Structural evolution of the TeamIndus spacecraft that will land on the Moon
How the lander design evolved for structural integrity and robustness
The TeamIndus spacecraft is soft-landing on the Moon in 2020. The lack of an atmosphere on the Moon makes it impossible to slow down the spacecraft using either air resistance or parachutes. To withstand the impact forces during touch down on the lunar surface, we’ve evolved a design that is lightweight and structurally robust.
The first structural design of our lander used aluminum based strut, a commonly used component in automobile and aircraft structures. Strut based structures are used to resist longitudinal (length-wise) compression, a desirable quality for soft-landing on the moon where the impact forces can be high. Here is what the lander looked like.
The main & top decks, the vertical panels connecting them, central cylinder and the landing gear+footpad used strut-based design as shown above. The landing gear primary leg had aluminum foam inside to absorb impact forces on landing.
But there was a problem. The aluminum foam wasn’t efficient enough in absorbing the landing impact forces.
Landing gear absorption material
After evaluating other materials, the team changed the aluminum foam to a aluminum based honeycomb-shaped structure that can be compressed. Called “crushable honeycomb” as it can compress longitudinally and absorb the impact forces on landing better than aluminum foam. Upon being crushed, the structure compresses layer by layer to fill the gaps.
Even though a honeycomb structure is easily compressible to form side-by-side layers, each individual sheet in the structure has high stiffness in the length-wise direction. This led to two significant advancements in terms of a space mission where every gram counts.
- Better absorption of impact forces meant that the landing gear height was reduced, saving more cost.
- Crushable honeycomb structure is lighter than the aluminum foam because it has a hollow structure.
The decks don’t crush at touchdown as they need to divide the impact forces evenly on landing. To reduce the mass of the frame while maintaining stiffness, the aluminum strut was replaced with a honeycomb structure
sandwiched between two thin aluminum sheets. Even a slim aluminum-honeycomb sandwich can take the weight of multiple people standing on it with ease.
The structural stiffness of the both the main and top decks were improved multi-fold while using ~85 times less the material. The main deck was made larger allowing us to reduce the inclination of the legs with respect to the deck. This means more of the impact forces can be absorbed along the length by the honeycomb crush in the landing gear.
This is what the spacecraft looked like after these changes.
This spacecraft design won us the GLXP milestone prize of $1 million to demonstrate our landing capabilities on the Moon.
The structure of the spacecraft plays an important role during the launch too. A lightweight and efficient design of the spacecraft allows for more payloads to be carried to the moon. Apart from the above changes, further refinements have been made that make the spacecraft lighter. Carbon fiber replaced aluminum strut in more places like the solar panel & thruster holders.
The design progress doesn’t stop there. Landing on the moon is a tricky business as it is difficult to predict the type of terrain the spacecraft might land on.
The spacecraft might land on an uneven surface or an inclined plain. The landing gear footpad thus needs to adjust to the terrain below it. This is why our spacecraft had footpads which can change their orientation depending on angle it hits the surface.
The flat footpad structure has since been changed to a semi-spherical one and is allowed to move along all axes. The footpads were made lighter by substituting aluminum for carbon fiber.
Upon landing, the spacecraft will have both vertical and horizontal velocities. While the impact due to vertical velocity has been taken care of by our design, some horizontal velocity still remains, the forces of which need to be absorbed. If it is not absorbed, the orientation of the spacecraft cannot be assured to be sun-facing for the solar panels to function.
The solution? Make the secondary legs of the landing gear use honeycomb crush too. This will absorb the forces along the horizontal velocity component and prevent the spacecraft from pivoting post-touchdown.
Our lander is thus capable of landing safely on a wide variety of terrains at the lunar landing site thanks to an incredible landing gear configuration. The structure of the spacecraft thus ensures the safety of the subsystems and the payloads during the landing.