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Old 05-05-2019, 04:26 AM   #46
Thunder Chicken
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Originally Posted by MaverickSawyer View Post
 I dunno. I'm still convinced they overpressurized the propellant tanks and, for lack of a better term, popped them.

Maybe, but I just doubt they would let CRS-17 fly if the tank pressurization system was at all suspect. It's not crewed, but it berths at the ISS which is.
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Old 05-05-2019, 06:58 AM   #47
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 Does anyone know if the fuel tanks for the Draco and SuperDracos are the same, or are they separate pressurized systems? I have heard that there is "cross-feed capability", but I don't know it that simply means the engines draw from the same tanks or that there is a transfer capability in event of a crisis.

Also, are the tanks pressurized all of the time, or do they need to be pressurized prior to use possible use?

From what I have heard, they use separate propellant systems, because of different head pressure requirement for the engines.
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Old 05-05-2019, 01:57 PM   #48
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And, given the pressure differences, they'd need to use different tanks from the Cargo Dragon anyways, even if they were shared.
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Old 05-06-2019, 02:57 PM   #49
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 And, given the pressure differences, they'd need to use different tanks from the Cargo Dragon anyways, even if they were shared.
And as far as I can tell - they couldn't use the same tank to supply fuel for different head pressures by sticking a pressure regulator between propellant tank and thruster.

There are no pressure regulators qualified to work with MMH or N2O4, I can only find models flight-qualified for inert gases.

A flow regulator would be no option, because of the interrupted flow. When the thruster is not firing the line between regulator and thruster valve would slowly approach the same pressure as before the regulator - it is usually deployed near the engines to control the flow into the engines directly, but with known head pressure.

Last edited by Urwumpe; 05-06-2019 at 03:04 PM.
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Old 05-06-2019, 10:45 PM   #50
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Does the Dragon capsule normally just use the standard Draco RCS for deorbit burns?

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Old 05-06-2019, 11:03 PM   #51
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Yes. The super dracos are for launch abort or landing emergency, if the chutes fail, but the latter is not approved as of yet.
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Old 07-16-2019, 12:46 AM   #52
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Update from SpaceX on DM-1 explosion:

https://www.spacex.com/news/2019/07/...-investigation

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On Saturday, April 20, 2019 at 18:13 UTC, SpaceX conducted a series of static fire engine tests of the Crew Dragon In-Flight Abort test vehicle on a test stand at SpaceX’s Landing Zone 1, Cape Canaveral Air Force Station in Florida.

Crew Dragon’s design includes two distinct propulsion systems – a low-pressure bi-propellant propulsion system with sixteen Draco thrusters for on-orbit maneuvering, and a high-pressure bi-propellant propulsion system with eight SuperDraco thrusters for use only in the event of a launch escape. After the vehicle’s successful demonstration mission to and from the International Space Station in March 2019, SpaceX performed additional tests of the vehicle’s propulsion systems to ensure functionality and detect any system-level issues prior to a planned In-Flight Abort test.

The initial tests of twelve Draco thrusters on the vehicle completed successfully, but the initiation of the final test of eight SuperDraco thrusters resulted in destruction of the vehicle. In accordance with pre-established safety protocols, the test area was clear and the team monitored winds and other factors to ensure public health and safety.

Following the anomaly, SpaceX convened an Accident Investigation Team that included officials from the National Aeronautics and Space Administration (NASA), and observers from the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB), and began the systematic work on a comprehensive fault tree to determine probable cause. SpaceX also worked closely with the U.S. Air Force (USAF) to secure the test site, and collect and clean debris as part of the investigation. The site was operational prior to SpaceX’s Falcon Heavy launch of STP-2 and landing of two first stage side boosters at Landing Zones 1 and 2 on June 25, 2019.

Initial data reviews indicated that the anomaly occurred approximately 100 milliseconds prior to ignition of Crew Dragon’s eight SuperDraco thrusters and during pressurization of the vehicle’s propulsion systems. Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve. The failure of the titanium component in a high-pressure NTO environment was sufficient to cause ignition of the check valve and led to an explosion.

In order to understand the exact scenario, and characterize the flammability of the check valve’s titanium internal components and NTO, as well as other material used within the system, the accident investigation team performed a series of tests at SpaceX’s rocket development facility in McGregor, Texas. Debris collected from the test site in Florida, which identified burning within the check valve, informed the tests in Texas. Additionally, the SuperDraco thrusters recovered from the test site remained intact, underscoring their reliability.

It is worth noting that the reaction between titanium and NTO at high pressure was not expected. Titanium has been used safely over many decades and on many spacecraft from all around the world. Even so, the static fire test and anomaly provided a wealth of data. Lessons learned from the test – and others in our comprehensive test campaign – will lead to further improvements in the safety and reliability of SpaceX’s flight vehicles.

SpaceX has already initiated several actions, such as eliminating any flow path within the launch escape system for liquid propellant to enter the gaseous pressurization system. Instead of check valves, which typically allow liquid to flow in only one direction, burst disks, which seal completely until opened by high pressure, will mitigate the risk entirely. Thorough testing and analysis of these mitigations has already begun in close coordination with NASA, and will be completed well in advance of future flights.

With multiple Crew Dragon vehicles in various stages of production and testing, SpaceX has shifted the spacecraft assignments forward to stay on track for Commercial Crew Program flights. The Crew Dragon spacecraft originally assigned to SpaceX’s second demonstration mission to the International Space Station (Demo-2) will carry out the company’s In-Flight Abort test, and the spacecraft originally assigned to the first operational mission (Crew-1) will launch as part of Demo-2.

Last edited by Thunder Chicken; 07-16-2019 at 12:51 AM.
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Old 07-16-2019, 01:41 AM   #53
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Good news !!
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Old 07-16-2019, 07:00 AM   #54
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I'm surprised that the reaction of titanium with N2O4 was a surprise, given that titanium will burn in pure N2.
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