Several tools can make the job of range safety officer (RSO) easier. One tool is a thrust-to-weight chart.
A thrust-to-weight chart allows an RSO to quickly check if a motor will provide sufficient thrust for a stable flight off the launch pad (rod or rail). The rule-of-thumb for “standard” launch rod and rail lengths is a thrust-to-weight ratio of 5:1. The guideline works in most cases. A lower thrust-to-weight ratio can be used when the wind speed is very low to zero or when a longer than typical launch rod or rail is used. Higher winds may require a higher thrust-to-weight ratio to ensure a stable flight.
The charts are used by finding the rocket’s weight (including motor) along the Y axis, and the motor’s average thrust along the X axis. If the intersection is on or below the green line then the rocket and motor combination fit within the recommended 5:1 thrust-to-weight guideline. Rockets that fall in the “marginal” area should use a longer than typical launch rod or rail or be flown when the wind is very low. Rockets that fall in the unsafe zone should not be flown. (The motor should be replaced with one that produces more thrust.)
A test of thrust-to-weight is not the only test that should be performed. A motor can have sufficient thrust to produce a stable flight off the launch pad, but insufficient total impulse to boost the rocket high enough to ensure a safe recovery. The expected altitude also needs to be checked against the cloud deck and FAA waivered altitude.
Two thrust-to-weight charts are provided that can be downloaded and printed. One chart is for small rockets weighed in ounces. The other chart is for large rockets weighed in pounds. Print them using landscape format.
Submitted by: Dean A. Roth