To cool the jet from the heat of hypersonic travel
It would channel some of the air, flowing at supersonic speed, through a nozzle located on the nose of the aircraft, producing a counterflowing jet of air that would induce LPM (a novel aerodynamic phenomenon called ‘long penetration mode), which would in turn lead to a drop in surface temperature due to aeroheating and a reduction of the shockwave and noise caused by breaking the sound barrier.
According to the designer, the most difficult part of building the Antipode would be the development of a stable and reliable scramjet engine—a feat NASA has yet to accomplish. There’s also the question of how to alleviate the paralyzing g-forces passengers would experience on board the craft.
Long Penetration Mode Counterflowing Jets for Supersonic Slender Configurations — A Numerical Study (18 pages NASA, University of Alabama), The Cooper Union for the Advancement of Science and Art, Ohio State University)
A novel approach of using counterflowing jets positioned strategically on the aircraft and exploiting its long penetration mode (LPM) of interaction towards sonic-boom mitigation forms the motivation for this study. Given that most previous studies on the counterflowing LPM jet have all been on blunt bodies and at high supersonic or hypersonic flow conditions, exploring the feasibility to obtain a LPM jet issuing from a slender body against low supersonic freestream conditions is the main focus of this study. Computational fluid dynamics computations of axisymmetric models (cone-cylinder and quartic geometry), of relevance to NASA’s High Speed project, are carried out using the space-time conservation element solution element viscous flow solver with unstructured meshes. A systematic parametric study is conducted to determine the optimum combination of counterflowing jet size, mass flow rate, and nozzle geometry for obtaining LPM jets. Details from these computations will be used to assess the potential of the LPM counterflowing supersonic jet as a means of active flow control for enabling supersonic flight over land and to establish the knowledge base for possible future implementation of such technologies.
Prior studies on counterflowing jets (supersonic) issuing from a central nozzle located on the nose of a bluntbody into a supersonic freestream, indicate two modes of jet interaction, namely the short penetration mode (SPM) and the LPM. A strong bow shock, a jet terminal shock, a free stagnation point, and a recirculation region characterize the SPM as shown in Fig. 1(a). Here, the jet does not penetrate into the bow-shock and results in a stable flowfield configuration. SPM is observed for jets operating with large jet pressure ratios (large mass flow rates). Operating the jet under pressure conditions slightly larger than the nozzle design conditions results in what is known as the LPM jet. The LPM is an unstable flowfield characterized by the familiar diamond-pattern jet plume that penetrates into the bow-shock. When the jet, issuing from the nozzle, penetrates the bow shock, the shock standoff distance becomes significantly higher than that found in flows without significant shock penetration (e.g., a SPM jet or a no-jet case) and the shock strength decreases. The ability of the LPM to disperse and weaken the bow shock has been confirmed experimentally as well as computationally. LPM jets, however, only exist for a narrow range of conditions beyond which the jet switches into SPM.
The Antipode concept comes just months after Bombardier unveiled his designs for the Skreemr, a four-winged scramjet that could carry 75 passengers at speeds of up to Mach 10 - so, 10 times the speed of sound and five times faster than today’s Concorde jets
The feasibility of achieving counterflowing LPM jets issuing from slender bodies against low supersonic flow conditions, as a means towards sonic-boom mitigation, was numerically investigated in this study. Two slender-body geometries of interest to the High Speed project under NASA’s FAP, namely a 6.48-degree cone-cylinder and a quartic geometry were the subject of investigation here. A parametric study on system parameters needed to achieve an LPM jet issuing from these slender bodies, was carried out using axisymmetric CFD computations. The parameters explored in the study include (i) the sizing of the jet, (ii) optimal jet mass flow rate /pressure conditions, and (iii) nozzle geometry. From the parametric study, the ratio of the diameter at nozzle exit to the diameter of the body was identified to be one of the key factors. The smaller this ratio is, the easier it is to establish an LPM jet. A pressure range that will sustain an LPM jet was identified for each of the chosen geometries; and based on that, the jet mass flow rates corresponding to these pressures were found to be relatively small. This information on smaller mass flow rate requirements augurs well in case this technology has the potential for an implementation in a real flight system. Nozzle exit angle, with the exception of large exit angles (over 30°), was found to not have much of an impact on the jet development.
Given the challenges in capturing the farfield pressure signature in the presence of the LPM jet and without any in-built mesh adaptation capability, plenty of work remains towards evaluation of the actual impact of the LPM jet interaction on sonic-boom mitigation. Towards this end, we may have to rely on using a signature propagation tool that will utilize the flow solution from the body near-field region (less than one body length). These efforts will be reported in a later work.
SOURCES: Forbes, NASA, Science Alert, imaginactive.org