Quote from: livingjw on 09/19/2017 07:20 pmrsdrvis9, Close, but not quite: - The combustion physics requires a certain dwell time in order to efficiently mix and burn. This dwell time is captured by the parameter "Characteristic Length", L* = Vc/Ath. RP1/Lox L* = 102-127 cm. CH4/Lox is probably a little higher maybe around 150-160 cm. So knowing your chemistry gives you an L* which allows you to calculate a combustion chamber volume, Vc = L* x Ath; hence, Vc is proportional to Ath. If you look at rockets of different thrust, you will clearly see that lower thrust rockets have proportionally bigger combustion chambers and higher thrust rockets have smaller chambers. The same goes for the pre-burners. Also note that these components handle the highest pressures. To summarize: - For a given cycle, a rocket engine weight scales roughly with thrust because: - Throat area and mass flow are proportional to thrust. - The pre-burners, combustion chamber and plumbing are pressure vessels proportional to throat area. - The turbines and pumps are proportional to pressure and mass flow. - Most of the expansion nozzle contains low pressure, its weight is dominated by minimal unit wall weight proportional to nozzle area.- A sphere 2x bigger has 4x the surface area and 8x the volume. At the same pressure,with the same material allowables it is 8x heavier, so the weight per unit volume stays the same. This is classic pressure vessel behavior.JohnJohn, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.
rsdrvis9, Close, but not quite: - The combustion physics requires a certain dwell time in order to efficiently mix and burn. This dwell time is captured by the parameter "Characteristic Length", L* = Vc/Ath. RP1/Lox L* = 102-127 cm. CH4/Lox is probably a little higher maybe around 150-160 cm. So knowing your chemistry gives you an L* which allows you to calculate a combustion chamber volume, Vc = L* x Ath; hence, Vc is proportional to Ath. If you look at rockets of different thrust, you will clearly see that lower thrust rockets have proportionally bigger combustion chambers and higher thrust rockets have smaller chambers. The same goes for the pre-burners. Also note that these components handle the highest pressures. To summarize: - For a given cycle, a rocket engine weight scales roughly with thrust because: - Throat area and mass flow are proportional to thrust. - The pre-burners, combustion chamber and plumbing are pressure vessels proportional to throat area. - The turbines and pumps are proportional to pressure and mass flow. - Most of the expansion nozzle contains low pressure, its weight is dominated by minimal unit wall weight proportional to nozzle area.- A sphere 2x bigger has 4x the surface area and 8x the volume. At the same pressure,with the same material allowables it is 8x heavier, so the weight per unit volume stays the same. This is classic pressure vessel behavior.John
Quote from: Semmel on 09/19/2017 07:25 pmJohn, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.It is not an analogy. Its engineering. L* does depend on mixing details and may be lower for gas/gas mixing in the main combustion chamber. That does not invalidate the engineering approach.
John, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.
Quote from: livingjw on 09/19/2017 07:34 pmQuote from: Semmel on 09/19/2017 07:25 pmJohn, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.It is not an analogy. Its engineering. L* does depend on mixing details and may be lower for gas/gas mixing in the main combustion chamber. That does not invalidate the engineering approach.Sorry, didnt mean to be emphasize the term 'analogy'. I enjoy your engineering approach (not only here) quite a bit. I however wanted to point out that your point about the length of the cumbustion chamber actually should be reversed. Hence a higher T/W ratio and the balancing factors do not apply here.Also, as ZachF pointed out, integrating the LOX preburner/turbine onto the head of the injector makes quite a lot of sense. I bet this design saved them quite a lot of grief.
- My post addressed why T/W tends to stay constant when you scale a rocket engine. I agree that integrating the LOX pre-burner/turbine into the head of the main chamber should increase T/W. I also agree that gas/gas mixing and combustion might have a lower L* than liquid/liquid or liquid/gas, which would also increase T/W. These are the types of details that can get you from 160 - 200 T/W. They won't get you to 350. Either way, the T/W trend with thrust will continue to be relatively flat.
Out of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.
Quote from: Semmel on 09/19/2017 09:31 pmOut of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.Rocket Propulsion Elements - SuttonMy copy is about 20 years old, but the 9th edition is Feb 2017.
Quote from: MikeAtkinson on 09/19/2017 10:14 pmQuote from: Semmel on 09/19/2017 09:31 pmOut of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.Rocket Propulsion Elements - SuttonMy copy is about 20 years old, but the 9th edition is Feb 2017.NASA SP-125 is still one of the most complete. I have attached the PDF (Don't you love the internet!). It covers theory to practical design details with lots of drawings and graphs. Couple this with NASA's online CEA program and you have a very good start. SP-125 mostly covers gas generator cycles. Your education won't be complete until you dig into the Russian staged combustion engines. They are truly phenomenal. In SP-125 a staged combustion cycle is referred to as dual combustion cycle. I had Sutton's book for an undergraduate course in 1971. It was OK, but not as much detail as in SP-125. SP-125 helped get us to the Moon.
I had the impresssion occasionally that developing rocket engines was a lost art in the US, reinvented only when SpaceX and BO appeared on the scene.So many threads on potential new launch vehicles over the years and they always looked which engines are available off the shelf, not developing a new engine tailormade for the needs.
Peter,Yes, they are doing very good work. More to the point, SpaceX is modeling the mixing and combustion chemistry. Combustion is mostly limited by mixing, and mixing as the video shows is fractal crazy! This kind of work goes back decades, but is now advancing rapidly with the advent of affordable massively parallel computers. The combustion reactions themselves go back even further and are well characterized. I just now went to NASA's old Chemical Equilibrium Analysis, CEA sight and ran a Methane-Oxygen case. See attached. The chemical equilibrium assumption works very well for rocket chambers, not so well for nozzles, but we can approximate non-equilibrium effects well enough. I doubt we will see any surprises, just lots of great detail to guide the design and development.John