Postby 93143 » Thu Nov 27, 2008 11:03 pm
I wonder if it is reasonably possible to use an MHD turbine on a scramjet exhaust in order to obtain large quantities of electrical power at a reasonable voltage?
In a QED engine, you'd probably want it in between the hot hydrogen injection and the electrical heating section, so that it would be operating with a reasonable plasma but wouldn't interfere with the magnetic shielding required downstream of main power input. You could use some of the resulting low-voltage power to run a heat pump to jack the hydrogen coolant temperature up past 2000 - 2500 K, in order to increase the effective Isp without having to run the magrid that hot (the refrigeration power ends up back in the engine anyway), and maybe use some of it to run an MHD or ion thrust system or something in an intake bypass, increasing efficiency the same way a turbofan does (more mass, less velocity).
I have no idea if MHD systems can be made efficient enough, when dealing with hypersonic partially-ionized airflow, for this to be useful. Historically, MHD hasn't performed to theoretical efficiencies... I don't personally like the idea of seeding the flow with metals, but it might help...
...I think I'm just about convinced that expelling hydrogen coolant is a good idea. It avoids having to get a DLC-coated tungsten/CNT composite radiator up to 3500 K or so (nickel-based reverse Rankine cycle anyone?), which is much more difficult materials-wise and results in not only the reactor waste heat but also the very substantial refrigeration power being pretty much completely wasted. This massive refrigeration power would thus be even more difficult to extract from the engine via MHD, since it would be a significant chunk of the total engine power. The combined losses could easily exceed half the reactor output, and unless large MHD pumps and turbines can be made really efficient, this scheme might not end up power-positive at all. Indefinite atmospheric flight at Mach 24 would be nice, but overall I think ARC wins...