Water Rockets - Science Olympiad

The Challenge

Science Olympiads provide an interesting set of constraints within which to work. Many of the rules are concerned directly with safety and rightly so, others make the competition fairer.

The object of the excercise is usually to keep a 'simple' water rocket in the air for as long as possible.

This page is not intended as a cheat, it simply discusses the issues and ways of working within the applicable rules - the applicant still has to use his or her skills to build one.

Rules that apply to rocket construction

The following are parts of the rules that apply to the construction of the rocket. They are not the rules in their entirity and when two appear in one section, they appear here as separate. [My comments are in blue.]

1. No commercially finished or model products may be used.
[So no ready-made nose cones, fins et cetera.]
2. The pressurized portion of the rocket must consist of one plastic 2 liter pop bottle.
[No joining two together.]
3. The manufactured structural integrity of the bottle cannot be altered.
[Apart from the safety aspect, no drilling of holes (for timers an so on) or modifying the shape of the nose or nozzle inlet (as has been done with the 1½litre 'Egglofter').]
4. No metal parts will be allowed on the pressurized rocket body.
[This really means no metal fins or nose cones for obvious safety reasons - in past years it has referred to any metal on any part of the rocket which excluded clockwork timers. We shall stick to the 'no metal at all' meaning just to make it interesting.]
5. The mass of the empty rocket assembly cannot exceed 250 grams.
[Safety. We don't want to do too much damage on impact.]
6. All energy imparted to the rocket must originate from the water/air pressure combination provided by the judges. No other potential or kinetic source of energy will be permitted.
[If you are going to use elastic bands, they have to be slack at the beginning - the beginning is the time of launch, not the time it is mounted on the launcher - any stretching has to be done after lift-off. This also disallows balloons as parachute release mechanisms as the pressure inside the balloon is not derived from the 'water/air pressure combination provided by the judges' - the only way that it could be would violate the third point.]
7. Timing of the rocket stops when the first part of the rocket hits the ground, or when the rocket disappears from the judge's sight, or when the rocket impacts or gets entangled with an object (e.g. the rocket collides with a tree).
[It has to stay up for as long as possible and if it is to remain in the sight of the judges, it should go straight up.]
8. All rockets will be launched at a pressure not to exceed 60 pounds per square inch.
[A physical limitation with safety in mind in addition to making the competition fairer. Note that this pressure may be lower in some competitions]
9. . . . but only one launch is allowed per rocket.
[It has to work first time.]
10. Though various rocket components may separate during the flight, all must remain linked together with a maximum distance not to exceed 3 meters. If a nose cone is used, it can separate, but should remain attached to the rocket body.
[The nose cone has to be tied to the rest of the rocket thus limiting parachute cord length but this is not unreasnable.]
11. All rockets will be launched using the launching pad provided by the supervisor.
[A fair standardisation. This does mean that before constructing a skirt for fins, confirmation of dimensions of the launcher should be sought.]
12. No materials will be allowed that can compromise the integrity of the plastic bottles (e.g. hot glues or super glues). Sanding or other abrasion of the plastic used for the pressurized body of the rocket is not allowable.
[Play safe and keep to tape.]

The actual wording may vary (though it will always be written in stone) but whether it does or not, the principle is an interesting one.

Discussion

Effectively, everyone starts off with a standard 2 litre bottle and has to fasten bits to it.

As blowing out the front of the bottle to make a hemispherical nose is not allowed, some sort of nose cone should be used. As the idea is to keep a bottle aloft for as long as possible, a nose containing a parachute seems like a pretty good idea.

Adding fins will keep the rocket aerodynamically stable and allow it to get higher and therefore fly longer. Fins should be mounted on a skirt in order to keep the weight down (pushing the centre of drag backwards without adding too much weight) - using fins that are mounted on an aerodynamic skirt will allow for less drag and a higher flight still.

However, some Science Olympiad launchers do not allow for anything protruding further back than an imaginary plane level with the flange. An example of this can be seen on Patrick Matthews' site at http://home.att.net/~pat.mat/rockets/bluemonster.htm which illustrates the point quite well. If your rocket design is intended to travel up the path of success through various competitions, you may encounter such a launcher in which case, you should mount the fins on the side of the 2 litre bottle - details later on.

Nose

The nose that I have chosen is the 'nose separates at apogee' nose cone which has its own page - the tapered nose cone should be the better type.

With this design, the nose cone may be packed before mounting on the rocket and stored separately. This means that the chances of the nose falling off when mounting the rocket on the launcher are liminated.

With a permenantly fixed chute, if the nose does fall off, it can take a few minutes to repack it properly. People usually get around this by taping the nose cone to the rocket body - and then forgetting about it only to see their pride and joy hit the Earth at around 50 mph with the parachute still inside the nose.

Pictured right is the actual rocket (with plenty of talc) mounted on the Copper Tube Launcher.

Far right is the diagram for how to fit the bits together. You should have no trouble with aerodynamic stability (see the stability section on the 1½ litre 'Egglofter' page for an explanation) with this design so you should not need at add any weight to the nose.

Parachute

The parachute should be as light as possible and, taking into account the fact that you may be allowed to have two launches and therefore need to have two parachutes (keeping the cost down), the Bin-Liner parachute is the best choice for the job.

This chute opens well and weighs very little. A further weight reduction can be made by using thinner cords although these will tend to pull out easier.

For a Science Olympiad, the drogue or nose cone needs to be joined to the rest of the rocket - see the 'nose separates at apogee' page for more details on various strategies and how to fold a parachute. Practice makes perfect.

Fins

The fins should be mounted on a tapered skirt and just such a device is detailed on the fins page.

If you want to keep weight low, you may like to try and find a lighter tape or maybe use a glue that is available locally and allowed in the rules.

If you are likely to encounter a launcher such as those shown on Patrick Matthews' page http://home.att.net/~pat.mat/rockets/bluemonster.htm you should design your fins so that they do not protrude beyond an imaginary plane level with the flange (this can still be on a skirt although a fairly short one). Attaching them to the side of the rocket and using a swept back parallelogram shape, such that the edge stuck to the rocket is on the straight sides but the distal edge (the edge furthest away from the rocket) goes back as far as the launcher will take (ie, level with the flange). Doing this will allow the centre of drag to be as far back as possible.


Assembly

Once you have built the bits, you should take care to mount the nose cone base and the fins unit centrally as this will make it fly better.

Weigh the rocket with the parachutes and calculate the best amount of water for 60 psi (or whatever is specified in your particular set of rules). Add that amount of water and mark on the side of the rocket where the level is (both upside down and right way up - making sure that you can tell which is which)

Decide on your chute packing strategy and you are ready to go.

Launching

  1. Pack the chutes in the nose cone and put it somewhere safe and dry.
  2. Overfill the rocket slightly and mount it on the lanucher.
  3. Add a little air and Bleed off any excess water down to the mark.
  4. Take the nose cone and screw the shock cord bottle top onto the nose cone base.
  5. Carefully position the nose cone.
  6. Pressurise to 60 psi (or whatever)
  7. Release.

If you have packed your chutes okay it should deploy the chute soon after reaching apogee and float safely to Earth.

Thanks to Patrick Matthews for providing a set of the rules and for taking and displaying the photographs of the launcher on his site (a healthy appreciation of the circumstances of the launch[er] will never go amiss).

Look at the pictures . . .


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