Complex sentence: Wallis's idea was to drop a large heavy bomb with a hard armoured tip at supersonic speed so that it penetrated the ground--an effect comparable to a ten-ton bullet being fired straight down.
Split into simpler sentences: Wallis's idea was to drop a large heavy bomb with a hard armoured tip at supersonic speed so that it penetrated the ground. This effect would be rather like a ten-ton bullet being fired straight down.