Coleman Hostetler
ENGL 202C
3/25/2015
Audience: High school graduates who have an interest in the space industry, but no specialized
knowledge in the field
Process Description of a Saturn V Launch
The Saturn V is a liquid fuelled rocket that was used in the Apollo program, and is the only vehicle to
ever carry humans to another world. The Saturn V is composed of several stages, as seen in Figure 1
below. When a stage runs out of fuel, it separates from the rest of the rocket and falls away. The next
stage then ignites its engines and the rocket continues onward.
Figure 1: Saturn V stages
“Diagram of Saturn V Launch Vehicle”, original image by NASA/MSFC, modified by Coleman Hostetler
http://commons.wikimedia.org/wiki/File:Diagram_of_Saturn_V_Launch_Vehicle.jpg
First Stage
The first stage, powered by five of the most powerful rocket engines ever made, produces over 7.5
million pounds of thrust. This stage uses a highly refined form of kerosene and liquid oxygen as fuel. The
first stage is mostly spent on lifting the Saturn V up through the dense lower atmosphere. The rocket
ascends straight up at first, but gradually pitches downward. This is because getting into orbit requires
horizontal velocity much more than it requires altitude. As the rocket ascends, its rate of acceleration
increases, for three reasons. First, the performance of the engines improves at higher altitudes. Second,
higher altitude means thinner atmosphere and less drag. Lastly, the first stage burns through 14 tons of
fuel per second, making the rocket lighter and easier to accelerate. After 2 minutes and 41 seconds, the
first stage has burned all 4.7 million pounds of its fuel, and the rocket is 42 miles up, travelling at over
6000 mph. The first stage then separates from the rocket and falls away. Then the second stage’s
engines ignite.
Second Stage
The second stage has five smaller engines and is fuelled by liquid hydrogen and liquid oxygen. The fuel is
different from the first stage because the heavier kerosene provides more thrust, which is very
important for getting the rocket off the ground. Liquid hydrogen is much lighter, which means it
provides less thrust, but also means that the first stage doesn’t have to lift as much weight, and
functions as a more efficient fuel. The second stage is intended to get the rocket very close to orbit, but
on a path that will still reenter the Earth’s atmosphere. This is to ensure that the stage will fall back to
Earth, landing in the ocean, rather than staying in orbit and adding to the incredible amount of orbital
debris already up there. The launch trajectory of the second stage, unlike the first, is not
preprogrammed. Instead, the most fuel-efficient trajectory is calculated in real time by the onboard
computer. The second stage burns for 6 minutes, and brings the rocket to an altitude of 109 miles and a
velocity of 15,647 mph, very close to orbital velocity.
Third Stage
The third stage has a single rocket engine, and unlike the first two stages, it is focused more on
efficiency than thrust. Like the second stage, it uses liquid hydrogen and liquid oxygen as fuel. The
engine fires for 2.5 minutes before being shut off, leaving the spacecraft in a temporary parking orbit at
an altitude of about 118 miles, travelling at 17,432 mph. This orbit is only temporary though, and the
third stage’s engine ignites again to put the rocket on course for the moon. The amount of time spent in
a parking orbit varied depending on the mission, and was around 2.5 hours for Apollo 11. The burn to
transfer to the moon lasts about six minutes, and brings the spacecraft close to Earth’s escape velocity
of 25,053 mph, the velocity needed to break free of Earth’s gravitational pull. Up until this point, the
rocket was just getting into orbit, so all it had to do was point forward and accelerate. Things work very
differently now that it is in orbit, and all maneuvers must be carefully calculated beforehand. In order to
put the spacecraft on an intercept course with the moon, it isn’t enough to simply point the spacecraft
at the moon and fire the engines. Instead, the spacecraft has to be put on an orbit that will cross paths
with the moon’s orbit when both the moon and the spacecraft are in that location. This maneuver is
known as a Trans-Lunar Injection, or TLI, and can be seen in Figure 2 below.
Figure 2: Trans-Lunar Injection
“Perspective View of a Lunar Transfer Trajectory” by Aresv, used under CC BY SA / new label added from
original
http://en.wikipedia.org/wiki/File:Tli.svg
Apollo Spacecraft
About 40 minutes after the burn to put the spacecraft on course for the moon, the third stage separates
from the Command Service Module (CSM). The CSM then turns back towards the third stage and docks
with the landing module inside the third stage. Once the spacecraft arrives at the moon, it isn’t in orbit
yet. If nothing is done, it will simply fly right past the moon. To get captured into a lunar orbit, the
spacecraft has to slow down by turning backwards and firing its engine, what is known as ‘burning
retrograde’.
Lunar Landing
After orbiting the moon a few times, the landing module detaches from the CSM, then waits until it has
drifted far enough away to fire its engine. Once it has reached a safe distance, the landing module
decelerates, dropping from lunar orbit and putting itself on course for the surface. Lunar landings are
performed on an area of the moon where it is either dusk or dawn, so that the long shadows cast by
rocks on the surface can allow the astronauts in the landing module to judge how high above the
surface their lander is. The initial phase of the landing was entirely computer controlled, reducing
velocity relative to the lunar surface to near zero, at an altitude around 10000 ft. Control of the landing
module was given back to crew for the final approach, which took them down to about 700 ft. The
descent stage had enough fuel to hover above the lunar surface for about two minutes, so the crew
could survey the landing site and ensure that it was safe to land. The pilot of the lander then had to
touch down safely, ensuring that the craft’s horizontal and vertical velocity were minimized so it didn’t
get damaged or tip over upon landing.
Getting to the moon is a lengthy process, spanning several days and consisting of atmospheric ascent,
entry into Earth orbit, Trans-Lunar injection, entry into lunar orbit, and finally descent and landing. All of
NASA’s moon landings were launched with a Saturn V, and went through these steps as well as several
more to return the astronauts on board the rocket safely back to Earth.