From: firstname.lastname@example.org (Steve Patlan)
Subject: The 'Last Word' on that Challenger Transcript
Date: Wed, 31 Jan 1996 18:12:42 -0600
Organization: NASA JSC
Where does the Shuttle's power come from?
To believe that the transcript is valid assumes that it was receiving
power after the explosion. The electrical system of the shuttle is
designed to be fail-safe - i.e, doubly redundant. There are three Fuel
Cell Powerplants (FCPs) which feed the three main electrical distribution
buses - MNA, MNB, and MNC. Some equipment is powered by redundant feeds
from two main buses. The FCPs are fed from multiple sets of cryogenic H2
and O2 tanks - at least three, and typically four. A failed fuel cell is
shut down and taken off-line, and a leaking cryo line can be isolated from
the rest of the system by closing valves (isolating the tank in the
process). The design philosophy of the shuttle assumes that at most two
FCPs could be lost before making an emergency landing, so there is no
"need" for battery backup-power, which is also prohibited by weight
considerations. (The shuttle uses 28V dc power. A typical on-orbit
current level is 570 amps. That's an awfully big battery.)
How is the Ops-2 recorder powered?
I consulted the Space Shuttle Systems Handbook (publication JSC-11174,
Revision E of October 28, 1994) Drawing 16.18, entitled "OPS RCDRS" shows
that power is applied to Ops-2 by Switch 12 on Panel A1A3. The recorder
has redundant feeds from the MNB and MNC buses, so at least one of FCPs 2
and 3 would need to be on-line after the accident.
So where are the FCP's located?
In the Rockwell shuttle body-axis coordinate system, the X axis is
along the vehicle length, positive out the tail. It is measured in
inches, with Xo236 at the nose and Xo1613 at the end of the bodyflap
(aerosurface between the elevons and below the main engines.) The aft
bulkhead of the crew compartment is at Xo576. So, here are the FCPs:
FCP1: Xo650 FCP2: Xo620 FCP3: Xo680
Here are the cryo tanks:
Tankset 1 | Xo890 | Xo780 |
Tankset 2 | Xo830 | Xo720 |
Tankset 3 | Xo1010 | Xo1100 |
Tankset 4 | Xo890 | Xo1070 |
All these are located beneath the payload bay. Tanksets 1 and 2 are
used on Ascent. Now, in the explosion video that I have seen, the crew
compartment can be unambiguously identified. It is clearly *not* attached
to 22 feet of keel. In fact, it does not appear to be attached to much of
anything. It is definitely *not* trailing a tangle of plumbing and cryo
tanks. (FYI, the O2 tanks are 33 inches in diameter and hold 781 lbs.
The H2 tanks are 41 inches and 92 lbs. I don't think I would have missed
seeing them.) There are two cryo pressure regulators for each FCP. The
first drops the line pressure from the tanks down to 120 psia, and the
second drops that down to 60 psia. Since the lines were severed during
the explosion, there wouldn't have been an appreciable amount left between
the regulators and the FCP. For reference, in the first 2 minutes and 30
seconds after liftoff, the shuttle uses 3.58 lbs of H2 (0.82 cubic feet)
and 28.6 lbs of O2 (0.42 cubic feet). So, the FCPs were not receiving
cryo after the vehicle broke up.
Yeah, but won't the FCP's "keep going"?
Well, this assumes that they were still attached. It's been a while
since I saw the video, but I don't recall seeing an extra five feet of
payload bay attached to the rear of the crew compartment. But I will
address this issue for the sake of completeness. In a nominal, non-
explosion scenario, an FCP will still operate if the flow of one of the
reactants is shut off. If H2 is cut off, the FCP will fail within 20
seconds. That's "fail" as in zero power - it will begin to drop off
immediately. If both H2 and O2 are cut off, I would expect it to drop to
zero in less than 20 seconds.
Making the shaky assumption that the FCP survived the explosion intact,
there is another consideration: Each FCP contains three substacks, each
with 32 fuel cells. The walls within and between these small cells are
rather thin. A regulator within the FCP keeps the coolant pressure the
same as the O2 pressure - if the O2 pressure is much greater than the
coolant, the walls will fail, destroying the internal structure. The
coolant line between the FCP and the heat exchanger would have been
severed in the explosion, so its pressure would have dropped to zero.
However, the O2 is still at 60 psia beyond the stage 2 regulator. Picture
a bear swiping a paw through a honeycomb. The FCP's water lines would
also have been severed in an explosion, causing another pressure
differential. It is not really reasonable to assume that the FCPs
produced electricity for more than a couple of seconds after the vehicle
broke up. But they weren't attached to the cabin anymore anyway, so it
Hey, we all know flight recorders have batteries. Sheesh!
Well, the systems drawing I mentioned earlier did not show any external
batteries. Besides, the Ops-2 recorder is not an airline-type "black
box", although it records the same type of data. I consulted the
Communications Systems Section head (who I've known for 15 years and is no
government shill) and his Ops Recorder system expert. They confirmed that
the recorder contains no internal batteries. Keep in mind that the
shuttle was designed in the early 1970's and uses a lot of old technology.
The Ops recorder is a large, heavy piece of equipment, and weight and
space are both at a premium on the shuttle. Given that the shuttle's
power system is assumed to be reliable, there is no reason to believe that
the Ops-2 recorder contains a battery. If you don't believe that last
sentence, re-read this article until you do. Thank you.
I welcome any reasonable, verifiable objections to the technical
arguments presented above. If you don't know diddly about Shuttle
hardware, don't bother. If you can only offer paranoia and skepticism,
that and a quarter will buy you a cup of jack-squat. If you want to argue
that since my Handbook was revised in 1994 it is obviously a battery
cover-up, I can only say: Get a life, you pathetic wanker. "Take my hand"
is a rancid red herring, so dont' EVEN talk to me about that.
Having said that, I offer the following comments on the supposed
transcript: Why didn't anybody at least *try* to signal Mission Control?
Why not more profanity? Why is it so dreadfully cliched and melodramatic?
("Not now, not like this"? Furrfu!) Why does somebody complain of being
hot? Yeah, it's real hot 6 miles up.
- Steve "furrfu" Patlan
NASA JSC/Electrical Power Systems
I speak only for myself. Like I had to tell you that.
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