NASA MESSENGER Mission News June 3, 2004

Awaiting the Big Push

With MESSENGER in the late testing stages, final launch preparations
will soon begin. A few days before MESSENGER is mated to the upper
stage of the Delta II launch vehicle, the team will fill the
spacecraft’s propellant and pressurant tanks.

Like many NASA deep-space missions requiring a lot of maneuvering
capability, the MESSENGER spacecraft sports a bipropellant main
engine, which, like an automobile engine, needs fuel and an oxidizer
to run. Automobiles use gasoline (or diesel) as fuel; the oxidizer
is simply oxygen from the atmosphere. Since MESSENGER will be
working in the vacuum of space, it needs to carry its own oxidizer
(in this case, nitrogen tetroxide) to combine with its hydrazine
fuel. Hydrazine is a clear liquid that smells like ammonia and
ignites when it meets the right oxidizer, making for a rather simple
motor design. Once the fuel and oxidizer are sprayed into the engine
nozzle in proper proportions – you have ignition! No spark plugs or
other devices are needed to keep the engine running.

Hydrazine is also the lone propellant for the 16 smaller thrusters
that help steady the spacecraft. No oxidizer is used in the
thrusters so there is no combustion; the fuel sprays out over a
heated material (a "catalyst") that causes it to break apart into
nitrogen and hydrogen gas, moving the spacecraft in the direction
opposite to where the thruster is pointed.(This is a practical
application of Sir Isaac Newton’s Third Law of Motion: "For every
action there is an equal and opposite reaction.") To simplify the
spacecraft design the MESSENGER team taps off the main hydrazine
tanks to run the small thrusters, thus avoiding the need for yet
another tank for yet another fuel.

MESSENGER carries 595 kilograms (more than half a ton) of
propellant; this includes 97 kilograms (214 pounds) of hydrazine for
control and 269 kilograms (593 pounds) to burn with the 229
kilograms (505 pounds) of nitrogen tetroxide in the high-
performance "biprop" mode. Helium (2.25 kilograms, or 5 pounds) is
used to maintain pressure in the fuel and oxidizer lines. The
hydrazine is stored in two large, custom-designed titanium tanks and
the oxidizer in a third, similar tank. A smaller tank holds the
helium, and an extra supply of hydrazine is carried in a small
auxiliary tank.