Integrated Spacecraft Platforms
Since the last edition of this report, some vendors have compiled the subsystems represented in the later chapters into complete, integrated spacecraft platforms, available commercially off-the-shelf (COTS) for rapid integration and delivery. Thus the state-of-the-art performance is commensurate with the subsystem performance listed below. A variety of small spacecraft buses available from various vendors have enabled integrated small spacecraft buses, including 12 and 27U CubeSat platforms. Recently, PocketQubes for Earth science missions have become more available, and many other vendors are providing engineering services to design turnkey small spacecraft platforms customized to specific mission requirements.
Each year small spacecraft power subsystems benefit from improvements in solar cell efficiency, battery chemistry, and the trend of electronics miniaturization. State-of-the-art solar cells are reaching between 29-33% efficiency and advanced lithium-ion and lithium polymer batteries are reaching 250 Whkg-1. Power management and distribution (PMAD) systems are still often customized per mission, but there are increasing numbers of lightweight, robust, commercially available PMAD systems from a variety of producers. Trends in consumer electronics and improvements in solar technology driven by a new focus on renewable energy are largely to thank for these advances, as the market for small spacecraft is still too small to drive large-scale research and development (R&D).
There are many promising photovoltaic technologies in development that will increase the efficiency and/or reduce the cost and weight of solar cells. These include 46% four-junction cells, lightweight flexible solar cells at 20% efficiency, and cells that make use of cheap organic electronics. While there continue to be advances in the thermo-nuclear and fuel cell power-generation areas, more development needs to be done (largely in miniaturization) before some of these promising technologies become available for use on small spacecraft.
Propulsion systems for small spacecraft have consistently increased their maturity and robustness with respect to the previous report. Several institutions have made a significant effort to design, develop and test of miniaturized thrusters. Versions of larger spacecraft systems have been adapted to satisfy the power, mass and volume constraints required in small buses. Fundamental components such as regulators, valves, feed systems or tanks have also been re-designed and currently several systems have higher TRLs.
Regarding chemical propulsion systems, low complexity technologies such as cold gas systems have already flown in small spacecraft and even CubeSats. Other options such as non-toxic propellant systems or solid motors have been incorporated into existing 50-150 kg class spacecraft or are ready to be flown in the next year. Electric propulsion systems have evolved by a series of continuous testing campaigns for a wide range of technologies. Electrosprays, Hall-Effect Thrusters, Pulsed Plasma Thrusters and ion engines are now ready to become fully integrated subsystems in small spacecraft missions (50 – 150 kg). This technology for CubeSats has matured since 2015, however further development is needed to improve these modules for such small platforms. Longer lead times have been associated with CubeSat propulsion systems even though several of these systems have been demonstrated in space. In regards to solar sails, recent successful demonstrations and tests have indicated a path towards the use of this propellant-less technology for both Low-Earth Orbit (LEO) and interplanetary missions.
Guidance, Navigation and Control
The current state-of-the-art for small spacecraft guidance, navigation and control (GNC) performance is 1.5 m onboard orbital position accuracy (using GPS) and pointing to better than 0.1° using reaction wheels, MEMS gyros and a star tracker. Component technology for Earth orbiting missions is mature and all key GNC components are available at TRL~9 from a variety of vendors for all small spacecraft classifications. Components for deep space small spacecraft missions have matured to reach high TRLs. Innovation in GNC is focused on miniaturization of existing technology, technology that can be sourced from a single vendor, and integrated, modular attitude determination and control units.
Structures, Materials and Mechanisms
The state-of-the-art for primary structures used for small spacecraft larger than 12U continues to be in custom in-house designs, or for tailored solutions offered by the industry to meet specific mission requirements. There have been recent attempts to establish a standard extensible bus and standard chassis in the 12U class of spacecraft. However, the benefits of this effort have yet to be realized. In the class of spacecraft smaller than 12U, there have been several unique solutions offered by a growing industry for COTS spacecraft structures and structural components. These COTS components complement the standard approach of custom designed frames (typically fabricated using milled aluminum) and have enabled a larger set of solutions for spacecraft designers. Most of the recent additions to the COTS market have been in the 3U class of CubeSats. However, there are now at least a few mature (TRL 9) COTS 6U chassis being offered. This is a class of spacecraft that has just recently begun to show signs of rapid acceleration in being adopted for flight missions. There are even 12U solutions being provided by many vendors, a sign of the industry’s desire to be ready for the next thing. 3D printed primary structures have just recently reached TRL 9 status with the launch of a few missions.
Thermal control management regulates the functional temperature range required throughout all spacecraft components. As small spacecraft design matures, the techniques that control the defined thermal environment must be able to meet these smaller volume and power constraints. Traditional thermal management may need additional testing and fabrication for small spacecraft applications.
Technologies such as passive louvers, non-metallic thermal straps, sunshades and cryocoolers are being designed for smaller spacecraft platforms. Better thermal management will expand small spacecraft design. Several thermal control mechanisms are currently being proposed, tested and fabricated for small spacecraft applications: thermal storage units for energy storage; stowed and deployable passive radiators; and miniaturized circulator pumps requiring minimal power input.
Command and Data Handling
Avionics solutions for small spacecraft and in particular, CubeSats, are abundant. Ongoing advances in the embedded systems industry have provided highly capable platforms and components that allow for rapid and low cost development of command and data handling (C&DH) systems. Embedded systems have paved the way for the development of highly integrated, low mass and low power processing and control systems. A lot of COTS hardware has successfully flown in the LEO environment over short mission durations. A number of commercial vendors are providing complete, integrated avionics systems on PC/104 boards, incorporating computer processor, memory, input/output (I/O) and electric power systems (EPS). A number of vendors source systems and components from a variety of manufacturers, which allows spacecraft developers to pick and choose components that will meet their design requirements. There are open source solutions available to those who are interested in investigating an entry-level means of developing spacecraft avionics.
As the CubeSat class of small spacecraft evolve into deep space and extended duration missions, there will be a need to address the impact of the space radiation environment. It will be necessary to develop radiation tolerant system designs to ensure mission reliability and success. Radiation hardened (rad-hard) hardware is available for a majority of the electronic components used in C&DH systems. However rad-hard devices can be significantly more costly when compared to standard COTS components. To keep development costs as low as possible, developers will undoubtedly use a combination of rad-hard components, COTS devices, shielding and mitigation techniques such as watchdog timers and memory scrubbing to reduce radiation environment impacts and improve system reliability.
Communication systems for CubeSats have largely used the VHF and UHF bands (primarily using whip antennas), or L- and S-bands (primarily using patch antennas), which have been adequate for lower-data-rate missions operating in LEO. CubeSat missions have also taken advantage of Iridium and Globalstar transponders to relay data to Earth via commercial constellations. COTS radios such as Bluetooth- and ZigBee-compatible radios also show promise for CubeSat missions.
X-band through Ka-band communication is gaining more traction as CubeSat missions become more sophisticated and require higher data throughput, with missions successfully operating beyond LEO. The higher frequencies offer more bandwidth and are less crowded, and the corresponding antennas can offer similar gain but with a smaller aperture. The drawback, however, is that the higher frequencies are more heavily attenuated by Earth’s atmosphere, requiring either more power to drive the signal or a higher number of ground stations. The development of CubeSat-compatible deployable dish antennas and other higher-gain antennas are also adding to the solution.
The advent of software-defined radio (SDR) has not totally replaced hardware-defined radio. Though an SDR can operate at various frequencies and various modulation schemes with a simple change in software, and generally has a smaller footprint than hardware-defined radio, it tends to consume more power, which is a large drawback on power-constrained CubeSats. However, a counter to this drawback is that a single SDR unit can function as multiple radios at multiple wavelengths, and it can be reprogrammed in-flight.
Laser communication (lasercom) for CubeSats is a TRL 8 technology that has been demonstrated in space. While lasers onboard CubeSats have a relatively high TRL status, asymmetric laser communication is a lower TRL concept whereby the laser is hosted by a ground station, and the laser signal is modulated and passively reflected by the CubeSat back to Earth. The development of X-band and Ka-band transmitters, arrayed and deployable high-gain antennas and lasercom systems represent the new frontier of CubeSat communication systems.
Integration, Launch and Deployment
More and more small spacecraft are launched every year. Technologies in launch vehicles, integration, and deployment systems are responding to the changing small spacecraft market. The traditional ride-share method where the small spacecraft hitches a ride in the leftover mass, volume, and other performance margins is still the primary way of putting small spacecraft into orbit. But new technological advancements show that the popularity of classical ride-sharing might decrease slowly in the upcoming years. Dedicated ride-sharing, where an integrator books a complete launch mission and sells the available capacity to multiple spacecraft operators without the need of a primary customer, is becoming more popular in the sector. Using an orbital maneuvering system which acts as an inter-stage on a launch vehicle and then propels itself after separation is another new approach.
Furthermore, ISS cargo vehicles are gaining additional capabilities to deliver secondary payloads to orbits higher than ISS altitude once their primary mission is complete. Beside any ride-share approach, more than twenty orbital launch vehicles are under development to carry payloads ranging from 5 kg to 500 kg to orbit. Some of these new systems propose to launch orbital payloads from airborne vehicles, suborbital systems, or even high altitude balloons. A wide variety of integration services and deployment systems are also under development to keep up with the increasing launch and deployment demand of small spacecraft.
Ground Data Systems and Mission Operations
Transmitting telemetry and scientific data back to Earth in the specified quality and quantity, and tracking and commanding the spacecraft to take certain actions depend on reliable telecommunications with ground stations. Although amateur ground stations have been essential for CubeSat missions in the past, small spacecraft and ground systems are rapidly shifting to non-amateur communications, as power systems become more effective, attitude control systems more accurate, and as higher data rates are needed for science or new technology missions. In the scenario of small spacecraft missions, many companies are developing new state-of-the-art systems for ground stations. While some of them focus more on single products (such as antennas, transceiver, and simulation software) that are the cutting edge technology yet to be validated in space missions, others consolidate and extend their services with turnkey solutions, which add more capability and availability to their already developed ground systems. Alternatives to common ground systems are inter-satellite communications, which relay data to the ground through constellations of satellites (such as Iridium or Globalstar). Still, there are a lot of new promising areas and technologies that ground data systems can explore and develop for future Small Spacecraft missions.
Passive Deorbit Systems
To constrain the amount of space debris orbiting Earth, a deorbit capability is often required. If a small spacecraft is unable to be parked in a graveyard orbit or naturally reenter Earth’s atmosphere in under 25 years, a deorbit system must be integrated. In the past decade, there have only been a few advancements on passive deorbit technologies, such as drag sails and electromagnetic tethers. NanoSail-D2, CanX-7, and several TechedSat CubeSats are all CubeSat platforms that have successfully demonstrated the use of drag sails for deorbiting in LEO within the 25 year post mission requirement. Terminator Tape is another deorbit option that uses electromagnetic tethers and is currently being flown on the Aerocube-V CubeSat. Additionally, both solid rocket and electric propulsion systems have been used to increase orbit decay rates.