(01 Feb 03)
Columbia on Mission STS107 sustained a debris strike on its left wing at some point short of max Q (max dynamic pressure) on launch (it looked to be around 35,000 to 45,000ft up and the event was caught clearly on the launch pad cine). At that point the Shuttle would have still been accelerating at a rapid rate.
The piece of debris was either assumed (or known) to be ice-hardened external tank foam-covering and is visually identifiable on the imagery (so it must have been quite large – and it would have been dropping through 50ft with the additional impact of the Shuttle’s launch acceleration). I’m not so sure that, even without being ice-hardened, that it would be all that light-weight or whether it has a more substantial substrate and adhesive base. The material is (I think) supposed to be an insulation therefore it would be fairly dense and quite solid once ice-coated. So it was probably capable of giving the area of the Shuttle that it hit a quite decent clout (left under-wing leading edge about halfway out it appeared). I cannot recall any earlier instances of such a large piece coming adrift, although early shuttle launches routinely lost tiles (but apparently only during re-entry). Unlike the external solid rocket boosters, the external tank is not recovered and refurbished by the United States Alliance (a conglomerate of contractors Boeing and Lockheed-Martin). On CNN it was being said that the Ground Control Mission Specialists had analyzed the available imagery and decided that the debris hit was not going to be significant. They may have misjudged that....or maybe that was just what was being released to the Press.
The Shuttle starts re-entry at an altitude of about 400,000ft and as the atmosphere thickens, maximum dynamic pressure occurs somewhere around 200,000ft at Mach 18– eighteen times the speed of sound or about 12,500mph. The breakup supposedly having occurred at 207,000ft, I would guess that it had to be related to that max q PEAK and one (or both) of two things. i.e. a weakening of the structure caused by that debris hit or a difficulty with roll-attitude control that quickly led to a loss of precise attitude control (which, at that high dynamic pressure, would mean an automatic catastrophic breakup).
As the atmosphere thickens during re-entry, there’s a
later transition from attitude control thrusters (Reaction Control
System or RCS) to conventional
aerodynamic control using elevons (see further on) It may even be that both control systems
are available together over some transitional period. But at the
stage that Columbia broke up, the computer was in control.
But at the stage that Columbia broke up, the computer was in control.
Because the angle of attack (of about 30+degrees)
for re-entry is so critical, the flight
control computers and autopilot are needed to make the very sensitive
fine adjustments so that the shuttle remains pointed precisely in the
direction that it’s going and critically, so
that the thermal tiles on the undersurfaces take all the frictional
heat-blast. I would guess that this particular thermal emergency is
not practiced in the simulator - as there would be no practical solution
available. More importantly, do they practice for
computer and system failure scenarios only
I would guess that this particular thermal emergency is not practiced in the simulator - as there would be no practical solution available. More importantly, do they practice for computer and system failure scenarios only? There’s something unhealthy about conceding the inevitability of casualties in a very mature program. It indicates a perfidy of the spirit of pioneering (or just a downright mean-spiritedness that has no place in a modern culture this long after the expediencies of the Cold War).
Obviously a significant hit that dented (or pierced) the left wing could affect either/both Lift or supersonic Drag and could create an aerodynamic asymmetry. That situation might have given rise to the impossibility of maintaining their critical re-entry pitch attitude.
If the scenario is correct, perhaps:
should always decree a space-walk to examine, photograph and transmit
any damaged section's imagery back to Mission Control for more precise
evaluation. The expectation that things would always just "be OKAY"
(post-launch) for re-entry seems to point to a significant gap in Mission
Safety risk-management. And based on 100 odd flights only, dismissing
the debris "hit" on launch as being inconsequential would
also seem to be a little "fingers-crossed" cavalier.
A 1997 Memo by
a NASA Engineer forecast the very problem that Columbia had.
A 1997 Memo by a NASA Engineer forecast the very problem that Columbia had.
2. In future, astronauts may be able to make boron-fiber putty-patch repairs during a space-walk (with an incorporated ablative outer skin - in order to one-time restore structural and aerodynamic integrity for re-entry).
a space-walk (EVA) walk-around should be a planned part of each mission
before re-entry (to check for any launch damage necessitating ad hoc
interim repair). Alternatively a remote control camera might be suspended
from the Shuttle’s robot arm (when carried) for an external inspection.
Perhaps a"flying" camera. It need be no more than an attitude control system and fuel tank with a radio and camera attached. It could also take great publicity photos.
4. Perhaps the Orbiter's heat-shield tile layer should be covered with an outer sacrificial spray-on cladding which may well burn away on re-entry BUT, prior to that, protect the tiles from any earlier debris hits.
No criticism intended here at all of the pilots.
The pilots were probably suddenly
confronted with a situation/failure scenario that was not system-failure
initiated (but, like all onboard fires, became system-related). It had probably NOT been included in their simulator syllabus (but
perhaps should have been). In fact it was Pilot McCool’s first Shuttle
Flight (and the Commander’s second). They were obviously trying for
the best outcome. But in such a highly sensitive flight-control regime,
they had no option but to let the computer work it out. However it is probable that the
process of heat degradation of the left wing and its systems was ongoing
and that, per any uncontrolled "thermal event" (i.e. fire) the Orbiter's structural integrity losses would
have rapidly caused
a loss of flight-control.
However it is probable that the process of heat degradation of the left wing and its systems was ongoing and that, per any uncontrolled "thermal event" (i.e. fire) the Orbiter's structural integrity losses would have rapidly caused a loss of flight-control.
Once the Orbiter had suffered significant uncommanded
roll and the autopilot had reached its control authority limit, it would have rolled but would
have also deviated significantly from its critical
angle of attack. At the high dynamic airspeeds, structural failure stemming
from loss of attitude control or further loss of structural integrity
would very rapidly lead to a catastrophic breakup in a matter of milliseconds. That
is evident from the
imagery of the contrail taken over Central to The many ground-shot videos
will obviously assist greatly.
The many ground-shot videos will obviously assist
These points may prove to be amongst the lessons
learnt - but they may also point to insufficient imagination being applied
in NASA's Risk Management assessment scenarios. Heat-rise and attitude
control loss is the main enemy. They must think in terms of contingency
plans that will insulate crews against known risks and enable recovery
from feasible external damage scenarios – and not just system failures.
Simulator training beyond system loss scenarios is obviously called
Simulator training beyond system loss scenarios is obviously called for.
|A timeline of the final
minutes of the shuttle flights and the hours following it (all times EST):
Additional data in italics from NASA press conference.
8:53 a.m. (Over California)
08:54 (Eastern California & Western Nevada)
08:58 (New Mexico)
08:59 (West Texas)
08:59 (East Texas)
NASA have interviewed the astronomer in Owen's Valley
(California) who reported debris coming from the shuttle. They have
his statement and believe it is an important contribution.