Possibly, but judging by the fact it's at least 125 blocks tall or more, perhaps a bit less. But, perhaps half(ish) the size for planets not only far, but intense with gravity and weather so it can hold up and have enough fuel. Like Jupiter!
Yes, the size and also the materials would need to be considered.
Perhaps to simulate pressure, glass panes would break if there was oxygen on one side and not the other.
Getting back to rockets, the Space centre and the NPC's
I will outline the principles involved:
The Blueprints for the Rockets.
Each section of the Rocket has to be build from a blueprint, so for example
The Thrusters, at the rear of the rocket, will vary depending on the situation.
The Fuel Tanks. these will be containers made of a certain block, that is either filled by a liquid, or fuel cells. So you either pump the liquid in or recharge the fuel cells.
The Body will surround everything, and will be made of a different blocks, it will contain insulators to protect the player from heat of either the planets or re-entry.
The Cockpit, this will be the heart of the rocket, it will contain a few special blocks that control everything, these are where you tell it which planet to go to, (if the rocket can survive there) as well as control take off and landing.
The NPC's
Lets say that there is a 'leader' NPC, this guy will be the main person in the entire project, first of all you need to research the planets, because how would you build the rocket.
The way you would research is first telescopes, then satellites, and eventually you would have enough information to fly there.
The more people you employ the faster the process.
This 'leader' NPC' will create groups of workers that research these areas.
Once all the research has been done, you can employ scientists to calculate what the rocket should be like, e.g. what blocks it should be made out of.
After that, the 'leader' NPC will employ workers and builders to build the rockets.
So just to outline You have groups of Npc's
Researchers
Scientists
Builders
and the 'leader' NPC who you tell what must happen, and he will inform the user of events.
The Structures.
Once the world has been generated, a small building spawns, of which the blocks cannot be broken (so no cheating) inside will be 1 NPC who will guide you to building your first space centre on the overworld.
In order to make the centre, you will have to complete certain criteria, which will be,
50 levels of experience
Recourses in order to build the centre.
Have employed 20 workers so far.
Complete a test ( which will be made later,) to show your understanding of Galacticraft.
A large amount of wealth,(can be decided later)
These are ideas at the moment
Once the criteria is fulfilled, the workers you have employed will build the structure, which as gamegeek1234 suggested will take several minecraft days.
The more workers you employ the faster the process.
The new structure will be made around the over the top of the existing one.
The NPC at the beginning will become the 'leader' NPC in the space centre.
Once the structure is completed, it will contain an area say 25 blocks by 25 by 100
Here is where the rocket will be constructed.
Once the research has been done on rocket thrusters, you have the ability to set builders to build this in the designated area.
Once research on fuel tanks have been done, they can be built, and so on and so on.
I think this idea is nice because you will be able to see it progress.
Once the rocket is completed, the rocket can be moved out of the building to the launch pad (which will be part of the structure) at this point you are ready for take off.
I think that the components of the rocket need to be tested for faults.
Lets say that the thrusters have a 1 in 20 chance of failing, so they need to be tested by another group of NPC's if they are good they can be used.
I think for the fun of it that if the player decides not to test the components, the rocket may spin out of control and crash, killing the player.