This is a bit of a clarifying post from the last one.
For instance, what's wrong with using solar dishes? The reason is that the concentration of light isn't at a place that's convenient for generating thrust. It gets in the way of the sunlight and requires that the hot gases be re-routed away from the heat exchanger. Much more convenient and simple to reflect the light toward a spot where it is convenient. Losses of energy are kept to a minimum.
The "power tower" concept works the same way as on the ground. It is set away from the where the light originally lands and is concentrated in a different location in order to do work. As on the ground, the power tower is at a 90 degree angle with the sunlight. Secondly, the concentration point needs to be as close to the rocket nozzle as possible. This is where the gas is the hottest and has the most energy. I think this is a good solution for the problem posed by the solar dish engine concept.
The main risk as I see it is the ability to keep the light focused on a single spot. An error here can be a catastrophe. Fortunately, the only moving parts are the heliostats which must track the sun, and then reflect the light to the "tower". In relation to the sun, there shouldn't be much movement even if you are flying pretty fast. That is because of the vast distances of outer space. The sun always appears to be in the same spot even when you are moving.
The one time that may present a problem would be in changing directions of thrust. That would entail the necessity of a great deal of movement somewhere- either in the booms or the "rocket".
A simple solution for that is to rotate the entire spacecraft assembly 180 degrees. Then you would be facing the opposite direction of travel, thus the thrust from the rocket engines would slow you down.
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