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264 MEMS and Microstructures in Aerospace Applications

11.4.1 PRINCIPLE OF OPERATION
The microsatellite obtains a thrust as the radioisotope emits the alpha or beta, or
both particles.
The thrust produced can be calculated as:
r ffiffiffiffiffiffi
2E
10
T ¼ _ mmv ¼ (A(3:7 10 )m P ) (11:33)
m P
with A representing the radioisotope activity in C i , m P standing for the particle mass
and E being the kinetic energy of the particle.

11.4.2 SYSTEM REQUIREMENTS
The system itself is extremely simple without feed mechanisms and only an
optional shutter. The propellant mass can be minimized by selecting the adequate
material. Alpha particles are easy to shield and therefore shielding should not
require significant amounts of additional mass. A summary of the radioisotope
propulsion system is shown in Table 11.11.

11.5 CONCLUSION
Ten different propulsion systems have been introduced, covering a large range of
requirements for small satellite propulsion. It should be mentioned that while
MEMS is an enabling technology for many of these systems, a significant amount
of research needs to be done on the systems level to approach high technical
readiness levels (TRL) that will lead to use of these technologies in upcoming
missions. In September 2003, DARPA had initiated an effort to push MEMS
technology into a different venue — that of high power electric propulsion. Field
effect thrusters like colloids and FEEP systems with power levels of up to 1 kW

TABLE 11.11
Radioisotope Propulsion Characteristics
6
3.9.10 sec 5.67 MeV alpha particles
I sp
I-bit (if pulsed) 1.55.10 4 mNsec Pulse rate is limited by valve — assume
1 KHz MEMS valve 1 msec
Rep. rate (if pulsed) 1 kHz 1 kHz MEMS valve is an option
Power 0 W—no valve/1 mW with valve 0 W required to generate particles
Thrust 0.155 mN 100 Curie of Pu-238
Thrust or power Infinity mN/W (0 valve), Without valve power consumption is zero,
155 mN/W (with valve) With valve operating at 1 mW
Impulse or prop. 1.58 10 5 Nsec/g
Total impulse/sys.-mass 428 Nsec/2g
Current system mass 2 g (with valve)

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