Rockets

Rocket Assisted Descent (RAD) rockets fire during entry, descent, and landing, slowing the spacecraft to a halt about 10-15 meters (33-40 ft) above the surface to ensure that the spacraft doesn't land on the surface at too high of a speed. They are a part of the aeroshell.

Engineers working on the RAD rockets benefitted from Pathfinder RAD designs, but had to invent a special way to prevent the backshell from tilting given winds and wind shear that are possible during entry, descent, and landing.

Due to winds and wind shear in the lower Martian atmosphere, it is possible that these winds could induce swinging motion and a backshell tilt. If the tilt is large enough at the moment that the rockets fire, the rockets alone may cause the spacecraft to accelerate horizontally (laterally) causing the lander/airbag combination to impact the terrain at a grazing angle at high speeds. While the airbags are quite strong, asking them to perform like giant aircraft tires landing on a rocky surface was just too much for them especially given that the lander and rover was some 50% heavier that the Mars Pathfinder system.

In order to help reduce the horizontal speed at touchdown (and help the airbags), a separate rocket system that was not part of the Pathfinder mission was invented, called the transverse impulse rocket system (or TIRS, pronounced ³tears²). It consists of three additional small rockets mounted on the backshell. Using the backshell interial measurement unit (or IMU), when the rover¹s software detects that the backshell may be tilted too far off vertical, the software will elect to fire one or two of these small TIRS rockets reduce the tilt effect. While not providing precision control, these little rockets can compensate for the swinging introduced by some of the worst wind predictable. Firing for a fraction of a second, these rockets provide just enough of a kick to the backshell to get it aligned with the vertical before the lander and rover are released some 15 m (49 ft) above the ground.