This conundrum creates multiple problems for the company. The first and most obvious is how they would be able to build a tank big enough to last the whole tree months while keeping it light enough to make the spaceship buoyant.
Elon Musk has come up with a solution for this forefront question by creating fuel tanks from carbon fiber. In essence, the tanks should be light enough, but would they be able to cope with the pressure and heat of space travel.
Elon says that they have done numerous test and the last test was done a week ago. The tank is supposed to have passed all its tests and is set for further development. "The enormous tank is a fundamental component of SpaceX's Interplanetary Transport System (ITS), the reusable rocket-spaceship combo the company is developing to take people to and from Mars," said Elon Musk. "Such tanks will house propellant for the Raptor engines that will power both the rocket and the spaceship."
But even if the containers can hold enough fuel to get the ship to Mars, the bigger problem would be to get back to earth. Most spaceships use nonusable fuel tanks, but SapceX is looking for a reusable system. They aim to create a fuel plant in Mars that they would use to create Methane and Oxygen, the main components in their spaceship fuel.