Comments on Rocketpunk Manifesto: On Gossamer Wings

I wonder how the potential for laser-assist system...

2008-06-10T16:41:00.000-07:00

I wonder how the potential for laser-assist systems could affect the costs of a regular shipping run. If you're already using this technology than booster lasers could be useful, particularly if a vessel is out beyond Mars orbit.

"The scenery out the viewports* won't change much ...

2008-06-08T19:36:00.000-07:00

"The scenery out the viewports* won't change much after the first week or so spiraling out from Earth. (In fact you probably ride a connecting bus up through the Van Allen belts.)"

Or maybe there won't be Van Allen belts. See this:
http://www.tethers.com/HiVOLT.html

Wasn't one of the post-Apollo missions, (before th...

2008-06-06T16:09:00.000-07:00

Wasn't one of the post-Apollo missions, (before the Shuttle killed it), a trip to one of the Near Earth Object(NEO's) asteroids? It seems like those would be good prospects for a visit from a solar-electric spacecraft. It would seem to be a low cost way to support an asteroid mine.
Ferrell

If you can get a 2 km diameter Mylar reflector to ...

2008-06-06T09:34:00.000-07:00

If you can get a 2 km diameter Mylar reflector to hold shape even approximately under milligee thrust, you could do both. The reflector doesn't need to be 'good,' because you're not imaging anything, just concentrating light onto a surface a few hundred meters across. The wings themselves are not just flat solar cells, but freznel strips, so in this case you have two stages of concentration onto the actual solar cells.

For solar-electric ships operating in the asteroid belt or out to Jupiter, maybe you just sacrifice some payload to carry a couple of these big parachute reflectors, jettisoning them rather than trying to re-furl the things. Little operational subtlety: The ship rolls 180 to use them, the wing upper side facing away from the Sun, not toward it as in inner-system operation.

Really belonging to another thread, but I just had a somewhat Moon-killing thought. For construction in Earth space, wouldn't it be easier to mine Earth-grazing asteroids? (If they have the stuff we want.)

Getting stuff off the Moon in large quantities requires a costly specialized transport infrastructure - high thrust lunar landers, mass drivers, or an elevator. The two preferable ones we not only have to build, we have to invent.

Earth grazers require less total energy to reach and return. No lift cost, because for a 1 km asteroid escape velocity is walking speed. Solar power doesn't go dark for two weeks of every month.

We always picture asteroid mining only in the asteroid belt, but that is one more way our perception of space was shaped in the rocketpunk era, when only one or two Earth grazers were known, and people just didn't think about them much.

I'm assuming that aluminized plastic is both m...

2008-06-05T21:49:00.000-07:00

I'm assuming that aluminized plastic is both much cheaper & much lighter per m^2 than the solar cells. To maximize the power output from a given area of solar cells you concentrate the sunlight so that what is hitting the solar cells is almost the maximum W/m^2 that doesn't damage the solar cells.

If you increase the distance from the sun you need more reflector material to collect the sunlight & concentrate in on the solar cells to maintain that same maximum design power.

This would mean you are carrying some extra reflector material that is only used far from the sun, but if it is light enough per m^2 you still come out ahead.

My guess is that the freznel lenses work just as w...

2008-06-04T21:08:00.000-07:00

My guess is that the freznel lenses work just as well, because you're already building the biggest collector practical on the mass budget. You're not going to unfurl it more as you move outward. (Does that make sense?)

Carbon nanotubes, if they pan out, look to be the duct tape of space. Always keep a few on hand.

I figured it was the same Bussard, hence my crack about what he did when not inventing fusion drives. He invented the Bussard ramjet long enough ago that it is in the class of techs I've 'always' known about.

The fresnel lens arrangement is rather neat, but I...

2008-06-04T19:35:00.000-07:00

The fresnel lens arrangement is rather neat, but I think movable reflectors would have a major advantage.

Since the solar cell is more expensive than reflective material, you would want your solar cells to always be illuminated with the maximum power that won't cause overheating & degrade the performance. As the distance from the sun of your solar electric spacecraft changes you would need to change the arrangement of lens or reflectors to keep the illumination constant. I think this would be easier with reflectors than the fresnel lens arrangement.

Re: heat engines in space & plumbing. I note that carbon nanotubes are claimed to have thermal conductivity over 10x higher than common metals. Using them in the radiator should reduce the amount of plumbing required.

Re: Bussard. Until he died recently he had been spending the last decade or two developing his polywell fusion device. http://en.wikipedia.org/wiki/Polywell Somewhere I saw an article by him pointing out how useful it would be for space craft if it was developed. BTW this is the same Bussard as in Bussard Ram Jet.

Really it has already happened, because 'spacecraf...

2008-06-04T17:50:00.000-07:00

Really it has already happened, because 'spacecraft' is the real world term. People romantically called Apollo craft moonships, but 'spaceship' may never jump from SF to actual space travel. If interplanetary transports are assembled out of modules on a per-mission basis - even if in practice they sometimes remained "coupled" through much of their service life - they probably won't be thought of as ships.

Likewise, people may travel in space without ever making a space voyage, because 'mission' may become the normal term for any prolonged activity in space, whether on orbit around one body or between two bodies. For a long time to come, surely, all space travel in any orbit will be thoroughly planned, as a 'space mission.'

In fact, note the space usage 'on orbit' instead of 'in orbit.' Something of the connotation of 'on course.'

Maybe 'spacecraft' is a good generic term for any ...

2008-06-04T17:16:00.000-07:00

Maybe 'spacecraft' is a good generic term for any space platform, ship, satellite, probe, ect. Not only is it non-specific, it also has the advantage of being readily identifiable.
Ferrell

Anita - interesting question about dust. Does it s...

2008-06-03T22:06:00.000-07:00

Anita - interesting question about dust. Does it stick, unless repelled as Ferrell suggested, or does it hit hard and pit the surface.

Given the enormous size of the wings, they'll get hit by less micro bits that will punch holes. The beauty is, you can punch multiple holes big enough for Amtrak trains with no significant degradation of performance. No radiator fluid to leak!

Ferrell - On drive types, especially considering that terms like solar/nuke-electric embrace a whole range of electric drives. I generalize them because these decisions don't drastically the nature of the ships, but obviously the details matter a lot to the people who operate them, refuel them, etc.

There's also the wide variety of spacecraft - we are biased toward big interplanetary liners and their military counterparts, the showboats, but only one perhaps fairly small component of space traffic.

Finally, I think 'spaceship' will turn out to be more ambiguous than we think, because in space everything is moving, including space 'stations,' and they can also move under their own power.

What do you call a space platform that spends most of its life in one orbit, serving a non-transport function, but every decade or two changes orbit or even goes to another planet? Station, ship, something in between?

Anita,
I believe a tiny electric charge would ...

2008-06-03T17:39:00.000-07:00

Anita,
I believe a tiny electric charge would keep dust off of the solar pannels. The pannels themselves would be replaced on a regular basis.
Ferrell

I think that for the first century (or less), of i...

2008-06-03T17:34:00.001-07:00

I think that for the first century (or less), of interplanetary travel, many different drives will compete. The ones that demonstate the best performance and cost effectiveness will win out over the others. My granddaughter might side in these ships!
Ferrell

OK re solar powered spacecraft: Isn't there a haz...

2008-06-03T17:34:00.000-07:00

OK re solar powered spacecraft: Isn't there a hazard of the sails being shredded by micro meteoroids or at least seriously compromised?

And if not that, wouldn't there be a build up of dust on the sail, the equivalent of barnacles, thus slowing the craft down over the long haul?

In the inner system, the performance of the two ty...

2008-06-02T21:25:00.000-07:00

In the inner system, the performance of the two types would be the same - even an edge to solar electric, if it is being actively developed while far fewer (if any) nuke electric ships are built.

So long as patrol craft have more coast guard than military missions, most of them may be chemfuel types for local rescue. If someone is in trouble in Earth or Mars orbit, you can probably reach it faster with chemfuel than electric ships of any type that take a week to change orbit.

In interplanetary space no fast emergency response is possible at this techlevel. I don't see why "the spacelanes" would be patrolled at all. Ships on an interplanetary trajectory can't stray off course, beyond that if the drive goes blooey during outbound acceleration it could leave the ship on an orbit to nowhere. In that case the only question is whether any other ship is positioned to make a rescue and then get anywhere itself.

Widespread activity in the asteroid belt calls for some sort of emergency services presence, and that could be a place for nuke electrics, since solar has sluggish pickup out there. But by the time we need 911 response in the asteroid belt, we'll probably have fusion.

Proviso to say that none of this means you can't have a good many nuke electric ships around in a midfuture solar setting. Just be mindful that they're likely to be competing in many roles against a drive with similar performance and much less cantankerous, and fiddle your setting details accordingly.

Solar-electric drives would appeal to commercal us...

2008-06-02T20:20:00.000-07:00

Solar-electric drives would appeal to commercal users for their lack of need for maintanance, and their low operating costs. For military (space guard) ships that need high velocity, nuclear-electric might be preferred. As far as Fusion drives go, the first one will probably use Helium 3, mined on Luna. Whether a direct drive or nuke-electric, fusion drives will be government operated until they become obsolete. And until, or if, nuke drives become as economical as solar-electric, for-profit interplanetary shipping companies will go for solar-electric ships in the inner system.
Ferrell

Jim - the arrangement described by Isaac Kuo invol...

2008-06-02T09:58:00.000-07:00

Jim - the arrangement described by Isaac Kuo involved concentration, but not by a big reflector - from any distance the collectors look like flat panels, but they are actually strip Freznel lenses that concentrate light onto the actual collectors. Nifty, huh?

Nuke space engines, I believe, would have to be much more amped up than sub reactors, for minimum mass, using pretty high-grade stuff and needing more frequent refueling. But what makes me so eager to dump nukes in favor of solar isn't Nasty Fission but nasty plumbing. With solar the whole power generating side has nothing to break or jam, and doesn't even involve high temperatures or pressures.

Even if fabricating the panels is expensive, spacecraft are expensive anyway. Jet planes cost about $1000/kg. Spacecraft aren't likely to be less, and the panels probably no more expensive than lightweight complex plumbing would be.

I don't know this Bussard fusion drive. What is his day job, when he isn't inventing fusion drives? 'Traditional' fusion electric (reactor separate from the drive itself) is no better in plumbing terms than fission electric, because you still have to get rid of so much waste heat.

A 'good' fusion drive, in classical terms, is direct drive, where the burnt fusion plasma is simply squirted out the back for thrust, and you only need to capture enough electric power to keep the fusion cycle running. But if a fusion power reactor is 90 percent efficient - thus only 10 percent waste heat - its heat management problem is comparable to direct drive.

In ship performance, the next quantum leap is about 10 kw/kg power density, with VASIMR style selectable exhaust velocity from about 100 to 500 km/s. (Not necessarily selectable on one ship; drive design may optimize for a desired balance.)

With that drive, you can whomp it up to about 5 milligees for fast, steep orbits in the inner system, or make a prolonged milligee burn to reach high transit speed for outer system travel.

An elegant fusion drive surely blows away solar or anything else for fast travel, though solar electric may be a long term choice for inner system slow freight because it is nearly maintenance free.

A further comment. If Bussard's Polywell fusion de...

2008-06-01T20:21:00.000-07:00

A further comment. If Bussard's Polywell fusion device works with the proton-Boron-11 reaction. That will make the whole discussion moot.

The claim being made is that with that reactor the output would be DC electricity at about 2 million volts with 90+ % efficiency.

Given that sort of performance a fusion powered electric drive would way out perform both solar & fission powered drives. There would be no need for either large solar arrays or large heat radiators.

Bussard also saw a way to use an electron beam powered by his reactor to heat any sort of fluid to run a high thrust rocket with much greater exhaust speed than chemical rockets. Thus making single stage to orbit rockets easy.

A few comments about solar-electric vs. nuclear el...

2008-06-01T20:02:00.000-07:00

A few comments about solar-electric vs. nuclear electric interplanetary spacecraft.

1) something that would tend to favor solar is that aluminized plastic is a lot cheaper per m^2 than any solar cell, so reflectors to concentrate sunlight on the solar cells would allow operation farther from the sun.

2) a point that would tend to favor nuclear, is that IINM radiation in space tends to degrade the performance of solar cells substantially over a decade or two. The reactors in nuclear submarines operate for 30 years without even refueling. Maintenance issues might favor nuclear.

3) I'm not sure what this favors, but the cheapness of a reflector compared to a solar cell has made me suspect that concentrating sunlight to run a heat engine, might beat solar cells for powering anything in space that has a crew that can do maintenance. OTOH that introduces plumbing issues.