How Reusable Rockets Work

Reusable rockets are revolutionizing

spaceflight. The movement may have begun

with SpaceX and their Falcon 9, but it's

quickly spreading across the world.

Chinese rockets are landing themselves

right now, just not always successfully.

Blue Origin is figuring out how to land

their own gigantic rocket booster, and

an underdog startup out of New Zealand

is looking to join the game this year

with a reusable rocket of their own.

For all of the many differences between

these vehicles and their builders, there

are fundamentals that tie them all

together, the key ingredients of the

reusable rocket, and this is what makes

them work. To begin, we should clarify

that when we say reusable rocket, what

we really mean is the booster stage.

Typically, that's the lower 2/3 of the

fully stacked rocket. A booster is

essentially just a big fuel tank with

some engines at the bottom. At this

moment, there is only one rocket booster

in the world that has proven to be

consistently reusable, and that is the

SpaceX Falcon 9. So, we are going to use

this as our reference point, but the

same process will apply to all of the

new rocket designs that are in

development. Now, you might also be

familiar with some reusable upper stage

vehicles, also known as orbiters. The

space shuttle is a famous example. The

SpaceX Starship is a more modern

example, and there have been a few

reusable space planes developed in

between.

These orbiters are significantly more

complex and diverse vehicles, each

deserving their own explanation. So,

we're going to put that subject to the

side for today and keep our focus on the

reusable rocket booster. While the space

shuttle may have been famous for its

orbiter, this 1980s era NASA rocket also

pioneered a method for reusable boosters

as well. If we look at how the space

shuttle was configured on the launchpad,

we have this big orange tank in the

middle that holds liquid propellant for

the main engines. On the back side is

the space plane and to the right and

left are the side boosters. These burn

solid propellant just like a model

rocket engine. And their job is to get

the shuttle off the ground and through

the most dense section of Earth's lower

atmosphere. At an altitude of about 45

km or around four times the height of a

commercial airplane, the side boosters

are released and allowed to fall back

down to Earth. The reason for this is

you don't want your orbiter to carry

around any dead weight. So once the

booster's job is done, we always

separate the stages of our rocket. Now

this is where most conventional rocket

boosters will simply fall back down into

the ocean. American space launches

typically lift off from the east coast

of Florida. So by the time stage

separation occurs, the rocket is far out

over the Atlantic Ocean. If you were to

dive down to the bottom, you'd find an

enormous graveyard with hundreds of old

boosters scattered across thousands of

acres of seabed. This is not an ideal

way to operate. It's bad for nature, but

it's also bad for us human beings, too.

We've taken months or even years worth

of highly complex engineering work and

expensive materials and then we've

converted that all into a pile of

garbage at the bottom of the ocean that

only ever contributed about 90 seconds

of actual usefulness. Now, wasted time

and resources is one thing, but it's the

wasted money that has really driven the

movement towards reusable rocket

technology. Every time you watch a

conventional rocket booster fall

helplessly back to Earth, that's

somewhere in the neighborhood of $50

million sinking down to the bottom of

the ocean. Something that you won't find

in the rocket graveyard are side

boosters from the space shuttle. Because

after stage separation, these rockets

actually deployed parachutes and floated

softly back down to the water where

they'd wait for a boat to arrive and tow

them back to port. Once all of the solid

propellant is burned up, you're

essentially just left with two empty

metal tubes, now they just need to be

cleaned up and refueled before they're

good to go for another shuttle launch.

And for many years, that was the only

way to reuse a rocket until SpaceX came

along. Falcon 9 introduced a new concept

to the world of rocketry. Vertical

takeoff and vertical landing. That means

we are not just parachuting our way down

into the water anymore. The booster will

guide itself back to the landing pad for

a controlled descent. Which begs a

question, why didn't SpaceX just go with

the parachute method? Sounds a lot

easier, right? And it is. But there's an

important distinction to make. On the

space shuttle, all of the important and

delicate flight hardware was inside the

orbiter, which landed itself on the

ground. It didn't get wet. Now, on a

conventional rocket that doesn't have a

space plane attached to it, all of that

flight hardware is located inside the

booster. So, you don't want to get water

in there. But since you already have all

of those computers and electronics on

board that guide the booster off the

ground and into space, you could

theoretically use that same hardware to

just run the play in reverse, guide it

from space back down to the ground. The

idea wasn't unique to SpaceX. Rocket

scientists had been pondering this

method for decades, but it was always

written off as being simply impossible

to accomplish in the real world. The New

Zealand startup Rocket Lab actually

tried to cheat the system with their

electron booster. What they had planned

to do was parachute the rocket down

through the sky and then just before it

hit the water, a helicopter would fly in

and snatch it out of the air. That was a

badass plan. But when they actually

tried to do it in real life, the

helicopter pilot quickly realized this

was way too sketchy to be safe and he

bailed on the catch attempt at the last

minute and the company never tried it

again. Anyway, back to our timeline

here. In 2011, SpaceX would begin their

quest to achieve the impossible. By

2012, they began flying a test vehicle

called the Grasshopper, a small fuel

tank with a single engine and four fixed

landing legs. It was designed to fly up

about 1 kilometer and then come straight

back down for a vertical landing. This

kind of maneuver is often referred to as

a hop. Grasshopper experienced some

trial and error, but it was able to

accomplish eight successful hops in one

year of operation. What this test

demonstrated was our first necessary

ingredient for a reusable rocket, an

engine that can be throttled down to a

very low power level. In order to land,

you need the engine to burn just enough

to slow you down, but not so much that

it starts to push the rocket back up

again. This is a delicate balance, and

that's exactly what SpaceX was trying to

find with Grasshopper. In 2013, they

began scaling this test up to the Falcon

9. That name tells us that this rocket

has nine engines, which is a lot. Most

rockets previous to this had four or

five engines at most, but SpaceX knew

that they wanted a reusable rocket

booster. So instead of using a small

number of big engines, they chose a

large number of small engines. This is

essential for achieving that balance we

talked about earlier. By the time our

rocket comes in for a landing, almost

all of the fuel in the tank has been

burned away. So it's mostly just a big

empty tube, nowhere near as heavy as it

was during launch. So, we only need a

little bit of power to slow it down. A

big engine, even running at its lowest

power level, is still going to push too

hard, and that would actually keep our

rocket in the air. It wouldn't be able

to touch the ground.

That is what we saw in phase one of

Falcon 9's experimental landing program

from 2013 to 2015. The rocket would fall

back from space over the open ocean and

it would relight that one center engine

to slow down for a very soft landing on

the surface of the water. That was the

first realworld test. And what SpaceX

was trying to prove was the ability to

restart their booster engines

mid-flight. This was not something that

any other rocket maker had really needed

to consider. You light your booster on

the launch pad. You burn until it runs

out of gas and then it winds up as

another addition to the rocket

graveyard. But in order to land your

booster, it actually needs to stop and

restart the engines multiple times on

the way back down. This is where

problems tend to arise. Rocket engines

are volatile by nature. When you light

them on the launchpad, you do so in the

ideal condition. But when you try and do

the same thing in midair, stuff tends to

explode.

Check out the second test flight of the

SpaceX Starship. The booster was going

to attempt a soft water landing just

like what we saw with Falcon 9. But as

soon as it tried to restart those

engines to begin slowing down, boom.

Unfortunately, this is the kind of thing

that you can only test by doing it live.

So, as more and more reusable rockets

join the party, we're probably going to

see a nice variety of midair explosions.

It's all part of the process. Now, we

know that reusable rockets need small

engines to land, but they also need a

large amount of power to lift off. For a

rocket to achieve reusability, it's

actually going to need more power than a

conventional rocket just to get the same

amount of mass into space. And that's

why you'll typically see a big cluster

of those smaller engines all tied

together. We have to remember that a

disposable rocket booster is free to

burn all of its fuel on the way to space

because it's just going to fall back

down and splash into the ocean. But a

reusable rocket needs to fly all the way

to space, reaching a top speed of around

8,000 km hour. And then it needs to slow

all the way back down again to reach a

speed of zero at the moment it touches

the landing pad. That requires a bunch

more fuel to be held in reserve for the

landing. We also have to consider that a

reusable rocket is going to be heavier

than a disposable equivalent because

there's a bunch of extra hardware that

is necessary for landing but needs to be

carried all the way into space before it

really gets used. Most of that excess

hardware is going to be needed to help

steer the rocket because going up only

requires a very minimal amount of

guidance. But getting back down is where

the real magic trick begins.

Following stage separation at an

altitude of around 80 km, our rocket

booster is going to begin the process of

slowing down for a return to the launch

site. In order to do that, we have to

flip the entire rocket around so that

the engines are pointing in the opposite

direction.

On Falcon 9, this is done using cold gas

thrusters, which are little jets of

compressed nitrogen gas. Thrusters are

located in two pods on opposite sides of

the rocket, right near the very top.

This position gives them the maximum

amount of leverage to push the booster

all the way over and get the top end

pointing back at the ground. Now, we

need to fire up all nine rocket engines

to start canceling out all of the

momentum that we just built up by flying

to space. This is called the boost back

burn. It's going to change our flight

path from a big arc that goes out over

the ocean to a loop that brings us back

down to where we started from. Once the

rocket is going slow enough that it

starts to fall back down to Earth, we

actually need to flip around again and

get the bottom pointing back toward the

Earth. As we begin to hit the

atmosphere, a lot of heat and pressure

is going to start building up underneath

the rocket. And the best way to relieve

that is actually to fire up the engines

yet again. The re-entry burn will slow

down the velocity even more and relieve

some of that pressure. But the exhaust

from the engines will actually push a

lot of that excess heat away from the

rocket and create a sort of protective

shield underneath, also known as the

jellyfish. Once we are through re-entry,

the engines are shut down again and the

booster goes into freef fall. This is

where steering becomes very important.

One extra item that you'll find on every

reusable rocket is a set of aerodynamic

grid fins. These are able to redirect

air as it flows over the booster. That

allows them to steer the rocket all the

way down through the atmosphere. This

steering is all done autonomously. There

is no person flying the rocket back

down. It's a flight computer that's

guided by something called an inertial

navigation system, which uses sensors on

the rocket to determine its position and

orientation. Then it combines that

information with the global positioning

system or GPS to see exactly where the

rocket is in the air. Then that location

is checked against a pre-programmed

flight path. And if the computer sees

any deviation from the plan, then it's

able to use all of this data to steer

itself back on course.

In the final moments before landing,

that single engine is going to relight

to begin slowing the booster down. And

as the velocity drops, so will the

engine's throttle setting until it's

down at the lowest power level. Then

landing gear is deployed. Three legs

made from a combination of aluminum and

carbon fiber are pushed out by

decompressed helium gas and locked into

position. Now, all that needs to be done

is steer the rocket onto the landing pad

and shut down the engine at the moment

the landing gear touches the ground.

SpaceX may be the king of reusable

rockets, but regardless of who builds

these new rockets, the fundamentals of

reusability remain the same. As radical

as the Starship Super Heavy might be, it

operates on a very similar game plan to

the Falcon 9. It just trades the landing

legs for giant robot arms, which is a

whole other video. Blue Origins New

Glenn will perform the exact same

routine as Falcon 9 once it gets going.

They've already attempted one landing

and the booster exploded when they tried

to relight the engines, which as we've

seen is not uncommon, but they will try

again very soon. And maybe by the end of

this year, we might see the Rocket Lab

Neutron make its own attempt. Will it

succeed? Will it explode in midair? Will

it smash into the landing pad like a

flaming lawn dart? We don't know. But we

get to find out. And that's our favorite

part of the space race.