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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