Elon Musk’s SpaceX stacked a Starship prototype rocket on top of a Super Heavy rocket booster for the first time Friday morning, giving a look at the scale of the combined nearly 400-foot-tall vehicle.

Musk, asked by CNBC what he thought of witnessing the milestone at the company’s facility in Boca Chica, Texas, responded simply.

“Dream come true,” Musk replied in a tweet.