Letter: All square

From David Stevenson

You illustrate a thruster array of a new engine for use in spacecraft with a square 46 × 46 millimetres marked “1 kW”; a smaller square, 11.5 × 11.5 mm, is marked “250 W”; the smallest, 4.6 mm to a side, reads “100 W” (24 March, p 43).

Since the power output will be directly proportional to the number of nano-thrusters, and thus to surface area, your two smaller squares should of course be 23 × 23 mm and 14.5 × 14.5 mm in size to give these power outputs.

Let’s have some fair play here. If a non-scientific publication had mixed up linear and square units in this way, Feedback would have had a field day.

• Got us. Above, right, is the diagram in the proportions originally printed; and below that, with the proportions it should have had.

Alloway, South Ayrshire, UK