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DISCLAIMER

The information presented here is meant to be used as an educational asset for the Space community, both for commercial, non-commercial entities and individual actors; the use case has been generated through IENAI’s GO™, an intuitive thruster design and mission analysis tool on the cloud, which is open-access and free-to-use for the community. External data and assets have been gathered in good-will and, to the best of our knowledge, are accurate, but are not intended to replace official information released by companies, institutions, or other stakeholders. All author’s rights have been reserved; reproductions shown here are intended with educational uses only.

The present use case is intended to showcase the capabilities of IENAI’s ATHENA™ electrospray thrusters in an application for constellation deployment of Planet Labs’ Flocks. The ultra-high efficiency and uniquely scalable nature of the thruster allows for a reduced and customizable form factor, tailored to the requirements and limitations of the satellite platform and the specific maneuvers for which it is intended. This application has not been sponsored by Planet Labs in any way.

TL;DR

  1. Electric propulsion provides faster deployment and extended mission lifetime, and offers the potential of improving revisit rate and optical resolution.
  2. ATHENA™ thrusters reduce the deployment time of the entire constellation from about 300 days to just 14 days (including altitude reduction).
  3. The resolution provided by the Earth Observation constellation may be improved by lowering 60 km of altitude, while still performing station keeping for up to 4.5 years, extending the lifetime of the mission.
  4. The propulsion system uses only 60 grams of propellant for 3 different maneuvers: Orbit phasing, Orbit transfer and Station keeping.

MOTIVATION

Planet Labs holds the record for the largest, and perhaps most successful, constellation of nanosatellites currently in use: 439 Doves and SuperDoves, which make up the companies “flocks”, have been launched since 2013, with more than 150 operating today (the constellation is well into its “replenishment” phase), providing with Earth Observation capabilities of the Earth at under 4m/pixel resolution. The average lifetime of each of the Doves is less than three years.

Typical launches send together tens of satellites (44 on the latest SpaceX Transporter 3 mission), which are deployed at close orbits and then require phasing in order to be uniformly spread around the Earth. In order to do this either propulsive maneuvers or differential drag can be employed; however, Doves are extremely packed (up to the tuna can!), as can be gathered from publicly available images, and therefore most propulsion systems on the market simply do not fit. Enpulsion’s NANO thruster flew in January 2018 in a 3U CubeSat Flock-3p from Planet occupying 0.8 U as publicly stated.

Figures
Figure 1: Cosmogia (later renamed to Planet Labs) Dove-1 render, launched in 2013 (left, source); Updated Dove schematic (right, source)

To date, Planet has employed differential drag in order to achieve True Anomaly phasing of their satellites. Foster et al. provides a methodology for the differential drag control of a large fleet of propulsion-less satellites deployed in the same orbit through a simple controller and two control modes: high drag (panels against the incoming flow) and low drag (panels parallel to the flow); the analysis was then compared to the real deployment of the Flock-2p batch of 12 satellites, launched into a circular 510 km sun-synchronous orbit in June 2016.

Diagrams
Figure 2: Low drag and High drag configurations for differential drag control of Planet Doves (source: Foster et al.)

The “area-based” ratio of ballistic-coefficients should be around 10:1 for the high and low drag modes; however, Foster concludes that the actual is closer to 3:1 due to a combination of satellite duty-cycle effects (differential drag may only be performed outside of the satellite’s intended operation modes), skin friction effects and attitude pointing inaccuracies. Due to this, the phasing maneuver is completed in well over a year, with only 150 degree separation achieved with regards to the reference satellite in the first 100 days, potentially leading to a suboptimal geometry of the constellation batch and long gaps in between revisits, potentially heavily impacting the quality of the intended service.

Chart
Figure 3: Planet labs Flock-2p differential drag phasing maneuvers (source: Foster et al.)

In order to avoid lengthy deployment times for a constellation such as Planet’s Flocks, improve the performance of its Earth Observation capabilities and provide longer operational lifetimes, this use case presents the “inverse propulsion problem”, as solved through IENAI’s GO™ tool, obtaining an optimal configuration of the ATHENA™ thruster for the mission’s requirements, during a theoretical preliminary design phase.

SIMULATING IN GO™

The mission presented can be easily translated into the required input parameters in GO™. The first step will be to define these parameters on the user-friendly Configuration view. This section allows the user to define the values for the platform, thruster envelope and initial orbit.

ParameterValue
Platform3U
Solar panelsTriple deployable
Thruster PointingOff-Axis
Power20 W
Mass440 g
LauncherPSLV by ISRO

The configuration used for this mission is given in the table above. Platform, deployables and launcher are inputted to be as close to publicly released data from Planet as possible. Thruster pointing is an assumption made due to the characteristics of this mission: since ATHENA can’t be placed in any of the On-Axis faces due to the Dove’s payload and Tuna-can, we assume it would be placed perpendicular to the main axis of the platform. While a mid-drag mode could be exploited here, we assume the worst case scenario, the satellite being always in high-drag mode. A more detailed explanation is provided in the results section.

For this mission we are simulating the Doves with the best and worst cases in terms of ΔV. The 12 satellites compose an equally-spaced, coplanar constellation, this is a satellite placed every 30º of true anomaly. The best case in terms of ΔV will be the first satellite placed at 30º from the injection phase; the worst case will be the Dove placed at 180º from the injection. For both cases, we simulate the following maneuvers:

  • In-plane phasing with RAAN compensation (deployment).
  • Orbit transfer to 450 kms to improve resolution and revisit rate.
  • Drag compensation to extend mission lifetime.

Applying these maneuvers allows Doves to have faster deployment while improving resolution, revisit rate and mission lifetime, as will be discussed below. Note that the maneuvers for True Anomaly changes larger than 180º can be calculated in GO™ by choosing the required figure, but will take less than the 180º maneuver, since they are achieved by performing the opposite maneuver in terms of altitude change.

MISSION CONFIGURATION

The first step in GO™, once the mission is created, is to select the platform configuration, taken from the Flock 2p Doves: 3U platform with triple-deployable solar panels. Once selected, the values of thruster pointing and thruster power are defined: we used a 20 W power budget for the thruster, assuming that said power would be available for a “propulsion mode” in the mission. Furthermore, as we will see in the results, this value will restrict the use of a single thruster module (on a 1U area), which is agreeable with the presumed lack of space in the platform. The thruster pointing is selected to be Off-Axis (High drag mode), since it is the only possible configuration to add a thruster due to the current requirements of Flock 2p. Once these parameters are defined, we proceed to select the initial orbit. In this step, the user has two options: Custom orbit or Launcher selection. Thanks to our partnership with Precious payload, we can provide real-time launcher selection to ensure our users have visibility on realistic launcher opportunities for their missions. For this particular case, we selected the PSLV launcher from ISRO since it was the original one that carried Flock 2p to its injection orbit. Once the launcher is selected, the configuration is saved by clicking on the “SAVE” button, after which we are redirected to the maneuvers scenario view.

Configuration view
Figure 4: Configuration used for Flock 2p

BEST CASE – Orbit Phasing by 30º

Once the initial configuration is saved, the next step is to sequentially add the maneuvers required throughout the mission. In the Maneuver Scenario view in GO™, we select “Orbit phasing” as the maneuver type and then set a 30º true anomaly shift; we toggle ON the RAAN shift compensation, since the semi-major axis change leading to the phase change generates a small RAAN difference, which can be balanced by adding a small out-of-plane thrust component, at the expense of slightly larger propellant consumption and longer maneuver times. Once these values are selected, the minimum duration is calculated automatically by GO™, for the shortest physically possible maneuver. In this case, we select the minimum duration, but the user can play with this value to allow for longer maneuvers at lower propellant costs. Clicking on “Calculate maneuver” shows the results and orbital element time profiles during the maneuver.

Maneuver scenarion - Orbit phasing
Figure 5: Orbit phasing selections

In the results shown in Figure 6, Planet maneuver for the 30º true anomaly change takes approximately 300 days after injection. In comparison, by using ATHENA™, the maneuver is reduced to 3 days and can be executed starting right after injection. These results represent a 100x reduction of maneuvering time. The time saved is indicative of how the deployment rate capabilities may be improved through propulsive maneuvers, which would also allow fast replacement of non-functional satellites in the orbital plane, puting quick remedy to the suboptimal coverage in the event of a failure.

Chart - Orbit phasing 30º
Figure 6: Orbit phasing 30º

One of the additional advantages of using on-board propulsion is the ability to modify the orbit altitude over time. In 2019, Planet lowered the altitude of Earth-observation SkySats from 500 kilometers to 450 kilometers to improve the resolution of SkySat imagery. Recently, Cognitive Space CEO Scott Herman claimed that “To get higher resolution and still play in a smallsat world, your only option is to go lower” as reported in SpaceNews. Unfortunately, for satellites lacking propulsion, lowering the altitude is not only a lengthy process but will also decrease the overall constellation lifetime, among other potential unwanted effects. Conversely, adding the propulsive capabilities of ATHENA™ allows for lowering the constellation altitude and maintaining it at that higher drag scenario, ensuring the required constellation operational life.

For this reason, we decided to simulate two additional maneuvers to improve Flock 2p resolution and extend their lifetime. First, we add an “Orbit Transfer” maneuver to perform the altitude lowering, with a target of 450km (decreasing the altitude 60km from the original injection).

Maneuver scenario - Orbit transfer
Figure 7: Orbit transfer selections

The results are shown next. The minimum duration possible, selected here, is 6 days, although a longer maneuver is possible to reduce the propellant consumed by selecting a longer time in the “maneuver duration” input.

Chart Orbit transfer
Figure 8: Orbit transfer from 510 km to 450 km

Once target altitude is reached, the second step is to maintain it for at least the original mission lifetime, and, if possible, longer. We add a third maneuver: “Station keeping”. We can impose an allowed altitude tolerance as a percentage of the current orbit altitude.

Chart station keeping
Figure 9: Station keeping selections

Even at the new lower altitude, with higher resolution and revisit rate, and using a tight tolerance of 3% (~13.5 km), maintaining a Dove satellite is possible for up to 4.7 years if all the remaining propellant on board is consumed, as shown in Figure 10. Since we saved time in the initial phasing maneuver, this implies that the total operational lifetime in the correct orbit of the satellite is extended considerably.

Chart station keeping
Figure 10: Station keeping 4.7 years (best case)

WORST CASE – Orbit Phasing by 180º

As stated before we are analyzing only two of the satellites in the constellation. Now we will dive into the most costly maneuver in terms of ΔV. Since we are sharing the same configuration as in the best case, obtaining the results for this case is as easy as changing the input parameters in the initial ”Orbit phasing” maneuver. We now choose a 180º input for the True anomaly shift, leaving the RAAN shift compensation toggled ON. The new maneuver requires at least twice the time than in the best case, for a total of 8 days, and consumes 2.45 times the propellant. Here the advantage over the differential drag maneuver performed by Planet is evident again, reducing the deployment time by a factor 37 by means of propulsion.

Chart orbit phase
Figure 11: Orbit phasing to 180º (worst case)

For the second maneuver, as before, we lower the orbit altitude to 450 kilometers. The maneuver lasts approximately the same amount of time as in the first case, about 6 days, as shown in Figure 12.

Chart orbit transfer
Figure 12: Orbit transfer from 510 km to 450 km (worst case)

Finally, we also simulate the “Station keeping” maneuver using the same tolerance (~13.5 km) from the nominal orbit. This maneuver maintains Flock 2p orbit altitude during 4.5 years, as before, allowing for a much longer time at the operational orbit than using differential drag; the effect of the longer initial phasing maneuver is negligible over the possible duration of the Station keeping maneuver.

Chart - Station keeping - 4.5
Figure 13: Station Keeping for 4.5 years (worst case)

CONCLUSIONS

The entire thrusting time for the worst case is about 220 days as shown in Figure 14, with deployment to the operational orbit achieved after the first 14 days. The propulsion system uses 60 grams of propellant, respecting the mass limitation imposed in the mission configuration. Once all maneuvers are calculated, you can click on “Go to Mission Overview” to see a summary of the maneuvers and propulsive requirements. GO™ automatically optimizes and builds a configuration of ATHENA™ that is optimal for the given mission thus solving the “inverse propulsion problem”. These results are good to go for a theoretical preliminary design phase such as Phase 0/A. In the “Mission Overview” view you may also find the final characteristics of the thruster in terms of power, masses and volume and even export the thruster as a “.step” file to be added to your satellite CAD. Figure 14 shows the mission characteristics aggregated in a visual representation and in table format. You can also download all the data presented in GO™ as a “.pdf” for your records or to share it with your peers.

Mission overview
Figure 14: Visual and table format results for the “Worst Case” study

On-board propulsion gives space companies the freedom of selecting different launchers, maneuvering in space, deploying constellations, and safely deorbiting their satellites. In Flock 2p, adding an ATHENA™ thruster would allow for reducing the deployment time of the entire constellation from about 300 days to just 14 days (including altitude reduction), ensuring correct operational capabilities, in terms of coverage, since the start of the mission. Furthermore, the resolution provided by the Earth Observation constellation may be improved, together with shorter revisit rates, while still performing station keeping for 4.5 years, considerably extending the lifetime of the mission. The operational and performance gains obtained by adding a propulsion system are thus clear and offset the potential cost of adding a custom propulsion system to the satellite. In addition, Figure 15 presents the final configuration of the device, with around 480 g of mass and a volume fitting in less than 0.35U; such small volumes are critical to potentially fit the propulsion system on the already full Dove platform. Furthermore, ienai SPACE can offer additional customization capabilities for the ATHENA™ thrusters, tuning the form factor in a way that would be more agreeable with the existing subsystems in the spacecraft, without requiring extensive re-engineering.

ATHENA generated
Figure 15: configuration of ATHENA™ generated for Planet’s Doves

 

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