HOUSTON — Ad Astra Rocket Co. is reporting advances in the performance of its 200-kw Vasimr plasma rocket engine prototype and its first demonstration of Constant Power Throttling (CPT), a unique feature of the experimental powerplant.

Representatives of the seven-year-old commercial space propulsion company plan presentations on the developments at the 48th annual American Institute of Aeronautics and Astronautics Joint Propulsion Conference in Atlanta on July 30. Ad Astra was founded by former NASA astronaut and plasma physicist Franklin Chang-Diaz with the goal of reducing interplanetary travel times.

June characterizations of the VX-200, Ad Astra’s 200-kw prototype, revealed a 10% improvement in efficiency at intermediate values of specific impulse below the 5,000-sec. optimal point demonstrated at the company’s suburban Houston lab in 2009 and 2010.

The efficiency improvements were achieved through design improvements in critical engine components, “fine-tuning” the radio-frequency power system settings and upgrades to the software that controls the engine during startup and firing, Ad Astra says.

CPT enables the VX-200 to vary the exhaust parameters for thrust and specific impulse while operating at a fixed power level. Achieved by changing the relative fraction of power reaching the engine’s two stages, while simultaneously altering the propellant feed, CPT would permit an operational version of the company’s Vasimr (Variable Specific Impulse Magnetoplasma Rocket) concept to both increase payload mass and decrease trip times, according to Ad Astra.

At relative power ratios of two or more, the June testing indicated no significant loss of efficiency.

“The recent data represent one more step in the full characterization of the engine and validate the assumptions on which our business models are based,” Chang-Diaz said in a statement.

In May, Ad Astra expanded its Space Act Agreement with NASA to initiate the safety, reliability and mission assurance phase of a prospective 2015 test flight of a Vasimr prototype, either to the International Space Station or on a free-flyer, for three years of in-space characterization.

Ad Astra envisions a role for solar-powered versions of Vasimr in the mitigation of orbital debris and the delivery of supplies to astronauts at lunar Lagrange points or exploring near-Earth asteroids. Theoretically, a nuclear-powered version could cut the 7-10-month transit to Mars using conventional propulsion to less than two months.