A 2-in by 4-in brick is heated by a welding torch in a test of a concept for making heat shields from the soil of other worlds. Credit: NASA |
An important test is coming up next week to see whether a heat shield
made from the soil of the moon, Mars, or an asteroid will stand up to the
searing demands of a plunge through the Earth’s atmosphere.
At stake is the possibility that future spacecraft could
leave Earth without carrying a heavy heat shield and instead make one on the
surface of another world and ride it home safely. The weight savings opens new
possibilities ranging from using smaller rockets to carrying many more supplies
on an exploration mission.
Michael Hogue, a researcher at NASA’s Kennedy Space Center
in Florida, came up with the idea during a brainstorming session last year
covering different ways to use extraterrestrial soils, known as regolith.
“Others were talking about how regolith can be used to
make bricks or landing pads and I said, ‘Well, if it’s good for that, why can’t
it be used to make atmospheric entry heat shields?’ ” Hogue says.
NASA funded the concept research through its NASA Innovative
Advanced Concepts, or NIAC, program.
Since then, a team of engineers has been trying out various mixtures
and techniques to find out whether the idea has any potential. So far, the
tests have been very successful, with small bricks of material standing up well
to the intense heat of a blowtorch. A sensor placed behind the brick recorded temperatures
of about 200 F compared to the approximately 4,000 F the front side endured.
“I expected
some to fail,” Hogue says. “There is an optimum range of density you
need to hit for each material where it’s light enough to have low enough thermal
conductivity, but also structurally strong
enough to survive the forces of atmospheric entry. All of our formulations that
we tested with a cutting torch at least passed that.”
The dome-shaped
bricks, each 2 in thick and 4 in in diameter and made of different combinations
of material, will face their toughest test next week when they are placed
inside the arc jet facility at NASA’s Ames Research Center in California.
There, they will be subjected to a scorching plasma stream that will put the bricks through heating conditions similar
to those seen during entry.
“That will ultimately determine whether this idea is
feasible or not,” Hogue says.
The concept, while promising, is far from becoming
operational. At this point, Hogue puts the concept at a TRL, or technology
readiness level, of 1 on a scale of 9, with 9 being an operational element.
Working it up the TRL scale will take a series of evaluations, adaptations and
inventions, including potentially trying out a sample disc on the bottom of a
cargo spacecraft returning from the International Space Station.
An artist’s concept of a spacecraft using a heat shield made from the soil of another world, called regolith. Image: NASA |
Hogue says his attitude has gone “from guarded
skepticism to hopeful enthusiasm” on the effort.
The potential weight-savings is too great to ignore, Hogue
says.
Making the heat shield in space would likely be the work of
a robotic device, or at least a heavily automated system to either mix the
regolith with a rubbery substance in a mold or heat a large disc of regolith
until the soil elements fuse together. The heat shield could then be cut and
shaped as needed.
The primary advantage is that getting the finished heat
shield off an asteroid or Martian moon would take very little force because the
gravity is so low. The heat shield could be as big as anyone would want. It
could be used to insulate a spacecraft whether it is going to the Martian
surface or back to Earth.
The weight savings is critical to the plan because the
regolith material is anything but light. A brick of regolith, actually made
from Mars and moon soil simulant instead of actual extraterrestrial dirt, feels
the same in the hand as a brick one builds a house from on Earth.
Also, the heat shield would not be reusable, but would be
designed to have some layers fleck or burn off, a process called ablating. All
heat shields except the space shuttles’ were made of ablative material.
“You can make it massive and if it heats up and ablates
off, all the better because the ablated mass takes heat with it,” Hogue
says. “After about five minutes you jettison the shield over water and
you’re done.”
Source: NASA