MSL Picture of the Day: T-4 Days: Mission Essentials

MSL Picture of the Day: T-4 Days: Mission Essentials

I think we all agree that without good wheel set-up and the power to operate to move those wheels over hard and diverse terrain this rover mission can not happen. Today we are going to explain how both work for Curiosity.

Curiosity’s wheels and driving systems are a scaled-up version of what was used on the three earlier Mars rovers: Sojourner, Spirit and Opportunity. Six wheels all have driver motors. The four corner wheels all have steering motors. Each front and rear wheel can be independently steered, allowing the vehicle to turn in place as well as to drive in arcs. The suspension is a rocker-bogie system.

note: the following explanation and image is courtesy of  Nick Sotiriadis,

Combined with a differential connecting the left and right sides of the mobility system, the rocker-bogie design enables a six-wheel vehicle to keep all its wheels in contact with the ground even on uneven terrain, such as with a wheel going over a rock as big as the wheel.
On each side, a bogie connects the middle and rear wheels and provides a pivot point between those wheels. The rocker connects the bogie pivot point to the front wheel.
The rocker’s pivot point connects to the differential across the rover body to the rocker on the other side.

Curiosity’s wheels are aluminum, 20 inches (0.5 meter) in diameter, which is twice the size of the wheels on Spirit and Opportunity. They have cleats for traction and for structural support.

The wheels also have the morsecode for JPL cut out in their surface. Curving titanium spokes give springy support. The wheels were machined by Tapemation, Scotts Valley, California. Titanium tubing for the suspension system came from Litespeed Titanium, Chattanooga, Tennessee.

The drive actuators — each combining an electric motor and gearbox — are geared for torque, not speed. As speed is not what we are looking for on Mars, but the ability to slowly move up a hill or over a boulder might be just the thing we do need.  The rover has a top speed on flat, hard ground of about 1.5 inches (4 centimeters) per second. (compared to 1 centimeter per second for Sojourner, 15 years ago)
Aeroflex Inc., Plainview, New York, built the cold-tolerant actuators for the wheels and other moving parts of Curiosity.

Under autonomous control with hazard avoidance, Curiosity achieves an average speed of less than 2 centimeters per second. The rover was designed and built to be capable of driving more than 12 miles (more than 20 kilometers) during the prime mission of the first 98 weeks. The actual odometry will depend on decisions the science team makes about allocating time for driving and time for investigating sites along the way.

Curiosity is the first rover that lands without a protective covering of man-sized airbags. Curiosity will land on its wheels, which will directly absorb the force of impacting the Martian surface at touchdown. As on the earlier rovers, the wheels can also be used for digging beneath the surface by rotating one corner wheel while keeping the other five wheels immobile.

Curiosity gets her power from a  multi-mission radioisotope thermoelectric generator (MMRTG) supplied by the U.S.Department of Energy. This generator is essentially a nuclear battery that reliably converts heat into electricity.

It consists of two major elements: a heat source that contains plutonium-238 dioxide and a set of solid-state thermocouples that convert the plutonium’s heat energy to electricity.
It contains 4.8 kilograms  (10.6 pounds) of plutonium dioxide as the source of the steady supply of heat used to produce the onboard electricity and to warm the rover’s systems during the frigid Martian night. Curiosity’s systems need to be kept warmer than -40 Celsius.

Radio-isotope thermoelectric generators have enabled NASA to explore the solar system for many years. The Apollo missions to the moon, the Viking missions to Mars, and the Pioneer, Voyager, Ulysses, Galileo, Cassini and New Horizons missions to the outer solar system all used radioisotope thermoelectric generators.
The multimission radioisotope thermoelectric generator is a new generation designed to operate on planetary bodies with an atmosphere, such as Mars, as well as in the vacuum of space. In addition, it is a more flexible modular design capable of meeting the needs of a wider variety of missions as it generates electrical power in smaller increments, slightly more than 110 watts. The design goals for the multi-mission radioisotope thermoelectric generator include ensuring a high degree of safety,optimizing power levels over a minimum lifetime of 14 years, and minimizing weight.

It is about 64 centimeters (25 inches) in diameter by 66 centimeters (26 inches) long and weighs about 45 kilograms (99 pounds).
Like previous generations of this type of generator, the multi-mission radioisotope thermoelectric generator is built with several layers of protective material designed to contain its plutonium dioxide fuel in a wide range of potential accidents, verified through impact testing.

The type of plutonium used in a radio-isotope power system is different from the material used in weapons and cannot explode like a bomb. In the unlikely event of a launch accident, it is unlikely that any plutonium would have been released or that anyone would have been exposed to nuclear material.

The plutonium is manufactured in a ceramic form, that does not become a significant health hazard unless it becomes broken into very fine pieces or vaporized and then inhaled or swallowed.
If there had been an accident at the launch of Mars Science Laboratory, people who might have been exposed would have received an average dose of 5 to 10 millirem, equal to about a week of background radiation. The average American receives 360 millirem of radiation each year from natural sources, such as radon and cosmic rays.

The electrical output from the multi-mission radio-isotope thermoelectric generator charges two lithium ion rechargeable batteries. This enables the power subsystem to meet peak power demands of rover activities, when the demand temporarily exceeds the generator’s steady output level. The batteries, each with a capacity of about 42 amp-hours, were made by Yardney Technical Products, Pawcatuck, Conn. They are expected to go through multiple charge-discharge cycles per Martian day.

Curiosity’s thermal control system was designed to enable the rover to operate far from the equator so that the mission would have a choice of landing sites based on science criteria. In a range of Mars surface temperatures from minus 133 degrees Celsius (207 degrees Fahrenheit) to 27 degrees Celsius (81 degrees Fahrenheit), the temperature-sensitive components inside the rover can be maintained between minus minus 40 degrees Celsius (40 degrees Fahrenheit) and 50 degrees Celsius (122 degrees Fahrenheit).

The rover’s heat rejection system has a pumped fluid loop that can deliver heat from the multi-mission radioisotope thermoelectric generator when the core electronics need heating and take heat away from the core if the rover is becoming too warm. Pacific Design Technologies Inc., Goleta, Calif., built the pump.

The fluid loop runs through an avionics mounting plate inside the insulated warm electronics box of the rover chassis. The multi-mission radio-isotope thermoelectric generator cools passively with its radiator fins when its heat is not needed for warming the rover. For heating needs beyond the circulation of the heat rejection system, the rover uses electrical heaters. This makes it possible to give extra heat when needed. Atop the remote sensing mast Curiosity has a secondary warm electronics box to maintain temperatures there above allowable minimums.

Radio-isotope power systems & heaters used in missions:

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