Liftoff from the Moon


  The moon has no atmosphere; therefore, there is no frictional drag on the spacecraft during liftoff.  Unlike the Saturn V rocket, the LEM did not have to have a streamline design to make it more aerodynamic; the LEM could be designed around functionality while on the moon's surface. 

On the moon's surface, the acceleration of gravity is 1.7 m/s2 which is 1/6 of the gravity on the Earth.  With the relative lightness of the LEM (compared to the Saturn V rocket), only a single rocket engine is needed to produce the necessary thrust to lift the LEM into lunar orbit.  The LEM's liftoff engine uses a hypergolic fuel, consisting of Aerozine50 (the fuel) and nitrogen tetroxide (oxidizer) . 

  What is meant by a hypergolic propellant and oxidizer? 


Why is using a hypergolic fuel important when attempting to liftoff from the moon's surface?


The mass of the LEM is about 5,000 kg.  With an acceleration of gravity of 1.7 m/s2, the weight of the LEM at liftoff is

W    =    m * g    =    5,000 kg  *  1.7 m/s2    =    8,500 N

The single engine on the LEM can produce 16,000 N of force.  (Remember, the Saturn V engines were measured in millions of Newtons of force - not thousands of Newtons.) With the LEM's weight at 8,500 N, the resulting net upward force is

Fnet    =    Fthrust  -  W    =    16,000 N  -  8,500 N    =    7,500 N.

The initial liftoff acceleration of the LEM is

a    =    F  /  m    =    7500 N  /  5000 kg    =    1.5 m/s2

The LEM will reach an orbital velocity of 1600 m/s at the orbital altitude of 110 km above the lunar surface.  While in orbit, the LEM will catch up to the command module and dock.  After docking with the command module, it returns to Earth.

The following two images show the LEM a fraction of a second prior to and after liftoff.  Only the top section leaves the Moon.  The base remains on the Moon.

  Watch a video of the lunar landing and liftoff.  

PHY280 General Physical Science   /   Webber International University