Monday, January 29, 2007

What do the numbers and letters on rocket engines mean?

Have you ever wondered what the numbers and letters on model rocket engines mean? Most people assume that the higher the letter (A, B, C, D, etc.), the more power the engine has. Although this is true, there is quite a bit more to these designations.

The letter is a power classification and refers to a range of total impulse measured in Newton-seconds. Each subsequent letter has twice the power of the previous letter. For instance, a B engine classification may have 5 Newton-seconds of impulse, C would have 10, D 20, and so on. These letters actually refer to a range of impulse and different values of impulse that fall in that range will all be classified under the same letter. For example, if the B engine had 5 Newton-seconds of impulse and the C has 10, any engine with an average thrust >= 5 and <= 10 would be classified as C. Any engines >= 10 but <= 20 would be a D, etc.

The first number following the letter is the average thrust of the engine measured in Newtons. The number after the dash is the delay time in seconds measured from when the engine burns out to when the ejection charge fires. There are high-thrust and low-thrust engines (number after the letter) in all alphabetical categories of engines. Here are some general facts about high thrust vs low thrust engines.

High Thrust Motors
  • Faster lift-off speed
  • Higher acceleration
  • Better for bigger or heavier rockets
  • Better for breezy days because they fly straighter
Low Thrust Motors
  • Flies higher
  • Slower lift-off speed
  • Lower lift-off speed
  • Better for smaller and skinnier rockets
  • Better for calm-wind days; they weathercock more into the wind.

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