Solar Thermal Rocket Series

This is becoming a rather interesting topic. So interesting that I'll make a series out of it. The preliminary posts have been done already, so I'll list the links to them below for anybody who is interested enough to follow the links and to see what the devil I'm talking about:

• First post:  This gave me the inspiration.   It is a video demonstrating concentrated solar power.  This wasn't new to me, though.  I've seen this in many places.  But something clicked in my mind.  It reminded me of the NERVA rocket in the Apollo program.  The NERVA was a nuclear thermal rocket.  The nuclear reactor heated a core, in which a reaction mass was passed through it.  The heated reaction mass provided the thrust for a rocket that could power a crew to Mars.
• Second post: More videos demonstrating concentrated solar power.  But these were mainly solar dish engines which generated electricity.  My idea doesn't require the generation of electricity.  The NERVA didn't.  It is an unnecessary step which only complicates the matter and increases inefficiencies.  Consider this post as a bit of brainstorming in order to weed out bad ideas which would leave us with a sound approach.  Solar dishes may not be workable in this concept.  But concentrated solar is on the right track.

Which brings me to this third post.  Now the series gets a name all its own.  It will also get a label of "history of the blog" which means that I consider this an important post.  Important enough to want to link to it in some future time.

The next thing that I want to explore is the use of mirrors to reflect sunlight toward a point, such as an array of heliostats in space.  These would focus the sunlight on to a small area which would become very hot.  Hot enough to melt anything on Earth.  Or maybe not quite that hot.  You want to be able to keep the heat exchanger from melting, so dial it down a little.  The heat exchanger would heat hydrogen up to the same temperature that the NERVA could, which would mean an ISP comparable to that of a nuclear thermal rocket.

How to get these into space?  I was thinking of using booms as it is done on the ISS.  Instead of photovoltaic panels on the boom, there would be heliostats.  The heliostats would reflect the sunlight to the heat exchanger as described.  The heat exchanger - rocket nozzle - hydrogen tank apparatus would look like a conventional rocket on the launch pad.  A risk to be avoided would be reflected the light onto the wrong place.  While avoiding this, the conventional-looking rocket would be attached to the space habitat, which would be propelled through space to a desired destination within the inner planets.  Mars would be as far out as you could go.

The ISS gets about a quarter megawatt of electricity out of their panels.  I'm thinking that a similar sized boom which would have heliostats would generate perhaps as much as twice the power.  Let's say four booms of this size could generate a megawatt of power.

I can't do the calculations for the thrust generated by this, but I suspect that with an ISP close to nuclear thermal, that it could provide sufficient acceleration to get a crew to Mars and back in a year.  Let's call a speculation alert on that, to be accurate.