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JF Mezei wrote on Sun, 7 Jul 2019
10:46:49 -0400: On 2019-07-06 10:57, Jeff Findley wrote: No. The escape systems are entirely different. Thanks. I was trying to find some reason why it was felt Starliner didn't need the abort test at MaxQ. Is it known who pushed for such a test? Is it possible that it wasn't part of NASA standards, but SpaceX offered to do it (since it has spare boosters)? NASA elected to do it for Orion, so I don't think that holds up. Curous to see if this would become part of "tradition" for subsequency capsules or whether this is more of a one-off thing. For any capsule that requires a full 'abort' envelope it's been a required test. Earlier capsules didn't require it because it was assume that a Max Q abort wouldn't be survivable. -- "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 Sun, 7 Jul 2019
10:55:31 -0400: On 2019-07-06 11:06, Jeff Findley wrote: Yes, at least from the command and control system's point of view. The same system which notifies the capsule that it must abort will also shutdown the engines. But that system is in the capsule, right? Unlikely, since the events requiring an abort are down in the booster. If the capsule has been commanded to escape, the engines have also been commanded to shutdown. Sending command to shut down engines from capsule doesn't garantee engines shutdown. (severed link scenario). And perhaps unicorn farts will destroy the vehicle, but it's pretty unlikely. Such a severance of communication would initiate an immediate abort of the capsule and shutdown of the engines because "something bad happened" which the command and control system could easily detect. If you cut the wires between capsule and engines, how long before each side detects that it hasn't heard from the other side and declares something has gone really bad, and the engine side decides to shutdown by itself ? You really don't seem to understand how such things work. I can understand constant flow of telemetry data from engines to capsule. So loss of such telemetry would be quickly detected by capsule. But would engines detect loss of connectivity with Capsule? Does capsule have to "ack" all telemetry packets, or is there a "hello" packet sent every 10 seconds? Without knowing exactly how they designed it, I can tell you how I would do it. Somewhere in the TM aggregator there is logic to look at selected pieces of TM and decide if the vehicle is so far off nominal that it needs to abort. If that logic determines that to be the case, a signal gets sent to both the booster and the capsule to initiate abort. Barring explosive disassembly of the booster, this works fine. Now, in the event of said explosive disassembly, you might lose that chunk of logic. In that case, about 50 msec later the capsule is commanded to abort (20 Hz is a common rate for things like TM). Even if the booster doesn't get or cannot obey a shutdown command, explosive disassembly pretty much shuts down the engine anyway. This is why how long it takes for engines to detect the capsule isn't there anymore matters. You're making assumptions about how things are put together that are not warranted. Someone mentioned that failure of tanks would result in lost pressurization and instant engine shutdown. Considering the acceleration and the turbopumps, wouldn't loss of pressurization of tanks still get fuel and oxydizer to engine turbo pumps for a few seconds? Again, look at how liquid rocket engines work. -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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Jeff Findley wrote on Mon, 8 Jul 2019
06:35:25 -0400: Yes, because these are liquid fueled engines, if you don't maintain tank pressure, the engines physically can't run. Even if they tried, without the "head" pressure in the tank, the pumps will cavitate and the flow of propellant would essentially stop anyway. They wouldn't be able to run as soon as the pressure in the tanks is released. We're not talking about a tiny pump here. The case is even worse for a staged combustion turbopump engine. The pressure in the fuel tank must be higher than that in the combustion chamber of the turbopump, which must be higher than that in the combustion chamber of the main engine. Lose tank pressure and fuel to the combustion chamber of the turbopump stops. That stops the turbopump, which stops the flow of propellants to the main combustion chamber. Engine out. -- "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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On 19-07-08 21:31 , Fred J. McCall wrote:
Jeff Findley wrote on Mon, 8 Jul 2019 06:35:25 -0400: Yes, because these are liquid fueled engines, if you don't maintain tank pressure, the engines physically can't run. Even if they tried, without the "head" pressure in the tank, the pumps will cavitate and the flow of propellant would essentially stop anyway. They wouldn't be able to run as soon as the pressure in the tanks is released. We're not talking about a tiny pump here. The case is even worse for a staged combustion turbopump engine. The pressure in the fuel tank must be higher than that in the combustion chamber of the turbopump, Doubtful, because the propellants are usually pumped into the pump's combustion chamber (preburner) too, raising the pressure. At least if we believe the Wikipedia description of staged combustion. which must be higher than that in the combustion chamber of the main engine. That, I do not believe. If the pressure in the tank is higher than in the engine combustion chamber, why would the pump be needed? For a pump to work, the *force* from the driving side must be at least as large as the *force* required on the pump side, but that does not imply the same relation for pressures. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
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JF Mezei wrote on Mon, 8 Jul 2019
12:03:23 -0400: On 2019-07-08 06:35, Jeff Findley wrote: No. It's most likely in the Falcon 9 because that's got all the computers which control it anyway. OK, so if the booster has the logic to detect abort conditions, it can self abort with or without capsule. Do you know if, in manned mode, falcon 9 would require the capsule "agree" to abort (akaL multiple computers needed to agree) ? You're looking at this wrong way around. Even if the capsule has parallel logic (which I doubt), you want to abort if there is any doubt, not fail to abort if there is any doubt. So in any sane system design a single 'abort' vote would cause an abort. Very fast. You do know how fast computers are these days, right? This really depends on implemenmtation. For TCP/IP for instance, a connection between two hosts can remain active for a fair bit before one or both sides detect that the "keep alive" has not been received and connection should be closed. For some apps, this can be in minutes. By default TCP/IP doesn't include a 'keep alive'. It was a later optional addition. There are three settable parameters that determine how it behaves (time, interval, and number) and a failed connection may not be detected for HOURS, not mere minutes. However, we're not talking about TCP/IP and things that are different just aren't the same. Let's start with the assumption that the implement ors are not incompetent boobs and know how to implement a safety-critical connection. you mentioned loss of power/light on a cable. If they have such hardware detection, then yeah, it can be instant. But if relying on data protocols, it all depends on the protocol and how quickly both sides detect the other is gone. Again, let's assume that different just aren't the same. Let's start with the assumption that the implement ors are not incompetent boobs and know how to implement a safety-critical connection. But one would also need to look at implementation. Does loss of layer 1 (in ISO 7 layers model) trigger immediate abort, or do they wish to gracefully handle temporary loss of it due to vribration etc and not needlessly trigger a dangerous abort? Again, you're looking at this wrong. 'Abort' is a safety function. If there is ANY doubt you want to abort. This is much less embarrassing than killing a crew because you were waiting to be absolutely sure. 'Temporary loss' is permanent loss because in the event of temporary loss the capsule is out of there. I provided the telemetry argument because it is a good example of one side sending constant data flow, so other side can very quickly detect loss of that data flow. But that doesn't mean the sender of telemetry detects loss of connection if the protocol only requires ACKs every few seconds. So if you assume the implementers are incompetent bozos there could be a problem. Perhaps I can reformulate the question: In the event the capsule decides it needs to abort at MaxQ. (say they realize they forgot the Columbian coffee): Does it make much of a difference if the booster continues to accelerate for a few more seconds, or does it need to stop ASAP in order to make it much easier for capsule to open a large distance gap between the two? It probably doesn't matter that much except insofar as being able to calculate safe separation distances. You'd like it to slow down because the capsule is at Max Q (which means if you need to push above that you are eating into mechanical safety margin and the more into it you need to push the more margin you have to eat). Yes, because these are liquid fueled engines, if you don't maintain tank pressure, the engines physically can't run. As I recall, the ET was barely pressurized. And when accelerating, the liquid fuel will be pushed down to the bottom where the pumps suck up the liquids right? The propellants in the ET were both only lightly pressurized. Each propellant feed had an LP/HP turbopump set with each pump having multiple stages to up the propel lent pressures by 100x or so. Considering the pressures the turbopumps create, wouldn't a minimal pressure difference in the fuel tanks (from normal to ambiant) be noticed by the turbo pumps as long as they can pull in liquid fuel/oxydizer? 'Ambient' is pretty damned low at the altitude of Max Q. When the tanks lose pressure do you still have liquid for the pumps to suck up? I was under impression that in the case of the ET, the pressurization with helium was done to reduce boiling off as the tank is being emptied by the engines. Does that tank pressure end up "pushing" fuel to the turbopump intake as fast as the turbopump sucks up those liquids? or does the turbopump still "pull" fuel from the tank? Would a drop in tank pressure result in instant "bad news day" with the engines, or could they run for some time before bad news happens? Would time be in milliseconds, seconds? 30 seconds ? a minute ? This all comes back to 'things that are different are not the same'. Why are you looking at the Space Shuttle when we're talking about Falcon 9? Falcon 9 tanks are pressurized at about twice what the ET used and the turbopumps only increase pressure by 30x. As Jeff noted, loss of pressure will cause pump cavitation and almost immediate loss of thrust. But even that loss of pressure isn't really what we're talking about. In a non-explosion there is no 'interruption' such as you postulate. In an explosion, the tanks, lines, and pumps are all wreckage. -- "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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Niklas Holsti wrote on Mon, 8 Jul 2019
22:46:38 +0300: On 19-07-08 21:31 , Fred J. McCall wrote: Jeff Findley wrote on Mon, 8 Jul 2019 06:35:25 -0400: Yes, because these are liquid fueled engines, if you don't maintain tank pressure, the engines physically can't run. Even if they tried, without the "head" pressure in the tank, the pumps will cavitate and the flow of propellant would essentially stop anyway. They wouldn't be able to run as soon as the pressure in the tanks is released. We're not talking about a tiny pump here. The case is even worse for a staged combustion turbopump engine. The pressure in the fuel tank must be higher than that in the combustion chamber of the turbopump, Doubtful, because the propellants are usually pumped into the pump's combustion chamber (preburner) too, raising the pressure. At least if we believe the Wikipedia description of staged combustion. Yeah, I was on my ass there. They do some pretty complicated things to get the pumps to drive. -- "Before you embark on a journey of revenge dig two graves." -- Confucius |
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JF Mezei wrote on Mon, 8 Jul 2019
21:14:26 -0400: On 2019-07-08 17:01, Fred J. McCall wrote: By default TCP/IP doesn't include a 'keep alive'. It was a later optional addition. There are three settable parameters that determine how it behaves (time, interval, and number) and a failed connection may not be detected for HOURS, not mere minutes. The argument had been made that because it is done by computers , it is very fast, I provided TCP example where, even you admit to it, detection of disconnection may take an eternityl being done by computer does not necessarily mean fast. TCP/IP was designed to be able to work over smoke signals (seriously, read up on it) so that speed of the transport layer didn't matter. Yes, if one implements stupidly it doesn't matter how fast the hardware is. Shall we assume that the developers aren't cretins? Again, you're looking at this wrong. 'Abort' is a safety function. If there is ANY doubt you want to abort. As I recall, in Apollo 13, they lost an engine after launch. Human decision was made to continue and just burn remaining engines longer to get the desired delta-V. Once again, THINGS THAT ARE DIFFERENT JUST ARE NOT THE SAME! Get that tattooed on the inside of your eyeballs or something. Would today's automated systems have seen "any doubt" and triggered an abort? or would they have predicted that loss of an engine above certain time would not trigger abort, but below that, would ? No, they would not. Again, let's assume that the folks implementing this stuff are NOT cretins, shall we? is an in flight abort considered safe? As safe as anything else in rocketry. Full envelope abort wasn't a requirement for Apollo. It is a requirement for Commercial Crew. Russian capsule can smack you with as much as 16+g on an abort (and they're not real friendly on landing, either). Crew Dragon is a much gentler 7-ish g. I don't know that the Russian capsules have full envelope abort capability (they may). Am asking in a context on whether they want automated "instant" trigger of abort, or whether there are situations where the computer might suggest an abort but leave it up to crew? No. People are too slow. The computer knows more and is much faster at making the right decision. Also, on the pad, with engines not running yet, but crew in capsule, what logic/sensors would detect the onset of an explosion requiring the capsule be thrown up real fast? Would that be a human with a finger on the big red button? Or severring of data lines to capsule? No to both your proposed possibilities. The same telemetry and logic that is used in flight will be used on the pad. This is much less embarrassing than killing a crew because you were waiting to be absolutely sure. For wide body commercial aircraft, using emergency chutes generally results in a dozen or so injuuries, (and in winter, frostbite for passengers idle on tarmac waiting for help). so unless there is an absolute immediate danger (fire), the crew will prefer to wait for airstairs. One more time for the slow of wit. THINGS THAT ARE DIFFERENT JUST ARE NOT THE SAME!!!!!!!!!! Are these capsule ejects considered "safe" with no ill effects on crew? or are there imminent risks wherher they would rather not eject unless it is a dire situation? First, it's not an 'eject'. Second, yes, it's safe, or at least as safe as any other part of riding on a rocket. G stress is lower than fighter pilots take and the landing looks like any other landing. So if you assume the implementers are incompetent bozos there could be a problem. No trying to understand how it is implemented in geheric terms. Mr Findley provided the information that the login in Falcon9 was in stage I, so that corrected many of my assumptions/questions. Frankly, I'd be surprised if the bulk of that logic wasn't in the second stage, since that's where most of the avionics, flight computers, and telemetry aggregation are. But what happens if stage II goes wrong at MaxQ? Will the logic in stage I detect this? What's to 'go wrong'? It's not doing anything. Since the second stage is where all the engine controllers are, if something goes wrong there I don't think anyone needs to 'detect' anything. 'Ambient' is pretty damned low at the altitude of Max Q. For the Shuttle ET, it was pressurized to roughly 35psi. So 2 ATM. My bike tires are at 110psi. Again, THINGS THAT ARE DIFFERENT JUST ARE NOT THE SAME!!!!!!!!!!!!!!!! Would assume that pressure generated by the turbopump was orders of magnitudes higher so a drop of 35psi shouldn't have had that drastic an impact initially/as long as there was liquid fuel the pump can suck from bottom of tank. I understand that Falcon9 has single turbopump stage and more sensitive to cavitation. In terms of kerosene, would kerosese vaporize if sucked in by pump at too low a pressure? All I could find was a value of 50psi on some reddit place (so not authoritative). Think about how a pump works. Do you understand what cavitation is? I can understand how liquid Oxygen would love to vaporize if pressure dropped as it was sucked into the pump at too high a rate (relative to its pressure in pipe/tank). At what PSI is the LOX stored in tank? Is a drop of 50psi once the engine is already running such a big deal? Yes. I can understand the need for pressure to push fuel into the turbopumps when engines are started. But once they are running, is the pressure as critical or less critical than at startup ? As critical. -- "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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