Inspecciones y ensayos 1 Inspección del funcionamiento

The maintenance of a cryogenic fluid’s low temperatures is one of the most critical design issues for space vehicles that run on high-energy fuels. Liquid-oxygen tanks frost over creating a natural thermal protection. Moisture on liquid hydrogen tanks, however, turns into liquid-air condensation which causes a temperature increase in the tank.330

Because liquid hydrogen is kept in a near-boiling state, even a slight increase in temperature can cause the propellant to evaporate. Heat sources include atmospheric conditions at the launch pad, aerodynamic heating during the launch, solar radiation, and heat from the rocket engines and electronics during spaceflight. The resulting hydrogen vapor is vented out of the tank, robbing the spacecraft of fuel critical to the completion of the mission.331 _{Lewis researchers expended more effort studying thermal retention} systems than any other aspect of tank design during the 1960s. Much of the work was conducted at the Rocket Systems Area.

James Dewar’s eponymous double-walled storage vessels were widely used for ground storage, but their weight made them impractical for spaceflight. So tank developers, dating back to Robert Goddard, turned toward methods of applying either interior or exterior insulation to standard single-walled tanks. There was only a minute amount of available data on lightweight cryogenic tank insulation in the 1950s. Lewis researchers compared the thermal properties of materials such as cork, balsa, glass laminate, and foam. Initial studies demonstrated the good performance of heavy corkboard and the benefits of using a thin external seal over the insulation.332 _{Subsequent investigations showed that low-density foam provided} an adequate thermal barrier for short-term missions, but questions remained regarding its form, application, and sealing.333

General Dynamics designed Centaur with four fiberglass insulation panels that were jettisoned during separation of the stages. Lewis engineers had been concerned with General Dynamics’s inattention to development of the insulation system for some time.334 _{During the unsuccessful first launch attempt, the} failure of the panels caused the tank to overheat which sparked the explosion.335 _{A Marshall postfailure} report stated that the “Centaur tank insulation material is not optimal, and the method of construction and mounting is poor.”336

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Porter Perkins and his colleagues from Lewis’s Materials and Structures Division began investigating different approaches to insulating Centaur’s tank, using three of the most effective methods from Lewis’s previous experiments—corkboard affixed to the tank, foam panels wrapped against the tank, and foam panels with a liquid- nitrogen film applied to the exterior.337

Perkins began by studying the heat transfer rates of the three systems on a small aluminum tank at the Rocket Systems Area’s Tank Test Facility (J–4). The staff filled the pressurized tank with cryogenic fluid then measured how long it took to evaporate. This could take hours or even days. Although all three of the insulation methods performed relatively well, the researchers decided to pursue the constrictively wrapped foam panels, which weighed the least. Unlike Centaur’s current insulation panels, the wrapped insulation would not be jettisoned during flight.338

Image 76: Perkins in 1958 (GRC–1958– C–47794).

Image 77: A subscale insulated tank having a full-scale Centaur tank radius with a constrictive nylon wrap. NASA contracted Goodyear to fabricate four of the tanks and wrap three of them with a 160-strand-per-inch nylon constrictive wrap, and the fourth with a 64-strand-per-inch fiberglass binding339 _{(NASA TM X–52004, Fig. 5).}

In the fall of 1963 Lewis obtained four specially designed 5-foot-long flask-shaped tanks that simulated the curvature of the Centaur tank. Goodyear Aerospace Corporation applied the rigid polyurethane foam panels to the tanks and sealed them with a Mylar- (DuPont Teijin Films) encapsulated aluminum laminate. They then used a large winding machine to wrap the tanks with a nylon constrictive wrap.340

The Lewis researchers measured the boiloff rates for these tanks in ground hold conditions at the J–4 vertical stand. They then repeated the tests in the Vacuum Environment Facility (J–3), which could simulate postliftoff conditions by lowering the chamber pressure and using infrared lamps to replicate aerodynamic heating. Perkins and his colleagues were pleased to find that, despite some minor leaks, the constrictively wrapped foam insulation performed well in both standby and launch phases. Alternative winding patterns did not impact the results.341

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The researchers used their findings to compare the wrapped insulation to Centaur’s jettisonable panels. They found that their new system did not provide the same amount of thermal protection as the current system and that it would require unwrapping the tank for defueling on the ground. Nonetheless, the constrictive wrap system would simplify the launch procedures, remove the need for a helium purge system, and eliminate the danger of a jettisoned panel striking the vehicle. Lewis felt that this made the comparison of the two systems competitive.342

General Dynamics continued to use the jettisonable fiberglass panels for nearly 30 years. The company resurrected the foam insulation concept in the 1980s and formally instituted it in 1992. Virginia Dawson and Mark Bowles noted, “The switch to foam insulation, not only increased performance and reliability but also reduced cost. As a result of successful implementation, the new philosophy of continuous improvement permeated the company.”343

Image 78: Constrictively wrapped full-scale Centaur tank in the J–4 test stand. Twenty foam panels were affixed to the tank, covered with the Mylar and aluminum laminate, an aerodynamic shield, and the constrictive wrap (NASA TM X–52004, Fig. 10).