In article ,
says...

JF Mezei wrote on Mon, 22 Apr 2019
20:34:34 -0400:

On 2019-04-22 07:05, Jeff Findley wrote:

Accident Claims SpaceX Dragon Abort Test Capsule
Apr 21, 2019 Irene Klotz - Aerospace Daily & Defense Report
https://aviationweek.com/space/accid...on-abort-test-
capsule

(sorry, it won't allow me to read that article).


You have to take the line break out so that it ends with '-capsule'.


Saw pictures of people on a beach with the big orange cloud at a
distance. How dangerous is burned hydrazine ? or would that cloud
contain unburned toxig hypergolics ?


Hydrazine burns to nitrogen and water. The color sounds to me like
nitrogen dioxide, which is a partial decomposition product of
dinotrogen tetroxide. Hydrazine and dinitrogen tetroxide are both
colorless.

I guessed it was a BFRC (big falcon red cloud) from the nitrogen
tetroxide converting to nitrogen dioxide. The picture on Wikipedia
shows some nice orange to red colors:

https://en.wikipedia.org/wiki/Dinitrogen_tetroxide

I assume thrusters on shuttle were a simple pump that pushed the 2
compienst into a small chamnber where they mix, combust and then push
themselves out through the thruster port.

Would Draco be the same or have actual turbopumps and high
velocity/energy moving parts ?


First, it's SuperDraco, not Draco. SuperDraco is something like 200
times the size of Draco. Most hypergolic engines use simple pumps,
although turbopumps are possible in very large engines. SuperDraco
uses ordinary pumps.

As far as I know, SuperDraco doesn't use pumps (too heavy and slow).
It's a "simple" pressure fed system. But because of the flow rates and
thrust needed in a short amount of time, I'd imagine that the helium
tanks for them are extremely high pressure and the valves and lines are
sized for high flow rates.

Are the hypergolic tanks highly pressurized or just enough to push the
liquids to the combustion chamber ?


They're usually pressurized to above the chamber pressure of the
engine.

Yep.


Trying to understand what sort of failure mode would cause the apparent
total destruction of the craft.


We don't know that there WAS "total destruction of the craft".

All we know, from the clearly unauthorized cell phone video, is that the
spacecraft was out of frame at the end of the video. It's not like it
was a high speed camera. The Dragon 2 capsule could be sitting on the
ground just out of frame or it could be in little pieces. The fact is
that we just don't know.

If the engines are simple pumps that push the hypergolics into chamnber
and they burn and exhaust, why would salt water cause such a failure?


Where did you get the idea that salt water had anything to do with it?
The engines successfully fired multiple times before the 'anomaly'.

That is still unclear. Some things I've read online say that they
earlier tests that day were of the Draco engines. That would mean that
the SuperDraco test at the end would have been the first time they were
fired since the DM-1 mission (where they were not used, but were
subjected to a spaceflight and ocean water landing).

If the engines are more complex with turbopumps and what not, wouldn't
they require fairly expensive inspections before being fired ?


They aren't and no, why would they?

Pressure fed engines with hypergolic propellants so they're as simple,
and reliable, as they can be. Every US crewed spacecraft following
Mercury has used similar engines.

If they find out that the cause of the accident was salt water, could
this allow "on time" continuation of programme using a new Dragon 2 as
test article for the pad eject test ? (aka: delay re-usability but don't
delay programme).


If they find out the cause of the anomaly WHATEVER IT WAS they will
then be able to continue the program. It looks to me like a pump
failure on the hydrazine tank, but that's just a guess and we don't
know why it failed if that was the case.

From the crappy video, it almost looks like a helium pressure vessel,
valve, or line ruptured first followed by the nitrogen tetroxide and
hydrazine tanks rupturing. What little burning did occur would be what
spread the cloud of propellants into the atmosphere (this cloud showed
up on weather radar!).

Or does this accident represent a hard stop to the programme
irrespective of cause until Space-X has designed changes to the
capsule/engines and proven it is safe to NASA ?


Regardless of the cause they are at full stop until they determine
what the cause was. They will then have to prove it's safe,
regardless of whether they have to make changes or not. There's a lot
of fire time on SuperDraco engines for me to believe there is some
fundamental design flaw.


Agreed. But in the meantime, preparations continue for the next Dragon
cargo flight to ISS. This should not be a concern since Dragon has no
SuperDraco engines and Dragon has been flight proven many times over.

Is it correct to assume that SpaceX would have tested engines after
having dropped them in sea water well before building the first Dragon 2
capsule ?


No, it is not, because there is no reason to do so.

Actually, the Dragon 2 pad abort test capsule was recovered from the
ocean and later used as the DragonFly test article (which performed
tethered hover tests on its SuperDraco engines).

Jeff

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