'Near space' similar to Moon?

sylvie369 said:

The equations for figuring out the force of gravity at various altitudes are pretty easy, and it's kind of instructive to run some numbers. Here's a page giving an example: https://www.mansfieldct.org/Schools/MMS/staff/hand/lawsgravaltitude.htm.

Jonny's point about the force of gravity 200 miles up is interesting. Most of your friends, family, teachers and so on probably believe that the reason things float around in the ISS is that "they've escaped the Earth's gravitational field" because they're so far up. But they're NOT that far up, and they haven't even come close to escaping Earth's gravity.

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Boom said:

I'm 15 years old and very interested in rockets. My father had a hard time with this one and recommended I ask here.

I notice rockets launching from the surface of the Moon are less substantial than those from Earth. I understand this is because of the reduced atmosphere and gravity.

My question is, how does that compare to Earth's 'near space'? What is the gravity and atmosphere like 60 miles up? The recent RedBull stunt got me thinking.. If it was possible to launch rockets from weather balloons at this elevation, how much smaller could they be?

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jadebox said:

A rather small rocket can reach space if launch from a high-altitude balloon. The main reason is that it doesn't have to fly through all that air below it to get started.

Researchers used "rockoons" (rocket balloons) back when sounding rockets were something new and many amatuer (and some professional) attempts to launch rockets into space are using them now.

But, there are many challenges to launching a rocket from a balloon.

A balloon doesn't provide a very stable platform. It's constantly moving. And, especially at high altitudes, it's hard to determine the orientation of the balloon. It's hard to point the rocket in the correct direction and it's hard to know which way to point it.

Igniting a rocket motor at high altitudes is challenging. The normal methods that that are used on the ground for igniting solid propellant and hybrid motors don't work well in the low pressure environment. Although the igniters are like rocket motors in that they don't need oxygen from the air to burn, the lack of air means that the igniter must be in direct contact with the propellant since there's no air to transfer the heat.

Once the rocket motor is ignited, it's difficult to ensure that the rocket goes the desired direction. At high altitudes, there is not enough air to make the fins of a rocket work effectively. So, a more complicated method must be used to ensure that the rocket travels in the correct direction and is stable.

But, as I said, there are many, mostly amateur, teams exploring this method of launching rockets to high altitudes.

Here are some links to articles about a few of the projects:

https://www.newscientist.com/blogs/shortsharpscience/2010/10/romanian-moon-balloon-gets-fir.html

https://wiki.seds.org/index.php?title=ISU_Rockoon_Project

https://www.jpaerospace.com/rockoons.html

Do an internet search for the word "rockoon" to find more articles about using balloons to launch rockets. You might also search this forum since there have been discussions with people involved in some rockoon projects.

-- Roger

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MaxQ said:

The phrase "falling around the earth" is also a pretty good description.

Just as an aside, I've had some interesting conversations with people (not rocketeers) adults and kids - that think going into orbit (and experiencing weightlessness) is a function of altitude, when in fact it is more so a function of speed (ie. "escape velocity").....granted getting above most of the atmoshere helps in dealing with the frictional heating produced by speed.

I'm sure you have all heard someone say something like: that thing went so high it must have gone into orbit....

I'm sure you've seen the trajectories of many sounding rockets in various books which kind of explains at a glance the fact that there is more to going into orbit than merely getting altitude.
Many sounding rockets were portrayed in comparative illustrations and graphs showing balloons, X-15s, space capsules etc., with the sounding rockets going very, very high but only achieving a ballistic trajectory and not going into orbit.

It's that velocity thing....got to have that speed to extend that ballistic trajectory and fall all the way out around the earth....

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