Ground Handling Loads — theory

Overview

Ground handling loads live in the shadow of flight and parachute loads, but ground failures happen, and even when repairable they cost schedule. Two cases are known: bending of a slender rocket carried from each end, and tearing the aft riding button off while the rocket is slid onto the launcher rail.

Carried by hand

A rocket of weight W with its centre of gravity at x_{CG}, carried at x_F and x_A, is in equilibrium when

Split the rocket into parts of weight w_i at x_i and make a phantom cut at a possible weak point x_J, such as an interstage joint. The cut carries the shear and bending moment of everything ahead of it; while the rocket is carried it also bounces, so the static values are multiplied by a load factor N\approx2:

Riding-button extraction

To load the rocket, the rail is lowered to horizontal and the aft button is slid into the end of the 80-20 slot. Inserting the forward button, often near mid-body, is hard to see, so the crew raises the nose. A pitch-up of only 1.5–5° brings the nozzle exit plane into contact with the rail; beyond that the rocket pivots on the nozzle, and the aft button is levered out of the rocket like a nail under a claw hammer — with almost no feel.

Before contact, the nose lift and the aft button share the weight:

After contact the nozzle exit carries a reaction F_A and the button pulls with F_L. Moment and force balances give F_F(x_A-x_F)+F_L(x_L-x_A)=W(x_A-x_{CG}). The angle changes too little to warn the crew, but the force does: if the crew stops at K times the normal lift, a button designed to

will not be extracted. Absent better knowledge use K=2, design both buttons to the same load, and do not give this job to crew members who do not know their own strength.

Verdict and fixes

The memo gives the loads; what the hardware can take depends on how it is built, so the calculator asks for it: the pull-out strength of the aft button attachment and the bending strength of the joint (from a test or the fastener data). Each load is compared with its strength: safe up to 80 % of it, marginal up to 100 %, unsafe beyond. The 80 % is this site’s choice, on top of Hoult’s factors K and N; it is not a published limit.

The button pull (eq. 6) is linear in the lift factor, F_L=aK-b with a=W(x_L-x_{CG})(x_A-x_F)/[(x_L-x_F)(x_A-x_L)] and b=W(x_A-x_{CG})/(x_A-x_L), so the largest lift factor the button takes is

Brief the crew to stop well before it. The other fixes: the lift point that keeps the pull within 80 % of the strength (by bisection, when one exists), and the strengths needed, F_L/0.8 and |M_J|/0.8. A third carrier at the joint takes its bending moment to zero. Hoult also asks that the aft button carry KW in shear when the rocket stands on the rail, and that both buttons be designed to the same load with countersunk screws.

Verification

The tests reproduce the original field tool’s lift forces and button loads for K=1 to 3 (241.8 lb at K=2 for the 54 lb example, the memo’s “about 243 lb”), check the statics of the carried rocket, and confirm that the joint shear and moment are the same whether computed from the forward or the aft piece.

Notes on the method

• Joint moment. The memo writes the joint moment as N(F_Fx_F-\sum w_ix_i), which is the moment about the nose tip; it equals the moment at the cut only when the shear there is zero. The calculator takes moments about the cut, and the test checks them against the aft piece.

References

1. C. P. Hoult, “Ground Handling Loads,” RS&T, 9 July 2014. View in library
2. C. P. Hoult, “Sounding Rocket Structural Design Loads (rev. 9),” RS&T. View in library