DPF for the Icarus Interstellar Spaceship project

[vansig]

Lungs are great! but as heat exchangers, they’re convective, not radiative.

Best emissivity for radiators cannot exceed 1 (a perfect blackbody), so I still get 1 km² area for a 37 GWt radiator at 900 kelvin.
at the thickness and density of household aluminum foil, 16 µm, double-sided, that’s 8 m³ x 3500 kg/m³ = 28t.

To make it useful for interstellar travel for the Icarus project requires bringing the combined mass of the radiators, 37 GW engines, fuel tank, and payload down to, say, 1t. That’ll be quite a trick, even with no radiator.

So once again, the only way to reject the excess heat is to vent plasma.

[Breakable]

Clearly you are not thinking about next generation technologies. I am trying to avoid speculation,
but I don’t think there is a hard limit on radiative output depending on an outside surface area.
You can probably sacrifice efficiency, and use heat-pumps, light-pumps (yet to be invented), light-pathways and make your radiator 3d.
Plasma venting is also an option, but maybe not-necessary.

[QuantumDot]

One way to cooling things that would massively reduce the use of radiators would be optical refrigeration,

“Optical refrigeration (also called laser refrigeration or anti-Stokes fluorescent cooling) is a technique for cooling a macroscopic crystal (or a piece of glass) with a laser beam. The crystal must be doped, e.g. with ytterbium or thulium ions, which are excited by the laser beam. The laser wavelength is chosen such that it is longer than the average wavelength of the resulting fluorescence. This means that the energy of the absorbed photons is lower than the average energy of the emitted photons, so that energy is removed from the crystal. Of course, it is essential that the quantum efficiency of the fluorescence is high, and that nearly all fluorescence light can leave the crystal without being absorbed, e.g. by impurities: a single absorbed photon would offset the cooling effect of many other photons.

Cooling a piece of ZBLAN glass in a “laser fridge” from room temperature down to 208 K has been demonstrated [2], and 155 K have been achieved with Yb:LiYF4 [6]. In theory, even temperatures of the order of 77 K (liquid nitrogen) should be reachable. Certain ytterbium-doped crystal materials, particularly tungstates such as Yb:KGW = Yb:KGd(WO4)2, appear to be suitable for this purpose.

Possible applications of laser refrigeration are the replacement of Stirling coolers and the like (avoiding moving parts, vibrations, etc.), but also radiation-balanced lasers, where the internal heat generation is essentially compensated by optical refrigeration.

It is instructive to consider entropy changes associated with laser refrigeration. The reduction in thermal entropy of the cooled device is more than compensated by the increase in entropy which arises from the conversion of narrow-band focused laser light into fluorescence light, which has a much higher entropy due to the many spatial modes and different frequencies involved in the emission.

See also the article on laser cooling, which deals with the cooling of microscopic particles, rather than macroscopic samples. The physical principles behind such cooling methods are rather different from those of optical refrigeration.”( http://www.rp-photonics.com/optical_refrigeration.html )

other sites to look at are
http://www.physorg.com/news183987251.html
http://encyclopedia2.thefreedictionary.com/Optical+refrigeration

[vansig]

I’d so-much like it to be true, but those laws of thermodynamics do get in the way, sometimes.

Even with next-generation technologies, we could probably only reduce the radiator’s mass to 22g per m². It would be a magic black cloth, having .999 emissivity, strong, flexible, lightweight, and double sided, and having a vascular structure, inflated with refrigerant. but it doesn’t radiate better than a perfect black body. So there must be 1 km² of it.

Laser cooling is a nice idea, but unless the lasers are separate from the ship, (strewn like breadcrumbs along the path), they add to the overall heat more than they take away.

[QuantumDot]

Pyroelectric and thermoelectric devices could be used to take some of the heat away and convert it to electricity.

[QuantumDot]

http://www.msnbc.msn.com/id/26294999/

Check out this spacecraft skin

i guess that it would need a combined solar sail/ radiator so could reduce the power level.

so a DPF could be used for additional propulsion for lasers for the solar sail as well as direct thrust and isn’t a DPF suppose to be 70 percent efficient not 50.

[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]

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

[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?

[vansig]

these are great ideas. structural forms for the ship should be multi-purpose. the skin can double as a whipple shield and radiator, if the vascular structure for the refrigerant has lots of redundancy and failsafe valves. we should note that micro-meteors at .084 c will conceivably do a tremendous amount of damage; and everything has to operate for 50 – 100 years.

now, is there any way to make thrust scale better than linearly with mass?

in terms of scaling, 7500 five MW electrodes, that have to be replaced every 90 days, is about the same as 2500 fifteen MW electrodes, that have to be replaced every month, unless worn parts can be repaired/refurbished aboard ship.

[vansig]

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

To prevent it from being vapourized, the anode must be kept cool. Beryllium melts at 1278 °C. If you can eliminate ablation, you can operate at higher temperatures for longer.

[vansig]

zapkitty wrote: …. 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?

If i remember correctly, Eric said we can expect ~50% system efficiency. Extraction of electric power from the fusion products and xrays is ~80%, but you must factor in the yield, and subtract the power needed to run the next shot. That’s for electricty generation.

For propulsion, the alpha beam and unburnt fuel goes directly to exhaust.

[Augustine]

Why would you want to carry the weight of 37GW worth of DPPF’s to the star of your choice? If you can make electricity cheaply from fusion then make antimatter and use that to power your spaceship as the thrust to weight ratio is probably notably better.

For me I would settle for a DPF/VASIMR powered ship visiting Ceres or doing an Apollo 8 flyby of Jupiter. Barring these developments, I would like to be able to tell my daughter that I remember a time before fusion power when people used to argue over so many silly things that were solved when power became cheaper AND cleaner.

[vansig]

Seeing as no one has, as yet, found a way to make or contain antimatter in sufficient quantity to use it for propulsion, let’s plan to go with something that has a chance of being feasible sooner.

[QuantumDot]

vansig wrote: Seeing as no one has, as yet, found a way to make or contain antimatter in sufficient quantity to use it for propulsion, let’s plan to go with something that has a chance of being feasible sooner.

That may not be true. people have been able to make fairly large abouts of positrons with a petawatt laser, and some think or claim to be able to store positronium with is a positron electron pair orbiting each other for years instead by using electromagnetic fields to increase the size or there orbits so that they don’t hit one another. And with the recent production of a BEC made of positronium the accusal handling of it to do work is improving.

But its still all between 4 and 2 in the technology readiness level, and none of it is efficient or ready for practical work in space.

production of positrons
https://publicaffairs.llnl.gov/news/news_releases/2008/NR-08-11-03.html

since the dpf would produce a beam of charged particles you could place undulators there instead of to convert it to a free electron laser and maybe you could then used chirped pulse amplifiers to but it in the petawatt range

storage of positronium
http://news.nationalgeographic.com/news/2006/05/0504_060504_antimatter_2.html

bec positronium
http://www.physorg.com/news191868695.html

other sites
http://thefutureofthings.com/articles.php?itemId=33/64/
http://www.centauri-dreams.org/?p=11444
http://atomicrockets.posterous.com/?tag=antimatter

[Author]

Posts