- 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. I of course also a huge thank
you to my Patreon supporters for helping me make all content possible, including traveling around all the time, trying to get as much content out there for you guys as I can. It's been a crazy time lately and you guys are definitely
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earth for everyday people. (upbeat music)
Whenever I get attached to a design they change it
Great video! But I'm more impressed by how quickly you put out this video. Cries in aerospike video
Great video, thank you! I'm curious why Elon said it's "controlled by (very) rapid movement of rear & fwd fins". Any speculation as to why the movement would be that fast?
I admit I had gotten attached to the latest design but it actually doesn’t look half bad in this new render.
I wonder why he didn’t choose to try out two different designs for these prototypes, then he could objectively test and compare them.
Ya know? I was still having trouble visualizing how the two top and two bottom fins/flaps/brakes would allow control of pitch, roll, and yaw... Until you basically said it's like a skydiver with arms and legs... That was, for me, kind of a wow moment. Yeah... Great job there.
The 3 fin design looks retro cool, but Imo the 2 fin design looks more badass..
[removed]
i think i like the two fins better. makes it more modern? missile looking like even?
any ksp models of the revised design?