An approximate expression is found for the penetration rate of a flexible penetrator, modeled as two masses joined by an elastic member with damping, with applied bias and harmonic forces. Modified Coulomb friction is used to characterize soil resistance. The approximate theory is compared with analog computer measurements and with an exact theory (developed for the limiting case of zero front mass and internal damping) and is found to give good agreement in lite range of small excess harmonic force amplitude. A comparison with a rigid penetrator is also made, and the flexible penetrator is shown to have the important design advantage of smaller weight or smaller applied harmonic force for equal penetration rates.