The problem came to light when a Propulsion Drive Electronics (PDE) control unit picked up a difference in pressures between the oxidiser and the fuel going through one of the ATV's four "propulsion chains".

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Conventional hybrids use mainly the gasoline engine for propulsion, drawing on the electric drive and batteries only occasionally to (impressively) improve gasoline efficiency.

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The system is tiny but has a grand name - Warp Drive (Water Alcohol Resisto-jet Propulsion De-orbit Re-entry Velocity Experiment).

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The ELR will run on an electric propulsion system made of a T-shaped lithium ion battery, an electric drive unit and a four-cylinder engine generator.

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