No, Voltes V is NOT a rip-off of Combattler V.
Tadao Nagahama, the creator of both shows, first created Combattler V.
The super robot show looks very much like an experimental proto-type; this means the characters were not thoroughly polished.
The main Battle Team leader Hyoma Aoi is an irresponsible, immature character who values watching cartoons and going to pubs before the defense and welfare of planet Earth.
The main coordinator/professor of Combattler V is a drunkard.
The Combattler V characters are a far cry from the characters of Voltes V, who are dedicated and responsible.
The very atmosphere of the Combattler V story is humorous compared to Voltes V’s dramatic story line which emphasizes more on human sentimentality.
In Combattler V’s ending, the Combattler V robot over-heats and bogs down after fighting off the last enemy robot. Unable to eject out of their giant super defender, the Battle Team awaits their fate from an oncoming alien armada.
It turns out that the said “armada” is only a large “clean up” committee sends by the planet from where the alien criminal came from. The alien planet’s government even apologies to the Earth People for the uncalled conduct of their fallen comrade.
In Voltes V’s ending, Voltes V along with The Solar Falcon: Camp Big Falcon turns to a space station’ (This feat was copied by Voltron in the last episodes where the Castle of Lions turns into a space ship) brought the war in Planet Boazania’s doorsteps.
Voltes V liberated the Boazanian slaves from the cruel reign of Emperor Zanbasil and crushed its haughty imperialistic ways.
Voltes V even battled with Boazania’s last defender, the “Boazanian God of War” piloted by Prince Heinnel (Prince Zardoz) where the Voltes team members, the Go brothers (Armstrong brothers) learned that their long time enemy is really their half brother!
Kennichi (Steve), Daijiro (Big Bert) and Hiyoshi (Little Jon) were reunited with their long lost father and Voltes V returns to Earth as its ever ready defender.
Since, Voltes V and Combattler V have a common creator; I can safely say that Voltes V is the IMPROVEMENT of Combattler V and not a mere carbon copy.