The shuttle Endeavour's countdown is ticking smoothly toward launch Saturday evening on a delayed space station assembly mission to attach an experiment platform to a Japanese lab module, to replace aging solar array batteries, and to deliver spare parts and supplies.
There are no technical problems of any significance at launch complex 39A, but forecasters are predicting a 60 percent chance of afternoon and evening thunderstorms and electrical activity that could trigger another delay. Launch is targeted for 7:39:35 p.m. EDT Saturday.
"Overall, our primary concern on launch day is for thunderstorms and showers to be in the area, particularly within 20 nautical miles of the shuttle landing facility and 10 nautical miles of the launch pad," said shuttle weather officer Kathy Winters.
Good weather is expected at NASA's emergency runways in the United States and Europe, Winters said, "so the primary concern will be here at the Kennedy Space Center."
The forecast improves to 60 percent "go" on Sunday with a 70 percent chance of acceptable conditions should launch be delayed to Monday.
"Bottom line from the team, everybody's go for launch, we have no major issues at all," said Mike Moses, director of shuttle launch integration at the Kennedy Space Center. "We're in really good shape for launch tomorrow.
"We do have some challenges with the weather, but we'll just work through those. I don't worry about things I can't control and I certainly cannot control the weather."
Endeavour's crew--commander Mark Polansky, pilot Douglas Hurley, Canadian flight engineer Julie Payette, David Wolf, Christopher Cassidy, Thomas Marshburn and space station flight engineer Timothy Kopra--flew to Florida on Tuesday. The countdown began at 10 p.m. EDT Wednesday.
Engineers loaded liquid hydrogen and oxygen into Endeavour's onboard storage tanks overnight Thursday to power the ship's three electricity producing fuel cells. Countdown milestones today include main engine checks, heat shield inspections, communications system activation, and retraction of a protective gantry that will set the stage for fueling.
If all goes well, engineers plan to begin pumping a half-million gallons of liquid oxygen and hydrogen into Endeavour's external tank beginning around 10:14 a.m. EDT Saturday. The hydrogen and oxygen sections of the huge tank should be topped off and in "stable replenish" mode by 1:14 p.m. EDT. The astronauts plan to begin strapping in around 4:20 p.m. EDT.
Two previous launch attempts on June 13 and 17 were scrubbed during fueling when a hydrogen vent line attached to the side of the tank began leaking as the hydrogen section of the tank neared its full load. Engineers ultimately determined the vent port housing was built into the tank slightly out of alignment, causing a quick-disconnect fitting to pull away slightly under cryogenic conditions.
After the second delay, engineers replaced a rigid one-piece Teflon seal with a more flexible two-piece seal and used shim-like washers on the vent line attachment plate to make it less susceptible to temperature-induced movement.
To make sure the repairs would work, Endeavour's tank was re-loaded with oxygen and hydrogen July 1. Sensors detected no measurable leakage and engineers are confident the system will work as required Saturday.