Why Microgravity?

Definition of Microgravity (µg). The Webster dictionary defines microgavity as "a condition in space of virtual absence of gravity".
Microgravity can also be attained on Earth and it refers to the condition of free-fall within a gravitational field in which the weight of an object is significantly reduced compared to its weight at rest on Earth (approaching weightlessness). There are five main microgavity facilities, with different characteristic times, ranging from a few seconds to several months (see next table).

Microgravity Facilities
Apparatus	Time	Drawback
Drop Tower (i.e. JAMIC)	~3 seconds	Short duration
Parabolic Flight (i.e. KC-135 aircraft)	~30 seconds	g-jitter
Sounding Rocket	~ minutes	g-jitter
Orbiting Spacecraft (i.e. Space Shuttle)	~ days	Availability
International Space Station	~ months	Availability

Why does gravity affect combustion? The large temperature gradients in the combustion flow due to the high exothermicity of the chemical reactions lead to significant density variations. In the presence of gravity, this density gradients lead to buoyant convection flows. In the absence of gravity, smaller effects, such as diffusion and thermophoresis, control the overall combustion process, altering it significantly (diffusion flames become spherical and less vigorous, hot air won't rise and paint will require much more time to dry). See Comparison of Candle Flames in Normal and Microgravity.
Why study combustion processes in microgravity? The absence of gravity simplifies the study of the controlling mechanisms in combustion because of the absence of buoyancy. This helps in the verification of theoretical models.
From the fire safety point of view, there is a lack of knowledge about the combustion behavior of materials in a low gravity environment. Further knowledge will improve our ability to predict and prevent fires on Earth and in space.

Comparison of Candle Flames in Normal and Microgravity

Microgravity (µg) produces a round, cooler candle flame with no soot. In 1g, the highest temperature is at the flame base. In ug, the highest temperature is at the flame tip. Near extinction, the candle flame in ug spontaneously oscillates.

	CPL NAVIGATION MENU WebMaster: Guillermo Rein reingu (at me.berkeley.edu) Last updated: March 2005
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Why Microgravity?

Microgravity Facilities

Comparison of Candle Flames in Normal and Microgravity