The SpaceX StarHopper is in one piece. SpaceX is targeting a first flight in four weeks if everything goes right.
SpaceX has put this together with astonishing speed.
The StarHopper will test technology that will be built into the second stage Starship. The Starship is a reusable launch vehicle second stage with an integrated payload section, and the ability to operate as a long-duration spacecraft on flights both in, and beyond Earth orbit.
It will be built in at least three versions:
Spaceship: a large, long-duration spacecraft capable of carrying passengers or cargo to interplanetary destinations, to LEO, or between destinations on Earth.
Tanker: a cargo-only propellant tanker to support the refilling of propellants in Earth orbit. The tanker will enable launching a heavy spacecraft to interplanetary space as the spacecraft being refueled can use its tanks twice, first to reach LEO and afterward to leave Earth orbit. This design reaches a Delta-v similar to three-stage rockets without needing the corresponding large mass fractions.
Satellite delivery spacecraft: a vehicle with a large cargo bay door that can open in space to facilitate the placement of spacecraft into orbit, or the recovery of spacecraft and space debris.
There will be on-orbit propellant transfers from Starship tankers to Starship spaceships or cargo spaceships.
A Starship and its payload will be able to transit to the Moon or fly to Mars after on-orbit propellant loading.
Stainless steel structure and tank construction. Its strength-to-mass ratio is comparable to or better than the earlier design alternative of carbon fiber composites across the anticipated temperature ranges, from cryogenic to the high temperatures of atmospheric reentry.
Some parts of the rocket will be built with a stainless steel alloy that has a cryogenic treatment. The metal is cold-formed into cryo-treated steel. It will be dramatically lighter and more wear-resistant than traditional hot-rolled steel.
The Heat-shields will be reusable. The same stainless steel alloy used to construct the vehicle structure and tankage, when unpainted and polished to a mirror-like finish, has a very high reflectivity facilitating atmospheric entry with much less heat shielding than would have been required for the earlier design of carbon fiber.