Is Dream Chaser’s Heat Shield Any Better Than The Space Shuttles?
When reentering the Earth’s atmosphere, different spacecraft typically experience temperatures around 3000 degrees Fahrenheit or 1650 degrees Celcius. This immense heat along with other forces make this part of the mission by far one of the most complex and dangerous. Decades ago the Space Shuttle changed how we accessed space by implementing a large spaceplane that could be reused.
One of the core pieces in this process was its heat shield which consisted of tens of thousands of individual and specific tiles. Unfortunately, this method was far from perfect and caused multiple problems including significant safety concerns along with time delays. Focusing on today’s technology, Sierra Space is working on a next generation spaceplane named Dream Chaser which is scheduled to launch for the first time later this year.
While Dream Chaser has different goals and constraints, the two spaceplanes share a lot of similarities. This includes a heat shield with thousands of individual tiles that Sierra Space is confident is a significant improvement from the past Shuttle design. Here I will go more in-depth into the problem with the Space Shuttles’ thermal protection, how exactly Dream Chaser compares, why this is important for the future, and more.
Space Shuttle TPS
The Thermal Protection System was designed to provide a smooth, aerodynamic surface while protecting the underlying metal structure from excessive temperature. Several types of TPS materials were used on the Orbiter. These materials included tiles, advanced flexible reusable surface insulation, reinforced carbon-carbon, and flexible reusable surface insulation.
Specifically, NASA used two categories of Thermal Protection System tiles on the Orbiter—low- and high-temperature reusable surface insulation. Surface coating constituted the primary difference between these two categories. High-temperature reusable surface insulation tiles used a black borosilicate glass coating that had an emittance value greater than 0.8 and covered areas of the vehicle in which temperatures reached up to 1,260°C (2,300°F). Low-temperature reusable surface insulation tiles contained a white coating with the proper optical properties needed to maintain the appropriate on-orbit temperatures for vehicle thermal control purposes. The low-temperature reusable surface insulation tiles covered areas of the vehicle in which temperatures reached up to 649°C (1,200°F).
Unfortunately, tiles often fell off and caused much of the delay in the launch of STS-1, the first shuttle mission, which was originally scheduled for 1979 but did not occur until April 1981. Each tile used cement that required 16 hours to cure. After the tile was affixed to the cement, a jack held it in place for another 16 hours. In March 1979 it took each worker 40 hours to install one tile; by using young, efficient college students during the summer the pace sped up to 1.8 tiles per worker per week. Thousands of tiles failed stress tests and had to be replaced. By fall NASA realized that the speed of tiling would determine the launch date. The tiles were so problematic that officials would have switched to any other thermal protection method, but none other existed. The tile TPS was an area of concern during shuttle development, mainly concerning adhesion reliability. Some engineers thought a failure mode could exist whereby one tile could detach, and resulting aerodynamic pressure would create a “zipper effect” stripping off other tiles. Whether during ascent or reentry, the result would be disastrous. Another problem was ice or other debris impacting the tiles during ascent. All various issues that made the Shuttle’s TPS system far from the ideal option.
Dream Chaser Changes
Now that we know more about the Shuttles heat shield and some of the main problems, we can take a closer look at Dream Chaser’s design and what has changed. The first main difference is the number of tiles. Dream Chaser features around 2,000 tiles in total compared to the more than 20,000 on the Shuttle. While obviously an improvement, it’s important to point out that these two spacecraft are not even close to the same size. Dream Chaser is 30 feet, or 9 meters long—roughly ¼ the total length of the space shuttle orbiters. This means relatively Dream Chaser features around half as many tiles considering the size difference.
The most recent image of these tiles installed on the spacecraft came a few months ago when the company tweeted saying, “As Sierra Space prepares Dream Chaser for missions to the International Space Station in 2023, we move another step forward in realizing our team’s mission to build a platform in space to benefit life on Earth.” This image highlights a few things including the fact that each tile is unique depending on its location and features a different design and even color. In this case, the white tiles reject more heat from the sun while on-orbit, which helps to keep the components within Dream Chaser cooler.
As far as upgrades, Sierra Space is quoted saying, “SNC engineers have been able to update the TPS tiles from what was used during NASA’s shuttle program with more innovation, better technology, and utilizing lessons learned. They use more modern manufacturing techniques to increase strength and reduce cost. Another difference between the tiles is Dream Chaser tiles are about 10 inches by 10 inches, while those on the shuttle were six inches by six inches. Dream Chaser tiles are stronger and lighter weight than those used during the shuttle program and meet all Micro-Meteroid Orbital Debris (MMOD) requirements to ensure safe entry, descent, and runway landings for crewed or cargo missions.
Back in 2015, Sierra Nevada Corporation (the parent company of Sierra Space) successfully completed several significant Thermal Protection System (TPS) material development tests for its Dream Chaser spacecraft. The TPS tests were completed at NASA’s Ames Research Center and Johnson Space Center under reimbursable Space Act Agreements (SAA). At the time, the corporate vice president of SNC’s Space Systems commented, “Safety of crew and cargo is most important to our team as we continue to mature the spacecraft design. For several years, we have worked collaboratively with Johnson and Ames, leveraging their existing infrastructure, materials, and expertise to mature and customize the TPS for our unique spacecraft. Our TPS is lighter, stronger, and more efficient than previous generations. We have met or exceeded all mission requirements. We are now prepared to enter the Critical Design Review phase for this system.”
As far as the tests themselves, over 100 arc jet cycles and radiant heat tests were completed at Johnson’s Radiant Heat Test Facility (RHTF) and Ames’ Aerodynamic Heating Facility (AHF). RHTF provided results supporting thermal characterization of the developmental TPS materials. The test data was then used for thermal modeling, analysis, and TPS sizing. The Ames AHF arc jet tests were performed as a second phase in the development testing to gauge the material performance in environments simulating Dream Chaser flight conditions. Valuable arc jet test results supported SNC’s certification of the manufacturing capability of a high-temperature material called Toughened Uni-Piece Fibrous Reinforced Oxidation-Resistant Composite TUFROC. TUFROC will be used on the high-temperature nose and wing leading edges of the Dream Chaser spacecraft.
Starting almost a decade ago in 2014 Sierra Space began testing tiles and heat resistant materials. These efforts included over 350 tests that allowed SNC to select the optimal TPS architecture for Dream Chaser to safely fly through the high-heat-load atmospheric environment during nominal return to Earth as well as during high altitude ascent aborts. In addition to the TUFROC testing, arc jet cycles and radiant heat tests in high-heating, simulated re-entry environments were conducted to measure the thermal performance of new silica tile coating developed by NASA and SNC. SNC’s assessments showed that these new coatings offer the same thermal protection as previously flown tile coatings, but at a greatly reduced cost.
This brings us to the current day as we wait for the first launch of Dream Chaser Tenacity. Currently, this mission is still scheduled for the third quarter of this year and will launch atop United Launch Alliance’s Vulcan Centaur. A report released just a few weeks ago from the company was quoted saying, “The first Dream Chaser, Tenacity, is nearing completion and will subsequently ship to NASA’s Neil Armstrong Test Facility in Cleveland, Ohio, for final space environmental testing ahead of its first mission to the ISS later this year.
Just as important, the Vulcan Centaur is only months if not weeks away from its first mission. Depending on the results of this launch it could have both very positive and negative effects on Dream Chaser’s maiden flight. This being said, Vulcan’s first launch is already a few months behind at this point. Not long ago the goal was to ship Vulcan to the launch site in November and complete pre launch testing in December. Only weeks ago was the rocket finally shipped and now is expected to complete testing sometime this month.
Conclusion
The Space Shuttle was an incredible piece of engineering that helped build and create various space infrastructure still in use today. Its heat shield and TPS system however were by no means perfect, something Sierra Space is trying to change with its next generation spaceplane Dream Chaser. We will have to wait and see how it progresses and the impact it has on the space industry.