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SpaceX Capsule Explosion
In article , says...
On 7/16/2019 7:41 PM, Fred J. McCall wrote: David Spain wrote on Tue, 16 Jul 2019 13:58:46 -0400: On Rand Simberg's blog, George Turner postulated they (titanium values) were to allow engine restarts back in the days when Dragon V2 was supposed to use propulsive landing. With burst disks you don't get that capability but don't need it because Dragon V2 will use its chutes and ocean landings only. I'd have to study it more myself to know for a fact if that is true... That sounds wrong to me. These engines are throttleable and it shouldn't matter if the propellant system is pressurized. Set throttles to zero and the engine shuts off. Open the throttles and the hypergolics hit the combustion chamber again and the thing lights. Yeah unclear to me as well. Why would these check valves be used for any purpose other that to close in order to refill helium tanks between flights? The abort system's Super Dracos require *much* higher chamber pressure than the Dracos, so the system is not pressurized until an abort is initiated. Why they chose to do it this way, I'm not sure. Perhaps NASA didn't want super high pressure hypergolic propellant tanks docked to ISS. For it to effect propulsive landing you have to postulate a scenario whereby the helium gets past the propellants and is expelled out the engine thus allowing the helium supply to get below the propellant supply. But I don't see how the helium gets past the liquid propellant being throttled. Physical chemistry is not my forte. Am I missing something here? Also what bursts the burst disks? I assume something will be used to over pressurize the helium? Or will they burst when the helium tank itself is pressurized? Thus the fueling operation would require hypergolic propellant loading before helium tank pressurization where that wasn't the case before. Correct? See above. They burst when the hypergolic system for the Super Dracos is pressurized, which is only when an abort is initiated. 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 Capsule Explosion
On 7/17/2019 7:43 PM, Jeff Findley wrote:
The abort system's Super Dracos require *much* higher chamber pressure than the Dracos, so the system is not pressurized until an abort is initiated. Why they chose to do it this way, I'm not sure. Perhaps NASA didn't want super high pressure hypergolic propellant tanks docked to ISS. That makes sense... See above. They burst when the hypergolic system for the Super Dracos is pressurized, which is only when an abort is initiated. Jeff I don't see how that answers my question. I must presume there is another (helium) valve upstream of the burst disk that holds back the helium pressure until an abort. It's the helium gas that's doing the pressurization correct? So I would assume that's implied in Fred's OP. Is helium used for pressurizing both bi-propellants? I would assume so. But always welcome facts if at hand. I can Google it too if anyone doesn't already know. Dave |
#13
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SpaceX Capsule Explosion
In article , says...
On 7/17/2019 7:43 PM, Jeff Findley wrote: The abort system's Super Dracos require *much* higher chamber pressure than the Dracos, so the system is not pressurized until an abort is initiated. Why they chose to do it this way, I'm not sure. Perhaps NASA didn't want super high pressure hypergolic propellant tanks docked to ISS. That makes sense... See above. They burst when the hypergolic system for the Super Dracos is pressurized, which is only when an abort is initiated. Jeff I don't see how that answers my question. I must presume there is another (helium) valve upstream of the burst disk that holds back the helium pressure until an abort. It's the helium gas that's doing the pressurization correct? So I would assume that's implied in Fred's OP. Yes, there is a valve between the super high pressure helium tank and the check valve (now burst disk). That's the valve that's opened to pressurize the propellant tanks for the abort system to operate. Is helium used for pressurizing both bi-propellants? I would assume so. But always welcome facts if at hand. I can Google it too if anyone doesn't already know. Yes. Both propellants need to be pressurized to about the same pressure for them to be injected into the engine's combustion chamber at about the same rate. 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 Capsule Explosion
On 7/17/2019 7:40 PM, Jeff Findley wrote:
Doesn't matter because NASA wants only new Dragon 2 capsules for crew. After their crew flight, they will be refurbished for commercial cargo. Part of that refurbishment is to remove the abort system to allow for more cargo up-mass and down-mass. Jeff Jeff that's a great point and I remember reading about that. Seems stupid on NASA's part, they lose the cost savings of reusable hardware. But hey this is the organization behind the SLS so what should I expect? Do you have a cite for the latter part (removal of abort system for more cargo)? I'd like to read up on that. First I've heard of it. Dave |
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SpaceX Capsule Explosion
On 7/16/2019 7:33 PM, Fred J. McCall wrote:
One story I read indicated (by an ex-SpaceX engineer) that it was for reusability, as burst valves would have to be replaced (with some difficulty) after any pressurization of the escape system. Some reports also made it sound as if the titanium parts were only used on the fuel side and if there had been no backflow everything would have ^^^^^^^^^^^^^ been fine (and we know they've successfully done this before). This is sort of supported by reported SpaceX comments that they had no reason to suspect this could happen. Fred just a nit, but I think you meant to write on the helium side... As for back flow: here's an excerpt from Henry (Spencer) from the Arocket mailing list on how that could happen and a possible fix for a reusable system with pressurization shut off between uses: By the sounds of it, this wasn't a case of oxidizer-fuel mixing -- just a slug of liquid in part of the plumbing where only gas was expected, leading to severe water hammer when that section got pressurized suddenly. The problem is that check valves don't reliably block slow reverse flow of *gas*, and so a volatile propellant can seep up past the check valve and condense in colder plumbing upstream. This is a known problem, and has been for decades! In the case of N2O4, such seepage can also corrode upstream components. (This is almost certainly what really happened to Mars Observer, whose helium pressure regulators were *not* rated for N2O4 exposure -- when the pressurization system was activated, the corroded regulators failed to control the helium flow, and the propellant tanks burst. Once this possibility was noticed, the regulator failure was successfully duplicated in the lab.) So just taking it slow on the pressurization is not sufficient. The fix is, *don't* rely on check valves to block volatile liquids from getting up into the pressurization system(s). For one-shot systems, burst disks will do. For multi-burn systems where you want to turn off active pressurization between uses, use actuated shutoff valves to positively, hermetically close the pressurization path. Henry Dave |
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SpaceX Capsule Explosion
"David Spain" wrote in message ...
On 7/17/2019 7:40 PM, Jeff Findley wrote: Doesn't matter because NASA wants only new Dragon 2 capsules for crew. After their crew flight, they will be refurbished for commercial cargo. Part of that refurbishment is to remove the abort system to allow for more cargo up-mass and down-mass. Jeff Jeff that's a great point and I remember reading about that. Seems stupid on NASA's part, they lose the cost savings of reusable hardware. But hey this is the organization behind the SLS so what should I expect? Oh, I don't know. This incident suggests perhaps They're right in being conservative. It may be design issues like this that convinced NASA they only wanted the single-shot with SpaceX. Remember, Boeing will be re-using their capsule. It IS in fact possible there are design issues that made NASA comfortable in one case, but not the other? Or... it could just be, "we don't trust the new folks, no matter what." Do you have a cite for the latter part (removal of abort system for more cargo)? I'd like to read up on that. First I've heard of it. Dave -- Greg D. Moore http://greenmountainsoftware.wordpress.com/ CEO QuiCR: Quick, Crowdsourced Responses. http://www.quicr.net IT Disaster Response - https://www.amazon.com/Disaster-Resp...dp/1484221834/ |
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SpaceX Capsule Explosion
David Spain wrote on Wed, 17 Jul 2019 12:06:13
-0400: On 7/16/2019 7:41 PM, Fred J. McCall wrote: David Spain wrote on Tue, 16 Jul 2019 13:58:46 -0400: On Rand Simberg's blog, George Turner postulated they (titanium values) were to allow engine restarts back in the days when Dragon V2 was supposed to use propulsive landing. With burst disks you don't get that capability but don't need it because Dragon V2 will use its chutes and ocean landings only. I'd have to study it more myself to know for a fact if that is true... That sounds wrong to me. These engines are throttleable and it shouldn't matter if the propellant system is pressurized. Set throttles to zero and the engine shuts off. Open the throttles and the hypergolics hit the combustion chamber again and the thing lights. Yeah unclear to me as well. Why would these check valves be used for any purpose other that to close in order to refill helium tanks between flights? Yeah, that's kind of where I am, too. Once you pressurize the system you can't really 'depressurize' it, no matter what you do. For it to effect propulsive landing you have to postulate a scenario whereby the helium gets past the propellants and is expelled out the engine thus allowing the helium supply to get below the propellant supply. But I don't see how the helium gets past the liquid propellant being throttled. Physical chemistry is not my forte. Am I missing something here? I don't think so. The 'backflow' problem occurs before full pressurization is reached. Once you're fully pressurized I don't think it can happen anymore. Also what bursts the burst disks? I assume something will be used to over pressurize the helium? Or will they burst when the helium tank itself is pressurized? Thus the fueling operation would require hypergolic propellant loading before helium tank pressurization where that wasn't the case before. Correct? The helium tank is where the pressure comes from. It is pressurized much higher than the propellant tanks, so opening the valve from the helium tank would be enough to blow the disk. At that point you have full pressure at the propellant tank inlets, so no backflow. -- "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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SpaceX Capsule Explosion
David Spain wrote on Wed, 17 Jul 2019 20:51:19
-0400: On 7/17/2019 7:43 PM, Jeff Findley wrote: See above. They burst when the hypergolic system for the Super Dracos is pressurized, which is only when an abort is initiated. I don't see how that answers my question. I must presume there is another (helium) valve upstream of the burst disk that holds back the helium pressure until an abort. It's the helium gas that's doing the pressurization correct? So I would assume that's implied in Fred's OP. It can all be one piece, hence 'burst valve'. The valve opens, which allows pressurized helium to hit the burst disk. When pressure at the disk is sufficiently high, the disk bursts. This prevents backflow into the helium system. Is helium used for pressurizing both bi-propellants? I would assume so. But always welcome facts if at hand. I can Google it too if anyone doesn't already know. From the descriptions I've read helium is used to pressurize both tanks and shares piping. Remember, the problem here was a cup of oxidizer getting over to the propellant side of the helium system and contacting a titanium valve over there under pressure. -- "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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SpaceX Capsule Explosion
David Spain wrote on Thu, 18 Jul 2019 08:39:31
-0400: On 7/16/2019 7:33 PM, Fred J. McCall wrote: One story I read indicated (by an ex-SpaceX engineer) that it was for reusability, as burst valves would have to be replaced (with some difficulty) after any pressurization of the escape system. Some reports also made it sound as if the titanium parts were only used on the fuel side and if there had been no backflow everything would have ^^^^^^^^^^^^^ been fine (and we know they've successfully done this before). This is sort of supported by reported SpaceX comments that they had no reason to suspect this could happen. Fred just a nit, but I think you meant to write on the helium side... Actually I meant what I wrote but it's a little unclear. Yes, the oxidizer was on the 'helium side', but it was at the check valve to the fuel system. -- "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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SpaceX Capsule Explosion
On 7/18/2019 2:43 PM, Fred J. McCall wrote:
David Spain wrote on Wed, 17 Jul 2019 20:51:19 -0400: Is helium used for pressurizing both bi-propellants? I would assume so. But always welcome facts if at hand. I can Google it too if anyone doesn't already know. From the descriptions I've read helium is used to pressurize both tanks and shares piping. Remember, the problem here was a cup of oxidizer getting over to the propellant side of the helium system and contacting a titanium valve over there under pressure. Thanks Fred. Dave |
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