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JF Mezei wrote:
On 2017-06-24 22:58, Jeff Findley wrote: Pretty much. But this stage started with a higher velocity than normal due to very limited fuel reserves. And this explains the "re-entry" being hotter and more stressful than normal. I get that. But by the time, the stage has fallen through atmosphere, wouldn't its vertical speed be comparable to other landings? No. hotter than normal). Also, due to lack of fuel reserves, this was a three engine landing, which is necessarily harder than a single engine landing. If you have X fuel left, is there an advantage of running 3 engines for 2 seconds instead of 1 engine for 6 seconds? Yes. Isn't there some fuel wasted during engine start-up? As such, woudln't starting a single engine and running it longer be more efficient? Hardly enough to matter (and in fact they restart engines TWICE on the way down). Also, how does re-entry burn get started? Since the stage is sort of free floating at that point, I take it the fins are not very useful? Do the nitrogen thrusters turn the stage around such that the engines are in the front, so when they fire, they decelerate the rocket's orbital velocity? Did you watch the video I've posted twice now? The stage is never 'free floating' Or does de-orbit burn happen once the rocket has fallen sufficiently that the fins are holding it vertical and the firing slows vertical descent rate? (from the video, attitude isn't so obvious relative to direction of travel). The stage is always pointed essentially 'down'. The OMS thrusters are used to make sure it is properly oriented for whatever burn of the main engine(s) is required. You can see them firing in the video. Also once the 'fans' come out, they, too, help keep the bottom of the vehicle properly oriented. Just as a caution, I've noticed that at one point in the video the vehicle suddenly gains like 18,000 kph of velocity. I suspect the tm changed its reference point for some reason, but from that point on 18,000 is essentially '0', as you can see on the velocity TM on landing. -- "Insisting on perfect safety is for people who don't have the balls to live in the real world." -- Mary Shafer, NASA Dryden |
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JF Mezei wrote:
On 2017-06-25 14:26, Fred J. McCall wrote: Did you watch the video I've posted twice now? The stage is never 'free floating' At MECO, it looks to me like free floating. In fact, the speed starts to drop off as the stage is free floating and still increasing altitude. At that point, the engine bells still point to KSC. Look at the velocity. Do you see it hit zero? Me either. Therefore never 'free floating'. Watching again, I see that they do turn the stage slowly so engine bells go from pointing at KSC to pointing towards europe, and this happens as the stage starts to gain velocity as it falls. And by the time the engine fire, the bells are almost aiming down to ocean. at the same time as its speed starts to increase (aka: starts to accelerate downwards). The burn drops speed from 8600kmh down to 6600kmh and after that, the drag slows it down to 5712 (last telemetry shown). Pardon? Velocity values are shown all the way to at rest on the barge on the video I posted. Are you talking about some other video? If so, you should provide a link to it so the rest of us know what the **** you're talking about. The mention the drone as AOS, but when they switch to the drove video, the telemetry doesn't get updated. Interesting that despite not having telemetry, they can announce that the landing burn has started. Since they do not show speed just prior to landing burn on this flight versus others, I can't learn by myself if the vertical speed was truly different for this one versus others. Hence my question on whether terminal velocity would normalize all landing approaches to roughly the same vertical speed. Not what you originally asked. And the answer is no. Just as a caution, I've noticed that at one point in the video the vehicle suddenly gains like 18,000 kph of velocity. I did not see that. Maximum speed was roughly 8600kmh before first burn. What you may have seen is when they switch the display of telemetry from stage 1 to stage 2. The video I posted never did that. It's Stage 1 TM all the way, which is why you see the altitude peak at around 120 km and then start decreasing as the first stage starts falling back. When the first stage is resting on the pad it show 18,000 kph and 0 altitude. If the display had switched to Stage 2 TM (and it's clearly labeled as Stage 1), just how the hell do you think it would suddenly jump 18,000 kph? Magic? -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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