Reply To: specific foreign languages ?

[zapkitty]

In honor of Cislunar Pirates Association Day I’m trying to set up an automated DPF-powered tug for you to terrorize the geosynch belt with but the thermodynamics are proving… interesting.

I have a set of four notional 200 KWe VASIMR drives running at an Isp of 5000 with 1 newton of thrust each. I have the engines massing the same as the 100 KWe versions at 300 kg apiece.

In order to cut down on radiator mass I have the standard 5 MWe DPF box operating at a yield of .89 MWe… 800 KWe for the engines and 9 KWe to run the tug. Not only does this decrease the needed cooling, but it should increase electrode life to match the expected time frames of missions for such tugs.

A DPF efficiency of 50% of electric gives us ~.45 MWt but the 40% efficiency of the 4 200 KWe VASIMR units gives us an additional .48 MWt that we need to reject.

So unless I botched the math (always a possibility 🙂 ) we have .93 MWt minimum to reject and I added an additional 5 KWt to reject for misc spacraft thermal output for a total of .98 MWt.

So that gives us a radiator of 188 m2 operating at 300 C. Double-faced it’s about 9.7 m2,

And then the kicker… while I’ve been working under the assumption that the DPF box could run at a temp of 300 degrees C with my water-based coolant, I realized that the VASIMR has at its core a superconducting magnet array that must be cooled by liquid helium at -268 C… and that that helium must be cooled in a separate array after cooling the magnets if it is to be reused.

And the lower the temperature the bigger the radiator… to the 4th power…

The helium could just be vented after cooling but that would put a strict time limit on deployments. A limit based on the capacity of your LHe dewars.

Fun times 🙂

There have been proposals to use helium to cool very high temperature reactors (up to 1000 C) and that would simplify things a little but the mass of the gear needed to move enough helium fast enough through such a system seems likely to be a severe blow to the mass margins of even a fusion-powered craft… and at least some of the helium must still be cooled back down to a liquid state after the high-temperature rejection phase…

… are we having fun yet?

Maybe there’s just not that much LHe needed to cool the magnets… I still have to work through the numbers. But it did occur to me that it would sure be nice if the drive version of the DPF could be implemented sans superconductors…

… but that would still delay out-of-the box implementation of fusion-powered spaceships as opposed to fusion-powered space stations.

Any corrections or suggestions?