Three great constants in life are death, taxes — and the costly rocket program that NASA always seems to have under development.

NASA’s current rocket of the future is called the Space Launch System. It is a heavy-lift vehicle that replaced a similar program called Constellation proposed by the Bush administration.

Both are quite a bit like the National Launch System of the 1990s. And all have a distinct similarity to a rocket that once actually existed — the Saturn V used in the Apollo program.

In addition to these brawny rockets, the space agency has, at different times, toiled on various smaller, futuristic rocket planes such as the X-30 (also known as the National Aerospace Plane) and the X-33.