This is the first of three articles covering NASA's "State of the Art of Small Spacecraft Technology" report.
NASA recently released an update to their “State of the Art of Small Spacecraft Technology” report. The report aims to give the most current information on the state of small spacecraft technology, mainly the CubeSat market. The report is divided into multiple sections, including power, propulsion, materials, and passive deorbit systems.
NASA uses the term “TRL,” which stands for technology readiness level, to indicate the current state of the technology. The TRL scale ranges from 1 to 9. Anything below a 4 is considered “on the horizon,” meaning the technology is theorized, but that no out-of-laboratory tests have been conducted. A TRL above 5 generally indicates state-of-the-art technologies that are in the testing phase, either in space or on Earth. A TRL of 8 or 9 means the system is currently or in the process of being flight qualified for active use.
Below are a few key highlights from different sections of the report.
Integrated Spacecraft Platforms
The CubeSat platform itself has matured greatly in the past 10 years, as the market has shifted toward the use of COTS (commercial-off-the-shelf) products. These offer a dual advantage, allowing for utilization of the most up-to-date commercially available technology, as well as a more plug-and-play mentality. This means engineers can expect a certain level of standardization and interchangeability, regardless of platform.
Additionally, recent advances in miniaturization have seen the advent of the picosat, which weighs less than 1 kg, allowing for smaller and cheaper one-off missions.
Using solar panels as the primary electrical power source for satellites has been the standard since the inception of the spacecraft itself, with nearly 85% of all CubeSats being powered by solar. While solar technology continues to improve, it's still costly for CubeSats, as about one-third of their mass is taken up by the electrical system.
Two on-the-horizon power systems of note are the TPV (thermo-photovoltaic) system and the nano-enabled power system. The TPV works by capturing infrared light from an internal heat source to generate power with a specific energy 6.5 times higher than the lithium-ion battery.
The nano-enabled power system is being developed by the Rochester Institute of Technology to incorporate carbon nanotubes into lithium-ion batteries. This will double current battery energy density while allowing the batteries themselves to act as a load-bearing structure, saving weight on the spacecraft.
The course-corrective propellant field is quite contentious. These propellants are used as thrusters on the spacecraft, allowing it to orient itself in space. Spacecraft engineers are very interested in monopropellants, which can oxidize themselves and produce thrust. The advantage of a monopropellant is that only one source of fuel is required, as opposed to the fuel/oxidizer combination of most rocket motors, necessitating separate storage and injection systems.
Currently, three monopropellants are vying for dominance: hydrazine, ammonium dinitramide (ADN), and hydroxyl ammonium nitrate (HAN). Hydrazine has been a mainstay as a thruster propellant for decades, and has the highest number of mature thrust systems in use.
Hydrazine’s main drawback is its toxicity to humans, which has spurred the development of what are known as the “green” propellants ADN and HAN. Green propellants are less flammable, and release nontoxic gasses such as water and carbon dioxide when combusted. ADN and HAN systems are in multiple stages of development; some systems with a TRL of 7 are currently being tested in orbit.
Electrical propulsion has improved recently, with the development of the resistojet and the electrospray. Resistojets are the simplest electric propulsion system; they work by simply heating a cold gas — such as xenon, butane, or nitrogen — electrically, and letting it expand out of a nozzle, producing thrust.
Electrosprays are a little more interesting. The electrospray takes advantage of capillary action through a negligible vapor pressure-conductive salt to accelerate ionized droplets of propellant through a thruster. (If that sentence confused you, you’re not alone.) Electrosprays produce an incredibly high thrust for their weight (~1000-2000 specific impulse), but the total thrust is very small as the mechanism itself cannot be very big. Resistojet TRLs range from 5 to 9, while electrospray TRLs range from 5 to 7.
Ion propulsion has also seen several advancements in the fields of pulsed plasma, hall effect, and radio frequency (RF) thrusters. At its core, ion engines use some method of electrically separating a propellant molecule into its constituent charged particles, and then magnetically accelerate them through a thruster.
Pulsed plasma thrusters use a high-voltage discharge arc to ablate material off a solid surface. The resulting charge imbalance between the ablated material and the fuel block creates a magnetic field, which then accelerates the material outside the spacecraft.
Hall effect thrusters are highly mature, with multiple engines already in various states of use on satellites both in orbit and in deep space.
Lastly, RF thrusters use oscillating RF induction to vaporize a propellant and then use magnetic fields to accelerate it. Pulsed plasma TRL ranges from 5 to 7, hall effect TRLs are as high as 8, and the highest RF thruster TRL is 5.
In recent years, the use of solar sails has been increasing. The solar sail leverages radiation pressure from the sun to provide motive force to the spacecraft. Solar sails have one key advantage over every other engine system: They are propellantless. So, all a spacecraft has to do to use a solar sail is deploy it in the right direction and wait. Solar sails are very popular and are seeing use in various satellites, both for course-corrective propulsive force as well as experimentation.
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