Artist's impression of a hypersonic weapon. (Raytheon)
Air Force and Army scientists are making progress in their effort to bring hypersonic weapons development to a new level of effectiveness by exploring ways to better manage “heat flow” during hypersonic flight.
The fundamental challenge with hypersonic flight resides in this need to manage the extreme temperatures reached at speeds five-time greater than the speed of sound, factors which can prevent, complicate or disable successful hypersonic flight.
An area of focus within this sphere of inquiry, Air Force Research Lab and Army developers tell Warrior, relates to several complex aerodynamic challenges, such as managing the airflow surrounding the vehicle in flight. Referred to by scientists as a “boundary layer,” the airflow characteristics of a hypersonic weapon’s flight trajectory greatly impact the stability of the system - much of which relates to temperature.
With this in mind, the Army Research Laboratory is now using 3D printing technology to explore new materials that might help scientists engineer weapons with optimal airflow characteristics.
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“You can print things like conformal heat exchangers, which are ways to integrate cooling within a structure itself to help maintain a lower temperature,” Dr. Brandon A. McWilliams, materials engineer, lead for metals added manufacturing, Army Research Lab, Combat Capabilities Development Command, told Warrior in an interview at Aberdeen Proving Ground, Md.
Working in close concert with Army scientists and weapons developers, the Air Force is moving quickly to better understand the many variables associated with heat flux on hypersonic weapons. This will allow us to do optimization on thermal management,” Tim Sakulich, Air Force Research Lab director of materials and manufacturing and lead on implementing the Air Force science and technology strategy, told Warrior in an interview last fall at an Air Force Association Symposium. “We are designing these systems to provide the speed, reach and lethality we are looking for.”
The science of airflow boundary layers is extremely complex, yet it does align with several key aerodynamic concepts related to hypersonic flight stability. Simply put, engineers are looking to create advanced hypersonic weapons that generate a “laminar” or smooth air-flow boundary layer, as opposed to a “turbulent” air-flow. The more movement, mixing or agitation in the airflow surrounding the air vehicle in flight, often consisting of movement or particle collisions in the airflow, the more turbulent it becomes, according to an essay from the University of Sydney’s School of Aerospace, Mechanical & Mechatronic Engineering. (Australia).
“A boundary layer may be laminar or turbulent. A laminar boundary is one where the flow takes place in layers...each layer slides past the adjacent layers. This is in contrast to turbulent boundary layers, where there is intense agitation,” the 2005 essay states.
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Of particular significance, the essay explains that turbulent boundary layers generate very high “heat transfer rates.”
“Packets of fluid may be seen moving across (in turbulent boundary layers). Thus there is an exchange of mass, momentum and energy on a much bigger scale compared to a laminar boundary layer,” the University of Sydney essay states.
In summary, as opposed to microscopic exchanges in mass known to occur in laminar boundary layers, turbulent boundary layers involve mixing across layers on a macroscopic level, the paper explains.
All of this leads to a current area of focus for scientists, who are experimenting with more ways to ensure laminar air-flow boundaries surround hypersonic flight vehicles. Laminar boundary layers are needed to advance hypersonic flight to a new generation, according to a significant paper from NASA, the Air Force Research Lab and Case Western Reserve University called “Recommendation for Hypersonic Boundary Layer Transition Flight Testing.”
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“In regards to a next-generation hypersonic vehicle, the design goal would be to maintain a laminar boundary layer for as long as possible in order to minimize heating. Small perturbations to the boundary layer can excite various instability modes,” the essay states. (NASA Langley Research Center, Air Force Research Laboratory, Case Western Reserve University… Scott Berry, Roger Kimmel, Eli Reshotko)
Increased heat can bring challenges; it strengthens the weapon's thermal signature, making it easier for sensors to track. Heat challenges can also introduce difficulties by creating a need to engineer a weapon able to withstand the heat levels and remain intact during high speed flight. For this reason, hypersonic weapons -- and ICBMs as well -- are constructed with specially engineered heat-resistant materials. Sakulich emphasized that current AFRL work is, along these lines, focused on finding newer composite materials.
Improving hypersonic propulsion will not only improve the effectiveness and resiliency of existing weapons but also enable different form factors such as larger, longer or differently shaped attack weapons. The NASA-AFRL-Case Western essay, for instance, introduces the additional technical complexity that might be needed to advance hypersonic flight stability for “re-entry” bodies, such as those used on a nuclear-armed missile.
“Generally, the application of this knowledge (boundary layer management) has been restricted to simple shapes like plates, cones and spherical bodies. However, flight reentry vehicles are in reality never simple,” the NASA, AFRL essay states.
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For example, rougher surface material or weapons vehicles with less linear configurations present additional complicating variables believed to impact the stability of hypersonic flight. Engineering scientific methods for increasing the laminar boundary layer properties of hypersonic vehicles, it seems apparent, could help lay a foundation for newer, next-generation hypersonic configurations, such as differently shaped drones or weapons with various warheads.
An interesting RAND essay, called “Hindering the Spread of a New Class of Weapons,” explains that heat signatures are impacted by the shape, size, velocity and trajectory of a weapon.
“The larger the nose radius, the smaller the heat transfer on the nose of the vehicle. Trajectory shaping, i.e., velocity and altitude, can also be used to manage the total heat transfer on an RV (Re-entry Vehicle) while meeting other input requirements and constraints, e.g.,range, maximum deceleration, and time of flight. Hypersonic weapons have different constraints and requirements compared with reentry bodies. HGVs (Hypersonic Glide Vehicles) and HCMs (Hypersonic Cruise Missiles) will tend to have sharp leading edges, i.e.,a small nose radius, which will increase the heat transfer,” the essay states. (RAND - Speier, Nacouzi, Lee)
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Also, most hypersonic weapons need to travel for long periods of time at high speeds, when compared to a re-entry body traveling at hypersonic speed … therefore …” two of the major parameters in the total heat equation, velocity and time, cannot generally be reduced,” the RAND paper states.
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