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A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The origin of the term comes from nautical vocabulary, where to sound is to throw a weighted line from a ship into the water to gauge the water's depth. Sounding in the rocket context is equivalent to taking a measurement.
The rockets are used to carry instruments from 50 to 1,500 kilometres (31 to 930 mi) above the surface of the Earth, the altitude generally between weather balloons and satellites (the maximum altitude for balloons is about 40 kilometres (25 mi) and the minimum for satellites is approximately 120 kilometres (75 mi)). Certain sounding rockets, such as the Black Brant X and XII, have an apogee between 1,000 and 1,500 kilometres (620 and 930 mi); the maximum apogee of their class. Sounding rockets often use military surplus rocket motors. NASA routinely flies the Terrier Mk 70 boosted Improved Orion lifting 270–450 kilograms (600–990 lb) payloads into the exoatmospheric region between 100 and 200 kilometres (62 and 120 mi).
A common sounding rocket consists of a solid-fuel rocket motor and a science payload. The freefall part of the flight is an elliptic trajectory with vertical major axis allowing the payload to appear to hover near its apogee. The average flight time is less than 30 minutes, usually between five and 20 minutes. The rocket consumes its fuel on the first stage of the rising part of the flight, then separates and falls away, leaving the payload to complete the arc and return to the ground under a parachute.
Sounding rockets are advantageous for some research due to their low cost, short lead time (sometimes less than six months) and their ability to conduct research in areas inaccessible to either balloons or satellites. They are also used as test beds for equipment that will be used in more expensive and risky orbital spaceflight missions. The smaller size of a sounding rocket also makes launching from temporary sites possible allowing for field studies at remote locations, even in the middle of the ocean, if fired from a ship.
Sounding rockets are commonly used for:
The term sounding rocket is sometimes colloquially used to refer to the firework rockets used during the Diwali festival in India. The reference derives from these rockets making a high-pitch whistling sound during their ascent.