Science Forums 2

[CraigD]

Originally Posted by Gardamorg

A good Science Fiction oriented space craft would be a super light spacecraft.

Very low mass space craft are a good idea, with an established literature. Perhaps the lowest mass of any proposed is the Starwisp, an unmanned interstellar spacecraft massing about 1 kg.

Originally Posted by Gardamorg

If we had nano assemblers we could essentially give Aerogel a nanostructure that fortifies it's strength by composing it's structure with nanotubes such as these.

Carbon materials not only promise to dramatically reduce the empty mass of spacecraft, but is already has in several designs, such as SpaceShipOne. As Gardamorg suggests, nano-materials – whether assembled by nano-robotic assemblers or other schemes – promise to allow even lower empty mass spacecraft.

Although the performance improvement possible with such materials is appreciable, it’s not extreme, because even old rockets already have a high fuel/frame+skin mass ratio. For example, the Saturn V’s first stage is 94% fuel by mass, of which 2% of the empty mass are due to its engines. So if its conventional metal frame and skin could be replaced with an advanced material massing nothing, its fuel/frame and skin would be improved to 98%/2%, a significant, but still only incremental improvement.

Originally Posted by Gardamorg

There is no limit to how strong we could make it, we just need to engineer a stronger structure, and this doesn't increase it's mass.

Although nano-materials promise to be many times stronger than conventional carbon fiber materials, which are already many times stronger than conventional metals, there maximum strength is not unlimited. They are limited by the strength of the intermolecular bond strength of the atom of which they’re made.

Originally Posted by Gardamorg

A spacecraft made of this material could be 40 km tall and travel just as fast as a modern chemical rocket, but hold a massive amount of fuel, and burn just a little bit of it to travel at these speeds.

The problem – obvious is you read the preceding quote – is that a massive amount of fuel has, well, a large mass. A rocket with a large mass needs a large thrust to lift off the Earth, or accelerate in space, which in turn requires a large thrust force, which requires a large motor power, which in turn requires burning a lot of chemical fuel. The only ways to partially escape the vicious circle of needing more fuel, which requires more thrust, which requires more fuel+oxidizer and larger motors, which requires more thrust, etc, is to increase the specific impulse of the fuel/motor system, yet keep it’s total thrusts high enough to lift the fully fueled vehicle, use a separable part of the system (lower stages, as with the Saturn V, or a carrier aircraft, as with SpaceShipOne’s White Knight), and/or get much of your reaction mass from outside the system (as with the air-breathing jet engines on White Knight) to do part of the lifting.

Simply scaling up a chemical rocket to be 40000 m tall would need more than super-strong structural material (note that 40000 m would be about 50 times taller than the tallest structure yet constructed, the to-be 818 m Dubai Tower). If shaped roughly like most rockets, it’s mass would be about 64 million times as massive as a Saturn V, the largest rocket ever launched. If it used motors similar to Saturn V’s 5 F-1 motors, the largest ever built, it would need about 320 million of them! It would need about (about 600 billion barrels) of kerosene – about equal to about 200 year of the current total world supply. That’s an awful lot of rocket motors and fuel!

Nanotechnology will, I suspect, be vital to future engineering projects of many kinds, including spaceflight. However, I don’t think a 40000 m tall chemical rocket is likely ever to be built.