  We report here on initial studies of a pulsed hydromagnetic plasma gun that
can operate in either a pre-filled or a gas-puff mode on demand. These modes
enable agile and responsive performance through tunable thrust and specific
impulse. Operation with a molecular nitrogen propellant is demonstrated to show
that the hydromagnetic thruster is a candidate technology for air-harvesting
and drag compensation in very low Earth orbit. Dual mode operation is achieved
by leveraging propellant gas dynamics to change the fill fraction and flow
collisionality within the thruster. This results in the formation of distinct
modes that are characterized by the current-driven hydromagnetic waves that
they allow, namely a magneto-deflagration and magneto-detonation respectively.
These modes can be chosen by changing the time propellant is allowed to diffuse
into the thruster based on the desired performance. Using time-of-flight
emission diagnostics to characterize near-field flow velocities, we find that a
relatively dramatic transition occurs between modes, with exhaust velocities
ranging from 10 km/s to 55 km/s in deflagration and detonation regimes,
respectively. Simulations of the processed mass bit offers a first glimpse into
possible thruster performance confirming a broad range and tradeoff between
specific impulse (2600 - 5600 sec) and thrust (up to 31 mN) when operating in a
burst mode.
