Robotic Lunar Lander Testing
Divert and Attitude Control System, or DACS, thruster comparison

(a) Test Setup in Vacuum Chamber at WSTF for DACS Thruster Hot-Firing and Comparison of Engine Envelope; (b) Conventional (LEROS 2B) Thruster; and (c) DACS Thruster. Image Credit: NASA
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On September 15, 2009, NASA successfully completed a series of hot-fire tests that demonstrated the ability of miniaturized thrusters to perform the descent and landing operations for a new generation of multi-use robotic lunar landers. The test included a mission profile representing a lunar lander duty cycle. This profile spanned 995 seconds and included pulses, coasts, and steady-state burns. The test program fully accomplished its objectives, including evaluation of combustion stability, engine efficiency, and the ability of the thruster to perform a lunar lander duty cycle.

This hot-fire test series, conducted at NASA White Sands Test facility, used a 100-lbf class Divert and Attitude Control System (DACS) thruster, shown in Figure (a). Figures (b) and (c) illustrate the dramatic size difference between a comparable, conventional thruster and the much smaller DACS thruster. In this test program, the DACS thruster fired under vacuum conditions to simulate operation in the space environment. The test matrix included various thruster pulsing durations, power levels, and propellant mixture ratios. The test series culminated in a 66-second continuous burn at full power.

To broaden the DACS thruster’s operating temperature range and evaluate its potential use in cold lunar environments, the test substituted MON-25 grade of nitrogen tetroxide for the originally intended MON-3 propellant. (MON stands for “mixed oxides of nitrogen.” The MON-25 grade includes 25% nitric oxide and has a much lower freezing point, compared to the more conventional MON-3, which nominally contains 3% nitric oxide.)

This test was part of a series of risk reduction activities undertaken by the Robotic Lunar Lander Development Team at NASA Marshall Space Flight Center (MSFC). MSFC engineers, with support from Pratt & Whitney Rocketdyne, are evaluating the use of missile-heritage, miniaturized thruster technologies to reduce spacecraft mass, volume, and production cost for future robotic missions. MSFC is considering this type of thruster as a descent engine for small lunar landers designed to be able to carry a range of science payloads and devices, including geophysical measurement instruments, volatile measurement instruments, or possibly lunar sample returns.