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Homemade Rocket Hits Record-Breaking 2,723 Feet Using Only Water and Air

posted by cmn32480 on Sunday October 11 2015, @06:17PM   Printer-friendly
from the damn-thats-high dept.

Phoenix666 writes:

There's a fairly robust community that builds and launches water rockets. You know, regular rockets in pretty much every way except for the fact that they use pressurized water and air to launch into the sky. One such water rocket from a South African team of students​ at the University of Cape Town just broke the world altitude record with it's most recent attempt.

The university team managed to grab the record thanks in part to an extremely lightweight frame. The rocket's frame weighs just over three pounds counting the on-board camera, parachute system, flight computer, and carbon fiber skeleton. The team had tried for the record two times before but had been plagued by equipment failure and air leaks. The third time took.​

The group launched their rocket, which clocks in at around nine feet in height, two different times on August 26th to get an average between the two. The first made it to 2,740 feet and the second to 2,707 feet for a world record of 2,723 feet, averaged. That handily beat the previous 2,044 foot record, set in 2007 by a U.S. group​.

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  • (Score: 3, Informative) by khallow on Monday October 12 2015, @01:05AM

    by khallow (3766) Subscriber Badge on Monday October 12 2015, @01:05AM (#248228) Journal
    Water-air rockets are governed by the rocket equation - delta v is proportional to the log of the mass fraction used (starting mass divided by final mass) and the exhaust velocity of the propellant. So if you can achieve a certain final velocity with your rocket, given 90% of the rocket's mass is used up, then you can get double the final velocity with 99% of the rocket's mass used up and triple the final velocity with 99.9% of the rocket's mass used up. If the exit velocity is low, you end up with a ridiculous mass fraction to get the desired final velocity. For example, a kerosene-LOX engine has an exhaust velocity of around 3000 m/s. Reading around, I see a calculation for one water-air rocket of 20 m/s. That means that for the same mass fraction consumed, the water-air rocket in question would reach delta v less than 1% of the delta-v of the kerosene-LOX engine. There probably isn't enough water in the world at that exhaust velocity to put a 3 pound rocket into orbit. Maybe I'll calculate it later.
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  • (Score: 3, Informative) by hankwang on Monday October 12 2015, @07:27AM

    by hankwang (100) on Monday October 12 2015, @07:27AM (#248306) Homepage

    20 m/s delta v for water sounds rather low. The exhaust speed is sqrt(2p/rho) with p the pressure and rho the density. With p in bar, it's Dv=14 [m/s] sqrt(p/[bar]). The pressurized air takes negligible mass and you surely can get more than 2 bar air pressure in a lightweight container.