Solar Thermal Rocket Series Part VIII

My latest thoughts on a solar thermal rocket.  Part 8 of an ongoing series.

When I left off last time, I started discussion of how the heat exchanger might work.  This post will refine some of the ideas.

I was thinking of using a fresnel lens to focus the concentrated light on a small point.  The thought occurred to me that this would not heat as evenly and as thoroughly as I would like, so I'm thinking of how to deal with that.  A new idea:  a ring of tungsten will rotate on top of molten aluminum.  An electric motor will turn the ring.  Gears will be on the side of the ring and the small motor that will turn it.  The ring will have the concentrated light energy focused on it while it rotates.  This is to give a more even heating, I hope.

The ring turning on the molten aluminum is to keep the heat exchanger still while conducting heat from the concentrated solar energy through the tungsten, then the aluminum, and into the heat exchanger.

Now, hook up a hydrogen feed line into the heat exchanger and a nozzle out the other end.

Now the question is this:  how big can we make this thing?  And how much energy do we need to focus upon the ring in order to transmit enough energy so as to heat the hydrogen to comparable temperatures in a nuclear thermal rocket?

My goal is to reach that temperature or one close to nuclear so as to have an ISP comparable to it, plus to use solar energy as opposed to chemical or nuclear energy.

Update:

Perhaps this is hard to express how it could work.  I'm thinking it could work like a wheel on a car.  In a car, the wheel rotates around a fixed spindle.  Adapting this concept as an analogue, the "spindle" would be a heat exchanger rotating around it.  It would not support weight as in an automobile.  After all, it is intended to work in a weightless environment.  What is supposed to do is to allow heat transfer between the ring and the heat exchanger while allowing rotation.

By the way, you don't have to use molten aluminum as a "lubricant".  Something else could work better.  I just don't know what that might be.

Update:

I wrote in the above update:  " heat exchanger rotating around it".  Sorry, a goof.  The "spindle" is the heat exchanger, the "wheel" is heat source.  The "wheel" is heated up while rotating- with the heat being transferred through the "bearings" and into the "spindle".  Hope that clarifies things.