Water rocket

Soap is also sometimes used to create a dense foam in the rocket which lowers the density of the expelled reaction mass but increases the duration of thrust.

Typically a single polyethylene terephthalate (PET) carbonated soft drink bottle serves as the pressure vessel.

Because water is essentially incompressible the divergent section does not contribute to efficiency and actually can make performance worse.

The increase in stability which well-designed fins give is worth the extra drag, and helps to maximize the height to which the rocket will fly.

[4] Stabilizing fins cause the rocket to fly nose-first which will give significantly higher speed, but they will also cause it to fall with a significantly higher velocity than it would if it tumbled to the ground, and this may damage the rocket or whomever or whatever it strikes upon landing.

Rubber bumpers, Crumple zones, and safe launch practices can be utilized to minimize damage or injury caused by a falling rocket.

Another possible recovery system involves simply using the rocket's fins to slow its descent and is sometimes called backward sliding.

[21] The Guinness World Record for the largest water rocket, measuring 7.72 metres (25.3 ft) tall and 72.5 centimetres (28.5 in) in diameter, is held by the NPO Showa Gakuen (Japan).

[22] The previous record holder was National Physical Laboratory (United Kingdom) with a water rocket measuring 3.40 metres (11.2 ft) tall and 40 centimetres (16 in) in diameter.

[23][10] The current record for greatest altitude achieved by a water and air propelled rocket is 2,723 feet (830 meters),[24] held by the University of Cape Town,[25] achieved on 26 August 2015, exceeding the previous 2007 record of 2,044 feet (623 meters) held by US Water Rockets.

[26] The rocket also carried a video camera as payload as part of the verification required by the competition rules.