2. EXPERIMENTAL METHOD

2. 1. IMPACT EXPERIMENT APPARATUS

Impact experiments of the penetrators on lunar-regolith analogue target were made using a gun facility at the Institute of Space and Astronautical Science. The most important feature of this facility is that a gun to accelerate the scale-model of the penetrator itself is mounted on a rotation wheel driven by a hydraulic motor system (Figure 1 ) .

Fig.1.(a)Photograph of the entire experiment failities is seen at the central part,gun barrel at the right side, and sand container at the liwer right side.(b) Photograph of th gun barrel mounted on the rotation wheel.A model penetrator and an ignitor are already set in the breech and not seen.

To accomplish a reliable shot (the incidence angle must be lower than 40^° , otherwise the penetrator will be shot outside the sand container) from the rotating gun, an appropriate triggering method and the recording system were developed as shown in Figure 2. A Iatching relay circuit is connected with an ignitor in the breech through a slip ring mounted on the spin motor. To trigger the ignitor, two optical spin sensors (Ono Sokki K.K., LG-916) and a pin-board sequencer (PRG-2018T) are used. Both the spin sensors are oriented parallel to the wheel's spin axis and point the edge of the wheel' 2 cm away from the surface (Figure 2b). They are set so as to make an angle of 120^° each other with respect to the spin axis.

Fig.2.Schematic diagram illustrating the trigger method and recording system. At the left side are shown the photographs of (a) an optical spin sensor and reflection markers to measure the wheel's spin rate, (b) two optical spin sensors for triggering, and (c) two magnet coil sensors to measure the impact velocity.In photograph (b), each reflection maker on the wheel's surface is viewed by separate optional spin sensors,which are set at the 120° phase angle.

Because LUNAR-A penetrator is planned to hit on the lunar surface at an impact angle of 90^° , the shot must be adjusted so that the impacts are made at normal incidence. Considering a slight delay time from the ignition to the releasing of a penetrator from the gun barrel, we intentionally make the ignition slightly in advance, whose timing was determined by preliminary experiments; the time lag was obtained to be 30 to 40 milliseconds, which is equivalent to the 63±9^° rotation angle in case of the spin rate of 5 Hz. This trigger method allowed us to control the impact angle ranging from 80^° to 100^° in almost all the experimental runs.