All language subtitles for 2021-05-11-210900

it will only have to do one

landing burn

and that right there is a substantial difference

but even for the final landing burn

staying in the belly flop as long as possible

still pays off

because notice

right before the falcon 9 lights attentions

its velocity is still about 310 meters per 2nd

that's a little

more than three times faster than starship

before it lights its engines for the landing burn

and it still hasn't even

reached the equilibrium of terminal velocity

because at this point

it's still experiencing aerodynamic drag of almost 2gs

so it didn't even reach terminal velocity period

so I guess that's maybe

a huge difference is that starship will

actually hit terminal velocity

and the falcon 9 just doesn't

okay well so what 220 meters per 2nd difference

that doesn't sound like that big of a deal

I mean to get into low earth orbit

you need to go about 7 800 meters per 2nd

so 220 meters per 2nd

that's only a small fraction of orbital velocity

why is this belly flop maneuver worth it

well here's the problem when you're falling

every 2nd you are trying to slow down propulsive

the first 9.8 meters per second of deceleration

are just wasted fighting gravity

so that 235 meters per 2nd can

actually be a lot more

because of something called gravity drag

or gravity loss

but in order to actually

understand gravity losses

we need to 1st

explain thrust to weight ratios and engine throttling

and here's where the fun begins

thrust away ratio perhaps you've heard of it

perhaps you've played lots of herbal space program

like me and you have a pretty decent grasp of it or

maybe you don't have

any idea what I'm talking about at all

and that's fine

so let's imagine a rocket hovering for now

let's completely ignore the atmosphere

if we separate these forces and concepts

it's going to make this a lot easier to learn

so in order to hover

the rocket engine needs to produce exactly as much

thrust as the rocket weighs

in order to explain this best we're going to use

newtons for both the weight

and the thrust of the rocket

since it's a unit of force

an object with a mass of one kilogram weighs

9.8 newtons on earth

this is because

earth's gravity pulls at one

kilogram with a force of 9.8 newtons

and just for fun on mars

the same mass would weigh 3.7 newtons

of course it be just as easy to use

pounds and pounds force in this example

but we'll use newtons despite me not

being very used to it

but it's all relative anyway

so if your rocket weighs 1

000 newtons otherwise known as a kiloton

and you're producing 1

000 newtons of thrust in the opposite direction

you would hover

because you have a thrust weight ratio of one to one

which means your thrust is

exactly counteracting gravity

and therefore your weight

your net acceleration is zero

because your thrust is exactly counteracting

earth's pole

on your rocket

produce 900 newtons of thrust with your 1

000 newton rocket

and your thrust to weight ratio will be less

than one to one

specifically 0.9 to one

and you'll go down

for each 2nd to your at this throttle setting

with that thrust away ratio of 0.9 to one

you'll go downward

faster and faster you would be accelerating downward

and if you throttle back up to one to one

you wouldn't go back to a hover magically

you'd actually continue to go down at the same

velocity a thrust weight ratio of one to one

just means your velocity is not changing

so in order to get back to a hover

we need to increase our thrust weight ratio to

over one to one

just to accelerate enough to reach zero velocity

so now let's throttle our engines to produce 1

100 newtons of thrust

which would be a thrust to weight ratio of 1.1 to 1

and we'll start canceling out the velocity

once we get back to

zero velocity

we can return to a thrust away ratio of one to one

if we want to hover

so now let's get back to where we started

let's go to thrust away ratio of 1.5 to 1

and accelerate quickly upwards

and again this is very important to remember

if you instantly throttled back

to a thrust away ratio of one to one

you would continue going up at the same velocity

you wouldn't magically hover

so to get back to hovering where we started

we'll hold our upwards velocity until we are close

to where we started

and then we'll reduce our thrust away ratio

below one to one

decelerate until our velocity is at

0m per 2nd

and then increase our throttle back

to a thrust away ratio of one to one

to maintain a hover

right back where we started

it's actually quite hard

to make a rocket hover and maneuver

but it gets even more complicated when you remember

that when a rocket engine is running

it's also burning fuel

so the rocket is getting lighter and lighter

as propellant is expelled

so in order to maintain a thrust to weight ratio of

say one to one

you have to be able to precisely

throttle your engine to produce exactly as much

thrust as your rocket weighs

even though it's getting lighter and lighter

and of course

throttling an engine is a big big deal for landing

otherwise if you couldn't throttle

you'd have to turn on your engines at the exact

right moment

huh that sounds like a really bad idea

right joe barnard