Researchers at NASA are using a novel thermal-imaging system on board a Navy aircraft to capture images of heat patterns that light up the surface of the space shuttle as it returns through the Earth’s atmosphere. The researchers have thus far imaged three shuttle missions and are processing the data to create 3-D surface-temperature maps. The data will enable engineers to design systems to protect future spacecraft from the searing heat–up to 5,500 degrees Celsius–seen during reentry.
“We want to understand peak temperatures, when they happen and where, because that determines the type of material for, and size of, a protection system,” says Thomas Horvath, principal investigator of the project, called Hypersonic Thermodynamic InfraRed Measurements (HYTHIRM), at Langley Research Center in Hampton, VA.
NASA has become more concerned with safety and developing tools for inspecting and protecting the shuttle since the 2003 space shuttle Columbia disaster, when damage to the shuttle’s wing compromised its heat-resistant shield, causing it to lose structural integrity and break apart during reentry, killing all seven astronauts aboard. Horvath, also a support team member of the Columbia Accident Investigation Board (CAIB), says the HYTHIRM project was developed in response to the Columbia accident.
“I certainly think [the researchers] can learn something about what causes the heating,” says Douglas Osheroff, a professor of physics and applied physics at Stanford University, and a member of the CAIB. He adds that the thermal images could also be used as a diagnostic tool to check the integrity of the shuttles tiles during reentry. Currently engineers must manually inspect the tiles upon the shuttle’s return.
To image the shuttle, the researchers used a novel optical system called Cast Glance on board a Navy P-3 Orion aircraft. The system is used mostly by the Department of Defense for missile defense missions and so had to be slightly modified for the NASA project. The Navy researchers added a high-resolution, off-the-shelf video camera and adjusted it to filter infrared light. They then calibrated Cast Glance’s optical sensors so that, by measuring the infrared radiation from the shuttle, they could calculate the surface temperatures.
The Navy aircraft flies to within 37 kilometers of the space shuttle when the latter is traveling at speeds of between two and three miles per second, acquiring eight uninterrupted minutes of data: approximately 10,000 to 15,000 images for each mission.
The researchers’ focus was the underbelly of the shuttle, which is covered by about 10,000 thermal protective tiles. The highest heating areas, near the nose and along the leading edge of each wing, are made of a material called reinforced carbon-carbon (RCC). As the shuttle pushes air molecules out of the way, says Deborah Tomek, project manager of HYTHIRM, a boundary layer or protective region, similar to insulation, forms around the shuttle where temperatures are between 1,093 and 1,649 degrees Celsius. Just outside that boundary layer temperatures can rise to a sweltering 5,500 degrees.
Any damage to the tiles, or a protrusion or bump on the underbelly of the shuttle, can cause a break in the boundary layer and allow in extreme heat. Of particular concern are the gap fillers, pieces of ceramic-coated fabric the thickness of a sheet of paper that fit between the tiles to provide cushion, which have been known to protrude. (NASA, however, says the fillers do not impose a safety concern.)
The Langley researchers imaged three shuttle missions: Discovery on March 28 (STS-119); Atlantis on May 24 (STS-125); and Discovery again on September 11 (STS-128). They also conducted two small flight-research experiments. “We added a tiny bump to Discovery’s wing, approximately a quarter of an inch, to better understand what is called a boundary layer transition or trip in the flow fields,” says Tomek. The researchers also coated two of the tiles with a material that is being developed for the heat shield of the Orion crew exploration vehicle.
The researchers are just beginning to process all the collected data into 3-D surface temperature maps, which they will compare with measurements from thermal sensors on the shuttle’s underbelly and with computational fluid dynamic models. Horvath says they will present their results at a conference in January 2010.
However, the researchers have already seen some unexpected results. A small imperfection, possibly as small as a tenth of an inch, on the opposite side from the bump purposely placed on Discovery’s wing, created high temperatures in a much larger area than what you normally see, says Horvath.
Osheroff says he is interested to see if the analysis finds different results for different orbiters. For example, Columbia was the first shuttle built and is 20,000 pounds heavier than the other orbiters. “Heating patterns depend on the attitude or orientation of the orbiter during reentry, so it would be beneficial to conduct tests for at least two flights of each orbiter.”
There are only six remaining space shuttle flights before the orbiters are scheduled to retire. Horvath says the researchers hope they can continue to image the remaining missions, but final approval is still pending. “Our ability to accurately predict thermal data will have a profound impact on designs for new vehicles,” he says.