BOTTLE ROCKET

[Jacob WorthingtonParticipant @jacobworthington64798]

Hello,
For a mechanics project I've been asked to design a bottle rocket.(pressurised air using water as propellant). The aim – to get the rocket as far horizontally as possible.

Now, for maximum flight time glider wings may be ideal, but won't survive the initial acceleration of the rocket considering I'm using a 2litre bottle at roughly 110 psi (subject to change).

For this reason I'm wondering what measures I can put in place to limit the acceleration of the rocket. Ideally the rocket/plane will accelerate slowly until it reaches maximum velocity. Perhaps a valve that regulates the flow of expelled pressures? Or a way of restricting the flow in a way that decreases as the pressure decreases.

Bit of a long shot but appreciate any advice.

Many thanks,

Jacob

[Jacob WorthingtonParticipant @jacobworthington64798]

[Speedy Builder5Participant @speedybuilder5]

I guess you meant "Increase the flow as the pressure reduces". How about a vane in the bottle neck that unscrewed as the water rushed past it. As the vane unwound, it would increase the nozzle cross sectional area. Having once said that, would you not require full pressure at lift off time ? I don't think that the 'toys' you can buy have any sort of proportional restricting valve in them.

It will be interesting to see how you make the latch / firing system. I think a lot of us will have children / grandchildren who would like one of these.

BobH

[Sandy MortonParticipant @sandymorton10620]

I've got one in the workshop which the gradweans think is brilliant. I did buy it and it wasn't expensive. I'll try and take some photos and post them on here but they will be inside on the bench – it's gusting over 60knots here just now

[Michael HornerParticipant @michaelhorner54327]

Hi Jacob

Make it look like Thunderbird 1, and as the forward velocity drops off the wings come out to give lift!

Cheers Michael.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Jacob ,

(1) If your water rocket has any significant amount of water in it then the acceleration characteristics given by a plain nozzle are best you can do .

This is not quite the place to discuss projectile dynamics but in essence take off is at maximum weight and initial thrust is maximum . As water is disipated weight comes down and reduced air pressure reduces thrust .

Given a bit of luck it all balances out for itself .

(2) Need to decide though whether you are building a water rocket as such or a powered aircraft – the design rules are very different .

(3) For a water rocket launch initially at a steep angle from horizontal and let rocket follow a modified parabolic path for maximum range .

(4) For a powered aircraft launch at a much more shallow angle .

I don’t see any practical problem in fitting normal wings . The wing load is going to be very small and speeds are going to be definately well subsonic so ‘barn doors ‘ will suffice and can’t see any attachment problems . May need to be very light weight though .

(5) Take care with this project – it’s only toy size I know but there is still potential to hurt someone quite badly if things go wrong .

Get back to me if you have specific questions .

Regards ,

Michael Williams .

[Jacob WorthingtonParticipant @jacobworthington64798]

Nice ideas, I like the idea of a coil that widens the aperture or the bottle neck over time, but how to make this without the whole lot flying out in one hit after the trigger mechanism is released? Spring loaded pop-out wings have been considered, and despite my love for thunderbirds I'm afraid it will have other weaknesses, and certainly difficulty during manufacturing.

Thank you Michael. I was hoping to go for more of a powered aircraft since it will travel further horizontally than a rocket launched mortar style. So for this distance long glider wings may be suitable, however the longer the wings the greater the moment and leverage the air resistance will have. Maybe I can compromise by having a long flat triangular wing like on the space shuttle or Vulcan bomber. I suppose it will depend on what materials I use for the wings but I'm afraid the shear lift could cause the wings to snap off. It may be subsonic but it packs one hell of a punch.

Many thanks,

Jacob

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hello again ,

Conventional thin aerofoil rectangular planform wings are probably all you need .

Do some numbers –

if you know the acceleration levels and wing load you can make definate decisions .

Needn’t be all that accurate – just ball park figures .

Regards ,

Michael Williams .

[Douglas JohnstonParticipant @douglasjohnston98463]

Is it just me or does the thought of pressurising a bottle to 110psi not sound just a tad scary?

Doug

[Martin WParticipant @martinw]

Jacob

Have a look on Utube, I seem to remember that they have videos of bottle rockets that have achieved altitudes of around 1000ft and descriptions/walk throughs showing launch rigs, release mechanisms, nozzles etc. Also shown are multistage rockets, using parts of a standard garden hose connectors that has been modified. Some of the launch videos are very impressive with a couple that that burst/exploded prior to launch completely destroying the launch rig.

Good luck

Martin

PS

Douglas, when one thinks what these bottles are designed to go through, hot environments and violent shaking and that before the kids get hold of them, I suspect they are designed to take pressures well above the 'normal' whatever that is. That said I agree with you that caution is required, especially after seeing the videos of some of these things exploding during pressurisation. Some rocketeers actually join bottles, cut and shut, by gluing a band made from another bottle around the join, to me that's inviting catastrophe.

Edited By Martin W on 23/02/2014 11:52:57

[John McNamaraParticipant @johnmcnamara74883]

Hi jacob

If you are building a plane would a smaller nozzle help? Small enough to restrict the thrust of the motor to just a bit more than the thrust required to keep the aircraft flying maybe at a smallish rate of climb.

As the water is ejected the weight will dramatically decrease. I wonder how long the thrust can be maintained?

Gee there is a wiki **LINK**

Regards
John

[Martin WParticipant @martinw]

Hi

If trying to use the rocket in a glider then there is going to/or is likely to be a major change in the centre of gravity as the water is expelled. Also if the angle of launch is too shallow and the acceleration low then there could be a possibility that the water will not be expelled when the pressure is at its optimum thereby reducing total effective power. In the extreme all the water may not be expelled and could just become a load to be carried during the free gliding stage of the flight.

Are you allowed to any form of active control i.e. electronics, on the model? If so then a simple multi-axis accelerometer and servo system may help to stabilise and extend the glider phase.

Cheers

Martin

[Michael HornerParticipant @michaelhorner54327]

Hi Jacob

If your allowed too have a ground based first stage, say a big spring so this provides the lift off as this "kick" dies away your second stage then takes it to its destination!

Cheers Michael.

[Gordon WParticipant @gordonw]

This is just a guess. Launch as a mortar for max. hight, at this hight the speed is nil for a moment. Have a mechanism to unfold wings and glide to earth. Balance before launch to suit empty bottle.

[Neil WyattModerator @neilwyatt]

Apparently a safe working limit for PET bottles is around 85psi, as they normally stand up to 100psi or even higher..

The great aeromodeller Dougg McHard made some air-powered models using pop bottl;es as reservoirs. He had a technique for re-inforcing them using self-adhesive fibreglass tape.

Simple folding cloth wings seem to be the easiest solution to me.

Neil

[frank brownParticipant @frankbrown22225]

launching at a shallow angle might result in the air escaping through the nozzle, while there is 25 degrees worth of water still in the bottle. You need a water pickup tube that is at the bottom of the almost horizontal bottle. Also as mentioned massive change of C of G. Years ago I saw a design which was a model air craft that basically had a second smaller one pivoting on its top. The idea was that changes in the relative trim between the two resulted in the elevators on the main wings being tweaked by the action of the smaller one, so as to keep level flight.

At School we had a craze on plastic rubber band launched delta planes. When launched because of the very high acceleration they would loop, then level out and fly normally. Very robust and long lasting. I think most ended up in the roof gutters!.

Frank

[Ian S CParticipant @iansc]

As you cannot compress water, I imagine that there must be a certain amount of air in the bottle, with this in mind, a vertical launch would put the air space in the right place, ie., the water in between the air and the nozzle. When at altitude some way of deploying a wing / paraglider or flexible delta wing as in hang glider, attached to the bottle so that the bottle is horizontal, given a good day this could fly quite a distance. For a bit more distance, get up a hill. Ian S C

[Russ BParticipant @russb]

I'd make a glider basically, gain maximum altitude, as quickly as possible, as the water is expelled the COG should shift to the front, ie. gliding angle, balsa wood is your friend here – I doubt it will snap, as others have mentioned the initial weight of the water is going to restrict acceleration,

The wing span obviously wont be large enough to support the weight of the water, but as it all exhausts and then levels its light weight gliding all the way.

I wonder if you could discharge the water bottle using a sort of level/pressure trigger & maybe a small weight could control small ailerons to keep her level…. I'm probably the king of impossible ideas, but hey, man's been to the moon and back……..

The direction of the wind will be a major factor in gliding, I guess you need control of the direction you launch in

[Jacob WorthingtonParticipant @jacobworthington64798]

Thanks for your ideas, very useful having other opinions. Yesterday I tested my prototype design. A balsa wood frame covered in tissue and shrinking dope. Weighted in a way that flew horizontal with no water in it since at 110psi all the water will be expelled in less than a second. The wing design I went for was a medium swept wing to cope with the acceleration, and a tail to give directional stability.

Filled with 550 ml of water, and at 110 psi at an angle of 45degrees launched with the wind, the thing went some! One wing and tail wing broke off in an upward direction suggesting too much lift(which is good) and not strong enough materials. Regardless it had a maximum altitude of about 25 metres before crashing down at the other end of the park (some 100 metres)

I have in mind to use software to laser cut 3.6mm ply wood as a substitute to the balsa frame, including additional supports to increase durability. Is there a stronger substitute to using tissue paper and shrinking dope that doesn't compromise weight?

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Mylar or basically cling film .

[jason udallParticipant @jasonudall57142]

“Solar film” from memory…light plastic film..shrink wrap type stuff..put on taught shrink with heat..no glue or paint…use white./clear..lightest ../square m..
Do remember the penny weight planes..1 m wingspan for the weight of a new penny…
shaved balsa and dope film..no tissue…much too light for this application. .

[Martin WParticipant @martinw]

Jacob

Any merit in using carbon fibre or similar composites in the more highly stressed areas. Some time ago I got hold of some thin walled tubular offcuts, about 5mm diameter, and while being very light they were almost indestructible.

Martin

PS

Plenty on Ebay at a reasonable price.

Edited By Martin W on 25/02/2014 14:05:40

[Chris ParsonsParticipant @chrisparsons64193]

I had a go at this a while ago – even made a 'flight computer' to release a parachute at apogee (ok, after a time delay!)

Here are some links if it helps?

**LINK**
**LINK**
**LINK**

Good fun, but you get pretty wet if you are not careful <g>

My 'launcher' was a garden hose fitting with a bit of string to pull the ring down, but now I am into this model engineering lark I did briefly consider modifying a brass hose fitting to make something a bit more professional
(perhaps another round tuit)

Regards

Chris Parsons

[jason udallParticipant @jasonudall57142]

As a kid I used car valve rubber thing..just fitted a well known washing up liquid bottle neck…add water add foot pump..and when the bottle feels the urge to fly..no release just when grip fails. ..best results found for quarter fill..but certainty discharged all reaction mass “in one”..13 mm nozzle..

[Howard LewisParticipant @howardlewis46836]

At school we were persuaded to follow through the proof, that for maximum range of a projectile, the launch angle should be 45 degrees, (as has already been reported, so practice seems to confirm the maths).

As soon as you add wings, the lift / drag ratio intrudes into the maths and aerodynamics

The propulsive effort on is based on the pressure difference between the contents of the bottle and the atmosphere outside, so maximum range, I would suggest maintaining that force at as high a level as possible greater than the drag forces.

This seems to have happened already judged by the structural failures!

The optimum would seem to be just strong enough to withstand the drag and lift forces, so the aim should be to minimise drag from any source, whether shape, surface finish, aerofoil section or angle of incidence.

Less parasitic drag ought to allow a reduced angle of incidence, and so less drag from theat source. Could be a virtuous circle, but don't believe in perpetual motion or anything like that1

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