SpaceX Falcon 9/Dragon COTS Demo (C2+) GENERAL DISCUSSION

[Antares]

One more question: is the engine controller typically in charge of initiating shutdown in the event of redlines/imminent failure or does it just inform the flight computer which then commands a shutdown?

Depends on the rocket.  Some leave the ground systems in charge until release.  Some have the engine controller tell the flight computer, "Hey, I'm aborting.  Tell everything else to abort too."  Some just relay the engine measurements to the flight computer.

[ugordan]

That's interesting because I was thinking that in the "hey, I'm aborting" layout you can get into trouble if you're not careful.

Suppose you're really unlucky and a redline is tripped a millisecond (or whatever the delay can be before the flight computer is expected to process queued signals) before T-0 and the controller commands shutdown. Meanwhile... the flight computer has just issued a launch commit, vehicle release *before* processing that controller signal and you're in a world of hurt.

Seems to me concentrating such critical functions into a single point would be the best approach.

I wonder how F9 does it. Shotwell's comment kind of gave the impression that the controller had some authority and the flight computer commanded the shutdown of the 8 remaining engines.

[Herb Schaltegger]

Do not believe it has been fired in a vacuum with a complete engine bell.

Flight 1?

Isn't the one when they sent a tech into the interstage with some snips and trimmed the nozzle prior to flight?

[ugordan]

That was flight 2.

[zerm]

For anyone wishing to read, here's my piece on this morning's scrub as seen from the press site- please keep in mind that this is written for regular folks in the aviation industry. Also- the photos were selected by the editor and not me.

http://www.aero-news.net/index.cfm?do=main.textpost&id=6e16e50a-9eb6-449b-b14b-528fa3b3a182

[zerm]

FYI, Ms Shotwell was FAR from depressed this morning right after the scrub. She was very up-beat in the press site, smiling and quite animated. (Charlie Bolden, however, really appeared to be wishing he could have slept in.) I thought she spoke as professionally as anyone could have wanted while in the presser- raised my opinion of her by several degrees (as if that matters). SpaceX had their act together this morning- 100%

[sdsds]

Faulty valve, or fault in the valve? That is, was the valve up to spec but exposed to out of spec environment?

[Comga]

For anyone wishing to read, here's my piece on this morning's scrub as seen from the press site- please keep in mind that this is written for regular folks in the aviation industry. Also- the photos were selected by the editor and not me.

http://www.aero-news.net/index.cfm?do=main.textpost&id=6e16e50a-9eb6-449b-b14b-528fa3b3a182

Nice.  Well said, especially for "regular folk".

You said "T-00:08:14: NASA PAO reports that, "The International Space Station is  passing over Florida now." A lot of us hustle outside and look up into  the night's sky, but we see nothing. Either the pass was too low to the  horizon for us to see it, or the PAO was late on their call." 

You were correct.  According to Heavens Above, the brilliant  (-2.7 mag) pass of the ISS ended by dropping below 10 degrees altitude at 4:46:02, which was T-9 minutes.  If only you had been alerted to be out looking ESE at T-12 minutes you might have seen the ISS emerge into the sunlight high (48 deg) in the sky.

You can check Heavens-Above for KSC before the next launch attempt.  However, if they launch on Tuesday, the ISS won't emerge into sunlight until it is only 11 degrees above the horizon to the NE.

[deltaV]

I've seen oxygen-rich combustion (relative to nominal mixture ratio, not relative to stoichiometric) as a conjectured cause of the combustion chamber pressure anomaly that caused the abort. If the same problem had happened to the upper stage Merlin (assuming vacuum Merlin has the same failure mode) I'm thinking loss of mission would be likely. Even if the engine avoided disassembly the unburned kerosene would screw up the burnout mass, leading to underperformance.

If the above is correct then this launch was only saved from failure by the anomaly occurring in the first stage. This suggests to me that Falcon 9 currently has a loss of mission rate quite a bit worse than the state of the art of around 1%. This isn't surprising for flight 3, but it seems worth mentioning since it helps deflate some of SpaceX's hype. For example the DoD seems quite justified in not letting SpaceX compete with EELV yet.

[go4mars]

I thought she spoke as professionally as anyone could have wanted while in the presser- raised my opinion of her by several degrees (as if that matters).

Good leadership definately does matter.  If you noticed, stakeholders and employees probably did too.  If she had been despondent with red eyes or a cloud of pessimism or fury...   

Well, the tone set by the leaders has a big impact on corporate culture and expectations.  It really does matter. 

And it doesn't mean its an act or anything.  She was probably hired partially based on a demonstrated understanding of the "It's okay to fail, but not okay to not try" mentality, and an optimistic worldview. 

[rickl]

That was a good article, zerm.

[jongoff]

The gas generator in a GG turbopump runs really, really rich to keep the combustion temperature low enough to not melt the turbine blades (I think they're limited to somewhere in the 1000-1200K range). If you had a leaky check valve depriving the gas generator of fuel, it would run leaner, which would cause it to run hotter, which could increase the turbopump power (you have less heat sponge soaking up combustion energy) and chamber pressure.  The problem is that you don't have to lean out the gas generator very far before the turbine gets too hot.

That's my best speculation based on the limited information.

~Jon

[deltaV]

The gas generator in a GG turbopump runs really, really rich to keep the combustion temperature low enough to not melt the turbine blades (I think they're limited to somewhere in the 1000-1200K range). If you had a leaky check valve depriving the gas generator of fuel, it would run leaner, which would cause it to run hotter, which could increase the turbopump power (you have less heat sponge soaking up combustion energy) and chamber pressure.  The problem is that you don't have to lean out the gas generator very far before the turbine gets too hot.

1. Are you sure that less fuel would cause increased turbopump power? The temperature would indeed increase, but there would be less mass flow rate. If oxygen mass flow rate is unchanged then combustion energy available would also be unchanged (edit: oops that doesn't follow since the extra fuel does react chemically). If turbopump efficiency is unchanged (I dunno if it would be) then turbopump power would also be unchanged.

2. If I understand your conjecture correctly only the main chamber would have the normal mixture ratio, just more propellants than usual. In that case my comments about the wrong mixture ratio causing too much burnout mass are probably wrong since an off-nominal GG shouldn't affect burnout mass much. I'm not saying that everything would be fine, just that an increased burnout mass probably isn't an issue.

[robertross]

The gas generator in a GG turbopump runs really, really rich to keep the combustion temperature low enough to not melt the turbine blades (I think they're limited to somewhere in the 1000-1200K range). If you had a leaky check valve depriving the gas generator of fuel, it would run leaner, which would cause it to run hotter, which could increase the turbopump power (you have less heat sponge soaking up combustion energy) and chamber pressure.  The problem is that you don't have to lean out the gas generator very far before the turbine gets too hot.

1. Are you sure that less fuel would cause increased turbopump power? The temperature would indeed increase, but there would be less mass flow rate. If oxygen mass flow rate is unchanged then combustion energy available would also be unchanged (edit: oops that doesn't follow since the extra fuel does react chemically). If turbopump efficiency is unchanged (I dunno if it would be) then turbopump power would also be unchanged.

2. If I understand your conjecture correctly only the main chamber would have the normal mixture ratio, just more propellants than usual. In that case my comments about the wrong mixture ratio causing too much burnout mass are probably wrong since an off-nominal GG shouldn't affect burnout mass much. I'm not saying that everything would be fine, just that an increased burnout mass probably isn't an issue.

More complete combustion (more products consumed) = more hot gases produced = more power.

Fuel rich means fuel just doesn't get burn't

[Rocket Science]

And generally produces a lot more power just before it all lets go…

[IRobot]

Sometimes you can check them and find nothing. It will only show its weakness when put to use.

[robertross]

SpaceX statement:

Today’s launch was aborted when the flight computer detected slightly high pressure in the engine 5 combustion chamber. We have discovered root cause and repairs are underway.

During rigorous inspections of the engine, SpaceX engineers discovered a faulty check valve on the Merlin engine.  We are now in the process of replacing the failed valve.  Those repairs should be complete tonight.  We will continue to review data on Sunday.  If things look good, we will be ready to attempt to launch on Tuesday, May 22nd at 3:44 AM Eastern.

The real question is whether SpaceX will investigate all of the similar check valves on the rest of the engines on this vehicle, as well as the rest of the MErlin Engines produced and valves procured.  THat was definitely something we would see on shuttle, but then again it is a commercial venture.

Would love to see a report on the issue just like shuttle, but that would probably be proprietary.

Definitely

One would think there would be a parallel effort to examine engines that have already been test fired at least a couple of times on the stand, and be 100% confident there are no similar issues. Easier to do that than on the vehicle itself.

[Vegeta]

Today’s launch was aborted when the flight computer detected slightly high pressure in the engine 5 combustion chamber. We have discovered root cause and repairs are underway.

I wonder if that means they have discovered root cause on why the valve failed and how it escaped detection even after the static fire.

[Jim]

To do cross range, falcon need to boost dragon into higher orbit, there, dragon wait for iss pass by, and then dragon decelerate into lower orbit to meet iss.

that is not the definition of cross range.  Yaw steering takes out cross range

[sunya]

The next launch time has been determined since the reason was found as they declare?