The Correct Rammsonde Formula

Sir, The Correct Rammsonde Formula

In his derivation of the theory of the Rammsonde, Haefeli ¹ gave two formulae for the ram resistance:

and

He proceeded to use the first formula, because it is somewhat simpler, and everyone since has followed his example.

However, impacts in snow or firn are almost completely inelastic. When the weight strikes the top of the tube, an elastic wave travels down it. A completely elastic impact would occur only if the cone at the far end were standing on a completely elastic and finite material; in such a case the wave is completely reflected, and on returning to the top of the tube it causes the weight to move upwards. There is thus a very significant rebound, which can be calculated using the impulse law.

This is obviously not the case in practice. The rebound is very small even when the cone is resting on an ice Iayer, and is zero when the cone is in soft snow. The amount of energy lost is variable and unpredictable.

This fact has no important consequences if we use the Rammsonde qualitatively, but is most important if we use the Rammsonde resistance for quantitative studies, as does Bull ² . The only correct method of using a Rammsonde would seem to be:

1. to use W ₂ instead of W ₁ for the ram resistance,

2. to put a ring of soft material (rubber or lead ?) at the point of impact to ensure that the blow is always inelastic.

The omission of the factor R/(R+Q) in the first term of the ram resistance formula leads Bull to very high values of the frictional resistance. In fact, as Haefeli says: “by choosing the cone diameter slightly larger than the outside shaft diameter, the friction between shaft and snow can be practically disregarded”.