The spacecraft

Each rover was carried to Mars in an entry capsule that was in turn attached to a cruise stage; disc-like structures over 2.6 m in diameter and 1.6 m deep. The identical cruise stages each carried photovoltaic arrays delivering 600 W of power at beginning of life (BOL) and had a dry mass of 870 kg. The propulsion system of the cruise stages used 31 kg of pressurized hydrazine and could give a 1V capability of around 50 ms"1, along with attitude and spin manoeuvres. The cruise stages provided power, attitude control via a pressurized hydrazine reaction system, communication support and thermal control for the encapsulated rovers. This last support function took the form of a pumped closed-loop refrigeration system using a chlorofluorocarbon that took excess heat from the cores of the rovers and disposed of it via radiators mounted on the cruise stage

exterior. This system was needed because each rover carried around 15 g of 238Pu oxide dispersed among eight radioisotope heater units (RHUs) to allow the rovers to survive the Martian night without impractically sized solar arrays and batteries. Consequently, the unwanted heat from the electronics boxes in the rover and from the RHUs (around 1 W per RHU) had to be dumped during cruise.

Upon arrival at Mars the cruise stage was separated from each lander capsule approximately 15 minutes prior to the nominal contact with the atmosphere. An ablating material (SLA-561), used first in the Viking mission, covered the entry shield and protects the 827 kg entry capsule and its contents. In response to the failure of the Mars Polar Lander mission during its entry phase, the MER craft both implemented a low-bandwidth telemetry system that operated throughout atmospheric entry. Set events during entry, such as airbag deployment, were signalled by the broadcast of one of 128 ten-second audio-frequency tones. The aim was to provide a clearer indication of which subsystem had failed, in the event of a catastrophe. A timeline of the entry, descent and landing (EDL) sequence is shown in Figure 27.1.

The mass of the MER rovers meant that a simple parachute decelerator, as used by the lighter Mars Pathfinder mission, could not provide a safe degree of speed reduction. Thus, along with a polyester/nylon parachute, a rocket-based decel-erator system was built into the backshell of the landing capsule. The rocket-assisted descent (RAD) system, consisting of three solid-fuel motors each developing around 10 kN for a little over two seconds, was used to bring the craft to a halt at a height of some tens of metres above the Martian surface. A modified solid-state camera, added late in the design sequence of the lander, allowed a smaller reaction system (three motors giving 750 Ns of impulse) to be fired to reduce the horizontal speed of the craft, and further reduce the risk of ripping the airbags. The airbags used in the MER missions were similar to, but tougher than, those of the Mars Pathfinder design. Protecting a lander twice as heavy as the Mars Pathfinder required the airbags' double inner pressure bladders to be wrapped with up to six, rather than four, layers of woven Vectran®, a synthetic polyester fibre having high tensile strength and flexibility at low temperatures. Pressurization of the airbags was performed by three solid-state gas generators delivering gas to a final pre-impact pressure of around 7 kPa; the cooling effect of

Entry attitude turn (L-77 min)

Cruise stage separated (L-21 min)

Peak heating (L-240s)

Peak heating (L-240s)

Parachute deployed (L-113s)

Radar acquisition and descent imagery taken (L-35s to L-22s)

Airbags inflated

Parachute deployed (L-113s)

Heatshield separated (L-93s)

Lander separated (L-83s)

Retrorockets fired (L-6s)

Radar acquisition and descent imagery taken (L-35s to L-22s)

Airbags inflated

Retrorockets fired (L-6s)

Multiple impacts till lander at rest. Lander petals opened (L>96min)

Airbags deflated and retracted (L+66min)

Figure 27.1. Significant events in the arrival sequences of the Mars Exploration Rovers.

the cold Martian atmosphere on the large airbag surface area necessitated the gas generators to continue to fire as the lander bounced and rolled to a halt.

The airbag system offered a low degree of dissipation for the impact energy, and thus the rover and its associated enclosure were exposed to around 30 consecutive impacts, with the initial impact speed being in the region of 10 ms"1. Each rover was fixed to the interior of a tetrahedral panelled enclosure formed from a space-frame carbon-composite material. The three panels around the lander's hold-down point were independently powered by hinge-motors, allowing the lander to turn itself to give the rover an upright orientation.

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