The engineering required to do this looks insane but can anyone the reasoning for such a massive undertaking?
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In a very dense atmosphere like Earth’s, it’s far and away the most efficient way of losing lots of speed as it requires no fuel – you’re just exchanging speed for enormous amounts of heat. So as long as you can withstand the heat, it’s far more efficient than turning around and blasting the engines in the opposite direction (something they *also* do). They do this because, unlike all other rockets, theirs need to be traveling at roughly 0m/s at the point they land.
(It’s the exact same process as capsules re-entering Earth’s orbit – they rely on the dense atmosphere to slow them down).
To be clear are you referring to the SpaceX Starship?
The reason Starship does a belly flop is so that it can use Earth’s (and eventually Mars’) atmosphere help slow itself down while using minimal fuel before killing it’s velocity before landing.
The goal is to create a fully rapidly reusable rocket system and bring down the costs of kilogram to orbit significantly.
There have been two successful ways of reentering from orbit.
Small crew capsules (Mercury, Gemini, Apollo, Dragon, and soon starliner on the US side, Soyuz and others on the soviet side)
Planes (Space shuttle, X-38, Soviet Buran) that can glide back into the atmosphere.
The problem is that SpaceX is trying to do something that has never been done before – they are trying to build a really big second stage back. You can’t build a capsule that big, and wings and wheels are really really heavy.
So SpaceX is trying something new and creative.
They are building something that will reenter like a capsule – sideways – with “fins” that can control it during he reentry. Then to land, it will flip back upright, relight its engines, and land vertically, the way the Falcon 9 boosters do.
In school physics it is often ignored, but air resistance does, in fact, exist. By doing a bellyflop maneuver the surface area in the direction of the movement (downwards) is greatly increased, which increases the total air resistance and allows the rocket to slow down greatly.
Things that fall back down from space are very very fast, so slowing down is a must and doing it by using air resistance is a very cheap method of doing it. They still have to reduce their speed further by using their rocket engines, but doing the bellyflop first reduces the speed and thus the amount of fuel needed to reach the landing speed.
All of this said, it is questionable whether the bellyflop maneuver is actually viable. The air resistance will cause a lot of heat and force, potentially weakening or even destroying the rocket. Even if the rocket survives, it will need some maintenance before it can be reused.
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