Engineering topics

Barnett, J. W.

Publications and source records attributed to Barnett, J. W..

3 records

NASA NTRS1990

The 30-kW ammonia arcjet technology

The technical results are summarized of a 30 kW class ammonia propellant arcjet technology program. Evaluation of previous arcjet thruster performance, including materials analysis of used thruster components, led to the design of an arcjet with improved performance and thermal characteristics. Tests of the new engine demonstrated that engine performance is relatively insensitive to cathode tip geometry. Other data suggested a maximum sustainable arc length for a given thruster configuration, beyond which the arc may reconfigure in a destructive manner. A flow controller calibration error was identified. This error caused previously reported values of specific impulse and thrust efficiency to be 20 percent higher than the real values. Corrected arcjet performance data are given. Duration tests of 413 and 252 hours, and several tests 100 hours in duration, were performed. The cathode tip erosion rate increased with increasing arc current. Elimination of power source ripple did not affect cathode tip whisker growth. Results of arcjet modeling, diagnostic development and mission analyses are also discussed. The 30 kW ammonia arcjet may now be considered ready for development for a flight demonstration, but widespread application of 30 kW class arcjet will require improved efficiency and lifetime.

Deininger, W. D.↗

NASA NTRS1985

Onset phenomena in MPD thrusters

An experimental study has clarified some aspects of MPD thruster onset phenomena. The steep increase in terminal voltage that occurs as the onset current is approached may have different causes, depending on the propellant injection geometry. For propellant injection at the cathode radius, terminal voltage increase corresponds to a growing anode fall voltage; for injection at a larger radius, the increase is related to the back emf in the near-cathode plasma. The formation of the onset current pattern within the arc has been mapped experimentally as the thruster responds to an input current step which rises from below onset to the onset value. The appearance of terminal voltage hash at onset correlates with the extension into the exhaust region of a significant fraction of the arc current.

Barnett, J. W.↗

NASA NTRS1984

MPD thruster operation with stepped current input

Experimental study of the response of a quasi-steady MPD thruster to an incremental increase or decrease in current illuminates the processes by which the discharge adjusts to changes in its operating level. For changes well below the onset condition, the thruster maintains quasi-steady operation throughout. Changes imposed near onset produce transition periods of non-quasi-steady operation. The duration of the transient appears to be determined by magnetic field diffusion in the portion of the discharge downstream of the thrust chamber.

Jahn, R. G.↗