Physics:Hypersonic speed

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Short description: Speed that exceeds five times the speed of sound (Mach 5 and above)
"Hypersonic" redirects here. For other uses, see Hypersonic (disambiguation).

CFD image of the NASA X-43A at Mach 7

In aerodynamics, hypersonic speed refers to speeds much faster than the speed of sound, usually more than approximately Mach 5.[1][2]

The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since individual physical changes in the airflow (like molecular dissociation and ionization) occur at different speeds; these effects collectively become important around Mach 5–10. The hypersonic regime can also be alternatively defined as speeds where specific heat capacity changes with the temperature of the flow as kinetic energy of the moving object is converted into heat.[3]

Characteristics of flow

Simulation of hypersonic speed (Mach 5)

While the definition of hypersonic flow can be quite vague [1][lower-alpha 1] a hypersonic flow may be characterized by certain physical phenomena at very fast supersonic flow.[4]

  1. Shock layer [1]
  2. Shock interaction - aerothermal:[5] aerodynamic heating[1] of the fuselage [6]
  3. Entropy layer
  4. Real gas effects
  5. Low density effects
  6. Independence of aerodynamic coefficients with Mach number.

Small shock stand-off distance

As a body's Mach number increases, the density behind a bow shock generated by the body also increases, which corresponds to a decrease in volume behind the shock due to conservation of mass. Consequently, the distance between the bow shock and the body decreases at higher Mach numbers.[7]

Entropy layer

As Mach numbers increase, the entropy change across the shock also increases, which results in a strong entropy gradient and highly vortical flow that mixes with the boundary layer.

Viscous interaction

High-temperature flow

Classification of Mach regimes

Although "subsonic" and "supersonic" usually refer to speeds below and above the local speed of sound respectively, aerodynamicists often use these terms to refer to particular ranges of Mach values. When an aircraft approaches transonic speeds (around Mach 1), it enters a special regime. The usual approximations based on the Navier–Stokes equations, which work well for subsonic designs, start to break down because, even in the freestream, some parts of the flow locally exceed Mach 1. So, more sophisticated methods are needed to handle this complex behavior.[8]


Regime Mach No Speed General characteristics Aircraft Missiles/warheads
Subsonic < 1 [1] <614 mph (988 km/h; 274 m/s) Most often propeller-driven and commercial turbofan aircraft with high-aspect-ratio (slender) wings, and rounded features like the nose and leading edges.

The subsonic speed range is that range of speeds within which, all of the airflow over an aircraft is less than Mach 1. The critical Mach number (Mcrit) is lowest free stream Mach number at which airflow over any part of the aircraft first reaches Mach 1. So the subsonic speed range includes all speeds that are less than Mcrit.

All commercial aircraft —
Transonic 0.8–1.2 614–921 mph (988–1,482 km/h; 274–412 m/s) Transonic aircraft nearly always have swept wings that delay drag-divergence and supercritical wings to delay the onset of wave drag and often feature designs adhering to the principles of the Whitcomb area rule. —
Supersonic > 1 [1] 921–3,836 mph (1,482–6,173 km/h; 412–1,715 m/s) The supersonic speed range is that range of speeds within which all of the airflow over an aircraft is supersonic (more than Mach 1). But airflow meeting the leading edges is initially decelerated, so the free stream speed must be slightly greater than Mach 1 to ensure that all of the flow over the aircraft is supersonic. It is commonly accepted that the supersonic speed range starts at a free stream speed greater than Mach 1.3.

Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behavior of flows above Mach 1. Sharp edges, thin aerofoil-sections, and all-moving tailplane/canards are common. Modern combat aircraft must compromise in order to maintain low-speed handling; "true" supersonic designs, generally incorporating delta wings, are rarer.

—
Hypersonic > 5 [1] 3,836–7,673 mph (6,173–12,348 km/h; 1,715–3,430 m/s) Cooled nickel or titanium skin; small wings. The design is highly integrated, instead of assembled from separate independently-designed components, due to the domination of interference effects, where small changes in any one component will cause large changes in air flow around all other components, which in turn affects their behavior. The result is that no one component can be designed without knowing how all other components will affect all of the air flows around the craft, and any changes to any one component may require a redesign of all other components simultaneously . |
High-Hypersonic [10–25) 7,673–19,180 mph (12,348–30,867 km/h; 3,430–8,574 m/s) Thermal control becomes a dominant design consideration. Structure must either be designed to operate hot, or be protected by special silicate tiles or similar. Chemically reacting flow can also cause corrosion of the vehicle's skin, with free-atomic oxygen featuring in very high-speed flows. Hypersonic designs are often forced into blunt configurations because of the aerodynamic heating rising with a reduced radius of curvature. —
  • Soviet Union 53T6 (Mach 17)
  • United States HTV 2 (Mach 20)
  • India Agni-V (Mach 24)
  • China DF-41 (Mach 25)
  • France M51 (Mach 25)
  • Russia Avangard (Mach 20–27)
Re-entry speeds ≥25 ≥19,180 mph (30,870 km/h; 8,570 m/s) Ablative heat shield; small or no wings; blunt shape. See reentry capsule.

Similarity parameters

Hypersonic flows, however, require other similarity parameters. First, the analytic equations for the oblique shock angle become nearly independent of Mach number at high (~>10) Mach numbers. Second, the formation of strong shocks around aerodynamic bodies means that the freestream Reynolds number is less useful as an estimate of the behavior of the boundary layer over a body (although it is still important). Finally, the increased temperature of hypersonic flow mean that real gas effects become important. Research in hypersonics is therefore often called aerothermodynamics, rather than aerodynamics.[9]


Wallace D. Hayes developed a similarity parameter, similar to the Whitcomb area rule, which allowed similar configurations to be compared. In the study of hypersonic flow over slender bodies, the product of the freestream Mach number M∞ and the flow deflection angle θ, known as the hypersonic similarity parameter:K=M∞θis considered to be an important governing parameter.[9] The slenderness ratio of a vehicle τ=d/l, where d is the diameter and l is the length, is often substituted for θ.

Regimes

Perfect gas

Two-temperature ideal gas

Dissociated gas

Ionized gas

Radiation-dominated regime

  1. Optically thin: where the gas does not re-absorb radiation emitted from other parts of the gas
  2. Optically thick: where the radiation must be considered a separate source of energy.

The modeling of optically thick gases is extremely difficult, since, due to the calculation of the radiation at each point, the computation load theoretically expands exponentially as the number of points considered increases.

See also

Engines
Missiles
  • 3M22 Zircon Anti-ship hypersonic cruise missile Russia (in production)
  • BrahMos-II Cruise Missile – India Russia (Under Development)
Other flow regimes

Notes

  1. ↑ is generally debatable (especially because of the absence of discontinuity between supersonic and hypersonic flows)

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Leishman, J. Gordon (January 2023). 53 Hypersonic Flight Vehicles. Embry-Riddle Aeronautical University. https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/hypersonic-flight-vehicles/. Retrieved 28 June 2025. "Hypersonic flow is typically considered to begin around Mach 5, but this threshold is not sharply defined" - "Sonic Booms in Hypersonic Flight.". 
  2. ↑ Galison, P., ed (2000). "The Changing Nature of Flight and Ground Test Instrumentation and Data: 1940-1969". Atmospheric Flight in the Twentieth Century. Springer. p. 90. ISBN 978-94-011-4379-0. https://books.google.com/books?id=qfrOBgAAQBAJ&q=hypersonic. 
  3. ↑ "Specific Heat Capacity, Calorically Imperfect Gas". NASA. https://www.grc.nasa.gov/WWW/BGH/realspec.html. 
  4. ↑ GÜlçat, Ülgen (2021). "Hypersonic Flow". Fundamentals of Modern Unsteady Aerodynamics (3 ed.). Cham: Springer. doi:10.1007/978-3-030-60777-7. ISBN 978-3-030-60777-7. Bibcode: 2021fmua.book.....G. https://link.springer.com/chapter/10.1007/978-3-030-60777-7_7. Retrieved 6 July 2025. 
  5. ↑ Fengshou Xiao; Zhufei Li; Yujian Zhu; Jiming Yang (19 October 2016). "Hypersonic Type-IV Shock/Shock Interactions on a Blunt Body with Forward-Facing Cavity". Journal of Spacecraft and Rockets (American Institute of Aeronautics and Astronautics) 54 (2): 506–512. doi:10.2514/1.A33556. https://arc.aiaa.org/doi/full/10.2514/1.A33556. Retrieved 10 July 2025. 
  6. ↑ J. Urzay (July 2020). "The physical characteristics of hypersonic flows". Center for Turbulence Research. p. 3. https://web.stanford.edu/~jurzay/hypersonicsCh2_Urzay.pdf. 
  7. ↑ Shang, J. S. (2001-01-01). "Recent research in magneto-aerodynamics". Progress in Aerospace Sciences 37 (1): 1–20. doi:10.1016/S0376-0421(00)00015-4. ISSN 0376-0421. Bibcode: 2001PrAeS..37....1S. https://www.sciencedirect.com/science/article/pii/S0376042100000154. 
  8. ↑ "Hypersonics". doi:10.1007/978-3-030-60777-7_6. https://link.springer.com/chapter/10.1007/978-3-030-60777-7_6. 
  9. ↑ 9.0 9.1 Anderson, Jr., John D. (2006). Hypersonic and High-Temperature Gas Dynamics. AIAA Education Series (2nd ed.). American Institute of Aeronautics and Astronautics. ISBN 1-56347-780-7. 



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