We hear speculation about voids, poor adhesion to the tank skin, cryopumping, "popcorning", and poor workmanship by those ole Cajun boys at the tank factory in New Orleans (even though each of them now has his own personal supervisor looking over his shoulder).

I have thought about this problem for almost ten minutes now, and have come up with a more straightforward explanation:

The foam falls off the tank because it is foam.

Foam is inherently weak and crumbly. It is composed of thin walls of plastic surrounding small gas bubbles. The foam on the ET has to be mostly bubbles in order to do its primary task of insulating the tank. This makes it unsuitable as a structural material.

The big foam chunk that narrowly missed smacking into the wing of Discovery popped off a ridge of hand-molded foam called the "PAL Ramp". The function of this structure is to protect the pipes and wires on the tank from a sideways airflow that develops during that awkward "transonic" speed range (Mach 0.8-1.2).

This is a common problem with conventional aircraft, and the solution is usually to add a "fence" or "strake" to block or straighten the unwanted airflow.

Now imagine yourself in a cubicle in Seattle or Toulouse, working on the latest Boeing or Airbus passenger jet. Your CFD program predicts an undesirable spanwise airflow on the wing, or a turbulent wake under the tail. You rough out a design for plastic foam ridges to correct this problem and present the idea to your manager.

What would happen? You would lose your job in an instant and spend the rest of your career designing Mach 0.2 homebuilt stunt planes. Nobody in their right mind would propose using bare uncoated spray-on plastic foam for a key aerodynamic feature of a transonic commercial or military aircraft. No government would certify such a design as safe. The very idea is ludicrous.

But the Shuttle has used hand-molded foam ridges to block and divert transonic airflows on all 114 flights. This is a clear example of how the Shuttle engineers operate in a fantasy world, totally divorced from the normal constraints of physics and finance.

Since NASA is self-regulating, they do not have to convince any independent technical authority like the FAA or the NRC that their vehicle is safe. Since it is self-insuring, no Lloyd's surveyors or Underwriters' Labs testers need pass judgment on their work. In this fantasyland, something as obviously unsound as blocking a 700mph air-blast with a hump of foam can be easily justified.

The use of some real structural material for these ramps has been considered and rejected. A consulting engineer to the CAIB recommended in May 2003 that all the hand-molded foam projections from the tank be removed. The CAIB did not pursue this issue because they were a political body looking for clubs to beat NASA management with, and better clubs soon came to hand. A Shuttle engineer has stated that many thousands of computer simulations of airflow over the ramps were run to justify this decision.

Why was such a huge effort made to avoid a design change that common sense and experience call for? Mike Griffin has stated that it was not due to schedule pressure or cost constraints. He carefully avoided discussing the real reason: weight constraints.

One of the lesser-known defects in the Shuttle system is that the Orbiters as built were much heavier than intended, and have gained even more weight over the years of modifications. By 2000 they were about 17% over their design 75-ton empty weight (21% for the prototype Columbia). This weight gain consumed almost half the system's payload and dangerously increased the landing speed.

The history of the Shuttle has been a constant struggle to overcome this problem with modifications to the Orbiter (stronger tires, carbon brakes, drag chute), SRBs (thinned motor casings, filament-wound casings, cancelled ASRM project), and most of all the ET.

The ET has been the principal target of these weight-reduction programs, since it is dragged almost all the way to orbital velocity. Every pound cut from the tank almost equals a pound added to the payload. Over the last 24 years almost every part of the tank has been omitted, thinned, simplified, or converted to an expensive Al-Li alloy. (The very first stage of this process was the omission of the ~600lbs of white paint that originally covered the orange foam).

So when a Shuttle engineer goes to his manager and proposes to replace the foam ramps with Al-Li or carbon-fiber composites, he faces a hopeless task. The Shuttle's ability to lift loads to the Russia-friendly orbit of the ISS is already so marginal that it is necessary to use the OMS space maneuvering engines for extra thrust during ascent. And most of the "payload" delivered on an ISS supply mission is really the hardware needed to package the supplies in the Shuttle's cargo bay. The Italian-built supply modules weigh about 10 tons empty but carry only about 3 tons of food and experiments if tightly packed.

So adding any weight anywhere in the Shuttle system, even for urgent safety upgrades is impossible without cutting its true useful lift capacity to almost nothing. And the European and Japanese science modules awaiting launch are large indivisible lumps that cannot be made any lighter. From the NASA managers' perspective, there is no point in making the Shuttle safer by adding modifications that would destroy the only reason for operating it.

All the bizarre proposals for fixing this problem proposed by frustrated Shuttle-huggers fall down on this point. They all add weight, and the Shuttles are maxed out on weight. Nets, giant sheathes, metal shields, stiffer foam etc. would all require omitting vital cargo, or a few of the mission specialists who serve as stevedores and repairmen during the Shuttle's short visits to the Station.

It's easy to take cheap shots at the current Shuttle engineers, e.g.: "If they are too dumb to solve this problem, how can they possibly send men to the Moon?" They aren't technically inept, merely trapped by bad decisions made back in the 1970s by other engineers wearing loud ties, wide lapels, and bell-bottomed pants. And those people were trapped by financial constraints and a general anti-technology mindset that today's young Space Cadets don't remember and can't believe when us old-timers try to describe it.

Where the Shuttle program is broken is at the managerial level. The problem is the managers' persistent refusal to face up to the simple fact that this design is inherently flawed and can't be fixed with the available resources. The real disaster of RTF-2 is the huge contrast between the confident statements made by these managers before the launch and the dismal post-launch reality. If the top Shuttle brass had been less dogmatic at a few press conferences, the political impact of the foam failure would have been far less.

There will be another volley of cheap shots fired against Mike Griffin when he announces the immediate retirement of the Shuttle. He will be denounced by angry Space Cadets as a lily-livered coward who has surrendered to the forces of evil.

In fact, he will be surrendering to the laws of physics and engineering.

And that's no disgrace; they always win in the end.

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