Water Rocket Problem Page

From time to time, I get asked various questions to some problems and sometimes, they have very similar answers although the answers are really too long to go on a FAQ.

On this page, I aim to provide the answers to some of these questions in full.

Covered here are:

See also the Frequently Asked Questions.

What is the best way to get the pop bottle label adhesive off of the bottle?
The glue is sticky because of localised static electric charge - the glue molecules are designed to have ths property. On the TV a while ago, there was a programme with some US weapons that were designed to incapacitate but not to injure and one of them squirted a foamy glue at the target which was then disabled like a fly on fly paper. One of the journalists covering the event noticed that when the military personnel got some of this stuff on them, they used a "special liquid" to get it off. On being asked what it was, they said they couldn't divulge that information. However, someone spotted a bottle with the label still on it - it was baby oil. The oil provides an insulating layer that shields things from the local electrostatic field.

To get the oil off a pop bottle, I warm it up by filling it with water at around 50 degrees centrigrade (this softens the glue), right up to the top and then screw the top on. This makes the warm bottle reasonably rigid. Then I use a knife (the sort of blunt knife you would eat at the table with - you don't want to scratch the plastic too much) to remove most of the glue. Then, I put some oil on the glue and rub at the edges to get the oil between the edge of the glue and the plastic bottle. Eventually, it all comes off.

Once the glue is off, you need to get the oil off. I use neat washing up liquid, making an emulsion out of the oil and once all of the oil has come away from the surface, I then wash it off with water.

If you need to have the bottle clean enough to glue things on such as fins, you can use a solvent (such as alcohol) to remove any residual oil from the surface of the bottle. If you are going to use gaffer/duct/duck tape, I have found that this last stage is not necessary.

Once the bottle is clean, dry it if necessary, and then take the top off. Carefully pour the water out - remember that the bottle is warm and therefore is quite flexible - otherwise it will collapse under even a slight vacuum and crease. Once it is empty, put some cold water in it to make sure it is hard enough to use.

You may wish to find a source of bottles that have not got labels on them :-)

It is possible to dissolve all of the glue in isopropyl alcohol but if you haven't got unlimited access to this chemical, using oil and washing up liquid is probably better.

I am currently doing a water bottle rocket. I have given it 5 small fins and a cone for the nose. Is there anytihing else I can do to improve its flying quality.
Make sure that the Cp is between one and two rocket diameters behind the Cg.


in English . . .

Cp - - -
The Cp is the centre of pressure. It is the effective centre of the area of the rocket as viewed from the side. One way of finding the Cp is to make a cardboard profile of the rocket and find where it balances.

To make the cardboard profile, you can either draw the outline from measurements, do it by eye or put it on the floor beneath a light in a room and draw around the edge of the shadow (a bare bulb works best). Once you have the outline on the card, carefully cut it out and then find the point where it balances.

You can find the point where it balances by sticking a pin in it with a piece of thread tied to the pin with a weight on the other end of the thread so that the thread is pulled down across the face of the cardboard. Draw a line with a felt tipped pen on the cardboard along the line of the thread and then take the pin out and stick it somewhere else on the cardboard, repeating the drawing exercise. This should give you two lines that cross. Repeat a third time and the third line should cross where the other two do. This point represents the Centre of Pressure and adding weight should not change its position (unless the weight is bulky).

Finally, put a mark on the rocket in the position that corresponds to the mark on the card. You now have your Cp marked on your rocket.

Cg - - -
To find the Cg, or Centre of Gravity, (sometimes called COG), hold the rocket on its side and find the point where it balances. The diameter of the rocket is just the diameter of the body of the rocket and does not include the fins (they are usually not bulky enough). The balance point of the rocket, the Cg, should be between one and two rocket diameters in front of the Cp for the rocket to be stable. If your rocket balances behind the point that is one rocket diameter in front of the Cp, then add some weight to the nose to bring the Cg forward. Once the Cg is at least one rocket diameter in front of the Cp, the rocket should be stable.

Testing without launching - - -
To check for stability without launching it, tie a piece of string around the rocket at the Cg (note that this test may not be practical if the fins come this far forward) and swing it slowly around your head. Starting from any direction, if it is stable, if should soon end up pointing in the right direction.

We are building a rocket for Science Olympiad and the competition is in three days. We made 10 launches today and never were able to get the parachute to separate from the nose cone. Is there some secret we have missed? What do you think about adding weight to the nose cone to make is pull away from the rocket. The nose cone is free falling and does not have a chute of its own. Any ideas are welcome at this point.
I think that the reason why you are not getting deployment is because the nose cone has no real reason to come off the chute, ie there is nothing pulling it off.

Solutions - - -

You suggest adding some weight to the nose cone. This would pull the cone off the parachute but it would have several detrimental effects as well:

  1. The extra weight would mean that the water in the rocket would have more work to do to attain the same height therefore the rocket would experience a loss of height (because of loss of performance) and therefore the flight would be of a shorter duration;
  2. Extra weight would also mean that the parachute would be pulled down through the air quicker and therefore the flight would be of shorter duration - remember that in the SO rules (if it is like previous years) there will be a condition that all of the parts of the rocket stay joined together so your nose cone needs to be connected to the rest of the rocket with a piece of cord of some sort that is packed so that it does not tangle with the chute during or after deployment;
  3. Extra weight in the nose cone would, after the water part of the thrust, cause the nose cone to separate early. Effectively the heavier nose cone would be pulling the rocket through the air and as there is nothing holding it onto the rocket, you would get separation fairly soon after the end of water thrust instead of between three and four seconds into the flight at apogee where you need the chute to deploy; and,
  4. Adding weight would mean that you would stand a greater chance of going over the weight limit of the Science Olympiad rules. If you are no where near this limit and the computer model says that you can increase the weight of the rocket, you could make the chute bigger but remember that a larger chute can cause problems deploying.

I suggest that you pack a drogue chute (about a foot in diameter) that is connected to the nose cone. This has several effects:

  1. At apogee, the nose starts to separate and the drogue pops out into the air stream. This then pulls the nose cone off the main chute. The drogue should be connected to the main chute (possibly to _one_ of the cords using a loop so that it can slide to the top of the cord or down to the bottom where all of the cords are grouped - this way if the drogue/nose-cone combination has a slower rate of decent than then main-chute/rocket combination, it will slide up and not get in the way of the main chute whereas if it is faster, it will slide down to the bottom and not get in the way) so that it remains as one piece; and,
  2. If the drogue/nose-cone combination has a slower rate of decent than the main-chute/rocket combination, this will slow the decent slightly as the drogue will go to the top of the cords on the main chute.

One point to note is that there is usually a limit on the total length of the rocket-parachute-etc. You should make sure that the drogue at the end of its travel along the main chute cord does not allow this limit to be exceeded.

This could be done by having:

  1. A small slip loop and a knot in the main chute cord alghough the disadvantage of this is that the main chute may stand a better chance of tangling as a result of the knot; or;
  2. Having the loop on the drogue go around the single main chute cord but also extend to the main chute eyelet (where all of the main chute cords go) so that the total length cannot exceed the limit. One disadvantage of this is that you could run into problems with main chute deployment so you could solve that by packing the chutes carefully. Remember to make the line linking the nose cone tot he main cute line longer than the length of the main chute so that it all pulls out (or pack it so that it comes out in a lump that then unfurls. The combinations are endless.

I also suggest that you line the sides of the nose cone with a PTFE oven liner (made by DuPont - they are expensive, around 3.00 (possibly $3 - 5 in the US) but half a sheet does nicely). Have a look at http://ourworld.compuserve.com/homepages/pagrosse/h2orrectef1pics.htm for pictures of where to put it. If you cannot get hold of one of these, use plenty of talc but make sure it is dry because the stuff is like cement when it is wet (this is where PTFE sheet comes into its own as it will work wet and dry - I have tried it ;-)

Also have a look at the packing strategies, half way down http://ourworld.compuserve.com/homepages/pagrosse/h2orrecsys1.htm

Also, you could put a series of 16 or so holes in the nose cone on the straight bit, just after the curve finishes (just above the PTFE liner). There is a low pressure area here and when the rocket is moving fast, the air is sucked out of the nose cone at this point, increasing its tendency to stay on while the rocket is in motion. At apogee, there is no motion (unless the flight was not particularly vertical) and the nose will separate.

For a school project I have to build a water rocket. I am building my out of a two litre pop bottle. For extra credit you can put a parachute that works on the rocket. I have decided to do that but I do not know how to have it work Please send me a . . . message saying how you would install a parachute on to a two litre bottle rocket.
Chute Design - - -
The parachute type depends on what you are after. I would recommend a circular chute (the parafoil is okay bot is rather fiddly and can fail to deploy sometimes.

Chute Materials - - -
The materials of construction depend on time available, materials available, wow factor required. The most reliable, lightest, easiest to make and one that requires least in terms of initial outlay is the bin-liner chute. These are lighter and pack smaller than the nylon ones which, I'm affraid has it for the wow factor (unless you can come up with brightly coloured, primary coloured bin liners:-).

Deployment Method - - -
The NSA (Nose Separates (or falls of) at Apogee method is reasonably reliable although some people go for clockwork timers (three or four seconds and then deploy the chute - those little timers that you get in some children's toys) or air speed flap design. These will be more reliable but require more in terms of setting them up in the first place. Perfecting the NSA technique may be all that is required.

General - - -
Use plenty of talc (to allow the chute to slip out of the nose cone).

I aim to have a go at the speed flap and timer methods but I suggest that you look at some other's web sites that have details on these methods. Depending on the tiem you have available, I suggest that you either have a go at building all three or pick the one that you think will give you best results.

Look almost half way down the page . . . http://ourworld.compuserve.com/homepages/pagrosse/h2orrecindex.htm . . . for some links.

Question on Momentum: John mentioned that [zero] mass is not the optimum for water rocket flight. Does anyone know how to calculate the ideal mass for maximum flight? It seems to me that for a given thrust/resitance, a reduction in mass will always yield a higher flight. Of course, dealing with short duration impulse thrust of the water rockets has thrown me way off in my understanding of thrust and rocket flight.
With a short duration inpulse (full open nozzle water rocket), the ideal weight for height is not zero. If you had a very light weight rocket (a couple of grammes), it would very quickly come to a stop as there was no real weight pulling it through the air. As you increase the weight of the rocket, the height increases until you reach a limit.

With longer duration water rockets, the ideal weight is very low and with 2 stage it is also low (I like to think of a 2 stage rocket as a variant on the launch tube/t-nozzle combination).

If you are going for distance, a heavier weight is more ideal than going for height.

To find the ideal weight of a rocket, use a computer model (WRCM). There are a number (PC, Mac, Online and so on), all but one is for free download (that one is a commercial version or the water rockets that are availble for free download within the group) or no download (an online version where all the calcs are done at the server end).

I have a link to the WRCMs on the following page but for more general browsing, use the most complete WR links list on the web at Clifford's site.

http://ourworld.compuserve.com/homepages/pagrosse/h2orckt.htm (screen shots and HTML based help link from this page)

My site's WRCM links list with details

Clifford's links list

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