|
|
|
Thread Tools | Display Modes |
|
#1
|
|||
|
|||
SpaceX Capsule Explosion
With the investigation roughly 80% complete SpaceX believes they know
what caused the explosion during a pad test in April. The current pressurization design uses titanium check valves between the helium pressurization system and the propellant tanks. Such valves can allow flow in both directions, since they can be commanded open before full pressurization is reached. That is apparently what happened in this case, where about a cup of oxidizer 'backflowed' onto the fuel pressurization side and came into contact with titanium valves there, creating a titanium fire/explosion. To correct the situation, the titanium check valves will be replaced with 'burst valves'; essentially a pressure plate that breaks when full helium pressure hits it, which prevents the backflow problem. Another case of 'simpler is better'. -- "Insisting on perfect safety is for people who don't have the balls to live in the real world." -- Mary Shafer, NASA Dryden |
#2
|
|||
|
|||
SpaceX Capsule Explosion
|
#3
|
|||
|
|||
SpaceX Capsule Explosion
On 7/16/2019 7:03 AM, Jeff Findley wrote:
The space people I follow on Twitter are already questioning why a titanium check valve was used in the first place (posting a link to an old paper on the fact that NTO can cause a fire with titanium parts if enough energy is present). I don't know how common titanium check valves would be in aerospace NTO plumbing (some on Twitter were saying it's common). I also don't know if this was such a big issue, why didn't NASA oversight catch this? Right now, I'm more confused than anything. To correct the situation, the titanium check valves will be replaced with 'burst valves'; essentially a pressure plate that breaks when full helium pressure hits it, which prevents the backflow problem. Another case of 'simpler is better'. This sounds like a sane solution, so that NTO will never get into the helium plumbing by mistake. Jeff 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... Dave |
#4
|
|||
|
|||
SpaceX Capsule Explosion
On 7/16/2019 1:58 PM, David Spain wrote:
On Rand Simberg's blog, George Turner postulated they (titanium values) ^^^^^^^^^^^^^ My bad, it was Edward M. Grant. Dave |
#5
|
|||
|
|||
SpaceX Capsule Explosion
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. -- "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 |
#6
|
|||
|
|||
SpaceX Capsule Explosion
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? 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? Dave |
#7
|
|||
|
|||
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. |
#8
|
|||
|
|||
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 |
#9
|
|||
|
|||
SpaceX Capsule Explosion
In article , says...
On 7/16/2019 7:03 AM, Jeff Findley wrote: The space people I follow on Twitter are already questioning why a titanium check valve was used in the first place (posting a link to an old paper on the fact that NTO can cause a fire with titanium parts if enough energy is present). I don't know how common titanium check valves would be in aerospace NTO plumbing (some on Twitter were saying it's common). I also don't know if this was such a big issue, why didn't NASA oversight catch this? Right now, I'm more confused than anything. To correct the situation, the titanium check valves will be replaced with 'burst valves'; essentially a pressure plate that breaks when full helium pressure hits it, which prevents the backflow problem. Another case of 'simpler is better'. This sounds like a sane solution, so that NTO will never get into the helium plumbing by mistake. 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... Good point which I saw several times on the Internet yesterday. Also, SpaceX has an aversion to single use hardware simply because you can't test it before flight. And, at the very least, if you use the system, you now have parts to replace rather than just refilling the system and going again. So SpaceX dislikes single use hardware in general. On the other hand, a burst disk is about as simple as it can get. And you can make several off a single sheet of metal and test several of the ones on the sheet, leaving the ones you're going to use. So they're about as reliable as you can get. And replacing one shouldn't be that hard to do, but you'll need someone to check off that it was done right (lest we have another DC-XA type failure). So, after a day of seeing posts on this, I'm not as confused as I was yesterday morning while drinking my first cup of coffee. ;-) 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. |
#10
|
|||
|
|||
SpaceX Capsule Explosion
Jeff Findley wrote on Tue, 16 Jul 2019
07:03:11 -0400: In article , says... With the investigation roughly 80% complete SpaceX believes they know what caused the explosion during a pad test in April. The current pressurization design uses titanium check valves between the helium pressurization system and the propellant tanks. Such valves can allow flow in both directions, since they can be commanded open before full pressurization is reached. That is apparently what happened in this case, where about a cup of oxidizer 'backflowed' onto the fuel pressurization side and came into contact with titanium valves there, creating a titanium fire/explosion. The space people I follow on Twitter are already questioning why a titanium check valve was used in the first place (posting a link to an old paper on the fact that NTO can cause a fire with titanium parts if enough energy is present). I don't know how common titanium check valves would be in aerospace NTO plumbing (some on Twitter were saying it's common). I also don't know if this was such a big issue, why didn't NASA oversight catch this? 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. Right now, I'm more confused than anything. To correct the situation, the titanium check valves will be replaced with 'burst valves'; essentially a pressure plate that breaks when full helium pressure hits it, which prevents the backflow problem. Another case of 'simpler is better'. This sounds like a sane solution, so that NTO will never get into the helium plumbing by mistake. If this does indeed compromise reusability they'll probably come up with a different fix later on, but this one works and will get them flying again. -- "Insisting on perfect safety is for people who don't have the balls to live in the real world." -- Mary Shafer, NASA Dryden |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
SpaceX Dragon 2 capsule destroyed in abort motor ground test | Jeff Findley[_6_] | Policy | 59 | July 12th 19 08:30 AM |
Latest candidate for SpaceX pad explosion | Fred J. McCall[_3_] | Policy | 50 | October 28th 16 06:54 AM |
SpaceX Falcon 9 ? Possible Explosion | Jeff Findley[_2_] | Policy | 22 | October 9th 13 09:54 AM |
SpaceX capsule has 'new car' smell, astronauts say | [email protected] | Policy | 5 | June 2nd 12 08:09 PM |
Space Station's Robotic Arm Successfully Captures SpaceX Capsule | Sam Wormley[_2_] | Amateur Astronomy | 0 | May 25th 12 03:36 PM |