Arachne spacecraft with SSPIDR (Space Solar Power Incremental Demonstrations and Research) payload
In December 2020, AFRL (Air Force Research Laboratory) in Kirtland AFB, NM, received the first flight hardware component of the Arachne spacecraft, from Northrop Grumman. Arachne is the flagship experiment within the SSPIDR project. 1)
Table 1: Some background of the name Arachne 2)
The component, named "Helios," is a commoditized spacecraft bus known as ESPAStar built from an Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adaptor (ESPA) ring, and will serve as Arachne's "bus," the part of the spacecraft that houses the necessary components for managing the power, communications, thermal, and attitude (or spacecraft pointing/orientation) control systems across the entire spacecraft. The spacecraft bus holds the payload in place as it travels to orbit and is the platform off which the experiments are conducted.
"Acceptance of the Helios spacecraft bus from Northrop Grumman is a key milestone in the development of the Arachne spaceflight experiment," said Kevin Bryant, Arachne program manager.
This milestone is significant not only for the Arachne program, but also for AFRL as a whole.
The Helios bus is the third vehicle in the Long Duration Propulsive ESPA product line, a technology transition effort made possible by EAGLE, an AFRL space experiment launched in 2018 to demonstrate assured access to space," Bryant continued.
As is the case with many AFRL technologies, the concept of using an ESPA ring to build a commoditized bus, as demonstrated by the ESPA Augmented Geosynchronous Laboratory Experiment (EAGLE) program, was developed at AFRL and transferred to industry. This technology transfer process allowed AFRL scientists to commercially purchase Helios at a substantial cost-savings.
As a commoditized bus, "Helios provides a known platform from which to demonstrate space-based power beaming technologies and concepts," explained Bryant. Instead of investing time and money to develop, build, and test this critical component, AFRL experts can focus on developing the science required for a successful demonstration of solar power beaming.
"The acceptance of the Helios bus is an exciting culmination of AFRL's journey with the ESPA ring," said Kyle Gleichmann, Arachne chief engineer, who has worked as part of the AFRL Integration and Test (I&T) Team on three AFRL flight experiments and watched the ESPA spacecraft bus concept evolve and eventually transition to industry.
"The idea to use the ESPA ring as the main structure of a spacecraft started with AFRL's [Demonstration and Science Experiments (DSX)] program and eventually led to AFRL's EAGLE program. As a result of EAGLE, the ESPA platform has successfully been transitioned to industry and can be purchased as a commoditized spacecraft. This essentially allows AFRL to buy a spacecraft off the shelf and eliminate it from the critical path of our flight programs, allowing AFRL to concentrate our time, money, and efforts in bringing cutting-edge technologies to our warfighters," said Gleichmann.
Like many ESPAs, Helios boasts six ports to host payloads, but it is currently only anticipated to host one – the Space Solar Power Radio Frequency Integrated Transmission Experiment (SSPRITE). SSPRITE is being designed and built by Northrop Grumman and will utilize four of the six available ports and interface with Helios to receive power for its subsystems.
The next major step is testing and learning what software, mechanical components, and electrical wiring or harnesses will be needed to best interface with the bus, and build a functional spacecraft. This also allows AFRL's I&T Team to begin making any necessary adjustments to the bus to support the mission.
As Helios will be AFRL's first use of an ESPA platform in Low Earth Orbit (LEO), AFRL will make some modifications to the bus.
"Some of the known modifications the team will make include upgrading the communications system, enhancing its ability to operate in Low Earth Orbit," said Gleichmann. "This will increase our ability to send commands to the spacecraft and receive routine telemetry from the vehicle; uploading flight software used on previous flight experiments, and optimizing the power and communication interfaces between Helios and the primary payload, SSPRITE."
Helios and SSPRITE make up the Arachne spacecraft, which is the first of the incremental demonstrations that comprise the SSPIDR project – a collection of ground and flight experiments designed to mature different critical technologies needed to build an operational space-based solar power beaming system. Arachne will demonstrate emerging technologies that support the ability to convert solar energy to radio frequency (RF) power, using innovative "sandwich tiles" as well as the feasibility of transmitting that energy to the ground and converting it to usable power.
The Arachne spacecraft, with the Helios ESPA ring and the solar power beaming experiment SSPRITE, are forecasted to launch in 2024.
Figure 1: The Arachne spacecraft, the first of AFRL's SSPIDR flight experiments, is comprised of Helios and SSPRITE (image credit: AFRL)
Figure 2: Artist's rendering of the Air Force Research Laboratory's Arachne flight experiment on orbit (imagecredit: /AFRL) 3)
SSPIDR (Space Solar Power Incremental Demonstrations and Research) project
SSPIDR is a series of Integrated Demonstrations and Technology Maturation efforts at the Air Force Research Laboratory (AFRL) Space Vehicles Directorate to address space-based power collection and transmission capabilities. 4)
Space solar power beaming is not a new concept; yet until recently, the technology did not have a clear path forward. In conjunction with primary industry partner Northrop Grumman, AFRL established the SSPIDR project to mature technology critical to building an operational solar power transmission system for providing reliable and logistically agile power to expeditionary forces.
Figure 3: SSPIDR is a series of Integrated Demonstrations and Technology Maturation efforts at the Air Force Research Laboratory (AFRL) Space Vehicles Directorate to address space-based power collection and transmission capabilities (video credit: AFRL)
How Does It Work?: SSPIDR is a collection of flight experiments designed to mature different critical technologies needed to build an operational solar power beaming system in space. With the end goal in mind, the SSPIDR project team examined the needs of the operational prototype and identified five critical technologies needing further development to make this system a reality. The technologies driving the effort are Deployable Structures, Energy Generation, Thermal, Radio Frequency (RF) Beaming, and Metrology.
Scientists and engineers will explore these areas in three planned experiments that feature scalable aspects of the required technologies. Additionally, SSPIDR pursues parallel technology paths – advancing multiple experimental possibilities to find the most innovative technological solution for further maturation efforts. These advancements will feed into the development of the large scale system.
The large-scale system will collect solar energy using highefficiency solar photovoltaic cells. Northrop Grumman is developing the "sandwich tile" which will convert solar energy to RF and beam it to a receiving station on earth. At this point, the receiving antenna, called a rectifying antenna or "rectenna" will convert the RF into usable power for the end user. AFRL is not developing the final system, but instead is collaborating with Northrop Grumman and the Naval Research Laboratory in researching, maturing, and demonstrating the technologies required to build the objective system.
Why Is It Important?: AFRL's main mission is to develop and mature technologies to benefit the warfighter. Ensuring that a forward operating base receives power is one of the most dangerous parts of a ground operation. Convoys and supply lines, which are major targets for adversaries, are the usual methods to supply power. To utilize the solar power beaming system, a service member would simply set up a rectenna to gain access to power, eliminating costly and dangerous convoys. Essentially, AFRL is enabling the relocation of those supply lines to space, which could save countless lives.
A high possibility exists that this technology could be a highly valued asset in the commercial sector as well. Much like the Global Positioning System (GPS), which started out as a military asset and transitioned to a technology now used by people everywhere, this solar power beaming system could transition to broader usage, providing solar energy regardless of weather, time of day, or latitude.
Figure 4: The image depicts AFRL's Space Solar Power Incremental and Demonstrations Research Project beaming solar power from space to Earth. SSPIDR consists of several small-scale flight experiments that will mature technology needed to build a prototype solar power distribution system (image credit: AFRL)
• December 21, 2021: AFRL is funding a $100 million experiment to collect solar power in outer space for use on Earth. — A satellite solar panel designed by Northrop Grumman to harvest energy in space to be beamed back to Earth performed successfully in lab tests, clearing the way for the technology to be launched to orbit in a military experiment planned for 2025. 5) 6)
- The demonstration — conducted recently at a Northrop Grumman facility in Linthicum, Maryland — was in support of an experiment funded by the Air Force Research Laboratory to collect solar power in outer space for use on Earth.
- AFRL said Dec. 21 that the "sandwich tile" designed by Northrop Grumman for this experiment successfully converted solar energy to radio frequency power, "a fundamental step required to pave the way for a large-scale solar power collection system in space." To make this work, receiving antennas on Earth would be used to transform the RF energy into usable power.
Figure 5: Project managers James Winter (Air Force Research Laboratory) and Tara Theret (Northrop Grumman) hold models of the photovoltaic and the radio frequency sides of the sandwich tile that converts solar energy to radio-frequency power (image credit: Northrop Grumman)
- Space-based solar power is a key focus of AFRL, which awarded Northrop Grumman a $100 million contract in 2018 to develop the payload for a demonstration called Arachne. The sandwich tile is a key payload component.
- Arachne is one of a series of flight experiments planned by AFRL under the Solar Power Incremental Demonstrations and Research (SSPIDR) project.
- Jay Patel, vice president of Northrop Grumman's remote sensing programs, said the successful conversion of sunlight into RF energy is a "significant step forward in delivering the technology building blocks to achieve the Arachne mission."
- The sandwich tile consists of two layers: a panel of photovoltaic cells collect solar energy and provide power to the second layer, which is populated with components that enable solar-to-RF conversion and beamforming.
- The ground demonstration used a solar simulator to illuminate the photovoltaic side of the tile and perform the solar-to-RF conversion process.
- "It's crucial we demonstrate this technology on-orbit as soon as possible to meet our nation's needs," said Kyle Gleichmann, Arachne's chief engineer.
- The testing of the individual tile for the Arachne experiment provides a building block for a payload that will have a square meter panel of tiles. Arachne is projected to launch in 2025.
Figure 6: AFRL and Northrop Grumman attendees gather behind an industrial-grade opaque tarp to shield them from the intense light of the solar simulator used in the Solar-to-Radio Frequency demo at Northrop Grumman facilities and view RF output data from the sandwich tile (courtesy photo/Northrop Grumman)
• July 6, 2020: The Air Force Research Laboratory Space Vehicles Directorate is collaborating with the U.S. Department of Energy's (DOE's) National Renewable Energy Laboratory (NREL) in maturing a technology for fabricating high-efficiency solar cells in a high-throughput, low-cost manner via a technology called D-HVPE (Dynamic Hydride Vapor Phase Epitaxy). 7)
- "We have been tracking the DOE's investments at NREL in this area for many years," said David Wilt, AFRL senior physicist. "The team at NREL invented the D-HVPE technology and are world experts in using this technology to create high-efficiency solar cells."
- According to a July 26, 2019 article on the NREL website (www.nrel.gov), researchers at NREL "have refined the D-HVPE process to produce solar cells more than 20 times faster than the process now commonly used called metalorganic vapor-phase epitaxy (MOVPE)."
- "Current solar cell and panel production is costly," said Wilt. "The end goal of D-HVPE technology, as well as the other related efforts, is to enable high-efficiency space solar cells and panels to be produced in large quantity and at lower cost to enable more and larger space uses, as well as a variety of Department of Defense terrestrial applications."
- NREL sees valuable benefits in collaborating with AFRL on the D-HVPE technology.
- "Partnering with the Air Force is important so that we can mature D-HVPE technology to the point where it could be transitioned to private industry," said Kelsey Horowitz, NREL lead researcher. "The Air Force also assists NREL in better understanding the solar cell technology requirements and needs for a range of defense applications."
- D-HVPE is promising for both defense and commercial use. "If we are successful in reducing all of the high cost solar cell fabrication processes, we may enable the use of these high-efficiency cells in broader civilian and commercial applications," Horowitz said. "These include applications that require higher power per area and value flexibility, like on ships, electric vehicles, or portable devices."
- The AFRL Space Vehicles Directorate has been a leader in space solar power systems for decades and in collaboration with industry partners, has made huge advancements in photovoltaic-based spacecraft power technologies.
- "The maturation of D-HVPE technology will build upon AFRL's other research developments with the goal of providing game-changing renewable power solutions for the warfighter," said Wilt. "For example, space-solar-beaming, central to AFRL's SSPIDR project, needs affordable high efficiency solar cells such as those produced by D-HVPE," Wilt said. "Scientists at AFRL anticipate using these next-gen solar cells to gather the sun's energy, convert it to radio frequency and beam it to a receiver on earth."
Figure 7: AFRL's ROSA (Roll-Out Solar Array) developed in partnership with Deployable Space Systems in flight-testing on the International Space Station. ROSA offers scalable solar power array technology from hundreds of watts to the megawatt range (photo credit: NASA)
Some background on a military spaceplane experiment
July 4, 2020: An experiment launched May 17 onboard the U.S. Air Force's X-37B Orbital Test Vehicle-6 is based on over a decade of work focused on a modular space solar satellite capable of beaming energy to Earth. 8)
Developed by the U.S. Naval Research Laboratory (NRL) in Washington, the hardware is called the Photovoltaic Radio-frequency Antenna Module (PRAM).
PRAM is an outgrowth of NRL effort in developing "sandwich" modules where one side receives solar energy with a photovoltaic panel, electronics in the middle convert that direct current to a radio-frequency signals and the other side has an antenna to beam power away.
NRL's Paul Jaffe, the Innovation Power Beaming and Space Solar Portfolio lead, said the PRAM aboard X-37B is not establishing an actual power-beaming link. Rather, the 30 cm square module is dedicated to evaluating its energy-conversion ability and the thermal performance of the device in Earth orbit. While PRAM does generate RF energy, that energy does not go to an antenna due to a potential for interference with other payloads aboard X-37B, he told SpaceNews.
Figure 8: PRAM is an experiment onboard the U.S. military's X-37B spaceplane, shown here in 2017 (image credit: U.S. Air Force)
We're testing a functional component that would be part of one class of solar power satellite that would ultimately send energy from space to Earth," Jaffe, PRAM's principal investigator, said. "We anticipate publishing something in several months once we get back some data and have a chance to analyze that information."
There will be regular data deliveries from the vehicle that's hosting PRAM, said Chris DePuma, NRL electronics engineer and PRAM program manager. "The advantage of their [X-37B] platform is that we don't have to create our own communication system. They collect our data in a package for us to analyze."
Given PRAM results, a next step would be fabricating a fully functional system on a dedicated spacecraft to test the transmission of energy back to Earth that could potentially help power remote installations like forward operating bases and disaster response areas.
Hitting a baseline
In an October report, "Opportunities and Challenges for Space Solar for Remote Installations," an NRL study group explored the concept of providing power to military and remote installations via solar power. The study determined that there remain significant unresolved technological, economic, legal, political, operational, organizational, and schedule challenges inherent in the development of a deployable space solar capability.
However, because of the potential game-changing nature of space solar power for terrestrial applications, the study team recommended investments in several critical areas, the foremost of which was power beaming technology.
Figure 9: The PRAM hardware is the first orbital experiment designed to convert sunlight for microwave power transmission for solar power satellites (image credit: U.S. Naval Research Laboratory)
Jaffe said there remain open questions with power beaming technology and its level of maturity, hence the X-37B experiment. The PRAM is viewed as the first orbital experiment designed to convert sunlight for microwave power transmission for solar power satellites.
"You can certainly make a case for solar power satellites in many circumstances where a laser link, not a microwave link, would be preferable," Jaffe said. "One application is getting energy into the permanently shadowed regions of the moon," he said, where expected water-ice there can be processed into drinkable quantities to sustain crews, as well as break that resource into components of rocket fuel.
NRL's DePuma said the main goal of the PRAM experiment on the X-37B is hitting a baseline and proving the concept works and they are not missing any major issues.
"The reason behind the sandwich module architecture is to modularize the space solar satellite system. You could send up components a few at a time and assemble them on orbit. You build a very large structure with multiple small launches," DePuma said. "It's a good way to approach getting to the larger systems.
1) "AFRL receives first component of solar-beaming project," AFRL News, 10 December 2020, URL: https://www.afrl.af.mil/News/Article/2442502/afrl-receives-first-component-of-solar-beaming-project/
4) "Space Power Beaming," AFRL, 2021, URL: https://afresearchlab.com/technology/space-power-beaming/
5) Sandra Erwin, "AFRL and Northrop Grumman test key hardware for space-based solar power experiment," SpaceNews, 21 December 2021, URL: https://spacenews.com/afrl-and-northrop-grumman-test-key-hardware-for-space-based-solar-power-experiment/
6) Rachel Delaney, "AFRL, Northrop Grumman demonstrate solar to radio frequency conversion," AFRL News, 21 December 2021, URL: https://www.afrl.af.mil/News/Article/2878401/afrl-northrop-grumman-demonstrate-solar-to-radio-frequency-conversion/undefined/
7) Jeanne Dailey, "AFRL collaborates in break-through solar power development," AFRL, 6 July 2020, URL: https://afresearchlab.com/news/afrl-collaborates-in-break-through-solar-power-development/
8) Leonard David, "Military spaceplane experiment sheds light on space solar satellites," SpaceNews, 4 July 2020, URL: https://spacenews.com/x-37b-experiment-sheds-light-on-space-solar-satellites/
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).