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What are the differences between a sounding rocket and a suborbital flight?

Sounding rockets have long been used for science purposes. Some of them have trajectories elliptic enough to do microgravity research during a few minutes. Sub-orbital is a term which mostly seems to be used when talking about crewed flights. Is the difference simply that sub-orbital flights are optimized for carrying a crew? Are sounding rockets too rough or small for carrying humans?

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Some sounding rockets are sub-orbital. Sub-orbital merely means going into space with less than orbital velocity. Going into space: That's a bit arbitrary, but a person who goes 50 miles or higher gets astronaut wings; another definition is the nice round figure of 100 kilometers (~62 miles) in altitude; that's the Kármán line. Yet another is the nice round figure of 400,000 feet; that's "entry interface" for NASA vehicles that reenter the atmosphere.

By whatever definition one uses, some sounding rocket flights are less than sub-orbital. Sounding rockets sent into the stratosphere may top out at much less than 50 miles altitude. A balloon is cheaper than a rocket, but balloons can only go to an altitude of 31 miles / 50 kilometers or so. If your experiment needs to go higher than that you need to use a sounding rocket.

As far as acceleration is concerned, sounding rockets go whoosh! and they're gone. However, their payload capacities are rather small. They aren't intended for carrying massive people and their even more massive environmental control and life support systems.

See the NASA Sounding Rocket Handbook for more info on sounding rockets and their capabilities.

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Very few of the things you describe are inherent to sounding rockets per se, rather that is how they have been to date. I.e. I do not think it is a definitional thing, rather an experiential thing. As you suggest, it is kind of a wishy washy definition. –  geoffc Mar 11 at 13:34
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If the sounding rockets go past the Kármán line at 100 kilometres (62 mi) above the mean sea-level that's now officially the edge of space since the Outer Space Treaty sort of ratified that, there is no difference, and they're all sub-orbital spaceflight. Meaning, they don't reach required orbital velocity (around 8 km/s) to stay in orbit and their flight is on a ballistic trajectory. If they don't even reach the boundary of outer space, then they don't really qualify as spaceflight and are just missiles.

So while suborbital spaceflight and sounding rockets aren't really directly comparable, and the former defines the speed and achievable trajectory and the latter identifies its purpose and often also size with usually small payloads for short-duration microgravity and/or upper atmosphere / ionosphere environment experiments, most sounding rockets are indeed suborbital. But it's not necessarily the other way around and not all suborbital spaceflights are sounding rockets, they could for example also be manned suborbital spaceflights, like say certain space tourism vehicles or point-to-point suborbital transportation routes.

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