Thrust Stand Micromass Balance for the Direct Measurement of Specific Impulse

A technique has been developed to directly measure the specific impulse from pulsed thruster systems. The technique is especially useful for propulsion devices that use solid propellants, for which a direct measurement of the propellant mass flow is extremely difficult. A torsion balance is used with a horizontal axis of rotation. A thruster is placed on the balance such that the impulse of the thruster firing and the change in mass due to the expelled propellant act in the same direction. A combined impulse and steady-state force measurement (due to propellant mass loss) can then be decoupled to assess the ratio of the impulse to the weight of propellant expended, or the specific impulse. A model has been developed to show the utility of the technique for pulsed systems with a firing time less than the natural period of the balance. An experimental proof of principle study was also undertaken using the laser ablation of engineering-grade Buna, Viton, and Teflon propellants. Specific-impulse measurements on the order of 200 s have been demonstrated with this laser ablation thruster.