SpaceX is working on building a fully reusable rocket-and-spaceship system called Starship.

Based on a presentation given by company founder Elon Musk in September 2018, the finished rocket may stand about 387 feet (118 meters) tall and 30 feet (9 meters) wide.

But Musk tweeted on Wednesday that he's already envisioning a "next gen" Starship that'd be 60 feet (18 meters) in diameter, or wider than an NBA basketball court.

A doubled diameter and height of a "next gen" Starship could multiple its overall size eightfold, enabling even more ambitious trips to Mars. However, either vehicle has yet to be built.

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A day after SpaceX's final flight of Starhopper, a stubby prototype of an enormous planned launch system called Starship, company founder Elon Musk dropped a clue about his supersize plans for the future.

Musk tweeted on Wednesday that he'd present SpaceX's latest thinking about the Starship system on September 28. The date marks the 11th anniversary of the company reaching orbit for the first time with its Falcon 1 rocket.

Starhopper — SpaceX's first Mars Starship prototype — hovers over its launchpad during a test flight in Boca Chica, Texas, on August 27, 2019. Trevor Mahlmann/Reuters Until then, most of what we know about Starship comes from Musk's latest presentation from September 2018. At that time, the vehicle was still called the "Big Falcon Rocket" and was supposed to be made out of carbon-fiber composites. SpaceX is now using stainless steel but appears to be keeping similar dimensions, based on several renderings posted by Musk (including one of a Starship spaceship on the moon's surface).

Read more: Elon Musk says SpaceX could land on the moon in 2 years. A NASA executive says 'we'll partner with them, and we'll get there faster' if the company can pull it off.

Those dimensions suggest the first operational version of Starship would be a vehicle about 30 feet (9 meters) wide and 387 feet (118 meters) tall. Yet Musk is already dreaming up an even bigger version of the system.

He revealed part of his grand plan on Wednesday when a Twitter user asked about a 39-foot-wide (12 meters) version of Starship. Musk replied that a "next-gen" version of Starship would probably be double that diameter: a width of 18 meters, or nearly 60 feet.

That's wider than an NBA basketball court. The footprint of such a rocket would also be 30% greater than the square footage of an average US home.

The following graphic shows about how big the first Starship system would be compared with its prototypes, such as Starhopper and Starship Mark 1. The Apollo-era Saturn V rocket and NASA's upcoming Space Launch System moon rocket are also included for scale.

A comparison of SpaceX and NASA rocket systems. Yutong Yuan/Samantha Lee/Business Insider

However, doing some basic math helps reveal the scale of what Musk is proposing to do with SpaceX in the far-flung future.

A 'next gen' Starship may have 8 times the volume of the first operational Starship

An illustration of SpaceX's Starship vehicle on the surface of Mars, with greenhouses and a nascent space colony in the distance. Elon Musk/SpaceX via Twitter

Simplifying the dimensions of the first-generation Starship into a cylinder (and ignoring its aerodynamic nose cone) gives a finished volume of about 7.5 million liters. The same calculation on the next-generation Starship — assuming its height also doubles, to about 775 feet (236 meters) — gives an approximate volume of 60 million liters.

So in effect, doubling the width and height leads to a launch vehicle about eight times as voluminous. Even if the next-generation Starship was just as tall as the first one, its increased girth would make it about four times bigger.

Boosting Starship' volume so much says little about how much payload or how many people a next-generation Starship could haul into orbit, or how deep into space such a gigantic spacecraft could go. But it's hard not to imagine the answers are "more, bigger, faster, and farther" since it could carry that much more fuel and make room for many more Raptor rocket engines.

Read more: SpaceX test-fired a giant rocket engine with 'insane power' for moon and Mars missions. The future of Elon Musk's company may ride on its unrivaled performance.

Certainly, a nose cone with basketball-court-width diameter is larger than that of any planned rocket's.

Such a size could accommodate space telescopes that astronomers may only dream of right now. For example, NASA's upcoming James Webb Space Telescope will fold up into a 15-foot-wide (4.57 meter) fairing of an Ariane 5 rocket around 2021. If a next-gen Starship is ever realized, it might fit six or more of the $10 billion space observatories inside its fairing.

However, Musk envisions building a city on Mars that will be self-sustaining by the 2050s (and eventually has pizza joints). If he hopes to see that mission accomplished before his time on Earth is done, though, SpaceX may need the biggest rocket its leader can dream up.