sentences:  Items other than consumables would therefore have to be launched only once.Two castles would be used, an outbound one on an Aldrin cycler with a fast transfer to Mars and long trip back, and an inbound one with fast trip to Earth and long return to Mars, which Aldrin called up and down escalators.The astronauts would meet up with the cycler in Earth orbit and later Mars orbit in specialised craft called "taxis".