The USSR was big on MHD in the past..
MHD introduces extra weight and extra conversion steps into the chain to drive a turbopump. Compare:preburner exhaust->turbine->shaft->turbopump impellerspreburner exhaust->MHD generator->electric motor->shaft->turbopump impellersPer Wikipedia, MHD efficiency is on the order of 30%. A turbopump can achieve 70-80% efficiency.For a large engine, you need lots of power. The Vulcain 1 needed 3.7 MW for the LOX turbopump and 11.9 MW for the LH pump. Electric motors that powerful aren't small.
Off on a tangent, but -* Superconducting electric motors are much smaller/lighter, provided they recieve coolant.
In some ways the MHD-driven-motor concept can be far simpler and more reliable, and also controllable, than the ingenious all-mechanical systems we use.
Against this, first of all are electric motors really much heavier than the alternatives we can actually use? Other methods all require high temperature gas turbines of some kind. J-2S used chamber tap-off to supply the turbine for the pumps, but had to dilute it with extra fuel (hydrogen) to cool it down to ranges that turbines could handle. I would guess that advances in high-temperature turbine engines over the past half century would allow a leaner and meaner version today but we still would take a hit in diverting fuel flow, and another hit in that we are tapping off the main chamber thus weakening it. Gas generators divert a portion of the fuel & oxidant to generate the power-the turbine in that system may well be far lighter than an equivalent power and RPM electric motor, but have you factored in the need for the gas generator, associated plumbing, and the diversion of loaded propellant mass around the main thrust cycle? Could it be that even if gears are included, modern electric motors can be competitively light?
Regenerative cooling recycles nozzle/chamber waste heat one way or another, by the heated propellant (usually fuel) feeding in to pre burners, gas generators, or the expansion turbine.
Gas generators may be very simple but turbines are not and they are especially pretty hard to manufacture given the tolerances (if you want them lightweight).Electrical pumps do have their merits in simplicity even if weight might not be their prime advantage.
There is no way that is true, see the H-1.a. how do you start a MHD-driven-motor? All it takes a turbine engine is head pressure, simple start cartridge, or gas bottle.
Gas generators may be very simple but turbines are not and they are especially pretty hard to manufacture given the tolerances (if you want them lightweight).
Yes, electric motor and pumps would be heavier than any gas generator engine. Rocket turbo pumps are the highest power per mass devices.
Again, nobody has yet fired a turbopump in space, after any significant loiter times.
If this would be a deep-space engine attached to a spacecraft, specifically designed for very long loiter times, you'd have a battery in the spacecraft anyway.
Quote from: savuporo on 09/14/2016 09:43 pmAgain, nobody has yet fired a turbopump in space, after any significant loiter times.Not an issue for the RL10
Quote from: Shevek23 on 09/14/2016 07:23 pmRegenerative cooling recycles nozzle/chamber waste heat one way or another, by the heated propellant (usually fuel) feeding in to pre burners, gas generators, or the expansion turbine. wrong, the heat picked up the propellant is not wasted. Propellant that going into the nozzle goes into the combustion chamber. Propellant for pre burners and gas generators is tapped off before going into the nozzle. Propellant going to an expansion turbine after the nozzle is using the heat energy to turn the turbine. It is not wasted and in fact, this means all the propellant will be burned in the main chamber. That is why this type engines have some of the highest ISP.