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Comment: Re:Not so fast (Score 2) 130

by Rei (#49385153) Attached to: World's Largest Aircraft Seeks Investors To Begin Operation

Most people's perception of how airships should behave from holes is wrong, and it's based on their experience with party balloons. The reason for the differences are:

* Party balloons are pressurized - the skin is stretched taught. The skin on airships are loose.
* Skin area (and thus leak rate) scales proportional to the radius squared, while the volume scales proportional to the radius cubed. Airships are many, many orders of magnitude larger than party balloons. Consequently the rate in which gas can leak out of a hope is drastically lower.

Even large holes in airships don't take them down quickly. Even a moderate sized airship can generally continue flying to its destination and then fix the damage and refill there.

Comment: Re:It is (Score 1) 124

by Rei (#49378241) Attached to: SpaceX's New Combustion Technologies

Wow, this is great to hear - I'd never heard of you guys before. :)

And looking at your site, I like what you're doing even more - direct 3d printed aerospikes? Pretty darn cool. What sort of 3d printing tech are you using? Have you looked into the new hybrid laser spraying / CNC system that's out there (I forget the manufacturer)? The use of high velocity dust as source material gives you almost limitless material flexibility and improved physical properties that you can't get out of plain laser sintering, and the combination with CNC yields fast total part turnaround times.

And you're working on turbopump alternatives? Geez, you're playing with all of my favorite things here.... ;)

What sort of launch are you all looking at - is this ground launched (and if so, do you have a near-equatorial site) or air launched? I'd love to see more details about your rockets, what sort of ISP figures you're getting so far, how you're manufacturing your tanks, and on and on. But I guess I'll have to wait just like everyone else ;)

I wish you lots of success! And even if you don't make it, at the very least you'll have added a ton of practical research to the world :)

Comment: Re:Wrong Focus (Score 1) 124

by Rei (#49378083) Attached to: SpaceX's New Combustion Technologies

Note that it's technically possible to have something like this with a slow reactor; you could for example use steam as a moderator, which will transmit a reasonable proportion of near infrared through it (the hotter you can run your fuel particles, the better transmission you'll get). But not only will you lose some light, but just the simple act of neutron moderation is a very heat-intensive process, meaning big radiators if you want big power (not to mention that the moderator itself for such a slow reactor is also far heavier than the core). The whole point of my variant is to avoid the moderator and avoid the ship having to ever capture anything but incident heat lost due to generation, transmission, reflection, etc losses.

One possibility for a slow reactor, albeit only directly applicable to the rocket mode above, is to have your propellant be your moderator, absorbing both IR and moderating fast neutrons. The fact that it's heating then becomes irrelevant (actually an advantage), since you're dumping it out the nozzle for thrust. If one wanted mission flexibility in such a scenario you could have such a moderator-ejecting rocket mode used to get to orbit, and then switch to retaining the moderator once in orbit and cooling it instead in order to make use of the fission fragment operating mode.

But a fast reactor would obviously be highly preferable so you don't have to worry about a moderator at all. :) I'm just pointing the above out because slow reactor versions have already been simulated.

Comment: Re:Wrong Focus (Score 1) 124

by Rei (#49378039) Attached to: SpaceX's New Combustion Technologies

Wait a minute, no, I entered it right into the calculator the first time around. Argh, this interface is confusing. Radiative equilibrium for Tunsten at its melting point 3300C according to the calculator is 92MW/m. A "cool" 1200C radiative temperature according to the calculator 2,6MW/m. According to the calculator, 10kW/m is about 380C.

Comment: Re:Wrong Focus (Score 1) 124

by Rei (#49375327) Attached to: SpaceX's New Combustion Technologies

The cornerstone of it is the dusty fission fragment rocket, so I'd start there. Another key aspect is the use of a accelerator-driven subcritical fast reactor rather than a critical slow reactor. Lastly it's a variant of a nuclear lightbulb, albeit (as mentioned) without the primary drawbacks of them (containment and radiation blackening of the chamber blocking the light). This latter aspect is due to the spectrum changes of fused silica (I can't find a paper on short notice that shows the IR spectrum, but you can see that for most types of fused silica / fused quartz, there's little loss of transmission on the red side of the spectrum; this holds true but is even more pronounced in the IR range).

Comment: Re:Wrong Focus (Score 1) 124

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) 124

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) 124

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) 124

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) 124

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) 234

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) 447

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".

"Success covers a multitude of blunders." -- George Bernard Shaw