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June 30, 2003

National Space Development Agency of Japan

                  NASDA REPORT NO.132

Part 2 of 3



The effects of gravitational pull of the Sun and Earth, as well as
the existence of very small atmospheric pressure make the satellite
change its orbit little by little. Hence, thrusters are sometimes
fired in order to make the satellite "stay" in the desired orbit.



When the wheel speed reaches its upper threshold values, it becomes
impossible to control the satellite's attitude. In this case,
thrusters are fired to dump the momentum accumulation in the wheel. 


Components of the Satellite Propulsion System

Next, we will give a brief explanation of the major components in the
satellite propulsion system. 



Thrusters are small rocket motors. The combustion gases, upon
acceleration, are exhaused through a thruster's nozzle, and thrusters
obtain thrust.

The right drawing shows a monopropellant hydrazine thruster. Although
hydrazine is very toxic and difficult to handle, its chemical
stability allows us to store the liquid propellant in tanks for a
long time in the space environment, which advantage leads to a wider
use. 

Hydrazine is decomposed into gas mixed with nitrogen, hydrogen and
ammonia when coming into contact with a catalyst bed (see chemical
equation).

N2H4 - x-N2 + y-H2 + z-NH3 (x, y, z vary depending on the thruster
design.)

Heat generated in the decomposition process raises the temperature of
the mixed gas to 800 degrees C. Thrust is developed by ejecting the
high-temperature gas. 

Although we will not discuss them here, there are other types of
thrusters, such as bipropellant thrusters fed by oxidizer and fuel,
DC arcjet thrusters and ion engines powered by electric energy. 



Valves are used for switching the flow of propellant on and off and
adjusting the flow rate. There are various types of valves, including
propellant valves, latch valves, fill and drain valves, pyrotechnic
valves, check valves and pressure regulating valves. These valves are
each suited for different purposes. 



Since tanks store propellant and gas, they are constructed robustly
enough to endure atmospheric pressures ranging from 20 to 300 MPa. In
addition, such advanced materials as titanium alloy and carbon fibers
are used in order to reduce weight. 

In a weightless space environment, propellant does not always settle
near the outlet of the tank, and if measures are not taken to guide
the propellant, droplets of the propellant will float free in the
tank or stick to the tank walls. In order to keep the propellant near
the outlet of the tank, the tank is provided with an inserted
rubber-made seat (diaphragm) of a built-in PMD to guide propellant
toward the outlet.


Research and Development for Improving Reliability and Performance

Next, we will present ongoing development and research activities
conducted on the propulsion system. 

First, we will describe research on a "contamination hardened"
propulsion system. 

Metal powders are often produced when fabricating tanks and piping.
Tiny contaminants such as these metal powders can become trapped in
the valves, causing propellant leakage or blockage. Accordingly, we
have devised a fabrication process to prevent the generation of
contaminants or have washed parts thoroughly after fabrication.
However, these countermeasures have yet to produce the desired
effect. 

Efforts have been made to solve the contamination problem and improve
reliability through research on valves having a structure not
vulnerable to contamination, implementation of anti-contamination
tests and disassembly of failed valves.

Second, we will provide an example of research on improving lifetime
and reliability. 

Due to recent demands for larger satellites with enhanced functions,
thrusters are used longer than their lifetimes. For developing
thrusters that can operate with higher reliability over a longer
lifetime, researchs has been conducted on understanding reaction
phenomena in thrusters and mechanisms that reduce the thruster's
lifetime. 

The photos show visualization tests conducted on ground and in the
zero-grabity environment to study chemical reactions in a glass-made
thruster. The catalyzer can be seen to glow in a reaction with
hydrazine. 


Development of Advanced Propulsion System for Future Space Activity

The propulsion system is critical to future satellites and manned
spacecraft. Accordingly, NASDA has conducted research for
contributing to the development of an advanced propulsion system
having capabilities that exceed traditional functions, such as
attitude control. 



NASDA has researched elements related to ion engines with extended
lifetime, that has performance superior to existing thrusters.



A low toxic, safe and environment-friendly propellant has been
investigated as a replacement for hydrazine. Chemical reaction tests
and material compatibility tests have also been conducted on the
promising propellant. 

 - Continued -

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