Hochstein’s Law

Alan Hockstein was the man most feared by pilot astronauts.  Well, except maybe for George Abbey.  Let me explain why.


The shuttle is the world’s largest glider.   The pilot has one and only one chance to make a landing; there is no “go-around” capability.  Obviously, good piloting techniques are studied exhaustively.  Much analysis and simulation has been completed to maximize the chance for a successful landing.


Alan was the senior landing analyst.  That means he studied more and worked harder than anyone to understand how the shuttle flies – especially in the final approach and landing phase.  One part of Alan’s job was to analyze the telemetry from each shuttle landing and see how that compared to the “ideal” landing.  So in a quiet office environment over a couple of weeks, Alan and his team would look at each telemetry point, every sample (up to 125 per second for some parameters) and compute how each one affected the landing. 


Every shuttle commander dreaded the day of the Entry, Descent, and Landing Debriefing.  Standing in front of a projection screen filled with data curves in the presence of a room full of folks, Alan would ask the commander something like:  “why did you deflect the hand controller here” pointing at a squiggle on the screen.  “That input caused a deviation of 12 feet high above the flight path which correlates to a 273 foot miss distance at the touchdown point.”  The commander would squirm in his seat and say “we had a wind gust” or some such.  Alan would point to another squiggly line on the plot and say “the accelerometer data doesn’t show a wind gust at that point.”  The poor pilot would then have to come up with some other lame excuse: “the visual scene was obscured by some wispy clouds.”  Alan would pull out the meteorological report “the lowest observed clouds were at 25,000 feet”  And so it would go.  Excruciating for the veteran test pilots who pride themselves on their steely nerved stick and rudder reactions.


Why did we go through this ritual?  One reason only: to learn what we could about flying techniques, how they affected the landing, what might work better.  All of this so that the next pilot would have a better idea of how to maximize the chance “for a happy outcome.”


At our Flight Techniques meetings, Alan was a frequent presenter showing what had been learned, advising of the best techniques.  At one period we experienced a number of landings that were shorter than desirable – still on the runway, but consistently closer to the threshold than comfortable.  Alan analyzed hundreds of combinations of factors over a several dozen landings looking for correlations.  Nothing seemed to correlate, except one:  “If you cross the threshold low, you are likely to touch down short.” 


Now that may seem obvious in retrospect.  If a glider comes in low, any pilot would intuitively expect a short touchdown.  But it was only obvious in retrospect.  And any number of other correlations that common sense might have suggested were simply not borne out by the data.  So we called this “Hockstein’s Law”:  If you cross the threshold low, you will touch down short.  The entire community worked very hard with the pilots to improve techniques to be higher at threshold crossing and thereby the incidence of short touchdowns was significantly reduced.  Well, that is the very short summary anyway.


Nowadays, I don’t spend my time studying shuttle landings like I used to.  Recently I’ve been a data gatherer and logistics helper to the Augustine Committee.  That group has been getting a lot of data and, among other things, looking at the cost estimates of various options for space flight.  I’m not well suited to work in that ethereal regime; nuts and bolts are more my specialty.   But it occurs to me that we need an Alan Hockstein to look at project development budgets for clues of how to improve the performance of future work.


Somebody who will look at each data point in depth, spend the time to think about it, calculate the consequences of each movement, and then provide those of us who may have to execute a project in the future with some guidelines that might lead to a better likelihood of a “happy outcome”.


Some of my experience suggests possible correlations between different events and poor program performance.  For example, continuing resolutions on the budget cause disruptions and delay planned activities.  It would seem that there might be a high correlation between lack of a firm budget (e.g., a continuing resolution) and poor program performance.  Then again, Norm Augustine himself kept saying that the secret to successful project management is reserves.  Perhaps the congressional prohibition against budgeting reserves for projects plays a role in poor program performance.  Then there is something called a “rescission.”  I never knew what a rescission was until I got into program management.  A rescission basically prevents a program from spending all the money budgeted for it.  I’m no analyst but it may be that rescissions play a role in poor program performance.


All of those things are just guesses on my part.  I’m no analyst.  But it seems like a good study if we want to have successful projects in the future. 


Now, where is Alan when we need him?

Tripping the Boundary Layer – Part 1

As I start this series, it occurs to me that “tripping the boundary layer” could be an article on social change – maybe I’ll do that. 

But for today it is an engineering subject.  So buckle your seatbelt and hold your hat, we are off on an adventure in rocket science!

Aviation has been driven by the desire to fly higher and faster.  Great strides have been made, especially up to the middle 1960’s.  But for the last few decades aircraft have been at a plateau in terms of speed and altitude.  With the exception of rocket powered X planes, the boundary of high performance jets has been just faster than Mach 3 and up to about 100,000 ft.  Even though there is the perennial dream of hypersonic transports carrying passengers across the globe in a fraction of today’s aircraft, we don’t seem to be advancing on that dream.

Part of the problem is we don’t understand how to avoid tripping the boundary layer.  There is precious little data at hypersonic speeds, and computer simulations are no good without data and the formulae derived from data to predict these things:  garbage in; garbage out.

So, to start this discussion off, let us define the terms.  (What the dickens are we talking about?)!   What’s a boundary layer and what does it mean to trip one?

In aviation, the boundary layer is a thin film of air closest to the wing, body, or engine of an aircraft.  At the molecular level, the air immediately adjacent to the airplane is dragged along with the plane.  Infinitesimally farther away, the air is being carried along at some fraction of the speed of the airplane, and at a longer way away from the airplane, the air is not moving at all, or at least not being dragged by the airplane.  That distant air is called the “free stream” and the close by air – which is affected by the passage of the aircraft – is called the boundary layer.  Typically aerospace engineers consider the boundary layer to be that close in part of the air that is being dragged along by the passing of the aircraft at a speed of 5% or more of the airplane.  These boundary layers are thin, inches or fractions of an inch.  They are important because the boundary layer causes most of the drag and most of the heating when an airplane is in flight.

Boundary layers, like all fluid flows, is either laminar or turbulent.  Laminar flow is smooth, turbulent flow is, . . . well,  . . . turbulent.  You can see a good youtube video of this here: 


And there is a really good wikipedia article on turbulence here:  http://en.wikipedia.org/wiki/Turbulence

So why is all of this important?  Exactly at this time there is a large effort by many companies and government agencies to develop hypersonic aircraft.  NASA has even sponsored a couple of test flights.  The problem, as it is for all types of aircraft flight, is drag and heating.  When the boundary layer over the wings or in the engine is laminar, there is low drag and low heating; and when the boundary layer is turbulent, drag and heating increase dramatically.  All boundary layers can be “tripped” or transition from laminar to turbulent flow.

In some of these experimental aircraft the engines [called SCRAM jets for Supersonic Combustion Ram jet engines] have only operated for a fraction of a second or a very few seconds.  Why?  Because the designers do not know how to cool them; they don’t understand when or whether the boundary layer inside the engine is turbulent or laminar. 

In some of these experimental aircraft, the engine begins to melt as soon as it is turned on; hence the extremely short operating times.

This is no good for a hypersonic passenger aircraft which might carry a hundred people from New York to Tokyo in a couple of hours. 

Why do we not understand this phenomenon?  Because it cannot be recreated in a wind tunnel or other experimental apparatus.  The wind tunnels that have long enough flow durations to study this phenomenon run only up to about Mach 6.  These hypersonic engines need to perform at Mach 8 or 10 or 12.  There are “wind tunnels” that operate at high Mach numbers but only for fractions of a second; not long enough to understand the way in which a boundary layer works.

No aircraft fly that fast, missiles can achieve it briefly, but there is one platform that spends a serious amount of time flying through the atmosphere at speeds above Mach 6: 

Its the space shuttle. 

Tomorrow I’ll talk about an experiment that will be on the next shuttle flight. An experiment which will study tripping the boundary layer.

With this knowledge, the designers just might be able to make a major advancement toward hypersonic passenger aircraft.

To hold your attention until my next post, here is a true story:

Around 1900 a young graduate student in physics was trying to do research on a problem that could earn him a doctorate degree.  He started out studying the transition from laminar to turbulent flow in fluids.  After months of work and study, he concluded that this problem was too hard.  He would concentrate on an easier subject:  atomic physics.  His name was Niels Bohr and he won the Nobel prize for physics in 1922 for his work in quantum mechanics.  And he was right; turbulence is harder.  And we don’t understand it yet.