1. Challenges of hypersonic air-breathing flight Dr. David M. Birch Head, Flow Control Research Group Technical Director, Surrey Sensors LLC*

2. Access to space Getting there is 90% of the problem. Ariane 5 775 tonnes Rosetta probe 1.5 tonnes

3. Access to space Getting there is 90% of the problem. Scania N230UD 18 tonnes

4. So why is it so difficult? Aerodynamic drag: minor problem. Gravity: Major problem.

5. Climbing out of Earth's potential well Low-altitude ideal rocket equation Velocity at MESAltitude at MESBurn duration

6. Climbing out of Earth's potential well It takes 50% of your fuel to get to 12% of your desired altitude.

7. Although small, drag is still there Drag is proportional to dynamic pressure Density drops exponentially with altitude ISO 2533:1975 ICAO Standard atmosphere

8. Although small, drag is still there M = 1 2 3 4 5

9. The verdict: You want to light your main engines at as high an altitude as possible. Carry rocket to altitude by other means Liftoff from "airborne launchpad"

10. Options available: Current standard: launch from another rocket. Saturn V stage 3 separation

11. Options available: UI "Rockoon" (1952) Launch from a weather balloon? Launch from an aircraft? Scaled Composites SpaceShip One (2003) NorAm X-15 (1959-1970)

12. Options available: Fire out of a giant gun? Jules Verne, De la Terre à la Lune (1865)

13. Options available: Fire out of a giant gun? HARP main gun firing (1965) Prof. G. V. Bull (1928-1990) Martlet 2G-1: 4-stage orbital insertion variant (1969)

14. Options available: Douglas D-558-2 "Skyrocket" Stick wings and a turbojet on your rocket, and fly it to altitude.

15. The "spaceplane" concept The single-stage-to-orbit concept has been around for a long time. Convair Spaceplane (1961) (1970) Space Shuttle original concept (1970)

16. The "spaceplane" concept (a) Oxidizer mass is a killer. Nearly half of liftoff mass is just oxygen (in one form or another). OK, so harvest oxygen from environment. The catch:

17. The "spaceplane" concept (b) Temperatures become a problem. The catch: Engine will melt before combustion even begins! Supersonic combustion Conventional combustion Subsonic combustion Burner inlet Mach number Flight Mach number Lines of constant burner inlet temperature: T ~ 1700 - 2000 C

18. The "spaceplane" concept (c) Flight envelope is limited. The catch: Aircraft breaks apart P > 90 kPa Aircraft drops out of sky P < 20 kPa Engine melts M > 6 P = 90 kPa 20 kPa h > 90,000 ft Engine suffocates Lines of constant dynamic pressure Flight Mach number

19. The "spaceplane" concept But you no longer need mechanical compression! Ramjets, scramjets No moving parts! Cannot self-start... On the other hand... Flight Mach number Available ram compression ratio Conventional turbofans Engine melts

20. Ramjets Ramjets are ideal for Mach 1 ~ 3, and are a well-proven technology. Proven technology LeDuc 010 (1949)

21. Ramjets Proven technology InletCombustorNozzle Fuel injectors / flameholders Ramjets are ideal for Mach 1 ~ 3, and are a well-proven technology.

22. Ramjets Proven technology Scramjets are still highly experimental, but work best from Mach 4 ~ 10 InletCombustorNozzle Fuel injectors / flameholders

23. Ramjets Proven technology InletCombustorNozzle Fuel injectors / flameholders Scramjets are still highly experimental, but work best from Mach 4 ~ 10

24. Supersonic combustion ramjets Scramjets are still highly experimental, but work best from Mach 4 ~ 10 Contoured inlet diffuser Internal compression and expansion Wall-mounted hypermixing fuel injectors Spike nozzle profile external expansion Not-so-proven technology MiG-21 (1959) M = 2.05 Rocketdyne RS2200 (1998)

25. Supersonic combustion ramjets Scramjets are still highly experimental, but work best from Mach 4 ~ 10 Spike nozzle profile external expansion Not-so-proven technology

26. Supersonic combustion ramjets Not-so-proven technology Scramjets are still highly experimental, but work best from Mach 4 ~ 10 NASA X-43 (2004)Boeing X-51 "Waverider" (2010)

27. Supersonic combustion ramjets So why the problems? The practical problems of employing supersonic combustion are very great: It is necessary to capture a stream tube of supersonic air, inject fuel, achieve a fairly uniform mixture of fuel and air, and carry out the combustion process--all within a reasonable length, and preferably without causing a normal shock within the engine. There is currently no conclusive evidence that these requirements can be met. Weber, R. and McKay, J. S. (1958) "Analysis of Ramjet Engines Using Supersonic Combustion," NACA TN 4386 There are still serious questions as to whether or not stable supersonic combustion is possible over the required range of burner entry conditions. Heiser, W. and Pratt, D. (1994) "Hypersonic Airbreathing Propulsion," Washington:AIAA

28. a)the fuel and oxidizer must be combined in the appropriate relative quantities; b)the fuel and oxidizer must be mixed; c)the activation energy must be exceeded, and d)the fuel and oxidizer must remain together for some finite time, since the reaction takes some time to occur. No problem  Big problem No problem ~ Medium problem Supersonic combustion ramjets So why the problems? Combustion is just an exothermic chemical reaction with a reasonably low activation energy. For it to happen,

29. Problems so far: Must increase speed with altitude, to hypersonic range Must combust supersonically for M > 6 or engine will melt Alternatively, must pre-cool inlet air before combustion Turbocompressors won't work above M > 2, and ram compression won't work for M < 2 Thrust available is limited by melting point of materials

30. Combined cycle propulsion One option Flight Mach number Turbojet Ramjet Scramjet Rocket

31. M < 0.8 Turbojet 0.8<M<2 Variable geometry inlet Combined cycle propulsion One option 2<M<5 Turbojet inlet closed Ramjet mode Ram inlet open Turbojet exhaust closed

32. Combined cycle propulsion One option 5<M<10 Scamjet mode Variable combustor nozzle geometry M>10 Rocket mode Ram inlet closed Oxidizer injection

33. Aspirating turbopumps Another possibility Comp Turb Fuel in Oxy in Burn Exhaust Burn Conventional staged combustion turbopump Rocketdyne RS-25

34. Aspirating turbopumps Another possibility Burn Comp Turb Fuel in Oxy in Liquid air Air in Mix Air-breating, staged combustion turbopump Balepin 2003 Burn Rich exhaust BIG PROBLEM

35. More problems... Supersonic heat exchanger Cross-flow fins for best heat exchange, but powerful shocks and losses (both potential and mass flow) result. Wall-flush heat exchangers minimally intrusive, but minimally effective. Heat exchange rate linked to cryogenic fuel flow rate: required fuel flow rate dictated by engine design, exchange rate dictated by altitude. Inlets must be HUGE for useful amount of air ingestion at altitude Again, the required oxidizer flow rates are HUGE. Fundamental problems of supersonic flows

36. So... We need high speed air-breathing launch platform for step- change in orbital insertion cost. High speed means high altitude and therefore low thrust; High speed means either supersonic combustion or heat exchange. We're getting there. What we have learned