Comparison of ablation performance in laser lightcraft and standardized mini-thruster

Experiments on laser ablation of black and white Delrin® with a 10.6-micron laser beam from a CO ₂ electric discharge laser are reported. Mass ablation and thrust generation (impulse) were accurately measured as a function of input laser energy in single-shot experiments. The efficiency of conversion of laser energy to jet kinetic energy depended on the geometry of the energy absorption/conversion zone. The standard geometry, an axisymmetric mini-thruster with 13-degree conical half angle and 8:1 expansion ratio, produced ∼65 % conversion efficiency. The extensively-studied 10-cm focal diameter Lightcraft (with inverted paraboloid, plug-nozzle geometry) produced ∼50% conversion efficiency. The upper limit to energy conversion was theoretically computed with a one-dimensional chemical equilibrium code to be 73% for the well-defined mini-thruster geometry. Thus, total losses amount to ∼8% in the mini thruster and ∼23% in the Lightcraft. In these experiments a near-exact match of coupling coefficients, ∼1%, was achieved in a 14-fold scale-down of the 10-cm focal diameter Lightcraft to the mini-thruster. These results helped validate the concept of "momentum calorimetry" in which heats of formation of energetic propellants are obtained by measurement of the momentum of the jets they produce when they are ablated with known laser energy. The performance of chemically-enhanced laser ablation propellants or other solid propellants may be inexpensively measured on a small scale. A matrix of impulse measurements, ablated mass, laser energy and plume absorptivity was carried out with various mini-thruster expansion ratios and chamber geometries.