Motor performance

Motor Performance

Total impulse (newton-seconds)
Letter code

1.26 - 2.50 A
2.51 - 5.00
B
5.01 - 10.00
C
10.01 - 20.00
D
20.01 - 40.00
E
40.01 - 80.00
F
80.01 - 160.00
G
160.01 - 320.00
H
320.01 - 640.00
I
640.01 - 1,280.00
J
1,280.01 - 2,560.00
K
2,560.01 - 5,120.00
L
5,120.01 - 10,240.00
M
10,240.01 - 20,480.00
N
20,480.01 - 40,960.00
O
Classification of motors according to total impulse

Whatever the type, there are a number of different aspects to a motor's performance - thrust (maximum and average), total impulse and burn time. Thrust is, of course, important, not least because it determines how heavy a rocket the motor can lift.

However, it is the total impulse that has most effect on the altitude the rocket will reach. Total impulse is simply the average thrust multiplied by the burn time. Different types of motor, as well as variants of a particular type, may have similar total impulse but different thrust profiles. For this reason, it is usual to classify motors by a letter code, representing its total impulse.

Each letter represents a doubling of the impulse range, so an E motor will have around twice the total impulse of a D. Note, though, that this does not mean the thrust is doubled - the impulse can be doubled by doubling the burn time for the same thrust. In fact, an Estes D motor has more thrust than an E. The letter codes are shown in the table.

The thrust profile of a motor is also important - it affects the capacity of the motor to lift a rocket of a given weight, and also how much load is applied to the motor mount. If the motor produces more thrust than the mount is able to withstand, it will break free in flight, usually destroying the rocket. This is known as a 'shred'.

Motors that produce a high thrust initially, then reducing thrust as the motor burns are called regressive motors, and those that show an increasing thrust with time are called progressive.

A motor that produces essentially constant thrust over the whole burn is called neutral. For any particular impulse (measured in newton-seconds), almost any combination of thrust and burn duration are possible, provided the product of the 2 equals the total impulse.

Thus a 280Ns H motor could produce 280N average thrust for one second, 140N for two seconds, or 560N for half a second, or any other combination, depending on how the motor grain and nozzle are designed.