"I've never heard of 'rocket' as a food. We do have 'sweet rocket' which is a tall, self-perpetuating flower. Not related?"

Ebbie,

Both are members of the Brassicaceae, the cabbage and mustard family. Hence both are 'crucifers' (i.e. they both have four equal-sized petals which, taken together, form a cross shape).

Sweet Rocket is Hesperis matronalis whilst (salad) Rocket is Eruca sativa or Eruca vesicaria (subsp. sativa) depending on which book you consult. 'Rocket' is supposed to be just a corruption of the Latin name 'Eruca'.

Last year I found, near Manchester (UK), another crucifer called Perennial Rocket (Sisymbrium strictissimum). But look in any reasonably comprehensive British flora and you'll find Annual and Perennial Wall Rockets, London Rocket etc., etc. You just have to remember that common names for plants are not particularly scientifically accurate and the 'Rocket' name seems to have been applied to several plants which are more or less similar and broadly related.