Problem Create one bottle rocket that will fly straight and remain aloft for a maximum amount of time. Materials Two 2-liter bottles One small plastic cone (athletic) Duct Tape Scissors String Manila Folder Large Plastic Trash Bag Masking Tape or Avery Paper reinforcement labels (you'll need 32/chute.) Hole punch Procedure Cut the top and the bottom off of one bottle, so that the center portion or a cylinder remains. Tape the cylinder to another bottle to create a fuselage (a place to store the parachute).
Transcript
Problem
Create one bottle rocket that will fly straight and remain aloft for a maximum amount of time.
Materials
Two 2-liter bottlesOne small plastic cone (athletic)Duct TapeScissorsStringManila FolderLarge Plastic Trash Bag Masking Tape or Avery Paper reinforcement labels (you'll need 32/chute.) Hole punch
Procedure
Cut the top and the bottom off of one bottle, so that the center portion or a cylinder remains.
Tape the cylinder to another bottle to create a fuselage (a place to store the parachute).
Get the manila folder; fins will be made from it. Cut three shapes out of the folded bottom in the shape that the diagram shows. Your fins will be triangular.
The next drawing indicates how the fin should look once folded.
Mark straight lines on the bottle by putting the bottle in the door frame or a right angle and trace a line on the bottle with a marker. Use these lines as guides to place the fins on the bottles.
Make three fins and tape them on the rocket. Be sure that the fins are spaced equally around the rocket body. This can be achieved by using a piece of string and wrapping it around the bottle and marking the string where it meets the end. Mark the string and lay it flat on a meter-stick or ruler. Find the circumference of the bottle by measuring the length of the string to the mark. Once you know the circumference, then you can divide it by three to find the distances the fins should be separated.
Use the athletic cone to make your nose cone. Use fairly rigid scissors and cut the bottom square off of the cone. Depending upon your project's mass limitations, place a golf ball sized piece of clay in the tip of the cone. This will add mass to the cone and give the rocket/cone more inertia. Then, using scissors, trim the cone to make it symmetrical. (Hint: the diameter of the bottom of the cone should be a little wider than the diameter of a 2-liter bottle.
Attach the cone with string to the top of the other two-liter bottles so that it looks like the diagram. Tie a knot in the end of each piece of string to give it more friction and tape it using a piece of duct tape to the inside of the cone and to the inside of the rocket body.
Many students have trouble with their nosecone getting stuck on the top of the rocket and not coming off. This can be prevented by making a pedestal for the cone to sit on. It should be high enough up so that there is space between the cone and the top of the parachute compartment. You can make a pedestal out of the same material you will
make the fins, the manila folder. Make three mini-fins, invert them and tape them on the rocket where the cone should sit.
Making the Parachute
Don't forget a good parachute has shroud lines that are at least as long as the diameter of the canopy.
Lay your garbage bag out flat. Cut off the closed end. It should look like a large rectangle and be open at both ends. Lay down the bag on a flat surface and smooth it out.
The bag has a long side and a short side and is open at both ends. Fold it in two so that the short side is half as long as it was originally.
Now fold it again. Fold the hypotenuse so that it lines up with the right side of the triangle in the above drawing.
Examine the base of the triangle and find the shortest length from the tip to the base. This is the limiting factor for chute size. The most pointed end will end up being the middle of the canopy.
For an example; if you want the diameter of the chute to be 34 inches then measure 17 inches from the center of the canopy (the most pointed side of the parachute) along each side, mark it and then cut it.
After cutting it, unfold it. If you have been successful there should be two canopies.
Fold the canopy in half, then into quarters, then into eighths. Carefully crease the folds each time. Crease it well and fold it again. Now the canopy is divided into 16ths. Unfold the parachute. Notice the crease marks. Get masking tape and put a piece around the edge at each fold mark. You may also use Avery reinforcement tabs. Place one on both
the inside and outside of every crease, making sure that they are overlaid on top of each other.
Punch holes through every piece of masking tape or Avery tab pairs and use these to attach the kite-string shroud lines.
As mentioned earlier the minimum length of the shroud line should be the same length as the diameter of the canopy.
After punching the holes fold the canopy in half. Pick four holes and tie the shroud lines to the holes. After doing this tie the four lines together at the end most distant from the canopy.
Repeat this four times until the chute is completed.
Once you have it complete attach it inside the fuselage. Generally a couple of pieces of duct tape will hold the parachute to the rocket. Pack the parachute loosely and put the nosecone on the rocket.
Run a bead of glue on the outside to cover the holes and create fillet.
Completed fin set that can be fitted to different rockets.
Nose
To modify the bottom of a pop bottle, you first of all need to have the connector described on the connector page. Attach this to the bottle (as described on that page) and pump up a fair pressure - between 1 and 3 BarG.
Unscrew the pump, leaving the hose (with its one-way valve) in place and start to rotate the bottle, holding the nose (a) approximately 9" above a gas ring (for those without gas rings, I have seen a barbecue mentioned as an alternative although I have not used one myself). After a while, the plastic will have softened sufficiently to form a hemispherical shape by itself (b), at which point, you should cool it down under a cold tap or in a bowl of cold water.
At this amount of pressure, there is little danger of a catastrophic failure of the bottle during this process as the place where it is most likely to fail is where it is hottest and thinest. When it does fail like this, it is with a Pfffff and not a bang. (you can always use the straight sides for fins - see lower down this page).
Body
Modifying the shape of the body is similar in many respects to the process of making the nose into a hemispherical shape. This time, however, the idea is a controlled collapse of the body, down to the required conical shape.
Fix the adapter onto the end of the bottle (a) as above and put a little less pressure in it this time. Make sure that the bottle heats up evenly in the area that you want to reduce by turning it continuously above the flame in the same manner as a glass blower. If the bottle is not showing signs of shrinking but is warm enough then loosen the adapter slightly to let some of the air out. You may have to do this a number of times but always have the pump nearby incase you let out too much.
Make sure that you keep the nozzle aligned with the axis of the bottle - if you do not, it may always fly in an arc. Do not heat too much near the connector part of the bottle as this may start to blow out making it impossible to fit properly on a launcher. If you have problems with bottles fitting launchers and cannot get hold of the correct internal or external 'o' rings, you can expand or colapse slightly the upper or lower part of the nozzle accordingly so as to get a good fit.
Again, once you have got it just the way you want it (b), put it in cold water to make it set.
Practice makes perfect and it may take a few attempts to get it just right.
Fins
Ideally the fins should be as far back as possible (without getting in the way of the launcher) and present as little drag to the rocket as possible when flying straight. they can be virtually any shape as long as the do the job of providing a sideway area at the rear of the rocket (see the next section on stability).
I have found the best material for fins to be the same plastic that the bottles are made from. There is enough of this material in two 2 litre bottles to make the 3 or 4 fins that you require.
Cut out the part of the bottle with the straight sides (1) and cut along one side (the bit with the glue on it is the best place to cut as this is unsightly)(2).
Flatten this bit out and fold back on itself so as to cancel out the tendency to curl (3).
Make a template of the fin shape that you want (sized so that you will get two out of each bottle with a minimum of waste) and cut them out (4). You can either glue the two sides of each fin together, or you can tape them around the edge - the latter making it easier to see the fins when in the air.
Fixing the fins
Two methods:
1. Glue. Glueing them has some problems. Holding them in the right place long enough for the glue to set and choosing a glue that will not break when the bottle expands during pressurisation (PL Premium is suitable for this). It can take days for glue to dry properly.Outline the position of the fins with a marker pen (you only need mark the corners) and apply the glue according to the instructions. Tape the fins in place until the glue dries and then remove the tape; OR . . .
2. Tape. The alternative to glueing them is using sticky tape of some sort (Gaffer Tape, Elephant Tape, Duct Tape et cetera) - this has the advantages of being instant and flexible although the tape can start to peal off. Taping can take only a few minutes.Cut out three or four thin pieces of tape and position them on the rocket where you want the fins to go, repositioning as appropriate. Put lengths of tape on each of the fins and position them on the rocket. Finally, put down some extra straps of tape around the fins to stablalise them.
1.
2.
3.
4.
Rough shape and position of fins
The real thing. Note that there is tape all around
the edge.
Once this is done, a further step to make the fins stable is to use a soldering iron to melt a hole through the fins near to where they join the rocket such that a cable tie can pass through all of them, linking them and holding them close to the rocket. This will stop the glue of the tape from creeping and keep the fins in place.
Stability
Ideally, you want the thing to fly straight as any deviation from this will reduce the rocket's performance. Once the thrust phase of the flight is out of the way, the rocket is essentially in free fall (even though the first part of this free fall is upwards). For it to maintain its attitude in the air, there are a few things that you will need to consider: the positions of the effective centre of drag (COD) and the centre of gravity (COG). With a small rocket such as a water rocket, it is fairly easy to find out where they are but you also need to know what to do with them. First, how to find them.
To find the COG, try to balance the rocket on its side so that you find a point that is reasonably stable. The COG is above this, on the axis of the rocket as in the diagram. (If it is not on the axis, you have a problem).
To find the COD, cut out a piece of cardboard the same shape as your rocket as viewed from the side and find its centre of gravity (it need not be the same size as the real rocket as long as it is to scale and that you remember to scale it back when you have found it). The centre of gravity of the cardboard model corresponds to the centre of drag.
Once you have put your fins on (which you should have done before you started trying to find the COD), the COD is going to remain almost in the same position, no matter what you are going to do with the rocket (within reason) wheras the COG may be moved by adding weight to the rocket. Ideally, the rocket should weigh as little as possible so you want to add only the barest minimum of extra weight.
So where does the COG want to be? The COG needs to be between 1 and 2 rocket body diameters (d) forward of the COD - in free flight, the COG effectively pulls the rocket forwards and the COD pulls it back - if they are between 1 and 2 rocket body diameters apart, they are able to exert enough of a couple (a couple is a pair of equal and opposite forces that do not share the same axis and therefore tend to have a twisting effect) to correct the rocket's
attitude during flight.
To move the COG forwards, make a mark with a pen on the rocket where the COG needs to be and then tape coins to the front of the rocket (at point w). Once you have found out how many you need, you can make a neat job of it with tape (or glue) so that the aerodynamic qualities that you have devoted so much of your time to are not lost.
I found that for the 2 litre bottle that I did this to, I needed 8 x 2p pieces - for the 1 litre and 250 ml, I needed 6 x 2p pieces. (A UK 2 pence piece weighs approximately 7 grammes or ¼ ounce).
See Bottle Modifications for Better Flights for an explanation of air and water flow.
Water
We now have a water rocket that is aerodynamically sound. We know that we will be able to pump it up to a pressure of between 4 and 6 BarG (between 60 and 90 psig) and we can measure it. So, how do we know how much water to put in it?
We need to know its tare weight, capacity, diameter and nozzle dimensions to be able to work out how much water it will need for a flight with the greatest height.
We can measure its nozzle, body diameter and weight it empty to get its tare weight but we have changed its capacity so we don't know that any more - the volume of liquid it had when you bought it was not the same as its nominal capacity either and in addition, there has to be a certain amount of ullage (head space) so as to take into account the expansion of the liquid when it gets hot so that the bottle doesn't burst in the shop. All we can do is measure it and the best way to do that is as follows. . .
Weigh the rocket empty (you will need this for the computer model anyway). Fill it to the top with water and weigh it again. Take the former from the latter and you have your capacity (close enough) as, for the purposes of water rocketry, 1 gramme equals 1 cm³.
These figures were then fed into my computer model and the weights in the table below and graph on the right were calculated to be the optimum for the pressure range considering the diameters of the different rockets.
To put them into practice, put a piece of gaffer tape along the side of the rocket and weigh in the optimum amount of water. Mark on the gaffer tape where the water comes to, screw a top on, invert it and make another mark (in such a way that you will not be confused - possibly using an arrow pointing upwards). This will make life easier when in the field and you haven't got access to the scales.
If your rockets have tare weights or capacities that are different to these, you can use the above graph to work out roughly the right weight of water optimised for height - this assumes that the rocket capacity and diameter are roughly in proportion.
Tests
I made 3 rockets for the purposes of testing this out. Two of them already had an almost conical neck (the 250ml and 1 litre) and therfore didn't need modifying there. I modified the 1 litre at the nose end to form a spherical end and, as it was originally a blackcurrant cordial bottle and had concertina style ribs, I blew them out to make a flat(ish) side. As a result, it shrank down to have only 800mls capacity. The 2 litre bottle was treated in accordance with all of this page - spherical end, tapered side, fins, weighted nose - and it shrank down to 1,500mls.
All of the rockets were pressure tested to around 1.3 times the normal pressure for twice the normal time of pressurisation. This was done by filling completely with water, attaching the connector described on the 250mls rocket page and pressurising. The reason for filling with water is that water is effectively incompressible and therefore can be taken up to quite high pressures without holding a dangerous amount of energy - the energy in the water rocket pressure tests is from the elastic deformation of the bottle during the test.
All forstandard
open nozzle
Size Rocket
Nominal 250ml 1litre 2litre
Approx 300 800 1,500
Measured
Tare weight g 50 100 120
Capacity ml 290 800 1500
Diameter cm 6.0 7.5 9.5
ComputerModel
Water g 110 270 450
Heightm
(feet)30.0(98)
41.7(137)
47.3(155)
Duration s 5.0 5.9 6.3
These computer predictions worked out to be correct as far as the times and heights were concerned. They were launched from the Copper Tube Launcher, timed and measured.
