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NASA Glenn Research Center

# Reinventing the Wheel

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## Exploration Requires Mobility

Exploration requires mobility. And whether you’re on Earth or as far away as the Moon or Mars, you need good tires to get your vehicle from one place to another. 

Tire development for space exploration has been a focus of research at NASA Glenn for a decade. Evolving since the days of Apollo, NASA engineers started examining tire designs back in the 1960's for use on the surface of the Moon.

## Wheels on the Moon

Three major  wheel designs were introduced by NASA and other international researchers for early exploration. While they were vastly different, they were all focused on moving hardware and astronauts across the lunar surface.

          -                                              LUNOKHOD rover and tire                                      
** Luna 17 - Nov. 1970, Luna 21 - Jan. 1973

The Russian vehicle *Lunokhod* (Moon Walker in English) used eight rigid rim wire mesh wheels with bicycle-type spokes and metal cleats for traction.

Photo courtesy of Serguei Matrossov
 
          -                                              MET-ROVER and wheel                                      
** Apollo 14 - Feb. 1971

The American Modularized Equipment Transporter (MET): a two-wheeled unpowered cart used smooth rubber tires supported by nitrogen filled inner-tubes and manufactured by Goodyear to make it easier to pull the cart through soft lunar soil and over rocks.

Photo courtesy of Dave Glemming
 
          -                                              Lunar Roving Vehicle                                      
** Apollo 15 - July 1971, Apollo 16 - Apr. 1972, Apollo 17 - Dec. 1972

The last NASA vehicle to visit the Moon was the Lunar Roving Vehicle. This manned vehicle used four large flexible wire mesh wheels with stiff inner frames to prevent over-deflection. Thin tread strips were attached to the carcass to enhance flotation in soft lunar soil.
 

## From the Moon to Mars

While NASA was developing rovers to send to Mars, there was also interest in taking a new look at returning to the Moon. NASA Glenn engineers rebuilt Apollo era tires and developed new designs using newer materials and technology to better function on a lunar surface.

              -                                              Spring Tire                                      
 During the mid-2000s, NASA Glenn engineer Vivake Asnani worked with industry partner Goodyear to develop the Spring Tire, an airless compliant tire that consists of several hundred coiled steel wires woven into a flexible mesh, giving the tires the ability to support high loads while also conforming to the terrain.  The Spring Tire has been proven to generate very good traction and durability in soft sand and on rocks. This innovation led to an R&D 100 Award for Top Inventions of the Year. 
 
              -                                              Mars Rover Tire Damage                                      
A little over a year after the Mars Curiosity Rover landed on Mars, engineers began to notice significant wheel damage in 2013 due to the unexpectedly harsh terrain, causing concern about the ability of the rover to drive far enough to complete its intended mission.
 
              -                                              Modifying the Spring Tire for Mars                                      
NASA Glenn engineers thought their work on Spring Tires might be a new and better solution for exploration rovers on Mars. So, they set about developing several Spring Tire prototypes to improve:

- traction in soft sand
- durability
- and reduce overall weight									  

 
              -                                              Durability Testing                                      
While the tires performed well, there was a problem with the steel wires deforming when rolling over punishing simulated Martian terrain at NASA's Jet Propulsion Laboratory (JPL). 
 

In one particular moment of serendipity, Engineer *Colin Creager* and Materials Scientist *Santo Padula* had a conversation that completely changed the path forward.

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The game changing material that dramatically advanced the development of spring tires was  nickel titanium, a shape memory alloy with amazing capabilities as explained by *Santo Padula*.

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After building the shape memory alloy tire, Glenn engineers sent it to JPL's Mars Life Test Facility.  It performed impressively on the punishing track.

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So, why are shape memory alloys the key to success for this new generation of spring tires? It has to do with the atomic structure. Take a look to find out. 

## The Path Forward

There are three major benefits to developing high performing compliant tires that are capable of performing in a Martian or Lunar environment.  First, they would allow rovers to explore greater regions of the surface than currently possible.  Secondly, because they conform to the terrain and do not sink as much as rigid wheels, they can carry heavier payloads for the same given mass and volume.  Lastly, because the compliant tires can absorb energy from impacts at moderate to high speeds, they can be used on crewed exploration vehicles which are expected to move at speeds significantly higher than the current Mars rovers. 

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 By: Nancy Smith Kilkenny              Graphics and Images by the Imaging Technology Center at NASA Glenn

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