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SpaceX Capsule Explosion



 
 
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  #1  
Old July 16th 19, 03:52 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default 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
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  #2  
Old July 16th 19, 12:03 PM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,959
Default SpaceX Capsule Explosion

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?

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
--
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.
  #3  
Old July 16th 19, 06:58 PM posted to sci.space.policy
David Spain
external usenet poster
 
Posts: 2,552
Default 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  
Old July 16th 19, 07:34 PM posted to sci.space.policy
David Spain
external usenet poster
 
Posts: 2,552
Default 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  
Old July 17th 19, 12:33 AM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 10,018
Default 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
  #6  
Old July 17th 19, 12:41 AM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 10,018
Default 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
  #7  
Old July 17th 19, 12:15 PM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,959
Default 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.
  #8  
Old July 17th 19, 04:41 PM posted to sci.space.policy
David Spain
external usenet poster
 
Posts: 2,552
Default SpaceX Capsule Explosion

On 7/16/2019 7:33 PM, Fred J. McCall wrote:

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.



That's an excellent point. I suspect more mods to Dragon V2 as SpaceX
and NASA gain experience with it.

Henry Spencer over on the Arocket mailing list has already suggested
that for re-usability actuated shutoff valves be used to hermetically
close the pressurization path in place of check valves.

I suppose it will come down to how easy it will be to replace the burst
disks between flights.

Dave

  #9  
Old July 17th 19, 05:06 PM posted to sci.space.policy
David Spain
external usenet poster
 
Posts: 2,552
Default 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

  #10  
Old July 18th 19, 12:40 AM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,959
Default SpaceX Capsule Explosion

In article , says...

On 7/16/2019 7:33 PM, Fred J. McCall wrote:

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.



That's an excellent point. I suspect more mods to Dragon V2 as SpaceX
and NASA gain experience with it.

Henry Spencer over on the Arocket mailing list has already suggested
that for re-usability actuated shutoff valves be used to hermetically
close the pressurization path in place of check valves.

I suppose it will come down to how easy it will be to replace the burst
disks between flights.


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
--
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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