The history of engineering is marked by some grandiose leaps which have enabled iconic projects. What will the future hold for engineering, especially as we are on the verge of expanding beyond our “cosmic shore”? Enter the Lunar Space Elevator.
The Tour Eiffel (1889) is the triumph of Euler’s beam theory, before which no slender structure was really understood. The silicon chip reflects the success of quantum mechanics, and Bernoulli’s principle dictated the careful construction of airplane wings to allow humans to soar in the air among birds.
Contemporary engineers might stare at these marvels of science and technology and wonder what there is left to do. What is left to get our hands on, for future generations to gaze at in awe?
In 1959, Nobel Laureate Richard P. Feynman delivered a lecture at Caltech with the witty title “There is plenty of room at the bottom.” He was clearly not only referring to students arriving late. He was instead suggesting what marvellous possibilities there are for physics and engineering at short length scales: nanometers, picometers (1/1000th of a nanometer), the scale of individual atoms. As a reference, a human hair is approximately 100 nanometers thick.
Many successes in this field have been achieved, and we can today, in Feynman’s own words, “arrange the atoms the way we want”.
Nevertheless, as impressive as these feats are, they are invisible to the naked eye. Regardless of how incredible the idea of pulling a single DNA strand in a nano-tensile machine is, it possibly does not fill us with child-like wonder as the colossal Mayan pyramids do.
But worry not, there is also, of course, plenty of room at the top!
Hardly any other engineering project is more flamboyant than the “Lunar Space Elevator (LSE)”. As the name implies, the plan is to tether a space station orbiting around the Earth to the Moon, which could then act as a transport relay of equipment and supplies.
Currently the sole means of transportation is rocket-propelled spaceships, which has limited our ability to colonise the Moon. Adding another option in the LSE could help render colonising the Moon far easier and faster.
Materials for Lunar Space Elevator
The tether would undergo gravitational and centrifugal forces, which are both a function of its density. Detailed calculations can be found in P.K. Aravind’s “The physics of the space elevator” [1], yet intuitively it is easy to understand that the critical parameter here is its specific strength. That is the ratio of tensile strength to density, also known as a strength-to-mass ratio.
There might be no benefit in having exceptional tensile strength if the density is too high, thereby increasing the maximum tension.
Interestingly, the specific strength index is one of the oldest known figures in engineering. By swapping a sign and considering compression instead of tension, specific strength would dictate the maximum height of a column before it crushes under its own weight. This is a very old technological problem, empirically investigated at least as early as 3000 BC by Stonehedge engineers and is still actively researched by sand-castle builders on beaches all over the world.
With the crucial material parameter clarified, Table 1 shows the ranking of materials based on specific strength.
| Tensile Strength (GPa) | Density (g/cm3) | Specific Strength (kN· m/kg) |
|
|---|---|---|---|
| Carbyne | 390 | 6×107 | |
| Graphene | 130.5 | 2.09 | 62453 |
| Carbon Nanotube | 50 | 1.34 | 46268 |
| Carbon Fiber | 7 | 1.79 | 3911 |
| Zylon | 5.8 | 1.5 | 3766 |
| Dyneema | 3.6 | 0.97 | 3711 |
| Aramid | 3.6 | 1.4 | 2514 |
| M5 | 5.7 (9 ?) | 1.7 | 3352 |
| Spider Silk | 1.4 | 1.31 | 1068 |
The table above contains some of the most recent advances in material science. It also summarises most of what is known on optimal material design-for-strength strategies.
In principle, high specific strength requires a very efficient resource allocation: strong bonds should be present and cooperating efficiently in alignment. There are, though, different ways to achieve the target.
Dyneema (spun ultra-high molecular weight polyethylene) uses long linear closely packed polymeric chains, yet there are regions where the alignment is less than perfect.
Aramid relies instead on a bulkier, rigid molecule. Zylon and M5 use a similar strategy, with an even bulkier molecule, which further improves alignment.
Climbing towards the top of the table, we meet graphene, with its beautiful geometric structure. Carbon fibers use graphitic nanosheets as reinforcement, but they are not as well organized as graphene itself.
Carbon nanotubes can be interpreted as rolled-up graphene sheets: the structure has an impressive regularity, yet the bending weakens the bonds by disrupting the planar geometry of 2D graphene.
At the top towers carbyne, a “nanowire” with perfectly aligned carbon atoms, all working towards the same aim [3].
It seems that the beauty of the nano-world celebrated in Feynman’s lecture appears even when attempting to lift a tether to the Moon.
It turns out that the “Lunar Elevator” is not a science fiction dream after all: the materials highlighted in bold in the table do possess a sufficient specific stiffness [2]. They can be produced in adequate quantities, with the level of quality control required. Once we master CNTs and carbyne, an Earth elevator to the moon might be possible. This is an even more demanding challenge, as Earth’s gravity is much stronger than its satellite’s.
The UHMWPE fiber from DSM Dyneema is a gel-spun, multi-filament fiber produced from ultra high molecular weight polyethylene, with main characteristics:
- High strength
- Low weight
- Low elongation at break
- Resistance to most chemicals
- Twaron® is a para-aramid, high-performance yarn which combines the following characteristics:
- High strength: excellent strength-to-weight ratio
- High modulus
- High dimensional stability: very low creep, small negative thermal expansion coefficient
Zylon® PBO HS (high modulus) is a super fiber that features one of the highest levels of strength and modulus of elasticity (resistance to deformation) with flame & heat resistance. Tee HM fiber differsn from the AS fiber with a higher modulus and lower elongation at break.
M5 Rigid-Rod Polymer Fiber (PIPD) – download property datasheet for free from Matmatch.
State-of-the-art chemical vapour deposition (CVD) methods produce exceptionally high quality graphene film for use as transparent conductors and in other innovative applications. Find graphene materials and suppliers here.
- Density: 1.76 g/cm³ at 20 °C
- Linear density: 4.00E+7 g/cm at 20 °C
- Number of filaments: 6000 [-] at 20 °C
- Sizing: 1.3 % at 20 °C
- Tex number: 400 [-] at 20 °C
- Elastic modulus: 238 GPa at 20 °C
- Elongation: 1.7 % at 20 °C
- Poisson’s ratio: 0.29 [-] at 20 °C
- Tensile strength: 3950 MPa at 20 °C
Check the complete list of properties and download the material datasheet for free.
- Graphitization level: 0.3 – 0.4 [-]
- Nanotube diameter: 30 – 90 nm
- Nanotube length: 300 – 1000 µm
- Oxidation threshold: 600 – 700 °C
Check the complete list of properties and download the material datasheet for free.
There might still be some “rough edges” to consider though:
- Will space radiation degrade tensile performance [3]?
- What about creep?
- What about taut tether dynamics: will not there be high-frequency oscillations and fatigue?
- And is there not at all any requirement for toughness?
Toughness is indeed a crucial property. Some technical ceramics have a specific stiffness index comparable to or better than carbon-epoxy composites, but they are not used that often for racing bikes, for example.
In nature, spiders have been building web “elevators” for millions of years: yet their silk does not seem to be optimised for specific strength performance. Not only is spider silk inferior to high-tech carbon-based materials, but it also performs well below common, everyday materials such as glass fiber.
This is certainly not encouraging for modern trends such as the design concept of “bio-inspired” or “bio-mimetic” materials. Are these marketing buzzwords? Do man-engineered materials already outperform spider silk?
Not so fast: if toughness were to be considered, a whole different outcome would result. Spider’s silk offers a yet unrivalled compromise between tensile strength and toughness.
Spiders clearly worry about toughness, should we not too for the lunar elevator? We would be glad to hear your views, do let us know at mat_team@matmatch.com.
References:
- The physics of the space elevator, P.K. Aravind, American Journal of Physics. 45 (2): 125
- Lunar Space elevators for cislunar space development: Phase I Final Technical Report, J. Pearson, E. Levin, J. Oldson, H. Wykes
- Carbyne from first principles: Chain of C atoms, a nanorod or a nanorope, Liu M., Artyukhov V.I., Lee H., Xu F., Yakobson I., ACS Nano, 10 Oct 2013, 7(11):10075-1008
- Comparison of High-Performance Fiber Materials Properties in Simulated and Actual Space Environments, M.M. Finckenor, NASA/TM—2017–219634
