1. Solid Propellant Micro-rockets: Application, Design and Fabrication ME 381 Final Presentation 12/12/02 Northwestern University Nik Hrabe Albert Hung Josh Mehling Arno Merkle

2. Motivation Mechanical-based MEMS High thrust-to-weight ratio Guidance systems Miniature satellites Integrated sensing systems (Smart Dust)

3. Outline Microrocket Comparisons –Turbine Engine –Gaseous Propellant Rocket –Solid Propellant Rocket Case Study: Solid Propellant Rocket –Fabrication –Materials Considerations –Geometric Considerations –Performance Conclusions

4. 3 Major Categories of Micro-Rocket Turbine Engine Gaseous Propellant Rocket Solid Propellant Rocket

5. Micro Gas Turbine Engine Characteristics –2 cm x 2 cm x 4 mm Advantages –Well Tested Disadvantages –Moving parts –External Fuel Supply Required –Complicated Design and Fabrication –Space Applications are Limited

6. Micro Gas Turbine Engine

7. Gaseous Propellant Rocket Characteristics –18 mm x 13.5 mm x 3 mm –Thrust-to-Weight Ratio = 85:1 Advantages –No Moving Parts –Efficient and Powerful Disadvantages –External Fuel Supply Required –Slow Fabrication Process

8. Gaseous Propellant Rocket

9. Solid Propellant Rocket Characteristics –1 mm x 1 mm x 1 mm –Energy Density = 5 J/mm 3 Advantages –No Moving Parts –Self Contained Fuel Supply –Preliminary Space Tested (STS-93, July 1999) –Straightforward Fabrication Process Disadvantages –1 Time Use Only

10. Solid Propellant Rocket

11. Case Study: Solid Propellant Microrocket

12. Fabrication Microheater/ Convergent Propellant Chamber Divergent Assembly of Parts –propellant filling –epoxy bonding of components propellant chamber convergent/ microheater divergent Rossi, C., et al., “Design, fabrication and modeling of solid propellant microrocket- application to micropropulsion”, Sensors and Actuators A, vol. 99, (2002) pgs. 125-133

13. Microheater/Convergent Microheater –Wet oxide growth –LPCVD SiN 1.2 –LPCVD Poly-Si for resistor –CVD gold electrical pads Convergent –KOH anisotropic etch SiO 2 /SiN 1.2 membrane Poly-Si resistor Gold electrical pad Poly-Si Gold SiN 1.2 SiO 2 Si Highlights: Microheater

14. Propellant Chamber DRIE –ASE (SF 6, C 4 F 8 ) Si Highlights:

15. Divergent Oxide growth Anisotropic Etch –45wt% KOH –80°C Si Highlights:

16. Assembly of Parts: Propellant Filling Localized Vacuum –consideration of air pockets Highlights: Rossi, C., et al., “Realization and performance of thin SiO 2 / SiN x membrane for microheater applications”, Sensors and Actuators A, vol. 64, (1998) pgs 241-245

17. Assembly of Parts: Epoxy Bonding Epoxy –EPO TEK H70 glue –cured at 60°C for 15 hours Array Fabrication Note propellant chamber convergent/ microheater divergent Highlights: Rossi, C., et al., “Design, fabrication and modeling of solid propellant microrocket-application to micropropulsion”, Sensors and Actuators A, vol. 99, (2002) pgs. 125-133

18. Material Considerations: Propellant Chamber Silicon –Amenable to established microfabrication techniques –High thermal conductivity (124W/mK) Ceramic (Macor®) –Low thermal conductivity (1.46W/mK) –Adaptable to microfabrication

19. Material Considerations: Propellant Heterogeneous solid propellant –Polymeric binder (PB), metal catalyst (Al, Mg), oxidizer (NH 4 ClO 4 ) –Relatively high energy density –Stable and viscous –Adaptable properties

20. Modeling Geometric Parameters: Chamber-to-Throat Area Ratio Determines pressure in propellant chamber and flow speed at throat –Maximize thrust when flow at throat is sonic A c /A t = 16 (chamber diam. = 1.0mm) –Subsonic under atmospheric, sonic under vacuum –Thrust force: 1.5 – 5.0mN –Burn time: 350ms A c /A t = 60 (chamber diam. = 0.85mm) –Sonic under all conditions –Thrust force: 4.8 – 5.8mN –Burn time: 250ms AcAc AtAt

21. Modeling Geometric Parameters: Divergent Guides expansion of exhaust gas from throat Unnecessary under atmospheric conditions (chamber pressure too low) Helpful under vacuum UnderexpandedOptimalOverexpanded

22. Conclusions Three microrocket designs: Turbine Engine Gaseous Propellant Rocket Solid Propellant Rocket Significant advantages exist for the solid-propellant design energy density fabrication techniques lifetime