Build it. Light it. Keep it pointed up.
The orbiter is your upper stage. Its three RS-25 main engines drink from the core tank below it. When that tank runs dry it drops away and the orbiter finishes on its two small OMS engines.
Real hardware uses published specs, rounded. The Saturn V preset flies its first two stages; the third stage is left off. Orion and Super Orion are real 1958–65 nuclear-pulse designs that were never built; their numbers are estimates. Moon and Mars trips start from Earth orbit and use real rough Δv costs. Antimatter rockets are real ideas that have never been built: so far people have only ever made a few billionths of a gram of antimatter, and it is very hard to store. The game’s antimatter numbers are estimates. The V-2 (Germany, 1944) was built during World War II as a weapon, much of it by prisoners forced to work. After the war its engineers and captured rockets helped start the American and Soviet space programs, and in 1946 a V-2 took the first photo of Earth from space. The real N1 failed all four of its launches (1969–72); here its engines just fail now and then. Sea Dragon is a 1962 design that was never built; its numbers are estimates. The Star Clipper (Lockheed, 1966) and UR-700 were also never built; their numbers are estimates too. The Cola Rocket is a game version of the real cola-and-mint-candy geyser; do the real one outside, with a grown-up. Ishyer 4, its Life Finder rover, @B@GHW, the ATP shuttle, NUT and its Builder rover, Chick and its Chick Builder rover, the Starwing, its Sky Builder rover, Planet Zorbia, NA-74, Peanut Planet, Planet Nowhere and the Ring World with its Ringfolk are made up (real spacecraft cannot warp) for this game; the real Area 51 is an aircraft test base and has never launched a rocket.