| From: [email protected]
Newsgroups: sci.space
Subject: The Return to KSC Decision
Date: 5 Jul 91 20:05:31 GMT
Organization: NASA Johnson Space Flight Center
If reference to the "Shuttling to the Cape" discussions, I'll try to shed some
light on why NASA has not been routinely utilizing KSC for Shuttle landings the
last few years, and why it has recently decided to do so. All this talk on the
net about thunderstorms has gotton too far off the mark.
Edwards is a large dry lake bed with many runways in many directions
painted on it (or, in a few cases, "concrete" poured to make some hard-surface
runways). It is very forgiving of short landings, blown tires, etc. should
they occur. The Shuttle Landing Facility (SLF) at KSC, however, consists of
one concrete runway surrounded by Florida swampland populated by alligators
that love to eat astronauts. As you can imagine, the SLF is not very forgiving
of imperfection. Since 1) we live in an imperfect world, and 2) NASA does not
like to have any of its employees eaten by alligators, it was decided years ago
that we would not *plan* to land nominal end of missions (EOM) at KSC until the
following four concerns were adequately satisfied:
o Dynamic weather conditions
o Limited lateral runway environment
o Rough runway surface
o Unknowns/unexpected events
In the ensuing years, each of these concerns has been worked upon. Let me try
to briefly touch on each one.
* Dynamic weather conditions*
The discussions on flying through rain (much less thunderstorms) are
correct--the Orbiter just can't do it. The tiles are the consistency of hard
styrofoam with a thin hard coating on top. They are waterproofed, but this is
to protect them while sitting on the pad in the rain. Nothing could protect
them against the sandblasting of over 200 knot rain. Thus NASA has a flight
rule which says we can't land when rain is within 30 miles of a landing site.
Another flight rule says the ceiling must be above 8000 feet and the
visibililty above 5 miles. These are primarily designed so that the crew will
be able to see the runway when they line up on final approach. (Yes, autoland
works fine, thank you. But if you were a first-class Mach 25 airplane pilot,
wouldn't you rather land the bird yourself? Autoland has landed successfully
thousands of times...in sims.) There are also flight rules covering crosswinds
on landing runways that I'll get into later.
So with all these weather flights rules that must be obeyed, and with the
dynamic, fast-changing weather conditions in Florida, just how well can the
future weather conditions be forecast? In June, 1988, NASA initiated a
forecast demonstration effort at KSC, Edwards, and Northrup (the third
continental US shuttle landing site). Practice forecasts were made 24 hours and
90 minutes (one orbit) in advance. For KSC, 714 90-minute forecasts were made
through October, 1990. Of these, 471 (66.0%) were predicted to be go for
landing. 90 minutes later the go forecast proved to be wrong 17 times
(for ceiling, visibility, precip, and/or crosswind violations). This 96.4%
90-minute prediction accuracy for KSC provided sufficient confidence to make the
go-for-KSC decision. For Edwards, out of 546 predictions, 500 (91.6%) were go
and the go was 99.4% accurate. For Northrup, 546 predictions were made, 461
(84.4%) were go and go was 97.6% accurate.
*Limited lateral runway environment*
This boils down to reliable braking, directional control, and tire wear.
Without getting into too much detail, the Orbiters used to have brakes made of
beryllium. These were good for 42-55 million ft-lbs of energy. A new carbon
brake has now been developed and flight tested, however. These carbon brakes
can provide 82 million ft-lbs of energy. The last shuttle flight, STS-40, was
the last flight with the beryllium brakes. From now on, all will be carbon. So
now NASA has additional confidence that the brakes won't break when landing at
KSC.
Direction control of the Orbiter during rollout consists primarily of nose
wheel steering and secondarily of differential braking of the left and right
main gear. These were improved upon in recent years by adding additional
electronic redundancy to the black boxes that control these functions. Flight
testing has also shown that differential braking is an adequate control method
for the Orbiter even if nose wheel steering totally fails.
Tire wear is important because blowing a tire during rollout on any runway
could make for a bad day. Remaining on the runway is doable, but it could be
tough. Tire wear caused by side force was taken into account in the near term
by reducing the amount of crosswind allowed at landing. With a vehicle weight
at landing <= 205,000 lbs (of which we *usually* are except when landing with
a satellite, LDEF or Spacelab on board), the allowable crosswind component was
reduced from 15 to 12 knots. In the long term, a new and improved tire has
been developed (and should soon begin testing) that will greatly improve tire
wear. Whenever it is certified, perhaps the crosswind limit can go back up.
*Rough runway surface*
This is one other aspect of tire wear. It figures that a lot of wear occurs
at touchdown during the spinup phase of the tire. The SLF runway used to
grooved transversely (like the yardlines on a football field) its entire 15,000
ft. length. This really helps the runway rapidly drain off water from those
sudden showers at KSC, but it also turned out to have a severe wearing effect
on the tires during spinup. To provide some help in this aspect, the first
3500 ft. of both ends of the runway, the touchdown area, were regrooved in the
longitudinal direction. This improved the spinup concern. More votes for KSC.
*Unknowns/unexpected events*
This is a catagory that mainly says "Put in as many improvements as possible so
that the unexpected can be handled." (My words.) This has been addressed by all
the improvements mentioned above except for one more addition to the Space
Shuttle fleet that is coming--drag chute.
As stated before, a big concern with landing at KSC is having a tire blowout at
landing or during rollout. Maintaining lateral control and staying on the
runway with a tire blown could be very difficult. Analysis has shown that a
drag chute attached to the aft of the Orbiter during rollout greatly improves
lateral stability, so NASA is adding drag chutes to the fleet. OV-105 was
built with it and will be the first Orbiter to use it on its first flight,
STS-49. OV-102 will get it during the coming months and will use it on its
next flight, STS-50. Likewise, OV-103 and OV-104 will eventually be modified.
As a result of all these improvements, NASA decided that KSC landing operations
would have sufficient margins of safety to resume landing there. I hope this
post has improved your understanding of that decision.
--
Stokes McMillan - Flight Evaluation Office (VF3) NASA JSC
Houston, TX 77058
[email protected]
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