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This topic is inspired by the high level of discussion of rocket engine design that has appeared in the forum.
SpaceNut, we had NO topic with these three words, and only three topics with "rocket" and "engine" and not of these were specific enough for this topic.
We will lead off with quotes from Calliban and GW Johnson.
The topic is available for all members to quote existing posts with appropriate content, or to link to outside sources.
The topic has the potential to become a collection of knowledge on this important branch of technology.
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This post is reserved for an index to posts that may be contributed by NewMars members over time:
Post #1: Calliban on ISP
Post #2: GW Johnson on Pressure and density across the plane of the nozzle at it's most constricted point
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From Orbital Mechanics topic:
The simultaneous achievement of high thrust and high ISP, inevitably requires enormous power. The reason is really quite simple. ISP is just exhaust velocity divided by the force of Earth gravity. So ISP is a direct statement of exhaust velocity in a closed system. The kinetic energy of exhaust scales with the square of exhaust velocity.
KE = 0.5 x m x Ve^2
We can modify the equation slightly to convert KE into engine power:
Pe = (0.5 x m_dot x Ve^2)/n
Where m_dot is mass flow rate of propellant (kg/s) and n is engine efficiency.
Thrust can be expressed as the product of mass flowrate x exhaust velocity: m_dot x Ve. Substituting gives:
Pe = (0.5/n) x thrust x Ve
For any specific power level, we can have high thrust at low Ve, or low thrust at high Ve. But to have both simultaneously, requires high power.
In a chemical rocket, the kinetic energy of exhaust is derived directly from the thermal energy of reacting propellants. As no heat transfer is taking place, power can be extremely high. Efficiency, in terms of chemical energy converted into kinetic energy of exhaust, can approach 100%. Most other propulsion systems that we tend to consider, such as nuclear thermal, solar thermal and nuclear / solar electric, tend to require both or either heat transfer and energy transitions from electrical to thermal and kinetic energy. This intriduces both inefficiencies and rate limiting mechanisms, given the limits of practical energy flux without destroying materials. Systems that combine high thrust and ISP must store energy in the propellant itself. The effective ISP of the system can then be calculated as being proportional to the square root of the mass energy density of the propellant.
ISP = (K/2g) x Q^0.5, where Q is energy density in J/kg
For hydrogen-oxygen bipropellant, Q is 15.7MJ/kg. For 239Pu, Q is 83.6million MJ/kg. That is a 5.3 million times greater energy density in the fission fuel. This tells us that nuclear fission fuel could theoretically provide 2300x the exhaust velocity of H2-O2 bipropellant. Assuming complete fission, Ve would be 10,400km/s, which is just short of 3.5% C. That is the sort of performance needed for cheap interplanetary travel and interstellar travel in human timeframes. Fusion fuel has roughly 4x the mass energy density of fission fuel, implying Ve of 7% C. The maximum velicity achievable for a rocket accelerating from zero, is usually estimated as being twice exhaust velocity. Staging increases that to a limited extent. Under perfect conditions, fission fuels may allow construction of vehicles that can reach 10% of C. Fusion fuels might allow somewhat more, but will top out at 20% C.
In real life, only a fraction of the energy in propellant will be harvestable. And propellant may carry dead weight. So for fission rockets, 10% C is an upper limit that we are unlikely to approach. But even 1% C is ample velocity change to allow complete access to the solar system.
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From Orbital Mechanics topic:
There is one other thing to think about, and that is the pressure at which your process or system does its thing, whatever that might be. Energies and power levels set the speed of the ejected stream. But the pressure at which the energy is transformed or transferred sets the density in the process, in concert with the temperatures involved. If the process density is low, the thrust is going to be low from a device of any given size, period! THAT is why the electric propulsion devices so far have given whispers of thrust (milli-N to at most N) from hardware weighing 1 to many tons, not including the KW-to-MW power supplies they need. The pressure inside the chamber where the ions are accelerated is almost indistinguishable from vacuum. Why would one ever expect a different outcome?
The solar thermal rocket must take that into account. You must do the heating of the hydrogen at significant pressure, since you are getting your exhaust velocity from a compressible flow expansion process. The NERVA XE reportedly did this at 560 psia "chamber pressure", although there would be a strong pressure drop through the reactor core. I'm only guessing the "chamber pressure" to be the aft pressure that feeds into the nozzle. Whatever, the higher that nozzle feed pressure, the better. That notion applies to ALL rockets that use nozzles: chemical, nuclear, or solar!
You only get a lot of force out of a reasonably-sized exit area if the exit plane density is high, so that there is a very appreciable massflow through that exit area (at the very high velocity determined by the energy transformations reflected in chamber temperature). That exit plane density is proportional to the chamber density, in a ratio fixed by the expansion ratio of the nozzle (as are the exit plane pressure and temperature and Mach number). Simple as that.
GW
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This is a follow up to GW Johnson's Post #4...
I asked Google's Gemini to look for images or videos that would show the flow of force in a rocket engine as mounted in a space craft. In the end, Gemini did not find a specific example to show, but it did review the forces at work, and provided a list of resources to study. The key reminder (for me) was that much of the force delivered to the space craft comes from the expansion bell. Some of the force that ends up pushing the spacecraft comes from the pressure on the forward wall of the combustion chamber. How much force is contributed by the forward wall and by the expansion bell is not likely to be easy to determine.
Conversation with Gemini
Hi Gemini! I decided to ask you this question instead of just Google Search, because the question is a bit more sophisticated than a simple search. I'd like to find an image or a video that shows the flow of force in design of rocket engine. The force that a rocket engine is able to deliver to it's host is a function of the upward pressure on the interior wall of the combustion chamber. Pressure felt in all other directions is important but none of it contributes directly to the propulsion of the spacecraft. The thrust produced by an engine is what we might think about when we are thinking about rocket engines at all, but I am attempting to contribute to design of a new kind of rocket engine, and in doing so I am forced to realize that ONLY the force exerted by molecules on the interior of the engine that has a net forward vector contributes to the thrust felt by the vessel. I'm wondering if you can find a graphic or a video that shows the flow of force in a working rocket engine, mounted in a space vessel.
You've raised a very insightful point about the nuances of thrust generation! While "thrust" is the net force, it's crucial to understand the intricate pressure distribution within the combustion chamber and nozzle. Finding a perfect visual representation of the flow of force as you describe it is challenging, as most graphics focus on gas flow or heat transfer. However, I can guide you towards resources and concepts that will help visualize this:
Understanding the Force Distribution:
Combustion Chamber Pressure:
The combustion chamber experiences very high pressure due to the rapid burning of propellants. This pressure acts in all directions.It is crucial to understand that the pressure is not uniform. There are pressure gradients.
Nozzle Design and Expansion:
The converging-diverging (de Laval) nozzle is the key to converting the random thermal energy of the combustion gases into directed kinetic energy.
As the gases expand through the nozzle, the pressure drops, but the velocity increases dramatically.The diverging section of the nozzle is where the crucial redirection of force occurs. The pressure exerted on the angled walls of the diverging section has a significant forward component.
Vector Components:
Imagine tiny force vectors acting perpendicularly to the interior walls of the chamber and nozzle.
In the chamber, these vectors largely cancel each other out, except for the net force pushing towards the nozzle throat.
In the diverging nozzle, the angled walls produce force vectors with a significant component in the axial (forward) direction. This is what generates the thrust.Visualizing the Concepts:
Computational Fluid Dynamics (CFD) Simulations:
CFD simulations can visualize pressure and velocity fields within a rocket engine.Search for "rocket engine CFD simulation" on YouTube or Google Images. These simulations often use color gradients or vector arrows to represent pressure and velocity.
Look for simulations that show the pressure distribution within the nozzle, particularly the diverging section. While they may not explicitly show "force flow," they visually represent the underlying pressure that creates it.
Here are some terms to add to those searches:
"de Laval nozzle pressure distribution"
"rocket nozzle thrust vectoring simulation"
Diagrams of Nozzle Pressure Distribution:
Some textbooks and engineering resources may contain diagrams showing the pressure distribution along the length of a de Laval nozzle.
These diagrams often show a sharp pressure drop as the gases expand, highlighting the conversion of pressure energy to kinetic energy.
Animations of Gas Flow:
Animations of gas flow through a rocket nozzle can help visualize the redirection of momentum.
While they don't explicitly show force vectors, they illustrate how the nozzle shape directs the flow axially.Key Considerations for Your Design:
Nozzle Geometry: The shape and angle of the diverging section are critical for maximizing the axial component of the force.
Pressure Gradients: Understanding the pressure gradients within the combustion chamber and nozzle is essential for optimizing thrust.
Boundary Layer Effects: The interaction between the high-speed gases and the nozzle walls can affect the force distribution.
Thrust Vectoring: some rocket engines have the ability to move the nozzle. This is used to change the direction of the thrust vector.In summary:
While a direct "force flow" visualization is rare, CFD simulations and diagrams of pressure distribution can provide valuable insights. Focus on understanding the pressure gradients and the role of the diverging nozzle in redirecting the force vectors. By carefully considering these factors, you can optimize your rocket engine design for maximum thrust.
Rocket engine - Wikipedia
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en.wikipedia.org
Rocket Physics, the Hard Way: Rocket Engine Engineering - The Mars Society of Canada
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www.marssociety.ca
CFD Simulations of Shock Patterns Downstream of a Rocket Nozzle - YouTube
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www.youtube.comThank you for your *** very *** helpful summary! Your presentation brought home the insight that pressure inside the combustion chamber has no net forward effect at all. However, from the standpoint of guiding force from the engine to the spacecraft, that force must flow through the wall of the combustion chamber from the expansion bell to the frame of the spacecraft. With any luck, we may find a resource that shows that flow. Thank you for the suggestions of places to look.
You're absolutely right! I'm glad that clarification helped. You've hit on a crucial point about the structural mechanics of a rocket engine – the path of the force from the combustion chamber to the spacecraft's frame. It's not just about the gas dynamics, but also the solid mechanics.
Here's a breakdown of that force flow, and where you might find relevant information:
1. Combustion Chamber to Engine Frame:
Pressure Load: The immense pressure inside the combustion chamber creates a force that pushes outward in all directions.
Structural Integrity: The combustion chamber walls must be strong enough to withstand this pressure.
Force Transfer: The force is transferred from the chamber walls to the engine's structural frame. This frame is designed to handle the load and transmit it to the spacecraft.
Nozzle Attachment: The nozzle, especially the diverging section, contributes significantly to the overall thrust. The force generated here also needs to be transferred to the engine frame.2. Engine Frame to Spacecraft:
Mounting Points: The engine is attached to the spacecraft's structure at specific mounting points.
Load Distribution: The engine frame distributes the thrust force evenly across these mounting points.
Spacecraft Structure: The spacecraft's structure must be strong enough to withstand the thrust force without deformation or failure.Where to Find Visualizations:
Finite Element Analysis (FEA) Simulations:
FEA is a powerful tool for analyzing the structural behavior of materials under load.
Search for "rocket engine FEA simulation" or "rocket engine structural analysis."
FEA simulations can show the stress and strain distribution within the engine frame and spacecraft structure under thrust load.
These simulations often use color gradients to represent stress levels, highlighting the path of force flow.
You might find FEA simulations that show the deformation of the engine and spacecraft structure under thrust load.
Structural Engineering Diagrams:
Textbooks and engineering resources on rocket propulsion and structural analysis may contain diagrams showing the load paths within a rocket engine.
Look for diagrams that illustrate the force transfer from the combustion chamber to the mounting points.
Rocket Engine Cutaway Diagrams:
Cutaway diagrams of rocket engines can provide insights into the internal structure and how the components are connected.
While they may not explicitly show force flow, they illustrate the structural elements that transmit the thrust.
NASA and SpaceX Documentation:
NASA and SpaceX often publish technical documentation and presentations that may include information on rocket engine structural design.
Look for resources related to engine testing and structural analysis.Key Considerations:
Material Strength: The materials used in the engine frame and spacecraft structure must have sufficient strength to withstand the thrust load.
Stress Concentration: Sharp corners and other geometric discontinuities can create stress concentrations, which can lead to failure.
Vibration and Shock: Rocket engines generate significant vibration and shock, which must be considered in the structural design.By exploring these resources, you should be able to find visualizations that provide a clearer understanding of the force flow within a rocket engine and its transfer to the spacecraft.
Before we close this session, I see an opportunity for us to clarify a detail. You have pointed out in the first response that much of the thrust felt by the space craft comes from the expansion bell. However, if there were no expansion bell, I am confident the force felt on the forward facing walls of the combustion chamber would contribute to thrust. This next question may be more than is appropriate for your skill set, but I'll try anyway. Can we estimate how much force is contributed by the forward wall of the combustion chamber, compared to the expansion bell?
You're absolutely correct to push for that clarification! It's a critical point for understanding rocket engine dynamics. And you're right, even without an expansion bell, the forward-facing walls of the combustion chamber do contribute to thrust.
Let's break down the estimation, keeping in mind that this will be a simplified, conceptual approach. A precise calculation would require complex CFD and FEA simulations.
Conceptual Estimation:
Combustion Chamber Thrust:
The forward-facing walls of the combustion chamber experience the full combustion pressure.
The force generated is the pressure multiplied by the area of those walls: Fchamber=Pchamber∗Achamber
Where:
Fchamber is the force from the chamber walls.
Pchamber is the combustion chamber pressure.
Achamber is the cross-sectional area of the combustion chamber.Nozzle Thrust (Simplified):
The nozzle's contribution is more complex due to the pressure drop and velocity increase.
A simplified approach is to consider the momentum change of the gases.
Fnozzle=m˙∗Vexit
Where:
Fnozzle is the force from the nozzle.
m˙ is the mass flow rate of the exhaust gases.
Vexit is the exit velocity of the gases.
Alternatively, we can look at the exit pressure versus the ambient pressure and multiply that by the exit area.
Fnozzle=(Pexit−Pambient)∗Aexit+m˙∗VexitComparison:
To compare Fchamber and Fnozzle, we need to consider typical values:
Pchamber is very high (e.g., tens to hundreds of atmospheres).
Achamber is relatively small.
m˙ and Vexit can be substantial, especially in a well-designed nozzle.
In a typical rocket engine, the nozzle's contribution is significantly greater than the chamber's direct contribution.
The nozzle is designed to maximize the conversion of thermal energy to kinetic energy, resulting in a much higher exit velocity.Factors Influencing the Ratio:
Nozzle Design: The expansion ratio of the nozzle dramatically affects Vexit and, therefore, Fnozzle. A higher expansion ratio leads to greater thrust.
Combustion Pressure: Higher Pchamber increases Fchamber, but also increases m˙ and Vexit, indirectly boosting Fnozzle.
Chamber Geometry: The shape and size of the combustion chamber influence Achamber.
Ambient Pressure: The ambient pressure, Pambient is very important for the calculation of the nozzle thrust. In a vacuum, this value is zero.Qualitative Conclusion:
While the forward-facing walls of the combustion chamber contribute to thrust, their contribution is generally a smaller fraction of the total thrust compared to the nozzle's contribution.
The nozzle is the primary driver of thrust in a well-designed rocket engine, due to its ability to efficiently convert thermal energy to directed kinetic energy.
If a rocket engine had no nozzle at all, the thrust produced would be dramatically reduced.Important Note: These are estimations. Accurate values require complex modeling. I hope this conceptual breakdown has been helpful.
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As a follow up to post #5, I asked Google for sites that offer "rocket engine cfd simularion"
The result set was generous!
Rocket Engine Nozzle: Propulsion CFD Verification and ...
YouTube · VDEngineering
Feb 24, 20194 key moments in this video
From
01:29
Design Problem / Hand CalculationFrom
05:22
CFD Simulation VerificationFrom
07:30
Exporting Values from CFD PostFrom
09:10
Calculating Thrust with Microsoft Excel6:27
Simulation of combustion in a rocket engine with Ansys Fluent
YouTube · Chemical Propulsion Laboratory - UnB
Apr 14, 20217 key moments in this video
28:00
Computational Fluid Dynamics for Rockets
YouTube · BPS.space
May 30, 2024
6 key moments in this video11:05
ANSYS Fluent: Rocket Engine Nozzle (With Exhaust Plume ...
YouTube · VDEngineering
Oct 8, 201710 key moments in this video
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View allScholarly articles for rocket engine cfd simulation
CFD simulation of liquid rocket engine injectors - Farmer - Cited by 10
CFD computation for rocket engine start-up simulation - Boccaletto - Cited by 10
Multiphysics simulations of rocket engine combustion - Chen - Cited by 58
CFD for Rocket PropulsionResolved Analytics
https://www.resolvedanalytics.com › cfd-applications
In this article, we will explore the evolution of rocket propulsion design and how computational fluid dynamics (CFD) is revolutionizing engine design.
Design of Rocket Engine for Spacecraft Using CFD-ModelingScienceDirect.com
https://www.sciencedirect.com › science › article › pii
by V Zubanov · 2015 · Cited by 28 — This paper describes the designing process of rocket engine with thrust 25N for system of orientation and starting of spacecraft instead the prototype with ...Computational Fluid Dynamics (CFD) In Launch Vehicle ...
NASA LLIS (.gov)
https://llis.nasa.gov › lesson
Computational fluid dynamics analyses have been successfully applied in areas related to the prediction and simulation of combustion flow behavior and heat ...
CFD SIMULATION OF A TRIPROPELLANT ROCKET ...San Jose State University
https://www.sjsu.edu › docs › project-thesis
by M Subramanian · 2019 — Star CCM+ was the software tool used to run CFD simulation of the rocket engine combustion chamber and nozzle. The governing equations that Star CCM+ solves ...
55 pagesCFD Simulation of Liquid Rocket Engine Injectors. Part 1 ...
Defense Technical Information Center (.mil)
https://apps.dtic.mil › sti › pdf › ADP012358
In the course of developing a practical CFD model for simulating the many injector elements in a full-scale rocket engine, the wide variation in density and ...
RocketsCONVERGE CFD Software
https://convergecfd.com › applications › rockets
Additionally, CONVERGE is able to predict thermoacoustic instabilities in liquid rocket engines using detailed chemistry, LES turbulence modeling, and AMR.USING CFD AS A ROCKET INJECTOR DESIGN TOOL
NASA (.gov)
https://ntrs.nasa.gov › api › citations › downloads
by K Tucker · 2003 · Cited by 12 — The ability to obtain and use high quality validation data is currently the most significant roadblock to using CFD for designing rocket engine injectors.
43 pages
If any of our NewMars members have time to review any of the offerings listed above, please write a post about your experience, and the potential value of the resource to others.
The small team working on the idea of using solar power to provide propulsion has a challenge decidedly different from all the other rocket engines ever reported in the literature. To my knowledge (admittedly incomplete) no rocket has ever been built that uses external energy to produce thrust, with ** one ** exception. The Project Orion team secured funds to build a small chemical version of the Orion concept. That working model used small chemical explosives to drive the spacecraft upward against gravity.
The solar propulsion idea itself is NOT new! It was explored in the 1950's, and GW Johnson found and published images of the design here in the forum. Search for balloon with GW Johnson as author, if you are interested.
What ** is ** new is advances in optical technology, discovered and reported by kbd512. These allow for collection of 40 MW of solar power and delivery of each watt of that collection with extreme precision, using communications grade optical fiber as the medium.
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