What are the different hypersonic propulsion technologies (e.g. Ramjet vs. Scramjet vs. Rotating Detonation vs. rockets)

• The Defense Department aims to field the first scramjet-powered cruise missile by fiscal 2027.
  • Northrop Grumman is the supplier of the scramjet engine for the Raytheon version of DARPA’s Hypersonic Air-breathing Weapon Concept (HAWC) demonstrator.
    • Northrop demonstrated a cracking-free combustor in 2015, which solved the challenge of providing efficient supersonic combustion without the need to crack the long strings of molecules in hydrocarbon fuel.
    • The breakthrough came in advance of the Raytheon-Northrop team’s successful HAWC demonstrator flight test at hypersonic speed in September 2021.
  • Northrop Grumman also demonstrated a significantly larger scramjet engine in 2019, the “10X” ground-test article that demonstrated more than 13,000 lb. of thrust inside a wind tunnel at the Arnold Engineering Development Complex in Tullahoma, Tennessee.
    • The 10X engine offers a path to a propulsion system for a large, reusable hypersonic vehicle, which could be used for space launch or other missions.
• Hydrocarbon fuels for hypersonic propulsion offer several operational advantages because:
  • They are easier to store and preserve.
  • They have a higher storage density.
  • When injected into the combustion chamber and regenerative cooling channels around the scramjet, hydrocarbon fuels are able to work at very high pressures above the critical pressure of the fuel, thereby increasing operational efficiency.
• Supersonic combustion without fuel cracking is only one of several critical challenges facing scramjet engine developers.
• Scramjets have several advantages over rockets for hypersonic propulsion, including:
  • Greater range and speed.
  • Improved fuel efficiency.
  • Reduced engine weight.
  • Reduced launch costs.
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• Video provides a basic introduction to ramjets and scramjets.
  • Ramjet engine: air enters through the front, compressed to supersonic speeds by the diffuser, fuel is injected and burned in the combustor, and high-speed exhaust is released through the nozzle at the back.
  • Scramjet engine: air enters through the inlet, does not slow down to subsonic speeds (that is, no diffuser), fuel is injected and burned supersonically in the combustor, and high-speed exhaust is released through the nozzle at the back.
• Hypersonic weapons and vehicles mentioned briefly
• A comment explains how a converging-diverging nozzle works in relation to thermodynamics.
  • For a converging-diverging nozzle to create supersonic exhaust, there must be a sufficient pressure difference between the inlet and outlet.
  • A nozzle does not add energy to a system.
  • If there is no pressure difference, a shock will occur in the nozzle, causing the exhaust to be slower than the intake (kinetic energy is lost to heat from the shockwave).
• Another comment clarifies that a difference in static pressure between the nozzle inlet and exhaust section is what is needed for a ramjet to operate.
  • A pressure drop is not required.
  • Pressure losses are the consequence of an imperfect flow (wall friction, turbulence, etc.).
• A third comment mentions a report that evaluates Russia and China’s hypersonic arsenal and states the United States is unlikely to field an operational system before 2023.
  • The report is called “Hypersonic Weapons: Background and Issues for Congress”.

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• Ramjet:
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• Scramjet:
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• Rotating Detonation:
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• Rockets:
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How do hypersonic propulsion technologies compare in terms of fuel efficiency?

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What are the environmental impacts of using hypersonic propulsion technologies?

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What are the key challenges in developing a rotating detonation engine?

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What are the advantages of a scramjet engine compared to other propulsion technologies?

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What are the key components of a ramjet engine?

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• Ramjet, Scramjet, Rotating Detonation, and rockets are hypersonic propulsion technologies.
  • Jet engines require air to be compressed before being mixed with fuel and ignited, so it shoots out the back to make the aircraft go.
  • In a turbojet, there is a fan to compress the air; in a ramjet, the speed of the aircraft moving fast compresses the air when it hits the engine.
  • A scramjet is a type of ramjet where the air moves faster than the speed of sound when it is compressed and mixed with the fuel to explode.
  • A rotating detonation engine is still in the experimental stages and still under the testing phase.
• A turbojet uses a turbine and compressor to compress the air before combustion: powered by a mechanical turbine after combustion. It creates thrust while sitting still.
• Ramjets use the velocity of incoming air, clever geometry, and have no moving parts to get compression. They generate thrust only at high velocity, so they need another way to accelerate the vehicle, make use of clever geometries or slow the air down.
• Scramjets are ramjet engines with different clever geometry, allowing the air to continue to flow through it at supersonic speed. They generate thrust with an air-breathing engine at higher Mach numbers but require an engine to get the vehicle into the supersonic range before it starts.

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What are the advantages and disadvantages of Ramjet, Scramjet, Rotating Detonation, and Rocket propulsion technologies?