If you like to build and launch model rockets that you purchase in the store, you
don't need to worry too much about rocket stability. And that's because the
rockets you buy are already designed to be stable.
However, if you want to design your own rockets, you should know a few things
about stability.
Basically, a rocket that is stable flies straight up, or straight in the direction
in which it is launched.
An unstable rocket doesn't.
A rocket that is unstable might fly in a circle or crash into the ground after it
leaves the launch pad.
So in this video I'd like to look at what makes a rocket stable. It's really not
all that complicated. Once you understand the basics, it's fairly simple to know what
you have to do to make a stable rocket.
First, let's take a look at the way a weather vane works.
Now bear with me because there might be a few people watching
who have never seen a weather vane.
A weather vane is a little device that mounts on a swivel on a roof
and it points in the direction that the wind is coming from. In other words
as the wind changes direction
the vane will simply point into the wind all the time
and it allows you to know what direction the wind is coming from at any given time.
Here's a diagram of simple weather vane.
Because of the shape of the vane and the position of the pivot point
it will always turn and point in the wind.
Let's take a look at why it does that:
Here we have a top-down view
Let's just say the weather vane is pointed like this and a gust of wind
comes at it from the side.
This creates two opposing forces.
The wind that hits the back end of the vane tries to turn the vane to point into the wind
However
the wind that hits the front of the pivot point tries to turn the vane away from
the wind which would be incorrect.
So which one of these opposing forces wins the battle?
The answer here should be pretty obvious:
Since there's more surface area behind the pivot point then in front of it
the wind that strikes the back of the vane has more leverage than the wind in
the front
and so the vane swivels and points into the wind as it should.
here again is the side view
For a weather vane to work there must always be more surface area to the rear
of the pivot point than in front of it.
Now let's get back to rockets. A model rocket works on exactly the same
principle as the weather vane.
In order to make a model rocket stable, you need to have more surface area to
the rear of the pivot point
than in front of it.
That way, the air that is moving past it keeps it pointing up instead of turning
it around.
Now you're probably thinking that a rocket doesn't have a pivot point because it isn't
attached to anything.
But that's actually not the case.
A free-flying object always rotates around its center-of-gravity
So to find the pivot point of a rocket
just use a string with a slip knot in it
and tie that around the tube of the rocket and position it and so the rocket balances.
You obviously need to do this with the engine installed because that's the way
the weight distribution will be
when you launch the rocket.
Now once you have it balanced, you've identified where the center of gravity is and
therefore
where it will pivot
and then you need to make sure that you have plenty of surface area, or fin area
behind or to the rear of the pivot point
so it will
overpower any
surface area that's in front of the pivot point that is working against you.
Because your rocket has fins, or at least most of them do,
it's a little hard to figure out whether you have enough surface area or not
because the fins obviously have different shapes and so forth.
So there's a simple test you can do to see if the rocket is stable
This is called the swing test and you can do this by tying a string around
the rocket
typically use a slip knot
and you
you want to put this right at the pivot point so you want to hang the rocket
from the string
and position it
and so it balances.
And once you do that you want to put a piece of tape on the string
on the rocket so it stays in place
and then you want to gently swing the rocket around your head
Once the rocket picks up a little speed it should fly straight
Now this could be a little tricky especially on smaller rocket. Sometimes
you'll get a small rocket that will actually want to fly backwards just because of the
weight distribution
but if you take your time and manually start it
in the straight-forward position you should be able to get it
to fly straight,
If you find that the rocket refuses to fly straight or especially if it tumbles
around as goes through the air
that is a sure sign that it is not stable.
If you find that your rocket is not stable you probably already know how to fix that
based on what I've told you.
The most common way is to just add a little bit of weight to the nose cone - that brings
the pivot point forward.
The other thing you can do is make the fins larger.
Obviously, you could also do the reverse of either of those things: for example you can
remove weight from the rear
or you could remove fin area from the front, but typically most rockets don't
have any fins in the front
unless they're some sort of decorative
fins. In that case, you could remove them; that would help.
In the case of taking weight off the rear there again that's kinda hard
unless you want to decrease the power of the engine. So most common fix is to
add some weight to the nose cone
or to just make the fins larger.
So hopefully you've found this helpful.
If so, let me know by pressing that like button or leaving me a comment.
And as always, thanks for watching, and I'll see you next time.