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Space Forum / Space Flight / November 2003Pressure fed versus pump fed rockets
The advantage of a pressure fed rocket, as I understand it, is much cheaper (and somewhat smaller and lighter) engines, but at the cost of very heavy tanks that hold propellant at 250-300 psi, compared with the 20-30 psi tanks used in pump fed rockets. Would it not be possible to build very cheap low pressure pumps with 250 psi instead of the 1400-3600 psi used in pump fed rockets, so we would get the low cost advantages of pressure fed rockets without the disadvantage of very heavy tanks? Or am I missing something? (Apologies if I have posted this twice -- email is acting up) -- Larry > The advantage of a pressure fed rocket, as I understand it, is much > cheaper (and somewhat smaller and lighter) engines, but at the cost of [quoted text clipped - 9 lines] > > -- Larry My opinion is that the main thing holding back the cheap pumps is the concept that they are complicated and expensive in the first place. A reasonable effort can show that the pumps you descibe are doable by changing a few of the assumptions in the design. One of the first assumptions to be changed is that the thrust chamber is served by a pump system that must be designed for it. By doing low level systems engineering of the entire thrust package in parallel, several options can be made available. One of the simple ones is leaving the thrust chamber ablative or radiative in cooling, with the pump rotors stacked directly above the injection manifold, feeding it out of coaxial bowl volutes. Drive the tip turbine with gasses taped off the combustion chamber. A little thought will produce better geometries. > The advantage of a pressure fed rocket, as I understand it, is much > cheaper (and somewhat smaller and lighter) engines, but at the cost of [quoted text clipped - 5 lines] > the low cost advantages of pressure fed rockets without the disadvantage of > very heavy tanks? Or am I missing something? You are missing something. Maximizing rocket engine power is all about maximizing mass flow rate of propellants. Basic fluid dynamics laws: . > m = Pv (mass flow rate = density * volumetric flowrate.), In other words, if you want to move the same amount of mass per unit time at a lower pressure (hence lower outlet density), you need a larger volumetric flow rate--i.e., the pump has to be physically larger, and thus weighs more. I know it's somewhat counterintuitive, but lower pressure = bigger pump. This has a double effect, because the bigger the volume is, the greater the surface area of associated piping, thus the thicker the piping has to be for the same pressure, thus mass of the engine goes up again. Counterintuitive and ugly, but true. For equal mass flow rates, the higher pressure system is smaller and lighter (although it requires exponentially more power to drive it.) Pressure fed systems take the performance hit to avoid having a pump at all, as that gets rid of both weight and complexity on the engine side, although adding mass to the tank. Two things that make turbopumps so ugly is that 1. they have to be powered by something--i.e. you need a turbine of some sort to power the pump, and 2. the pump impeller has to be VERY carefully machined,and staged to prevent cavitation, as this would under most circumstances shake the engine apart and destroy the pump. Tom Merkle > 2. the pump impeller has to be VERY carefully machined,and staged to > prevent cavitation, as this would under most circumstances shake the > engine apart and destroy the pump. It's my understanding that quite a lot of rocket impellers cavitate over the whole flight. It's only got to last a few minutes... > Tom Merkle > > The advantage of a pressure fed rocket, as I understand it, is much > > cheaper (and somewhat smaller and lighter) engines, but at the cost of [quoted text clipped - 13 lines] > . > > m = Pv (mass flow rate = density * volumetric flowrate.), I believe you are missing much of his point about creating cheap components as opposed to high performance ones. You are also confusing gas compression and flow with pressurising incompressable liquids. A one inch pipe will flow 10 times as much gas at ten times the pressure at constant temperature. The same size pipe will flow almost identicle masses of incompressable liquid at 10 times the pressure. A <1% difference is not a consideration. > In other words, if you want to move the same amount of mass per unit > time at a lower pressure (hence lower outlet density), you need a [quoted text clipped - 7 lines] > and lighter (although it requires exponentially more power to drive > it.) The pump impeller and volute are about the only fuel handling components that increase in size, though not necessarily in mass as they can be thinner materials for lower pressures to be handled. The thrust chamber increases in size for lower pressures. However, he was discussing 250 psi pumped vs 250 psi pressure fed, which makes that point a non issue. Liquid flow pipe diameters are a function of of velocity and density, so for the same velocity, the same mass will flow. With similar masses flowing at lower pressures, thinner pipes can be used. > Pressure fed systems take the performance hit to avoid having a pump > at all, as that gets rid of both weight and complexity on the engine [quoted text clipped - 4 lines] > prevent cavitation, as this would under most circumstances shake the > engine apart and destroy the pump. A low pressure pump fed system does not have to be much more complex than a full up pressure fed system. A slightly higher thrust chamber pressure can save mass on the thrust chamber, canceling the mass penalty of the pump. Just as long as you stay off that slippery slope of max possible performance at all costs. Tank masses are only part of the problem, pressurant gas and systems are a mass and cost driver. I am willing to argue that they are more expensive than some pumps. XCORs' EZ-Rocket had helium as one of the main cost drivers per flight. On point 1. So what if you need a turbine. Turbines are not as super tech as some people tend to believe. The turbine off of a semi truck engine turbocharger makes a dandy test bench model for a small rocket engine turbopump. Mine came from a blown engine for $20.00. On point 2. Carefully machined impellers are available at your local industrial pump supplier. Scapped ones can be machined locally in an hour on old lathes at a normal rate of ~$55.00 an hour. Designing around cavitation is more of a consideration for very high performance pumps than for the ones under consideration here, though some attention must be paid to it. Also, it needs to be pointed out that the impeller tip speeds required here are 300-350 feet per second. These are the tip speeds of hand held demolition saws with $5.00 abrasive blades that are frequently out of balance and always loaded off center when working. > Tom Merkle John Hare > "Tom Merkle" <merkletr@msn.com> wrote in message > > > I believe you are missing much of his point about creating cheap components > as opposed to high performance ones. nope, got the point but questioned it's applicability to engine design. > You are also confusing gas compression > and flow with pressurising incompressable liquids. Guilty as charged. > A one inch pipe will > flow 10 times as much gas at ten times the pressure at constant temperature. > The same size pipe will flow almost identicle masses of incompressable > liquid at 10 times the pressure. A <1% difference is not a consideration. [snipped my now pointless blather] > The pump impeller and volute are about the only fuel handling > components that increase in size, though not necessarily in mass as > they can be thinner materials for lower pressures to be handled. oops. good point. Again, I was apparently lost in the weeds between higher and lower pressure engines, as well as gas vs. incompressible liquid flow. > The thrust chamber increases in size for lower pressures. However, > he was discussing 250 psi pumped vs 250 psi pressure fed, which > makes that point a non issue. Liquid flow pipe diameters are a function true. I have no response for that. > of of velocity and density, so for the same velocity, the same mass > will flow. With similar masses flowing at lower pressures, thinner [quoted text clipped - 15 lines] > turbocharger makes a dandy test bench model for a small rocket engine > turbopump. Mine came from a blown engine for $20.00. Were you able to hook an engine up so that its turbopump was not independantly powered? That's amazing if so. Using a turbocharger to work as a turbine for a bench model is a far cry from turbines being 'easy,' though. A lot of research, money, and years has gone into getting combustion engine turbo chargers right. I'm sure if as much commercial money was poured into turbopumps, they'd be every bit as cheap and reliable. If turbine technology was really that generally easy, though, we'd all be driving jet cars, wouldn't we? (cheaper fuel, higher fuel efficiency!) > On point 2. Carefully machined impellers are available at your local > industrial pump supplier. Scapped ones can be machined locally in an > hour on old lathes at a normal rate of ~$55.00 an hour. Designing around > cavitation is more of a consideration for very high performance pumps > than for the ones under consideration here, though some attention must Won't that make interesting (bad) things happen in the thrust chamber? > be paid to it. Also, it needs to be pointed out that the impeller tip speeds > required here are 300-350 feet per second. These are the tip speeds of > hand held demolition saws with $5.00 abrasive blades that are frequently > out of balance and always loaded off center when working. Although those bearings never have to work in a LOX environment, right? John Hare, thank you for the informative and unsarcastic correction. >From now on I'll leave explaining the engine science to people who have obviously built one. (not me) (chagrin) Tom Merkle > > On point 1. So what if you need a turbine. Turbines are not as super tech > > as some people tend to believe. The turbine off of a semi truck engine [quoted text clipped - 3 lines] > Were you able to hook an engine up so that its turbopump was not > independantly powered? That's amazing if so. I was trying to work out a propellant supply system. At the time I tried that, I was not interested in building the actual engine. If a supply system can be built on the cheap, I have friends with engines, stands, and experience. One spontaneous disassembly due to ignorance is enough for me. > Using a turbocharger to work as a turbine for a bench model is a far > cry from turbines being 'easy,' though. A lot of research, money, and [quoted text clipped - 3 lines] > really that generally easy, though, we'd all be driving jet cars, > wouldn't we? (cheaper fuel, higher fuel efficiency!) I deal with people that do jet dragsters, and others that deal with turbine drives through gear boxes. Turbines are not a good match for automotive velocities for the most part. In many ways, turbopumps for the low end pressures under discussion are easier than turbochargers. > > On point 2. Carefully machined impellers are available at your local > > industrial pump supplier. Scapped ones can be machined locally in an [quoted text clipped - 3 lines] > > Won't that make interesting (bad) things happen in the thrust chamber? Yes it would if it gets out of hand. The lower speeds here are just easier. > > be paid to it. Also, it needs to be pointed out that the impeller tip speeds > > required here are 300-350 feet per second. These are the tip speeds of [quoted text clipped - 3 lines] > Although those bearings never have to work in a LOX environment, > right? Right. Though I believe that may not be much of a problem. > John Hare, thank you for the informative and unsarcastic correction. > >From now on I'll leave explaining the engine science to people who > have obviously built one. (not me) I have not yet built succesful ones. The #1 problem I share with other$, i$ lack of fund$. I have been agressively addressing that issue to the exclusion of hardware for a couple of years. Homework and theory has not stopped. In many of the texts, they assume everyone is after max performance and you can find the easy, discard solutions with minimal effort. The easy ones are good enough for now. When I $olve problem #1, it's hardware again. > (chagrin) Unearned, I overstated my case. > Tom Merkle > Larry Gales <larryg@u.washington.edu> wrote in message > You are missing something. Maximizing rocket engine power is all about > maximizing mass flow rate of propellants. [snip] > In other words, if you want to move the same amount of mass per unit > time at a lower pressure (hence lower outlet density), you need a > larger volumetric flow rate-- This is not quite right. Most fuels and oxidisers can be considered incompressible. So the density is a constant. The only exception IIRC is liquid hydrogen. Going down in pressure won't always mean a smaller pump, but always a lighter pump. Pumping power is proportional to presser and volume (for incompressible liquids) so lower pressure means smaller turbine, low tip velocity etc. and thus everything can be made lighter. greg merkletr@msn.com (Tom Merkle) wrote in message > 2. the pump impeller has to be VERY carefully machined,and staged to > prevent cavitation, as this would under most circumstances shake the > engine apart and destroy the pump. > > Tom Merkle correction to my post(thanks Ian Wollard): I oversimplified quite a bit. should read "2. the pump impeller has to be VERY carefully machined to minimize cavitation", not prevent it. Some cavitation takes place in every pump, and is only disasterous when it occurs a lot. The damaging effects are not just from vibration but also from affecting the output. The impeller is also just one piece of an expensive pump. The turbine driving it, bearings, and associated piping are just as demanding in terms of tolerance as the impeller. Tom Merkle Larry Gales <larryg@u.washington.edu> wrote in message > Would it not be possible to build very cheap low pressure pumps with 250 > psi instead of the 1400-3600 psi used in pump fed rockets, so we would get > the low cost advantages of pressure fed rockets without the disadvantage of > very heavy tanks? Or am I missing something? The main reason that very high chamber pressure is used is because it produces an overall lighter engine for a given thrust. Well for sea level engines anyway. Higher chamber pressure means higher expansion ratio and hence better performance, it also means a smaller engine although stronger (hence heaver per unit area, its still a net win). But high chamber pressure means heaver turbo-pumps and as the SSME have shown they can be problematic to develop and operationally expensive (although this could have a lot to do with hydrogens low density ie the same performance turbo-pump for RP1 should be lighter/cheaper). One problem is just how much pressure gets wasted in cooling passages etc. IIRC the SSME turbo-pumps run at a pressure of about 5000psi but the thrust chamber runs at only 3500psi or thereabouts. I think that if commercial considerations are more of a design driver you would end up with pump fed engines, but with lower chamber pressures. To 'recover' some performance, some kind of altitude compensation is probably going to have lower operation costs than very high chamber pressure engines. This is not really going to work for a SSTO where sea level T/W of the engines is much more critical and turbo-pumps really are worth there weight in gold. Personally i think if you stay away from liquid hydrogen then high performance turbo pumps with reasonable operational and development cost should be possible. But so far history has proved me wrong. greg > Larry Gales <larryg@u.washington.edu> wrote in message > [quoted text clipped - 6 lines] > produces an overall lighter engine for a given thrust. Well for sea > level engines anyway. Higher chamber pressure means higher expansion How much better would it be if they, say, launched at 12,000 feet? > > Larry Gales <larryg@u.washington.edu> wrote in message > > [quoted text clipped - 8 lines] > > How much better would it be if they, say, launched at 12,000 feet? Yep. Its reasonable gain, which is why there are so many proposals using some form of high altitude launch. But 12,000 feat seems a little low if your going to all that effort. > Date: 14 Nov 2003 21:29:27 -0800 > From: Greg <gewi001@phy.auckland.ac.nz> [quoted text clipped - 35 lines] > > greg ------------------------ Thanks for your very informative reply -- I learned a lot. -- Larry Greg> One problem is just how much pressure gets wasted in cooling passages Greg> etc. IIRC the SSME turbo-pumps run at a pressure of about 5000psi but Greg> the thrust chamber runs at only 3500psi or thereabouts. Even worse than that: >From page 166, Design of Liquid-Propellant Rocket Engines: HPFTP discharge: 6024.8 psia HPOTP discharge: 6952.2 psia Page 92: (following the H2) Main combustion chamber Coolant inlet pressure: 5890 psia Coolant exit pressure: 4800 psia Back to page 166: HPFTP turbine inlet: 4933 psia (?!?) HPFTP turbine exit: 3376 psia Throat stagnation pressure: 3010 psia I don't understand the rise in pressure from chamber coolant exit to HPFTP turbine inlet. Presumably this happens through the fuel preburner... I would have expected a pressure drop there (mostly from the injectors). On page 113 the book suggests that a 20% pressure drop across the injectors is a good value because it gives good isolation between chamber pressure variation (NASA says 5% variation is "stable combustion") and propellant flow rate. It looks like the SSME gets by with a 12% drop instead, but maybe that's because they're injecting hot turbine exhaust rather than cold dense propellants. [...] >From page 166, Design of Liquid-Propellant Rocket Engines: > HPFTP discharge: 6024.8 psia > Page 92: (following the H2) > Back to page 166: > On page 113 the book suggests that a 20% pressure drop across the Iain, I haven't seen this one listed on the group (though I missed Scott's jpegs this time around). Could you add author info, and maybe date? Tnx /dps > [...] > >From page 166, Design of Liquid-Propellant Rocket Engines: [quoted text clipped - 6 lines] > Scott's jpegs this time around). Could you add author info, and maybe > date? Huzel and Huang, "Modern Engineering for Design of Liquid-Propellant Rocket Engines", Fourth printing, 1992, AIAA I may have misled you by dropping the first three words. Sorry about that. Where (and what) are Scott's jpegs? BTW, I read http://yarchive.net/space/rocket/rocket_design_books.html and then hit amazon.com. My wife was NOT happy with the bill! > Huzel and Huang, "Modern Engineering for Design of Liquid-Propellant > Rocket Engines", Fourth printing, 1992, AIAA Thanks! > [...] Where (and what) are Scott's jpegs? His bookshelf pictures. Scott Lowther, that is. > BTW, I read > http://yarchive.net/space/rocket/rocket_design_books.html > and then hit amazon.com. My wife was NOT happy with the bill! But you'll actually read the books, and that's even scarier! /dps |
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