evening news report of a disaster in space might be enough to halt the Moon mission altogether.
The technician who reported the excess propellant usage had on his screen an algorithm that constantly told him what propellant would be required to perform an abort and an estimate of the trajectory and time required to recover from the abort so that another attempt could be made. Bill knew that the console tech was ready for his question.
“Well, Bill,” Stubborn Stu started, “if laser ranging data is correct and we have to perform a burn to take out that velocity and then fly around a few orbits to try again, we will be at the minimum propellant margin for the rendezvous. But it still won’t meet mission-success criteria. The EDS will have been on-station too long. Propellant boil-off will exceed TLI commit.”
Though never actually uttered, virtually everyone in the room heard the expletive that Stetson thought to himself. Bill pulled his headset free for a second and adjusted his thinning hair while making a motorboat sound with his lips. Not being able to do a TLI, or Trans-Lunar Injection, burn of the rocket engines would mean not going to the Moon. The longer the EDS had to wait, the more propellant would evaporate—reducing the total burn-time possible for the engine. They had to fire before too much had boiled off.
The EDS was powered by one of the most energetic rocket fuels known—liquid hydrogen. When combined with an oxidizer, in this case the ultimate oxidizer, liquid oxygen, the combustion produced the rocket thrust that would propel the EDS toward the Moon. It was these same propellants that powered the three main engines of the old space shuttle, producing much of the cloud of steam that was the hallmark of a successful launch.
Unfortunately, to keep hydrogen liquid, it had to be kept cold. In fact, the temperature had to be kept to about minus four hundred twenty-three degrees Fahrenheit. To do this, the huge hydrogen tanks in the EDS were kept wrapped in the best thermal insulators known and placed in the vehicle so as to minimize the heat they might receive from the sun and that reflecting back into space from the Earth. It was, in effect, a large thermos bottle in orbit. It was also an imperfect thermos bottle; some heat inevitably would always get through to warm up the hydrogen. As the volatile gas warmed, it boiled and evaporated and then vented into space. Hence the phrase “boil-off.”
Engineers designed the EDS tanks and propulsion system to have enough liquid-hydrogen propellant remaining—after boil-off—to complete the mission even if the craft had to remain in orbit for a few days before beginning its trip to the Moon. If its time in orbit exceeded the design limit, then there would simply not be enough fuel remaining to complete the mission. Since there had been some minor glitches before this one, the allowable time in Earth orbit was close to being over, and a further delay would mean that the burn to send the Orion to the Moon would not happen—at least not on this test flight.
“Oh, well,” Bill Stetson responded with an audible sigh as he readjusted his headset. He then straightened himself in the seat and barked, “Release the automatic docking system to manual control. Give me real-time data from the laser ranger and don’t give me any more data from the damned GPS!” This, too, they’d practiced in training. A manual docking was something the astronaut corps had insisted upon since the Shuttle-Mir program of the 1990s. This was what the pilot and mission commander
Switching views on the monitor in front of him and seeing the requested data feeds appear on the secondary monitor to his right, Stetson prepared to take manual control of the Orion. Forgetting about the paperwork that would be required should he be successful, and the probable reprimand should he fail, Stetson gave the order to release the vehicle to manual control.
“Alright, give her to me,” he said.
To a detached observer, it would have appeared that Bill Stetson was beginning to play a video game. With a controller that looked like the technological cousin of a PlayStation game controller and an LCD screen with a simulated 3-D rendering of both the Orion and the EDS, he assumed manual control.
At first, he saw no discernible effect from his efforts. He’d begun by firing the thrusters on the Orion that were responsible for making a rendezvous possible. But he didn’t fire them to accelerate the craft; rather, the opposing sets of thrusters were used to slow it down. Newton’s laws are unforgiving. Every action has an equal and opposite reaction. To speed something up, you fire rocket engines. To slow that same something down, you fire rocket engines that point in the opposite direction. And it takes the same force to accelerate to fifteen feet per second as it does to slow down by fifteen feet per second.
“You should be seeing something. The propellant in the tanks is starting to drop rapidly,” Stubborn Stu said calmly. “You still have plenty remaining, but I’m definitely seeing it.” He, like just about everyone else in the room, was starting to perspire. It was a stereotypically hot Houston afternoon in a room with stereotypically cold Houston air-conditioning doing nothing to prevent the perspiration from coming. It just made the sweat feel uncomfortably cold.
The sweat starting to bead on Bill’s forehead glistened in the control-room lighting. He kept a watch on the laser-ranger data, and finally the velocity numbers began to decrease. The velocity dropped from an excess of five meters per second to four. Then to three and finally to a closure rate that should permit safe docking. This happened not a minute too soon—as Stetson fought to bring the closure velocity down, the distance between the Orion and the EDS continued to dwindle. They were now only a hundred meters away from one another and in desperate need of fine guidance for the final rendezvous. This, too, was a maneuver that the team in mission control had practiced manually, and their training not only took over for these last few minutes of the rendezvous, but it alleviated the stress and allowed the heart rates of the console techs to fall back to normal.
“We have manual docking in three…two…one.” Marianne Thomas provided the countdown. Bill could tell from the tone in her voice that she was grateful he had overcome the problem and that it wasn’t something she’d done. He figured that the engineer already was beginning the mental construction of a fault tree that would help the mission-review team find out why the automated system had failed and why the GPS data was suddenly blank.
“Phew,” Stetson said, relieved at completing the docking maneuver successfully. He then declared, “We’re not finished here yet, people. Need I remind you, we’ve got a vehicle that needs to be checked out and sent on its way to the Moon. No dinner and bar just yet.” And he knew that he was correct. If nothing else went wrong, the flight was supposed to continue to the Moon, with the EDS lighting its engines to escape Earth’s gravity in just a few hours.
The two concurrent failures still needed to be explained and corrected. While the GPS measurements would be useless at the Moon—which had no Global Positioning System satellites—the onboard computer that was supposed to make sense of the laser-ranging data would be used again when the Altair lunar lander returned from the surface of the Moon to rendezvous and dock with the Orion in lunar orbit, allowing the crew to transfer back to the Orion for their trip home. Yes, this was an unmanned test flight, but the systems nonetheless had to work or the launch of the actual mission would be postponed indefinitely until the problem was resolved. The public and political pressure was mounting to kill the space program, and having to scrub so close to launch could be a public-relations nightmare. Bill hoped to circumvent all that.
