People are heavy. At least when you're trying to put them into space. Even healthy, fit astronauts weigh considerably more than you would imagine when you're putting them in a rocket, because you also need to pack the food, water and life support necessary to keep them alive.
That's why the Saturn V rocket used in the 1960s and 70s to send the Apollo astronauts to the Moon is the largest that we've ever built - 110 metres tall. It's also why the new rocket designed by Jeff Bezos' Blue Origin, "New Glenn", will be the second-largest that we've ever built, at 95 metres tall.
Practical, operable, reusable
Because New Glenn - which is named after John Glenn, the first American to orbit Earth - is designed to carry people too. Its seven BE-4 engines are capable of 17.1 million Newtons of thrust, giving it enough oomph to get humans into orbit or beyond. Plus, the booster portion is reusable - just like the ones on SpaceX's newer rockets.
"Building, flying, landing, and re-flying [Blue Origins' current-generation rocket] New Shepard has taught us so much about how to design for practical, operable reusability. And New Glenn incorporates all of those learnings," Bezos said in a statement.
The competition for New Glenn will be fierce. SpaceX is due to launch its 68-metre-tall Falcon Heavy for the first time in early 2017, while Nasa's Space Launch System (SLS), due to fly for the first time before November 2018, will dwarf both of them with a height of 117 metres.
But Bezos doesn't seem especially bothered by his rivals, saying that his rocket has been in the works for four years and will launch before the end of the decade. "We plan to fly New Glenn for the first time before the end of this decade from historic Launch Complex 36 at Cape Canaveral, Florida," he said.
"In the long run, deliberate and methodical wins the day, and you do things quickest by never skipping steps."