Space Shuttle orbiter windows are made of high-purity fused silica glass, designed to withstand extreme temperatures during reentry into the atmosphere and cabin pressure during space flight.

Within the shuttle, there are also glass panes known as pressure and thermal panes that are made from tempered alumino-silicate glass.

Thermal Panes

Glass has been an essential tool in humankind’s exploration of space since Galileo created his first telescope. By layering curved pieces of glass, he could magnify an image of a distant object several times its actual size.

That discovery established the basis for modern observational astronomy and our growing comprehension of our solar system’s place within the cosmos. It has also played a pivotal role in space exploration ever since – from Mercury to Apollo 11 and now to the International Space Station.

NASA has long relied on Corning for its space flight windows, which are essential to the safety and performance of spacecraft. These glasses are constructed from a special low-expansion thermally stable material that can withstand both the extreme cold of outer space and the hot reentry of a spacecraft back into Earth’s atmosphere.

Space windows must not only withstand thermal shock, but they must also withstand mechanical stresses and be durable enough for long-duration flights in space. A broken window would seriously compromise crew health and safety, potentially jeopardizing mission success.

Engineers have been searching for a solution to this issue for years, and polycarbonate has been suggested as one potential option. Unfortunately, polycarbonate panes lack the optical properties NASA requires so cameras pointing through them can capture high-resolution imagery.

Thankfully, engineers have identified another glass-based solution. Acrylic, which is much cheaper than the specialty glass needed for Orion’s windows, will be tested by engineers back on Earth to see if it can withstand sustained loads over nine months in space.

To test its durability, the acrylic panes will be subjected to a creep test – simulating what astronauts would experience during an extended mission. Once confirmed reliable, more of it can be introduced into Orion’s windows in order to save money and mass, making the spacecraft more accessible for commercial interests.

Sutton and his team have undertaken one of the most fascinating and difficult challenges they’ve ever encountered – which has served to make him so proud to work on NASA for so many decades.

Pressure Panes

On the space shuttle, windows must withstand both cabin pressure and high temperatures during reentry into Earth’s atmosphere. That is why NASA uses two types of glass: synthetic high-purity fused silica thermal panes outside to protect against reentry heat; and an inner tempered aluminosilicate glass pane called a pressure pane for maximum strength.

On the inside, a middle pane known as a redundant pane acts as backup to the pressure pane. Meanwhile, an outer debris pane shields the pressure pane from orbital debris when the Cupola shutters are opened.

The windows are also shielded from solar radiation by a special dome, or Cupola. This dome, which can be opened to let in daylight and closed to block out UV rays and micrometeoroids, weighs 1.6 tons and is made of forged aluminum.

During the Columbia crash that claimed all seven astronauts aboard, damage to its exterior windows from debris and melting was a minor inconvenience. But this was only a blip in the overall picture.

Larry Sutton, Corning’s North American manager for semiconductor materials, confirmed that every American manned space flight from Mercury through the Space Shuttle program has used corning’s windows. For Apollo 11, for instance, corning created a “full set” of optical-quality triple paned windows for both shuttles and their crew modules.

One of the most challenging aspects of these windows is their inability to withstand both high pressures and temperatures. Therefore, space shuttles use multiple panes of glass (or sometimes acrylic) in order to ensure they survive an intense journey into space.

Estes’ team is working towards a solution to this issue. The initial step will be conducting more experiments on the thermal integrity of acrylic panes. If these tests prove successful, they can be added to Orion’s windows, reducing their total number of panes from three to two and saving more than 30 pounds from its mass.

But if the acrylic panes fail, then the spacecraft will need costly repair and redundancy work. This could disrupt ground schedules and put two orbiters back on the manifest for an extended period.

Frit

Space Orbiter Glass uses specialized glass to craft the windows on their orbiter spacecrafts. These glasses must be resistant to extreme reentry temperatures in space – no small feat!

Space Orbiter Glass utilizes a special type of fused silica called frit, which is then compacted and baked at ultra-high temperatures to create an optically clear and heat-resistant material.

Space Orbiter Glass uses aluminum oxynitride (AlON), a special glass material that starts as a fine powder. This frit is then tamped and baked into an armor-piercing ceramic that can stop 50 caliber rounds.

AlON for windows is an example of why special glass is necessary in spaces that must be highly reliable and strong. A 1.6 inch thick piece of AlON can completely stop a 50 caliber round, making it the ideal material for spacecraft windows that will be exposed to harsh environmental conditions.

In addition to fused silica, the orbiter windows are made with a special glass called Macor, developed by Corning Inc. This material is an advanced ceramic glass ceramic that can be machined like metal – perfect for space shuttle windows!

To guarantee the tiles and thermal blankets on orbiters are securely attached, tile holders on the Columbia shuttle were glued together with special adhesives that could withstand space’s extreme temperature changes. After being coated with a protective layer, these pieces would prevent moisture absorption – adding weight to the orbiter.

The orbiter windows consist of three panes of glass, each with its own special properties. For the outer pane, fused silica is used to withstand extreme atmospheric reentry temperatures; inside is a pressure pane reinforced for vacuum in space; and finally, middle pane is thicker and stronger glass reinforced to withstand high cabin pressures in space.

Tile Retainers

The Space Shuttle orbiter is the vehicle responsible for transporting astronauts and payloads into low Earth orbit before returning them safely back on Earth. Its primary defense against heat is its Thermal Protection System, a set of ceramic tiles designed to shield it from thousands of degrees Fahrenheit during re-entry.

Each tile is custom-cut to fit the orbiter, which then gets installed at Florida’s Kennedy Space Center. They range in thickness from half an inch up to four inches depending on how much heat resistance is necessary.

Workers attach tiles to flexible felt-like pads attached to an orbiter in order to hold them firmly in place. These prevent the skin of the orbiter from shifting during reentry as it contracts and expands.

Another method for installing tiles is to leave small gaps between them. However, these openings can still allow plasma leakage through, so installers plug them with fabric sheets known as gap fillers.

Other methods for ensuring the tiles don’t come off during reentry include inspecting them before and after each flight, replacing them as necessary (about 30 to 100 tiles are replaced per mission), and repairing and refurbishing damaged ones.

These repairs may involve the use of an emittance wash, a chemical that looks like shoe polish and has been used by astronauts during spacewalks. This mixture, composed of silicon carbide fibers and special glue, can increase the radiant heat emitted by damaged tiles by up to 70 to 160 degrees Fahrenheit.

Last summer, NASA conducted an emittance wash test aboard STS-114 without incident or concern. This marks the first time a repair material had ever flown on board a shuttle flight, according to NASA spokesman Scott Hodge.

On board the orbiter, crew members are trained to detect when a tile needs repair or replacement. They can then contact a technician to inspect its condition, which plays an important role in safeguarding against high temperatures and air deflections during re-entry.