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SpaceX's New Combustion Technologies
An anonymous reader shares this story that takes a look at some of the advances SpaceX is working on. "Getting a small group of human beings to Mars and back is no easy task, we learned at the recent GPU Technology Conference in San Jose hosted graphics chip and accelerator maker Nvidia. One of the problems with such a mission is that you need a very large and efficient rocket engine to get the amount of material into orbit for the mission, explained Adam Lichtl, who is director of research at SpaceX and who with a team of a few dozen programmers is try to crack the particularly difficult task of better simulating the combustion inside of a rocket engine. You need a large engine to shorten the trip to Mars, too....Not only do you need a lot of stuff to get to Mars and sustain a colony there, but you also need a way to generate fuel on Mars to come back to Earth. All of these factors affect the design of the rocket engine....As if these were not problems enough, there is another really big issue. The computational fluid dynamics, or CFD, software that is used to simulate the movement of fluids and gases and their ignition inside of all kinds of engines is particularly bad at assisting in rocket engine design. 'Methane is a fairly simple hydrocarbon that is perfectly good as a fuel,' Lichtl said. 'The challenge here is to design an engine that works efficiently with such a compound. But rocket engine CFD is hard. Really hard.'"
They think what they're doing is "hard"? What the hell do they know? I once had to scale a Ruby on Rails web app so it'd handle more than 8 requests per second. Let me tell you, that makes fluid dynamics and rocket engines and trips to Mars look easy-peasy!
This is the first article I've seen that explains well how GPUs can/are being used for practical applications along with what can be achieved and some of the issues. Well worth the read even if you're not into this stuff.
I'm sure that there is a significant cost in developing this new approach to CFD (as well as pushing the envelope on GPU operation) but the result is going to be usable for different applications. TFA says there's irony in what SpaceX is doing here as it has applications with automotive Internal combustion engines but I see that as SpaceX/Musk having a secondary revenue stream for this work that doesn't mean he's helping out his direct competitors.
Along with that, they are driving the development of high speed inter GPU communications which I'm sure has value as well.
All this means is that Musk returns to his home planet, not only is the trip going to be fully funded, but he's going to have some money to throw around when he gets there.
myke
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You cannot leave earth or mars surface using an ion engine.
It's not exactly brain surgery, though.
https://www.youtube.com/watch?...
Umm, rocket science is...rocket science?
Combustion CFD is a very difficult area. The problem is that there are so many interlinked phenomena all requiring special modeling methods that one really isn't quite certain of the accuracy of the result unless they can compare it to a physical model test, which is what is frequently done. Simply getting the correct boundary conditions can be very challenging. Failing to apply appropriate modeling and boundary situations leads to a garbage in/garbage out situation, but the numerical solution may look plausibly correct.
CFD is not use exclusively in design work except for very basic cases where the modeling accuracy is well understood. However, CFD for more complicated situations is still useful as it may illustrate behaviors and trends in performance in situations where physical observations are difficult (like in a rocket nozzle). The CFD results can be used to guide and interpret the results of physical testing.
Understanding CFD really requires PhDs who understand fluid dynamics as well as the limitations of the numerical models used. This is true in many industries, not just rocket surgery.
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As it happens, back in the '80s I worked at a company (Commonwealth Scientific) that built ion-beam guns based on the Kaufman duoplasmatron, which was the basis of the mercury-vapor thrusters that NASA had developed in the 1960s. The company was trying to make the aperture of the guns as wide as possible, and the difficulties included neutralizing the ion beam on the way out, keeping the plasma inside the gun stable, and keeping the beam density even. Basically, the bigger the gun, the harder it was to make it run steadily. When I was there, they had 8" apertures and were working on scaling them up to 12" apertures.
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The gravitational constant is G, and is the same everywhere...it's a physical constant. The surface gravitational acceleration of Mars is different because of its lesser mass. And apart from the problem of the atmosphere, having surface gravity of about 1/3 of Earth's is nowhere near enough to make ion propulsion useful for launch, an ion propulsion system with a nuclear reactor and propellant would easily weigh around ten thousand times what it could actually lift on Mars. The only bodies where launch could be usefully performed or assisted by ion thrust are asteroids and comets.
Ion engines use very high amounts of power and very low flow rates of propellant. They provide a benefit when you need low amounts of thrust for a long period, and have either plentiful solar power or a nuclear power source. They could be used for shipping bulk supplies ahead of a manned expedition, but a manned expedition itself or any other mission with tighter than usual time constraints will use chemical propulsion, or at most nuclear thermal propulsion. These relatively low-Isp systems require more propellant for a given delta-v, but can achieve accelerations millions of times higher than ion engines, and do so without heavy power systems and gigantic radiators.
All real fluids have a finite Reynolds number, which tells you offhand how much grid refinement it takes to resolve the smallest scale directly. Since for supersonic flow in a rocket engine R is usually stupid high, the small scale turbulence is too small for direct resolution so you resort to turbulence models (e.g. RANS - Reynolds Averaged Navier Stokes) which is in itself an entire industry.
That part is relatively well developed and it's actually approaching the point that (with a team of experts who can recognize the defects on sight) things like CFD wing design are approaching predictive rather than "hey, the CFD actually got it right for a change. Woo!" The challenge for rocket engines is that you're not considering a single fluid, or even a two-phase flow, but a reactive flow which (if you look at all the paths even methane combustion goes through) contains about a hundred components, meaning a hundred flows, with 100 godawfully stiff nonlinear rate equations coupling them - in every single cell! This is the crux that largely stymies effective CFD of combusting flows.
I'm an astrophysics guy so I mostly get to watch from a distance and cringe in horror. We consider ourselves to be Doing Well if we look at gas/dust or neutrals/ions. Really good is looking at neutrals/ions/electrons. We do have our own 100-coupled-rate-equations horror show in examining the nucleosynthesis going on behind a supernova blast front.
The matter of computability is this: Watch a river flow, a prototypical turbulent system if ever there was one. Below the mercurial, ever fluctuating turbulence, you notice persistent, standing structures. Many flows of interest have a similar structure. The flow of water in to a nuclear reactor plenum, air over a car, the atmosphere - Turbulence superimposed on a coherent larger structure. Trying to model the exact turbulence is, as you say, chaotic and pointless: Paths depart exponentially. But if you can model the chaotic part you can still learn about the underlying nonchaotic structure.
In spectrum space, what I'm describing are systems where the turbulence lives in high-wavenumber modes and interacts in some relatively predictable way with the lower wavenumber modes describing the structures of interest. When something breaks down into complete turbulence (e.g. a Rayleigh-Taylor unstable turnover in the atmosphere - Have you ever been in a placid afternoon, then out of nowhere, huge gusts in random directions out of nowhere? R-T overturn), whole new animal...
I hope they simulate propane too, not just methane. Propane has some really interesting properties as rocket fuel but have (like methane) never gotten much research. But now there's a big rush to research methane as fuel based on the concept of generating it on Mars - so propane still gets left in the dark.
Methane's ISP is only very slightly better than propane's - 364,6 vs. 368,3 at a 100:1 expansion into vacuum and 20MPa chamber pressure. But propane at around 100K (note: not at its boiling point, 230K) has far higher density (782 kg/m^3), closer to that of room temperature RP-1 (820 kg/m) then that of boiling point methane (423 kg/m^3), which reduces tankage mass and cost. 100K propane's ISP is of course better than RP-1's 354.6 in the same conditions as above. Plus, its temperature is similar enough to your LOX that they can share a common bulkhead, which reduces mass further and simplifies construction.
Hydrogen generally is the easiest fuel to synthesize offworld. Methane is generally second, and propane third. Hydrogen is often rejected as a martian fuel because of the tankage and cooling requirements. Methane can be kept as liquid on Mars with little cooling in properly designed reflective / insulated containers - but so can 100K propane, in similar conditions, but with significantly smaller tankage requirements.
It seriously warrants more research, I tell you what.
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