Prototypes of micro nozzles were fabricated using the web-based system. The fundamental process of micro nozzle is mechanical micro machining (figure 10) using High Speed Steel (HSS) tool. The smallest tool diameters used in this study were 100 §­ ~ 500 §­ for flat endmill (roghing), and 200 §­ for ball end mill (finishing). Aluminum 6061 T6 rod was used as the nozzle material. The 3-axis micro stage (the precision is 1 §­) and high speed spindle (max. spindle speed is 42,000rpm) was used to fabricate the rocket nozzle.
The total machining time for the rocket nozzle was about two hour and 30 minutes. The spindle speed used in this machining was 23,000rpm, and the feed rate was 20mm/min.
For the combustion chamber, Aluminum 6061 T6 rod with 12mm diameter and 40mm length was machined to have 5mm diameter and 35mm deep blind hole. A drill of 5mm was used with a CNC lathe to fabricate the hole.
For the solid propellant, a gun powder for firework (Hunan Provincial Fine Crackers & Fireworks) was used. Gun powders usually have rapid explosion, thus more effective at high altitude or at the space environment than ground level. To prevent an accidental disassembly of the nozzle and the combustion chamber during combustion test, mechanical fastening by threads was applied.


Micro machining

Micro endmills used in this test
Among prototypes, two types were tested to measure thrust properties. The nozzle with1mm throat diameter was referred to Type 1 while the nozzle with 0.5mm throat diameter was referred to Type 2.

Specification of the micro rocket



Aluminum nozzle and chamber
The combustion chamber was filled with the gun powders. The nozzle was fastened to the motor case by the threads. Igniting gun powders from the same manufacturer were filled space inside of the nozzle, and a wick was connected to the ignition gun powders.
The assembled rocket was supported by a plastic foam. Then nozzle was faced upward and was fixed on a digital scale that measures up to 0.001g resolution. A digital camcorder was used to record the experimental process during the combustion tests.
As intended, the combustion generated a three-dimensional exhausted gas. A RS232C interface of the precision balance with PC was used to collect test data.


Exhaust gas from the micro nozzle

Thrust characteristics of the micro rocket (type 1, throat = ¥õ 1mm)

Thrust characteristics of the micro rocket (type 1, throat = ¥õ 0.5mm)
Since the thrust-to-weight ratios were greater then 1, the rockets could be launched vertically. For simplicity of measurement, however, 45¡Æ launch was tested.
Type 1 rocket showed flight distance between 49m~50m. Type 2 rocket compared with Type 1 had higher specific impulse and maximum thrust. However, effective thrust was similar for both types. In terms of actual flight distance, both types showed similar results.

Set up for launch tests. The flight distance was measured from the horizontal distance of micro rocket launched at 45¡Æ from the ground.

Test result of the micro rockets