Sky & Telescope - August 2022 - 15

the researchers would get was from Mars's weak gravity -
about a third of Earth's - which would make it a bit easier to
lift the solar panels, rotors, and landing gear, not to mention
the computers, batteries, electronics, heaters, cameras, accelerometers,
and laser altimeter housed inside the fuselage.
All of that work had to be done with the clock ticking and
with the team knowing that any major test failure or other
mishap could derail their ride to Mars. The team referred
to itself as WENDY, for " We're Not Dead Yet, " Matt says,
because NASA only gave them a little bit of money at a time,
tied to a deliverable that then led to the next short-term
funding with a new deliverable. " So we never knew if we
would actually get to the launch pad. "
But they had their eyes on the final prize. " Our team
ect manager, bringing her expertise with spacecraft-to-spacecraft
communications technologies as well as knowhow from
having led JPL's Guidance, Navigation, and Control Sensors
section. MiMi describes the helicopter team's journey from
the initial concept through design and testing as starting
with a basic question: Is it possible to build and fly a helicopter
on Mars at all? The team members took a fast-paced - yet
also incremental and systematic - approach to answering the
question, starting with prototypes and engineering models
that they operated remotely inside JPL's " space simulator "
thermal vacuum chamber, set up to simulate Mars with its
thin and cold carbon dioxide atmosphere.
To achieve lift in an atmosphere with a pressure comparable
to 80,000 feet above Earth's surface (where existing
helicopters can't fly), they realized that they would need a
super-lightweight fuselage and large, specially shaped rotor
blades that could spin about 10 times faster than what is
needed for terrestrial helicopters. They brought fans into the
testing chamber to add wind, making flying even tougher but
more realistic. They worked out how to communicate back
and forth with the rover, essentially creating a mini local
Wi-Fi network between the heli and its rover " base station. "
And they figured out how to enable the helicopter to fly
autonomously: Relying on its own cameras, computers, and
other systems, the craft has to make decisions in real time
about how to stick to the flight plan, including adapting to
changing conditions like wind gusts.
The team did its own research as well as leveraging ideas
from the terrestrial robotic drone community. The only break
really, really wanted to get to fly the helicopter at Mars, "
MiMi says. " As a result, as we accomplished each major milestone,
we dared not celebrate fully - we had the attitude of
'not yet, not yet' at each step. "
The resulting helicopter design is both familiar and
alien. The dual lightweight, carbon-composite rotor blades
each span 1.2 meters (almost 4 feet) from tip to tip. They
counter-rotate nearly at the Martian speed of sound in order
to generate the required thrust. (A similar rotor spinning on
Earth could lift more than 50 times as much mass, according
to JPL's Håvard Grip, Ingenuity's chief pilot.) The fuselage is
a relatively simple box a little smaller than a typical toaster
and coated with Kapton film for thermal control. Rather than
wheels or skids like most helicopters, this one's landing gear
consists of four flexible, 38-cm-long (15-inch-long) angled
legs made of tapered carbon-fiber tubes, with golf club-like
feet designed to prevent the legs from digging into soft soil.
Springy hinges at the legs' tops help soften the shock of landing.
Power comes from six lithium-ion batteries inside the
fuselage that are trickle-charged during the daytime by solar
HELICOPTERS 101
Aircraft fly thanks to the inverse relationship between
air's speed and pressure: The higher the speed, the
lower the pressure. The curved top and flat bottom of
aircraft wings force air to flow over the top faster than
it does over the bottom, which causes suction and lifts
the craft higher. An airplane zooms through the sky to
move enough air over its wings to fly; a helicopter does
the same thing by spinning its rotor blades.
sk yand tele scope .o r g * AUGUST 2022 15
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Sky & Telescope - August 2022

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