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Glossary

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A300 Zero G

A modified Airbus A 300, used for flying parabolas and so as to generate short periods of medium quality microgravity up to a maximum of around 20 seconds. The quality of the microgravity is not particularly high (10-2 g) but it is adequate for many experiments and the scientists responsible for the experiments can fly with them and carry them out directly.

A300 Zero G

A300 Zero G Aircraft

Absorption

The process by which atoms, molecules or ions enter the body of another material (either gaseous, liquid or solid). It should not be confused with adsorption. The term sorption covers both absorption and adsorption together with ion exchange.

Activation

Used to describe the process by which certain catalysts (including spongy nickel) are converted from a solid material in to a porous sponge by a chemical process. The term is synonymous with leaching. With spongy nickel a block of nickel aluminium alloy is reacted with concentrated sodium hydroxide leaving a skeleton of nickel together with some aluminium.

Adsorption

The process by which atoms, molecules or ions collect on the surface of another material (either liquid or solid). It should not be confused with absorption. The term sorption covers both absorption and adsorption together with ion exchange.

Aging

The final stage in the process of precipitation hardening by which the work piece is heated to and held at a temperature, well below it’s melting point, that allows impurity atoms to come out of solution in the alloy and coalesce into ‘impurity islands’. These islands provide barriers to the motion of dislocations and so increase the yield stress of the material.

Ah (Ampere Hour)

A unit of electric charge commonly used in fuel cells, batteries and other DC power supplies. It is the charge delivered by a current of one amp flowing for a period of one hour. The SI unit of charge is the Coulomb but it is inconveniently small. 1 AH = 3600 C.

Air Speed

The speed of an aircraft measured relative to the surrounding air which can itself be moving.

Alloy

A partial or complete solid solution of one or more elements within a metallic element. The minority elements need not themselves be metallic, for example the principal alloying element in steel is carbon. Almost all metals used in engineering are alloys rather than pure elements as alloys have enhanced engineering properties such as yield stress.

Angular Acceleration

The rate at which the rotation of an object changes. It is an exact parallel of ordinary (linear) acceleration. If the initial angular velocity changes from ω0 to ω1 in a period Δt then the angular acceleration α is given by:

Equation

Angular Impulse

In the same way that ordinary (linear) impulse is the change in momentum of an object, so angular impulse is the change of angular momentum of an object:

Equation

Apogee

Normally apogee is the highest point in an object’s orbit about the Earth, perigee being the lowest point. A sounding rocket however does not enter orbit and so its apogee is just the highest point in its trajectory.  

Artificial Gravity

Artificial gravity can be generated with a centrifuge. To date no centrifuge has been flown that is large enough to provide artificial gravity for astronauts but a number have been flown in sounding rockets to study the effect of reduced gravity (less than one g) on plant and small animals.

ATV

The Automated Transfer Vehicle. One of Europe’s contributions to the International Space Station. The ATV is a combination of delivery lorry, orbital booster and rubbish disposal system. Once launched the ATV is totally autonomous and approaches and docks with the ISS without any human intervention. There are plans to extend the ATV into a manned launch vehicle.

Automated Transfer Vehicle

Automated Transfer Vehicle (cut away artist's impression)

Austenite

Also called gamma phase iron. A metallic, non-magnetic solid solution of carbon in iron that, in plain carbon steel, is only stable between around 900 and 1400 °C. In other steel alloys the temperature range varies and can reach as low as room temperature in ‘austenitic stainless steel’.

Austenite can dissolve a maximum of around 2% carbon (by atomic percentage) at a temperature of 1150 °C. Below this temperature the carbon will normally come out of solution resulting in pearlite, that is alternating layers (lamella) of ferrite (alpha iron) and cementite (the intermetallic Fe3C). If however the steel is rapidly quenched then the carbon is forced to stay in solution but distorts the crystal lattice structure, this produces the hard but brittle martensite.

 Body Centred Cubic Structure of Austenite

Body Centred Cubic Structure of Austenite

Auxetics

These are a rare group of materials with a negative Poisson Ratio. This means that when a sample is stretched it gets thicker. The easiest way to understand it is to look at the following diagram. The sticks in the two diagrams represent the bonds between the atoms in a hypothetical auxetic material. As the material is placed under tension the blue strands straighten out and this pushes against the red cross connections. The overall result is that adjacent blue strands are pushed apart and so material becomes thicker.

Auxetic Materials

Auxetic Structure

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