We Don't Know As Much As We Think We Do

We Don't Know As Much As We Think We Do

One of the issues with building reusable space transports are those of maintenance and inspection. The Shuttle is a nightmare in terms of the things that have to be done to it between flights, and the question arises--is this intrinsic to reusable orbital launch systems, or was it a bad design? It's some of both, but mostly the latter. The development budget for the Shuttle was severely constrained, resulting in a lot of design decisions that proved to be very costly down the road, when it came to operating it. And the technology at the time the design was frozen (early seventies) is three decades behind ours of today.

A major issue is inspecting structure for fatigue between flights. We have quite a bit of experience with aluminum and other metals, and their behavior after repeated stress, and we know how to inspect for it. But one of the ways that we hope to get launch costs down in the future is to shift from metal structure to composites, which are much lighter for a given level of strength. That's an area that we understand much less well, as demonstrated by the fact that rudders are, apparently inexplicably, falling off of Airbuses:

Composites are made of hundreds of layers of carbon fibre sheeting stuck together with epoxy resin. Each layer is only strong along the grain of the fibre. Aircraft engineers need to work out from which directions loads will come, then lay the sheets in a complex, criss-cross pattern. If they get this wrong, a big or unexpected load might cause a plane part to fail.

It is vital there are no kinks or folds as the layers are laid, and no gaps in their resin coating. Holes between the layers can rapidly cause extensive "delamination" and a loss of stiffness and strength.

Airbus, together with aviation authorities on both sides of the Atlantic, insists that any deterioration of a composite part can be detected by external, visual inspection, a regular feature of Airbus maintenance programmes, but other experts disagree.

In an article published after the flight 587 crash, Professor James Williams of the Massachusetts Institute of Technology, one of the world's leading authorities in this field, said that to rely on visual inspection was "a lamentably naive policy. It is analogous to assessing whether a woman has breast cancer by simply looking at her family portrait."

Williams and other scientists have stated that composite parts in any aircraft should be tested frequently by methods such as ultrasound, allowing engineers to "see" beneath their surface. His research suggests that repeated journeys to and from the sub-zero temperatures found at cruising altitude causes a build-up of condensation inside composites, and separation of the carbon fibre layers as this moisture freezes and thaws. According to Williams, "like a pothole in a roadway in winter, over time these gaps may grow".

Commenting on the vanishing rudder on flight 961, he pointed out that nothing was said about composite inspection in the NTSB's report on flight 587. This was an "unfortunate calamity", he said. Although the flight 961 rupture had yet be analysed, he continued to believe Airbus's maintenance rules were "inadequate", despite their official endorsement.

Barbara Crufts, an Airbus spokesperson, said visual inspections were "the normal procedure" and insisted Williams's case was unproven. "You quote him as an expert. But there are more experts within the manufacturers and the certification authorities who agree with these procedures." She disclosed that the aircraft used in flight 961 -- which entered service in 1991 -- had been inspected five days before the incident. She said did not know if the rudder had been examined.

How applicable is this cautionary tale to the design of space transports? Well somewhat, but not quite as much as one might think. Fatigue is (usually) a phenomenon that occurs as a result of a large number of cycles (assuming that the stress is reasonable--obviously, one can fatigue a paper clip to failure in just a few extreme twists back and forth with a pair of pliers). It's a real concern for aircraft that are in the air a lot, with many takeoffs and landings, and continuous buffeting from the air.

A space transport has two things going for it. First of all, it spends little time in the atmosphere, which is where most of the structural stress occurs, at least that due to aerodynamics. In space, it's actually a quite benign environment, from a structural standpoint. Second, if we ever get to the number of flights of a single space transport that even start to approach the cycle life of an air transport, we'll have clearly solved the problem of space access, even if we occasionally (as in the aircraft industry) lose a vehicle to structural fatigue.

But regardless of what this means for spaceship design, I think that Airbus has some big problems, until they understand this issue better. And now that Boeing is also using composites for primary structure, they need to get on top of it as well.

This article was taken from http://www.transterrestrial.com/archives/004973.html#004973