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.