Interview with Starliner astronauts reveal that the situation was more precarious than many realised. The failure of so many thrusters violated the mission's flight rules. In such an instance, they were supposed to turn around and come back to Earth.
But what if it was not safe to come home, either?
"I don't know that we can come back to Earth at that point," Wilmore said in an interview. "I don't know if we can. And matter of fact, I'm thinking we probably can't."
'If we lost thrusters, we could be in a situation where we're in space and can't control it.' That's what I was thinking. And oh my, what happened? We lost the first thruster."
When vehicles approach the space station, they use two imaginary lines to help guide their approach. These are the R-bar, which is a line connecting the space station to the center of Earth. The "R" stands for radius. Then there is the V-bar, which is the velocity vector of the space station. Due to thruster issues, as Starliner neared the V-bar about 260 meters (850 feet) from the space station, Wilmore had to take manual control of the vehicle.
Wilmore: "As we get closer to the V-bar, we lose our second thruster. So now we're single fault tolerance for the loss of 6DOF (6 degrees of Freedom, forward/back, up/down, left/right, yaw, pitch, and roll) control.
The condition of being "single fault" tolerant means that the vehicle could sustain just one more thruster failure before being at risk of losing full control of Starliner's movement. This would necessitate a mandatory abort of the docking attempt.
Wilmore: "We're single fault tolerant, and I'm thinking, 'Wow, we're supposed to leave the space station.' Because I know the flight rules. I did not know that the flight directors were already in discussions about waiving the flight rule because we've lost two thrusters. We didn't know why. They just dropped."
From the outside, as Starliner approached the space station last June, we knew little of this. By following NASA's webcast of the docking, it was clear there were some thruster issues and that Wilmore had to take manual control. But we did not know that in the final minutes before docking, NASA waived the flight rules about loss of thrusters. According to Wilmore and Williams, the drama was only beginning at this point.
Wilmore: "We acquired the V-bar, and I took over manual control. And then we lose the third thruster. Now, again, they're all in the same direction. And I'm picturing these thrusters that we're losing. We lost two bottom thrusters. You can lose four thrusters, if they're top and bottom, but you still got the two on this side, you can still maneuver. But if you lose thrusters in off-orthogonal, the bottom and the port, and you've only got starboard and top, you can't control that. It's off-axis. So I'm parsing all this out in my mind, because I understand the system. And we lose two of the bottom thrusters. We've lost a port thruster. And now we're zero-fault tolerant. We're already past the point where we were supposed to leave, and now we're zero-fault tolerant and I'm manual control. And, oh my, the control is sluggish. Compared to the first day, it is not the same spacecraft.
At this point in the interview, Wilmore went into some wonderful detail.
Wilmore: "And this is the part I'm sure you haven't heard. We lost the fourth thruster. Now we've lost 6DOF control. We can't maneuver forward. I still have control, supposedly, on all the other axes. But I'm thinking, the F-18 is a fly-by-wire. You put control into the stick, and the throttle, and it sends the signal to the computer. The computer goes, 'OK, he wants to do that, let's throw that out aileron a bit. Let's throw that stabilizer a bit. Let's pull the rudder there
.' And it's going to maintain balanced flight. I have not even had a reason to think, how does Starliner do this, to maintain a balance?"
Essentially, Wilmore could not fully control Starliner any longer. But simply abandoning the docking attempt was not a palatable solution. Just as the thrusters were needed to control the vehicle during the docking process, they were also necessary to position Starliner for its deorbit burn and reentry to Earth's atmosphere. So Wilmore had to contemplate whether it was riskier to approach the space station or try to fly back to Earth. Williams was worrying about the same thing.
Williams: "There was a lot of unsaid communication, like, 'Hey, this is a very precarious situation we're in.' I think both of us overwhelmingly felt like it would be really nice to dock to that space station that's right in front of us. We knew that they [Mission Control] were working really hard to be able to keep communication with us, and then be able to send commands. We were both thinking, what if we lose communication with the ground? So NORDO Con Ops (this means flying a vehicle without a radio), and we didn't talk about it too much, but we already had synced in our mind that we should go to the space station. This is our place that we need to probably go to, to have a conversation because we don't know exactly what is happening, why the thrusters are falling off, and what the solution would be."
Wilmore: "I don't know that we can come back to Earth at that point. I don't know if we can. And matter of fact, I'm thinking we probably can't. So there we are, loss of 6DOF control, four aft thrusters down, and I'm visualizing orbital mechanics. The space station is nose down. So we're not exactly level with the station, but below it. If you're below the station, you're moving faster. That's orbital mechanics. It's going to make you move away from the station. So I'm doing all of this in my mind. I don't know what control I have. What if I lose another thruster? What if we lose comm? What am I going to do?"
After Starliner lost four of its 28 reaction control system thrusters, Van Cise and this team in Houston decided the best chance for success was resetting the failed thrusters. This is, effectively, a fancy way of turning off your computer and rebooting it to try to fix the problem. But it meant Wilmore had to go hands-off from Starliner's controls.
Imagine that. You're drifting away from the space station, trying to maintain your position. The station is your only real lifeline because if you lose the ability to dock, the chance of coming back in one piece is quite low. And now you're being told to take your hands off the controls.
All of this training meant Wilmore felt like he was in the best position to fly Starliner, and he did not relish the thought of giving up control. But finally, when he thought the spacecraft was temporarily stable enough, Wilmore called down to Mission Control, "Hands off." Almost immediately, flight controllers sent a signal to override Starliner's flight computer and fire the thrusters that had been turned off. Two of the four thrusters came back online.
Wilmore: "Now we're back to single-fault tolerant. But then we lose a fifth jet. What if we'd have lost that fifth jet while those other four were still down? I have no idea what would've happened. I attribute to the providence of the Lord getting those two jets back before that fifth one failed. So we're down to zero-fault tolerant again. I can still maintain control. Again, sluggish. Not only was the control different on the visual, what inputs and what it looked like, but we could hear it. The valve opening and closing. When a thruster would fire, it was like a machine gun."
Mission Control decided that it wanted to try to recover the failed thrusters again. After Wilmore took his hands off the controls, this process recovered all but one of them. At that point, the vehicle could be flown autonomously, as it was intended to be. When asked to give up control of the vehicle for its final approach to the station, Wilmore said he was apprehensive about doing so. He was concerned that if the system went into automation mode, it may not have been possible to get it back in manual mode. After all that had happened, he wanted to make sure he could take control of Starliner again.
Wilmore: "I was very apprehensive. In earlier sims, I had even told the flight directors, 'If we get in a situation where I got to give it back to auto, I may not.' And they understood. Because if I've got a mode that's working, I don't want to give it up. But because we got those jets back, I thought, 'OK, we're only down one.' All this is going through my mind in real time. And I gave it back. And of course, we docked."
So it was quite a tricky situation it turns out. I can see why NASA were reluctant to return Butch Wilmore and Suni Williams on the misbehaving Starliner!
https://arstechnica.com/space/2025/...cial&utm_campaign=dhfacebook&utm_content=null
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