Well, sort of. It's all engine research. And I hope that we'll continue doing research, even if we get a working space plane. There's allways more to learn, new discoveries to be made, system to optimize.

What makes the Skylon concept different is the engine. Instead of a SCRAM-jet air breathing engines they're going for a traditional rocket engine that is fed pressurized air, while in the atmosphere. This is advantageus to the SCRAM-jet and rocket approach, since only a single engine is needed for getting into orbit.

I'm not sure I believe in the Skylon space plane, but the SABRE engine is definitly interesting. If nothing else, researching the concept will teach us something about what is possible with our current technology.

While what you say about Galilean invariance is true, it has no implication on the current discussion. For each inertial frame you only have one acceleration per ridgid body.

Your words were:

Clearly you were talking about the "accelerations" in one inertial frame, not in multiple. So sorry, but you can't weasel yourself out of this discussion by refering to Galilean invariance.

Now, let's use the above on your example: Me sitting on a chair. According to you there is an acceleration from the pull of gravity, a1(t) = -9.8 m/s^2, and an acceleration from my chair, a2(t) = 9.8 m/s^2, and we can sum these to get the "resulting acceleration": 0 m/s^2. So far so good. What you say seems to work.

But, since I have two "accelerations" a direct consequence is that I will have two distinct "positions". A position at p1(t) = -1/2*9.8 m/s^*t^2 and a position at p2(t) = 1/2*9.8 m/s^*t^2. (In the same inertial frame!) At the moment I am still sitting on my chair. My position is neither above my chair nor below it, so what you claim is still wrong.

Yeah, SpaceX are cheap. If I remember correctly, then the cost to low earth orbit in the last sattellite launch I was involved in was something like $12 - $14k/kg.

You're welcome!

I'm not convinced that a pioneering effort would really be that beneficial, but that doesn't mean I'm not all for it. If for nothing else, then just to see how far we (as a race) can go.

SpaceX: Falcon 9 Pricing information

Grandparent isn't talking about unobtainium, but ordinary resources not in this gravity well. The cost of getting something into orbit is prohibitively high, in the neighbourhood of $10000/kg.

This is high school physics, more specifically Newton's laws of motion. It's not as if this is really that hard to understand:

First Law:

The velocity of a body remains constant unless the body is acted upon by an external force.

Second Law:

The acceleration a of a body is parallel and directly proportional to the net force F and inversely proportional to the mass m, i.e., F = ma.

Thrid Law:

The mutual forces of action and reaction between two bodies are equal, opposite and collinear.

Nowhere in Newton's laws of motion do I find any mention of "net acceleration" or "accelerations" (plural), but let's for a moment assume you're right.

Acceleration is defined as the rate of change of velocity, dv/dt, so, each of these "accelerations" will result in "velocities" (again plural), and using the definition for velocity, i.e., rate of change of position, or dp/dt, we get multiple positions. So, a direct consequence of your "accelerations" is "positions" (plural!), all for the same mass! I've never observed anything like that, nor have I ever heard of anybody observing it. If you have any evidence of this, I'd very much like to see it.

Now, it can be advantageous to calculate the acceleration of a body by summing the individual forces divided by the mass of the body. But that's all it is. A computational trick. There is no such thing as "accelerations".

It's rather easy to infer the acceleration from the resulting force (and mass). But why do you keep talking about "accelerations" (plural)? A ridgid body can only have one acceleration. (As the grandparent previously correctly stated.)

Just to be a unit-pedant: Acceleration is measured in m/s^2. (or other equivalent distance per time squared measure)

Did you factor in the conversion from energy bound in the fuel to kinetic energy of the car?
If your didn't, then with a conventional four stroke engine you get about 30% efficiency, so petrol is only about four times better than a battery, while with a PEM fuel cell you can get efficiency up to about 50%, making hydrogen 1 to 1.5 times better than batteries.
And that's just by volume. Even though volume certaintly factors in, I'm not convinced there isn't more, e.g. weight, to the problem of energy storage.

But why would you build a power plant like that?

You're proposing a cycle that goes something like:
solar mirrors -> zinc reactor -> hydrogen furnace -> turbine

Why not just use the old and tried method of:
solar mirrors -> hot steam -> turbine

It would be simpler and far more efficient.

Now, the story is interesting because it's about creating hydrogen using sunlight, without converting the sunlight to electricity first.

That is a consequence of the Danish education system. Education is free in Denmark, and in addition students automatically receive a stipend large enough to support them through the education. This results in danes having quite high levels of education. For example, commonly an ordinary programmer holds a master's degree. Because of this, it can be quite hard for people without at least the equivalent of a bachelors degree to get a job.