Special thanks to the author : Johanna De Witte
The following information is the basic format that I used for my bottle rocket. Due to difficulties in getting parts I had to modify my launch pad. The valve stem I used was from a Tire shop and it sat nicely in a 1/2 " hole on the launch pad. There are drawings on the web site listed below. Good Luck, Johanna DeWitte
copyright February 1997 by Brigham Rees. Copies may be made by educational institutions. Otherwise contact
Austin, Texas 78753
Even bare bottle experiments can be dangerous, but a finely tuned rocket can reach speeds in excess of 150 miles per hour. Major league baseball pitchers pitch fastballs between 80 and 95 miles per hour (Bob Feller was clocked at 145 feet per second--99 MPH.). Imagine getting hit without (or with) a helmet at speeds of that magnitude. Cars can be dented, windows broken, roofs damaged. The good news is that with some precautions bottle rockets are relatively safe, despite their awesome power.
0. Equipment. Reliable equipment is absolutely critical. Using poor equipment can not only damage your bottle rocket, launcher, houses, cars, or other property, it can also damage people. A bottle rocket is danger and power in a pop bottle. See the appendix on equipment and make sure yours is up to standard. Don't launch until your equipment is good enough.
1. Adult Supervision. Although not a guarantee of better precaution than youth, adults have had more years to add to their alarm systems. However, anyone's alarms, bells, and whistles should be enough to at least pause a launch for more consideration. A sanity check should be made before any rocket is pressurized. If you don't check your stuff before pressurization, you will almost be guarantied of having to deal with a dangerous situation sometime. Adults should be able to deal with emergencies. They should also familiarize themselves and the students with the following guidelines:
2. Tools. Some rocket ideas may require using tools the students are not trained to handle safely. Watch the students' abilities with tools. Either training the students or doing the part of tool usage that they cannot do may be in order. (For their sake, only the unsafe parts should be done for them, or they will not learn as much). Although individual parts of the rocket may need adult help, the overall rocket should be made and assembled by the student.
3. Metals and sharp objects. This is obviously an alarm bell because a bottle can explode, misfire, or have an errant flight path that would cause someone to get hurt or maimed. In addition, a rocket which has excellent flight characteristics and a good flight trajectory on the way up, may become very dangerous on the way down-especially if a parachute doesn't exist or fails to deploy. (Excellent flight characteristics will mean the rocket comes down almost as fast as it went up). See the huge precaution of the next to last paragraph of Inertia in Flight.
4. Launch area. Although the size of the launch area can be as small as a front yard for no wind and bare bottle experiments, once good flight characteristics are achieved, the launch area will need to be clear for the size of a football field size. If parachutes are added to a good rocket (you will get tired of making disposable nose cones.), a half mile or more of fairly clear field space may be needed, depending on the size of the chute. In any case, nothing should be overhead of the launch. Period.
5. Observer. The adult should be free to be observing during every launch to check for safe and unsafe practices and conditions. Any launch should be stopped immediately if any suspicion of an unsafe condition is observed.
7. Before pressurizing. Someone other than the one who pinned the rocket should check it. Although the launch pad and clamp design included at the end of this has yet to have a misfire due to bad pinning, anything is possible.
8. While pressurizing. The rocket and pad should be observed by the adult and the launching student for the first sign of any problems. 2-liter soda-pop bottles which have no modifications to the water/pressure chamber should solid above 90PSI, but I won't guarantee them. Soda-pop bottles have a rumored minimum specification of 90PSI and a few are valid to 150PSI or more. Water bottles have no known specification. Some appear to be exactly like soda bottles, but I had a sun bleached 20-oz water bottle (unknown time in the sun) explode on me at 55PSI. Sharp shards from the bottom didn't hit anyone, but upon examination, if they had... Any bottles that have not been pressurized previously should have a containment box over them with 20 lbs. or more of weight on top of the box. The box should be heavy duty cardboard (like an appliance box, but only a few inches to a foot taller than the rocket on the launch pad. This prevents the rocket from turning potential energy into dangerous kinetic energy. Leaning over a pressurized rocket or fooling around with one are obviously invitations for injury. Parents won't like any injuries. Any rocket which has multi-pressure-chamber bottles joined in any manner should be pressurized in increments of 5PSI with wait times of 10 seconds between each increment--(pressurized in the containment box). These may be dangerously explosive. The first signs of problems at pressures above 35 PSI may not have enough action time between the observation and the resulting explosion to do anything. Safety goggles are recommended.
9. Problems while pressurizing. If a leak is detected during pressurization, a blowout may be eminent. Stop pressurizing immediately. Clear the area immediately. The leak may be with the launcher or with the rocket. If the leak is at the launch-pad clamp of the design specified in the rear, it may be adjusted carefully. Otherwise, go directly into a launch procedure at the current pressure. The leak should be fixed before launching again.
10. After pressurizing. All other activities should stop. Everyone should be facing the rocket without the sun in their eyes.. Permission to launch should be asked for, permission given, and a countdown started. Everyone should understand that until the blastoff is reached, the countdown can be stopped by anyone.
11. No one catches rocket parts after a launch. Bare bottles have no need to be caught, and other rockets have the possibility of good flight characteristics. They are dangerous on the way up or down.
12. Launch failure. I have never seen this happen with our launch pads. One book recommended the adult jiggle the rocket 3 with a long stick to cause it to release. Maybe a plastic bucket over the rocket would be better (hold on tight. During the first moments no kinetic energy level of danger has been reached, but loosing grip on the bucket may allow that to happen.). Some other procedure may be better still. I have no experience with this and don't believe it happens with this launch pad design, but I welcome inputs.
Up In ;Smoke: Money
The glamour of bottle rockets is that they are something fun to do with an item that is normally trash or, at best, recyclable material. It is possible to spend a lot of money for fancy parts for rockets, but we have achieved best-of-class results from refuse items. An open eye while looking at trash, packaging material, etc. will provide phenomenal results using plastic material that is normally thrown away. After this has been achieved, you will know what a few dollars may possibly achieve towards the super-phenomenal. Don't send your money up in rocket vapor. The most expensive items we have spent money on so far are the launch pad and air compressor. You would be wise to do likewise. See the equipment appendix.
Work and Energy
What makes a bottle rocket fly? (Water and Air Pressure)
If some water and air pressure works, will a lot of water and-or air pressure work better? (Yes and no.)
The water has no magic. It is just a convenient material to push out the bottom of a bottle. Bottles without any water will still fly, just not as far. Adding water gives us a much larger transfer of mass as well as a pressurized container. Let's look at the bottle as the water escapes. We will look at the first little piece that comes out. (You are right, it flows out smoothly, not in pieces, but if we make our pieces small enough we get down to the molecule level.) But I can't draw pictures that small, so we will look at bigger pieces.
A. When the first piece of water comes out of the rocket at a tremendous velocity, it has an equal and opposite reaction on the rocket mass (including other water). But since the other stuff is so big, the little mass only causes a small velocity change to the whole mass. The other possibility is that it could have a full reaction on a singular piece of the large mass. That would involve a small piece pumping up through the water at a high velocity. Imagine a small piece hitting the top inside of the rocket at full force. In all reality, this idea of what happens is more realistic since water is fluid. But the water friction, viscosity and air pressure on the water mass transfer the velocity to the whole mass. Viscosity is how &127;sticky&127; the fluid is with itself. Honey is more viscous than water.
B. After the first little masses have left, 2 things are happening. The big mass is beginning to pick up speed. As it does, the little masses are already traveling in the up direction, so their down velocity coming out of the bottle is less. This is their relative velocity with the rocket. In addition, as more water escapes, the air volume gets bigger. So the pressure on the water decreases in proportion to the increase in volume. This has a direct bearing on the question of whether a lot of water (D.) would be better than a little.
C. As the last water leaves, it is approaching a one-to-one mass transfer with the water left behind. Only the weight of the bottle prevents it from that ratio. And the force imparted may still be very high if the proper water ratio was used in the beginning. 4
D. If the bottle is filled almost full, the amount of air volume needed to get a high pressure is very small. And as the air expands as the water leaves, the pressure drops dramatically. This is still an interesting experiment to do, and has interesting results.
Determining the best water volume. Experiment # 1:
Since we want to determine the best water volume, we must only vary water volume. We can check our results by doing the experiment on another bottle. The first set of experiments might be done using a 2-liter bottle pressurized at 75PSI. A second set might be done using a 20-oz. bottle. We want to know which one reaches a maximum height. We could use a sighting method to determine height, but the best result will be timing the bare bottle from the moment it is launched to the moment it hits the ground.
Typical bare bottle times for a two liter bottle at 75PSI are in the 5 to 6 second range for optimal water volume. We simply put a mark and its number with a permanent marks-a-lot on the side of the bottle before each launch. Then you can go back and number other lines after you see the probable best volume.
Newton's Laws of motion: Mass transfer .vs. Gravity
Once the ideal volume of water for the best thrust .vs. mass transfer ratio has been found, it would be nice to know what forces are acting on the rocket in order to optimize performance.
The sum of all the forces on any object equals zero, so Fv + Fg + Fr = 0. This section will be completed later.
Fins in the Air Stream
It would now be great to begin improving the rocket performance. The first thing the students invariably suggest is to install fins. We began with paper fins, then went to paper with lamination over them. Then came cardboard fins, then cardboard with lamination over them. We devised a method of marking our bottle with perfectly vertical lines which were used as guides to have highly tuned fins. Rockets still tumbled tremendously in the air stream. This was obviously not yet the answer to good flight.
In our fin quest, we discovered a very convenient and cheap fin material that is easy to cut and attach. The plastic in which many store items are packaged is a fairly tough plastic that must be cut with scissors to get the part out of the package. Usually there is a small side and a much larger side at right angles to it. The right angle makes it easy to tape it to the rocket. Duck tape works, but clear packing tape can give a clean smooth edge. A little finesse with the tape yields beautiful fins. This is even easier than the 5-minute epoxy method we originally used with plastic fins. But since this is not yet the answer to good flight, why bother mentioning it now? Because it is time for experiment number 2.
Cut fins and tape them onto the bare bottle, then do a time test. More importantly, do an observation test. I have yet to see a bottle that performs any better with only fins attached, but who knows, maybe someone will surprise me. If it does, share your fin design with me. In any case, fins aren't yet the answer, so rip them off and proceed to experiment #3.
Inertia in Flight
Experiment number 3 involves a wooden meter stick or other thin &127;pole&127; to which you tape some large object. It should not be large enough to bend or break the stick when the taped object is on top and the bottom of the stick is on the floor. It should be as heavy as (or heavier than) a large apple. See the diagram. Now get the students to guess whether it is easier 5 to balance the stick with the weight on their finger, or the bottom of the stick on their finger. The mind naturally moves toward the big end down. Now let them try it.
This is a quick fun experiment of inertia. Your hand is racing to move either the stick or the weight before gravity can get past you. Now ask yourself. Which is easier to move: your dad's car in neutral or a little tricycle? The car is very massive and takes more effort, even though its rolling friction is more efficient than the tricycle. The reason is called inertia. So it takes more effort for you to move the heavy object, and gravity can move the yardstick rather easily. But if you let gravity try to move the heavy object and leave yourself with the easier job of moving less mass, you can easily outperform gravity. Now you know how a circus acrobat can balance a man on a chair on his head. And the heavier the man, the easier it gets (up to the point where the man on the bottom gets crushed, anyway).
So what does this have to do with our rocket? We are trying to balance it in the air stream. This is why spears have a big heavy head on the front and a relatively thin staff behind them. This is why arrows have arrowheads. Get the big guy moving and the rest will trail behind.
We can actually find out how balanced our rocket is by finding the center of mass in relation to the &127;center of pressure&127;. The next experiment must now move to a football size field. No one can get in the way of the rocket coming down. Nothing should be on the football field that can be dented, broken, or destroyed.
Put a small handful of clay on the top end of a bare bottle rocket. (The top end of a bottle rocket is not the nozzle end.) Shape it as symmetrically as you care to. Alternately, tape a small handful of dirt onto the end of the bottle. Blast it off. It should bring some form of stability to your rocket, although probably not a perfect result that would be desired. The tail end (nozzle end) probably began oscillating in the wind, but never swung to the top side of the now heavy top end. Now you have managed to orient it into the wind, what can be done to keep it straight so it doesn't oscillate?
Yes, bring out the largest fins you made and tape them on again. Six inches is a large enough fin, but not the optimum size. Use a smaller pressure of 40 or 45 PSI, because poor fin design makes for rockets that zig and zag like balloons and may be very dangerous. Check the wind. If there is anything more than a whisper, move your launch closer toward the windward side of your &127;football field&127;. Do not launch in heavy winds. For this launch, everyone should be paying strict attention to the rocket and be able to keep it in sight throughout the entire flight. This rocket will go 5 to 10 times higher than any rocket you have launched. It will come down almost as fast as it went up and can dent cars, break windows, damage houses, and maim people. From here on out you must not launch your rockets anywhere but clear areas larger than football fields.
Now go and experiment with fin sizes. Find out how small they can be and still be effective. Do they need to be bigger in length or width? Neither? Two fins? Three fins? Four fins? More fins? The answers fit within the laws of flight, but you will be required to find these on your own. Too small a fin causes a balloon zig/zag kind of flight; too large a fin drags too much head wind if even slightly misaligned, and always catches the wrong amount of side wind.
Velocity Calculations and Height
Now you will be wanting to know just how high that rocket went. We will assume it has virtually no air resistance. If your round-trip time exceeded 10 seconds for a one bottle rocket at 75PSI, you are close enough. Calculations will be made over the distance traveled coming down from the top. Only gravity affected the rocket, so it is a constant acceleration problem. Suppose your flight was 10 seconds round trip (up and down). We can only calculate from the peak of flight down, since that is the distance gravity moved the rocket by itself. So gravity had a &127;pull&127; time t = 5 seconds on the rocket.
distance: y = (1/2)gt**2 ; g = 9.8meters/sec**2
y = (1/2)(9.8meters/sec**2)(5sec)**2
y = (4.9meters/sec**2)(25sec**2) = (4.9meters)(25)
y = 122.5 meters = 401.9 feet 6
velocity: v = gt = (9.8meters/sec**2)(5 sec) = 49meters/sec = 160.8 feet/sec = 109.6miles/hour
As promised, this rocket will fly faster than a baseball form a major league pro pitcher. Now let's see how trigonometry compares with calculations.
Height from Trigonometry
Trigonometry in this case is simply a tangential calculation. We need to step back 300 feet from the launch site sideways from any wind. Otherwise our angles will carry an error of windage. A protractor can be used to site the rocket at the top of flight and find the height angle at 300 feet. Since the rocket slows to zero velocity at the top of flight, then accelerates with our tuned design, this will be much easier than with bare bottles or finned-only bottles. Those rockets stop much more abruptly due to misalignment with the air stream.
Only the part of the protractor that goes from 0 to 90 as shown is needed. If your protractor doesn't number like this, you can subtract your reading from 90degrees to get the height angle. Or use the co-tangent instead of the tangent. So in our example time of 10 seconds of flight, the height was 401.9 feet. So the angle we should measure will have a tangent = 401.9feet / 300 feet = 1.33967. This angle is about 53degrees, so if we had made an observation of 53 degrees we could calculate the height:
tangent 53 degrees = 1.327 = y/x = y/300feet , y = (1.327)(300feet) = 398 feet
If we had been off by 3 degrees (and measured 50 degrees) when we sighted, the calculations would be made by looking up the tangent of 50 degrees either in a trigonometry table or by using a scientific calculator:
tangent 50 degrees = 1.1918 = y/x = y/300feet , y = (1.1918)(300feet) = 357.5 feet
The amount of error can be lessened with larger protractors or smaller tangent angles. Stepping back far enough to keep the angle under 30 degrees will lessen the amount that a site error affects the calculation. This is because the tangent has smaller increments at those angles.
Increasing height is a big challenge, and you will want to experiment with nose cones as well as the fins and &127;spearhead&127;. Nose &127;cones&127; can be an form of symmetry: round, cone, split point, ... Cutting either nozzle end or the bottom end of bottles is a valid thing to experiment with. Taping nose weight inside them is an easy thing to do. Use tape to balance the nose &127;cone&127; and the amount of nose weight until the balance point you have discovered is correct. Then tape the weighted nose cone down solidly to the rocket. The challenge will be to find a nose cone/weight that provides a great round-trip time. But soon after getting the rocket to time well, you will get tired of crashing nose cones with each flight. Finding a way to put a parachute on it and maintain rocket dynamics will be a challenge. Usually this involves making a nose cone that will come off at the top of flight (and not before), when the rocket changes direction. And inside that nose cone would be something to return the rocket to earth without wrecking the nose cone. Finding a way to put a parachute in it, maintain rocket dynamics, and not deploy the nose cone too early will be a challenge. Most of our problems with nose cones come from having the cone deploying early or having it throw our rocket dynamics off balance. Maximum air pressure may not be best for your nose cone design. A proper nose cone and parachute will not throw the rocket off course nor pop off before normal peak height. The gauntlet is down. Let the quest begin.
But I suppose you would like a few hints about parachutes. When I was a kid with Moses, we used to take dry cleaner bags, split them squarely from the large opening on the bottom to the shoulder with a smooth pass of sharp scissors. Then we 7 folded them in half diagonally. This allowed for an easy guide to cut a square. Cutting the corners off equally yielded an octagon. This is a good parachute start. Checking into real parachutes and determining how to duplicate them in very thin plastic is more design work the students will be able to jump into. And then improving them for bottle rockets will be the next challenge.
Toward New Heights
When you reach for new heights, you will probably want to try to hook more than one bottle together to reach for the stars. Although this is tempting, I would like to tell you that I have seen a one bottle rocket go almost as high as 2-bottle rockets. Getting a good rocket flight is more important than 2-bottles. Now if you have plenty of time left to mess around with 2-bottles, the cautions are simple. I have yet to see a rocket that was cut on the outside cylinder hold up to 75PSI. I have heard rumors, but actual proof is lacking. I have seen 4 different successful versions of rockets that have been joined end to end in some fashion. All of them had mechanical means of joining the bottles, with another method of sealing the chambers. I have not yet seen a successful joint based solely on glue.
Now for safety concerns. Never pressurize a joined chamber without the new bottle containment box mentioned in safety rule #8. Pressurize it to 85PSI and wait 5 minutes. If it doesn't explode, tilt the pad over sideways with someone holding the containment box in place over the pad. Unfasten the air hose and release some of the pressure. Tilt the box back upright and carefully remove the containment box. Now you may launch. If the rocket works properly and doesn't crash (the parachute works right), you may perform future launches without doing a pressure test. After any crash, check the rocket, then do a containment test before putting it back in service.
Air pumps: A handheld bicycle pump with a built-in pressure gauge runs anywhere from $40 to $80. We have found a sealed cell battery operated pump that hold up very well at Walmart. A Campbell-Hausfeld portable air compressor launched 60 rockets at our state tournament, then worked every Saturday for the next 9 weeks for 3 to 7 launches. The only caution is that it is powered by a sealed cell lead-acid battery. Unlike ni-cad batteries, they don't have memories, so recharging them too soon is not a problem. Running on low charge too long is a problem and can short out the battery cells. This unit ran about $40 to $60, depending on the model. The rocker switches were the weakest part of the design. I wound up popping the pump switch out after it broke and simply held the wires together on the weekend it broke. I put in longer wires and a push-button switch. Not good for a 3-minute tire inflation, but fine for a 15 second rocket pressurization. The rocker switches may have been replaced with better slide switches since then.
Launch Pads: We took the information from the Nationals Coaches Manual and Rules page and did some ingenious modifications. We used a plastic short rework electrical box, turned it upside down, and used an adjustable hole cutter to cut a 1-3/4-inch hole in it. This provided the ability to slip the rocket pin yoke under the hole edge, then wiggle it into the holes without any difficult struggle. One of our coaches suggested adding an o-ring to the valve stem. This helps provide an easier seal. Additionally, a custom valve stem (the ones used for Mag wheels) was used as the valve stem. This gave a further advantage. With a little adjustment to the washer nut, the o-ring tension could be made to just the right amount. The valve stem could then be fitted to the bottles right side up. When the bottle was turned upside down, none of the water leaked out even if the valve stem was let go of. This makes water measurements very accurate, and allows for easy guiding of the rocket over a launch guide rod if desired. The launch platform size is not specified, and when it is built large enough and passive restraints are used, it becomes stable enough to have no need of being staked down to the ground.
Assembly time of all components is about 4 to 6 hours. Mass production cuts that down somewhat. Parts alone run about $7-$9. Completed launch pads can be obtained for $25 from the authors (unpainted). (Not including shipping.) I have sold 7 launchers at the 1996 Cen-Tex Regional Tournament, and am just publishing this paper, so I haven't yet had to determine how much it would cost to ship the pad. My guess would be between $6 and $10. Paint $5 extra. Specify or sample color.
If you are ambitious, here are my plans as complete as I can get them:
Cut a piece of 3/8 inch plywood. I built the first platforms 24inches by 24inches. For that size, I cut four 2inch by 2inch white pine studs to lengths of 22-1/2inches. Successful pads have also been cut 24inches by 18inches. (Cut two 2x2inch studs to 16-1/2inches, and two to 22-1/2inches.) The smaller pads seem to be as stable during launch as the larger sizes. I drilled and screwed the 2x2's to the platform and each other as shown, using &127;yellow&127; wood glue liberally in all the joints. You can also use Elmer's white glue. The glue makes the launcher more solid, for more reliable launches. Refer to the next page for a better visualization. I used 3inch &127;drywall&127; screws in the 2x2 corners, and 1-1/2inch drywall screws to hold the top of the pad to the 2x2's. Drilling 3/32inch holes for the screws prevents splitting the wood. Drywall screws aren't necessary, but drilling appropriate holes for nails also prevents splitting.
A plastic &127;rework&127; type electrical box is used for the launch site. It measures 2-1/2 x 3-1/2 inches x 1-1/4 inch deep. Two 5/16inch mounting tabs stick out on each long end. The ones I can get here are all made from blue PVC plastic. I'll call it a bluebox from now on, even though yours may be a different color. When the launch pad is dry (an hour should do), center the bluebox from side to side, but within 3 inches of one end of the platform. Make sure it is &127;squared&127; with the edges. (Meaning that each edge of the bluebox is parallel to each edge of the platform.) Draw on the pad around the bluebox, and draw a circle inside all four mounting tabs. Set the bluebox aside. On the pad, draw a diagonal from one of the mounting tab circle marks to the opposite mark. Then draw a diagonal from the a third mark to the last opposite (fourth) mark. Where the two diagonals cross is the center hole where the valve stem will go through the pad. Drill it with a 5/8inch bit. Drill the four mount tab holes with a 9/64th inch bit. However, you may find it easier to drill one hole; screw the blue box down with a 10-24 by 3/4inch screw through the pad wood (getting it started may be a little hard, but should not be too hard.); finally drilling the next holes using the bluebox as a hole guide, screwing each down in turn. If you are careful, you really don't need nuts on the back side of the wood as the wood will form sufficient &127;treads&127; to easily hold the 75PSI.
Now a 1-7/8inch hole must be cut in the bottom of the bluebox. Refer again to the next page. Depending on your hole cutter, the hole that comes out can be used in the valve stem assembly later. The side holes for the locking/release pin should be drilled so as to just barely miss the bottom of the blue box. They are 1-1/4inches center to center. Use a 1/4 inch drill and a round file to give them a smooth clearance of the 1/4 inch &127;locking/release&127; rod. Usually 1/4 inch rod comes in 3 or 4 foot lengths. Cut a foot of it, bend it carefully without pliers into a U shape. If you use pliers, you will leave scars on the rod that will not allow a smooth pull. I drilled a 3/8inch hole in a 2x4 stud and used the hole to bend with. As you start to complete the U, you will want to use a pop bottle to help get the proper diameter of the U. (Bend the last part around the pop bottle neck.) Work the U until it slides smoothly in and out of the bluebox locking holes (that are 1-1/4 inch center to center). You will probably want to file the U ends until they are round enough to smoothly enter the bluebox.
Now for the valve stem. Buy a straight chrome Hi-performance valve stem (They usually come blister-packed in a box of four.) Western Auto sells them for about $5.00 a box. They are used for chrome or Mag (&127;high performance wheels). We will not need the bottom rubber washer. Take off the other washer as well. It should have a &127;lip&127; on it. Cut it off as carefully as possible without cutting the surface it is on. It is now a flat washer. Now put the metal washer on the stem, followed by the flat washer you cut. Push them all the way to the top. You will need a metal fender washer with a 7/16inch hole, or the hole cut from the bluebox with a 7/16inch center hole. Use it as a pattern to cut a piece of an old inner tube. A standard paper hole punch will punch a sufficiently large hole in the inner tube. Stretch the inner tube over the valve stem, working it up all the way up to the flat washer. Put on the &127;fender&127; washer (or bluebox hole &127;washer&127;). Screw on the valve stem nut. Now comes the O-ring. Any hardware store should have these. Inside diameter is 9/16inch. Outside diameter is 3/4inch. They usually come in a box of six. They can pop off during launch, so get a box. Work it onto the top of the valve stem over the high-performance metal washer onto the flat rubber washer. This should now fit into a pop bottle very snugly. If not, loosen the valve stem nut a little. If you fill a bottle half-way with water and put the valve on tight, it should hold all 9 the water in the bottle when you turn the bottle upside-down. If not, tighten the valve stem nut a little until it does. Now you can do accurate experiments with water quantities. You should be able to lower the whole assembly (with a bottle rocket on the stem) into the bluebox, and clamp it to the pad with the U lock/release pin.
Now for safety. Learning a carefully aimed but swift pull might not result in any accidents from the U-pin hitting shins or other body parts, but a restraint would be advisable. A 2x2 stud secured to the top of the pad on the opposite end of the launch bluebox will do. Drill a 9/16inch hole towards the top side and smooth it out so it won't wear the rope out. Or make a passive restraint from two large strips of inner tube about 3 foot long each. screw each to opposite corners of the end away from the bluebox site. These allow you to not need to put stakes in the ground in the pull direction of the pad to guarantee that the box doesn't get pulled into the rocket on a good hard pull. (The solid 2x2 stud will catch the rope and U-pin solidly, and can pull the pad into the rocket). Inner tube strips will be used at Texas State Tournament 1997.
One last requirement. Unless you like building pads, take the bluebox off now. Paint the entire wood surface with high gloss outdoor latex paint. (It sheds very well). Let it dry for 2 days, then mount the bluebox again. May I suggest a half-pint of your school color. Happy launching.
If this sounds complicated, you may contact the author, or see him at one of the Texas tournaments about buying one pre-built. I do have water bottle launch pads. Since these are largely used by schools who prefer to paint it their school colors, the launch pad will be unpainted and should be painted in a good exterior gloss latex paint (or other good water-proof coating) before using. Otherwise it WILL warp. If the order request comes on school letterhead with a photocopy of a 1998 Science Olympiad Coaches Manual cover, the cost is $25 plus $5 shipping. Otherwise, it is $30 plus $5 shipping (continental U.S.). I am not in a business, so I cannot do charge card stuff.
You may order a launch pad by sending check or
money order to:
Austin, TX. 78753
Current supply will determine speed of delivery.