[MUSIC PLAYING]
Welcome to the NASA High Power Rocketry video series.
In this video, we'll discuss the basics
of high-powered rocketry.
What makes a rocket high powered?
Most of us are familiar with the small model rockets
that are popular with children.
High powered rockets are similar in theory,
but are larger and more complex, and use more powerful motors,
and more robust materials.
They often use electronics to deploy parachutes,
and track the rocket's location.
For these reasons, high power rockets
are more hazardous to build and fly.
A high power rocket is defined as a rocket that
uses a motor with more than 160 Newton-seconds
of total impulse.
The total impulse is the total amount
of energy in the rocket motor.
Mathematically, it is the thrust of the motor integrated
over time.
These motors are H motors or larger.
We'll go over what H means in the propulsion video, which
is number 4 in the series.
High power rockets also use a cluster
of motors that together exceed 320 Newton-seconds.
They use a motor with more than 80 Newtons average thrust.
They use a motor with more than 125 grams of propellant,
or they weigh more than 1,500 grams, including
the weight of the motor.
High power rocketry is regulated by several federal agencies.
The agency that regulates the air space through which
high power rockets fly is the Federal Aviation
Administration, or FAA.
The FAA regulates all types of aviation,
including airplanes and helicopters, gliders,
parachutes, balloons, and unmanned rockets that we build.
Any FAA required paperwork for high power rocket launches
is usually handled by the local section
of the National Association of Rocketry,
or the Tripoli Rocketry Association.
Most basic HPR flights follow one of two flight profiles.
The first flight profile is referred to
as either single deploy or apogee deploy,
or pop at the top.
This involves using an ejection charge
to deploy a parachute at apogee, or the highest
point during the flight, just before the rocket arcs over
and starts to descend.
The parachute then brings the rocket down for a safe landing.
Apogee is the safest point for deploying the parachute,
because the rocket is traveling at the lowest air speed.
The apogee deployment event is usually
controlled by an ejection charge built into the rocket motor.
The single deploy method is what is commonly
used in low power rocketry.
This is the most simple flight profile.
But it has some drawbacks.
First, if the apogee of the flight is high enough,
or if the winds are strong enough,
the rocket may drift off the launch field.
Second, the timing of the ejection charge in the motor
may not coincide with apogee.
In this case, the rocket may be traveling too fast,
and the ejection event may damage
the rocket due to the forces exerted by the parachute.
A flyer can use an altimeter that
senses the change in air pressure
to determine apogee and avoid the problem.
We'll discuss recovery techniques
in the recovery video, which is number 5 in the series.
The second flight profile, called dual deployment,
solves the problems associated with single deployment.
In a dual deployment flight, the rocket travels to apogee,
where an electronic barometric pressure-sensing altimeter
deploys a drogue parachute.
This small parachute is sized so that the rocket
will fall at a velocity high enough
to prevent excessive drift-- generally between 70
and 100 feet per second.
Then, at a much lower altitude-- generally between 300 and 700
feet-- an electronic altimeter deploys a larger main parachute
that slows the rocket down to a safe landing speed.
Dual deployment solves the drift issues found
in single deployment flight, but it
comes with added complication of flying electronic altimeters.
Most HPR flights that use smaller motors
or fly to lower altitudes use single deployment.
HPR flights with the larger motors,
or that fly to higher altitudes, use dual deployment.
For level 1 or 2 HPR certification,
you can use either method.
In order for your flights to be safe and reliable,
the rocket must be stable.
Stability is determined by a stability margin and a rail
exit velocity.
To determine the stability margin and the rail exit
velocity, we need to understand some important parameters.
Center of pressure, or CP, is the point
on the rocket where the aerodynamic forces are
balanced.
There are many ways to calculate the CP,
and the most common method in HPR
is the use of the Barrowman Equations.
There are several sources online that show
how to use the Barrowman Equations.
And your local HPR mentor should be
able to explain a simplified method to use these equations.
The biggest factors in the calculation of CP
are the length of the rocket and the size, shape, and location
of the fins.
The purpose of having fins on a rocket
is to bring the CP further back on the rocket.
The next parameter is the center of gravity, or CG.
CG is the point on the rocket where the mass of the rocket
is balanced.
Simply put, you should be able to hold the rocket at the CG
without it tipping on one side or the other.
One should always verify where the CG is on the rocket
by balancing the rocket once the motor, recovery
system, and full electronics package are installed.
One rule of thumb for stability in HPR
is that the CG must be at least one body diameter, or 1
caliber, in front of the CP.
This is the stability margin.
Mathematically, the stability margin
is the distance between the CG and CP,
divided by the airframe diameter.
So if you are flying a rocket with a maximum diameter of 4
inches, your CG needs to be at least 4 inches in front
of your CP, giving 1 caliber of stability,
or a stability margin of 1.
If your CG was 8 inches in front of your CP,
you would have 2 calibers of stability,
or a stability margin of 2.
Most fliers recommend stability margins of two or higher.
Another important factor for stable flight
is the speed at which the rocket leaves the launch rail.
This is the rail exit velocity.
The launch rail keeps the rocket in a vertical configuration
while the rocket is accelerating to a speed at which the fins
cause the CP to be driven back behind the CG
and keep the rocket stable.
This exact speed is hard to quantify,
and varies based on rocket size, mass, and fin shape.
A rule of thumb is that 50 feet per second
is the speed at which the rocket is
stable without the help of the launch rail.
It's important to note that rockets connect to the launch
rail with at least two rail buttons--
one near the rear of the rocket, and one
further up the airframe-- usually near the CG.
Once the forward rail button has left the launch rail,
the rocket can rotate if it has not reached a safe rail exit
velocity.
It's important that the safe rail exit velocity
be reached before the forward rail button leaves the launch
rail.
Rail exit velocity can be increased
by using a motor with higher initial thrust,
or by using a longer launch rail.
Now let's briefly talk about how a very general high power
rocket is constructed.
The basic part of the rocket is the air frame.
This is the long cylindrical part that
gives the rocket its shape.
At the top of the rocket is the nose cone.
Nose cones can be conical, or they can
be a more complicated shape.
Nose cones can be empty, or they can house electronics.
In the center of the air frame is the avionics bay,
a compartment that stores things such as batteries, sensors,
or scientific payloads, and fits inside the diameter of the air
frame.
The avionics bay also houses the electronics used for recovery.
At either end, bulkheads seal the avionics bay
by tightening threaded rods with washers and nuts.
On the outside of the bulkheads, U-bolts or I-bolts
are used to anchor the recovery harnesses.
At the bottom of the rocket, there
is a subsystem called a motor mount,
which is a motor tube and motor retainer
to hold the motor in place, and a set
of centering rings that keep the motor mounts
concentric with the air frame.
The component that really makes a rocket look like a rocket
is the fin set.
Fins can be mounted directly to the surface of the air frame,
but are generally mounted through the airframe
and against the motor tube.
This is called through-the-wall fin mounting.
And it's much stronger than surface mounting.
Some rockets may have additional fiberglass
or carbon-fiber reinforcement over the fins and air frame
to increase the rigidity and lower the likelihood of failure
due to increased aerodynamic forces.
Long recovery harnesses keep the rocket sections tethered
together after the recovery separation events.
The ends of the harnesses are anchored to the air frame
sections with hardware.
Rocketry can be used to demonstrate
many different real world aspects of physics
and classical mechanics.
Motion is most commonly described
in direction, distance, velocity, and acceleration.
The relationships between these things
make up most physics classes, and can be seen and measured
in the flight of your rocket.
We discussed earlier the importance
of attaining a safe exit rail velocity.
The exit rail velocity of your rocket
is something you should know before you
attempt to launch it.
You can use a rocket simulator to estimate this velocity.
But you can also use standard high school physics equations
to figure it out, also.
We'll demonstrate how to determine the exit rail
velocity of a sample rocket by taking just a few measurements,
and then working through some common equations.
First of all, we will need to keep our units in mind.
For simplicity's sake, most of our calculations
will be using metric units.
But we will need to do some conversions between Imperial
and metric due to the standards of the hobby.
For instance, we will need to convert our 8 foot launch
rail into meters, and our minimum exit rail velocity
into meters per second from our known
value of 50 feet per second.
Let's begin with our known values and these conversions.
First of all, you will need to weigh your rocket
to determine its weight.
Since this is the weight as it sits on the launchpad
you will want to include the weight of the motor.
If you are using this calculation
to help you pick which motor you're going to buy,
you will most likely not have a motor available to weigh.
But motor weights can be found on the internet.
So weigh your rocket as it will sit on the pad,
and add the weight of the motor.
This total weight will be what we use in our calculations.
Now, if your mass is not already in kilograms,
convert it to kilograms.
So we'll use a mass of 3 kilograms for our example.
We will use a J300 as our potential motor.
As you will learn in an upcoming video,
the 300 following the J in the motor designation
is the motor's average thrust measured in newtons.
We will use this value, but label it
as force for our calculations.
The mass of our motor is 607 grams,
taken from the thrustcurve.org, making our total weight 3.607
kilograms.
Using Newton's Second Law, we can calculate the acceleration
of the rocket from the force and mass,
or force equals mass times acceleration.
Rearranging, the acceleration value
is equal to force divided by mass.
So we have 300 newtons divided by 3.607 kilograms,
for an acceleration of 83.17 meters per second squared.
The acceleration gets us one step closer to our exit rail
velocity.
But we still need to calculate the amount of time
between the ignition of the rocket motor
and when the rocket leaves the rail.
To calculate the time, we can use this equation--
d equals initial velocity multiplied
by time plus 1/2 of the acceleration multiplied by time
squared, where d is the effectiveness of the launch
rail.
The effective length is the length
of the launch rail minus the distance between the two rail
buttons on the rocket.
Using a 10-foot launch rail with a rail button distance
of 2 feet, we have an effective length
of 8 feet, or 2.44 meters.
The i is the initial velocity, which is 0 in this case,
since the rocket started at rest.
Since initial velocity is 0, time
cancels out, leaving the equation
d equals 1/2 of the acceleration multiplied by time squared.
Rearranging this gives us time squared
equals 2d divided by acceleration.
Simplified, this becomes time equals the square root
of 2d divided by acceleration.
Filling in our values, we have time
equals the square root of 2 multiplied
by 2.44 meters, divided by 83.17 meters per second,
giving us a time value of 0.24 seconds.
We now have all of the information
we need to calculate the exit rail
velocity with this equation-- acceleration equals
final velocity minus initial velocity divided by time.
Initial velocity is still 0, since the rocket began at rest.
And we can rearrange the equation to become,
final velocity equals acceleration
multiplied by time.
Final velocity equals 83.17 meters per second multiplied
by 0.24 seconds, giving 19.96 meters per second.
Finally, we need to convert this value
from meters per second two feet per second
to see if it meets our minimum requirement of 50
feet per second.
To convert our final velocity of 19.96 meters per second,
we'll multiply times a conversion factor of 1 mile
for every 1,609 meters times 5,280 feet for every mile.
This gives us 65.5 feet per second,
which is well above our minimum exit rail velocity.
The J300 motor should provide enough thrust
to get our rocket to a safe velocity
before it leaves the launch rail.
Another concern when selecting your rocket motor
is the thrust to weight ratio.
The hobby standard is to launch only rockets with at least a 5
to 1 ratio, meaning that the thrust is at least 5 times
the weight of the rocket.
This is pretty easy to calculate with the info we already know.
1 kilogram is equal to 9.8 newtons.
3.607 kilograms equals 35.35 newtons.
The thrust, 300 newtons, divided by the weight, 35.35 newtons,
gives us a value of 8.48.
This means that we have a thrust to weight ratio of 8.48 to 1,
which is well above the safe minimum value of 5 to 1.
It is always a good idea to verify that your rocket
motor will be a good match to your rocket
before attempting to fly it.
With these two calculations, you should
be able to make that determination
before purchasing your motor.
Good luck in all of your rocketry endeavors.
And thank you for watching the NASA High Power Rocketry video
series.