THE SMALL SATELLITE REVOLUTION
Are Flying Discs the Future of Small Satellites?
An alternate approach to the CubeSat provides the benefits of a small satellite with large surface areas that can be dedicated to antennas or instruments needing exposure to space.
A defining feature of a standard CubeSat is its containerization — the shape, volume, and design — which makes it rideshare-friendly. This shape has been important since these devices have historically comprised a minor part of a total payload. The small number of standardized sizes and shapes greatly simplifies integration of the satellite with the launch vehicle, providing ride flexibility and substantially reduced costs for both the satellite builder and the launch provider.
Containerization has ensured that CubeSats would not endanger a launch vehicle or primary payload on rideshare missions. Without the need for rideshare, satellite design options can be expanded beyond a cube. DiskSat is an alternate approach to containerization, providing the benefits — standardized launch interface, low launch costs and simple mechanical design — with large surface areas that can be dedicated to large antennas or instruments that need exposure to space, and high power.
The plate shaped DiskSat satellite measures 1m in diameter and 2.5cm thick and has a volume comparable to a hypothetical 20U CubeSat. For launch, several DiskSats can be stacked to fit within a launch vehicle’s fairing and deployed one at a time after the launch vehicle reaches orbit — an ideal approach to building large constellations of small spacecraft, allowing 20 or more satellites to be containerized in a single small launch vehicle.
We spoke with Dr. Richard Welle, Aerospace Senior Scientist, about DiskSat and its implications for future small satellite constellations.
How did the DiskSat concept come about? Was there a particular inspiration?
The idea of the DiskSat arose out of a study of a potential mission for a large constellation of small satellites, each of which needed quite a bit of power, but not much mass. The study team wanted to build a well-structured constellation of CubeSats using small expendable launch vehicles. They would not be able to get the power they wanted in a traditional CubeSat format but were thinking of using the entire launch capacity of a small launch vehicle, so adhering to the traditional CubeSat shape for rideshare wasn’t necessary.
The mission requirements naturally led to the idea of stacked disks that would make efficient use of available launch volume. This maximizes the surface area of individual satellites without increasing their mass beyond that typical of CubeSats.
Once we had the DiskSat idea, we realized it could be useful to support a variety of missions, including rideshare with other DiskSats.
Does the form factor of DiskSat give it any distinct advantages?
The most obvious advantage of the DiskSat is the high power-to-mass ratio, the initial motivation for the shape. Beyond that, there are several distinct advantages worth noting.
First, the high power-to-mass ratio suggests the use of electric propulsion (EP) to give a satellite capable of significant orbit changes. For example, available EP systems could make it possible to fly a DiskSat from GEO to lunar orbit, something unprecedented in a satellite under 10 kg.
Second, the shape of the DiskSat makes it possible to fly in an orientation that produces very low atmospheric drag. Combining low-drag flight with EP for drag makeup enables mission operations in very low orbits. While typical satellites do not operate below 300–400 kilometers in altitude, DiskSat’s low drag means it can operate down to about 250 kilometers with electric propulsion — an ability that can provide better resolution or sensitivity for Earth-observation or commutations missions.
A closely related benefit is that the DiskSat offers a form of automatic orbital debris control. To maintain low-drag flight, the DiskSat must be operated in a particular orientation. If the DiskSat should fail for any reason, the attitude-control system will stop working and the DiskSat will tumble. A tumbling DiskSat will experience something like ten times more atmospheric drag than a satellite flying in low-drag orientation, and the increase in drag will result in the satellite rapidly falling out of orbit, thereby automatically removing non-functioning satellites.
A final advantage worth noting is the ease of fabrication enabled by the DiskSat format. In contrast to typical satellites that tend to be complex three-dimensional structures, the DiskSat is more of a two-dimensional structure with a large surface area allowing easy access to all of the internal volume used for satellite components. Integration, test, and any necessary repairs are much easier than with traditional 3D satellites, which should shorten design, build, integration, and test schedules accompanied by lower cost.
Are there any challenges with this new design?
As with any new satellite design, there will be benefits and challenges. Two challenges often noted are thermal management and attitude control. Thermal management is somewhat simplified by the presence of a large surface area. Often with CubeSats and other power-limited small satellites, the developer wants to cover every available surface with solar cells to maximize power availability, so the area dedicated specifically to thermal management may be quite limited. With the DiskSat, the large surface area means it is possible to reserve a reasonable amount of surface area specifically for thermal management. This gives the satellite designer the ability to tune the thermal properties of that surface area as needed to manage heat dissipation and control temperatures.
The attitude-control challenge is driven by the large moments of inertia of the DiskSat. Without a substantial increase in reaction wheel size and/or speed, a DiskSat is not going to be as agile in pointing as a CubeSat. However, the impact of this is mission-dependent. For example, an Earth-observation mission will likely require either pointing an instrument either in a particular direction so that it tracks across the ground or pointing an instrument at a fixed location on the ground as the satellite passes overhead. In the former case, there is little need for agility; the satellite just has to maintain a constant orientation. In the latter case, the satellite may have to rotate at up to one or two degrees per second depending on how high it is. This means the reaction wheels will have to be sized to provide the necessary torque and momentum storage. The DiskSat, of course, will have plenty of power to run the reaction wheels.
How will the DiskSat launch dispenser mechanism work?
The key challenge with the DiskSat is developing a dispenser that will contain the satellites during launch and then release them one at a time on orbit. We are still working out the details of the dispensing system, but we expect that the satellites will be released one at a time from the top of the stack with sufficient speed to ensure that each satellite is well clear of the dispenser before the next satellite is released. The design of the dispensing system will be finalized and tested in the laboratory over the next few months.
What kind of missions would be well-suited for DiskSat?
Although it is possible to envision a DiskSat taking on almost any mission that can be flown in CubeSats, the large surface area, high power-to-mass ratio, and the stackability, suggest a few missions that cannot easily be flown in CubeSats. For example, there have been many proposals for satellite constellations that involve hundreds to thousands of satellites. These constellations typically require multiple satellites deployed in multiple orbital planes and requiring multiple small launch vehicles. Because it is stackable, the DiskSat is ideally suited for this application since it makes very efficient use of launch volume in small launch vehicles.
Missions that require high power and large RF apertures, for example communications relays or radar missions, are a category that can particularly benefit from the DiskSat geometry. Also, missions requiring orbit agility and/or long duration in high-drag environments can benefit from the high power-to-mass ratio and low drag of the DiskSat. This opens a new mission space for low-altitude imaging, radar, or other Earth-observation missions as well as beyond-LEO applications, including lunar and planetary missions.
When will DiskSat be tested on orbit?
The Small Spacecraft Technology Program within NASA’s Space Technology Mission Directorate is supporting Aerospace to build and fly a demonstration mission. Four spacecraft will be deployed in LEO to verify baseline DiskSat performance and validate the launch dispenser mechanism — the latter being a key goal since dispensing must be done in a way that prevents contact between satellites.
The four demonstration satellites will have electric propulsion and operate in pairs: one pair will fly at low altitude, and the other will demonstrate high-altitude operations, showcasing DiskSat’s maneuverability. This demonstration mission is expected to be ready to fly by the end of 2023 and will fly as soon thereafter as a launch opportunity can be identified.
Dr. Richard Welle is a senior scientist at Aerospace. With more than 35 years of experience in his field, Welle is an expert in microsatellites and microsatellite systems, thermophysics and heat transfer, reacting/radiating flows, multiphase and cryogenic flows, electric propulsion, and mechanical systems.
