Thrusters, in addition to engine and control systems, typically require hydrazine rocket fuel, which they must store in tanks, making the entire setup physically rather large. Recent innovations are under development, and they use ion and electric propulsion systems. Although the newer thrusters are physically smaller, they are still too large for Nano and Pico satellites, which weigh between 1 and 10 kg, and between 0.1 and 1 kg respectively.
These small satellites require miniature satellite thrusters. These are rocket engines with combustion chambers of 1 mm size. They use only electricity and ice to create thrust. Manufacturing such tiny coin-sized thrusters requires MEMS fabrication techniques.
Miniaturization of electronics is leading to increased accessibility of orbital launch capacity. In addition, small satellites are experiencing fast growth. But, along with electronics, many other things need to shrink too.
For small satellites, thrusters and other equipment for stabilization must also proportionally shrink in size. Although satellites for special purposes are getting smaller, some key components, especially thrusters, have not kept pace with the downsizing.
Enter The Imperial College of London, where a team has designed a new micro thruster especially meant for Nano and Pico satellite applications. The ESA or European Space Agency, who tested the new thrusters, has dubbed them as ICE-Cubes or Iridium Catalyzed Electrolysis CubeSat Thrusters. ICE-Cube thrusters use the process of electrolysis to separate oxygen and hydrogen from water.
The thruster then recombines the two gasses in a combustion chamber less than 1 mm long. The miniature size of the chamber requires a MEMS fabrication process to create it. In laboratory tests, the thruster delivered 1.25 millinewtons of thrust, and it could sustain it for an impulse of 185 seconds.
Although a fast-growing category of space vehicles, Nano satellites are a relatively new breed. While 2012 saw only 25 launches, a decade later, there were 334 launches in 2022. By 2023, that number has nearly doubled.
Being tiny, Nano satellites have little room to spare. That means, conventional tankage carrying corrosive and toxic propellants, such as hydrazine, is no longer practical. While there are forms of propulsion available on a smaller scale, and they typically use compressed air, ions, or steam, these are neither energy-efficient nor do they offer sufficient lifetime. The highest energy efficiency comes from using oxygen and hydrogen in a combustion system.
Nano satellites typically store their propellant as water-ice, because it is safer and less expensive as compared to holding it in liquid or gaseous form. The electrolysis process requires only 20 Watts, which storage batteries or solar cells can easily produce. Therefore, the satellites typically convert solar energy into thrust using ice.
The Imperial Plasma Propulsion Laboratory of the college fabricates the above devices in-house, using their own MEMS process. They create the shape of the device with a reactive ion etching technique using a refractory metal. Then they sputter-deposit an indium layer, which acts like an ignition catalyst, while simultaneously creating a protective oxidation layer for the walls of the device.
The college laboratory has developed two types of micro thrusters—the ICE-200 producing a design thrust of 1-2 N, and the ICE-Cube, generating a thrust of 5 mN.
