Lasers help MIT make Cube Satellite engines
Researchers around the world are developing small, artificial satellites that can be launched into orbit. These miniature satellites, often called “CubeSats,” are designed to be inexpensive alternatives to earlier-generation satellites.
The first satellite was Sputnik 1, launched by the Soviet Union on Oct. 4, 1957. Besides initiating the space race with the U.S., Sputnik 1 led to a host of other satellites being launched and positioned in various levels of the atmosphere. The largest artificial satellite presently orbiting Earth is the International Space Station.
CubeSats cost less than $100,000 to launch. Image courtesy NASA.
Most satellites are powered by onboard systems that incorporate solar panels. Their power subsystems regulate, distribute and store energy in batteries for use when Earth is shadowing the solar panels. The downsides to power subsystems are that they are expensive, large (in relation to the size of the satellite) and heavy.
CubeSats are designed to keep launch costs under $100,000, which is far less costly than launching a conventional satellite. CubeSats measure 10cm × 10cm × 10cm, weigh about 1kg and their primary payload consists of instruments for conducting one or two scientific experiments.
The satellites need small, light engines that don’t sacrifice performance because of their size. Massachusetts Institute of Technology’s Space Propulsion Laboratory has developed such a propulsion system.
Called the ion-Electrospray Propulsion System (iEPS), each tiny engine fills a volume of approximately 5.4 cm3, weighs a few grams and has a high specific impulse, which should promote precise attitude control of a CubeSat while in orbit.
An iEPS differs from a conventional rocket engine in that its propellant isn’t combusted. Instead, it is a conductive liquid that responds to applied voltages (Figure 1). In vacuum, applying sufficiently high voltage between a propellant within a beaker and an extractor electrode above it causes minute droplets to break away from the propellant reservoir and race toward the extractor. The fraction of droplets passing through the hole in the upper electrode provides a downward thrust. The strong bias voltage creates tiny fluid instabilities along the surface of the propellant, called Taylor cones (inset), which eject droplets from the propellant reservoir.
This scheme is refined within the iEPS thruster head, where, by design, each Taylor cone is assigned an aperture of the extractor electrode. To accomplish this, a hexagonal array of laser-milled, needle-like tips are cut in a porous emitter substrate (Figure 2). When the emitter tips are aligned and glued beneath a perforated extractor electrode, with the apex of each tip coincident to, and centered on, the corresponding entrance aperture of the extractor, the thruster head assembly is complete.
Figure 2: A laser is used to mill needle-like tips into a porous emitter substrate of the engine’s thruster head (left). The apex of each tip is coincident to, and centered on, the corresponding entrance aperture of an extractor electrode (right). Image courtesy MIT.
These Taylor cones are extremely small (< 5µm at the base), making them difficult to manufacture by any method other than lasing. Ultraviolet wavelength and short-pulse-length lasers are the best options for this application. With these lasers, a variety of materials can be handled, the desired precision and ultimate achievable feature size can be obtained, and the cleanliness of the laser processing minimizes post-laser cleaning or additional operations.
After its manufacture, the thruster head is attached to a fuel reserve (Figure 3). This propellant delivery system further distinguishes the iEPS from more traditional engines that rely on cumbersome valves to control the rate at which propellant is delivered. Instead, when the fuel tank is attached to the emitter chip, capillarity passively controls the rate at which propellant is delivered to the apex of the tips. Then, when the extraction grid and fuel reserve are connected to the power processing unit (developed by Espace Inc., Hull, Mass.), an applied voltage between the tips and the extractor electrode concentrates the electric field, thereby constraining the Taylor cones to the apex of the emitter tips.
Figure 3: After the thruster head is manufactured, it’s attached to a fuel reserve. Image courtesy MIT.
With a large enough bias, propellant is emitted from the tips. Figure 4 shows a single tip biased to emit a spray of propellant (an ionic liquid) from the Taylor cone formed at its apex. In this case, the spray contains both droplets and ions. The shape of the emitter tip and its porosity determine the amount of droplets to ions that are emitted; ions provide more control and higher efficiency than droplets.
Figure 4: A Taylor cone biased to emit the engine’s ionic-liquid propellant. Image courtesy MIT.
MIT’s iEPS engine is scheduled to be deployed in a CubeSat slated for launching into space this fall. µ
Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H. Corey Fucetola, Ph.D., is a postdoctoral associate at MIT. E-mail:email@example.com. Telephone: . E-mail: firstname.lastname@example.org.
— R. Schaeffer