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RGClark

Member

From: Philadelphia, PA

Registered: 2006-07-05

Posts: 744

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Re: Are radically reduced costs to space imminent with hypersonic flight?

…
EXACTLY those technologies were in ASALM-PTV,  a ramjet cruise missile prototype,  flight tested successfully in 1980.  This thing was boosted to ramjet takeover (NOT scramjet!!!) at Mach 2.5,  then flew in ramjet power to cruise at Mach 4 and 80,000 feet.  It would then dive onto its target at an average Mach 5. In one flight test,  we had a throttle runaway failure,  and it reached an unintended Mach 6 at only about 20,000 feet!  It would have melted and broken up,  had this been more than a several-seconds-long transient event. 
…

This articles discusses a scramjet research program that initially thought it had accomplished scramjet propulsion at Mach 6.4:

https://www.linkedin.com/posts/activity … member_ios

But later analysis showed it was actually subsonic, thus ramjet, combustion. This is interesting because it means ramjet propulsion can work at least to Mach 6.4. Most commonly ramjet propulsion is described as limited to ~Mach 5.5. If this test really did ramjet propulsion to Mach 6.4, then it raises the interesting question of how far can we push ramjet speeds.

  Bob Clark

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Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):

      “Anything worth doing is worth doing for a billion dollars.”

GW Johnson

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From: McGregor, Texas USA

Registered: 2011-12-04

Posts: 5,612

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Re: Are radically reduced costs to space imminent with hypersonic flight?

Bob:

I'm not sure what the modern consensus is,  being long retired from the rocket and ramjet business.  But long ago,  in the 1970's-early 1990's,  the consensus was about Mach 6 as the speed limit for subsonic combustion ramjet.  That's a fuzzy number for a limit,  and Fred Billig at JHU-APL thought it might be closer to Mach 7.  Fred is no longer with us,  but I sat in a hotel bar swapping ramjet tales with him in 1987.

The real problem is what kind of models can you use for a design analysis as the speed exceeds about Mach 6?  The inlet air is becoming hot enough to start ionizing,  at which point it is no longer really air,  and the standard compressible flow analyses begin to fail,  because their fundamental assumptions are being violated. 

Something similar is happening in the combustor at about that same fuzzy speed,  where the energy release of combustion is no longer just going into gas internal energy,  but also into its ionization.  Again,  the onset of ionization starts violating the fundamental assumptions of compressible flow analysis. 

At least back then,  the consensus was that ionization energy was not recovered as flow acceleration in a conventional nozzle.  Only the internal and pressure energies were recovered (together,  those are the enthalpy). 

The point is,  beyond something like Mach 6,  you start needing another design analysis model.  Compressible flow is becoming increasingly inaccurate,  and rather rapidly with speed increases beyond about Mach 6. That's not to say that ramjet won't work at higher speeds,  but you have little way to model and predict it.

What you will notice as you look around is that there is very little in the way of design analyses that adequately model scramjet,  other than CFD codes,  whose input requires that you already have design drawings from which you can set up the analysis grids.  The same sort of thing would be needed to model a Mach 7+ ramjet. 

And even then,  many crucial processes (such as physical chemistry) cannot really be included in a CFD code.  Turbulence models seemed to be a fatal bugaboo in CFD codes when I last worked in the industry,  but that seems to be resolved now.  But without transient physical chemistry,  you cannot model flameholding and ignition adequately.  Which is why actual test is so often,  and so vastly,  different from what the computer predicted.

GW

Last edited by GW Johnson (2024-06-27 10:02:22)

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GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

tahanson43206

Moderator

Registered: 2018-04-27

Posts: 18,104

Re: Are radically reduced costs to space imminent with hypersonic flight?

Here is an attempt by ChatGPT to evaluate the situation in discussion in this topic ...

Understanding the Physics of Combustion at Mach 6

1. Fundamental Chemistry and Physics:

• Review Basics of Combustion:

  • Understand standard combustion reactions, focusing on hydrogen and oxygen.

  • Study the principles of oxidation-reduction (redox) reactions.

• Ionization:

  • Learn about the ionization process, how molecules lose or gain electrons, and the energy levels involved.

  • Study how high temperatures and pressures affect ionization rates.

2. Hypersonic Flight Dynamics:

• Aerodynamics:

  • Understand the behavior of air at high speeds, including shock waves and boundary layers.

  • Study the effects of high velocity on air properties, such as temperature and pressure changes.

• Thermodynamics:

  • Delve into the principles of thermodynamics as they apply to hypersonic flight.

  • Study how high-speed airflow leads to significant aerodynamic heating.

3. Combustion at High Speeds:

• Reaction Mechanisms:

  • Investigate the changes in reaction pathways at elevated temperatures and pressures.

  • Understand the role of radical species and intermediates in high-speed combustion.

• Combustion Instability:

  • Study potential instabilities that might arise due to the altered chemistry at hypersonic speeds.

  • Learn about strategies to mitigate these instabilities.

4. Experimental and Computational Studies:

• Literature Review:

  • Read existing research on high-speed combustion, particularly studies involving ionized gases.

  • Look for experimental data from hypersonic wind tunnels and flight tests.

• Simulation and Modeling:

  • Use computational fluid dynamics (CFD) and chemical kinetics models to simulate combustion at Mach 6.

  • Explore how different ionization states of oxygen affect the reaction with hydrogen.

5. Interdisciplinary Collaboration:

• Work with Experts:

  • Collaborate with specialists in aerodynamics, thermodynamics, and combustion chemistry.

  • Engage with mathematicians for advanced modeling techniques and engineers for practical applications.

6. Application of Knowledge:

• Design and Testing:

  • Apply the theoretical and computational insights to the design of combustion chambers for hypersonic engines.

  • Test these designs under controlled conditions to validate the models and theories.

Detailed Explanation of Electron Transfer in Combustion:

Standard Combustion Reaction (STP):

• Reaction: 2H₂ + O₂ → 2H₂O

• Electron Transfer:

  • Hydrogen atoms donate electrons to oxygen atoms.

  • Oxygen accepts electrons, resulting in the formation of water.

  • Oxidation state changes: Oxygen is reduced (gain of electrons) to -2, and hydrogen is oxidized (loss of electrons) to +1.

Ionized Oxygen Combustion at High Speeds:

• Ionized Oxygen: O₂ → O₂⁺ + e⁻

• Increased Electron Need:

  • Ionized oxygen (O₂⁺) is missing electrons, making it a stronger oxidizing agent.

  • This increased electron affinity could enhance the reactivity with hydrogen.

• Combustion Dynamics:

  • Enhanced Reactivity: The stronger electron-accepting nature of ionized oxygen could lead to more vigorous reactions with hydrogen.

  • Thermal Activity: The energy released during these reactions might be higher, increasing thermal activity and potentially leading to greater thrust.

  • Pathways: New reaction intermediates and pathways might form, altering the combustion characteristics.

Conclusion:

Understanding combustion at Mach 6 involves studying the interplay of high-temperature chemistry, ionization effects, and high-speed aerodynamics. By focusing on these areas, you can build a comprehensive picture of how ionized oxygen affects combustion dynamics and apply this knowledge to hypersonic propulsion systems. This pathway integrates theory, simulation, and practical testing, providing a holistic approach to this complex problem.

(th)