ESA Demos MEMs Rocket Thruster That’s as Small as a Coin
Miniature satellite thruster rocket engine with 1 mm combustion chamber requires MEMs fabrication techniques to manufacture. Uses only ice and electricity to create power.
With the continued miniaturization of electronics and increasing accessibility of orbital launch capacity, small satellites have been experiencing fast growth. However, it’s not just the electronics that must shrink.
Thruster and other stabilization equipment must also drop proportionally in size. While special purpose satellites keep getting smaller, key components, such as thrusters have not kept pace with downsizing.
Traditional thrusters use hydrazine rocket fuel, which requires tankage in addition to the engine and control systems. Newer ion and electric propulsion systems have been released or are under development, but they are still too large to be an effective solution for the smallest two satellite categories: Nano satellites, which are between 1 and 10 kg (2.2 to 22 lbs.) and Pico satellites, which weigh between 0.1 kg and 1 kg (0.22 to 2.2 lbs.).
ICE-Cube thruster designed for nano satellites. Image courtesy of ESA
The ESA (European Space Agency) has now tested a micro rocket thruster aimed at Nano and Pico satellite applications. It was designed by a team at The Imperial College of London. The new thruster, dubbed ICE-Cube (Iridium Catalysed Electrolysis CubeSat Thruster), uses electrolysis to separate hydrogen and oxygen out of water.
It then recombines the gasses in a combustion chamber less than 1 mm in length. So small, in fact, that it requires use of a MEMs fabrication process. The thruster has been demonstrated, in laboratory tests, to deliver 1.25 millinewtons thrust at a specific impulse of 185 seconds sustained.
The Small Satellite Race
Nano satellites are a relatively new but fast growing category of space vehicle. Just 25 were launched in 2012. A decade later, 334 were launched in 2022 and nearly double that number have been announced for launch in 2023.
Nano satellite. Image courtesy of NASA
These satellites have little room to spare. The requirements for tankage of conventional toxic and corrosive propellants, such as Hydrazine quickly become impractical. Other small scale forms of propulsion like ion, compressed air or steam are not as energy efficient as a hydrogen + oxygen combustion system. Many don’t give sufficient lifetime when limited to Nano satellite size.
Recombining Hydrogen and Oxygen for Thrust Generation
Storing the propellant in the form of water ice is safer and less costly than holding it as gaseous or liquid gas form. The electrolysis process uses just 20 Watts that would be generated via solar cells and or storage batteries. In a sense, the device converts solar energy into thrust by way of ice.
ICE thruster in action. Image courtesy of Imperial College of London
Unconventional Manufacturing
The devices are fabricated in-house in the college's Imperial Plasma Propulsion Laboratory with a MEMs process they developed. They use reactive ion etching with a refractory metal to create the shape of the device. The next step sputter-deposits an iridium layer which both acts as an ignition catalyst and creates an oxidation protective layer to the device walls.
The thruster would work equally well with a tanked gaseous hydrogen and oxygen supply system. However, that would take up more space, be vulnerable to leaks and explosions and would require additional regulation hardware. By utilizing ice, considerably fewer components are required and the fuel/oxidized combination is stable and safe.
It might seem inefficient to separate the two components, only to recombine them an instant later. However, with solar power used for the electrolysis, no onboard resources are required and minimal hardware components are needed to support the whole system.
When both gasses are mixed in the combustion chamber, the indium catalyst starts the burn which produces thrust. While the net power used is more than gained by combustion, more power is produced than would be with other electric and ion based thruster systems.
The college lab has developed two micro thrusters, The ICE-200 with final design thrust of 1 - 2 N (Newtons) and the ICE-Cube, mentioned above, with 5 mN design thrust.
