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Abbreviations and Terms of Water Rocketry

Gyro ooops



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 Scientists!" (GD)


Barrowman Center of Pressure
Calculating the 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 are:

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.
5. 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, Australia. (GD)


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 of 90°.
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 about it.

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

see CCR


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. (VCP)
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. (CH)

Here are some of the real guppies - click on the image to see more:
Super Guppy Airplane       guppy fish 1guppy fish 2

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 bands. (GD)


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 this server.

String Test


Swing Test

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. (RM)


"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 here. (GD)


Vertical (parachute) Deployment with a Tomy Timer
First 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)
(otherwise called Water Rockets)

Contributors to this definitions (not the inventors of  the items unless otherwise noted) in alphabetical order:


Clifford Heath <cjh"at"managesoftxxxx>


Gordon McDonough <vapor"at"trailxxxx>


Russell MacMahon


VCP help file, Copyright © 1995, Gary A. Crowell Sr.

The program  VCP, a rocket stability calculator, is available at (if this link does not work, use the right mouse key on it to download).
Tip for European users: if the program behaves in a strange way, go to the windows control panel, select "Countries, English, US". Change back after using VCP.



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Stand / Last Revision:  17.2.2002   email addresses anonymized: 13.11.2002

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