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alternate working fluids for nuclear thermal rockets?
Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. In particular, why is it CO2 doesn't seem to be discussed as a working fluid? It's cheap. I suspect low ISP is the answer but as I say my method turns out to be utter crap. James Nicoll -- It's amazing how the waterdrops form: a ball of water with an air bubble inside it and inside of that one more bubble of water. It looks so beautiful [...]. I realized something: the world is interesting for the man who can be surprised. -Valentin Lebedev- |
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alternate working fluids for nuclear thermal rockets?
James Nicoll wrote:
Is there some nice concise source for the ISPs of nuclear thermal rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. In particular, why is it CO2 doesn't seem to be discussed as a working fluid? It's cheap. I suspect low ISP is the answer but as I say my method turns out to be utter crap. CO2 is bad for two reasons. First, it has a high molecular weight, and does not easily break down into components of lower molecular weight. Second, it is oxidizing at high temperature, so it is not compatible with ultra high temperature refractory materials like carbon and carbides. Realize that ammonia will decompose into nitrogen + hydrogen at high temperature and low pressure, so the molecular weight of the exhaust of a nuclear thermal ammonia rocket is lower than you might expect. Paul |
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alternate working fluids for nuclear thermal rockets?
Mercury would be a nice candidate, but the folks down range wouldn't
appreciate it much... James On 9 Nov 2003 12:49:13 -0500, (James Nicoll) wrote: Is there some nice concise source for the ISPs of nuclear thermal rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. In particular, why is it CO2 doesn't seem to be discussed as a working fluid? It's cheap. I suspect low ISP is the answer but as I say my method turns out to be utter crap. James Nicoll |
#5
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alternate working fluids for nuclear thermal rockets?
In article ,
James Nicoll wrote: Is there some nice concise source for the ISPs of nuclear thermal rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. To a sloppy first approximation, it scales inversely with the square root of the molecular weight. However, that's not as simple a calculation as you might think, because some of the molecules in question break down -- in particular, an ammonia NTR will have a lot of nitrogen and hydrogen in its exhaust. With current technology, solid-core NTR is no better than chemical fuels unless it is running on LH2 or just possibly something (e.g. ammonia) that breaks down easily to a mix containing a lot of hydrogen. That's one reason why you don't see a lot of interest in alternate propellants. In particular, why is it CO2 doesn't seem to be discussed as a working fluid? It's cheap. I suspect low ISP is the answer but as I say my method turns out to be utter crap. Propellant cost is utterly insignificant for near-future NTR systems. CO2, with a molecular weight of 44, gives performance grossly inferior to a LOX/kerosene chemical rocket. Moreover, at high temperatures it shows some tendency to break down to CO and O2. This improves performance a little, but that free oxygen is very hard on an orthodox NTR core and chamber. (There are materials that can stand it, but they're very different from the ones in LH2 or ammonia NTRs, and the technology for oxidizing-propellant NTRs is poorly developed.) -- MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | |
#6
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alternate working fluids for nuclear thermal rockets?
(James Nicoll) wrote in message ...
Is there some nice concise source for the ISPs of nuclear thermal rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. www.astronautix.com has a lot of engines, and includes the ability to look at them by propellant. http://www.astronautix.com/props/nucmonia.htm http://www.astronautix.com/props/nuccohol.htm http://www.astronautix.com/props/nucarlh2.htm But there's no systematic method there for calculating Isp, just examples of nuclear thermal with various reaction mass types. Mike Miller, Materials Engineer |
#7
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alternate working fluids for nuclear thermal rockets?
(James Nicoll) wrote in message ...
Is there some nice concise source for the ISPs of nuclear thermal rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. In particular, why is it CO2 doesn't seem to be discussed as a working fluid? It's cheap. I suspect low ISP is the answer but as I say my method turns out to be utter crap. To the first approximation, the main requirement for the working fluid of a nuclear thermal rocket is: low molecular weight. The Isp of a NRT using a fully expanded exhaust flow (i.e., infinite area ratio nozzle) is: Isp = Sqrt[2/(g-1)] * ao (units: m/s. Divide by 9.8 m/s/s if you want Isp in seconds.) ...where ao is the speed of sound of the gas in the heated propellant before expansion, and "g" is the ratio of specific heats: g = cp/cv. Since sound speed is given by: ao = Sqrt[g R To / MW] ...where "To" is the initial temperature, R is universal gas constant (8314 J/kg-K), and MW is the molecular weight of the gas. "To" is usually set by operating conditions of reactor. So, if you want a high Isp, then you want high sound speed, and therefore low molecular weight, e.g., hydrogen, ammonia, methane. I do not see any conceivable advantage of CO2 over ammonia or methane. The dependence on the specific heat ratio "g" would suggest you want a small "g", but "g" is inversely related to the degrees of freedom a molecule has (think of it as the "squishiness" of the gas: for a monatomic line helium: g = 1.666, for diatomic like oxygen or nitrogen: g = 1.4, for CO2: g = 1.3, etc.), so you can only lower "g" by raising the molecular complexity of gas, which raises the MW unacceptably. Thus, to the first approximation: you want a propellant with low MW and you take the "g" that gives you. Then you starting thinking about storing the working fluid and the logistics of actually using it... -- Andrew J. Higgins Mechanical Engineering Dept. Assistant Professor McGill University Shock Wave Physics Group Montreal, Quebec CANADA http://www.mcgill.ca/mecheng/staff/academic/higgins/ |
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alternate working fluids for nuclear thermal rockets?
James Nicoll wrote:
Is there some nice concise source for the ISPs of nuclear thermal rockets using various working fluids, like ammonia or methane? I thought I had a method of guessing what the ISP might be but it seems to be giving me values that are wildly wrong. In particular, why is it CO2 doesn't seem to be discussed as a working fluid? It's cheap. I suspect low ISP is the answer but as I say my method turns out to be utter crap. CO2 is not as inert at high temperatures as it seems at low ones. Ignoring all else, the important thing is the weight of the molecules/atoms coming out of the exhaust. Both NH2 and CH4 are good as they have lots of hydrogen. CO2 by comparison is relatively heavy. |
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alternate working fluids for nuclear thermal rockets?
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#10
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alternate working fluids for nuclear thermal rockets?
The original context for this was a thread on "ultimate rockets",
rockets where the dominent cost was fuel rather than labour. Obviously that doesn't describe the current state of affairs. -- It's amazing how the waterdrops form: a ball of water with an air bubble inside it and inside of that one more bubble of water. It looks so beautiful [...]. I realized something: the world is interesting for the man who can be surprised. -Valentin Lebedev- |
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