Fly to the three planets where the other knights have gone, and battle the mechanical creatures there.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.
Your built spaceship should be on top of the castle.
on the castle
It's on the castle, climb it and you'll find the rocket.
Swap your wrecked saucer for a new holo-built ship by talking to the head mechanic at Astrozone (building on the right). You should have the coordinates to the three planets where the other knights have gone (X-15 Y-15, X-73 Y-83, and X-83, Y-20) and they will all appear on your Spacemap aboard your new spaceship. You will need to defeat all three planets to rescue the Princess.
While the hovercraft built by Mordred runs on manure, the saucer is advanced technology and its green Fuel Rod is empty. You will need another Fuel Rod. (see related question)
I like to study about the inside of a spaceship, and how its built.
Mordred was a scientific genius, inventor, and former ruler of the Kingdom of Arturus. His experiments with cyborg organisms was decried by the populace and he was deposed. Years later, the solar system is raided by the robot forces of the Binary Bard, who has built creatures much like Mordred's designs. When the Princess is kidnapped from Arturus, space knights are sent out to rescue her.
The wall between Mordred's lab and the tunnel to the dungeon is dirt. Push hard on it. The cell in the dungeon is accessble from the underground lab built by Mordred. You should feed the mouse from the castle to the robot owl, who will then follow you and retrieve items for you.
The original coordinates are X-56 Y-52, which takes the Excalibur to the Pewter Moon. When you have beaten the three planets (jungle, fire, and ice), you can travel to the Asteroid Belt where Mordred has built his portal, near X-11 Y-84.
Once you have spoken to the Astro Zone manager and built your new ship, there is no reason to go back there. If you leave the island anytime after building a new spaceship, you will find it parked on top of the castle in Arturus.
Mordred (aka the Binary Bard) is hiding on the secret asteroid in the asteroid belt (X-11 Y-84). He has built a portal that leads to his castle in a distant galaxy. Once you defeat the three planets (fire, ice, and jungle), you can go there to confront him. (see related questions)