SpaceX’s Big Falcon Rocket (BFR) won’t be making its maiden voyage to the moon and back until 2023. In the meantime, CEO Elon Musk intends to test some of the BFR’s most innovative designs using the Falcon 9 rocket.

On Wednesday, Musk took to Twitter to announce that SpaceX would construct a miniature version of the BFR’s winged space shuttle to launch on the Falcon 9 as early as June 2019. It’s unclear if this means that all future Falcon 9 flights will include this upgrade or if this will specifically serve as a proof-of-concept launch for the BFR.

“Mod to SpaceX tech tree build: Falcon 9 second stage will be upgraded to be like a mini-BFR Ship,” tweeted Musk.

The BFR is slated to be a two-stage vehicle consisting of a booster and a spaceship. The booster — the first stage — will propel the entire craft past the atmosphere. That’s where the spaceship — the second stage — will detach itself and continue onward into the great unknown.

The Falcon 9's second stage will look a lot more like a spacecraft in 2019. SpaceX

Currently, the Falcon 9 is also a two-stage vehicle, though it doesn’t have a second-stage spaceship. Instead, its second stage is powered by a single Merlin vacuum engine meant to take the rocket’s payload the extra mile once it’s in space. This is what Musk said will be replaced for a “mini-BFR Ship.”

While this could serve as a way for SpaceX to get the hang of launching rockets designed like the BFR, this small-scale trial won’t be able to test all of the ship’s components.

That’s because the final design of the BFR’s second stage, for example, is supposed to be capable of propulsive landings. This means that once it reenters the atmosphere it’ll reposition itself vertically and fire its engines to gracefully touch down.

How the BFR's second-stage sizes up next to a person. SpaceX

The Falcon 9’s second stage will be unable to do that because it makes use of a vacuum engine, which only works properly in space. Without a landing, SpaceX won’t be able to see how the ship’s heat shields and exterior fared during missions.

“Won’t land propulsively for those reasons,” wrote Musk. “Ultra light heat shield & high Mach control surfaces are what we can’t test well without orbital entry. I think we have a handle on propulsive landings.”

This is one of the many initial steps SpaceX will take before the BFR takes flight with commercial space travelers aboard. With June 2019 only eight months away, expect to hear updates from Musk about more imminent Falcon 9 upgrades soon.