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Thrust vs altitude question
JF Mezei wrote on Fri, 11 May 2018
16:50:59 -0400: On 2018-05-11 01:22, Niklas Holsti wrote: What/where are those "documents", please? From what you say, they seem wrong. Commentary from several launches has said that Falcon 9 first stage engines are throttled down around the Max-Q point, to reduce the stresses, and the engines are also throttled down for the landing. And, of course, re-ignited two times (entry burn, landng burn). I know about throttling back at MAX-Q and as stack gets lighter. Which is why the following commentary during yesterday's (and today) launch got me confused: (roughly 3 minutes prior to launch) Block 4: Constant chamber pressure which means thrust increases as atmpsphere density went down. (they mentioned by 10 to 15,000lbps PER engine. I'd bet what you're missing is that that's thrust increases FOR A GIVE THROTTLE SETTING. Block 5: constant thrust of 190,000lbs till cutoff, which means we'll be slowly decreasing chamber pressure over time to maintain constant thrust. Which means it's essentially an 'auto-throttle'. So I started to look and: https://www.spaceflightnow.com/falcon9/001/f9guide.pdf Table 2-1 , page 10, states Throttle capability of "NO" for Stage1 ,and "yes" for stage 2. However, this appears to be an older document (not that I looked more carefully) and while originally from SpaceX, is hosted by spaceflightnow and is for Block 2. I really have to question that, since I don't see how they could do the Max Q throttle reduction without, well, being able to reduce throttle. However, this document is for Falcon 9 v1.0 with Merlin 1C engines. The Payload User's Guide for Falcon v1.1 specifies that the first stage is throttleable. See the chart on Page 11. This indicates, though, that it's been throttleable for a long, long time (since back in Block 2) and this is not something new for Block 5. http://www.spacex.com/sites/spacex/f...de_rev_2.0.pdf https://en.wikipedia.org/wiki/Merlin...ily)#Merlin_1D Something wrong with your link. It doesn't go to the rocket engine. (I assume the 1D is block 4). It does mention that the engine gained variable thrust, initially from 100 down to 70% and later down to 40%. If the Wiki page is right (always something of a crap shoot) the Merlin 1C actually existed in two different variants and they say that the Sea Level version is NOT throttleable. This still leaves me puzzled about how they get through Max Q, although that may not have been a problem because 1C has much lower thrust than 1D. The 1D first flew on Falcon 9 1.1, which corresponds to Block 2 (as I make it). Full Thrust v1.1 is Block 3. Then we have Block 4 and Block 5. So I make it that the Merlin 1D engine was used for Block 2-5. There was an upgrade going to the 'Full Thrust' vehicle, but they still call it the 1D (upgrade) engine. There's a chart here that shows which engines go with what. https://en.wikipedia.org/wiki/Falcon_9#Capabilities If block 4 had variable thrust, but constant chamber pressure, how did they maintained pressure if they reduced fuel flow? They didn't. As I said, I suspect the 'constant chamber pressure' is at any specific throttle setting, not across throttle settings. Does the chamnber (I assume combustion chamber?) have an outflow valve/iris that allows it to maintain pressure with variable flow of fuel/oxidizer going in ? Almost certainly not. What would be the point? -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
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Thrust vs altitude question
JF Mezei wrote on Sun, 13 May 2018
15:47:35 -0400: On 2018-05-11 23:24, Fred J. McCall wrote: They didn't. As I said, I suspect the 'constant chamber pressure' is at any specific throttle setting, not across throttle settings. Wouldn't that be a simple software change? There is no such thing. And if I were doing what it seems is being claimed I'd want some additional hardware, as well. Just so I understand, would it be correct to state that there is a direct relationship between throttle level and chamber pressure? (whether linear or not). That's certainly what logic seems to tell me, but we're a bit outside my field of expertise. -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
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