Interdisciplinary Applied Mathematics

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Three different classes of microspacecraft have been proposed by (Mueller, 1997) depending on their size, mass, and power ranges. Class I spacecraft have mass in the range 4-20 kg and use mostly conventional hardware components with some possible MEMS components. The power density for Class I is of order 1 W/kg, and characteristic dimensions are in the range of 0.3-0.4 m. Class II microspacecraft are based on MEMS components, and

FIGURE 6.30. Sketch and typical dimensions in microns of a micronozzle.


they have mass of 1 kg and characteristic dimensions of order 10 cm. They have extremely miniaturized propulsion systems, both for delta-v maneuvers and for altitude control. Finally, Class III refers to microspacecraft with mass less then 1 kg and characteristic dimensions of order 3 mm or less.


The most important component for such microspacecraft is the propulsion system, which in the new mission scenarios plays an even more important role than previously. For example, propulsion capability is required on board the microspacecraft to adjust its trajectory. This points to the need for low thrust and quick response. The latter is expressed quantitatively by the    impulse    bit    (/bit).    This is the    minimum    impulse    obtained    once    the


thruster is given the command to fire, and it is the integrated thrust over the faster valve cycle time.


There are a number of micropropulsion designs built or proposed that are reviewed by (Mueller, 1997) and (London, 1996). They range from ion engines to hall thrusters to pulse plasma thrusters. These technologies have been used    mostly in    the    macro    domain,    but    cold    gas    thrusters    have    been


recently miniaturized successfully, and similar plans exist for microplasma thrusters (microPPTs) (Gulczinski et al., 2000). In particular, the cold gas thruster offers a low    Ibit    and a    thrust    in    the    range    from    1    p,N    to 0.2 N

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