Looking for a book pre-1994

[rmse17]

Book starts with a person in modern times (time when book was written), where a male main character finds himself in another body, done by a mind switch, temporarily. This body is of an heir to some galactic empire, which stands on a brink of war with another force. The facility where the experiment was performed gets attacked by the hostile faction, the scientist that performed the mind transfer - being the only one who knows about it, gets killed, and so main character from the 20th century finds himself stuck with everyone thinking he is the heir of the galactic empire, and ultimately is forced to lead the force into a war, against the hostile faction, where he has to use a secret weapon called the destroyer - I think, I read it in Russian, so not sure of the word. In the middle of the book are all kinds of adventures....