[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?

[zapkitty]

“Ah, I see…” said the blind kitty 🙂

My cheap and tawdry space pr0n has worked against me.

The VASIMR units are not permanent attachments to the FOOF station.

They represent a multi-megawatt interplanetary drive configuration that someone has sent to the station for testing after the reactor has been proven to be more online than off.

Let us omit the temptation for joyriding that those drives embody.

Delete the drives! 🙂

And instead for the FOOF initial secondary research projects (the primary research project is the reactor itself) we add two important testing and research rigs… one is for improved radiators via a liquid droplet radiator unit and the other is a radiation deflection scheme that uses electrically-powered fields to deflect both solar particle events and galactic cosmic radiation away from the station.

Both are vitally important to the future of spaceflight and both can be tested as soon as a station capable of hosting and powering them is fielded.

(And that isn’t going to be ISS, perhaps not for a decade… if even then. It’s not designed to handle the changes projects like these would entail.)

As before these are placeholders, and are not intended as CGI work for the sequel to Avatar. Do not confuse the playing pieces with the game. Specifically, the LDR rig is an upside down shower head and the deflector antennas are hydrophone emitters swiped from an old project. I’m lazy 🙂

But these changes should help curb the temptation to hijack the station… I hope 🙂

Attached files

[zapkitty]

Why would you build a deep-space ship before building a space station?

Testing a 200kw drive by charging batteries for weeks just to get a few minutes operation gets data… needed data… but it’s not the same as testing the drive at full power for weeks on end.

And that’s the kind of testing it’s useful to do before entrusting lives to those drives for an interplanetary journey.

You can rush off to Mars in your fusion-powered torchship… but you’ll be missing a few vital technologies. Just to start with you’re still going to be hauling that big-ass hot water radiator around. And there’s much more…

We Americans skipped over building a proper space station in our rush to get to the moon before the Soviets and we’ve been paying real and measurable engineering penalties for that ever since.

In-space refueling depots using whatever fuels we choose should have been a fact of life… half a century ago.

The radiators and support technologies for high-powered solar and nuclear systems should be an accomplished albeit still-developing art as well.

And radiation countermeasures for extended deep-space missions should have been topic one rather than swept under various rugs over the decades.

… the first generation of ships assembled in space should have been nearing their retirement age by now…

Instead we’ve finally got ISS… decades late even from its late start, underpowered and understaffed… and it can’t do all that needed research if at the same time it’s trying to keep its ongoing research agreements among the partner nations.

If you want fusion-powered ships roaming the solar system then you need to do your “homework” first. And a fusion-equipped station that has power and room and crew to spare is the key to testing and validating current ideas for such ships and researching new ideas.

This particular station concept is not only a way to try to get fusion on orbit and validated ASAP… it is a neccesary precursor to those high-powered ships you want to build.

We shouldn’t try to skip the basics this time.

[zapkitty]

vansig wrote: ok, brainstorming here…

Yay! 🙂

vansig wrote:
refrigerant
isnt ammonia the more common coolant in space?
i hear that helium would be used to cool the DPF anode?

Ammonia is used in current external ISS radiators because it stays liquid at very low temps. But there is a drawback if you want to operate at higher power levels because ammonia is not quite so good as water at handling higher heat loads (although it’s better than anything else but water at the temperatures and pressures we currently deal with)

In fact the interior coolant loop of the ISS uses water in the hab modules, which dumps the heat to an outside ammonia loop via heat exchangers.

But engineering an ammonia loop to handle megawatts of heat at hundreds of degrees is not properly matching the coolant to the project and results in notable engineering inefficiencies. In fact even with the minimal fusion plant discussed here we’re pushing the limits for unpressurized water… but it’s a well understood tech and is both quicker and cheaper to implement than other, more robust options that will require research on orbit before they can even be used.

We are not escaping the eventuality of liquid metal coolants when it comes to fusion plants in space, but water cooling can keep our initial units running while we hash out the more advanced tech… and a lot of time and money will be saved by having an operational reactor in a genuine space environment ready and willing to test out new cooling ideas and systems.

btw, quite a few proposed nuclear spacecraft designs use water as well… so I’m not exactly apostate here 🙂

As for inside the DPF: I’ve been treating it as a black box that puts out electricity and heat. (also magnetic fields but that is another subject) The heat is supposed to be delivered from the reactor by a helium loop but my own little project stops there. Is the helium stored cryogenic? Is it liquified after heating? Are there sufficient hamsters to run the pumps? I don’t know.

I’ve just been told that a certain amount of heat will be delivered via helium gas at a certain temperature.
(well I presume it’s a gas at that temp, I think someone would have mentioned a high-pressure system otherwise… I hope…)

vansig wrote:
VASIMR
is the shape realistic?
pictures i’ve seen show the accelerators as longer cylinders

It’s a placeholder, not a miniature 🙂

And a serious note: I did the pics because others find them useful but I can’t really see very much of them at one time myself. They are just mass and volume estimations assembled as needed… the veriest epitome of the epithet “lego spacecraft” 🙂

The Bigelow modules use the exterior dimensions and wall thickness given by Bigelow.

The nodes are approximate in size based on the CBM hatches that Bigelow uses on all its modules and nodes. (Imperial NASA strikes back!)

The SLASR arrays use the 2.5x5m 4kw units developed by the SLASR people so they’re in the ballpark at least.

Dragon Crew and Dragon Cargo w/ extended trunk are copyright for all eternity by SpaceX.

The station radiator is an external ammonia loop radiator swiped directly from ISS… I wonder when they’ll notice it’s gone…

The arcjet propulsion module uses 4 commercial ammonia arcjets operating at 30kw for 2.37 newtons each at an Isp of 1012 seconds. The implementation thereof is a figment of my imagination.

The CMG box is something else that the inhabitants of ISS will notice is missing… sooner rather than later, I think…

The ISPR racks are done European style instead of the more curved style used elsewhere… (socialist science racks! 🙂 )

vansig wrote:
xenon propellant tank for VASIMR?

… xenon, argon… as long as it’s something easily distributed through the gas or fluid interfaces of the CBMs it doesn’t otherwise matter. LH2 is right out for the initial tests 🙂

vansig wrote:
if opposing nozzles, then each VASIMR engine would connect at about the centre of mass?

Rather, the center of mass of the axis of the station node that they are mounted on… they can be located most anywhere otherwise. I stuck them next to the fusion radiator.

vansig wrote:
can the VASIMR nozzles be positioned?
if they hinge/twist at or near the station’s centre of mass, then they could be
positioned quickly and effectively for acceleration in any direction

They are there to be tested and measured.

They are not there to be a part of station operations.

This is a good thing 🙂

vansig wrote:
mass of the modules?

Usually less than the mass of whatever is put in them… usually…

vansig wrote:
total station mass?

As above. I will have better estimations later on but this will do for a start. The only certainty is that this station will be quite a bit more massive than what Bigelow envisioned the original Sundancer assemblies as weighing in at, thus my substitution of the high-ISP arcjets for the Bigelow hypergolic propulsion module.

[zapkitty]

…. and I’d thought that FFDPFs had 50% of their watts electric come put as watts thermal…

… which would give your classic 5 megawatt FFDPF an additional 2.5 megawatts thermal (to use, if on the ground… to contend with, if on orbit…)

… is that ratio incorrect? Or has it changed?

[zapkitty]

… btw if the FFDPF can actually be run at a helium outlet temp of 627c that would be very useful to know… 😉

[zapkitty]

*sigh* 🙂

If you must insist on these multigigawatt power levels (which are overkill at the moment for anything but an non-airbreathing orbital launcher) then what you are looking for is external liquid radiators.

The liquid is sprayed directly into space in droplets or sheets and is then collected and reused. The droplets or sheet vastly expand the radiative surface for a relatively small volume and mass budget compared to a solid surfaced radiator.

These actually have had research and some testing done on them but no actual deployment in space.

(Researchers have tried to get test units aloft on the shuittle but it’s another necessary tech that NASA couldn’t quite budget for..)

http://www.5596.org/cgi-bin/dropletradiator.php

… cools 37+ gwt with a “radiator” only 14326 square meters … that’s a triangle with an emitter slot 191 meters wide and a collection point 191 meters from the emitter.

Ah well… Project FOOF disdains such pie-in-the-sky fusion applications 🙂

… although FOOF would be a great place for onsite testing of LDR and LSR prototypes…

[zapkitty]

Either you or I have misplaced a decimal point somewhere…

gigawatts (electrical)
37.95
(gives you .95 gw to actually operate the ship)

gigawatts (thermal)
18.97
(assumes 50% efficiency)

stefan’s
5.67E-008

emissivity
0.85

temp (c)
627
(That’s 900k)

area (m2)
600000
(over 774 meters on a side)

Specific Mass (kg/m2)
10
(assuming a lead/bismuth eutectic for coolant the usual 10kg per square meter seems plausible given advanced construction and economies of scale)

Total Mass (kg)
6000000
(So I make that as 6000 tons worth of radiator… hows your mass margins? 🙂 )

But… the Aero-person said over on talk-polyglennbeck that FF DPFs should run at temperatures of approx 444c so molten lead wouldn’t exactly be a coolant…

Use NaK (Sodium Potassium mix) and run it at 410c… and the radiator size goes up proportionately… the 4th power kills you… (… you were warned about the dangers of reading Perry Rhodan when you were young…)

temp (c)
410

area (m2)
1809000
(over 1344 meters on a side)

Specific Mass (kg/m2)
10

Total Mass (kg)
18090000
(Over 18000 tons of radiator…)

… now if I could only figure out why either of you think 37 gigs electric is needed for interplanetary travel i could die happy…

[zapkitty]

Hmmm… how about something less ambitious?

Let’s try powering a small commercial space station in LEO first.

There you can learn about whatever quirks lie within the system in a working environment of vacuum and microgravity… but you have auxiliary systems on hand in case of teething pains… and Earth is only an hour away by capsule if things go really south on you.

Attached is a rorschach test… see what you can make of it… I don’t see so well myself but everything should be in place…

Attached files

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