- Hi, it's me Tim Dodd,
the Everyday Astronaut.
SpaceX is starting to release
more and more pictures
of their Crew Dragon
capsule as they get closer
and closer to finally testing it,
hopefully at the beginning of 2019.
And I just had this awesome question
from Twitter user, Joe Scibelli, who asks,
what's the reason for the
fins on the service module?
That's a great question, and
this is one of those topics
that's super fun to
dive into really quick,
'cause it's a cool
demonstration of physics.
So first I'm gonna show you a spacecraft
that you might be familiar with,
that has to address this
with some fins already.
Then I'm gonna show you a spacecraft
that I think probably
could use these fins.
And then I'm actually gonna actually whip
up a quick Dragon Capsule
in Kerbal Space Program,
and we're gonna recreate exactly why, how,
and what the mechanisms and
the physics are behind this.
So first off, what is going on here?
Well, an object in flight
wants its center of mass
to be in front of its center of lift.
So you can think of something like a dart,
where if you throw a dart, no matter what,
the pointy ends gonna go first
because that's the
heaviest part of the dart,
and the fins are gonna
want to be in the back
and that's the center of
lift or center of drag.
Speaking of pointy end first,
of course rockets are the same way.
So a rocket, typically you want
the pointy end goin' first.
You might see some older
rockets had some fins
at the bottom of them,
but also definitely model
rockets often have fins
at the bottom of the rocket
Unless its of course a Joe
Barnard BPS space rocket,
that has a gimbal, but other
than that, ignore that rocket.
All other model rockets
will typically have fins,
to maintain stability
and correct orientation.
And that's why it points,
and continues to go pointy end first.
So, although that's great for rockets,
there is one part of the space ship
that you don't want to
go pointy end first,
and that's the capsule when
it's coming back from reentry.
When a capsule comes back through reentry,
the heat shield is on
the exact opposite side
of the space craft as the pointy end.
So therefore, we want it
to go heat shield first.
So the smart people who
design these things,
designed them in a way
that's passively stable
through reentry, heat shield first.
Okay, not necessarily
designed that way per se,
it's really more of an inherent property
of the truncated cone shape
like the capsules are.
As a matter of fact, in the 1950's,
Harvey Allen had to convince
his colleagues at NASA
that this was indeed
the most stable design
for space craft and also the best
as far as reentry heating goes as well.
So that means that even if, say, it comes
in crooked or say, you don't have control
and you can't keep it
pointing heat shield first,
due to the physics, it'll
basically, no matter what,
passively come back heat shield first.
And this is great, this is
a really really good thing,
unless you're trying to abort.
So what happens if you're
trying to abort from something?
Well then in general, if
you were just to abort,
straight off the launchpad
with, say, a standard capsule,
it would actually want
to go heat shield first.
And that's not necessarily
the way the thing's gonna
be pointing on the launchpad.
If it's going heat shield first,
it might end up shooting
straight into the ground.
So they have to be clever.
So, SpaceX isn't the only
company that's ever had
to deal with this phenomenon.
As a matter of fact, a rocket and capsule
that you're probably really familiar with,
also has something on it,
and that's the Soyuz Capsule.
Have you ever noticed
that the Soyuz Capsule,
actually has gridfins on it.
Yeah, that's right, let me say that again,
the Soyuz Capsule has gridfins on it,
to keep it stable in
the event of an abort.
Now notice that they're folded up
and tucked away on ascent,
but if there would need to be an abort,
that entire upper fairing comes
along with the abort motors,
and those gridfins pop down
and that makes it so the center of lift is
behind the center of mass.
Now say those gridfins weren't there,
it would probably cause the center of mass
to be awfully close to the center of lift,
somewhere in the middle of that fairing.
And it would probably be very
un-aerodynamically stable
so a couple of gridfins
just solve it perfectly,
An almost identical version of this,
is also with India's
space agency, the ISRO.
Now they're working on the
Gaganyaan crew capsule,
which did an abort test on July 5th, 2018,
and notice it has almost
that exact same system,
some gridfins, which again
keep that center of lift
behind the center of mass.
Now a vehicle that I think could
use some fins of some kind,
would be Blue Origin's New Shepard.
So we actually saw them do
an in-flight abort test,
on October 5th, 2016, with
their New Shepard program.
Right at max Q, they lit off
the solid rocket motor trip
inside the capsule that
is their abort system.
And the capsule actually took off,
and actually in an
absolute feat of rocketry.
It was really cool, the
booster ended up still landing
after having a rocket
engine ignite on top of it.
But that's another story.
The crew capsule, meanwhile, took off
and at first it maintained a
decent amount of stability,
but pretty quickly it starts to oscillate
and swing back and forth,
and eventually even tumble.
This is an example of where it'd probably
make it a little bit more stable ride,
and pointing continually
in the right direction,
if it had some fins, or gridfins,
or something that deployed out
that kept the center of lift
behind the center of mass.
Now while we're looking at abort footage,
lets not forget that SpaceX
has already done a pad abort,
with their Dragon 2 Capsule.
They did this way back in May 2015,
and you'll notice, it went pretty well.
It slightly underperformed,
one of the SuperDraco engines shut down
and when one does, the opposite one has
to shut down as well, so
it slightly underperformed.
But it still made it out
to the ocean basically,
where it was safe and it
did what it needed to do.
It passed, it was certified.
Don't forget in 2019, we're going
to see them do an in-flight abort test.
That one's going to be crazy,
because they are going to
detonate the Falcon 9 beneath it.
I think that's gonna be a launch,
I absolutely have to go see.
Normally, I always say,
scrubs are cheaper than booms,
but in this one, the boom is definitely
going to be the show stopper on this one.
Okay, I'm sure you have it
all pieced together now,
why there's fins on the
trunk of the Dragon Capsule.
But, just for fun, I've
gotta show you what I did
in Kerbal Space Program
that represents this,
absolutely perfectly.
It's a great demonstration
so check this out.
Alright, welcome to Kerbal Space Program.
I always say this is like
half game, half simulator
and 100 percent explosion factory.
So I'm just playing back
a really quick build,
a nice quick, dirty build of
the Dragon Spacecraft-ish.
It's obviously not going
to be a perfect replica,
but I'm attaching a few things,
like adding some fuel tanks
and some basketballs here
to the bottom portion
of the Dragon Capsule.
Now in real life, SpaceX
really does have tanks
of propellant tanks, and oxygen,
and all these things kind
of in the bottom portion
of the pressure vessel.
I'll pull up this video quick,
so you can see what that looks like
when there's not a skirt around it.
But here's what is looks like,
I'm gonna pop the skirt on there kind of.
This is pretty close to a Dragon Capsule.
Now of course we still
need to add those engines,
and SpaceX actually
has four different pods
of SuperDraco engines, but
each pod has two SuperDraco
so there's actually
eight SuperDraco engines,
on the Dragon 2.
This is about the right amount of thrust,
just having four of these engines,
so it's not a perfect representation,
but it's close enough.
It's representing what
we need it to represent.
We're gonna do a few things
here to this command module
to make sure it acts like how
a crew command module would.
And that first thing we did
was turn off the control wheel,
the reaction wheel, control authority,
because in Kerbal, the
reaction wheel can steer
and stabilize a spacecraft
a lot, like way too much.
In real life, reactions
wheels don't have nearly
that much control authority during ascent
or any other time.
So I turned that off completely,
so we get the true passive
effects of this vehicle.
The other thing I'm going to do
is turn off the Gimbal of the engines.
Now in SpaceX's case, the
engines can steer the Dragon 2.
As a matter of fact, they
can even make the Dragon 2
hover perfectly, maintaining
its orientation perfectly,
using thrust differentials.
That's where they can
throttle individual pods
to maintain an orientation or guide it,
and point it, and steer it
in directions using thrust differential.
Kerbal Space Program
doesn't actually let us do
thrust differential but this at least
is a good demonstration of
what would happen if the system
was totally passive and I think
you have a pretty good idea
of how this is going to end up.
So let's just take it out
to the launch pad as is,
and see how we do.
Okay, and here we go,
three, two, one, and go.
See, like I said, this is exactly
what you'd expect would happen,
it's incredibly unstable,
and that would not be a
very good day for anyone.
I'm just gonna go ahead and
stick on kind of a trunk here,
to the Dragon Capsule.
I'm just gonna use a structural tube here
in Kerbal Space Program, an empty tube.
And I'm gonna stick some of
those deluxe fins on there,
and I'm gonna stick four of them,
I'm gonna push them in quite a
bit, so that they look right.
One of the other things
I'm gonna be doing here is,
they will still be able to
spin or steer a little bit.
Obviously in real life,
SpaceX doesn't have
any control authority using the fins,
they're completely passive.
Since I don't have thrust differential,
or any mod to use thrust differential
in Kerbal Space Program,
I will not be able
to steer, any other way.
So I'm going to use these
to steer a little bit.
Here, you can see the yellow
ball is the center of mass,
so that's where the weight is,
in the middle of the vehicle.
And the blue ball is the center of lift.
Now, you would think that in general
that the center of lift in
that, looked like it was behind
but it's so close in the
standard capsule design,
that's it very unstable.
By adding the trunk and these fins,
you can see how much lower
our center of lift gets
compared to our center of mass.
Now clearly the center of mass is well
ahead of the center of lift.
This should be nice and
stable, let's check it out.
Okay, here we go, pad abort,
three, two, one, and go.
Now notice, you can see a
little a bit of RCS firing,
those don't really steer too much.
The only that's able
to steer a little bit,
again in my case, is those fins,
and they can steer just a tiny bit.
The big thing is, notice that
it's pointing the correct way,
the whole way through ascent here.
Also once that trunk is jettisoned,
it instantly starts to flip
and try to roll heat shield first.
That's just again, its passive thing.
The craziest thing is if we
compare that side by side
to SpaceX's pad abort, it
is shockingly identical.
So I think that's just
showing how cleanly,
Kerbal actually represents
the physics here.
And that right there ladies and gentlemen
is exactly why there's
fins on Dragon Capsule.
Now, we're heading into the year 2019,
where we're going to finally
see the crew Dragon capsule fly
as well as Boeing CST100 Starliner.
Of course I'll be doing a
comparison video of them,
pretty soon, hopefully before
either one of them launches.
Let me know what other
questions you guys have
about either one of those vehicles
or about next year's
commercial crew program,
or if you have any other questions
that I forgot to talk about in this video.
I hope it all makes sense.
I just wanted to throw this thing
together real quick for ya.
And don't forget, if you want
to see more exclusive stuff
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including, shirts, hats, prints, mugs,
grid fin not-a-coasters,
and lots of other fun stuff,
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And while you're there, make
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where you can download and stream
all the music you've been
listening to in this video.
It's off my first album called
Maximum Aerodynamic Pressure,
which you can check out
and listen to anywhere.
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)
