Replies

Let me repeat, this work does not come from nowhere. It isn't magic. You use fuel and chemical reactions.

Of course ... to do otherwise would violate the First Law of Thermodynamics.

If you are going to exponentially increase the amount of power of your rocket then you are going to have to exponentially increase the size of the support systems in your rocket ...

No, you have to increase the size of the support systems if you want to increase the thrust. As the speed of the rocket increases, the power produced by the engine increases because the ft-pounds (or newton-meters) per second increases. But this doesn't increase the acceleration because the F and m of "F=ma" haven't changed. As the rocket accelerates, it's producing more power, which is convenient because it needs to produce more power consistent with the exponential increase in kinetic energy with increases in speed.

Here is a graph of the solid rocket boosters on the Space Shuttle. They separate at 126 seconds (where the graph ends). The initial spikes are caused by mass loss and atmospheric interference. After about 80 seconds (and finally above most atmospheric interference) the profile is clear. Thrust is not even close to constant nor is it trying to be.

The Space Shuttle SRB (or any other solid rocket motor) is not a good example of the point you're trying to make.

Design of the cross-section of the "hole" in the center of the motor is a near-black-art; thrust is proportional to the surface area of the propellant; if the hole starts out circular, the area increases and the thrust increases - sometimes good, and sometimes not. A more typical hole-shape is "star", which gives a more uniform surface area as the propellant is consumed. The variation you show in the SRB thrust is designed in via the propellant configuration - it has nothing to do with speed. Also, the plotted thrust is probably from a test stand at the manufacturer, where the motor isn't moving at all. From the NASA website:

The propellant is an 11-point star- shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.

The motor described in the original article which precipitated this discussion would produce uniform thrust indefinitely as long as it has power. Liquid-fuel rocket motors are the same - they produce approximately uniform thrust as long as they've got fuel.