Tag Archives: weather

Monday – Another Flight Director Story

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NASA has the best weather forecasters in the world.  Both the Spaceflight Meterology Group – who produce landing weather forecasts for all the shuttle landing sites all around the world – and the USAF 45th Weather Squadron – who produce the launch weather forecasts for all the launch operations at the Kennedy Space Center and Cape Canaveral Air Force Station — are the best.  Not only do they make forecasts every day and every hour of the year, but they actually check to see if the forecast was accurate, and keep score.  And their forecasts are really micro-forecasts — exactly what will happen at the launch pad or at the runway; not a general area forecast, which is much tougher. 

America has lost a number of space launch vehicles over the years due to poor weather decision — Atlas/Centaur-76 is the case study example — and the weather community is striving to make sure that we never lose another. 

I’ve talked about the shuttle landing before, and you know it is the toughest decision that a Flight Director ever makes.  The shuttle gets only one shot and it has to be right.  The FD goal is to provide the shuttle commander acceptable weather — not perfect or the shuttle would never land.  The decision has to be made an hour and a half before landing.  There have been some long waits between the deorbit burn and landing. 

Early on, the shuttle was supposed to have jet engines so, among other reasons, it could fly multiple approaches or divert to different runways.  However, the weight of the orbiter in the design phase kept growing, from 150,000 pounds empty to 200,000 pounds and more.  And the ops guys kept asking (as they always do) “what if the engines don’t start during entry, don’t we have to protect against that situation?”  So fairly early on, the jet engines, the fuel tanks, and all that stuff got deleted from the design.  So the orbiter is the world’s heaviest and only hypersonic glider.  One shot at landing is all the commander gets. 

The auto landing capability that was built into the shuttle is not perfect.  It could work, if necessary, but engineering analysis shows that there are more times than we would like where the auto landing system would fail.  On a commercial jetliner, this is accommodated by an auto go-around feature.  But then the shuttle . . . well, see the paragraph above.

So the commander flies the vehicle on final through touchdown and rollout.  Visual cues are very important even though there are electronic navigation aids, a head-up display, and all sorts of redundancy.  But being able to see the runway and the PAPI lights are mandatory.  With the steep approach required by the brick like flying qualities of the shuttle, a minimum altitude of 8,000 feet to see the PAPI lights is required (for end of mission in the daylight — different rules apply to other situations).  (OK, PAPI stands for Precision Approach Pilot Indicator — four lights which show white at a certain angle and red if you are below that angle).

Mother Nature, of course, makes this difficult.  There are these things called “clouds”.  One meterologist called them “hydro-meteors”.  Occasionally these “clouds” have been known to obscure the navigation aids.  Since there are almost always some clouds in the sky below 8,000 feet, long studies were undertaken to determine how many clouds below the limit constitute a violation.  See what I mean by giving the commander acceptable – not perfect – weather?  After many approaches in the training aircraft, long arguments, a decision was finally reached:  if the clouds obscured 50% or more of the area, that was no-go, if it was less than 50% then it is go because the commander could see the “runway environment”.

STS-53 was a “classified” mission but what happened at landing is not secret.  In fact several things happened on that landing so it could be grist for more than one story, but today I’m going to talk about the weather.  We wanted to land at KSC – saves the ferry flight with its expense and risk and it saves a week or more of time.  So we started looking at KSC. 

On the particular day in question there was a front coming through Florida – not unusual.  Associated with this were low overcast (100%) clouds – at 3,000 feet the bases of the clouds would clearly violate the landing rules, if they got to the Shuttle Landing Facility.  On the satellite photos it looked like there was a ruler running diagonally across the peninsula:  north and west were socked in and south and east were crystal clear.  Weather at the SLF all during the deorbit preparation was perfect: no wind, crystal clear skies, beautiful.  Just that threat.  The inexorably approaching line of low overcast clouds that would block the commander’s view of the field and the all important PAPIs until the last minute — actually last 20 or so seconds — before landing.  Much too late.  Very scientifically our weather forecasters plotted the approach and predicted that the clouds would cover the SLF about a half hour before orbital mechanics would allow the shuttle to arrive there.

So we turned our attention towards Edwards AFB in the high desert of California.  Perfect weather there.  Virtually no clouds, winds were reasonably low, everything was good to go.  Except one cloud.  There was one cloud hovering over the PAPI lights on the approach and its base was 3,000 feet.  The astronaut pilot flying reconnaissance in the Shuttle Training Aircraft reported that all was go with the exception of that one cloud. 

Well, it met our criteria; clearly less than 50% of the area was obscured; in fact probably 90%+ of the area was crystal clear.  And in an hour and a half, there is no way that cloud would remain there.  I gave a GO for deorbit.  The astronaut in the STA had a conniption:  in his opinion it was clearly no go.  We had a short discussion (see above).  I reiterated my GO to the crew.  The deorbit burn happened on time for the EDW landing. 

Want to guess what happened in the next hour? 

In defiance of all the known laws of nature, the front threatening Florida stalled out well northeast of KSC.  The KSC center director and his technical staff were later shown on the runway at what would have been the landing time pointing up at clear blue sky.  It would have been a perfect day to land in Florida.

In defiance of all the known laws of nature, the one cloud in the sky at Edwards air force base not only did not move but stayed exactly where it was and grew a bit.  Still technically within limits, it totally obscured the line of sight for the PAPIs and the runway threshold.

As the Commander later debriefed “we never saw the PAPIs or the runway until we broke out at 3,000 feet.  Somebody has some ‘splaining to do”

That would be me.

The landing was a good one; all the electronic navigation aids backed up the visual ones and the commander touched down on speed and on distance.  Hey, any landing you can walk away from is a good one.  If you can fly the machine again, as the saying goes, it was a great landing. 

I stand by my initial statement, we have the best weather forecasters in the world.  But you can’t fool Mother Nature.  No wonder Flight Directors get gray or bald . . .

 

Monday – Time for another true story

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The Space Shuttle is the only manned winged vehicle to fly hypersonically.  I have seen it fly almost directly overhead at Mach 15 – extraordinarily impressive.  No other flying machine comes close.  But the price to fly at hypersonic speeds is the subsonic L/D, near landing approaches the aerodynamics of the common brick (L/D is the ratio of lift to drag, one of the most important properties of any flying machine).  Which is to say, the shuttle comes down fast; its glideslope on final approach is 7 times steeper than a commercial airliner.  Think dive bomber.  And the pilot only gets one chance to get it right.  No go-around capability. 

 

The number one job of the Entry Flight Director is to make sure the Commander has the very best situation for landing.  No, that’s not right; if we were to wait for the very best situation the flights would have to be a lot longer than they are.  No, the number one job of the Entry Flight Director is to make sure the Commander has an adequate situation for final approach and landing.  Defining the dividing line between an unacceptable situation and marginally adequate situation takes about 25 pages of Flight Rules.  The worst part about this whole thing is it revolves around the weather.  Or more precisely – and even worse – the weather forecast. 

 

The law of conservation of energy means that all the energy that went into getting the shuttle from the launch pad to earth orbit must be removed to get the shuttle from earth orbit to wheelstop on the runway.  99.8 % of the energy is taken out by air friction – which is why it gets really hot during re-entry.  Any glider pilot will tell you that making a good landing is all about energy management.  Energy management on the shuttle is particularly complex.  Stay with me now.  This is rocket science.

 

After deorbit and throughout entry you always want to keep just a tad more energy than you need to get to the runway, but not too much or you will overshoot the runway and plop down in the middle of nowhere without a runway in sight – considered to be poor form.

 

The band between too much energy and not enough energy gets tighter and tighter as the shuttle approaches the runway.  Like baby bear, you want don’t want to be too hot or too cold, but just right.  The tail of the shuttle acts as a conventional rudder but also splits down the middle to make what is called a “speed brake” or in pilot speak: the “boards.”  As the speed brake opens up, the shuttle will slow down faster and faster.  The minimum (closed) is 15% — don’t ask why, it’s a long story.  At settings of the “boards” about 60%, the handling qualities for the pilot degrade.

 

Periodically before the predicted landing time, balloons are launched near the runway and tracked up to 50,000 feet altitude or so.  This gives a profile of wind speed and direction.  This information is fed into a computer program which assumes the shuttle is flown perfectly and it computes all kinds of interesting results.  Among the most important results are predicted speedbrake setting on final approach and the predicted touchdown location on the runway.  The touchdown target is 2,500 feet past the painted threshold stripe on the runway.

 

The shuttle touches more than twice as fast as a commercial airliner, 205 knots for heavy weight returns and 195 knots for lighter weight returns when the payload bay is empty.  Only slightly faster and the tires will come apart.  Not very much slower and the tail will drag before the wheels touch down.   

 

So there I was, the rookie Entry Flight Director, in charge on my very first shuttle entry.  Watching the weather, sweating, over-caffinated, about to be sick to my stomach, but radiating calm confidence to my team.  In other words, it was just like every other time I was in charge at the Flight Director console.

 

I had studied all the previous shuttle entries, knew all the procedures by heart, had all the rules memorized, and had watched several previous landings sitting right beside an experienced Entry Flight Director.  The secret is, of course, that no two landings are alike, and I was about to learn that the hard way.

 

It was a windy day at Edwards AFB.  All real test pilots like to fly early in the morning before the winds build up.  Shuttle landing times are determined by orbital mechanics and this was going to be an afternoon landing.  Stiff winds started blowing hours before landing and the forecast for surface wind speed was just below the limit.  No other runway would do, only the big concrete runway that runs in a southwesterly direction, straight into the prevailing winds.

 

The onboard computed guidance which the Commander would be following would try to get the shuttle to a landing 2,500 feet down the runway at 195 knots; the speedbrake would be automatically adjusted accordingly.  With the winds that were measured aloft, the computer model predicted that even with the speedbrake closed (15% — not dissipating any excess energy) would come up short on the minimum distance past the threshold allowed by the rules, 1000 ft.  Now, at the very end game, a pilot can trade airspeed for lift and thus distance down the runway.  The rule of thumb is 10 knots of airspeed difference results in around 1000 ft of distance along the runway.  If you land 10 knots too fast, touchdown will be short by 1/5 mile.  If the pilot stretches the landing by holding off until the airspeed reads 10 knots lower, the tires will hit the pavement about 1/5 mile farther down.

 

The crew is ready to return, all the other parameters are GO, just that pesky wind and short touchdown prediction.  I checked with the weather man; conditions were predicted to be worse later in the day and unacceptable tomorrow.  

 

Time to go to the coffee pot and think this over.  It is a myth to believe that all the important decisions are made at the Flight Director console or maybe in some conference room down the hall.  All the really important decisions in Mission Control are made by the coffee pot in the hall right outside.  Lots of ex-Flight Directors and other management types seem to show up when the Flight Director on duty stops by for a cup.  Lots of good advice can be had there.  On this particular day, the place was deserted.  Nobody to help.  Well, back to the console, the clock is ticking.

 

The Flight Dynamics Officer points out that there is an exception in the rules; for a lightweight orbiter, landing at 10 knots slower – at 185 rather than 195 knots – with a predicted touchdown at 1000 or more feet past the threshold is considered adequate.  By making that adjustment in the computer model, with the balloon measured winds, the prediction is touchdown at 1100 feet with 185 knots and CLOSED SPEEDBRAKE.  No energy reserves other than that 1100 feet back to the start of the runway.  Any EXPERIENCED Entry Flight Director would have seen warning flags all over this!  

 

But, I was a rookie.  All the flight rule criteria is met – GO FOR DEORBIT!

 

During the hour between the deorbit burn (no turning back now) and landing, the winds got stronger.  They got stronger on the surface and the balloons showed the winds increasing aloft.  There was nothing we could do about it but tell the Cmmander.  He did not sound happy when he acknowledged the call.  The Capcom stopped making eye contact with me.  

 

Mission control can do a lot of things, but it can’t make the wind blow less.  

 

The landing looked great on TV.  You can’t tell on the video where the touchdown spot was.  A good landing and the crew was healthy.  I was feeling good about life.  The tag line on all the news reports was that the shuttle landed safely.  As far as the public knew it was all routine and there had been no danger.  My stomach stopped doing flip flops.

 

A couple of hours later, I got THE PHONE CALL.  The commander was NOT HAPPY.  As I’ve told you before, it is never a good thing to have a commander who is NOT HAPPY.  He thought he wasn’t going to make it to the runway.  He really had to stretch to make it over the threshold.  Just what did I think I had been doing to put him in that situation?  I had a low moment.

 

The next day the official numbers came in.  The rubber marks where the main gear tires kissed the runway were 1176 feet past the threshold.  Yeah!  That is OK!  But wait, further down the form, the speed of touchdown was 176 knots.  Oh no!  Not 195 knots which is the standard target or even the 185 knots special exception, the shuttle touched down at 176 knots!  The Commander really did have to stretch it out.  The computer models did the math: if touchdown had been at the target speed of 195 knots, the wheels would have hit the ground 130 feet BEFORE THE RUNWAY THRESHOLD.  Not good.

 

There are a lot of things in space flight that can kill you.  Having a rookie Flight Director is one of them.  If you ever get assigned to a space flight, check to see who is sitting in the big chair in mission control before you agree to go.  If it’s their first flight, or maybe even their third, you may want to ask for a ticket on another flight.

 

Oh, and that wasn’t the worst thing that I ever did to a commander on landing.  But that’s another story for another day.

Starting a conversation – June 17,2008

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Starting a conversation is hard.  Geting off on the wrong foot can happen so quickly and then any chance of a meaningful encounter is lost.   I am really interested in starting a conversation.  Not a conversation on any subject, but a conversation about space exploration and why that effort is meaningful.  Perhaps you agree, perhaps you disagree, but you certainly know some things I don’t.  And I am really interested in finding out what I don’t know (which is a lot!)

So I’ll try to hammer out some thoughts every day or so –  maybe with pictures if I figure that out –  and you can write me back and tell me how its going..

So for the first post, I’ll start with the reason I came to work late — my grandkids (oh, no, you didn’t want to hear that kind of story did you?!)

Not that my two grandchildren are the smartest and brightest kids ever born (they are),  but what having grandchildren – or children – can mean.  Children make me thoughtful.  (After they are in bed, anyway).  Little children make me think about the future.  What will the world be like when these wee ones inherit what we have done? Will the world be a better or a worse place than it is today?  And the hardest question of all: what can I do to make it better?

 I hold a stubborn and passionate belief that space exploration is important for that future.  Just one example for today: understanding our own planet.  Without satellites to monitor the weather, climate, pollution, crop growth patterns, and many other things we would be blind and deaf to what is happening.  The earth monitoring satellites built by many countries and many US agencies are giving us vital information about our world every day. And that is information that cannot be gained anywhere else but from earth orbit.

Beyond all that, studying other planets have helped understand our own.  In particular, how our complex atmosphere works.  Our weather and climate are influenced by the interaction of air and water, sometimes too complex to understand directly.  Venus and Mars don’t have oceans but they have weather. Studying those planets helps us understand what happens without oceans.   Jupiter and Saturn have planet circling oceans (not water, certainly!) underneath their atmosphere.  By studying them we understand better how gas and liquid interact to affect climate and weather.  The payoff is better weather forecasts, better understanding of climate change, and more impetus to change our own future.  Don’t stop there.  The Sun drives all our weather and climate, everything else is just a tweak around the edges by comparison.  The Ulysses probe is going silent after 17 years of service.  Ulysses, launched on the shuttle Discovery, studied the sun.  Ulysses and the other solar satellites are helping us to understand how changes in our unstable star occur.  Better understanding of the sun is mandatory to understand our future.

All of that to make sure that our children and grandchildren have a better future.  Or, at least better than what would occur if we didn’t know what was going on and therefore couldn’t do anything about it.

So, a first example and discussion topic.  I’ll have more in days to come. 

Let me know what you think!

Wayne