Abstract The use of a percussion hammer can provide high penetration rates through hard rocks when air drilling. The work of this paper was partly sponsored by the US Department of Energy to develop hammer drilling techniques for drilling deep hard rocks, using real drilling muds. Unfortunately, development is complicated because understanding rock breakage and cuttings removal under the dynamic loading conditions of hammer drilling is difficult. Historically, empirical relations have been used to relate hammer blow energy and blow rate with drilling. This paper presents results of carefully measured single cutter impact—as well as high rate and ‘static’—rock indentation tests under high borehole pressure conditions, using real drilling muds. The results include cutter force-displacement and crater volumes for both first stress wave (initial impact) and long time (quasi static) rock indentations. From this, specific energy for rock breakage (i.e. the energy to excavate a unit volume of rock) can be calculated. The specific energy values measured in these single cutter impact tests show the rock destruction efficiency that can be achieved by impact loading. They are compared with specific energies measured during full scale hammer and rotary drilling experiments, under relatively similar conditions. For a given rock type, impact stress, cutter configuration, wellbore pressure, and mud type are shown to be critical parameters. A key observation is that most of the rock breakage occurs during the first stress-wave, and for higher impact stresses above the dynamic confined indentation strength, relatively little additional rock breakage seems to occur.