When the first Orion astronauts stare back at Earth, they'll be looking through windows made mostly of plastic. Because Orion is designed to carry out manned deep-space missions and even a possible Mars voyage, NASA decided it was time to replace the conventional glass windows with panes of acrylic that are lighter, less expensive, and more structurally sound than previous designs, and is more suited to long-duration missions.
Until now, all US manned spacecraft have used tempered and annealed glass windows to let astronauts see out while protecting them from the vacuum, extreme temperatures, and UV radiation of space, as well as from the heat of reentry. Anyone who has ever bumped a window pane too hard or dropped a water tumbler might think that glass was a poor choice, but it's actually quite a remarkable material.
Technically, glass isn't a solid. It's a supercooled liquid, which means it doesn't have a crystalline structure, but it is isotropic and elastic. It's also surprisingly strong, with the ability to withstand stress up to 3 million psi. In everyday terms, that means that you can make (and people have made) a spring out of glass that can support a doubledecker bus. The problem is that without a crystalline structure, glass needs to be flawless for maximum strength.
The reason why the bulk glass found in building windows and other items is so weak is because it's full of micro flaws due to manufacturing and handling. When under stress, a tiny scratch in a piece of glass can suddenly turn into a crack that spreads at the speed of sound. Tempering, where the glass surface is compressed, and annealing, where the glass is heated and allowed to cool slowly, can make it stronger, but annealed glass suffers from static fatigue and degrades under stress over time. Of course, a pressurized deep-space capsule puts the window glass under a lot of stress for a lot of time.
Orion has six windows in all – four windows in the crew module and two more in the docking and side hatches – so what to make them out of is a major engineering decision. There are many ways of solving the window problem in spacecraft. In fact, back in the Mercury days of the 1960s, the engineers wanted to leave them out entirely as an unnecessary structural weakness, but the astronauts weren't having it. One solution is to use a different transparent material, such as a polycarbonate, instead of glass, but because of the need for astronauts to take high-resolution images, the material has to have the optical qualities of glass.
For Orion, engineers chose an acrylic plastic material that is stronger than glass, but has similar optical properties. It's the same material commonly used in aquariums ranging from small home tanks to giant public displays. These not only handle the constant pressure of thousands of gallons of water, but also touches by the public numbering in the millions.
NASA says that acrylic windows not only have an engineering advantage, they are also lighter, which means that they reduce launch costs. Just replacing some of the glass in the Orion test flight in December 2014 saved over 200 lb (91 kg). In addition, the acrylic panels are less prone to damage so they don't require the same level of redundancy, meaning fewer panels are needed. The acrylic panels are also less expensive per volume than glass. NASA is very cost-conscious in the this regard, as is shown by its recycling of glass stock originally ordered for the Space Shuttle for use in making the windows – a move that has already reportedly saved US$2 million.
For Orion, each of crew capsule's six windows are made up of three panes to provide maximum redundancy. During Orion’s first flight test, the innermost was made of acrylic, so the design is moving off the drawing board. Since the acrylic panes have already flown in space, the next step is testing for thermal integrity, as well as creep tests to assess how well the material stands up to pressure loads over a period of 270 days.
The plan is that if the acrylic panels pass muster, Orion's window will be upgraded to a set of two acrylic panels and one glass panel, which will save another 30 lbs (13.6 kg). This new technology is also being shared by NASA's Commercial Crew Program partners for incorporation into their own manned spacecraft