variable thrust/ISP rocket engine via variable mix LOX+LH2?

[gunsandrockets]

High thrust (but lower ISP) boost phase switching to high ISP (but lower thrust) sustainer phase for a chemical rocket. A technology that could aid SSTO, or reduce the reliance on solid rocket boosters that some multi-stage launch vehicles have.<br /><br />This was an idea I just thought of, and knowing of course that when it comes to spaceflight there doesn't seem to be ANY original ideas, so I did a google search. I found the information below but not much else. It seems like such a simple and practical idea I'm surprised there hasn't been at least some test program or development project to exploit the idea. If a rocket engine can be throttled than surely adjusting the fuel mixture ratio shouldn't be that difficult to achieve. Maybe this is another area for some alt-space company to exploit, just as Bigelow Aerospace has taken up the cast aside technology of inflatable habitats.<br /><br /><br /><br />http://books.nap.edu/openbook.php?record_id=1976&page=62<br /><br />Variable Mixture Ratio Liquid-Oxygen/Liquid-Hydrogen Engines: <br /><br />The concept is to run the engine on an oxidizer-rich mixture ratio (12:1) for boost operations, then shift to a fuel-rich mixture ratio (6:1) for the high-performance, sustained operation to orbit. The problem is that to make such a shift, the engine must transition through the stoichiometric mixture ratio which yields the highest temperatures of engine operation. Engine control and the means to affect the mixture ratio transition are critical technologies.