Sorry for the dearth of posts, been a bit busy lately with some really interesting work. Here's a repost of an interesting little application for computing the letter class of your solid rocket motor, from 1/4 A sized up and beyond that of the SRBs.
Have you ever launched a model rocket? If you have, then you know that when you buy
your rocket motors, they have a letter classification associated with
them. This classification system was developed and refined by the National Association of Rocketry. For example, you might buy a
“C6-3” motor from the store. The letter C
tells you the impulse class of the motor (the “6” in this example is the
average thrust in Newtons, and the “3” is the time delay in seconds between
motor burn-out and the firing of the parachute ejection charge).
Each time you go up a letter, you double to total impulse
delivered by that motor. Thus, a “C”
motor would have roughly twice the total impulse of a “B” motor, etc.
A through D class motors are considered “Low Power”
motors. E through G are considered “Mid
Power” motors. Motors starting at the H
class and above are considered “High Power” motors, and require a certification
procedure to purchase and use. Rocket
motors as high as an O class can be purchased commercially. Above O, and you’re in the realm of
professional and military solid propellant rocket motors.
However, using the same classification logic, one can calculate the motor class of any solid rocket motor, given its average thrust, and it burn duration. Thus for example, each of Aerojet motors used as boosters on the Atlas V has a nominal thrust of about 1270 kilonewtons (285,500 lbf ), and a burn duration of about 94 seconds. With a total impulse of around 135,000 kN-sec, this motor could be classified as a AA motor (once you get up to "Z", then you start over again with "AA", etc.) The Solid Rocket boosters on the Space shuttle (the SRBs), would be about an EE class motor.
Attached is a little spreadsheet for determining the letter classification of a solid rocket motor given its average thrust and burn duration. Like all my spreadsheets, the blue numbers are ones you enter, the black ones are the calculated values. Simply enter the thrust and the burn duration, and it’ll tell you the letter classification of your entry.
Download Motor class calculator
Exercise for the interested student: What is the motor classification for the Ares I 5-segment SRB?
I would say the 5 segment is about the same as the 4 segment, both EE. My computer didn't access the calculator for some reason, probably operator error.
Posted by: john hare | August 28, 2009 at 01:54 AM
John, my friend, there's no need to guess at the answer, that's what the calculator's for!
For the 5-segment SRB, try using 3,600,000 lbf of thrust and a burn time of 120 seconds.
I'll email you the .xls file.
Posted by: John Bossard | August 28, 2009 at 07:13 AM