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Comment: Re:Wrong Focus (Score 1) 115

by Rei (#49372263) Attached to: SpaceX's New Combustion Technologies

Used an online calculator earlier but clearly I had entered something in wrong last time because the results it's coming back with this time are different (and much lower). Tungsten could radiate around 10kW/m around its melting point. Graphite could do 14,5kW/m at its sublimation point. Hafnium carbide, 17,2kW/m at its melting point (though ceramics are brittle and probably not suitable).

An ideal near-term radiative solution for minimizing mass in this regard would involve a working fluid in carbon tubes carrying a thermal fluid out to carbon radiators.

There's also radiator concepts that don't use solids at all - various kinds of droplet radiators.

Comment: Re:Wrong Focus (Score 1) 115

by Rei (#49369195) Attached to: SpaceX's New Combustion Technologies

How exactly are radiators that can radiate tens to hundreds of kilowatts per square meter supposed to be mass-prohibitive but solar panels that generate a couple hundred watts at best per square meter not mass-prohibitive? Okay, they're not exactly the same, solar cells are inherently going to be heavier than whatever minimum thin aluminum sheeting is needed for radiating, but the heat pipes leading up to it will be heavier than solar power booms... regardless, I can't see how solar wins this competition.

Comment: Re:Wrong Focus (Score 1) 115

by Rei (#49369185) Attached to: SpaceX's New Combustion Technologies

I assume because sunlight is only 1kW/m at Earth, less at Mars, and of that you only capture a few hundred watts (using very good, ridiculously-expensive spectrolab cells, otherwise only 150-200W or so, assuming full coverage), and space-borne solar panel booms aren't as light as one would desire? If you envision thermal radiators in place of solar panel booms, which can radiate a *lot* more heat per square meter than the couple hundred watts of a solar panel boom, then you can see how a nuclear reactor has the potential to have a much better power/mass ratio where cooling is the reactor's limiting factor (which in most cases it's expected to be)

Comment: Re:Wrong Focus (Score 1) 115

by Rei (#49369123) Attached to: SpaceX's New Combustion Technologies

My personal "dream rocket" is to combine a dusty fission fragment rocket with the nuclear lightbulb concept. You have a subcritical fast dusty core which achieves criticality via a spallation neutron source rather than a moderator, using a compact linear accelerator powered by the reactor's fragment deceleration grids (no Carnot losses). The core radiates intensely in the mid-IR range. The core is suspended electrostatically in a fused silica chamber, which while it will steadily blacken in the visible from neutron radiation, is resistant to blackening in the infrared, and can tolerate quite high temperatures. Outside of the core are mirrored aluminum walls. The particles of nuclear fuel in the core being a fine dust, their ability to radiate quickly is extreme; if the process is designed suchly that they tend to radiate and absorb in different bands (a strong reverse greenhouse effect) then you can have ridiculous optical power output despite the radiative temperature only being in the infrared.

Such a craft could operate in several different modes.

1) Clean airbreathing: Air is shunted into the engine between the transparent chamber and the reflector. "Starter" microwave beams (powered by the deceleration grids) help ionize a thin sheath of air to plasma, making it more opaque to IR, allowing it to heat even more, generate even more plasma, absorb even more IR, and so forth. The superheated air exits out the rocket nozzle.

2) Rocket: Hydrogen or other fuel is shunted in instead of air; otherwise, the process is exactly the same. #1 and #2 can be hybridized, and also get a little more boost from any combustion that occurs in the process.

3) VASIMR-like: Only a low flow rate of fuel is injected. The low flow rate and high degree of ionization allow it to reach a much higher temperature and be directed out of a magnetic nozzle rather than being in contact with the physical nozzle.

4) Fission fragment rocket: The bottom of the core is opened up and fission fragments leave the rocket freely. This is of course dirty and low thrust, and would only be useful in space, but would yield absurdly high ISP while still achieving thrust levels comparable to today's ion engines.

5) Photonic rocket: If you want to go really extreme, you could simply just radiate the intense IR beam from your core running as hot as you can get it without melting the silica chamber or mirrored reflector. But I'm not sure if you'd actually get better performance, as you wouldn't be tossing your waste (thus lightening up the craft), and 3/4ths of the energy is already in the fission fragments. On the other hand, if you're willing to accept even less thrust, the simple decay of any short-lived isotopes inside the core will provide some thermal output even when your reactor is not engaged.

Another neat part of this is that being a fast reactor, it could breed its own fuel. So mined natural offworld uranium or thorium could be purified and milled into appropriate dust and then injected into the reactor; with time it'd breed into the fuel needed to power the craft. No need for offworld centrifuges or anything like that. Another capability would be to work around the anti-nuclear crowd on launches: if you face too much opposition you could launch your craft loaded non-fissile fuel, just natural uranium or thorium, and then mount it to a (very) large space-borne solar power source. You could then breed your fuel in space using the craft's linear accelerator. Of course, it'd be far better to just load it with fissile fuel on earth and then ascend in airbreathing mode.

A fission fragment reactor is expected to produce no waste when operating in fragment rocket mode excepting what fragments you decelerate for power generation. When operating as a closed system (with all fragments decelerated), the waste will still be low, as with any fast reactor, assuming that fragments are decelerated in an area well exposed to the core's neutron flux.

This is not the only "nuclear lightbulb" concept, but it avoids the problems with all of the others. It uses a practical, proven way to keep the fuel from contacting the "bulb" (electrostatically repelled dust) rather than a lot of hand-waving, and neutron blackening is not a problem due to the use of IR rather than visible or UV light. Dusty fission fragment reactors have been researched and simulated; however, that which was simulated was a slow reactor with a water moderator, not a fast subcritical reactor. So I can't say how well that aspect would play out. Also I've done no simulations on the rate of absorption of air or various fuels to absorb the IR on their way out of the rocket. I have little doubt that some configuration would work in that regard, but it's not something I've calculated out.

Comment: Re:Wrong Focus (Score 1) 115

by Rei (#49369043) Attached to: SpaceX's New Combustion Technologies

Not true. Look up MPD thrusters. The thrust to weight ratios are incredible, the only limiting factors are cooling rate and power supply. If we're proposing an "infinitely powerful battery", then that takes care of the bigger challenge. A MPD thruster with such a battery and, say, an isotopically pure diamond radiator, could conceivably lift off from the surface of a planet.

Comment: Re:WIMPs (Score 1) 230

by Rei (#49366735) Attached to: Dark Matter Is Even More of a Mystery Than Expected

In fact all forces should get weaker with distance faster in an expanding space than in flat space.

That seems like quite an assumption on your part, if I'm understanding you correctly. We can't just assume that all properties of spacetime are scaling evenly - if they did, then we'd perceive no effect at all.

But perhaps I'm misunderstanding you.

Comment: Re:Still photos (Score 1) 412

by Rei (#49366707) Attached to: Why the Final Moments Inside a Cockpit Are Heard But Not Seen

Some pilots would probably still want the ability to override the limits in an emergency if they feel that they can handle the situation better than the autopilot (for example, if the plane is crashing and the pilot wants better control over where/how to bring it down). If so, then you should make it a possibility to disable the limits, have it such that only *ground* can disable the limits. This would of course impose a delay, but at least overriding the limits would remain a possibility.

Of course, a pilot may try to trick ground into disabling limits (such as pretending to be going down or pretending to have a malfunction), so ground would need as much data as possible to assess whether the situation is legit. Might be tricky... best would be to err on the side of caution and only remove limits if everyone is absolutely sure that this is appropriate, if there's any doubt the answer should be "no".

Comment: Re:Wrong Focus (Score 1) 115

by Rei (#49366605) Attached to: SpaceX's New Combustion Technologies

Not today. But maybe in the future. If you can develop a crazy-power-dense energy source and cooling system, you could probably do it with a MPD thruster. The research I've seen on MPD thrusters operating in pulsed mode yields crazy output relative to the mass of the thruster. But you can't run it continuously because it'd overhead and take way too much power. But who knows about the future? There's the potential for extreme heat conductors like isotopically pure diamond, maybe a some kind of fission fragment reactor with a deceleration grid for power...

(of course, if you have a fission fragment reactor, at least when you're in space itd be best just to jet your fragments rather than use them to power a MPD thruster...)

Comment: Re:It is (Score 5, Interesting) 115

by Rei (#49366565) Attached to: SpaceX's New Combustion Technologies

I hope they simulate propane too, not just methane. Propane has some really interesting properties as rocket fuel but have (like methane) never gotten much research. But now there's a big rush to research methane as fuel based on the concept of generating it on Mars - so propane still gets left in the dark.

Methane's ISP is only very slightly better than propane's - 364,6 vs. 368,3 at a 100:1 expansion into vacuum and 20MPa chamber pressure. But propane at around 100K (note: not at its boiling point, 230K) has far higher density (782 kg/m^3), closer to that of room temperature RP-1 (820 kg/m) then that of boiling point methane (423 kg/m^3), which reduces tankage mass and cost. 100K propane's ISP is of course better than RP-1's 354.6 in the same conditions as above. Plus, its temperature is similar enough to your LOX that they can share a common bulkhead, which reduces mass further and simplifies construction.

Hydrogen generally is the easiest fuel to synthesize offworld. Methane is generally second, and propane third. Hydrogen is often rejected as a martian fuel because of the tankage and cooling requirements. Methane can be kept as liquid on Mars with little cooling in properly designed reflective / insulated containers - but so can 100K propane, in similar conditions, but with significantly smaller tankage requirements.

It seriously warrants more research, I tell you what.

Comment: Quote (Score 1) 147

by Rei (#49363093) Attached to: Russia Wants To Work With NASA On a New Space Station

"We are pleased Roscomos wants to continue full use of the International Space Station through 2024 -- a priority of ours -- and expressed interest in continuing international cooperation for human space exploration beyond that. The United States is planning to lead a human mission to Mars in the 2030s, and we have advanced that effort farther than at any point in NASA's history. We welcome international support for this ambitious undertaking.

Yeah, that's basically "Go F* yourselves" in diplomatic speech.

And as it should be.

Comment: Re:WIMPs (Score 1) 230

by Rei (#49360117) Attached to: Dark Matter Is Even More of a Mystery Than Expected

One thing that dark energy can't be is *all* fundamental constants, plus position, velocity, etc scaling up evenly. Because if such was the case then there would be no perceptible change.

If youe saying that for example what is ground state would change too then it seems like you're arguing that things at the quantum level *aren't* moving into higher energy states. But things at the macroscopic level absolutely are moving into a higher energy state. So are you arguing that dark energy doesn't act on the quantum scale? I find that difficult to accept if so.

Comment: Re:WIMPs (Score 4, Interesting) 230

by Rei (#49358535) Attached to: Dark Matter Is Even More of a Mystery Than Expected

That the thing about dark matter... it has a perfectly reasonable explanation (WIMPs). It's not that weird of a "thing".

Dark energy on the other hand, that's just WEIRD ;) It doesn't act like any "energy" as we know it, even though everything is clearly moving into a higher energy state. A question I've had for a while... if space itself is being inflated (or any sort of mathematically equivalent scenario) - everything inflating in all directions at all scales - wouldn't there be some sort of weak radiation signal from electrons expanding into a higher energy state due to dark energy and then collapsing back down? But I have trouble picturing how to reconcile an absolute, varying distance at the atomic scale with quantization of energy states, positions, etc...

All great ideas are controversial, or have been at one time.