Welcome to NASA's High Power Rocketry video series.
In this video, we'll be discussing recovery.
The rocket recovery system is vital
because it returns all of your hard work to the ground,
to be flown again.
And more importantly, it ensures the safety of you,
fellow rocketeers, and surrounding property.
Developing a reliable recovery system
is a challenging, but important, skill to master.
The recovery system consists of a black powder charge that is
ignited, and pressurises the air frame where a parachute
and recovery harness are housed.
The pressurization separates the airframe,
and pushes the parachute and the recovery
harness into the atmosphere.
The parachute is sized for the weight of the rocket,
the desired descent rate, and with proper packing will lower
the rocket to a safe landing.
There are two conventional high power rocketry recovery
methods-- single deployment and dual deployment.
Single deployment method deploys the parachute system
in one single event.
And it's also called apogee deployment, or pop at the top.
This involves using an ejection charge
to deploy a parachute at apogee-- or the highest point
during the flight.
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.
It is simple, 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 initiate the black powder
charge, instead of using the motor ejection.
This method avoids the challenge of guessing the correct engine
delay.
The second flight profile, called dual deployment,
helps eliminate the problems associated
with a simple deployment flight.
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 the rocket
will fall at a velocity high enough
to prevent excessive drift.
Then, at a much lower altitude, an electronic altimeter
deploys a larger, main parachute,
that slows the rocket down to a safe landing speed.
Most high-power rocketry flights that use small motors,
or fly to low altitudes, use single deployment.
HPR flights with large motors, that fly to high altitudes,
use dual deployment.
For level 1 HPR certification, you can use either method.
Because dual deployment is crucial to safe rocket
recovery, we will focus on this method.
Dual deployment uses an electrical circuit,
housed in an isolated compartment within the rocket.
The circuit consists of an altimeter
to control the recovery events, a power
source for the altimeter, a switch to open and close
the circuit, and electrical matches
to ignite the ejection charges.
The ejection charges are generally
made from a small amount of black powder, which, when
ignited, pressurize the air frame,
and push the parachute into the atmosphere
to slow the vehicle down to a safe landing velocity.
The altimeters generally have two outputs
to fire an electric match.
One is for the drogue parachute, and the other
is for the main parachute.
The output for the drogue parachute
is usually programmed to fire the electric match at apogee,
or slowest velocity, but can be programmed to fire
a few seconds after apogee.
The output for the main parachute
is usually programmed to fire the match between 300 and 5,000
feet, depending on the size of the rocket and the recovery
area.
To make parachute packing easier, and more reliable,
the main parachute and the drogue parachute
are in separate compartments.
As such, the rocket separates at two different sections--
once when the drogue parachute is deployed,
and again when the main parachute is deployed.
There are systems which house the parachutes
in the same compartment, but those are less common.
Information regarding those systems
can be gained from an experienced rocketry mentor.
Several steps can be taken to increase proper and reliable
parachute deployment.
First, flyers generally use a separate redundant electrical
circuit to control the ejection events.
Instead of using just one altimeter that
controls an injection charge for the drogue parachute,
and an ejection charge for the main parachute,
the rocket uses a second altimeter
to back up the first altimeter, which controls a dedicated
set of ejection charges for the main and drogue parachute.
Each altimeter needs its own switch and own power supply,
as well.
In this configuration, there are two ejection charges
for the drogue parachute, and two ejection charges
for the main parachute.
This way, if an altimeter, switch, power supply,
or ejection charge fails on one circuit,
the separate circuits serves as a backup
to ensure good deployment.
Another way to ensure good deployment
is to offset the altitude at which the charges go off,
and to increase the size of the ejection charges
in the altimeter circuit that fires the charges at the lower
altitude.
If the first charge for the apogee deployment event
is programmed to deploy at apogee,
the second apogee charge may be programmed
to deploy one second past apogee.
The altitude offsets are to ensure that the charges don't
go off simultaneously, because this could over-pressurize
and damage the airframe.
Essentially, the second charge serves as a backup.
If the parachute gets stuck in the airframe,
or if the first charge is undersized,
the backup charge would further assist the parachute
deployment.
One of the most important actions
for the proper deployment is to ground test
the deployment system in its final flight configuration.
The rocketeer prepares the ejection charges,
packs the parachutes, and assembles the airframe
in the flight configuration.
Then the ejection charges are ignited remotely,
via long electrical leads.
Using long electrical leads, and standing well away
from the rocket, helps ensure that you
won't stand in the path of the ejecting parachute.
It's important to ensure the parachutes successfully eject
from the airframe.
If the parachute gets stuck in the airframe,
or the deployment is weak, the rocketeer
should examine the size of the charge
and check the method of parachute packing.
It's better to find mistakes in the recovery system
on the ground, than in the air when safety is at stake.
Plus, improper deployment could result in a lost or destroyed
rocket.
Assessing a properly sized ejection charge,
and ensuring a good ground test, are
skills that are taught by an experienced rocketry mentor.
Let's briefly discuss how to size
an ejection charge and parachute.
The sizing of an ejection charge is based on the ideal gas law.
The ideal gas law relates pressure, volume,
and temperature of a system.
Mathematically, the ideal gas law
states that pressure, or P, multiplied by volume, or V,
is proportional to the mass, or m, multiplied
by the temperature, or T, in a gas constant, or R. Or,
PV equals mRT.
R is the gas constant for burning black powder,
and T is the combustion temperature
of the black powder.
Essentially, the combustion of the black powder
will raise the pressure inside the airframe,
and this will create a force which
will push the parachute out of the airframe.
In addition to the ideal gas law equation,
we need a few parameters from our rocket.
The diameter of the parachute compartment,
the cross-sectional area of the parachute compartment,
the length of the parachute compartment, and the force
needed to eject the parachute.
The amount of force necessary to deploy the parachute
is dependent on the design of the rocket,
the size of the parachute, and how it's packed,
and how the airframe is held together.
The necessary ejection force for your rocket
will need to be determined with the help of your mentor.
As an example, we have a rocket that
is four inches in diameter, with a 3y-inch long parachute bay.
We've assumed that we need 200 pounds of force
to eject the parachutes.
We solve for the mass of black powder
by rearranging the ideal gas law to solve
for m, where m equals the pressure multiplied
by the volume divided by the gas constant multiplied
by the temperature.
Next, calculate the cross-sectional area
of the parachute compartment.
This is calculated with the formula
for the area of a circle.
A equals pi divided by 4 multiplied by the diameter
squared.
This gives a cross-sectional area of 12.56 square inches.
Pressure is a measure of force per unit area.
So we can substitute F divided by A for P
in the ideal gas law equation to obtain
m equals force multiplied by the volume divided
by the cross-sectional area, multiplied
by the gas constant multiplied by the temperature.
The volume is found by multiplying
the cross-sectional area by the length of the airframe.
Substituting the values into the rearranged ideal gas law
equation gives a black powder mass of m equals 0.0082 pounds.
The amount of black powder used in an injection charge
is generally expressed in grams.
So using the conversion factor of 1 pound equals 454 grams,
gives m equals 3.72 grams.
It's important to understand this calculation is only
a starting point for the ground testing of an ejection system.
During the ground test, you may find
you may need more or less black powder.
But you should never fly a dual deployment rocket that has not
been ground tested first.
Always test what you fly, and fly exactly what you've tested.
Now let's talk about the parachute sizing for a rocket.
To pick a proper parachute, you need
to know two things-- rocket mass,
and desired descending speed while under the parachute.
With a drogue parachute, the rocket
should descend quickly enough that it doesn't
drift excessively, but not so fast
that the forces of the main parachute opening
will damage the rocket.
Under the main parachute, the rocket
should descend slowly to prevent damage as the rocket touches
down.
A common descent rate for drogue parachutes
is 70 to 100 feet per second.
And common descent rates for the main parachute
is 17 to 22 feet per second.
The governing equation for this exercise
is the drag force equation.
The drag force equals 1/2 the density multiplied
by the velocity squared multiplied
by the coefficient of drag multiplied by the reference
area.
Since we are sizing the chute for specific descent
rate at a constant speed or terminal velocity,
it's important to realize that under this steady state
condition, the force of the drag equals the force of gravity.
Or more simply, the drag on the chute
is equal to the weight of the rocket.
Since the force of gravity equals mass times acceleration
due to gravity, we substitute and mg for fg
in the drag equation.
As a result, mg equals 1/2 times the density of the air
at that altitude multiplied by the square of your target
descent velocity multiplied by the chute's coefficient of drag
multiplied by the chute's reference area.
We rearrange the equation to solve
for our area, A. The most commonly
used parameter for parachute sizes in the hobby
is the diameter.
So once we have found the area, we
can find the diameter by using the equation
D equals the square root of 4 times the area divided by pi.
Let's pretend that we're designing
a drogue parachute for a 10-pound rocket,
and we want the terminal velocity
under this drogue parachute to be 85 feet per second.
Note that the density of air is 0.075 pounds
mass per cubic foot, and the acceleration due to gravity
is 32.2 feet per second squared.
The exact coefficient of drag for a parachute
varies by design, but a common coefficient
is 1.5, which we'll use in this example.
After rearranging the force equation
as discussed, and substituting all variables,
we solve for an area of 0.79 square feet.
As we said earlier, it's more common to refer
to rocketry parachutes in terms of the diameter.
So using our equation for diameter,
and substituting our variables, we
obtain the diameter of 1 foot, or 12 inches.
Inches are the common unit of measurement for parachutes.
To finish our example for a 10-pound rocket,
a 12-inch diameter drogue parachute
will bring our rocket down at a descent
rate of 85 feet per second.
Chute manufacturers will calculate their reference areas
differently, so it's important to ask
how they calculated this value.
This video provides basic insight
on high-power rocketry recovery, but it's vital
that you seek the guidance of an experienced rocketry mentor
to learn the aspects of designing and testing
the recovery system.
This video is only a supplement to the guidance
of a mentor, not a replacement.
The most important aspect of recovery
is to always test what you fly, and fly only what
you've tested successfully.
One of the most exciting things you
will get to do in high-power rocketry
is testing the ejection charges.
Calculating your ejection charge is a great way
to teach the ideal gas law.
You can use the model we demonstrated earlier
to calculate the ejection charge of your rocket,
using your data and measurements.
A few things to remember as you work through the calculations
are that the ideal gas law, as it deals with rocketry,
is PV equals mRT, where P is the desired pressure,
V is the internal volume of your parachute compartment,
and m is the mass of the black powder in pounds.
Additionally, the temperature of the burning black powder, T
and R, the gas constant, do not change.
It's extremely important to remember
that you are calculating the black powder amount in pounds,
and it must be correctly converted to grams.
This will give your students extra practice
converting units.
But all calculation should be double and even triple
checked before testing.
The descent rate calculation that we demonstrated
is also an excellent classroom activity.
Predicting a descent time, and then
comparing it to actually flight data can be very rewarding
and fun for the students.
A related activity, which will also
aid in recovering your rocket, is calculating
the drift of the rocket.
Predicting the flight of your rocket can be fun,
and testing it can be a whole lot of fun.
Please remember to follow all safety practices in the NAR
Rocket Safety Code, as well as the motto, "Test what you fly,
and fly only what you've tested."
Thanks for watching the NASA High-Power Rocketry video
series, and good luck in all your rocketry endeavors.