Newsgroups: sci.space.policy
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Horizontal glide landing fans, take this simple test:
Date: Sun, 7 Jan 1996 02:51:05 GMT

In article <4cknef$lqk@starman.convex.com> schumach@convex.com (Richard A. Schumacher) writes:
>How will your opinion change, if at all, after a Shuttle first
>breaks its gear upon landing and slides to a stop? ...

This isn't very hard to arrange either:  just blow a main-gear tire early
in landing.  That's all it takes.  The worst-case main-gear loading is at
nosewheel touchdown, because the nose gear is short and the wings push
*down* once the nosewheels are on the ground.  (This is why the pilots
hold the nose up as long as possible, to minimize airspeed at nosewheel
touchdown.)  If one main-gear tire goes before nosewwheel touchdown, it's
very likely that the other tire on that side will go then -- the loads
exceed the capacity of one tire.  There have been studies of adding either
a skid or a roll-on-rim capability to the main gear, to avert disaster in
such cases, but last I heard, nothing has actually been *done*. 

Start with an unexpectedly high crosswind (which is hard on the tires),
throw in a pilot who's not at his best -- perhaps spacesick, perhaps
having trouble coping with 1G after a long flight -- and land on one of
the hard-surface runways, and it's not at all inconceivable to blow a tire
at or shortly after main-gear touchdown.  Scrape, skid, crunch. 
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu

Newsgroups: sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: shuttle landing burst tyre - which mission?
Date: Tue, 14 Oct 1997 14:30:19 GMT

In article <344361B1.6D5A@students.stir.ac.uk>,
Donal Mountain  <dm065@students.stir.ac.uk> wrote:
>I remember hearing a news report on the radio a few years ago about how
>one of the space shuttles tyres burst during landing
>can anyone tell me which sts mission this was?

That was flight 51D, which burst a main-gear tire just before stopping,
due to a brake failure.  (Two later flights also had serious tire damage,
but not to the point of actual failure.)

Incidentally, the tires are Criticality 1 items -- failure can cause loss
of orbiter and there is no redundancy.  The peak load on each main gear --
at nosegear touchdown -- is too high for a single tire to carry it, so if
one of the tires fails at or before that time, the other will probably
blow too.  The crew might well survive but the orbiter would be wrecked.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Door falling off...(calm down!)
Date: Sat, 31 Oct 1998 19:08:28 GMT

In article <71figp$cac@starman.rsn.hp.com>,
Richard A. Schumacher <schumach@convex.com> wrote:
>CNN said last night that the drag chute will not be used,
>presumably because the missing door has exposed the chute
>to heat damage during ascent. This, uh, opens the door to
>the possibility of breaking the nose gear after touchdown...

Not really.  Remember that the first 40-odd shuttle landings were made
without the drag chute.  The drag chute isn't necessary, it just increases
the safety margins, especially if something else fails.

The thing to worry about in a bad landing, by the way, is not the nose
gear but the main gear.  The main point of the drag chute is to postpone
nose-gear touchdown, yes, but that's not because of the nose gear -- it's
because peak load on the main gear is at nose-gear touchdown, and the
slower the orbiter is moving at that time, the lower the peak load is.
(Because the nose gear is relatively short, when it's on the ground, the
orbiter is nose-down and the wings are pushing *down* on the main gear.
Because aerodynamic forces are proportional to the square of velocity,
even a small reduction in velocity reduces the forces considerably.)
--
Mass-market software technology has |  Henry Spencer   henry@spsystems.net
been deteriorating, not improving.  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Door falling off...(calm down!)
Date: Mon, 2 Nov 1998 15:06:00 GMT

In article <y4lnlufpfj.fsf@mailhost.neuroinformatik.ruhr-uni-bochum.de>,
Jan Vorbrueggen  <jan@mailhost.neuroinformatik.ruhr-uni-bochum.de> wrote:
>> (Because the nose gear is relatively short [...]
>
>So why is it that short? Not enough space beneath the crew compartment?

These things are complex compromises and I don't think I've ever seen a
detailed explanation of why a short nose gear was chosen.  I'd guess it
was some combination of limited space and limited mass (landing gear is
quite heavy).
--
Mass-market software technology has |  Henry Spencer   henry@spsystems.net
been deteriorating, not improving.  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Retirement
Date: Fri, 16 Apr 1999 14:14:28 GMT

In article <ant160821b49M+4%@gnelson.demon.co.uk>,
Graham Nelson  <graham@gnelson.demon.co.uk> wrote:
>> in particular, probably the likeliest single shuttle accident is a bad
>> landing which totals an orbiter (the crew might survive, depending on
>> details).  There have been two or three close calls.
>
>Much of the talk in the early days of the Shuttle programme was
>about the landing phase being the tricky part, but we do seem to
>have a perfect record on Shuttle landings now.  Just how close
>were the "two or three close calls"?

The margins got pretty thin on some of them.  Off the top of my head:

STS-37 landed 600ft short of the runway threshold, due to a bad call on
winds aloft.  (The Edwards runway has a paved underrun area, so this wasn't
a disaster.  I'm told that heads rolled among the weather people.)

There has been one tire burst late in the landing roll.  If that happens
before nosegear touchdown -- the time of maximum load on the main gear --
then the other tire on that side will almost certainly blow, and the
orbiter will probably end up in pieces.  The landing gear is a rather
marginal design, originally meant for a somewhat lighter orbiter.  (The
surface of the KSC runway has been re-done to reduce tire wear.  There
has been talk of adding a roll-on-rim capability to the wheels, but I
haven't heard of it being done.)

On at least one occasion, the weather guys have sent a landing to Edwards
because Cape weather looked a bit unstable and they didn't quite trust it,
although it wasn't clear-cut... and lo and behold, there's been a
thunderstorm over the KSC runway at what would have been landing time.
There is pressure to avoid diverting to Edwards, because of the extra
delays and costs; it would not take much to make some of those borderline
calls go the other way.  (Still a problem; Florida is a lousy place for
a spaceport in many ways, the local weather is very erratic.)

About two years ago, a minor instrument failure in the shuttle simulator
(real failure, not a deliberate part of the simulation, and in hardware
that's the same as that of the orbiters) confused both the crew and the
ground completely.  This was a dress-rehearsal simulation, not an early
training exercise; the crew was flight ready.  Had it happened in flight,
the outcome would have been the same:  unsurvivable crash.  (This scared
people.  Training and procedures have been revised.)

>Of course just because the only serious accident to date involved
>the SRBs, it doesn't follow that the SRBs are necessarily the
>most risky part of the system...

Quite so.  As Feynman pointed out during the Challenger investigation,
just because you've gotten away with it so far doesn't mean it's safe.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Retirement
Date: Sat, 17 Apr 1999 04:26:10 GMT

In article <3717B278.10165CB6@aspentech.com>,
Reed Snellenberger  <reed.snellenberger@aspentech.com> wrote:
>> ...although it wasn't clear-cut... and lo and behold, there's been a
>> thunderstorm over the KSC runway at what would have been landing time.
>
>Wouldn't a thunderstorm be more of a "we'd rather not" situation than a
>guaranteed opportunity to crash (within limits, of course -- nobody
>wants to fly through a tornado...)?  I realize the tiles would take a
>beating, and that it'd be a  "difficult" thing to do...

The problem is not cloud penetration or the wear and tear on the tiles,
but updrafts and downdrafts.  Airliners do not land in thunderstorms, at
least not if their pilots are halfway sane, and they've got much bigger
safety margins than a shuttle orbiter.  There is a particular nasty
phenomenon known as a microburst, not uncommon in thunderstorms, which can
basically make the aircraft fall right out of the sky.  (Airline pilots do
now have a procedure for dealing with a microburst, which starts with
"FULL POWER ON ALL ENGINES!" -- an option not available to the orbiter.)

>...In any event, isn't the escape-pole option still
>available during descent?  IOW, they'd lose the orbiter, but the crew
>could still bail out, if necessary?

Bailout has to start at about 20,000ft if everybody is to be out before
impact.  (That's why the bailout gear includes oxygen bottles.)  The
orbiter comes down *fast*; in fact, it beats the crew to the ground.

>If they realize the landing site's messed up after the de-orbit burn,
>how much lead time would they need to redirect to another site
>(Orlando/Orlando/Tampa come to mind)?

They could divert to Orlando until pretty late, I believe.  The risk is
that they'd keep hoping for clear air on the KSC runway until too late;
this sort of wishful thinking kills pilots every year.

>I seem to remember there's a 500
>mile cross-range capability (thanks, Air Force...).

More than that, but the maneuvering has to start early in reentry.  By the
time the orbiter is down to Mach 3 or so, its options are very limited.

>I'm assuming that
>there are procedures in place for them to declare an emergency and get
>the orbiter down *somewhere* if they need to...

Definitely.  The tricky part, as with any sort of piloting, is having the
sense to chicken out early, when it means risking your career on the
strength of a nagging feeling that those clouds look a little too black.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Retirement (& powered landings)
Date: Fri, 16 Apr 1999 14:30:14 GMT

In article <7f7at4$lfm$1@nnrp1.dejanews.com>,
 <mlindroo@my-dejanews.com> wrote:
>> The most desirable safety enhancement:  Powered flight for landing.
>
>...For the Shuttle, you would need jet
>engines which introduce new problems. E.g., what happens if your landing
>jets *must* ignite for a safe landing and they don't?

Same situation as for other aircraft:  normal landings are powered, but
unpowered landings must be possible *as a high-risk emergency procedure*.

Incidentally, it's not unheard-of for military aircraft to be very nearly
unlandable without engines.  Not that this is *encouraged*, mind you...

>...The design requirement (safe restart after up to ten
>days in space) is not very common either...

There were plenty of uncommon design requirements in the shuttle.

>Finally, the weight penalty will
>be totally unacceptable since the payload would be reduced by one-third or
>so. NASA can't afford this now, with ISS deployment just around the corner.

As already noted, this change isn't practical as a retrofit.  It really
needed to be done at the start, so that the necessary weight would be
figured into the orbiter's dry mass, rather than counting against payload.
(Indeed, the shuttle has suffered badly from having its size and general
design frozen too early, which repeatedly led to undesirable tradeoffs
being made to maintain the promised payload capability in a vehicle that
was really a little too small for it.)
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Retirement (& powered landings)
Date: Sat, 17 Apr 1999 15:30:14 GMT

In article <371921b9.455372501@hurricane.ispnews.com>,
Derek Lyons <elde@hurricane.net> wrote:
>>Incidentally, it's not unheard-of for military aircraft to be very nearly
>>unlandable without engines.  Not that this is *encouraged*, mind you...
>
>Actually it is encouraged...  In fighters at least.  Unstable aircraft
>= highly manuverable aircraft.

Unstable aircraft can still land without engines; for example, the F-16
has a little hydrazine APU to provide hydraulic and electrical power long
enough for an emergency landing.

However, there have been military aircraft which were almost unlandable
without *engine* power, and not because they were unstable.  Most notably,
the F-104 used blown flaps -- flaps whose airflow was guided by sheets of
high-pressure air supplied by the engine -- which meant that an engine
failure also disabled the flaps, on an aircraft that needed them badly,
making landing very tricky indeed (I believe the recommended procedure was
to eject if you couldn't restart the engine).  There have been others
which had marginal landing performance to begin with, making engine-out
landings distinctly hazardous.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy,sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Retirement
Date: Mon, 19 Apr 1999 19:44:59 GMT

In article <371B54C2.C861F5BB@aspentech.com>,
Reed Snellenberger  <reed.snellenberger@aspentech.com> wrote:
>Stupid/naive question for the day... is it even *possible* fire up the
>OMS engines during the descent?

Not useful, alas.  For one thing, they have long high-expansion nozzles
which would experience severe flow separation, maybe even structural
damage, if fired in atmosphere.  More fundamentally, their thrust is
simply too low to be very useful even if they did work.  If dim memory
serves, the idea was investigated once, and abandoned.

>> Bailout has to start at about 20,000ft if everybody is to be out before
>> impact.  (That's why the bailout gear includes oxygen bottles.)  The
>> orbiter comes down *fast*; in fact, it beats the crew to the ground.
>
>? Wouldn't that be the point of using parachutes... :|

Picky, picky, picky...  More precisely, the orbiter would beat the crew
to the ground even if their parachutes didn't open.  (And I think at least
those who leave early do delay chute opening, to get down into breathable
air first.)
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)