I should specify that I am looking at the 8 ton thrust engine for the Blok-DM, not the newer 5 ton thrust engine. Even with the NK-33 powered Soyuz core, gravity losses for the initial Blok-DM burn would be significant. The Blok-DM/double Glonass stack is going to have a mass of about 19 tons, and the engine would have to burn for 7 minutes before the stack has a thrust to mass ratio greater than 1. The first seven minutes of Blok-
DM operation or so would effectively be a "coast", except that the engine would be burning, the same as during a SeaLaunch mission.
This is my reference point for the Soyuz 3/Blok-DM:
http://www.russianspaceweb.com/vostok_lv.htmlthe Vostok rocket similarly dispensed with the Soyuz "3rd stage", and was used for Atlas-F class missions: putting relatively light payloads in high LEO orbits. In particular, this vehicle put 1,500 kg Meteor weather satellites into ~800 km orbits.
In my hypothetical Soyuz 3/Blok-DM, the core stage is powered by NK-33 and is larger than the Vostok core. The strap-on boosters are the same. The larger and more powerful core lifts the larger Blok-DM upper stage (compared with Vostok), so the upper stage mass is some 18 tons, compared with ~6 tons for Vostok.
There are some incremental savings of mass for Soyuz 3, some 200 kg for not carrying the old Soyuz analog flight computer and some lighter avionics systems (I would imagine that Block DM would control the entire flight with its Bizer).
≤1.4 Mt to GLONASS orbit.