The Design of Space Craft.

This post was  triggered by the following BBC report.

What should spaceships look like?

http://www.bbc.co.uk/news/magazine-14291992.

Science fiction (SF) has always fascinated me since I was a child. It is the type of fiction that opens the mind of the reader to concepts that are far outside the mundane realities of everyday life. Many of the SF writers also have an impressive grasp of psychology that could make other main-stream writers envious.  Note that there is a considerable difference between SF and Science fantasy in that SF writers tend to remain within the realms of scientific possibilities whilst science fantasy writers tend to wander into the areas of the ridiculous.

Science fiction illustrators nearly always produce artwork that is total fantasy in having no scientific basis. This is particularly obvious in their portrayal of spacecraft. The designs are generally totally unworkable as spacecraft and are mainly just strange shapes full of odd bits stuck on to make them look more ‘scientific’.

The design of the “Starship Enterprise”, the most famous spaceship design ever produced, would be be totally useless for space flight outside the influence of a gravity source.  Note; I am not a ‘Trekie’ so I am expecting lots of repudiations of my comment.

Why is this?

It is the same reason that bullets, war-shells, guided missiles etcetera all have symmetrical shapes. This allows balance in the forces acting.

Even the spin on bullets is there to there to counterbalance any imperfections in manufacturing that would cause them to deviate from their planned trajectory.

Aircraft have all sorts of imbalanced forces operating on them such as tail fins, landing gear, wing tanks, outboard weapons systems, etcetera that all affect the flight of the aircraft. However gravity exerts a major force that means that providing the imbalances are equal on both sides of the vertical plane, they do not upset the handling too much whilst travelling horizontally.

In space flight outside gravitational force, even slight imbalances become more serious. During acceleration the power must be applied through the centre of mass and in the direction that you want to travel. Any slight variation in the centre of mass will cause the spacecraft to rotate. If it rotates by I degree in the first second of a 10 second ‘burn’ it will rotate by 10 degrees (Approximately) during the 10 second burn.

To prevent this from happening a compensating force will be required. which wastes fuel.

How would the pilot know exactly where the centre of mass is? Generally he wouldn’t know, he would only have a calculated location. If a pilot put out his hand to operate a control, this would change to centre of mass of the spacecraft. If you had people walking about this would cause the centre of mass to continually change.

Let us now consider the starship Enterprise. Let us assume that it has lots of computer equipment to handle variations in the centre of mass and everyone on board is tagged to allow the computer to be aware of their locations.

The shape of the enterprise is quite pleasing and is not embellished with too much external junk. It could fly in Earth’s atmosphere or under a gravitational force. However, the position of it’s drives means that it would be impossible for the force to act through the centre of mass. It would need a large amount of energy to correct the imbalanced forces attempting to spin the spacecraft.

Another major problem with shapes of spacecraft designs is that they are not streamlined. If space was a total vacuum then the shape would not matter if the driving force operated through the centre of mass. However, space is not a vacuum and therefore there is something there to cause resistance. Due to the non-streamlined nature of most designs this resistance of the various ‘stuck on’ appendages would cause deviations in the path of the spacecraft.

Author

Brian Williams