I thought it'd be helpful to explain the Starliner thruster problem here.

So here's the Starliner system. It works like the Apollo spacecraft. There's two parts. The crew module that comes back to Earth, and the disposable service module that is jettisoned before the crew capsule re-enters the atmosphere.

You see those slim white boxes spaced at 90° around the service module? Those are the thruster packs, also known as the "doghouses".

Each one has 11 thrusters. 8 of them are reaction control system thusters, aka RCS. These are the very-low-power thrusters that are used for ultra-high-precision maneuvering, like you need during docking. Each doghouse has 2 pairs (for redundancy) each aimed in four directions - forward, aft, starboard, and port.

If you've ever seen the Apollo spacecraft, they do the same job as those little blocks with the tiny engines sticking out.

The remaining 12 thrusters (3 per doghouse) are more powerful ones called OMAC (Orbital Maneuvering and Control). They're basically the "main engines" of the spacecraft. Here's what a doghouse looks like without the cover.

The blue circles are those OMAC "main engines". The red boxes are the aft-facing RCS thrusters.

The five RCS thrusters that failed were all aft facing. They are the same model of RCS thruster as the forward, starboard, and port thrusters, all of which are working fine.

The speculation is that the aft RCS thrusters are too physically close to the OMAC thrusters, which is causing uncontrollable overheating while the spacecraft isn't docked and the engines are ready. If this is indeed the cause of all the problems, then that means this is not a software or assembly line or QA problem, but a design flaw with Starliner's thruster doghouses. And that could be extremely expensive for Boeing, with significant time delays.

But what about the RCS thrusters on the crew module, you may ask? Well, here's the fun part: it is unclear if they are also packed too close to other systems that may cause overheating issues. Even if the two astronauts are able to fly the spacecraft away from the ISS (which is by no means certain with temperamental service module RCS thrusters), it could mean a loss of control during re-entry. Or in plain english, a dead crew.

Further clarification on overheating in spacecraft. Obviously there's no atmosphere to carry away heat by convection. Spacecraft rely on radiator panels to radiate away heat. For example, the Space Shuttle's radiators were on the interior of the big cargo bay doors. Space Shuttles had to open their cargo doors almost immediately on achieving orbit or they'd risk overheating. If a shuttle couldn't get its doors open, it meant the flight would be aborted and they'd have to return immediately to Earth. In practice that never happened on an actual flight, but it was a standard emergency procedure. The International Space Station's radiator panels are those giant white slightly-corrugated panels that are at 90° to the solar panel arrays. SpaceX's Dragon has its radiators panels covering 180° of it's "trunk", the disposable aft unpressurized-cargo cylinder section, with solar panels covering the other 180° of the cylinder.

Even a small amount of unexpected heat buildup adds up over time, especially in situations involving thrusters. If Boeing's engineers didn't properly account for the heat interaction between the RCS and OMAC thrusters it could easily lead to the overheating issues on this flight. There's no overheating now because all the thrusters are shut down while docked. But the moment they undock, that problem could come back, and it might happen even faster and be more severe if there's permanent heat damage to the thrusters.

The lunar rovers had an amazing heat management system. I love to tell this story, it's one of those classic Apollo so-simple-it's-genius ideas. They couldn't afford the size or weight or battery power for a complex pump system or huge radiator panels, and the electric motors did build up quite a bit of heat. So they just had boxes of paraffin wax acting as heat sinks. When they stopped the rover to do work, they unfolded a few small metal flaps from the top of the paraffin wax box to act as radiator panels. They knew everything was cool (figuratively and literally) when they saw with their own eyes that the wax had solidified. No power needed. Didn't take up much mass or volume. Just boxes of wax and some metal sheets. Those rovers were amazing pieces of engineering.