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SpaceX Dragon 2 In Flight Abort Test
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SpaceX Dragon 2 In Flight Abort Test
On 2020-01-20 3:28, Jeff Findley wrote:
Still, [the Falcon 9 first stage] looks to have been torn apart by aerodynamic forces. The second stage was intact when it hit the ocean (and created quite the fireball). Interesting that the interstage seems to have stuck with the second stage and not with the first stage, or so the videos of the falling second stage seem to show. The connection between the interstage and the second stage, which is where the stages normally separate, seems to have been tougher than the join between first stage and interstage, which is meant to be permanent. There seemed to be a bulge at the lower end of the interstage, which perhaps was the top bulkhead of the top propellant tank of the first stage. So perhaps the split was the bulkhead coming loose from the tank walls -- corresponding to the seam that failed in the recent Starship "Bopper" test. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
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SpaceX Dragon 2 In Flight Abort Test
In article ,
says... On 2020-01-19 09:57, Jeff Findley wrote: Firstly, Merlin engines are designed to throttle. Also, in an abort situation, you don't care one bit if you destroy the engines (i.e. turbopumps) in the process of quickly shutting them down. I would assume that the mechanical design of the valves was done to ensure that engine shutdown command was survivable. Adding a new mode of shutdown that is quickler but destructive would introduce an unnenssary failure cause. I'm quite sure Merlin relies on software to throttle the engine. This isn't an engine designed in the 1940s or 1950s with a mechanical engine controller. Again, in order to have a fast throttle response, for things like landing, Merlin no doubt has very fast actuators on the valves leading to the gas generator. Slamming those valves shut is something which can be done in software. And in the video, there was a fair gap in time between the crowds counting down to 0, a progressive shrinkage of flame bhind engines till no flame. Not like an on-off of a light switch. I've not gone through the video frame by frame, but when I watched it, it was on the order of seconds at most. But Musk stated in post test press conference that the command to Dragon is sent milliseconds after the "shutdown" command to engines. A delay measured in milliseconds is still quite fast. It's an order or magnitude faster than any human could ever react (this is why when you drive a car you should have a two second following distance). He also stated that Dragon has enough power to escape even with Falcon9 still producing thrust. Mentions theoretical capability up to 6Gs. "Theoretical capability" is an interesting way to word it. In this test, both he and NASA were impressed that Dragon experienced no worse than 3.5Gs during escape. I've literally been on "thrill rides" at the Kings Island theme park with higher G loads than that. One of the steel coasters had a manufacturers plaque on it that, among other facts, listed the maximum G load. At the time I noted that was quite a bit higher than max G during a space shuttle launch. Cite: http://www.tecumseh.k12.oh.us/Downlo...rPoint_9_3.pdf So, far less than any military ejection seat. Should be quite survivable, especially considering the orientation of the seats in Dragon 2 is far more favorable than an ejection seat which puts an incredible compressive load on the pilot's spine. If you're in an abort situation, you're *already* having a "bad day" and are trying to save the crew's lives! If the abort is for reason other than engines exploding (aka: just off course or enough engines shutdown) , you may not wish to cause an emergency "abrupt" shutdown that initiates engines exploding. Making the "decision tree" complex introduces its own possible failure modes. Keeping it simple is almost always better, which means slamming the valves shut as fast as possible. Again, the lives of the astronauts are always more important than trying to save the Falcon 9 first stage. Best not to take chances with complex logic, IMHO. SpaceX and NASA would have the gory details. I'm just saying what makes the most sense from an engineering point of view. #1 requirement is to save the crew in an abort. Nothing else even comes close. Regardless of the situation, Dragon 2 will leave *immediately*. Which is to say it will start pressurizing the tanks for the Super Dracos the instant that it receives the abort signal. I thought those were pressurized at the time lauch abort is armed on the pad, before launch and in case of abort they just needed to open a valve to let fuel and oxydizer flow , meet, fall in love and create fireworks. No. But at the press conference, Musk mentioned that the tanks are pressurized as first step in an abort. Correct. You normally don't want the tanks at abort pressure because that's far higher than needed for the Dracos. The human body can take that jerk to 4 Gs. Look up the USAF ejection seat tests done decades ago. This acceleration is quite tame compared to what e-seats expose USAF pilots to. I've worked out the logic (I think). Assuming fixed thrust on Falcon9 and progressively increasing thrust on Super Dracos. As Super Dracos increase thrust, the weight Dragon exerts on Falcon9 is reduced, allowing Merlin engines to accelete the stack at higher rate. Eventually, Dragon accelerates faster than Flacon9 and separation happens. Yes, basic physics here. So, if Super Dracos go from 0 to 4G acceleration "instantly", the crew will go from existing 3G to 4G acceleration. So escaping from a running Falcon9 wouldn't result in a big jolt. It's actually called jerk. Jerk is defined as the rate of increase in acceleration. In this scenario, jerk is essentially caused by the ramp up in thrust of the Super Dracos. The faster they ramp up in thrust, the higher the jerk. But if they wait for engines to shut down, they woudl go from 3G to 0G to 4G at which point it is a big jolt. The pressurization and firing of the Super Dracos takes less than a second. That is happening simultaneously with Merlin shutdown. Would be interesting to see two graphs: 1. the overlay of normalized thrust (0-100%) of the Merlins overlayed with a graph of normalized thrust (0-100%) of Super Dracos. This would show what I'm talking about above. 2. the overlay of acceleration (actually deceleration) of Falcon overlayed with a graph of acceleration of Dragon 2. However, in the press conference, Musk alluded to the thrust from Super Dracos is adjusted based on conditions. (Crew safety mentioned). So if Falcon has shut its engines, Dragon wouldn't have thrust has high from Super Dracos than if the Falcon9 engines are still running. So it appears to be a fairly smart system. I saw that. Still, Dragon 2 won't wait for the Merlins to shut down. They're going to fire as quickly as they can. What they will change is the thrust of the Super Dracos. If the computers think the engines can be safely shut down, the Gs on Dragon 2 won't be as high as the scenario where Dragon 2 determines that the Falcon went "boom". In the case of a "boom", I'd expect Dragon 2 to GTFO as quickly as possible. safety comes next. But in many cases, like this abort test, the abort will result in the destruction of the Falcon 9 launch vehicle before the range safety system attempts to fire. An abort in the pad prior to launch does not necessarily result in the Stage1 being destroyed. Prior to launch if the Falcon isn't in the process of being destroyed, why are you aborting? Aborting Dragon 2 is dangerous. Like ejection seats, you only do it because it's literally attempted suicide to avoid certain death. This situation simply *can't* happen. Dragon 2 would abort *long* before "imminent danger" to anyone on the ground. Also, this is why Falcon 9 flies *east* out over the ocean. Nothing launched from KSC flies towards Orlando. I realize that. But NASA stll feels it necessary to have range safety. (and it will be very interesting to see how that policy is adapted to Starship) You're not getting it. Range safety provides for an area around the launch pad and along the flight path where debris will drop in case the launch vehicle destroys itself or is destroyed by range safety. As such, you don't have to destroy Falcon 9 at the first sign of trouble. You have to first abort, which means shutting down the engines and telling Dragon 2 to GTFO. Deliberate destruction of Falcon 9 will therefore happen after the abort. Now, there is always the chance Falcon 9 goes "boom", in which case destruction of Falcon 9 becomes moot (impossible, actually since it would already be in pieces). Consider the Shuttle. They had range safety on it with crew on it. So obviously NASA had considered the issue of having to initiate range safety with crew still on board. But also note that the shuttle orbiter itself had no destruct charges. Only the SRBs (to unzipper them and terminate thrust) and the ET (to disperse propellant) had destruct charges. Range safety is to protect people on the ground, so putting charges on the orbiter, or Dragon 2, would make no sense because they need to protect the crew. This is why both Dragon 2 and Starliner have already performed pad abort tests (both passed). Were those tests done from an actual rocket at the pad, or were they done on the ground with the Dragon/Starliner on a cradle and it just fired its engines to pop up, deploy parachutes and come back down? On the ground. There was simply no reason to stack either of them on top of an expensive launch vehicle to perform this test. It is a simple answer. Either your sensors indicate the engine is working properly, or they don't. Was hoping for answer one whate xactly do those sensors sense to gauge thrust being produced by an engine. For now, I will take explanation by someone selse of messuring pressure in combustion chamber. (I assume there is a pipe that leads to a sensor far enough to not burn). On something as complex as a liquid fueled rocket engine, there are actually lots of pressure sensors, temperature sensors, accelerometers (to measure vibration), and etc. There are also sensors in the tank (certainly pressure and sometimes a "low level" sensor as well). I simply don't want to go down the rat hole of talking about every possible sensor and what it would mean if it read "lower" or "higher" than expected. BTW, during the press conference, Musk mentioned that they are looking at p]reventing Dragon 2 from landing in water by catching it i flight. They are precticing with fairings and if that ends up being reliable, may try with a Dragon (not sure if mentioned Dragon or Dragon 2). I heard about that. I think NASA would nix that right away. SpaceX would certainly want to test this with Cargo Dragon and Cargo Dragon 2 before trying it with a Crew Dragon 2. The problem here is, what if you barely miss it? Say you smack the capsule with one of the steel "arms" on the fairing recovery ship as it's falling. What kind of damage would happen? Just like testing propulsive landing with Cargo Dragon 2, NASA won't like the idea of things like essentially irreplaceable EMUs being lost at sea due to testing of this sort. Not sure how realistic this is, but I guess this really means SpaceX won't try to convert Dragon 2 to powered landings. That was nixed by NASA long ago for the reasons stated above. Also, for those who are interested, here's a link to the transcript: IFA Post-Presser Presser 2020-01-19 https://gist.github.com/theinternetf...25229fb6a17195 From above: Q2: Elon, does the press deserve a dance today? [room laughs, groans] Elon Musk: I can't take these expectations of dancing constantly. I'm not that good of a dancer. I haven't ever worked on my skills. Q3: Dance! Elon Musk: No! I am not your dancing puppet! Jeff -- All opinions posted by me on Usenet News are mine, and mine alone. These posts do not reflect the opinions of my family, friends, employer, or any organization that I am a member of. |
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SpaceX Dragon 2 In Flight Abort Test
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SpaceX Dragon 2 In Flight Abort Test
On Tuesday, January 21, 2020 at 1:30:51 PM UTC-5, JF Mezei wrote:
On 2020-01-21 07:28, Jeff Findley wrote: Could be. I'm sure SpaceX and NASA will take a close look at when and how it came apart. This is a unique opportunity to validate that the "computer models" are correct. During press conference, Musk was asked about studying how Falcom9 behaved and he seemed quite dismissed with "we knew it woudl bloc it, it blew up, we won't bother with recovering pieces". He also said in another question that telemetry stiopped when it blew up". (terse answer). The answer could have been "for the X seconds between engine shutdown and loss of telemetry, we got exciting data and we are bound to look at the Falcon9 performance" type of answer. Instead of was quite dismissive. I think that illustrates the difference between NASA and the private space ventures. The latter will not freely share information that might benefit numerous competitors. Think "Capitalists in Space!". |
#17
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SpaceX Dragon 2 In Flight Abort Test
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