Materials & Applications

Materials for Lunar Space Elevator: Accessing the Moon Ever More Easily

Lunar Space Elevator - Matmatch 3d rendering

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.

Lunar Space Elevator - Moon Exploration

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”.

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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.

A lunar space elevator or lunar spacelift is a proposed transportation system for moving a mechanical climbing vehicle up and down a ribbon-shaped tethered cable that is set between the surface of the Moon "at the bottom" and a docking port suspended tens of thousands of kilometers above in space at the top.

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.

Lunar Space Elevator
A space elevator to the moon could be doable — and surprisingly cheap. New study suggests that a lunar space elevator could be built for about $1 billion using existing technology.

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)
Carbyne3906×107
Graphene130.52.0962453
Carbon Nanotube501.3446268
Carbon Fiber71.793911
Zylon5.81.53766
Dyneema3.60.973711
Aramid3.61.42514
M55.7 (9 ?)1.73352
Spider Silk1.41.311068
Table. 1: Specific strength values for selected materials.

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.

Lunar Space Elevator - 3d rendering Matmatch
A lunar space elevator (also called a moonstalk) is a proposed cable running from the surface of the Moon into space. It would instead be constructed with its center of gravity in a stationary position above the surface of the Moon, providing a controlled means to transport people and/or materials between the surface and lunar orbit.

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.

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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

Download datasheet for free from Matmatch.

  • 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

Download material datasheet for free from Matmatch.

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.

Download material datasheet for free from Matmatch. 

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. In addition to offering standard samples, Goodfellow can assist customers with the transfer of graphene film onto their own substrate. Get in touch with the supplier of graphene for questions, samples, inquiries – message directly from Matmatch.

  • 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, download material datasheet for free, get in touch with carbon fibre supplier on Matmatch.

  • 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, download material datasheet for free, get in touch with carbon fibre supplier on Matmatch.

lunar space elevator - the concept
A lunar elevator could massively reduce the costs for reliably and cheaply soft-landing equipment on the lunar surface.

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.

lunar space elevator - spider silk
Spider silk has a tensile strength on the order of 1 GPa (it's a natural fiber and so varies) and that's only about 4% of the strength needed to build a minimal earth-based space elevator.

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.

Kevlar vs Spider fiber@4x

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.

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References:

  1. The physics of the space elevator, P.K. Aravind, American Journal of Physics. 45 (2): 125
  2. Lunar Space elevators for cislunar space development: Phase I Final Technical Report, J. Pearson, E. Levin, J. Oldson, H. Wykes
  3. 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
  4. Comparison of High-Performance Fiber Materials Properties in Simulated and Actual Space Environments, M.M. Finckenor, NASA/TM—2017–219634

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