Is it more efficient to assemble spacecraft in orbit, or assemble on the ground and launch?

Question

My friends and I were trying to figure this out a while ago.
I was thinking that by transporting smaller pieces into space via a lighter re-entry spacecraft, there would be overall less fuel used because there would be less "feedback" (ie more fuel needs more fuel to lift that fuel, etc).

My friends were thinking it would be the same because the same overall mass of the craft would require the same amount of fuel.

Would it be more efficient to transport a spacecraft into space in pieces, or only once it is assembled?

Answer 1

There is clearly a trade off that needs to be made.

If your components are only 2 feet cubed, then it is not more efficient to build a ship for humans out of it.

If your components are 10,000 cubic feet each, then it is not more efficient easier, or conversely, you only need one or two.

It depends on many things.

If you have cheap launch for a useful size chunk (say Falcon 9 gets reusable and dirt cheap) but a larger launch does not linearly scale (Say Falcon Heavy does not get as cheap) then it might still be cheaper to launch many times.

Propellant is cheap compared to launch cost. Elon Musk has claimed F9 propellant cost is on the order of 3% of total cost.

Answer 2

There are a couple of things that have to be considered in this assessment.

1. What is the total launch cost?
2. How does the cost of the components scale, with changes in the launch?
3. How difficult is the assembly?

Big rockets typically are more efficient, especially if they are aerodynamically shaped. However, there is somewhat of an optimal ratio of diameter to length. The object you are going to get in to space will pretty much have dimensions of a can, scaled to an appropriate size, no matter the rocket.

Multiple launches were required to build the ISS, simply because it couldn't fit in to any rocket available, and it probably wouldn't hold together without it. However, some things can be made cheaper if they are a single entity, like a big mirror for instance.

Bottom line is this:

1. Cost typically goes down per pound the larger the rocket.
2. The volume and shape of the object vary considerably after you get to that stage.