The Mars Helicopter Ingenuity: A deep dive into its 6-pack

Given the recent excitement about the Mars Helicopter Ingenuity’s first powered flight on another planet, I wanted to share my research into its power source: 6 lithium ion cells.

Background

Ingenuity rode under the belly of the Perseverance rover which landed on Mars on the 18 February 2021. Perseverance dropped the Ingenuity from its underside, and then drove away leaving it completely on its own to conduct its first powered flight on another planet. Once this manoeuvre was complete, Ingenuity has been completely self-reliant for all of its energy needs. As Bob Balaram, Chief Engineer for the Mars Helicopter Project wrote: “No more free power from the rover!”.

The main goal of Ingenuity is to provide a stepping stone to further powered, autonomous flying on another planet. The learnings from this mission will feed back into future missions where an autonomous aircraft could carry instruments and bring back samples from further afield.

The 6-pack

You may be surprised to learn that the Ingenuity’s energy source is built from six off the shelf 18650 lithium ion cells from Sony, the VTC4. This cell has been around for several years - I found a data sheet from 2012 while researching for this article. Given the general high power and low energy specifications of the VTC4, it has probably been used in power tools here on earth. Furthermore, it comes as little surprise that the team at the JPL went for something “tried, tested, and true” for a mission critical component.

The cells are configured in a 6S configuration, meaning they are all connected in series. This yields a pack voltage of 25.2 V when fully charged and 15 V when fully discharged. The nameplate (using numbers from the cells’ datasheet) capacity and energy rating of the pack is 2 Ah and 42.1 Wh, respectively, when discharged with a constant current of 10 A. The team at Nasa designed Ingenuity assuming that there is about 36 Wh of energy available for use every cycle of the cells, allowing for a bit of derating.

Weight is everything in space flight, therefore while designing the Ingenuity every single component was scrutinised for its weight. On earth, Ingenuity weighs 1.8 kg and of that, 273 g are the Sony VTC4 cells, or about 15%. For an aircraft or even a road vehicle, this is a small portion of weight dedicated to a battery energy storage system, so the engineers had to manage this energy very wisely.

Energy management

One of the most interesting things about Ingenuity is that the helicopter will be completely self-reliant for its own energy production, storage, and management as soon as Perseverance drops it onto the Martian surface. There are three components to its energy system 1) a solar panel providing an energy source, 2) a Lithium-ion battery pack for its energy store, and 3) control electronics to manage all of the energy in the system and the energy within the battery pack.

Ingenuity relies on its Lithium-ion battery pack to store enough energy to keep it warm through the extremely cold Martian nights (-90 deg C). Ingenuity completed its first night alone on Mars on the 6th of April, 2021, a big milestone towards its first flight. From the 36 Wh of energy available for use during every cycle of the battery pack, 21 Wh will be used every night to keep the cells and electronics warm enough so that they do not fail. Lithium-ion cells are a ‘Goldilocks’ chemistry – they do not like to be too hot or too cold, and can fail catastrophically if not treated properly. The cells are directly heated with strips attached to them as shown in the figure. The cells are surrounded by circuit boards, and the circuit boards are encased in a foil jacket. Mars’ atmosphere of CO2 will provide enough insulation to keep everything warm. Keep in mind that Mars’ atmosphere is less than 1% as dense as Earth’s which means it is more difficult for heat to transfer from an object into the atmosphere – in other words, in some ways it is easier to keep something warm on the surface of Mars as compared to Earth.

Ingenuity was designed so that it can use 10 Wh of energy every flight, which is…not a lot. 10 Wh is equivalent to the amount of energy held in the average smart phone battery, or about 10 large apples. As a result, the flights will be restricted to 90 seconds at a time and “only a few” are part of the planned mission. However, I would be willing to bet that the Ingenuity team will do as many flights as possible beyond the mission plan given the extended missions of their Spirit (2269 earth days) and Opportunity (5498 earth days) rovers, both of whom had planned missions of only 92 earth days. In order to extend Ingenuity’s mission, its energy system will be one of the critical systems that must be working to its energy and power specifications.

Power

Given the energy and flight constraints, the battery pack must deliver the required power for the flights themselves. This is an important characteristic in any battery pack design, particularly for any type of vehicle. The internal chemistry of Lithium ion cells can be tuned by a certain degree for different applications to optimise for high power (fast acceleration) or high energy (further range). However, there is no such thing as a free lunch: high power Lithium ion cells usually have a lower energy density than high energy Lithium ion cells (if you want to accelerate quickly, you won’t be able to go as far). Therefore, there is always a trade-off in battery pack design between power density (how quickly can we accelerate a car) and energy density (how far can we drive a car). The cells chosen for Ingenuity are designed for power delivery over energy density. From the 2012 data sheet, they are rated all the way up to 30 A discharge per cell, or 15 C. This is quite impressive.

A note about ‘C-rates’: most packs and cells are specified with a maximum ‘C-rate’. It is a normalised quantity determined by dividing the maximum cell current by the capacity of the cell, so in the case of the Sony VTC4:

30 A / 2 Ah = 15 C.

As designed, Ingenuity’s battery pack is capable of 480 W of continuous power, and a peak power of 510 W. This is works out to be below the 15 C rating of the cells, most likely limited by design. The helicopter itself will have a peak power requirement of 350 W, well below the full capabilities of each cell, and below the designed capabilities of the pack. This will ensure that Ingenuity can perform several flights even as the cells degrade.

Cell degradation not only effects the capacity of the cells (how long the flights are), but also the peak power the cells can deliver (the ability to generate enough lift for flight). Therefore, the end of life scenario for Ingenuity could be that it just cannot generate enough power to perform any more flights.

That's it!

I hope you found this very small part of the Mars 2020 rover mission as interesting as I did. The energy system of Ingenuity will play a big role in the success of its mission, paving the way for scientists to explore further afield from a single landing spot on Mars and other celestial bodies.

References and more Information

• A great video about all aspects of Ingenuity from Veritasium.
• Poor Spirit
• https://www.popularmechanics.com/space/moon-mars/a35353442/ingenuity-mars-helicopter/
• https://en.wikipedia.org/wiki/Mars_Helicopter_Ingenuity
• https://www.jpl.nasa.gov/news/press_kits/mars_2020/download/ingenuity_landing_press_kit.pdf
• https://rotorcraft.arc.nasa.gov/Publications/files/Balaram_AIAA2018_0023.pdf