The United Launch Alliance canceled the planned launch of the Atlas 5 missile on Wednesday at Cape Canaveral to solve the fuse problem in the launch pad, while a SpaceX team a mile and a half to the south equipped the Falcon 9 rocket to take off Thursday evening with the US Army’s GPS navigation satellite.

The ULA launch team canceled the attempt to launch Atlas 5 on Wednesday before 6 PM ET (2300 GMT) after failed attempts to fix the valve issue, first remotely and then with a team of technicians dispatched to the Atlas 5 coastal launch pad.

The Atlas 5 missile will carry a classified payload into orbit for the National Reconnaissance Office, the US government’s spying agency. The launch is designated NROL-101, and the NRO has not disclosed any details about the payload mission, other than that it will assist the agency in its mission to collect and disseminate information to government intelligence agencies.

ULA began the countdown to Atlas 5 late Wednesday morning, switched on the missile, and proceeded with testing of the guidance system and other checks before loading the cryogenic thrusters into the launch pad prior to planned takeoff at 5:54 PM EST (2254 GMT) .

In a statement, ULA said the launch team stopped the countdown clock after “an unexpected system response from remote-controlled ground system liquid oxygen valves”.

“The team is continuing to analyze the system and will protect its next launch attempt no later than November 6,” ULA said.

ULA said the next opportunity to launch an Atlas 5 missile would be Friday. The exact date for the attempted launch on Friday was not immediately announced, but the mission’s launch time had moved roughly four minutes earlier per day. That will set the launch date for Friday at around 5:46 PM EST (2246 GMT).

The launch of the Atlas 5 was previously scheduled for Tuesday, but ULA has returned the missile to its helicopter hangar near the launch pad to replace the environmental control system runway that feeds conditioned air into a highly classified NRO payload on the top of the 206-foot (63-meter) vehicle.

Ground crews returned Atlas 5 to the launch pad late Tuesday in preparation for the launch attempt on Wednesday.

Ahead of the next Atlas 5 launch opportunity, SpaceX plans to launch a Falcon 9 rocket from the nearby 40 platform during a 15-minute window at 6:24 PM EST (2324 GMT) on Thursday.

The 229-foot (70 m) Falcon 9 rocket was standing at Platform 40 on Wednesday in preparation for launch Thursday evening. Pad 40 is located 1.5 miles (2.5 kilometers) south of the Atlas 5 launch pad in Cape Canaveral.

The Falcon 9 rocket is preparing to project the US Space Force’s GPS 3 SV04 navigation satellite, to replenish the fleet of positioning and timing stations used by billions of military and civilian users around the world.

SpaceX attempted to launch the GPS satellite on October 2, but an engine problem led to an automatic miscarriage just two seconds before take off.

Engineers who investigated the October 2 miscarriage found that two of the nine first-stage engines on the missile had a tendency to ignite a fraction of a second earlier than expected. Inspections showed that a closed relief valve in the gas generators of the two engines caused pressures to rise sooner than designed at start-up, and the sensors in the motors detected the problem and stopped the countdown.

SpaceX engineers Select a masking treatment It was inadvertently left in two Merlin engines as the reason to cancel the countdown last month.

“When we looked at the data, we saw that two of the engines tried to start early, and spontaneous abortion prevented it,” said Hans Koenigsman, SpaceX’s vice president of construction and flight reliability. By doing this, it prevents a potentially difficult start that could damage the engine hardware. “

Merlin engines are powered by an ignition fluid known as TEA-TEB – or Triethyl Aluminium – Triethylboran – which gives a bright green flash at the start of the ignition sequence.

“Then we have liquid oxygen, and we have kerosene, or RP-1 as it is called,” Koenigsman said last week in a conference call with reporters. And you need to enter these fluids in the correct order. “If I do it in the wrong order, if I happen to receive liquid oxygen, RP-1 and liquid ignite, what will happen is that we call it a tough start.”

In most cases, Koenigsmann said a hard start would “jolt” the engine, but it could cause damage. “So in general, you don’t want that. You want a good startup.”

SpaceX shipped the Merlin engines back to a test site in Central Texas, where inspections revealed a substance blocking a line leading to the pressure relief valve in the gas generator on two of the engines.

Koenigsman said the vent, which means one-sixth of an inch wide, was blocked by a hooded varnish. He said the liquid varnish – similar to red nail polish – is used by an outside vendor that anodizes aluminum engine components for SpaceX.

The coating protects certain parts during the anodizing process, but the vendor – which officials has not specified – is supposed to remove the material before shipping components to SpaceX to manufacture the engine.

The gas generator in each Merlin engine drives a turbine pump that feeds kerosene and liquid oxygen fuel into the main combustion chamber.

Engineers at SpaceX’s McGregor test site showed that the engines were running normally after removing the blockage from the vent valve. The problem is “very subtle, but it is clear that it could have some negative impact on engine operation,” Koenigsman said.

“The GPS 3-4 mission will continue to use the same booster used for the first launch attempt,” said Walt Lauderdale, the GPS 3-4 mission manager from the Space Force Space and Rocket Systems Center. “The two motors that caused the launch abort were replaced by one that was confirmed by examination and review of proportions as there is no masking varnish remaining.”

SpaceX and Space Force officials have verified that all nine Merlin engines on the Falcon 9 rocket were ready to fly after a test launch on Platform 40 on Saturday.

Besides the GPS mission missile, the engine problem also affected vehicles for a pair of upcoming NASA launches. So far, the issue has only affected the missions that are scheduled to be used by the new Falcon 9 Boosters.

SpaceX’s Crew Dragon spacecraft’s first operational flight is scheduled for November 14 from the Kennedy Space Center with three NASA astronauts and a Japanese mission specialist to start a half-year expedition on the International Space Station.

SpaceX replaces two Merlin engines on a Falcon 9 rocket for the Crew Dragon mission that engineers found suffered from the same early start-up trend that the engines on the rocket showed for the GPS mission. The issue delayed the launch of the Crew Dragon from October 31 to November 14.

Steve Stitch, NASA’s Commercial Crew Program Manager, said last week that the agency’s engineers wanted to analyze engine data from the Global Positioning System (GPS) launch before allowing the Crew Dragon to take off later this month.

The engine problem also delayed the launch of the US-European Sentinel-6 Michael Freilich oceanographic satellite from Vandenberg Air Force Base in California. This mission was originally supposed to launch on November 10, but is now slated to launch on November 21.

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