This is a simple conjecture of a possible future method for cutting down the cost of building a spaceship. It was an idea I mooted in a book I wrote a number of years ago. I never published the book, although it had some good elements it was more time for self-indulgence than anything else and not good enough for print. One day I might rewrite it - one day. In the meantime, I thought it might be fun to disseminate this idea of how to build a spaceship for longer-term consideration! It may have been mooted many times for all I know. I'm sure anybody reading this, who thought so, is bound to let me know. However, I wrote the concept in the book in 1986 so at worst this might be an early contender. Since that time I can update to some more recent facts and figures. Not that there will be much in this short piece; you will be pleased to know.
The spaceships and I am taking into consideration here are never ships that will land on the planet but will build in space for space travel only. The big difficulty of building large spaceships for much longer journeys is the cost of transporting materials into low Earth orbit. You only need to look at the cost of the International Space Station to get an idea. At the time of writing the space shuttle is on its very last mission. Since it costs about $450 million per mission every kilogram taken to orbit costs over $10,000. The new Ariane V rocket reduces this to about $5000 per kilogram. So at today's prices to build a spaceship by sending everything up from Earth is therefore just a tad expensive. It has been discussed recently that having a permanent base on the moon would make this kind of task somewhat easier. Local mined resources, lots of energy available (solar), no atmosphere and only one sixth gravity. This would be a massive step forward and much easier to lob any craft off into the solar system -or even further.
My thought is that the next step would be to use the resources already available within the solar system. We (the human race) are already looking at ideas to stop asteroids hitting the Earth. One method, being nudging them into a slightly different orbit so that they miss Earth altogether. So why don't we take this idea to enable us to build a spaceship?
There are three main types of asteroid S-type, C-type, and M-type. The ones that may be of most interest are the M-type as many are made of nickel iron and have a strong metallic composition. So the idea is; you find a suitable asteroid, survey it, and insert various gas filled containers. This can be achieved by using drilling or natural contours or fissures. An inert gas would be preferable. You then nudge the asteroid into an orbit that takes it into a predetermined close orbit to the sun. When it reaches perihelion, the asteroid would have heated up to just the right amount to make the metallic rock into a viscous constitution. It is at this time the gas filled chambers expand, blowing the rock up like a balloon and making some very useful chambers, for equipment, machinery or accommodation. When back in orbit around the Moon, now somewhat cooler, it could then be fitted out ready to send off on its first mission. A thought for some time in the future, perhaps.