Air Flap Release; initially it was a
way of releasing a nosecone covering a chute, developed
by Dave Johnson.
This mechanism was later used by
Robert Youens to trigger the HDTT
with an air flap instead of the older method, a string
tied to the launcher.
Angle Of Attack
The angle between a rocket and
the relative wind. A stable rocket in still air
is flying most of the time with an AOA of 0° or
very close to that.
you don't see this one much any more,
but it is the bottom line: "Actually, We ARE Rocket
Barrowman Center of Pressure
exact CP of a rocket for a small AOA
is a complex process. However in the mid-1960's an aeronautical
engineer named James Barrowman, reduced the problem
to the major factors that affect the subsonic CP
of a typical rocket, and simplified the equations to
the point where they can be applied by the rocketry
hobbyist. The set of equations that he developed are
now commonly known as the "Barrowman Equations".
The Barrowman Equations give us the ability to calculate
a very close estimate of the actual CP
location, as long as we adhere to the assumptions upon
which those equations are based. Those limiting assumptions
1. The angle of attack of the rocket is near zero
(less than 10 degrees).
2. The speed of the rocket
is much less than the speed of sound (not more than
660 km per hour = 183 m/sec).
3. The airflow over
the rocket is smooth and does not change abruptly.
4. The rocket is thin compared to its length.
The nose of the rocket comes smoothly to a point.
6. The rocket is an axially symmetric rigid body.
7. The fins are thin flat plates.
Fortunately, these assumptions encompass the large
majority of hobby-type rockets. The VCP program implements
the Barrowman Equations in a form that is relatively
easy to use. (VCP)
A bulkhead is usually a pressure containing end on
an FTC (q.v., (Latin for which see)),
although it could be interior, I suppose, or fit on
any other tubing. Typically one bulkhead has the nozzle,
and the other bulkhead is at the nose cone end. (GD)
Clark Cable Release launcher system that doesn't
involve the cable but rather the cable ties that would
otherwise go around the cable, if we weren't misusing
them to hold our rockets down during pressurization.
Named for their originator, Ian Clark, of Melbourne,
Center of Gravity.
The CG of a rocket is the
point where all of it's mass seems to be concentrated.
How do you find it? Tie a string tightly around the
body of your rocket. Move the string loop to that position
where the rocket body balances horizontally. The CG
can be thought of sitting at that point where the string
is, on the longitudinal axis of the rocket.
Center of Lateral Area
The CLA is equal to the
CP of a rocket at an AOA
How can the CLA be determined by simple
means? An "OK" method is to draw the outline
of you rocket on cardboard, cut it out and then find
the centre of gravity of the resultant shape. This can
be established by balancing it on a pin and moving the
pin until you get proper balance. The "outline"
here is the shadow that you get if you shine alight
from a great distance from the rocket with the rocket
side on to the light direction - eg shape of the shadow
at midday in summer.
This works (or is said to work)
because the air acts against the cross section of the
rocket in applying aerodynamic forces. If the force
exerted on any section is proportional to area (as it
roughly will be) then area is equivalent to pressure
so centre of the area will equal to the pressure centre.
There are various effects which will interfere with
this simple model. If you have eg a 4 fin rocket you
will note that when you look at its cross sectional
view from side on, that it varies as you rotate the
rocket. You will get more area when 1 fin is pointed
straight outwards and least when the rocket is rotated
45 degrees from this point. In practice the rocket will
rotate in an airstream so that the greatest fin area
is away from the air flow. This makes sense if you think
Here comes an ASCII picture of 3 fin rocket.
It assumes the rocket turns side on to the airflow (AOA=90°)
and you certainly hope this does not happen in flight
Rocket turns so one fin faces into flow
so that maximum fin area is AWAY from flow.
R O O O O <--- Airflow
Second order aerodynamic effects on fins and body
will alter this somewhat.
A ring fin tends to be rotationally symmetric but
the way the gap between body and ring fin appears to
airflow alters with angle of attack.
The "real" cross section presented to the
airflow occurs when you look at the rocket birds eye
on (you are a flying bird and the rocket is coming up
to meet you and THEN moving off centre just slightly.
When you are directly head on the fins will appear as
thin radial lines (thickness only seen) and as you move
off centre you will see a reduced projections their
areas. Again, a shadow would show the same shape as
you see. This is arguably a more realistic shape to
use for CP calculations and is called
the Barrowman Center of Pressure (BCP).
In practice if you use the "side on" projection
and get a result which is two diameters away from the
CG you are probably doing OK. (RM)
See my Java-Script based CLA-Calculator
for calculating the approximate position of the CLA.
Clark Cable Release
Center of Pressure
Just as the CG
is where a rocket will balance, there is also a point
on the rocket where all of the aerodynamic forces balance.
However, this point depends
on the AOA of the rocket. So, CP
is often used as a general term. For small AOA (up to
10 degrees), the CP is the BCP, for
an AOA of 90° the CP is the CLA.
See also "String test".
FTC- Fluorescent (light) Tube Cover, a sturdy clear
plastic (polycarbonate) tube that we Americans put over
light fixtures so that if we hear the bulb break, we
can look upwards. In other countries, FTC refers to
anything similar, and I presume you know better than
to look up when a fluorescent tube breaks. (GD)
We call reshaping the bottom of a bottle
into a more rocketlike shape, Guppying. The result is
a Guppied Bottle.
The term "Guppy" (and
the technique!) was first used and promoted by Clifford
Heath. This term was chosen with a mind to the shape
of the fish which it was named after. Another relationship
(or the same?) leads to the tip of the hat to the Super-Guppy
airplane built to accommodate bizarre loads during the
American Apollo Project. Originally Clifford used it
for a rocket which had also had the tail reshaped (shrunken/tapered),
but when everyone started using it for reshaped nose-cones
he called those fully-reshaped rockets Super-Guppys,
and the rest of the water rocket society followed him.
Here are some of the real guppies - click on the
image to see more:
And well, it's no secret any more how
to build those guppied WR nosecones.
Horizontal (parachute) Deployment with
a Tomy Timer. It is a highly evolved way of shooting
a parachute out a horizontal tube through the side of
your rocket using TT's and rubber
Keep it simple, stupid - one of the best design rules
for technical development.
Poly Ethylene Terephtalate. The material our bottles
are made from. Only the ones which hold gaseous beverages
are used - bottles for plain water usually are not apt
for uses as WR or other high pressure applications.
PLP- short for plop, the sound made when one of our
favorite adhesives, PL Premium, a polyurethane based
water activated construction adhesive that is used by
many water rocket enthusiasts to hold together rockets
where enormous strength, flexibility, and a variety
of materials is involved, drops on our brand new US$200
wing-tip shoes. PL Premium can be hard to find overseas
(from where I sit), but ask your neighbors for suggestions.
PL Premium comes in a tube for a caulking gun. (GD)
Simple, but Exact Altitude Measurement
A description can be found on
A method used by the hobby pyro rocket community
is to load up the rocket as per flight conditions, connect
a string to the CG and then to whirl
the rocket in a circle at speed on the string. A stable
rocket will fly with correct orientation. An unstable
rocket will take up a funny angle. This seem a less
than perfect method but would give a fair idea of stability.
Note that with a water rocket the water would move to
the outside in this test and make the balance different
than actual. Using ice frozen in position would probably
give a good result.(DON'T freeze a full bottle of water
if you value the bottle).
Note that a water rocket is MOST UNSTABLE at some
stage near BUT NOT At the water exit stage. When fuller
or when completely empty the rocket is more stable than
at this critical point.Testing with about 10% of final
water in position (frozen) would probably be in order.
"That Looks About Right".
Also known as a "calibrated eyeball", this
method of determining stability is used more often than
people like to admit. It simply means to look at a rocket
configuration and judge its stability via common sense
and experience. In the hands of a competent operator,
this method can work reasonably well. After observing,
building, and crashing, a sufficient number of rockets,
it is possible to cultivate a feeling for the potential
success of a configuration. Occasionally, with particularly
odd configurations, TLAR may be the only method that
can be applied. Use with caution.
Tomy Timers are the motors from small
and amusing wind-up toys that are increasingly hard
to find at reasonable prices. They are used to time
recovery system deployment as they wind down. Examples
Vertical (parachute) Deployment with a Tomy Timer
development by Robert Youens. It uses the acceleration
during liftoff as a trigger, a magnet to
keep the release mechanism from interfering with the
Tomy Timer during the negative G's after liftoff. The
TT finally releases a rubber
band which kicks out a plunger to kick out the chute
and the nose cone away from the rocket.
What have I left out? (GD)
WR- stands for Hydro-Pneumatic Tropospheric
Reaction Research Vehicles. (GD)