Dynamic Materials Testing for Ultrahigh-Speed Spaceflight

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Recent technological developments have signaled that a new era of very high velocity spaceflight is emerging, with the potential to enable rapid interplanetary transport and a roadmap to eventual interstellar flight. Spaceflight at these speeds opens up new regimes of dynamics as the spacecraft encounters micrometeoroid impacts and high launch accelerations. For example, the Parker Solar Probe will encounter dust grain impacts at speeds of 300 km/s, more than 30 times faster than laboratory testing of micron-sized impacts is capable of simulating. The impacts occurring under these conditions will enter previously unexplored regimes of high energy density; the coming years will see even greater spacecraft velocities. At present, the most promising technology for rapid space travel appears to be directed energy, wherein a ground-based phased-array of lasers act as a single, diffraction-limited optic capable of applying energy to a target out to distances of millions of kilometers. This technology has been enabled by the photonics revolution that has seen a Moore's Law-like decrease in the cost of fiber-optic-based laser amplifiers, which permits the beamer to be developed in a modular fashion. The energy can be used to power high-thrust electric propulsion on-board the spacecraft or can apply direct photon pressure onto a thin film. The former concept is the subject of a proposed NASA-funded institute for Revolutionary Propulsion for Rapid Deep Space Transit, while the latter approach is the basis of Breakthrough Starshot. In the direct drive approach, dynamic photon loading onto thin (sub-um) films can result in accelerations of 100,000 g. At present no testing methodologies exist to test the response of the thin films to dynamic loading that would be used. This research program will pioneer the development of testing capabilities for the complete simulation of dynamics in this ultrahigh velocity regime, including the dust grain impact problem and the response of thin films to high dynamic loading. Gasdynamic techniques enable transient loads to be applied to thin films that are quantitatively matched to the conditions of the driving laser, but without having to use a 10 MW-class laser array. Specifically, a shock tube results in dynamic loads being applied to the sail that are essentially identical to those encountered with a direct laser-driven lightsail. Using techniques pioneered in my lab, including Photonic Doppler Velocimetry, would enable the dynamic response of thin film materials under representative loading to be investigated experimentally. The use of energetic materials to implode a thin-walled evacuated cylinder has the potential to drive a jet of wall material forward at velocities greater than 100 km/s. This program will systematically study this phenomenon using modern diagnostic tools and computer simulation, with the goal of being able to use these jets to reproduce impact events at velocities of interest in the laboratory.