Anyone rejoicing over the space shuttle’s safe return, as a catharsis of earlier disaster, would do well to recapitulate a mite. Consider the following:
a. NASA ultimately launched on the basis of the mission entailing acceptable risk, a stance only adopted after having been unable to resolve the issue of foam detaching from the external tank on launch. The Launch Director later pointed out somewhat misleadingly that the small pieces of iced up foam that detached did so this time at a much higher altitude–i.e., where the dynamic pressures were insufficient to accelerate debris against the orbiter and damage it. In other words, it was a return to the familiar status quo ante of the pre-Columbia era where foam had regularly detached, but mostly without any significant after-effects.
b. Inspections in orbit by the arm-mounted camera and photos taken from the International Space Station (ISS) confirmed, as expected, that the Orbiter Discovery was undamaged. However, not all future planned missions will have the arm or go to the ISS (or have the capability of injecting into the ISS orbit for the crew to obtain safe haven while awaiting rescue).
c.Cracks and actual foam detachments had been found even prior to Discovery’s launch. These were said to be related to the thermal stresses of cryogenic fuelling/defuelling associated with Discovery’s delayed launch. However, such delays and weathering during launch delays have always figured in Shuttle operations.
c.The threat of foam detaching and damaging the Orbiter’s sensitive wing leading edge remains. The successful laying on of a caulking compound during trials in orbit falls far short of an ability to repair any lethal sizeable defect in the reinforced carbon tiles (RCC) wrapping around the Orbiter’s very brittle wing leading edge. But hey, the space walks and experiments had PR value for NASA.
Soft Armor Plating
A much earlier ASW suggestion that the vulnerable sections of RCC ahead of the main landing gear wheel wells would be best protected by a layer of silastic compound went unanswered by NASA (ASW May 2, 2005 & July 18 2005). A photocopy of the Administrator’s Return to Flight Plan was their response. The sacrificial glove concept envisages protection of the very brittle RCC against iced foam impact by overlaying the relevant area with up to 1.5 inches (at the leading edge) of a particular Dow Corning RTV – – as a conformal coating.
This depth of silastic would taper away aft on the upper and lower surfaces of the wing and serve to soak up the blunt direct (versus glancing) impact of any detached tank debris. During launch, because of the low Mach numbers attained, the coating’s effect on the Shuttle’s flying qualities would be non-critical, which means it would not have to be 100 percent conformal. The fact that the overcoat would rapidly burn up ablatively with no effect on the Orbiter’s re-entry posture and lower atmosphere aerodynamics is the significance of it being called “the sacrificial glove”. The compound could in fact be applied anywhere (nose, fin, etc.) that there is an underlying vulnerability – particularly to accelerated direct impact. However, the lethally susceptible area (and the area most likely to be struck) remains the wing leading edge just ahead of the main-gear wheel-wells. Being struck there is what killed the Columbia.
A successful launch and recovery doesn’t mean that an iced foam-strike is no longer a lethal threat. One gets the impression that NASA is listening but rarely hearing. NASA is still in therapy. For more, go to: http://www.iasa.com.au/shuttle.htm. Also look for news updates on http://www.aviationtoday.com.