> So it shouldn't matter at what speed the spacecraft is traveling since the thing it's doing work against (propellant) is always traveling at the same speed as the spacecraft?

That is exactly right.

From the spaceship, when I expel some exhaust, I gain some constant amount of energy. So, it makes sense to me that me acceleration is steady.

From the ground, for a spaceship to have steady acceleration, it must be gaining more and more energy per second. The faster I am going, the more energy I need to gain a little bit more speed. This extra energy comes from the fact that the fuel is moving along with the spaceship - from the ground, the fact that it's moving means it has extra kinetic energy to expend on propelling the spaceship. If you do the algebra, you'll find that the extra quadratic terms on the spaceship's energy and the fuel's energy exactly cancel out.

So the actual effect (constant acceleration of the spaceship) looks the same, but the accounting of which object has how much kinetic energy looks completely different.

>Thanks, I think this is starting to make sense. So when the car changes speed, it's applying work against the ground/earth and that's the frame of reference? I think that's what I was missing

The frame of reference is just what you define as being stationary for your calculations. It is something that just exists in the model you use for the calculation.

The speedometer of the car measures the speed of the car relative to what is rolling on. Let's assume that it is a day with no wind. let's say the speedometer shows 120, the unit does not matter, that is the relative speed between the car and the ground.

So you can use the ground as the frame of reference then the car moves forward at 120 or you can use the car then the ground and the air move backward at 120. So any calculation you do need to include both parts

So the situation is like if you drive a remote-controlled toy car on a treadmill and add a fan to get the air moving. Compare that car to a remote-controlled car on the floor beside it. From the behavior, it is quite clear that the moment of the treadmill has an effect on the behavior of the car.

So if you use a car that is more relative to the earth as the frame of reference you need to include that earth is moving backward just like if you drive on a treadmill and the frame of reference is the ground you need to include the motion of the band on the treadmill

For spacecraft or any other rocket engine, the propellant is moving at the same speed as it. They will both have the same speed regardless of what frame of reference is used. What speed at both moves depends on the frame of reference you use but it will always be the same.

The result is it works out the same regardless of what framer of reference you use, what can change it how hard the calculation is