- Hi it's me, Tim Dodd the Everyday Astronaut.
We're less than a week away from this year's
Starship update from Elon Musk,
and before the party gets started,
we're already catching some major changes
happening before our eyes.
That's because as we speak SpaceX is building
the Mark 1 Starship prototype, right there in Boca Chica
where the presentation will be held on September 28th.
And of course, because it's being built
out in a field in public,
we're seeing some major updates to the hardware.
The most obvious change is the fact
that the Starship has only two finney flappy
air-brakey things now instead of three,
and the previous version
touted just a year ago at the Dear Moon event
showed Starship sporting three fins
that were also its landing legs.
So now that we're seeing it only has two,
the question has come up, and it's come up a lot.
Why, why just two?
So today, we're gonna do a really quick rundown
on why two might be better than three,
show you some potential problems that three fins had,
and then we'll show you exactly how they'll control
the re-entry and landing with just two rear fins.
Let's get started. (rousing music)
- [Mission Control] Three, two, one,
and liftoff! (upbeat music)
- [Neil] That's one small step for man.
- [Mission Control] That's Houston contact for the test one.
- Okay, first things first.
I think it'd be fun to do a quick rundown here
on this fun little bit of engineering
while we get ready for the event on the 28th,
so, when it gets revealed, we can be like, "Ah, yeah."
And since I've been way too busy now for over two months,
working on what's become
literally a documentary on the aerospike engine,
I figured I'd better crank out
a nice easy video here in the meantime.
Okay, so now before we get going,
I wanted to remind you these are not wings.
They are really just giant air brakes,
or body flaps, as Elon once called them.
Ew. (laughing)
They are perpendicular to the airstream and not parallel,
but we'll call them fins or flaps
interchangeably in this video.
And another note. We're gonna use
Kerbal Space Program here in this video.
And if you're triggered by a game, settle down, settle down.
We're gonna use Kerbal to illustrate some of the problems
and demonstrate some of the solutions,
but we're doing this knowing this isn't an engineering tool
for validations or argument sake.
Regardless, it's still a great demonstrator
of the basic physics, and it definitely helps visualize
some of the challenges involved.
Okay, so let's start off with the obvious reason
of why there's now two fins instead of three.
Wait, according to Elon Musk,
"Current analysis, which I'm not fully bought into,
"suggest that two rear fins
"with separate airframe-mounted legs will be lighter,
"so this is the plan for Mark 1 and Mark 2."
So it sounds like there may be
some internal debate here on what's lighter,
and for now, two fins seems to have won out.
But part of me thinks that Elon
really loved the old-school look
of the three fins at the rear.
I know he mentioned how it reminded him
of Tintin's rocket many times,
and considering how much he loves aesthetics,
this might actually be kind of true.
- I think it looks beautiful.
And I love the Tintin rocket design,
so I kind of wanted to bias it towards that.
- And I'm a little torn up on this one actually,
because I thought it was super cool
that the landing legs were also the control surfaces.
But then again, there was that third leg
that didn't articulate at all
and would've had a lot of additional dead weight,
and not to mention, making your control surfaces
also handle the weight and impacted the entire ship
would've likely been rather difficult to engineer.
Now, I'm not saying it couldn't be done in the future
and it might not be something that they update to later on,
but for now, the easiest path forward
for these first prototypes,
if they separate the landing legs from the fins,
it might have simplified things quite a bit.
Not to mention, say you had a rough landing on Mars
and broke a landing gear, at least it's not
integrated into your primary control surface
that helps you survive re-entering Earth's atmosphere.
I mean, that's unlikely, but having the two systems separate
might not be a bad thing,
especially as they develop the vehicle.
And after all, don't forget, these first prototypes
are really just technology demonstrators.
They're flying science experiments
that are built as inexpensively and quickly as possible,
to start shaking out some of these new technologies.
Nothing on Starship is set in stone.
I mean, look at how much the Falcon 9 evolved
in such a short timeframe.
Starship is taking that concept up another notch.
So, if the easiest path forward for now
is landing legs on the body and the flaps separate,
sweet, yeah, get it going.
At last year's Dear Moon event,
it stood out to me that Elon said a few times
how the third leg is only a leg
and it's not a rear stabilizer of any kind,
and mostly because on re-entry
that fin is entirely in the wake of the vehicle
and it doesn't even touch the airstream.
Later on, if it does prove to actually be lighter
to integrate the landing legs onto the flaps,
nothing's going to stop SpaceX
from evolving Starship to do exactly that.
But in the meantime, I actually think
that the aerodynamics team at SpaceX
was likely begging engineering to reconsider three fins
for a few reasons.
Now, here's where we're going to get
into speculation land for a little second,
but I've always been questioning how three would've worked,
because of some basic physics.
About exactly your ago I did a video
about the belly-flop maneuver,
and in order to help me get it to work well
I had to keep the fins tucked in on ascent,
and I needed to keep them
behind the center of mass on descent.
Here, let me show you something.
The general rule of thumb for anything flying is,
you want your center of pressure or lift
behind your center of mass.
Otherwise, if the vehicle moves away
from its heading at all, the surfaces will induce lift
or torque on the vehicle,
and since it's in front, it'll become a feedback loop.
Say it pulls up just a little,
that's going to change the angle of attack on the surface,
which will make it pitch up a little more,
which will change the angle of attack on the surface,
which will make it want to pull up a little more,
which will change the angle of attack on the surface,
which will make it wanna, et cetera, et cetera,
until you lose control,
and that surface will want to flip around to the back
where it belongs.
That's because, in the back,
those forces actually do the exact opposite thing
and work to stability's advantage.
When a craft starts to pitch away from its heading,
the fins apply torque to recenter the craft.
The further away from your heading you pitch
the more torque it applies,
which makes it very inherently stable.
This principle applies to Starship
during three different phases, ascent, re-entry and landing,
and it does so in three very different ways.
That's why, when we started seeing renderings and animations
of the three-fin Starship
ascending on the super-heavy booster,
my eyes got a little wide and I got a little worried.
When I saw that, I thought about throwing a dart backwards.
It's probably not gonna fly backwards very long,
it's probably gonna flip right over
and orient itself so the fins are in the back.
Now, that's not to say it can't be done,
because the engines have an awful lot
of control authority with their gimbal,
and they can help maintain proper orientation.
But in general, it's much, much more difficult to fly
with large aero surfaces
that far out in front of your center of mass.
Luckily, computers are really good
at managing crazy aerodynamic properties,
which is why all modern fighter jets
are inherently unstable and are fly-by-wire,
where computer actually interprets the input from the pilot
and moves whatever surfaces necessary
to move the aircraft as the pilot intended.
(upbeat music)
Okay, so why would two be better than three on ascent?
Won't two still be an issue?
Besides the fact that there's less fins in the first place,
there's something two can do
that three can't really do easily,
and that's get tucked in away from the airstream.
While on ascent, a pair of fins
can easily tuck in against their airframe
and mostly negate their aerodynamic properties.
Both the top and bottom fins will do this.
Whereas three fins, especially when one was
an immobile landing leg only,
can't really tuck in out of the airstream entirely.
I mean, sure, they could've tucked two in
and left the third one out
and kept that more in the wake of the vehicle
during its gravity turn,
but it's still just not the most stable configuration.
Okay, but forget ascent.
I'm sure they can do that no problem
either way if they had to.
The bigger challenge is actually
with the descent of Starship, with three versus two fins.
And again, this has everything to do
with tucking the fins in.
Let's actually watch the real physics simulation
that Elon showed at last year's Dear Moon presentation.
You'll see the vehicle does the belly-first
to tail-down flip at only about 1.5 kilometers in altitude,
while still traveling at mach one for landing on Mars.
Now notice, during this landing phase,
the rear fins are tucked in all the way
until the very last second.
But even so, the vehicle has one giant fin
fully eating the wind stream in front of the vehicle.
Any movement away from the vehicle's heading
will induce a lot of torque.
And again, the gimbals on the Raptors
and the attitude control thrusters
can offset all of this torque
and hopefully maintain stability,
but if you have the option
of eliminating something that's going to actively try
and make your vehicle lose control,
it might not be a bad idea to change it.
And Elon did go on to say,
"Stability is controlled by very rapid movement
"of rear and forward fins during entry and landing,
"as well as ACS thrusters," or attitude control thrusters.
"The smaller leeward fin would simply be used as a leg."
But I think he's mainly talking about re-entry,
because during the actual landing portion,
the legs and upper fins can only articulate
on their dihedral axis.
Now, if these fins had the ability
to also act like canards, which would be able to produce
two axes of control through the x-axis of the rocket,
I can see where the statement would be true.
But during the final tail-down landing portion,
they are essentially passive.
Notice, with just the two fins on the bottom,
again, just like on ascent,
we can tuck them in against the fuselage
and mostly negate their aerodynamic effects.
Not to mention, there aren't any portion of these new fins
that are sticking out below the engines,
which would put them in front of
the airstream of the vehicle as it descends,
which again is a bad thing.
The other problem with the three landing legs
is a little more complicated,
but I'll bet the aerodynamics team had a pretty great time
going over the properties of the three-legged re-entry.
When re-entering, the vehicle will be
completely belly-flopping, and that is,
coming in broadside with its belly facing the airstream.
Well, hold on actually.
It'll mostly belly-flop.
But it sounds like, according to Elon Musk,
"It does actually generate lift in hypersonic regime,
"which is important to limit peak heating."
In other words, it could probably pitch down a little bit,
which would allow the body to create
a little additional lift, which can also limit peak heating
and make sure it doesn't ever get too hot.
With three legs, they're attached
at a 120 degrees apart from each other,
which puts the two fins that articulate
awfully close to the center of mass of the rocket.
Now, this isn't necessarily a bad thing.
In fact, a traditional capsule shape
intentionally has a center of pressure
in front of the center of mass.
This is known as blunt body dynamics,
and in 1952, H. Julian Allen of NACA,
now NASA's Ames Aeronautical Lab,
discovered a capsule was most stable
and dissipated the most heat
when it had a blunt surface facing the wind stream.
So, perhaps the fins could function in this manner,
but considering how much
they actually need to move throughout re-entry,
I could see it again being less stable
than having those fins behind the center of mass.
I even wonder if they do end up going
with integrating the legs back onto the fins someday,
if they'll have a retractable third leg
that would be in the front of the vehicle,
so facing windward,
and let the other two fins remain behind the center of mass
for better passive stability.
So this is another time we're having just two fins,
when partially retracted through the re-entry,
would have a better, more passively stable configuration.
But again, I'm sure they really cool ways
to deal with some of these things.
But I'm thinking they actually made
a significantly more stable vehicle
by only having two fins in the rear.
If we see them tucked in on ascent and tucked in on landing,
that kind of proves that this is indeed advantageous.
Okay, so that's a quick overview
of why I think two is better than three.
So let's have some fun, and I'm gonna show you
just how exactly you can control all three axes of control
with just two upper fins and to lower fins.
Now, this method of control hasn't actually changed at all,
because that third fin on the back
was really completely passive and out of the wind stream,
so everything we talk about here
is the same way it would've worked
with three fins in the back too.
It's actually a really cool method of control,
I've never really seen anything like this.
Remember, these flaps are perpendicular to the wind stream
and not parallel like the wings or ailerons of an airplane.
The only thing we can do is increase or decrease
the amount of drag each one produces.
Remember, a plane typically uses
a change in lift or deflection
to induce pitch, yaw, and roll.
Starship will essentially use drag brakes
to control its pitch, yaw and roll,
and most aircraft do not do this.
My favorite example of this is the B-2 Stealth Bomber,
which doesn't have a tail stabilizer.
So, in order to induce yaw,
it actually opens up what's called a split aileron,
which can induce drag on one wing or the other,
which then controls the jet's yaw.
Here's how this works.
Now, we're still gonna consider the cockpit
our frame of reference for pitch, yaw, and roll.
To induce pitch, the top and bottom fins
can move opposite each other.
One extends more while the other retracts,
which can help pitch the nose up or push it down.
To induce roll, the left and right flaps
can move opposite each other.
Extend one side more than the other
and the vehicle will roll.
But you can even induce yaw.
For this, we move opposite corners of each other.
Top left and bottom right can move
opposite of the top right and bottom left.
This will induce yaw
as it belly-flops through the atmosphere.
And this is exactly why Elon keeps saying
this vehicle is more like a skydiver than an airplane.
A skydiver needs to use their limbs
to control themselves in a very similar manner,
something I once found out I'm awful at.
It's a lot harder than it looks.
Every tiny bit of movement I make creates a lot of torque,
thanks to the grids on my grid fin.
So, throughout re-entry, the computer will be
continually adjusting the flaps
to maintain orientation and a stable configuration,
but they can also adjust their total drag coefficiency
to help perfectly nail their downrange distance.
And don't forget, the vehicle can also use
attitude control thrusters throughout re-entry.
But with that being a limited quantity,
the more work the flaps can do the better.
Now, it's gonna maintain this orientation as long as it can,
because that belly-flop does an awesome job
of slowing it down without using any propellant,
so they really wanna do that flip as late as possible.
After all, as soon as it goes tail down,
it's actually a lot more aerodynamic
and will fall a lot quicker,
so the engines actually have a lot more work to do.
Once the vehicle's ready to do its flip,
they're going to tuck the rear fins in
and extend the upper ones.
Then they'll light up three of its Raptor engines,
and alongside with the ACS,
they're going to reorient the rocket
so it basically ends up landing kind of like a Falcon 9.
(uplifting music)
And then, when the timing is just right,
they're going to extend those landing legs,
throttle the engines just perfectly
and hopefully make a perfect, perfect touchdown.
Yes, there we go.
Okay, that's the one.
(laughing) Yes!
That's what I'm talkin' about, right there. Holy cow.
Now, that will be something to see.
I mean, come on, I can't help but laugh
about how ho-hum Falcon 9 landings have become already.
God, we really do have the attention span
of a bunch of five-year-olds, don't we?
And in case you're wondering,
yes, I do have the craft file link in the description.
It's all stock, but it does utilize
Kerbal Breaking Ground DLC for those robotics.
So, to summarize, I truly don't think
aerodynamic considerations was the primary reason
we're now seeing two fins on Starship,
but I can't help but think it's inherently more stable now.
And when dealing with something
that's as big as a skyscraper falling out of the sky
with hundreds of tons of explosive propellant on board,
it's probably a good idea to have physics on your side.
It sounds like the primary reason is weight and simplicity.
And just again a friendly reminder,
I don't think I know better
than anyone on the subject matter.
I'm literally the last person
you wanna have working on your spaceship.
But teaching some of these fun principles
and applying them to new designs is super fun.
And knowing just how much Elon Musk cares for aesthetics,
I still think his love for the retro styling
was a big factor in having three fins,
because quite frankly, it did look really cool.
But what do you think?
Do you like the two fin version, or are you gonna miss
the old retro-throwback three fin version?
Do you think they'll stick with the two fin for a while,
or do you think we'll see them go back to three
after these Mark 1 and Mark 2 prototypes?
Do you think they'll tuck in on ascent
and portions of descent as well?
Let me know what you think and let me know
if you have any other questions about re-entry,
landing burn, Raptor engine, Starship,
or just rockets in general, in the comments below.
And be sure and stick around, because not only
am I going down to Boca Chica for the Starship presentation,
I'm also going to do a video about why Starship
doesn't have an abort system.
And of course, I'll hopefully have my long long long video
about aerospike engines done soon as well.
I promise, it's coming along great,
and hopefully, you learn as much as I did making it.
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Thanks, everybody. That's gonna do it for me.
I'm Tim Dodd, the Everyday Astronaut,
bringing space down to earth for everyday people.
(upbeat music)