Model Forum / General / Rockets / November 2005

Thanks guys.... this really helps. I guess what I am after then is total
thrust an engine produces from start to finish...
Am I right in assuming... ("C" and "D" engine comparisons) total thrust in
this example?

Can I assume this is correct for getting total thrust?

"C"... 10/4.45=2.247 Total thrust (lbf)
"D"... 20/4.45=4.494 Total thrust (lbf)

Richard

You need to look at the thrust curves and other data on the NAR website.
This will help clarify the difference between "thrust coming out of the
motor at any given point in time" vs. the "total impulse delivered by the
motor over the entire burn".

The letter class of the motor is only the POSSIBLE total impulse range *if*
they packed the motor to the maximum allowed in that class. You can have a D
motor that is *not* a "full" D (like the Estes D12). Look at the NAR data
for various Aerotech D motors for a good comparison. Many are a full 20
Newton-seconds of total impulse, but some burn slower with a lower thrust
level for a longer time, while others burn faster with a higher thrust level
for a shorter time. That is why the number after the letter is important: it
tells you the average thrust in Newtons. Divide that number by 4.45 to get
the average thrust in pounds.

BUT you should also look at the curve, since many motors have large
variation in thrust over time, and the initial peak thrust is also very
important to get the rocket off the launch rod fast enough to fly stable.
Compare many curves on many motors to see the wide variety available.

Heck, look at the not-so-subtle differences between the D12 and D11 and the
C11 and E9 from Estes. They get the differences with different nozzle
diameters and different counterbore depths.