If the problems with Briz-M are due to the long burn time of the engine required to go through the 15 tons of prop (it's a very small engine, so it has to burn for a long time to get satellites to GTO), then another approach would be to somehow affix small engines to the large auxiliary propellant tank (APT). Even a series of 100 kg engines would significantly reduce Briz-M engine burn time.
Unless they greatly increase the ISP, the increase mass fraction will adversely impact your system performance. Especially if the extra engines have lower ISP.
If the problems with Briz-M are due to the long burn time of the engine required to go through the 15 tons of prop (it's a very small engine, so it has to burn for a long time to get satellites to GTO), then another approach would be to somehow affix small engines to the large auxiliary propellant tank (APT). Even a series of 100 kg engines would significantly reduce Briz-M engine burn time.
Unless they greatly increase the ISP, the increase mass fraction will adversely impact your system performance. Especially if the extra engines have lower ISP.
Yep, reliability at the expense of performance.
OK, after checking various launch vehicle manuals, I have figured out the story concerning the Briz high pressure and low pressure prop tanks.
The Briz main engine is fed by a turbopump, which allows tank pressure for the main engine to be maintained at a relatively low pressure. This include the Briz main engine tanks and the APT as well.
There are also 4 small rocket engines (40 kg), which are pressure fed. Therefore, their prop tanks are maintained at relatively high pressure. Note that this "high pressure" is the same as the Russian segment of ISS prop system.
Therefore, it would be virtually impossible for the 4 small thrusters to feed from the large prop tanks, since there would not be enough pressure for effective thrust.