back to article Firing a water rocket to 1km? Piece of cake

Our report yesterday that a water rocket formed from "2 x 2l fizzy drink bottles" may have ripped past an Airbus A321 departing Birmingham airport last year prompted some readers to mull the possibility of having a pop at the water rocket world altitude record. The current "Class A" (single stage) record stands at 825m (a …

  1. Unep Eurobats
    Mushroom

    Zoooosh

    Shirley the first rule should be that the hull is based on a commercially available fizzy-drink container?

    1. MyffyW Silver badge

      Re: Zoooosh

      My money is on a Diet Pepsi bottle, because sending it into space beats drinking the stuff.

      Irn-bru is made in Scotland, from girders, so probably wouldn't be appropriate.

      I'll resist any tasteless Morton Thiokol 7-Up jokes.

      1. Message From A Self-Destructing Turnip

        Re: Zoooosh

        That rule would rather defeat the key engineering task to the challenge, e.g. design an air receiver that will take the pressure needed to go the distance, without making it so heavy that it won't.

    2. Anonymous Coward
      Anonymous Coward

      Re: Zoooosh

      Does beer count as a fizzy drink? If so we can try a draft beer keg. May not go very high but at least we get to empty it first.

      And don't call me Shirley!

  2. Rich 11

    Zippy

    at this speed, it could cross a rugby field in three-quarters of a second

    Almost as fast as Jonah Lomu, back in the day.

    1. Vinyl-Junkie

      Re: Zippy

      Perhaps the width of a rugby field should be added to El Reg's units of measurement? The unit should, of course, be named the Lomu.

      1. 2460 Something
        Headmaster

        Re: Zippy

        They were talking about it's length rather than width. We already have appropriate measurements for this. It would have, of course, been significantly better if they had reported the distance travelled accurately though. This sloppy journalism mathematics isn't appropriate for El Reg.

        I have kept all to 4 decimal places for ease of typing. As the rocket was travelling at 550km/h (or 152.7778 m/s) in 0.75s it will have covered 114.5833m.

        A ruby pitch is between 112m and 122m, this article is suggesting that the speed could have been anywhere between 537.6km/h and 585.6km/h. Accuracy is key. It would have been much more appropriate to have stated that it would have covered a distance of 15 double decker buses (or 1 Brontosaurus) in a princely time of 0.9051s.

        I wonder why El Reg even maintains its conversion page, I really do.

    2. Doctor Syntax Silver badge

      Re: Zippy

      How long to cross Wales?

      1. Anonymous Coward
        Anonymous Coward

        Re: Zippy

        >How long to cross Wales?

        Who would want to? Wales is best left to one side.

        1. Steve Davies 3 Silver badge

          Re: Zippy

          Wales? Rugby?

          gotta be the side in a Gareth Edwards 'side-step' in that famous Ba-Ba's match.

      2. TRT Silver badge

        Re: Zippy

        By any mode of transit, fucking aaaaaaaaaages. The roads go round and round the mountains, the trains are non-existent except up the coast if you don't mind them being some miniature gauge, the bus follows the road, on foot you'll be cold, miserable and exhausted (see mountains), by air you have to go up and up to avoid the mountains and the rain clouds... really, it's not worth even setting out.

        1. MyffyW Silver badge

          Re: Zippy

          @TRT @Chris_W But at the end of all that you've reached Aberystwyth, the Las Vegas of Cardigan Bay, home to "Treadle Trollops", Druids and the resident of 22/1b Stryd-Y-Popty.

          1. Steve Davies 3 Silver badge

            Re: Zippy

            Aberystwyth? Ah, I remember it well as a child. Closed on Wednesdays even in Mid summer.

            As my family comes from Lampter we have a little local rivalry...

            1. MyffyW Silver badge

              Re: Zippy

              @Steve_Davies_3 - Lampeter? The third largest urban area in Ceredigion after Aberystwyth and Cardigan [according to Wikipedia].

              Well if that's not big city enough for those boys from the east I don't know what is ;-)

          2. Anonymous Coward
            Anonymous Coward

            Re: Zippy

            @MyffyW, Rob Brydon and Max Boyce, proof the Welsh do not have a sense of humour

            1. MyffyW Silver badge

              Re: Zippy

              @Chris_W have an upvote hun, just to prove we do :-)

      3. Stoneshop
        Devil

        Re: Zippy

        How long to cross Wales?

        Just tell it it's English.

    3. Trigonoceps occipitalis

      Re: Zippy

      Yes, but only when he was slowed down by half the English backs hanging onto him.

      "The man is unbelievable. He's very balanced, has that incredible power and anyone coming on to the ball like he did is almost impossible to stop."

      Sir Colin Meads

    4. Gavin Jamie

      Re: Zippy

      Impressed. Do they give the pressure required?

  3. chivo243 Silver badge
    Happy

    I had this as a kid

    http://www.retroland.com/water-rockets/

    It would go high enough that we couldn't see it anymore. A few times it landed on the neighbors roof...

    1. John Brown (no body) Silver badge
      Mushroom

      Re: I had this as a kid

      Back in the day when a standard type valve from a bike tyre was just the right size to jam into the nozzle of a washing up liquid bottle, Fairy Liquid being the best, and everyone had a bike pump, it was easy enough to make your own at no cost. Lego or meccano was useful for building launch towers too. It worked best with an adapter and dads car foot pump.

      It wasn't exactly predictable when it would launch since it depended how hard the valve was jammed in.so there were one or two "rapid unexpected dis-assemblies". In my defence, I was only about 9 at the time.

  4. Anonymous Coward
    Anonymous Coward

    Love the footer:

    Sponsored: Are you risking security with the cloud?

    (yes it does change)

  5. Vinyl-Junkie
    Headmaster

    I don't quite see....

    "the team made extensive and creative use of carbon fibre materials, due to their amazing strength"

    ..what the amazing strength of the team had to do with the materials used...

  6. Clive Galway
    Boffin

    "featherweight record-breaking rocket that is 2.68m tall yet weighs less than 1.5kg"

    "The rocket produced 550kg of thrust – enough to lift a small car off the ground – and blasted off to 550km/h in under 0.5 seconds"

    I am guessing the 1.5kg weight is without fuel - even so, 0 - 550km/h in 0.5s sounds like way too fast too soon. Wouldn't you be better off distributing that thrust over a longer time? Are they not wasting a lot of DV trying to go too quickly through the atmosphere?

    1. Mike Moyle

      Using a lower acceleration means that you're wasting impulse lifting the remaining mass of your fuel for a longer time, hence you'll get a lower altitude. Blasting it out all at once means that more of that potential energy stored as pressure is used to push the vehicle alone, getting you to a higher altitude.

      As long as you're not worried about the effects of too-rapid acceleration on fragile payloads (Fleshy water-bags like passengers, say) you're best off using as much of your launch fuel as possible to accelerate you as quickly as possible as early as possible in your flight.

      Note: I am not a rocket scientist.

    2. Paul Crawford Silver badge

      Not in this case. More generally there is an issue for rockets not wanting to reach too high a speed low down in thick lower atmosphere due to drag (and possible heating) but they ain't getting high enough for that!

      1. Clive Galway

        Draw a graph of atmospheric pressure over altitude. Is the line straight or curved?

        http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

        It's curved. Ergo, expending all your thrust at launch is not the most efficient way to gain altitude.

        According to that graph, at 1km up (Which is how high the vehicle got), there is ~2.3% less pressure than at sea level.

        "Using a lower acceleration means that you're wasting impulse lifting the remaining mass of your fuel for a longer time, hence you'll get a lower altitude. Blasting it out all at once means that more of that potential energy stored as pressure is used to push the vehicle alone, getting you to a higher altitude."

        If you expend 100% of your fuel in the first 1% of altitude, it means that you have to travel the remaining 99% (Through the least efficient part of the graph) with no power at all, and that you have way less inertia. So expending all the fuel at the start is not all good - there are up-sides and down-sides.

    3. John Robson Silver badge

      How do you propose to distribute the thrust?

      The only way to distribute the thrust is to reduce the pressure (so the water takes longer to be spat out of the 'fire end'). And that reduces the energy stored.

      Additionally, distributing the thrust means that you have to lift some of your reaction mass (water) - whereas dumping it all in the first 0.1s means that none of it is lifted beyond about 4-5m of the ground. That leads directly to more energy in the rocket.

      Yes, the atmosphere is a bit of a pain - I wonder if they can take a water rocket with them to the moon next time - see how well it goes there ;)

      1. The First Dave

        The best way to distribute the thrust is to use a tighter orifice.

        Since friction depends on the square of speed, I think some fairly complex calculations are required to decide whether or not max speed is better than prolonged thrust.

        1. John Robson Silver badge

          D'Oh - of course you are right - that is the way to distribute the energy.

          My brain got stuck on the new Ro-Kit I got for the kids (cough) recently.

          But on the basis that all the serious (single stage) rockets still use very short "burn" times, I'll stand by my suggestion that lifting the reaction mass is probably a bad idea.

  7. Yag
    Trollface

    "uses only water and air – very environmentally friendly"

    Let's not talk about the gasoline powered air compressor back there.

    1. Annihilator
      Headmaster

      Re: "uses only water and air – very environmentally friendly"

      "The water rocket competition is very exciting, as it uses only water and air – very environmentally friendly"

      Technically speaking, the Space Shuttle Main Engines only used water (well, its component parts anyway). Wouldn't call it environmentally friendly though.

  8. Anonymous Coward
    Anonymous Coward

    Oddly reminiscent...

    ... of my first girlfriend after a 6 year dry spell.

    1. MyffyW Silver badge

      Re: Oddly reminiscent...

      @AC is that you?

      Wasn't so much the altitude reached as the record-breakingly short countdown sequence, as I recall.

      1. TRT Silver badge

        Re: Oddly reminiscent...

        @MyffyW. I can't upvote you enough for that.

        1. MyffyW Silver badge

          Re: Oddly reminiscent...

          @TRT you are a total sweetie. Have an upvote on me.

  9. allthecoolshortnamesweretaken

    550 kg thrust

    is impressive, but would only lift a VERY small car - a Smart weighs in at 880 kg (empty).

    1. Sorry that handle is already taken. Silver badge

      Re: 550 kg thrust

      This one puzzled me too. Maybe a matchbox car? They're pretty small.

      1. Sgt_Oddball

        Re: 550 kg thrust

        A lotus/Caterham 7 weights in around that weight. Now those things shift even with the sort of weedy engine that annoy the most leisurely of Sunday drivers.

        1. Sorry that handle is already taken. Silver badge
          Headmaster

          Re: 550 kg thrust

          A lotus/Caterham 7 weights in around that weight.

          But it's not a small car...

    2. Stoneshop

      Re: 550 kg thrust

      More than enough to lift a Fiat 500 (the original): 499kg, 118.80952 jubs. It would also easily, although briefly, significantly increase its acceleration*.

      * They're remarkably spiffy when fitted with a Moto Guzzi engine, but still sluggish compared to a Goggo with a 9 cylinder radial.

      1. allthecoolshortnamesweretaken

        Re: 550 kg thrust / Fiat 500

        Fiat 500 - one of my favourites. The Abarth versions move very nicely, too. The Fiat 500 Giardiniera is the original minivan IMO.

        There is one original Fiat 500 that was recently converted to accommodate a V12 Lamborghini motor. Bit of an abomination, but fast...

        http://www.spiegel.de/auto/fahrkultur/fiat-500-getunt-mit-einem-motor-aus-einem-lamborghini-murcielago-a-901161.html

    3. CrazyOldCatMan Silver badge

      Re: 550 kg thrust

      > is impressive, but would only lift a VERY small car

      Indeed (looks meaningfully at Morris Minor outside, rusting gently in the sunlight).. 770Kg dry weight. Although I suspect that rust is slightly heavier than unnibbled mild steel.

      1. The First Dave

        Re: 550 kg thrust

        Rust is pretty close to half as heavy again as bare steel, until it falls off that is.

        1. Stoneshop
          Boffin

          Re: 550 kg thrust

          Rust is pretty close to half as heavy again as bare steel, until it falls off that is.

          Per unit volume, that is. But if you let a piece of steel turn to rust, it'll still contain the same number of Fe atoms, plus the now added O, so the total weight will have increased.

          Its weight will indeed be lower when it falls off, but only during the fall.

          1. Sorry that handle is already taken. Silver badge
            Headmaster

            Re: 550 kg thrust

            Per unit volume, that is. But if you let a piece of steel turn to rust, it'll still contain the same number of Fe atoms, plus the now added O, so the total weight will have increased.

            Hence "half as heavy again" I'd have thought.

    4. DropBear

      Re: 550 kg thrust

      "is impressive, but would only lift a VERY small car - a Smart weighs in at 880 kg (empty)."

      Nonsense - it's widely known that the only space-worthy car is the Robin Reliant...

    5. allthecoolshortnamesweretaken

      Re: 550 kg thrust

      And another thing, shouldn't one use a proper unit like Newton for this kind of thing?

      1. Stoneshop
        Go

        FTFY

        And another thing, shouldn't one use a proper unit like Newton for this kind of thing?

        s/Newton/Norris/.

  10. Anonymous Coward
    Anonymous Coward

    Not a cheap record to beat

    I'd budget at least £20,000 in manufacturing costs to build a rocket to *perhaps* beat the current water rocket record; a filament wound carbon fibre or aramid pressure vessel is almost certainly needed to contain the pressure and keep the weight down, given the thrust to weight ratio needed. That figure assumes the design is done for free, and that someone is available who is capable of designing the nozzle for the waterjet, which is really a job for an expert in the field.

    1. Anonymous Coward
      Anonymous Coward

      Re: Not a cheap record to beat

      Not sure about your pricing. Carbon fibre is really quite a home tech thing, you can buy a 2kg reel of carbon fibre string for 80 quid, a bucket of epoxy would hardly break the bank. There's some tricky tooling and you'd probably need to be able to borrow a decent lathe for the nozzle and cone, but again possible for most students although harder for the rest of us.

      I also wonder about the nozzle, given the reaction mass is an incompressible fluid is there very much you can do with it?

  11. Richard Parkin

    Rules don't say how much water

    Rules quote dry weight but say nothing about quantity of water, because water ...

POST COMMENT House rules

Not a member of The Register? Create a new account here.

  • Enter your comment

  • Add an icon

Anonymous cowards cannot choose their icon