Suppose NASA, SpaceX or any other aerospace company has managed to build a technology that allows them to fire a rocket to the moon, from "space". Basically, they have created a huge weather balloon that can reach altitudes so great that atmospheric pressure is effectively zero compared to ground-level - "space".

Considering that the shape of the rocket does not matter (it doesn't have to be shaped like a cylinder with a nose cone etc.) because it isn't affected by air drag, what would a rocket launched from "space" look like?

Note: The highest point where the weather balloon (launch site) can reach, still has gravitational force on the rocket. Approximately as much as gravity would affect it at ground level.

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    $\begingroup$ For the current space rockets, first stage separation occurs at altitudes of 40+ km. This means (assuming second stage is able to go into orbit without a running start) you still have to lift at least 10+ ton second stage to the stratosphere. The balloon will have to be enormous. $\endgroup$ – Alexander Mar 26 '18 at 17:29
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    $\begingroup$ FYI in terms of size, it would just about the same. what-if.xkcd.com/58 - the problem isn't getting to space, it's staying there. $\endgroup$ – Rob Watts Mar 26 '18 at 18:55
  • $\begingroup$ @RobWatts Getting back safely is also a bit of an issue. $\endgroup$ – JAB Mar 26 '18 at 22:50

We actually did it. Six times we went to the Moon, and six times we launched a rocket from the Moon's surface into space, to return the astronauts to the Command Module waiting for them on Lunar orbit. It looked like this:

Lunar Module ascent stage

Apollo 17 Lunar Module Ascent Stage. Picture by NASA, available on Wikimedia. Public domain.

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  • $\begingroup$ This is an awesome answer! $\endgroup$ – The Square-Cube Law Mar 26 '18 at 18:43
  • $\begingroup$ The whole stack was launched from Earth orbit to the moon, and the Command & Service modules were launched back to Earth. $\endgroup$ – jamesqf Mar 27 '18 at 4:26

First, a balloon cannot get to 0% air pressure. Balloons float in the air.
But you can have a ship built from small pieces in orbit, either by using materials shipped up in a space elevator, or harvested from asteroids, or salvaged from derelict satellites. So I will assume one of those scenarios.

You are right that lack of air drag means no need for a smooth aerodynamic shape.

But you will still need to deal with acceleration from the ship's engines, which has an effect similar to gravity. Your ship will have to be like a house or tower - the (main) engines are the foundation, and everything will have to be on top of that, or supported so it does not fall off or cave in when the engines fire. You cannot have airy-looking sprawling solar arrays. If you were to take the current ISS and accelerate it with a single engine, the solar arrays would fall off, followed by modules on the sides. If you look at lunar landers from the Apollo program, they did look a bit like a pyramid.

You can have multiple engines supporting a sprawling structure, maybe like a castle with multiple towers, but then you would need to balance the thrust from each engine to the weight of the module that it pushes.

Finally, you can have very weak acceleration, e.g. from ion thrusters or a solar sail. Then you can have "airy" structure with weak supports; I wouldn't call it a "rocket" anymore, though.

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  • $\begingroup$ First, balloon cannot get to 0% air. B/c balloon is floating in the air. So what happens if the balloon is so well engineered that doesn't pop on low pressure levels? Will it just hover around at the top of the atmosphere? That would mean there's no more air around. $\endgroup$ – Coto TheArcher Mar 26 '18 at 17:42
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    $\begingroup$ @CotoTheArcher - balloons work on the principle of buoyancy, like things floating in water. They only rise as long as the density of the air around them is greater than the density of whatever is filling them. So unless your balloon is full of vacuum (and this is, of course, neglecting the weight of whatever it's lifting), you're not going to have a balloon make it to 0 atmospheric pressure. You can get to the mesosphere, but it would, as has been pointed out, need to be a gigantic balloon. $\endgroup$ – jdunlop Mar 26 '18 at 18:00
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    $\begingroup$ "you can have very weak acceleration" - not true. The spaceship still has to reach orbital speed from near standstill. With low thrust engines, the ship will simply fall down to Earth. $\endgroup$ – Alexander Mar 26 '18 at 18:18
  • $\begingroup$ @Alexander. I do assume that the ship is built in orbit, so it has orbital speed to begin with. $\endgroup$ – Bald Bear Mar 26 '18 at 19:29
  • $\begingroup$ You could accelerate the ISS with a single engine, as long as the acceleration is low. In fact, this is regularly done - see space.stackexchange.com/questions/9087/… - otherwise the orbit would eventually decay and it would fall to Earth, as Tiangong 1 will do in a few days. $\endgroup$ – jamesqf Mar 26 '18 at 19:40

Orbit is a function of velocity as well as altitude

The Reason balloon/rocket orbital launches aren't made is because no matter how high you get your rocket, you still have to get it moving with enough velocity along the proper vector to circularize an orbit. To be sure, balloons with sounding rockets attached are a great way to break out of the atmosphere, but they are a terrible way to achieve an orbit. Achieving a high enough altitude to leave the atmosphere is actually quite easy, but if the proper velocity is not gained in doing so then the object will simply begin plunging back to earth in a ballistic trajectory. About 9.4 km/s delta-v is the minimum required to establish a stable orbit. This means that even if you float it up there on a balloon your rocket isn't really going to end up any smaller or cheaper than a regular ground launched one, its going to have to burn nearly the same amount of fuel (not enough less to offset the cost of your gargantuan balloon) except now its for the purpose of gaining velocity instead of altitude.

So, to answer your question, unless you achieve enough velocity to establish an orbit a rocket launched from a balloon will look like this: enter image description here

If you DO have enough velocity to establish an orbit it will look the same as a regular rocket launch, it will just have a higher altitude for its starting point.

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  • $\begingroup$ You're missing the point of the question: that the rocket is launched from high enough that aerodynamics are irrelevant. $\endgroup$ – Mark Mar 27 '18 at 22:07

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