For me one of the more compelling elements of an engineering education and
then also practicing engineering is this idea of being a detective. Of being able to look at the underlying code
of how the world works and deciphering it. Being able to see what works, what doesn't,
and understanding the why behind it.
And so, for the journey of engineer, it's a lot about observation.
It's looking for the problems that are out in the world. It's also being able to marry those observations
up with analytical modeling, being able to characterize how things work,
and fusing those two together to be able to test things to be able to design and iterate
and to improve over time as well.
I think on top of this, engineers are also kind of an unsatisfied lot. We see problems everywhere.
We not only see the problems we feel personally compelled to help solve them.
And so to do that, we design we look for those problems, we iterate and brainstorm solutions.
And then we go out and try things. And some of those are going to work, some will not.
But we move forward iterating and improving and making things better.
And for many of this, this sort of this idea of engineering and observation starts very early in life.
Finding one's place in life can be difficult, but I think for engineers often happens quite early.
I can remember an instance of small child in a car,
my mom had rolled down the window, and I put my hand out the window,
and I could feel the airflow go through my hands.
And that was really my first taste of aerodynamics.
And so this idea that some sort of invisible force could reach out and press against my hand amazed me.
And I wanted to understand it. I wanted to see why was it working the way it was.
And one of the things I found was not only that that air pressure or the forces on my hand,
it was also that I could control it, which was even more amazing.
Depending on the angle of my hand in the wind; if I had a fist, or if I cupped it, or had a flat hand, or an open hand
all changed the forces. Both the magnitude of the force and the direction of the force
in a way that I could control even as a small child.
So from early observations like this, as I grew, I learned a little bit later
at about age 10, or 11, this world of model rocketry.
and model rockets for me were amazing.
It was something where I could take elemental materials, paper, wood glue,
and then be able to put those together in a very specific configuration and be able to launch that into the sky.
And so for a 10 or 11 year old it's very empowering,
you could actually take materials and they would do something. They just didn't sit there on a table.
And so I must have participated in or witnessed hundreds of launches through my
kind of early rocketry career.
and I found some differences.
The first was some went straight and very high up into the sky performed flawlessly. Others not so much.
They either didn't gain much altitude, some of them even tumbled in the sky
and then ultimately fell onto the ground unexpectedly.
And so I wondered what what made some things successful and other things unsuccessful.
And as I learned a little bit more about engineering and a little bit more of mathematics,
I learned that there are some fundamental forces that affect how vehicles like a rocket actually work.
And if you can understand those, forces, if you could understand those various quantities,
you could design better. And I wanted to design better.
And so if you look at some of the fundamental forces that are acting on a rocket,
they're fairly straightforward, but it's important to understand a little bit about how they work,
why they work and what the limitations are of that knowledge.
And so the first is just gravitational force, we all intuitively understand that as a weight of an object.
And so this rocket absolutely has weight.
If you can imagine this rocket in a vertical configuration that weight is always acting downwards
and pulling the rocket earthward.
Counteracting that force is the thrust from the rocket motor that counteracts that weight.
In addition, there's also aerodynamic forces. And so these are the forces that result from the pressure
very much like they have your hand outside the window that you feel, only done at much higher velocity.
And so you can see those forces those are real and they exert forces on the rocket.
So quantities like lift and drag are also important as well,
in addition to the thrust and the gravitational forces that you have for a rocket.
And so what's challenging is that throughout our rockets flight, these forces are always changing.
So for example, as the rocket motor fires and the fuel is expanded, it gets lighter
and the overall rocket gets lighter over time.
In addition, the thrust in the motor itself can be designed to vary over time, so that's constantly changing.
And in addition, as the rocket accelerates and goes up into the sky,
the pressures and forces like lift and drag that are acting on it are changing all the time as well.
In fact, those lift and drag forces tend to change the square of the velocity
and can become very large over time as well.
And so if the flow field around this rocket can be very complicated. And so
somehow getting your hands around these forces is very important.
And so fortunately, one of the things you can do is to simplify matters.
And one of the things that you can do is really look at the forces that I just mentioned,
as acting through single points on the rocket. So instead of it being this big field of forces all around the rocket,
let's just take those down to simple points on the rocket that we can deal with.
And so the first is called the center of gravity.
And we're probably all used to something like center of gravity,
if we've ever tried to balance a pencil or another object on our finger or on a tabletop
we know that's just the balance point of an object with a gravitational force.
So very easy to find that. For the aerodynamic forces,
a little bit different, but they can actually be placed in a center point called the center of pressure.
And so the center of pressure, very much like the center of gravity,
is just where all of those aerodynamic forces tend to balance.
And so it's a little hard to find that center of pressure on a rocket.
So one of the things we did very early on, when I was young, we would make a cut out of our rocket.
So we'd have this two dimensional profile of our rocket. And we balanced that.
And so that was a way of kind of estimating or defining what that center pressure might be.
And it's actually a pretty good method, but it's not perfect.
The three dimensional airflow around a rocket is very involved, very complicated.
So what you'd find as you get into either higher performance rockets like this, or actual rockets
that are taking astronauts up to the space station, and so forth
they use much more complicated tools or analytical tools called computational fluid dynamics, or CFD.
And CFD is used to be able to measure the pressure and the forces and on a rocket and be able to find things like
the center pressure that we were just talking about here, but do it obviously at a much larger scale.
Now that we've simplified the gravitational forces and pressure forces into single points,
the center of gravity and the center pressure.
It's also important to look at the relationship between those two points
the location of those points on the vehicle matter.
And so the relative position between the center gravity and center pressure affects something called stability.
And stability is the ability of a vehicle such as a rocket to be able to return to straight flight
if it's disturbed in some way. Say as if it gets hit by a wind gust, for example.
And so stability allows that vehicle to self correct automatically to straight flight.
And so in general, vehicles are designed or rockets are designed to have the center of pressure
be to the rear of the center of gravity.
And why is this? And so if you look at that relationship of the center of gravity and center of pressure
let's take a look at this rocket and imagine that it gets perturbed in its flight.
So let's move it a little bit. So it rotates around its center of gravity and is perturbed.
And so what you find is that the center of pressure has now moved,
and the forces that are acting at that center of pressure tend to return the rocket back to its original flight path.
And so by having the center pressure behind the center of gravity, you have a restoring force.
or restoring what's called a moment or torque
that allows the rocket to rotate, rotate back to a stable flight.
And so this is called static stability or positive static stability,
So that the rocket can automatically change itself back to that stable flight.
What happens if we change the location of the center of gravity?
Let's bring it in a little bit closer to the center pressure.
So in this case, center of gravity has moved to the rear, aft. How can that happen?
Well we might have lightened up the nose area, we could have added an even heavier motor at the back
and that would shift the center of gravity backwards.
It's still in front of the center pressure but nowhere near as far as it was before
and because of that, it is still stable.
However, any type of perturbation like we had before, will take much longer to correct.
In fact, the nose may wander back and forth and oscillate
before it finally goes back to static into that static or stable flight
And so once again, still stable but not as stable as it was before.
What happens if we move the center pressure in front of the center of gravity?
So how can that happen?
Let's say we have an even heavier motor that moves the center of gravity back further,
or we for whatever reason, put some fins up onto the front of the rocket
that moves the center of pressure forward.
And then let's take a look and see what happens why this is now what we would call an unstable configuration.
And what happens is, let's let's perturb the rocket by the wind gusts like we did before,
and the center pressure still acts the way it did before but because of its position now
in front of the center of gravity, that resultant force
causes even more rotation of the rocket away from the flight path.
And as that rocket moves away, there's even higher aerodynamic forces causing it to move even further.
And as that progresses, you often can get a tumbling or unpredictable motion out of the rocket.
Something that looks a little bit like this:
But what you really want is this:
So as I mentioned at the beginning, engineering is about making observations.
It's about tying those observations to analytical tools, making predictions,
going out and trying things testing and ultimately, as you iterate make better and better designs.