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Throughout August, Sydney Do conducted dozens of "drop tests" of a dummy attached to the airbag system. The system was dropped from different heights that ranged from one foot to 10 feet. Photo: William Litant
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Plans for Orion were changed in February
when President Obama cancelled Constellation, and then announced two months
later that NASA would continue to develop Orion as an escape vehicle to be
docked at the International Space Station for emergencies.
While it appears that Orion will
eventually take flight, NASA continues to struggle with one crucial aspect of
its design: minimizing the violent impact that astronauts would experience
during landing. Although NASA initially designed Orion’s crew seats to be
mounted onto a stiff structure supported by shock absorbers—essentially the
same technology used to cushion Apollo’s water landings—this 1,100-pound
structure would be too heavy to cushion astronauts if the vehicle landed on
land. Whereas the Apollo capsule was designed to land in water, and Orion would
likely do the same, NASA wants to make sure that Orion can land on land in case
of an emergency.
A graduate student in MIT’s Department
of Aeronautics and Astronautics has helped design a smaller alternative: a
reusable, 700-pound air-bag system that could inflate during launch and
landing, deflate for storage purposes, and partially inflate to provide seating
while the vehicle is in space. Not only would the system be lighter than the
one NASA originally proposed, but it would also be entirely mechanical, meaning
not controlled by computers.
This is important because “the vast
majority of accidents and failures in engineering systems” can be traced to
computers misinterpreting situations, says Sydney Do, who helped design the
air-bag system and spent several weeks in August testing a full-sized prototype
designed to protect one astronaut. “Our goal was to see if it was possible to
design a landing system that was purely mechanical.”
According to a paper presented at the American Institute of Aeronautics
and Astronautics Space 2009 conference by Do and his thesis adviser, Olivier de
Weck, an associate professor of aeronautics and astronautics and engineering
systems, the air-bag system was inspired by the structure of seeds. Just as a
fluid surrounds the embryo in seeds to provide protection as the seed is
distributed, the Orion air-bag system would surround each astronaut in “a personal
cushion of air,” according to current NASA astronaut Charlie Camarda, who seeks
to develop more innovative space-engineering concepts that veer from the
traditional. In 2008, Camarda helped organize a group of students from Pennsylvania State University
and MIT, including Do, to explore how the physics of seeds could be applied to
engineering principles. Do’s design for an Orion air-bag system, Camarda says,
represents “a very novel” approach to mechanical design that could inspire more
biological-based solutions in engineering.
Valve analysis
NASA’s Engineering and Safety Center agreed to fund the study by the Penn State
and MIT students to explore the feasibility of an air-bag system that Orion
astronauts could inflate before re-entering Earth’s atmosphere. The students’
first step was to conduct tests to observe how the inflated bags behave when
they are dropped from increasing one-foot increments while supporting an object
that weighs about the same as an average male head—such drops simulate the
impact velocity that an astronaut would feel upon landing.
These tests revealed how important
timing is in terms of releasing gas from an air bag. Unlike car air bags, which
inflate when hot gas is injected into them upon impact, the inflated Orion air
bags already contain gas upon impact. If the air bags are either not big enough
or don’t have enough air in them, the astronaut’s seat will directly impact the
ground. Alternatively, if there is enough gas inside the bag, but it’s not
released before the seat hits the ground, the impact will cause the seat to
bounce upward, which could injure the astronaut. That’s because as an astronaut
falls into the bag during the landing, the kinetic energy created from this
motion is combined with the energy of the gas molecules moving inside the bags.
This increases the pressure of the gas inside the bag, which could cause
bouncing.
To prevent this bounce, enough gas needs
to be vented between the point at which the floor of Orion impacts the ground
and the point at which the seat and the astronaut impact the ground so that the
kinetic energy caused by the falling seat and occupant have been removed. But
even after some of this gas is vented, there still needs to be enough gas
remaining in the bags to prevent direct impact between the seat and the ground.
To get this balance right, the students decided to design valves that are
triggered to open at a low pressure, which would allow gas to vent as soon as
Orion’s floor comes to rest, but before the seat can impact the ground.
Drop-test survival
When NASA decided to fund the research for another year last spring, Do took
over the research for his master’s thesis and began testing a valve for the
system. He then developed a computer model to analyze how certain variables,
such as air-bag size, would affect the risk of astronaut injury upon impact.
This helped him configure a prototype seat that would have four air bags—each
about one foot long by two feet wide—containing two rectangular valves about
six inches wide. Do then built the air bags from vectran, a high-strength
material that was used to make the air bags for several rovers that landed on
Mars.
Earlier this month, he tested the
prototype through a series of drop tests conducted from as high as 10 feet
involving a crash dummy that measured the acceleration of each drop. While Do
still needs to analyze those results before presenting his final design to NASA
later this fall, he says that the fact that the system survived dozens of drops
suggests that certain variables he chose for the prototype, such as the
material and manufacturing of the air bags, are adequate for an Orion landing.
According to Camarda, future research could explore ways to ensure “a robust
and fail-safe” system in the event that a valve malfunctions.
Do cautions that the air-bag system has
one drawback: It’s likely only effective for vertical drops, meaning that the
air bags could tip over if Orion descended at a sideways angle. But he says
this might not be an issue if Orion is designed to land vertically. Although
whatever NASA decides to do with Do’s research ultimately depends on the future
of human spaceflight, he is hopeful that even if Orion never takes flight, his
research could be used to guide designs of similar capsule-type spacecraft that
commercial companies might be interested in building.
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