If you mean the "popping noise" the TM65 engine that Copenhagen Suborbitals testet made at startup, then it was a bit of engine oscilations. It's most likely caused by the engine being run at a low fuel pressure. The fuel will ignite in the ignition chamber, causing the pressure to rise, giving a higher exhaust flow, causing the pressure to drop, giving less exhaust flow, resulting in more fuel in the ignition chamber, that ignites, ...

At higher fuel pressures the oscilations are dampened. (But they do sound awsome!)

I don't know if that's the case with SpaceX's Falcon, but I'm pretty sure that if they have engine oscilations it's nothing they can't handle.

Well, sort of, yes.

When I get paid enough to cover my needs I don't really care about money. If I had to choose between two jobs: one with huge pay and dreary work, and another with adequete pay and interesting work, I'd go for the second job.

And, wire wrapping was at that time better at handling vibrations and bumps than solder. Perfect for avionics!

Oh, I miss wire wrapping.

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.

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".