Venusian Voyagers

Universal Event | Exploring Venus Together

Venusian Voyager Mission: Dual Spacecraft Power Beaming System

High-Level Project Summary

Our Venusian Voyager Mission will be composed of two spacecrafts: an aerial vehicle (AV) and a surface lander. Both will be equipped with GaInP/GaAs 2J multijunction photovoltaic solar cells (33% efficiency) and LMBs composed of Ca–Mg||Bi cells which can operate under Venusian temperatures and pressures. Through cycles of energy harvesting and ytterbium-doped fiber laser (YDFL) power beaming, the AV will ascend and descend below the Venusian cloud layer to transmit energy to the receiver (rectenna) on the surface lander. Thus, a sustained energy cycle will be created to provide for the surface lander and successfully complete the 60-day mission.

Link to Final Project

https://syonagupta.notion.site/5c3a65e4c53445eaaa2da046529b1653?v=bbd00e41312242cdad6ae0aefdfa38ac

Link to Project "Demo"

https://docs.google.com/presentation/d/1lPbMIbVtCgetwSX-xiMAsjCCMkpKKZFRs4aHW76-08k/edit?usp=sharing

Detailed Project Description

Introduction

We propose a dual spacecraft system composed of a surface lander and aerial platform. Both spacecrafts would be covered in multijunction photovoltaic cells made up of gallium compounds (GaInP/GaAs 2J solar cells). Because Venus’ dense CO2 atmosphere and sulfuric acid clouds reflect up to 80% of all radiation, the surface lander would not be able to receive adequate solar energy to power itself on its own. Less than 100 W/m2 are available at the surface of Venus compared to ~2622 W/m2 above the atmosphere, due to absorption and scattering of radiation by Venus’ thick atmosphere and cloud layer. To support the surface lander and provide enough energy, an aerial vehicle (AV), at a cruising altitude of ~60km will be utilized, as it will be able to receive significantly more energy. Our proposal has four general steps: 1) AV harnesses solar energy, 2) AV descends below cloud layer to ≤ 47km altitude, 3) ytterbium-doped fiber laser (YDFL) power laser is beamed to the receiver (rectenna) on the surface lander for a two-hour transmission duration, 4) AV ascends to cruising altitude to harness more solar energy before its next descent. Both the AV and lander would contain liquid-metal batteries (LMBs) that harness Ca-Mg alloy as a negative electrode and can operate at 550 degrees C.

Aerial Vehicle (AV)

The aerial vehicle (AV) will function as an energy harvesting platform. A solar array will be mounted on the maneuverable platform, whose course can be altered based on Venusian weather conditions. With a design modeled after the Venus Atmospheric Maneuverable Platform (VAMP) designed at Northrop-Grumman, the AV will be an aeroshell-less hypersonic entry vehicle that transitions to a semi-buoyant, maneuverable, solar-powered air vehicle for flight in Venus' atmosphere. It will hold a cruising altitude of 60 km and a transmission altitude of 47km or lower. The AV will continuously cycle between transmitting and recharging every two hours. It will descend beneath the cloud layer to transfer solar energy through a ytterbium-doped fiber laser (YDFL) power laser and then ascend to continue charging. Ascent and descent will occur at a speed no greater than 5 km/h.

The AV will require 2 hours of charging to perform 2 hours of transmission. At ~60 km above the surface, ~2300 W/m2 is available to the AV. The solar arrays will have an efficiency of ~33%, leaving ~766 W/m2 available for storage in the LMBs. The battery has a storage efficiency of 99%, so very little energy will be lost during storage. With an energy loss factor of 20% for the power beam, ~150 W/m2 will be received by a 10m2 rectenna for every 2-hour transmission. The aircraft will descend beneath the cloud layer in a circular pattern. To function through significant temperature fluctuations the AV will be coated in a thin-layered blend of alumina and silica aerogels. The solar energy received by the AV will be stored in LMBs and will be converted into a ytterbium-doped fiber laser (YDFL) for transmission and beamed to a receiver on the lander. Fiber lasers (FLs) demonstrate very high electrical to optical conversion efficiency and they will convert the electrical energy from the solar cells into the power laser. Afterward, the aerial craft would ascend to its original position above the cloud layer to continue receiving solar energy.

Aerogels

Coated in alumina and silica aerogel, the aircraft will not be harmed by increasing temperatures during its descent. Aerogels have been used by NASA ever since the 1990s. Aerogels are a type of nanomaterial that consist of an open-cell network and are very porous. Silica aerogels are unique as they possess a lower density, thermal conductivity, refractive index, and dielectric constant than any other solids. Aerogels make a good insulator due to the fact their pores are in the nano range. NASA has used monolithic silica aerogels previously. Specifically, the silica aerogel was first used as an insulator on the Mars Rover, Sojourner, to protect the primary battery pack against low temperatures. However, in this instance, the properties of alumina-silica-based aerogels will be leveraged for Venus’ high-temperature environment. The blend of alumina and silica allows the aerogel to withstand temperatures up to 1200-1400 °C.

Ytterbium-doped fiber laser (YDFLs)

The power laser will be a ytterbium-doped fiber laser (YDFL) working in the near-infrared (NIR) emitting pulses at 1022 nm. This wavelength holds the lowest possible energy loss factor: 20%. This loss factor is due to atmospheric scattering and absorption. YDFLs are capable of high-power operations because of their femtosecond-paced pulses. The laser beam would be received by silicon carbide rectennas.

Surface Lander

A rectenna is a rectifying antenna, a special type of antenna that is used to directly convert microwave energy into DC electricity. Its elements are most often arranged in a multi-element phased array (a phased array usually means an electronically scanned array, a computer-controlled array of antennas) with a mesh pattern reflector element to make it directional (i.e. receive the high-energy laser beam from any direction). We will be using these special antennas to convert the laser beam directed from the AV to DC (Direct Current) for supply to the surface lander. Silicon carbide will be used for making the electronic components of the surface lander and of the AV. This is because it can withstand the high temperature and pressure of the Venusian surface. On the surface lander, a photovoltaic laser power converter (PVLPC), will transform the laser beam into electricity. Implementation of these laser power converters (LPCs) is possible because they are semiconductor devices that convert optical energy into electrical energy. Additionally, we plan to use 304 stainless steel and titanium alloys to make the protective layer for the energy storage system, along with the heat-resistant aerogel.

Liquid-metal batteries (LMBs)

We will utilize LMBs developed by Donald Sadoway and a team at MIT as our energy storing system on both the AV and surface lander. These batteries operate at temperatures ranging between 450 and 900oC. The Ca-Mg alloy would act as the negative electrode, while Bi would act as the positive electrode. The battery will use a LiCl–CaCl2 multi-cation mixture as an electrolyte. The Ca–Mg||Bi cells are designed to suppress the self-discharge current density to <1 mA cm^ -2 while maintaining high cell voltage at optimal operating temperatures.

Multijunction photovoltaic solar cells

GaInP/GaAs 2J multijunction photovoltaic solar cells will be placed on the rover and the AV. These solar cells utilize gallium in place of silicon and will work by capturing photons that bombard the panel. These photons will knock off electrons from atoms causing a flow of electricity. Two regions in traditional solar cells that are made up of silicon will house this reaction. Replacing silicon with gallium arsenide will also improve the solar array’s ability to operate in temperatures ~300°C. Gallium arsenide is a type of semiconductor that is highly resistant to radiation and heat. Gallium arsenide solar cells have higher efficiencies compared to traditional solar cells because the electrons are able to move faster through the structure as well.

Multijunction solar cells are photovoltaic (PV) cells that have multiple p-n junctions, which are the conversion places for photons to electricity. Due to our PV cells being multijunction cells, this allows the junction to absorb different frequencies of light. When waves of light are received, they will go through multiple layers. This means that if the first layer doesn’t fully convert all the photon energy, the next layer will capture the remaining photons. The combination of gallium-arsenide and the multijunction properties enables a 33% solar cell efficiency.

Details of energy storage system

A typical Venus rover requires a power supply of 100 W for 1 hour every 24 hours, which results in 100 W-hr of total energy consumption. For 8 hours, 91 W is required for a total of 728 W-hr of energy. This is followed by an 8-hour recharge period and then a 91 W data transmission period for 7 hours. This results in total energy utilization of ~1545 W-hr per day of operation for the surface lander.

The LMBs on the rover will receive 12 hours of beaming per day in 6 cycles of 2-hour transmissions. With each 2-hour transmission providing 150 W/m2, this gives the rover a total of 900 W/m2 per 24 hours from the AV. The solar array on the surface lander itself will provide energy as well. The surface of Venus only receives ~8 W/m2. With a 10m2 array composed of solar cells with 33% efficiency, it will receive ~26 W/m2 every hour, equating to 624 W-hr per 24 hours. (One day on Venus is 243 Earth-days, so for a mission duration of 60 Earth days, it would receive continuous sunlight for the entire duration of the mission). In total, our system will generate 1524 W-hr per day. The battery system will be 4.7 L in volume. This value was calculated by the 329 W-hr/L volumetric discharge energy density for the LMB containing Ca-Mg alloys. With a total energy usage of 1545 W-hr each day, this would require a battery system of ~4.7 L. The mass of the battery was calculated by dividing total energy usage (1545 W-hr) by gravimetric discharge energy density for the LMB (45 Wh kg-1), resulting in 34 kg. The Ca-Mg||Bi cells are designed to suppress the self-discharge current density to <1 mA cm-2 while maintaining high cell voltage at optimal operating temperature.

Conclusion

To summarize, our Venusian Voyager Mission will be composed of two spacecrafts: an aerial vehicle and a surface lander. Both will be equipped with GaInP/GaAs 2J multijunction photovoltaic solar cells and LMBs containing Ca-Mg alloys. Through cycles of energy harvesting and power beaming, the AV will ascend and descend below the Venusian cloud layer to transmit energy to the surface lander. Thus a sustained energy cycle will be created to provide for the surface lander and successfully complete the 60-day mission.

Space Agency Data

To gain a general understanding of Venus we relied on a NASA Solar System Exploration page that provided both an overview and an in-depth approach to learning about planetary conditions on Venus.

Venus. (n.d.). NASA Solar System Exploration. Retrieved October 2, 2022, from https://solarsystem.nasa.gov/planets/venus/overview

To familiarize ourselves with past missions and their feats and failures, we relied on:

Energy Storage Technologies for Future Planetary Science Missions. (n.d.). NASA Solar System Exploration. Retrieved October 2, 2022, from https://solarsystem.nasa.gov/resources/549/energy-storage-technologies-for-future-planetary-science-missions

We based our design on the "Power Beaming for Long Life Venus Surface Missions" NIAC Phase I Final Report published by Brandon et al. March, 2020. Using the dual spacecraft system data from this report as our foundation, we constructed our proposal for the best-equipped energy storage system for a 60-day mission to Venus.

Brandon, E. J., Bugga, R., Grandidier, J., Hall, J. L., Schwartz, J. A., & Limaye, S. (2019). Power Beaming for Long Life Venus Surface Missions. 46.

Hackathon Journey

This Hackathon experience was a one-of-a-kind journey of growth for all of us. As a group of six high school students -- three from the United States and three from India, none of us knew every single person on the team prior to the challenge. Through early morning research sessions, late-night technical conversations, and all-day calls, we are all happy to say that throughout this challenge, we became closer than we anticipated. We learned the virtues of perseverance and patience and how they can be leveraged to improve team dynamics. We gained invaluable experience with scientific communication and literacy. And above all, we were able to collaborate with like-minded individuals from the other side of the globe, brought together by our shared vision for the future of space exploration.

We approached this project by researching already established ideas and trying to find ways to improve them. We initially struggled with coordinating meeting times, as our time zones are over nine hours apart. This initial struggle did not last long and once we fell into a pattern by syncing our schedules, we found that our time differences could be utilized as an advantage (half of our team could be working while the other half was asleep! ).

We are so grateful to have had the opportunity to participate in the NASA Space Apps 2022 Challenge. This experience has been remarkably unique and rewarding.

-- The Venusian Voyagers