Here's a great technical analysis I submitted the other day.

Rocket engine with electric pumps: http://www.scienceforums.net/topic/73571-rocket-engine-with-...

Cost, according to another article I found. The article also mentioned that electric is 95% efficient while turbine-pumps are ~60%, but that is still definitely not worth the weight penalty.

I suppose it makes sense, as it is very difficult to have a turbo with rocket exhaust on one side and cryogenic oxygen on the other. Its quite creative if it is actually cheaper. It might also allow finer control over throttling back, but thats not useful unless you're spaceX.

The other factor with turbo pumps in open cycle designs is you end up burning a fair amount of your fuel and oxidizer in them, and throwing it overboard at fairly low pressures/velocities, which hurts your specific impulse.

Staged combustion motors run the exhaust of the turbo-pump into the main combustion chamber, and have far better specific impulse, but are much harder to design and build.

The the electric pump design, you can have the simplicity of and reliability of the open cycle main engine, with the efficiency inherent in not effectively throwing a percentage of your fuel overboard.

Turbopumps powered by rocket fuel require quite a bit of plumbing and design work; you're essentially building a second rocket motor to drive the bigger one. By using an electric turbopump, you just need a place to put the motor and batteries, which is a lot more simple in comparison.

Here's an article about test firing a Saturn V turbopump power plant:

http://arstechnica.com/science/2013/01/saturn-v-moon-rocket-...

Yes, the turbopump is run off its own rocket engine --- which drove a 40MW turbine, which ran a pump that could push three tonnes of rocket fuel a second.

Admittedly, the Saturn V was a little bit bigger than this rocket's going to be, and had special needs, but it goes to show that rocket fuel powered turbopumps are scary.

(Incidentally, SpaceX use a rocket fuel powered turbopump for the Falcon 9. I believe the scary-looking plume of flame that comes out sideways is the exhaust. It's a mere 2MW. Per engine.)