Water rocket
A water rocket is a rocket that ejects water and carries its energy in the form of compressed air or hot water.
It is propelled by a jet of water that is forced through a nozzle under pressure and leaves the rocket with an opposite impulse . Depending on how this pressure is generated, a distinction is made between cold water rockets and hot water rockets; the latter are also known as steam pressure rockets. What both have in common is that the propulsion energy is carried in physical form and not as chemical energy, as is the case with most rockets .
Cold water rockets are also offered in simple form as toys , serve to demonstrate the rocket principle and are mainly used in the leisure sector. Hot water rockets are mainly found in university research and development.
Cold water rocket
A cold water rocket is a water rocket in which the rocket body, which is partially filled with water , is propelled with the aid of the previously compressed air above it . When the nozzle is released, the water exits the rocket body at high speed with the help of compressed air and accelerates it through its recoil .
Due to their lower performance, cold water rockets are used primarily in the leisure sector as a hobby or in school lessons as a demonstration model to illustrate the recoil drive.
Water rockets, available as model construction and toy items, are inflated with a bicycle air pump and can reach a peak height of up to 80 meters. For beginners and hobby pilots, there are already half-finished kits from mostly Asian manufacturers.
As pressure body for self-construction of cold water rockets are suitable PET - bottles . These hold partially 8 bar ( disposable -PET bottles) or even up to 20 bar (burst pressure of multipath -PET bottles) pressure from. With several extended and combined PET bottles ( boosters ) heights of up to 830 meters can be reached.
The return of smaller water rockets from disposable PET bottles is largely safe, even without a parachute , as long as no massive payloads are transported. Since the overpressure in the bottles as well as the thrust and acceleration are considerable when starting, handling it remains dangerous. The launching rocket takes an uncontrollable trajectory and can hit spectators, for example. A disposable PET bottle without a screwed-on, narrowing nozzle develops 213 Newtons of thrust at 6 bar and thus, depending on the payload, a starting acceleration of over 15 times the acceleration due to gravity , which even increases after a few meters.
Construction
You can build a cold water rocket yourself from a PET bottle. One-way bottles are used, but also the thicker-walled reusable PET beverage bottles. A plug-in coupling for garden hoses and its counterpart is often used as a starting mechanism. A large number of building instructions for rockets and other launch mechanisms can be found on the Internet. Extended rockets or multi-stage models are described with spacers.
To increase the compressive strength, wrappings of the cylindrical sections of the PET bottles are described, for example made of glass fiber fabric. Hand air pumps or compressors are used to compress the air.
In order to achieve a more stable flight, fins / guide wings are also glued to the rocket, which partly cause a stabilizing twist by being tilted .
The bottom of the bottle flying ahead is sometimes (especially with the heavy reusable PET bottles) protected against too hard an impact with a tennis ball cut in half.
As early as 1975, before PET drinking bottles appeared, there was a water rocket as a school teaching aid. The approximately 20 cm long tank was about 6 cm in diameter halfway up, was welded together from the upper and lower part and ran up to a buffer and down to the nozzle symmetrically and arched to a rather pointed end. The upper half was cloudy and transparent, the lower half, including the three guide surfaces protruding from the bottom, dark red. The nozzle was hooked into the launch base, the rocket was fed with compressed air with the hand pump protruding to the side, launched by hand with the trigger in the open air and rose about 20 m.
Amount of water
Without water, the rocket is already very light at launch and the air flows out very quickly, but the mass that is ejected is only a small fraction of that of a water filling. (Rule of thumb: air at 1 bar has 1/800 the density of water; 8 bar: 1%; 2% compared to water filling to half the volume)
If 8 bar excess air pressure is used and only 1/9 of the bottle volume is reserved for compressed air, it will have already relaxed from 9 bar to 1 bar absolute pressure when the last portion of water is expelled, i.e. the last water will no longer accelerate. Air also cools down a bit when it expands.
As a rule of thumb, the filling of the “pressure vessel” of the rocket to 1/3 of the volume with water before it is locked on the launch pad and inflated with air has proven itself. With this amount of recoil agent one moves in the range of a broad maximum of the achievable height.
With computer programs, the flight of a rocket can be calculated from the following parameters:
- Pressure tank volume
- Start air (over) pressure
- Net weight (vulgo: weight) of the rocket including payload
- Nozzle inside diameter
- Water loading
Liquid density
The liquid density - assuming the same viscosity - has only a relatively minor influence on the height of rise. Calculations are complex here. With alcohol and sugar water, densities of 0.8 and 1.25 kg / liter can be achieved.
Alternative recoil media
Bread crumbs or granulated sugar (density about twice that of water) are pressed to the nozzle by gravity and rocket acceleration and loosened in the nozzle by the expansion of the air while flowing through the powder matrix, individual grains are loosened and blown downwards. The process leads to a liquefaction of the air-powder mixture in the nozzle. Fine noodles in water also work.
The non- Newtonian liquid made from corn starch, which solidifies under shear stress, clogs the nozzle, flows out very slowly, and leads to a later start due to the recoil of the air supported only by small liquid residues.
Videos and pictures from the startup process
- Series of images from a high-speed video of the launch of a cold-water rocket.
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Hot water rocket
A hot water rocket (also called a steam pressure rocket) is a water rocket in which the water in the rocket's body is heated to a high temperature (around 250–500 ° C) with the help of mostly electrical heating. The water, which remains liquid due to the pressure in the closed water tank, is let out through the thrust nozzle released for this purpose with a valve, evaporates in this and emerges from the nozzle at high speed. The recoil accelerates the rocket against the outflow direction.
The water tank of water rockets must be able to withstand high pressure and high temperature.
Hot-water missiles are occasionally used as a jump starter and for experimental purposes. They are superior to the cold water rockets, because the ejected mass itself contains the energy and the thrust remains almost constant until the "burnout".
However, the performance of solid and liquid fuel rockets is much higher.
Return / recovery systems
For larger water rockets or their payloads, it is advisable to install a parachute. This is necessary for reasons of security and intrinsic value, for example when electronics (height measurement / altimeter ), video camera and batteries are carried.
The parachute is triggered, for example, with spring mechanisms from wind-up toys ( Tommy Timers ), timers or an altitude or time-controlled servo release by a microcontroller .
Of course, depending on the mass and value, all other common recovery systems are also possible, such as u. a. Swivel wing, rocket glider, flutter tape .
Records
The Water Rocket World Achievement World Record Association (WRA2) has set up an extensive set of rules, including a class system, for setting altitude records with cold water rockets.
Within Germany, members of the Rocket Modeling Forum developed their own set of rules, which is based in part on the WRA 2 system, but also takes into account German records that were set up before the set of rules was established.
- The “Ascension III” model reached a height of 830 m (2,723 feet ) on August 26, 2015 and thus holds the current record of the WRA2. The model belongs to class A (single stage, cold water rocket, air as a compressed medium).
- On May 6, 2016, the “Überflieger Mini Evo” model reached a height of 290 m and thus currently holds the current German record.
See also
Web links
- H. Adirim: Performance limits of hot water rockets (PDF; 174 kB) Student thesis at the Institute for Aerospace at TU Berlin
- Water Rocket engl. detailed instructions
- Burst pressure test 1l Coca-Cola bottle burst test
- The water rocket (PDF) physikfuerkids.de, 14 pages, practical instructions, sketches, formulas
- High speed video of a water rocket launch
Individual evidence
- ↑ AQUARIUS hot water rocket projects
- ↑ Korean water rocket "Shootinger"
- ↑ Extend the PET bottle by sticking it on YouTube
- ↑ Gigant Water Rocket 2008 on YouTube
- ↑ Official Water Rocket World Record Standings. Retrieved May 9, 2017 (American English).
- ↑ Gardena coupling
- ↑ Advanced water rockets | Raketfued. (No longer available online.) Archived from the original on May 9, 2017 ; accessed on October 24, 2019 .
- ↑ How much water to use in a water rocket YouTube, Video 4:28, March 2, 2016, AirCommandRockets; accessed October 20, 2016.
- ↑ Water Rocket Liquid density experiment YouTube, Video 6:49, January 1, 2016, AirCommandRockets; accessed October 20, 2016.
- ↑ Water Rocket Alternative Fuels YouTube, Video 4:56, November 15, 2011, AirCommandRockets; accessed October 20, 2016.
- ↑ Oobleck in Water Rockets YouTube, Video 4:56, April 14, 2016, AirCommandRockets; accessed October 20, 2016.
- ↑ vapor pressure table . Wikibooks
- ↑ Picture of a "Minialt" altimeter ( memento from July 24, 2011 in the Internet Archive )
- ↑ Water Rocket Super Computer ( Memento from May 15, 2009 in the Internet Archive )
- ↑ electronic parachute opening on YouTube
- ↑ hgv.purespace.de (PDF; 316 kB)
- ^ The Water Rocket Achievement World Record Association. Retrieved May 12, 2017 (American English).
- ↑ Forum - set of rules: establishment of water rocket height records in Germany. In: Raketenmodellbau.org. Retrieved May 12, 2017 .
- ↑ youtube.com
- ^ The Water Rocket Achievement. World Record Association
- ↑ Our water rockets. Raketfued, accessed May 12, 2017 .
- ↑ German water rocket record 2016 (290m) - launch day No. 45. Raketfued Rockets, July 13, 2016, accessed on May 12, 2017 .
