Johnsen now faces the task of proving SpaceX can deliver on its promises as a public company.

The winners of SpaceX's blockbuster IPO—valuing the company at $1.75 trillion—might not just be investors, but also the ones who welded the rockets.

Founder, CEO, Chairman, and Chief Engineer of SpaceX, Elon Musk, speaks via videolink on the day of SpaceX's initial public offering (IPO) at the Nasdaq MarketSite in New York…