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SpaceX Dragon 2 capsule destroyed in abort motor ground test



 
 
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  #51  
Old May 5th 19, 07:22 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

JF Mezei wrote on Sat, 4 May 2019
18:45:28 -0400:

fron:

https://youtu.be/s6a9Ct9O_nY

(NASA press conference on launch of cargo Dragon, they discussed Dragon
2 "anomaly".

"just prior, before we wanted to fire the Super Draco, there was an
anomaly and the vehicle was destroyed"

"anomaly occured during activation of the Super Draco system".

"we have no reason to believe tht there is an issue with the Super Draco
themselves", "those have gone thorugh about 600 tests".


I'm pretty sure several of us told you that right after the anomaly
happened.


The way I interpret the SpaceX spokesperson's words:

-The engines are not at issue.


You don't know that. "No reason to believe" does not mean "not". It
just means that like the COPVs it's a lower probability. But we're
not playing "guess the most probable thing". We're playing "what the
**** went wrong so we can prevent it from happening again, regardless
of how likely it was".


-use of word "system" points to when they "booted" the system, not when
they pressed the the big red button to get fuel to the engines.


Right conclusion but wrong reason. Your key is 'activation', not
'system'. And that still doesn't guarantee it wasn't a combustion
chamber failure. For example, you pressurize the propellant tanks and
a throttle valve fails. That puts propellant under pressure in the
combustion chamber.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
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  #52  
Old May 5th 19, 10:53 AM posted to sci.space.policy
Anthony Frost
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Posts: 242
Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

In message
Jeff Findley wrote:

I doubt NASA is going to put anyone at risk, so the cleanup and
collection of debris and data will be slow and methodical. They're
going to want to document every bit of debris including exactly where it
was found. It's hard to do that when you're in a hazmat suit.


Elsewhere I've seen it said that the residual hypergolics isn't the main
reason for diverting the first stage landing, there are after all two
pads some distance apart. It's because it is very tricky to document the
position of any bits of debris when they've been blown about by the
exhaust from a descending Merlin.

Anthony

  #53  
Old May 5th 19, 02:43 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

In article ,
says...

On 2019-05-04 17:05, Jeff Findley wrote:

Those were numbers for the space shuttle, not Super Draco.


OK, had been lead to believe that Super Dracos were same ballpark as the
OMS engines on Shuttle. 1000psi is much much higher.


No you weren't. When I posted the shuttle RCS/OMS info, I said that the
because the thrust of Super Draco is so high the flow rate also had to
be high. So that necessitates higher pressure in the propellant tanks.

They're similar overall, but in some of the details, they're quite
different.

Assuming (for simplification) the fuel has to be at 1000psi, roughly
speaking at what PSI would helium tank need to be such that at end of
engine firing, there would still be 1000psi in the fuel tanks?

are we talking 1500psi, 2000 psi ? 5000psi ? (I have no ideas of size of
tanks involved and how much helium needs to be displaced as fuel tanks
empty to combustion chamber).


Depends on the details of the design. Plumbing, control valves, check
valves, and etc. will all cause a pressure drop from the tanks to the
combustion chamber. This is all internal flow type engineering that a
mechanical engineer would perform. That's the high level summary.
Trying to dig down into details isn't something we're going to be able
to do on a forum like this.

This another reason why I hate idle speculation. You're essentially
trying to reverse engineer the entire design to come up with pressures
of everything. Good luck with doing what a team of engineers likely
took months or years to do.

Just curious if in case of regulator failure, the fuel tanks may be
overhwelved with intense pressure from helium or whether the maximum
possible helium pressure would be well within reasonable pressure
capability of fuel tank.


Depends on the details of the design. Regardless, it shouldn't go
"boom", hence the investigation. A mechanical engineer wouldn't
knowingly design in a "feature" which could cause the whole system to go
"boom". This entire system is supposed to be fast, reliable, and fail
safe.

Although arguably an abort system which "fails safe" still leaves the
astronauts potentially dead because if the escape system has been
activated, Dragon 2 is potentially trying to escape a fireball caused by
the Falcon going "boom".

Would they design the hypergolic tanks to widthstand worse case scenario
in terms of helium pressure being fed into it ?


Depends on the design. There could be something like a blow off valve
to vent helium in case of over-pressurization of the plumbing. Air
compressors/tanks typically have a blow off valve so that if the
pressure switch for the compressor motor goes bad, the tank doesn't
over-pressurize.

Also pressurized tanks often have similar features designed into them
such that if they are over-pressurized, they'll more benignly vent
instead of exploding violently. Of course if you're venting hypergolic
propellants, you're already having a "bad day".

Again, we have no idea if the tanks had a feature like this or not.
More idle speculation.

Would the thrust level for Super Dracos be determined by the regulator
between helium tank and hypergolic tanks, or are there variable
regulators between hypergolic tanks and the combustion chamber ?


Most likely thrust level is controlled by valves between the propellant
tanks and the combustion chamber. That would give the fastest throttle
response time.

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.
  #54  
Old May 6th 19, 09:46 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

JF Mezei wrote on Sun, 5 May 2019
12:11:21 -0400:

On 2019-05-05 09:43, Jeff Findley wrote:

Depends on the details of the design. Plumbing, control valves, check
valves, and etc. will all cause a pressure drop from the tanks to the
combustion chamber.


OK, let me reformulate the question:

Would there be an expectation that the tanks holding hypergolics be
built to widthstand operating pressure (say 1000psi) with some safety
margin, or would they be designed to support the same pressure as what
the helium tanks are built for (say 10,000psi) in case a regulator fails
and equalizes between helium of hypergolic tanks ?


There is no such expectation and it would be a poor design that did
so.

This another reason why I hate idle speculation. You're essentially
trying to reverse engineer the entire design to come up with pressures
of everything.


No, trying to understand how these engines are built so that I can see
various way they can fail and better understand the meaning of what
SpaceX does say.


You seem mostly concerned about the former and mostly so you can
second guess.


For instance, by being told here that the Draco and SuperDracos are
separate systems operating are different pressures and from different
tanks, it put into perspective the statements from the SpaceX engineer
about why they weren't worried about Dragon 1's launch even if they have
similar Draco engines.


So by pointing out the obvious (that the Crew Dragon anomaly was in a
system that Dragon I doesn't even have) you understood something? You
shouldn't need help with that sort of thing. Here's a summary for
you:

Draco - 90 pounds of thrust, around 100 psi chamber pressure
Super Draco - 15,000 pounds of thrust, 1000 psi chamber pressure

There are 18 Dracos arranged around the capsule and they are used for
attitude and orbital maneuvering. They're essentially identical
regardless of which Dragon you're talking about.

Note that even if Crew Dragon used some Rube Goldberg system of
pressure reducers and conduits everywhere so as to be able to feed
Draco engines from Super Draco tanks, that wouldn't change your
'understanding', SINCE DRAGON I DOESN'T HAVE ANY SUPER DRACOS OR ANY
SUPER DRACO TANKAGE OR ANY SUPER DRACO PRESSURIZATION SYSTEM.

"boom". This entire system is supposed to be fast, reliable, and fail
safe.


Sometimes one has to make compromises to maintain the balance of fast,
reliable and fal safe. And sometimes you forget a possible failure mode
or are told to not worry about it because component X has never failed
and considered reliable.


You really don't know anything about engineering, do you? Yes,
compromises get made (but not the kind you seem to be thinking of).
And it doesn't matter what I, as an engineer, am 'told'. You pay
attention to and analyze all the possible failure modes. And when
something goes wrong you do it all again, using the data from what
went wrong. I had an understanding with my Program Manager. If he
suggested something and the words "But that would compromise good
engineering practice and rigor" came out of my mouth he would nod and
we would do it my way. That didn't happen very often (good
management).

Depends on the design. There could be something like a blow off valve
to vent helium in case of over-pressurization of the plumbing.


Would it be safe to have a blow off valve between helium tank and
hypergolic tank or would this have too high odds of hypergolic liquid
venting? (or is that considered a necessary evil to prevent hypergolic
tank from rupturing?


If I'm understanding what you're saying, if such a system was in place
I would expect a check valve between it and the propellant tank to
prevent 'backflow'. The biggest problem with 'blowoff valves' is they
have to blow off TO somewhere. That is probably going to involve
extra piping and a hole punched in the skin of the vehicle. In the
case of a helium tank you could use the fill connection. To put one
where you want it requires another vent.


In terms of pressurizing the helium tank, wouldn't there be a blow off
on pad equipment instead of being on-board since overfilling can only
happen on pad? (ag: not have to carry weight of blow off valve for
whole flight when it is only needed while on pad when tanks are filled.


You're assuming there's no way for the helium tank to overpressure
other than being overfilled. I see no reason to assume that.


--
"Insisting on perfect safety is for people who don't have the balls to
live in the real world."
-- Mary Shafer, NASA Dryden
  #55  
Old May 6th 19, 12:15 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 1,843
Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

In article ,
says...

On 2019-05-05 09:43, Jeff Findley wrote:

Depends on the details of the design. Plumbing, control valves, check
valves, and etc. will all cause a pressure drop from the tanks to the
combustion chamber.


OK, let me reformulate the question:

Would there be an expectation that the tanks holding hypergolics be
built to widthstand operating pressure (say 1000psi) with some safety
margin, or would they be designed to support the same pressure as what
the helium tanks are built for (say 10,000psi) in case a regulator fails
and equalizes between helium of hypergolic tanks ?


I believe Fred already answered this.

This another reason why I hate idle speculation. You're essentially
trying to reverse engineer the entire design to come up with pressures
of everything.


No, trying to understand how these engines are built so that I can see
various way they can fail and better understand the meaning of what
SpaceX does say.

For instance, by being told here that the Draco and SuperDracos are
separate systems operating are different pressures and from different
tanks, it put into perspective the statements from the SpaceX engineer
about why they weren't worried about Dragon 1's launch even if they have
similar Draco engines.


Because the systems are completely separate and the test of the Dracos
passed. It was the test of the Super Dracos that failed. Therefore,
using basic logic and reasoning, the Dracos on both Dragon and Dragon 2
are just fine. The problem is with the Super Dracos on Dragon 2.

BTW, Dragon 2 was just grabbed by the SSRMS. I was watching the NASA TV
live-stream on YouTube. Once they get Dragon 2 into the hold position,
the SSRMS moves relatively fast to grab the thing. Amazing really.

"boom". This entire system is supposed to be fast, reliable, and fail
safe.


Sometimes one has to make compromises to maintain the balance of fast,
reliable and fal safe. And sometimes you forget a possible failure mode
or are told to not worry about it because component X has never failed
and considered reliable.


Compromises? No, it's got to be fast, reliable, and fail safe. Those
simple terms are better captured by the actual engineering requirements
specifications for the Super Draco *system*. It either meets the specs
or it doesn't. So when something fails during testing, you figure out
why, fix it, and move forward.

Depends on the design. There could be something like a blow off valve
to vent helium in case of over-pressurization of the plumbing.


Would it be safe to have a blow off valve between helium tank and
hypergolic tank or would this have too high odds of hypergolic liquid
venting? (or is that considered a necessary evil to prevent hypergolic
tank from rupturing?


I don't even know if the system has a blow off valve. Because if that
valve did activate, you've got bigger problems (i.e. your escape system
isn't going to pull you away from the fireball engulfing a failing
Falcon launch vehicle).

In terms of pressurizing the helium tank, wouldn't there be a blow off
on pad equipment instead of being on-board since overfilling can only
happen on pad? (ag: not have to carry weight of blow off valve for
whole flight when it is only needed while on pad when tanks are filled.


From what I could see from the cell phone video, there wasn't any "pad
equipment" attached to Dragon 2 during this test. It was just sitting
on top of a test fixture.

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.
  #56  
Old May 6th 19, 04:01 PM posted to sci.space.policy
Niels Jørgen Kruse[_2_]
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Posts: 16
Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

Jeff Findley wrote:

From what I could see from the cell phone video, there wasn't any "pad
equipment" attached to Dragon 2 during this test. It was just sitting
on top of a test fixture.


I wonder how they add the vibrations for the test. I read that they
simulated the launch environment.

--
Mvh./Regards, Niels Jørgen Kruse, Vanløse, Denmark
  #58  
Old May 7th 19, 04:00 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 9,924
Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

JF Mezei wrote on Mon, 6 May 2019
17:03:13 -0400:

On 2019-05-06 14:58, Fred J. McCall wrote:

What we used to do was what we called "shake and bake" testing.


In the context of a highly instrumented ground test, wouldn't they first
fire the engines without vibration so they can establish a baseline
where they work, and then do a subsequent test with vibration where if
there is failure, it bcomes easier to narrow the possible causes ?


That doesn't make it easier.


In terms of re-usability, would the mugh higher thrust for Super Dracos
introduce some limits to how often they are designed to be fired versus
the Drasos that are fired hundreds of times to navigate in space and
then de-orbit (a firing that lasts 15 minutes).


No.


--
"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
 




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