NASA Scrubs Launch Due to Faulty Fuel-Tank Sensor

VUSE g-EE-k writes "NASA has scrubbed Wednesday's launch of Discovery due to a faulty fuel-tank sensor in the external fuel tank. They are going to begin the troubleshooting process. They have not released details as to how long this delay will last. The crew have begun to get off the shuttle. For more information, see the NASA TV site. Drudge Report has some initial coverage of the scrub." Reader adefa adds a link to NASA's Space Shuttle launch page with more info.

Comments

Redundant system

[Fiz Ocelot]

One of four sensors used to detect a low level of hydrogen propellent. They need 2 to work of the four. After they detank, it can be diagnosed, possibly launching tomarrow.

Re:Redundant system

[b1t r0t]

Cause the last thing they want is to launch, and then run out of gas while in orbit!

That's pretty close... actually they don't want to run out of gas before reaching orbit. I was watching NASA TV and they mentioned that the purpose of the sensors is that the main engines don't take very well to suddenly running out of fuel when they're going at full throttle. This would allow them to know when fuel was low so that they could throttle back.

Re:Redundant system

[Anonymous Coward]

From the Shuttle Operational Data Book:

There are eight propellant-depletion sensors, four each for fuel and oxidizer. The fuel-depletion sensors are located in the bottom of the fuel tank. The oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect. During SSME thrusting, the orbiter general-purpose computers constantly compute the instantaneous mass of the vehicle due to the usage of the propellants. Normally, main engine cutoff is based on a predetermined velocity; however, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down.

The locations of the liquid oxygen sensors allow the maximum amount of oxidizer to be consumed in the engines, while allowing sufficient time to shut down the engines before the oxidizer pumps cavitate (run dry). In addition, 1,100 pounds of liquid hydrogen are loaded over and above that required by the 6-1 oxidizer/fuel engine mixture ratio. This assures that MECO from the depletion sensors is fuel-rich; oxidizer-rich engine shutdowns can cause burning and severe erosion of engine components.

Re:Redundant system

[Rei]

You run the SSMEs fuel-rich for two main reasons.

* It gives better ISP by reducing the number of atoms in the exhaust stream molecules (H2 vs H2O), thus increasing the amount of directional translational energy compared to rotational and vibrational energy (not because of the lower mass/higher velocity of the output gas, like some sites and even textbooks mistakenly state, because you get less energy output from the reduced reaction rate at the same time and thus over-cancel out the effect)

* It strongly reduces corrosion (not that a mixture containing incredibly hot hydrogen isn't corrosive; it's just less corrosive than a mixture containing incredibly hot oxygen, as far as the combustion chamber and nozzle linings are concerned)

The latter issue is undoubtedly the reason for concern of running out of hydrogen.

Another article link

[JLSigman]

In case others get Slashdotted, Here's CNN.com's article.

Better safe than sorry

[ravenspear]

Hopefully they will get the issue fixed soon.

They have until July 31st in the current launch window if I recall correctly.

Troubleshooting to begin shortly

[Alizarin Erythrosin]

NASA engineers are waiting for the astronauts to disembark so they can troubleshoot the sensor while the tank is still full. The way they were discussing it on NASA TV, the tank will be defueled at some point, so they wanted to run some tests before that.

Orlando Sentinel shuttle blog

[kingrat]

Here.

Re:Launch window?

[UMhydrogen]

NASA has through July 31 to launch Discovery or else wait until September 9 due to the need to lift off and separate the external tank in daylight.

Re:Begin the countdown!

[eggoeater]

The external fuel tank burns up in the atmosphere after launch. They are never reused or AFAIK even recovered. The external fuel tank on the pad now, and every part in it, is brand new.

Re:Horrible Quality

[timster]

I would hope F, as there is no such thing as -400 C.

Re:Launch window?

[InfiniteWisdom]

There's a daily lauch window of about 10 minutes when the ISS is in the right place. Those 10 minutes occur during the night between July 31st and September 9th.

Re:Launch window?

[saider]

Actually, yes.

The ascent path to the ISS will be partially or completely dark for the next few months after the window. This is because of the mechanics of the ISS orbit. Basically they have to make sure that the ISS passes reasonably close to the launch pad and that that pass occurs early enough in the day, so that it is still daylight a few thousand miles downrange.

Re:Begin the countdown!

[NeoThermic]

>Why not just put in 10 sensors and as long as 3 or 4 are working then go ahead launch.

They did. There's four fuel sensors, of which one was faulty. You can launch on two, you can use it on just one. Its not a critical system in most respects, but since this fault is a bit odd, they decided to scrub the launch and check it over.

NeoThermic