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Water Rocket Glider

  1. Jul 26, 2011 #1
    Hi, we are doing a project in school where we have to launch a water-rocket at 45 degrees and get it the furthest distance from the launch pad with the 'egg-stronaut' still intact.

    Here is my idea. We make the wings in the shape of a glider, similar to the shape of the space shuttle's wings and add a crumple zone into the nose cone so that it absorbs most of the impact in the case of a bad landing.

    The thing is that I am only a high school student and have no idea of where to start to calculate the wing size/surface area/shape. The rocket is a standard 1.25 litre coke bottle and will be filled with approx.420ml of water. can you provide me with the details to create wings of the right size so that the rocket can land at an adequate/safe speed and glide the furthest distance. This is due in 1 week so please answer quickly.

    P.S Post any other suggestions you may have. More than 1 way to skin a cat they say, and i may be completely wrong so correct me if I am.

  2. jcsd
  3. Jul 26, 2011 #2
    Short answer, you need an airfoil.
    Long answer, and with any luck you've had some basic physics and know some of what I'm talking about:
    Generally speaking, four forces are at work on any given aircraft:
    Gravity: the mass of the craft times the acceleration of gravity
    Thrust: in your case a result of expelled water
    Drag: the force of air pushing back against the wing and other surfaces
    Lift: The force perpendicular to drag, which generally speaking, fights gravity

    You have no control over gravity, and little or no control over thrust. That leaves us with Drag an Lift to play with. We want to maximize lift while minimizing drag. You control the two by varying something called angle of attack, the angle between the wing and direction of travel through the air. http://en.wikipedia.org/wiki/Airfoil" [Broken], specifically the graph relating angle of attack to lift and drag. There's a sweet spot there that you want to aim for.

    Now, there's no way you're going to be able to get the design anywhere near optimal with the time and other restrictions you have. The biggest thing is that you need to keep the nose oriented in the direction of travel, otherwise your angle of attack will be off and you'll stall. Stalling would be bad for two reasons, first, you loose the glide part of the trajectory, and second, the egg will smash. I think canards and tail fins are the way to go there. As far as size, look at pictures of a 747 and try to make things proportional.

    Lastly, and I speak from experience here, the best way to save the egg is a few inflated zip-lock sandwich bags. They have negligible weight, and distribute the impact very symmetrically about the egg. I'd say place two in front of the egg, and three in a triangle around the egg.
    Last edited by a moderator: May 5, 2017
  4. Jul 26, 2011 #3


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    I just looked up diet coke and mentos rocket on youtube, and they seem to be stable once they are in the air, and while they still have 'fuel' left. (Once they run out of fuel, they seem to pretty much fall out of the sky). But If you're doing it at 45 degrees, hopefully it will come down nose-first.
    Looking at those videos on youtube, I'd say your biggest problem is protecting the egg, and keeping it attached to the rocket. Although maybe your rocket won't have as much thrust as a diet coke and mentos rocket?
    My general advice is to keep your thinking practical and common-sensical. In this situation, air resistance is one of the biggest factors, which is not easily modelled by physics. Probably the best way would be to try out a few different designs, then choose the best one. But I'm guessing you don't have the time/resources to do that..
  5. Jul 26, 2011 #4


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    If it's allowed you could try attaching the bottle to a model glider, although that's somewhat expensive. Again if it's allowed, you could use a radio controlled glider, or at least use one for testing the theory. For free flight (no radio control), the glider would need a polyhedral (bent wings) for roll stability, and be trimmed for pitch stability (center of mass a bit in front of center of lift, with some up elevator to compensate).
  6. Jul 27, 2011 #5
    Thanks guys. You've helped alot. How about making the rocket come down with a soft landing without the actual 'glide to a further distance'. Could I use wings without airfoils to achieve this? IF you need clarification I mean something like a paper plane which sort of 'glides' but doesn't actually stay on a level flight.
  7. Jul 27, 2011 #6


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    Paper planes and small balsa gliders use flat wings, and they glide reasonably well. I'm not sure how you'll be able to trim the wings to prevent the rocket from rolling (rotating on it's primary axis). If you can prevent the roll, you'll need something like an horizontal stabilizer (elevator) to keep the rocket from pitching down and nose planting.
  8. Jul 27, 2011 #7
    So you think it'll work? I mean, ill use flat polyhedral wings to prevent rolls and use tail fin to keep it going straight and some canards for horizontal stability. This should come down like a paper plane right?
  9. Jul 27, 2011 #8


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    Yes, but getting such a system trimmed out would probably involve having to rebuild stuff after each crash landing.

    The end result will be a bottle powered glider. You could also consider a setup to detach the bottle from the glider once it stops generating thrust to reduce drag, since the goal is to carry the egg and not the bottle.

    You didn't mention if the rules would allow a bottle powered glider.
  10. Jul 27, 2011 #9
    Yeah were allowed the glider but the egg had to stay on the rocket part of the glider. So no detaching. And the trim adjustment wont be a problem because we get plenty of test flights.

    I'll just cut some flat wings from a sheet of Styrofoam.
  11. Jul 28, 2011 #10


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    Sounds like a lot of drag. Why not get a plastic egg warmer and attach it firmly to the bottle? Eventually fill it with paper so the egg is fixed within the warmer.

  12. Jul 28, 2011 #11


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    Actually this can be tricky, because you can have very different velocities during the different phases of the flight. If you trim it for optimal glide, it might over pull during the powered phase.
  13. Jul 28, 2011 #12


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    An simpler configuration might be just four big delta fins at 90° (like a glide bomb). Then you don't have to worry about rolling stability. Here what I would try:


    Attached Files:

    Last edited: Jul 28, 2011
  14. Jul 28, 2011 #13
    Why don't you stick the aerodynamics to the egg itself (inside one of those cups), then the bottle is basically just a device to get the egg as high as possible? Once the thrust runs out the bottle and egg will fall separately, so if you put some decent wings and a nose/tail on the egg, it should hopefully glide a lot further in the right direction.
  15. Jul 28, 2011 #14
    Yeah, I was thinking about making it like a glide bomb. I would buy a spare wing for the plane in the picture below (because I used to own one and know for a fact that it glides well enough), cut it to size and glue to my rocket. Tthis would be more like a http://en.wikipedia.org/wiki/HOPE/HOSBO" [Broken] glide bomb.

    [PLAIN]http://www.rc-airplane-world.com/images/yellow-bee-electric-rc-plane-21296156.jpg [Broken]

    I am a member of many forums and never in my entire life, have I got this many replies on my first post. You guys are legends!
    Last edited by a moderator: May 5, 2017
  16. Jul 28, 2011 #15


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    The bottle + egg might be heavier that this plane, so you will need more speed. The transition to glide flight without stalling might be critical here. Can you use angles lower than 45°

    Useless nonsense like this is the most fun. Post a video of the trials.
  17. Jul 29, 2011 #16
    I think your going to end up ripping the wings from bottle during thrust. You could buy the wing, cut it in half hinge them so they pop out at 90 degrees from the fuselage of the bottle. Kind of like when a fighter plane untucks it's wings when it comes out of mach, to make it more stable at slower speeds. If you use a strong elastic band and some kind of clip to keep the wings locked while it lands it should work. You might need to attach some fishing wire to release the mechanism, then tie the wire to the ground. when the bottle gets a certain distance the wire goes taught and pulls the release mechanism, the wings then pop out and lock in place and the bottle can glide down.
  18. Aug 12, 2011 #17
    Okay, so we had the launch. Finally after a few weeks of our teacher being sick and on leave we got the opportunity to launch our rockets. We decided to go with flat delta wings and tail fins to keep the rocket gliding. On the first 2 test flights on previous days, the trim was not adjusted correctly and the angle of 45 degrees was too high, so, on the final launch, I had to adjust the trim by feel.

    The first group to launch got their rocket really far, but the egg broke so they were disqualified. The second group got really far also and their egg stayed intact. This made me think our group would lose because of our fails on the previous days.

    When our turn came to launch, I adjusted the trim to the way I felt it would counteract the thrust from the rocket and at the last moment adjusted the angle to 30 degrees.

    Guess what? our group won by about 30cm and the egg stayed intact. Thanks for all your help guys. Sorry I couldn't post a video but thanks anyway.
  19. Aug 12, 2011 #18


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    Yeah, as I expected those are the tricky paramaters, that have them most impact, regardless of how well designed the rocket-glider is.

    Awesome! Did you put it into a plastic egg warmer? This would almost seem like an unfair tactic, because hardly any drop would destroy it.

    Thanks for the feedback.
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