One of the most critical parts of the system in limiting the peak accelerations experienced by the occupant's head is the head rest.
Designing the head rest is a good introduction to the differences between occupants. The head of a mature man weighs approximately 10 pounds. The head of a pre-teen weighs approximately 6.5 pounds. A system designed for a 12 G decel of an adult male head will result in an 18 G decel in the pre-teen's head...a nogo condition.
Plastic hinge theory suggests that a rectangular, ductile beam in bending will max out at a moment equal to 0.25*Width of beam*Yield stress of material*thickness^2.
The problem is that younger, shorter, lighter occupants lower the headrest and the bending moment DECREASES, thus raising the force level generated by the plastic hinge.
Having a stepped or tapered guide rod partially addresses this issue.
|Position of head rest with a tall person, presumably with a heavy head.|
|Position of head rest with short person. The effective beam length is longer due to the increase in effective length.|
|All units are inches. Bogey is 288 inch-pounds for a six pound head and 8" guide rods.|
One might reasonably expect to get a travel of 4" from 8" long guide rods in bending. From a budgeting standpoint, we need about 30" of launch-pad to accelerate the occupant to the velocity of the bullet vehicle and a well designed head-rest gets us almost 15% of the needed distance AND gives us a way to positively discriminate between low-mass and high-mass occupants. And it is cheap and does not weigh much.