Northrop Grumman and the Air Force Research Laboratory have begun a four-year project in an effort to produce reusable space launch vehicles that are less costly and more durable than current systems, an industry official tells sister publication Inside the Air Force.
The program, known as Advanced Development of Integrated Warm Structures, will demonstrate the relative weight and performance benefits of using different types of thermal protection systems (TPS) and composite materials to produce reusable, two-stage-to-orbit launch systems, Tod Palm, lead engineer for the Northrop team, said in a Dec. 4 telephone interview.
The concept research and development contract is worth $2.9 million.
“We’ve been talking with the Air Force for some time on thermal protection systems,” Palm said. “They are particularly interested in thermal protection systems that they can turn around quickly on the ground so that, if anything happens in terms of an impact or any damage, they can remove and replace the thermal protection systems quickly to get a responsive cycle time on the missions.”
Under the contract, Northrop and the Air Force will design and produce hardware that tests the performance and durability of both metallic and non-metallic TPS materials on traditional graphite-epoxy composites as well as a more heat-resistant composite called polyimide.
According to a NASA fact sheet, one derivative of polyimide, PMR-15, is “widely used” in both military and civilian aircraft engines, including the B-2 bomber, and “offers the combination of ease of processing, low cost, and good stability and performance at temperatures up to” 500 degrees Fahrenheit. However, PMR-15 is made from methylene dianiline, a carcinogen, and NASA researchers have been developing suitable replacements for the derivative, such as 2,2′-dimethylbenzidine, which has relatively the same sheer strength at both room temperature and 550 degrees Fahrenheit.
“The reason why temperature became an issue [in the current contract] is that you have this re-entry condition that you have to survive, and that results in high aerodynamic heating as you come in and re-enter just like the space shuttle,” Palm said. “A lot of Air Force systems are looking at a two-stage system and they want to recover both the lower stage and the upper stage. The lower stage might be at a lower Mach number — Mach 4 to Mach 10 — that you have to have the thermal protection system for.
“The upper stage, the orbiter, would be at a much higher Mach number when it re-enters,” he added. “These adverse thermal environments are really driven by the Air Force’s desire to have a reusable launch system and that makes it a much more severe thermal environment that you need to design for.”
In 2001 to 2004, Northrop did some development work on metallic TPS panels for NASA when the civil space organization was looking at reusable launch vehicles for its missions, Palm said. The defense giant will use some of the hardware designed for that project in this new contract “because it is very applicable to the Air Force mission set,” Palm said.
The first phase of the project is to mate the existing assets onto the graphite-epoxy carrier structure — which has a heat threshold of 250 degrees Fahrenheit — and test them, and the second phase will be to mate the TPS panels onto an airframe composed of polyimide, Palm said. The applications of non-metallic TPS will also be researched.
Metal alloys tend to be dense and heavy, whereas the non-metallic TPS panels add weight savings, which can improve the performance of the launch vehicle by cutting down the amount of insulation and fuel it would need, Palm said. This could lead to the service being able to put more payload into a particular launch or use a smaller launch vehicle than it would use with current spacecraft.
“The real question is which system is more durable to external impacts,” Palm said. “There’s a lot of confidence in metal being durable as an exterior surface of the airframe. There’s less confidence in the non-metallic material, and that’s something we need to consider in the selection of those systems.”
Actual cost savings would be designed specific to the launch vehicles, and, since this research contract is a ground panel-level test program, Northrop has not run through the cost parameter applications of this capability, Palm said.
Northrop is “putting all the pieces in place” and conducting initial design activity in order to begin hardware development next year, according to Palm.
Research will be conducted at an AFRL facility at Wright-Patterson Air Force Base, OH, which is capable of applying both the thermal condition and vibro-acoustic loading onto the panels to create the type of environment the materials would be in during a re-entry condition, Palm said. “That will improve confidence in the ability of this assembled system to perform and then the next step on that would be to scale it up to apply [it] on an actual vehicle and put it into a flight test type of environment,” he said.
This research also has applications for parts of aircraft near engines that operate at high temperatures, Palm added. Several years ago, Northrop and NASA used this research in a program called High-Speed Civil Transport, the goal of which was to create a supersonic commercial transport — with cruising speeds at Mach 2.4 — for which a high-heat-resistant airframe was needed due to the speed.
