NASA may have isolated shuttle glitch

New Scientist.com ^ | July 21, 2005 | Kelly Young

[Tunehead54]

Thank goodness at least SOMEONE recognizes the dangers to our pigeons and seagulls! Does PETA Know about this?

---

Oh and by the way - I figure the power density variation was only +/- 0.46. Check your work - you too Brush)! ;-)

[burzum]

Being a person who has worked on nuclear reactor instrumentation and control equipment (I&CE) systems, I feel qualified to comment a little on the redundancy issue.

NASA has said that this system is 2-failure redundant and provides coincidence protection. This is a fairly insane standard to meet in operation because this means that any 2 sensors, instruments, control circuitry, power supplies, sources of power, or combination thereof can fail and the system would still operate. It also means that 2 sensors must be giving the same signal to initiate a protective function (typically if an instrument fails it will not be within this band). To give you an idea of how hard it is to meet a 2-failure redundant system, I will briefly describe a 1-failure redundant system.

In case 1: you have 3 sensors. Each sensor has an associated instrument (which sends an electrical signal to the sensor, receives it back, and interprets it). Each instrument is powered off of a single electrical bus and is isolated by fuses or breakers. This single electrical bus is considered indestructible (normal in engineering systems since it takes alot to break a bus bar). The electrical bus can be powered one of two ways. It can be powered by a DC auctioneered power supply, where you have 2 DC input sources, 2 diodes, and one output source. This will supply power to the load with both sources energies or either energized. The second way is by an automatic bus transfer device (ABT). If one AC bus goes away, it will automatically switch to the other (sort of like the auctioneered power supply except it is AC and it is built differently). This case 1 system will be able to provide coincidence protection in any case except the failure of a bus bar or a failure of the ABT (or auctioneering power supply).

In case 2: The problem with the potential failures of a bus bar or ABT (or auctioneering power supply) is fixed by providing 4 sensors. 2 sensors and their associated instruments are powered by one power supply, which is electrically isolated from the other 2 sensors and their associated instruments. In this case any 2 failures will allow the system to operate normally.

Now try to do it with 2 failure redundant and have coincidence protection. This requires at least 4 sensors and 4 separate power supplies. If one has failed prior to launch you are now down to 1 failure redundant. But, I don't think the shuttle is built this way. Even though the engineers have stated that it is 2 failure redundant, I believe that has caveats. I do not believe that the shuttle has 4 separate power supplies (from a cursory look at the electrical schematics they had on the mission site a week ago). I believe they have 2. This means its probably the same as case 2 that I described. Consider if they have a failed sensor #3. Then if they lose power supply 1 that powers instruments and sensors #1 and #2, they only have sensor #4 operating. They no longer have coincidence protection (and only a hack like a loss of power signal can allow a protective function). This means that one additional failure can render the system inoperative.

It seems wise to me to fix the problem on a redundant, coincidence protected system. Working on nuclear reactors I would never consider operating a safeguards system with a broke sensor. I would be less likely to consider it if I was an astronaut.

[The Brush]

I figure the power density variation was only +/- 0.46.

It's not my fault, I was distracted by a noisy group of carollers at my front door demanding figgy pudding.

[Tunehead54]

I hate it when that happens - it really does make it hard to concentrate aside from the fact that the price of figgys these days is ridiculous!

---

;-)

[Chode]

outstanding reply... even more so because i believe i understood it all 8^)

btw, have you ever worked with IST incore/transcore/excore or visual systems???

[Nachoman]

The problem was isolated to the top three layers of NASA management.

[burzum]

I worked as a reactor operator on a S5W submarine. My job included both maintaining and repairing all of the reactor instrumentation and control equipment as well as operation of the reactor itself. Compared to civilian reactors, military reactors tend to be fairly simple, but very expensive. Oddly enough, simple tends to be more expensive in some engineering projects (look into the cost of reduction gears for a ship if you are curious). Of course simple is good if you need to be able to withstand depth charges or a potential grounding incident (like what occurred to the USS San Francisco--reactor kept online). Unfortunately due to different regulations (EO 12958 as amended, Atomic Energy Act as amended, and the International Export in Arms Regulation) I can't say which if any or if all systems were used to monitor my reactor. Nonetheless, I am familiar (have been trained) with most of the systems you have listed.

[Rockitz]

I do not believe that the shuttle has 4 separate power supplies...

Extremely doubtful. It has to fly- nuclear reactors typically don't, but certainly have- Cassini most recently.

[null and void]

So, if their fuel gauge read empty halfway home, what were they going to do? Stop by the nearest "Shuttle Gas & Go" and fill up?

As I understand it the turbine pumps need to be shut down before they go dry, otherwise they will spin up so fast that they will fail spectacularly, with loose blades flying everywhere. The fuel sensors are there to gracefully shut the pumps down in the event of a leak.

This would cause an early main engine shutdown and an abort to low orbit, secondary landing strip, or a swim.

All of these are much easier on the crew than having the tail of the orbiter disintegrate at high Mach numbers and altitude...

[jetson]

I just watched the video " Space Station" narrated by Tom Cruise...AWSOME! It is a documentary on the building of the international space atation. Having done all that...What is the problem with our latest shuttle flights? Is this a reflection of our new work force? Not as educated and not as thorough?

[jetson]

I just watched the video " Space Station" narrated by Tom Cruise...AWSOME! It is a documentary on the building of the international space atation. Having done all that...What is the problem with our latest shuttle flights? Is this a reflection of our new work force? Not as educated and not as thorough?

[Chode]

cool, my question about are you being familiar with IST(Imaging and Sensing Technology)brand sensors and controls was because i used to work there and the nuclear Navy was one of our biggest customers... 8^) the last systems we did while i was were for the SeaWolf.

[Right Wing Assault]

The B52 has redundant systems. For the first two minutes of flight the Shuttle is a projectile with no redundancy. One merely has to overcome gravity and windspeed ... the other gravitational forces and the atmosphere. Big differences.

That's not the point. Of course a plane is different from a spacecraft. I was responding to the statement that was made concerning age. He implied that the Shuttle wasn't worthy due to age. I was responding that age can be repaired. An unmaintained B-52 will crash as will an unmaintained Shuttle.