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SpaceX Capsule Explosion



 
 
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  #11  
Old July 18th 19, 12:43 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 1,959
Default 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
--
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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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  #12  
Old July 18th 19, 01:51 AM posted to sci.space.policy
David Spain
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Posts: 2,552
Default 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  
Old July 18th 19, 12:15 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 1,959
Default 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.
  #14  
Old July 18th 19, 01:17 PM posted to sci.space.policy
David Spain
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Posts: 2,552
Default 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
  #15  
Old July 18th 19, 01:39 PM posted to sci.space.policy
David Spain
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Posts: 2,552
Default 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
  #16  
Old July 18th 19, 06:44 PM posted to sci.space.policy
Greg \(Strider\) Moore
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Posts: 658
Default 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


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  #17  
Old July 18th 19, 07:38 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default 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
  #18  
Old July 18th 19, 07:43 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default 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
  #19  
Old July 18th 19, 07:47 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default 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
  #20  
Old July 19th 19, 11:33 AM posted to sci.space.policy
David Spain
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Posts: 2,552
Default 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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