Quote from: livingjw on 09/17/2017 07:26 pmPeter,- Engine T/W does not go up as thrust goes down. It just doesn't. T/W tends to be about constant from 100K lbs to 1M lbs thrust class. ISP isn't much effected by thrust either. - You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.JohnHi John,- For other rockets you're probably right, for the Raptor I think it's different, and to my opinion an advantage that will leave other rocket companies like Blue Origin behind in the dust.- I'm obsessed with filling the base yes, because I know my rocket can be morI e heavy/higher.Rocket weight = combined thrust devided by 1,25.
Peter,- Engine T/W does not go up as thrust goes down. It just doesn't. T/W tends to be about constant from 100K lbs to 1M lbs thrust class. ISP isn't much effected by thrust either. - You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.John
.....I know my rocket........
If I remember correctly, people were saying the same thing about a 9 engine cluster around 2008-2010.
Peter,- Engine T/W does not go up as thrust goes down. It just doesn't. Engine T/W tends to be about constant from 100K lbs to 1M lbs thrust class. ISP isn't much effected by thrust either. - You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.John
Quote from: livingjw on 09/17/2017 07:26 pmPeter,- Engine T/W does not go up as thrust goes down. It just doesn't. Engine T/W tends to be about constant from 100K lbs to 1M lbs thrust class. ISP isn't much effected by thrust either. - You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.JohnI would have thought that engine TWR would increase with increasing size/thrust although SpX seems to think otherwise. For a given Pc rocket engine the plumbing wall thickness would remain constant so the OD/ID ratio gets smaller as a rocket engine is scaled up so in theory a larger rocket engine should have a higher TWR than a smaller one if all other variables are constant. So a future larger BFR should have the no more than the same no. of engines as mini-BFR. Less engines = less plumbing on the bottom of the BFR.
Quote from: livingjw on 09/17/2017 07:26 pmPeter,.......You also seem to be obsessed with filling the base of the vehicle with rocket exhaust.......Obsessed is right. Too much armchair amateur rocket designing based on personal belief rather than on science, engineering, and technological understanding.
Peter,.......You also seem to be obsessed with filling the base of the vehicle with rocket exhaust.......
Quote from: DJPledger on 09/17/2017 08:02 pmQuote from: livingjw on 09/17/2017 07:26 pmPeter,- Engine T/W does not go up as thrust goes down. It just doesn't. Engine T/W tends to be about constant from 100K lbs to 1M lbs thrust class. ISP isn't much effected by thrust either. - You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.JohnI would have thought that engine TWR would increase with increasing size/thrust although SpX seems to think otherwise. For a given Pc rocket engine the plumbing wall thickness would remain constant so the OD/ID ratio gets smaller as a rocket engine is scaled up so in theory a larger rocket engine should have a higher TWR than a smaller one if all other variables are constant. So a future larger BFR should have the no more than the same no. of engines as mini-BFR. Less engines = less plumbing on the bottom of the BFR.No, wall thickness is proportional to diameter. Once you reach a certain minimum size TWR does not change much with thrust.
They've talked about the engine being scaleable -- 'easily' was the adjective used IIRC -- so they will grow whatever size engine(s) they need from the Raptor family. My bet is to see second size at this IAC... sub-scale is likely in or near flight qualification testing.
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Nice logarithmic chart!T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size wouldn't that not imply a T/W ratio of around an unimaginably good score of around 500-600?
Quote from: Peter.Colin on 09/18/2017 12:54 amNice logarithmic chart!T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size wouldn't that not imply a T/W ratio of around an unimaginably good score of around 500-600?Benifits of Full Flow Stage Combustion, in that by adding 50% more turbopump machinery (which is by far not the entirity of the mass of the engine) you can get closed cycle efficencies with a chamber pressure higher than most open cycle engines.
Re scalability: I can't remember where but I read there is a principal limit to chemical rocket engine sizes. Something to do with fluid and combustion dynamics. As the limit is approached, the engine becomes more and more complex. Surely the cycle type and fuel type influences the limit, so raptor is hard to compare with existing engines in this regard. Larger might not be better.
Nice logarithmic chart!T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size would that not imply a T/W ratio of an unimaginably good score of around 500-600?