect?
For NASA, it’s critical—85% of the space shuttle’s
take-off weight is fuel!
In our case, only 12 g of
the 105-g take-off weight is fuel (11%). So, we can treat
the rocket mass as a constant, which gives us the expected
acceleration curve shown as the solid line in Figure 1.
Our students made this calculation
on the first day of class. Their task for the semester
was to determine, by direct measurement, if Figure 1 is
correct.
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