Its critical component is an ingenious device that can cool the incoming air almost instantaneously.

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Unlike the more familiar turbojet, which uses a turbine-powered fan to compress the incoming air before it is mixed with fuel, a ramjet compresses air by forcing it down an ever-narrowing funnel.

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Incoming cooler ambient air from the other end of the piston causes the helium to contract and move the piston to generate AC power.

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Last week, America suddenly began requiring most incoming international flights to carry armed air marshals.

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Meanwhile, Heathrow Airport has been closed to all incoming flights that are not already in the air, while several outbound services have been cancelled.

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