I don’t know if it is still the case today, but when I first starting reading sci-fi/s.f. (gulp – more than 25 years ago), Isaac Asimov, Robert Heinlein and Arthur C. Clarke were grouped together as ‘the Big Three’. I’ve never really been quite sure why this should be, given that there have been some truly astounding writers in the same general space (if you’ll pardon the turn of phrase), but they are certainly big personalities, with big ideas, who wrote intriguing novels which captured the imagination – with some of these novels forming the basis for films. All were also unusual in that they invented words and phrases that entered the language, and were prescient to the detriment of more than one patent application.
All came at s.f from a particular standpoint: Asimov was a chemist, Heinlein an Engineer, and Clarke primarily a physicist (although his career included significant experience in the education sector). Slightly surprisingly, perhaps, or perhaps not, Heinlein never really played fast and loose with materials. His stories may have included a significant number of alien species and life on Mars and Venus, but for the most part (at least from memory), his engineering is pretty sound, with one YA novel, ‘Have Space Suit, Will Travel’, including a teenager with a penchant for home projects, fixing up a space suit that he won as a bit of a joke prize, to the point that the suit is space worthy.
So let’s not spend anymore time on Heinlein, because he doesn’t offer anything that is particularly useful for my AtoZ theme. Asimov we looked at in passing, fittingly right (write?) back at the beginning of the alphabet. So today I’d like to look at a classic Arthur C. Clarke novel: The Fountains of Paradise. I must confess that this is a book that I haven’t actually read myself, although it is on my “I really must read that” list (not to be confused with my TBR pile). (I have a slightly on/off relationship with Clarke’s writing, and whilst I have read all of the Odyssey novels, and most of the Rama sequence, his writing has never quite grabbed in the same way as Asimov’s and Heinlein’s). Luckily, this doesn’t stop me from pontificating, as the reason that the book is indelibly etched on my brain (as a book that I really should read), is that at its heart it is a story about the construction of a space elevator.
Whilst the imagery around a space launch is undeniably awesome, the cognoscenti will realise that, with the multiple stages to loft a comparatively small ‘mission payload’ into orbit, such a launch is almost a high-tech ‘Xeno’s paradox’: successive booster stages are used to get the next bit high enough that it needs less oomph to get it higher still. In this way we get personnel and equipment out of the gravity-well of Earth and up, up and away. A more elegant solution (for a more civilised age), would be to have some sort of pulley arrangement that would use counterweights to bring stuff up from Earth to geostationary orbit and down again afterwards. There are several advantages to this, and whilst it would not be a completely ‘free’ form of launching material into space, the energy requirements over the lifetime of the elevator would be miniscule compared with rocket based launches for the same equipment. Further, such a system would make manufacturing in micro or zero gravity environments feasible, opening the way for all sorts of exotic materials that would be difficult or impossible to manufacture on Earth. One, that I can remember from when I was doing my A-level exams, was an alloy of Aluminium and Lead. I can’t, now, remember the properties, and whether such a material would even be useful, but the point was that the different densities of the elements made mixing very difficult, but in zero-g you got some interesting microstructures.
But how do you make a space elevator? With difficulty! There have been some suggestions that you could simply(!) nudge a carbonaceous (carbon rich) asteroid into geostationary orbit and effectively use this as the terminus, and this is almost inevitably what will need to be done. Why carbonaceous? We’ll come back to that in a moment.
Here we are, nearly ¾ of the way through the alphabet, and I haven’t really talked about some of the properties of materials most important to me personally as Materials Scientist! The core part of my day job is to break things. As such I’m interested in four key properties: the strength of a material, i.e. the amount of load you can place on something of a given cross section before it breaks; the strain to failure (strain being a dimensionless property defined as being the change in length divided by the original length), the stiffness of a material, i.e. (crudely) the resistance to deformation of a material before it yields (changes shapes in a non-recoverable way or as stress x strain, in the recoverable portion of loading, at least), and the toughness, i.e. the amount of energy something can absorb before it fails. (In practice, what I’m interested in is a little more complicated than this, but this is a good enough working description to be going on with). On that basis, one can therefore design something for use on a number of considerations. Often, it is a case of saying “it needs to be able to carry this load, so it needs to be this strong”. Sometimes though, it is necessary to say that it must not deflect more than this much, and so the stiffness becomes important. Sometimes, it is necessary to be more specific still and to say that the component must not extend by more than a certain amount – we must keep below a working level of strain. This last, is perhaps the most demanding, and when we are dealing with a component that is required to be of the order of 36, 000 km, then even a mere 0.1% strain represents an elongation of some 36 km.
Coming back to our carbonaceous asteroid, Clarke described the use of a ‘hyperfilament’. Fountains of Paradise was written before the discovery of C60 and the other buckminsterfullerenes, and so his hyperfilament was a form of diamond, although following the discovery of Smalley, Kroto et al, he went on record as saying that he thought hyperfilament was a continuous form of carbon nanotube. The stiffness of the carbon-carbon bond is one of the largest known: theoretically this should come in at about 1 TPa, or about 15 times the stiffness of aluminium.
I’m a little out of date on research in this area, but I remember looking at this (for interest, not a particular need) when I was doing my PhD. People had tried to measure carbon nanotubes and come up with values under and over this value. Whatever form of carbon that you want to use, the idea is that you use your carbonaceous asteroid as feedstock, with whatever is left over becoming your header station, and the counterweight.
I don’t have the time, space or inclination to do the calculations for the stress that would have to be carried by the nanotube, but there are a few things that we do need to take into account when taking this proposal forward. One of the absolutely brilliant things about carbon nanotubes is that depending on the angle of the carbon bonds relative to the ‘long’ direction, you can change the nanotube from being an electrical conductor to a semiconductor, which gives you all sorts of possibilities in terms of power and communication. If you are very clever in the growing of your tubes you can even introduce changes in direction so that different parts of the tube act in different ways.
A significant problem is that, to date, the longest tubes manufactured are of the order of millimetres in length, i.e. about 12 orders of magnitude too small for a space elevator (you’d need to multiply the longest nanotube available nearly 1,000,000,000,000 times). The longer that you try and make nanotubes, the more chance there is of a defect creeping in. In this context, we really want nice even hexagons, but in the same that you can get transcription errors in DNA the more times it is copied, the more hexagons that you try to add, occasionally you get pentagons or septagons. This can, if it can be controlled, be turned to account, and you can actually make a nanotube spring by carefully introducing these ‘defects’ in the right place – but the same objection applies in terms of quality control of the overall structure: making sure you get every atom where it is supposed to be every time, is going to be…tricky.
In practice therefore, whilst carbon nanotube based solutions are likely to be at the forefront of options for building a space elevator (both because of material properties and the availability of the raw material), the overall structure is likely to be more substantial and more complex than a simple ‘hyperfilament’.