The Nebutron is the main name of the Sun Riders' battle machine, which Chiro himself once labelled the coolest super robot ever built. After joining the Hyperforce upon discovering the Super Robot, he quickly corrects himself by claiming the Nebutron as the second coolest. Judging by how it's labelled, there are eight designs of Nebutron. While models 1-6 remain a mystery, and are most likely toy designs anyhow, models seven and eight are the only two actually seen.
This is the model that Chiro points out when introducing the rest of his team to his favorite TV show. On the actual show, it appears equal in size to the Super Robot. Next to the Super Robot, the Nebutron 7 is the same scale size as an action-figure next to a fully grown human adult. Though it did have the show's props, like fake legs, to make it look bigger on TV.
The weapon-system on it was a load of baloney as well. The mace/hammer that extends out of it for the Comet Clobber attack might be fitting for special effects in the show's battles, but it just falls off during a real fight, which is utterly useless. Which is why the Sun Riders abandoned it when the Skeleton King gave them real superpowers. Surprised that their biggest fan, who's also a target, would ask about the Nebutron 7. Johnny Sunspot lied to Chiro saying it's kept in a secret military base until further notice. After the reveal of the Sun Riders' intentions, Chiro's disappointed to the point of wanting to give up. When it's revealed that the Nebutron 7 is really at a long forsaken studio and is just as fake as his so-called heroes. To cheer their leader up and insure they have a battle machine, since the Sun Riders stole the Super Robot, the Hyperforce pitched together to fix up the Nebutron 7. Making it functional enough for battle in spite of all the flaws that the Sun Riders underestimated to the point of cruel laughter. Aurora Six even called it "an old junker."
To make up for that, when the Hyperforce fixed it up, they used the Nebutron 7's frame as an extension of their own weapons and powers. They even used their rocket-packs to make this battle machine fly, taking the Sun Rider's themselves by surprise. While the Nebutron 7's crushed instantly, it lasted in the fight long enough for Otto to shut down the Super Robot from the outside.
After regaining their own robot, the Hyperforce used what's left of the Nebutron 7 to seal up the Sun Riders, and banish them to another part of the cosmos with a throw.
In The Sun Riders Return, it seemed that the reformed Sun Riders used up the parts of the Nebutron 7 to make the Nebutron 8. It has a drill system with underground navigation, camouflage to make it invisible when not in use, and a self-destruct button. Johnny Sunspot programmed some of the features on a remote. It doubled as keys to his seat of the cockpit. Despite being so powerful, it cannot be activated, let alone driven, unless all three Sun Riders are in the cockpit.