Why Not Space? [Op-Ed]

[K. A. Pital]

If fusion reactors turn out to be poorly scalable, what would you use? Stuck with nuclear-electric something? Moon-based lasers to shoot at sails and propel shit whenever you want and wherever you want? *laughs*

All in all, a very good read even if you don't understand most of the physics like me.

I'm not that bad at physics though certainly owing to the fact that higher education had little to do with the physics-related math, most of my physics studies have been own initiative. I'm reading MIT courses on aerodynamics, but haven't found a decent one which helps with space. Atomic Rockets is the best place to go, but there's just so much stuff it simply doesn't cover.

To get the numbers I told you in my last post I used the numbers from Atomic Rockets (which are also similar to the ones I've seen thrown around in other places), that is the Orion's relatively realistic performance.

I read the AR table on propulsion systems and from that table I gathered that the fusion Orion designs are pretty formidable independent transfer systems.

What interferences you were thinking of?

Someone screwing the guidance of the systems in the ground control centers equals lots of dead people or a completely stranded ship if there's a systems failure, right?

It's hard to jam them all due to sheer distance involved, and they can easily overcome any long-range jamming if the ones not jammed relay the message to others with their own short-range comms (likely much stronger than any realistic long-range jamming).

What if they aren't jammed but actually hijacked? I presume them to be fully automatic and ground-controlled, right?

MCF

There's no bottle MCF due to the engineering challenges involved. As even your paper stipulates that MCF is inferior to Orions, perhaps I'll stick with them after all. The fusion-bomb ones.

The issue with ion-engines in general is simple: what gives you the hellacious amounts of power to operate them? Rememeber the formula of the thrust power from Atomic rockets. What engine gives you X performance is irrelevant. That energy for that must come from somewhere.

Flying fusion reactor plants also work, if they can offer the necessary transit speeds.

[Simon_Jester]

I'm not that bad at physics though certainly owing to the fact that higher education had little to do with the physics-related math, most of my physics studies have been own initiative. I'm reading MIT courses on aerodynamics, but haven't found a decent one which helps with space. Atomic Rockets is the best place to go, but there's just so much stuff it simply doesn't cover.

You might contact the site owner and ask him where he gets his information from, then ask them for where to go. Past a certain point the information isn't found in single repositories online, which is a pity.

[K. A. Pital]

It's not the hard physics par se, that's solvable with a textbook and sticking to the hardest possible calculations. It's the minor bits, like where and how to place maneuver engines, how to dispose of waste and collect it. The BIOS enclosed biosphere studies done in the USSR are pretty hard to get, at least with a detailed description of what goes where and how.

Shit, I've never read that much construction books in my entire life, but the materials science is something that takes time to master. Just lots of auxiliary knowledge without which you find it hard to understand what's feasible with what and so on.

[Skgoa]

The problem with Atomic Rockets is that it's only a collection of what SciFi authors (plus a couple of armchair space-generals on UseNet) think. Huge parts are simply wrong. Some of the pages feature 100% pure ass-pulls presented right next to actually realistic stuff - with no inherent clues as to what is what. And on a stylistic level, I don't like that AR constantly bangs the reader on the head with unnecessary(ly complicated) math in lieu of substantial arguments. I forgot which, but one of the pages even has the math show the exact opposite (if you actually apply it right) of what the text is trying to make you believe.
So yeah, it's a great website if you want to have an overview of what other SciFi media has already done and/or want inspiration for your own work but it's not a dependable source.

Absolute fastest? Some kind of fixed external accelerator, e.g. a huge railgun. You would simply launch the interplanetary shuttle from the orbit (or an airless moon) of the planet you start on and right into the railgun in orbit around the destination. Since none of the critical parts have to move, you can build them as big as you want to. It would start to make economic sense from a certain volume of travel onwards.

Uh...the critical parts would ABSOLUTELY have to move. First, since geometry of the transit would keep changing due to planets going around their orbits, at the very least you'll need a way to orient the decelerating railgun to catch the incoming ships. Second, it would need the ability to alter its own orbit for a variety of reasons, like avoiding debris impacts or again orienting the decelerator towards incoming traffic (it has to be perfectly synchronized with the target's arrival to catch it, including position in orbit) - and since it's virtually certain that the ship will need to do correction maneuvers duirng transit (because you'd need to hit a relatively tiny target in orbit around the destination), that's gonna be a huge concern.

And then, of course, there's the problem of heat management which becomes seriously problematic in a structure "as big as you want it to be", especially one that accelerates ships to absurd speeds over the length of the barrel. You really don't want uneven heating from the Sun to deform the barrel ever so slightly while a ship is rushing through at 30 km/s

But storywise, it's a really cool concept. Hell, the engineering difficulties might be mentioned as a way to make your hypothetical civilization look more awesome for having solved them

Yes, of course it would move a little bit. But not nearly as much as having the drive on board would mean. And well, he asked about what I thought was physically feasible. This is. Simon_jester is right, I just put this foreward as an example of what I would see as the ultimate solution, if I weren't constrained by any other factors.

[K. A. Pital]

I asked for a realistic solution, though. However, thanks for the idea, now I have a railgun on the moon (probably works 100% percent, as opposed to coilguns).

[Sky Captain]

VASIMR is a bit of a fraud, to reach Mars in so little time it requires fission reactors with a power-to-weight ratio that makes no sense whatsoever as discussed here.

Well, there have been very little research into developing high power space reactors so it may be difficult to tell what could be possible if more resources were thrown at the problem. For Earth - Mars transit solar power is also viable. With huge ultra lightweighr mirrors concentrating sunlight on highly efficient solar cells it may be possible to reach or exceed required 1kw/kg power to weight ratio. Since the ship would never land and accelerate gently structures supporting those mirrors and mirriors themselwes could be extrmely huge and lightweight.

[Seggybop]

Well, there have been very little research into developing high power space reactors so it may be difficult to tell what could be possible if more resources were thrown at the problem. For Earth - Mars transit solar power is also viable. With huge ultra lightweighr mirrors concentrating sunlight on highly efficient solar cells it may be possible to reach or exceed required 1kw/kg power to weight ratio. Since the ship would never land and accelerate gently structures supporting those mirrors and mirriors themselwes could be extrmely huge and lightweight.

Thin-film solar cells have already been developed that exceed 10kW/kg -- here is a recent publication, although not necessarily the best example. There's little point to bothering with space nuclear reactor research at this stage when solar has already been developed to this extent and lacks fission's political issues. At this power density, even things like SBSP begin to be plausible.

[K. A. Pital]

Wouldn't thin film be damaged by small particles, debris etc.? Let's say we get an ungodly huge solar power collector and an array of gridded electrostatic ion thrusters, the most efficient ion engine type as of now. How massive should the ship be for that to work? I presume the engine grid would have to have occupy a really enormous area to ensure fast transit. Which would make the ship look like plate with a needle and huge "wings" - a thin habitable environment inside, wide and massive thruster array plate and really wide solar collector units spread out from the habitable environment and the plate itself to shorten the powerlines and minimize power loss.

[dragon]

Well, there have been very little research into developing high power space reactors so it may be difficult to tell what could be possible if more resources were thrown at the problem. For Earth - Mars transit solar power is also viable. With huge ultra lightweighr mirrors concentrating sunlight on highly efficient solar cells it may be possible to reach or exceed required 1kw/kg power to weight ratio. Since the ship would never land and accelerate gently structures supporting those mirrors and mirriors themselwes could be extrmely huge and lightweight.

Thin-film solar cells have already been developed that exceed 10kW/kg -- here is a recent publication, although not necessarily the best example. There's little point to bothering with space nuclear reactor research at this stage when solar has already been developed to this extent and lacks fission's political issues. At this power density, even things like SBSP begin to be plausible.

They have been doing a lot of work on RTG power plants still low power but they last quite a while. Did you know that in 40 years NASA has transfered over 1600 space related technologies to the civilian markets which in turn lead to over 30,000 commercial applications of space technology that have entered the consumer market since the 1950s. That allone is makes it worth it to develope the science and engieering to go.

link

[MrDakka]

I'm not that bad at physics though certainly owing to the fact that higher education had little to do with the physics-related math, most of my physics studies have been own initiative. I'm reading MIT courses on aerodynamics, but haven't found a decent one which helps with space. Atomic Rockets is the best place to go, but there's just so much stuff it simply doesn't cover.

The go to book for anything rocket propulsion; had to buy this book for one of my classes and is probably the only textbook I'll keep after I graduate. Its mostly about nozzle equations for chemical rockets, but they can be applied to NTR (probably only solid core) nozzles as well.http://www.amazon.com/Rocket-Propulsion ... 0471326429 As for magnetic nozzles, the theory is the same, but I doubt any of the equations will be.

As for nuclear reactors, NASA did some work with Project Prometheus and the related JIMO craft, which was supposed to use an actual fission reactor and several ion thrusters http://trs-new.jpl.nasa.gov/dspace/bits ... 5-3441.pdf

[someone_else]

I read the AR table on propulsion systems and from that table I gathered that the fusion Orion designs are pretty formidable independent transfer systems.

Yeah, but the fusion ones use significant amounts of handwaving (like how to make the bombs small enough or fully-fusion for example). The difference between that and pulling numbers from your ass is very very thin.

Someone screwing the guidance of the systems in the ground control centers equals lots of dead people or a completely stranded ship if there's a systems failure, right?

They are pretty slow to change course until they are in the final run (under the control of the client's ship), so unless your saboteurs can keep screwed the (multiple) control centers for 4-5 months, the ship is safe.

Anyway, if you want to make it SAFE, you can. It's not horribly difficult to do even now.

If you program them to just receive the coordinates of their client from the ground control ONCE and allow them to home in by automated celestial navigation, which is impossible to jam, then for the final approach (to slam on the pusher plate with a decent accuracy) they home on a beacon on the ship they have to push.

If they are not designed to take any more orders than the first coordinates, and assuming none screwes up the spacecraft's homing beacon (jamming it is not really practical unless you are also onboard), it is completely safe.

But this way you lose lots of the flexibility of the system, like the ability to reuse the unused bots, or clear the way from something.

What if they aren't jammed but actually hijacked? I presume them to be fully automatic and ground-controlled, right?

As above. To go anywhere the hiijacker has to keep control over them for multiple months. Any half-decent government can retake control of the facility in less than a week. Assuming they don't have a bit of redundancy and just shut the captured station down.

It's like trying to steal a oil tanker.

Flying fusion reactor plants also work, if they can offer the necessary transit speeds.

If you miss the inherent stupidity of converting power from a fusion generator to electrical (60% efficiency) and then from electrical to plasma again in a VASIMR-like engine (80-85% efficiency), and ignore the levels of wasted heat you have to get rid by doing this, I think I understand more physics than you.

Besides, kinetic impactors offer any level of performance you want, since adding more "fuel" does not add to the ship's mass nor require idiotic amounts of power generation systems.

Let's say we get an ungodly huge solar power collector and an array of gridded electrostatic ion thrusters, the most efficient ion engine type as of now. How massive should the ship be for that to work?

By using AR's stats for an ion engine (210 km/s Ve and 10'000 newtons of thrust, mass of 400 tons, requires 800 freaking megawatts), and a vessel massing 1000 tons you'd have a murderously high acceleration of a=F/m = 0.01 m/s.
To reach a speed of 50 km/s (which is the very low-end of high-speed transit) you must keep thrusting for 5000000 seconds or around 60 days. Then you have to slow down in another 60 days.
Assuming that a course like that is feasible at all.

[K. A. Pital]

Yeah, but the fusion ones use significant amounts of handwaving (like how to make the bombs small enough or fully-fusion for example). The difference between that and pulling numbers from your ass is very very thin.

I never said pure fusion. However, over 90% fusion is pretty good in my view.

But this way you lose lots of the flexibility of the system, like the ability to reuse the unused bots, or clear the way from something.

Another thing I thought about: you're basically spending lots of ion thrusters or solar sails or whatever you're using to make those things move. Isn't that... not much different from spending lots of uranium?

It's like trying to steal a oil tanker.

Which pirates do.

If you miss the inherent stupidity of converting power from a fusion generator to electrical (60% efficiency) and then from electrical to plasma again in a VASIMR-like engine (80-85% efficiency), and ignore the levels of wasted heat you have to get rid by doing this, I think I understand more physics than you.

I'm not thinking about a VASIMR-like engine. There are other options.

Besides, kinetic impactors offer any level of performance you want, since adding more "fuel" does not add to the ship's mass nor require idiotic amounts of power generation systems.

That's reasonable. Though solar batteries plus sails can also offer lots of power.

Assuming that a course like that is feasible at all.

Nice. Now I just need to design a reasonable on-board power generation system which would allow me to have that much power.

[Sky Captain]

If present high end solar cells can reach 10 kw/kg (with ultra lightweight mirrors concentrating extra sunlight even better power to mass ratio may be possible) then getting required electrical power amounts in Earth - Mars space should be relatively easy. If we assume 10 kw/kg then 1 GW power plant would have mass of only 100 tons and certainly it would be much cheaper than fission reactor with similar performance. Stick it to your best electric rocket engine and you have spacecraft with better performance than small Orion craft and much more cost effective.

[K. A. Pital]

And the scale of such a ship? At least the mirror area that's needed for like 1 GW or so. But hell yeah, this works for interplanetary transport. I would guess also that the farther out you get, the less efficient the whole solar thing would be. So there's still a use for Orions to haul stuff and/or people towards outer planets.

[Sky Captain]

At Earth orbit distance from sun in space 1m2 receive ~1300 W. Assuming 20 % efficient solar cells to collect 1GW would require area ~4 km2. Yeah it is huge surface area, but since it always stays in space and ship likely would accelerate no faster than loaded freight train framework supporting that huge solar cell/mirror could be extremely fragile and light something that would never survive in full gravity environment.

[K. A. Pital]

Hot shit! My ship's almost making it. It's a 700 m (or perhaps 1 km) long, 20m wide "needle" with a "flower" of solar batteries on one side and a massive DS4G thruster block on the other side. I'm now sort of deciding that the main ship body itself is a tad shorter, but the solar power block is long and massive, expanding to a 3-4 million sq. m area like a massive space umbrella. I called them 'solars' and I like the name.

However, for passenger flights to farther planets (transneptunian missions, etc.), especially considering the asteroid belt passage, Orions still run. Solars are, however, the primary passenger transport inbetween Mercury and Mars, possibly reaching out as far as Ceres, and they also can work for Jovian missions, unless transit time is absolutely essential.

Besides, the image of a glittering flower opening up in space is a nice one and much more fitting for the "peaceful spacefaring explorers" image of the Commune than massive nuclear boom ships. Thanks for the tip. The very fact that we could be building ships like that in 10-20 years from now if we'd have a nice heavy booster to lift parts of the ship up to space is quite amazing itself.

I guess though that with the lightweight batteries the civ should also operate a solar plant on the Moon and possibly contemplate one on Mercury.

[PeZook]

Yeah, imagining a ship like that undocking from a space station and unfurling its solar array to get on its way is pretty powerful. It's kind of like a tall ship setting full sail, a very inspiring and poetic start to a long journey.

[HopDavid]

I'm not that bad at physics though certainly owing to the fact that higher education had little to do with the physics-related math, most of my physics studies have been own initiative. I'm reading MIT courses on aerodynamics, but haven't found a decent one which helps with space. Atomic Rockets is the best place to go, but there's just so much stuff it simply doesn't cover.

One of the beefs I have with Atomic Rockets is its delta V tables. The figures are for low circular orbit to low circular orbit.

There are a wide spectrum of parking orbits. In terms of delta V, low circular orbits are at one end of the spectrum and capture orbits are at the other. By capture orbits I mean periapsis as low as possible and apoapsis as high as possible without the sun's influence tearing the space ship from the planet's capture orbit.

Here is a graphic contrasting the delta Vs:



The above delta Vs assume Hohmann transfer orbits between planets. The blue bars are close to the quantities in Atomic Rocket's mission tables.

If a low circular orbit is desired, park in a capture orbit whose periapsis is in the planet's upper atmosphere. Each time the spacecraft passes through the upper atmosphere, velocity is shed lowering the apoapsis. Aerobraking can change capture orbits to low circular orbits with virtually no propellant.

With propellant sources high on the slopes of a planet's gravity well, a spacecraft can depart from a highly elliptical capture orbit. Departing from EML2, Trans Mars Insertion (TMI) can be less than 1 km/s rather than the 3.6 km/s needed for TMI from low earth orbit.

Given propellant high on the slopes of planetary gravity wells, chemical is more than adequate for interplanetary trips. Planetary Resources hopes to park a water rich asteroid at EML2 or high lunar orbit. And there are also lunar volatiles only 2.5 km/s from EML2.

[Skgoa]

Just a little note: you meant "highly eccentric" orbit.

[HopDavid]

Just a little note: you meant "highly eccentric" orbit.

That term misses the mark. If an orbit has eccentricity 1, it's a parabola. Greater than one it's a hyperbola. So a highly eccentric orbit might describe something other than an elliptical capture orbit.

I can see why "highly ellliptical orbit" might seem silly. An ellipse with eccentricity .5 is just as much an ellipse as an ellipse with eccentricity .7.

However "high elliptical orbit" is a common term for an elongated orbit whose eccentricity nears 1. See HEO.

[Skgoa]

Ah, ok. I guess we use the terms slightly differently in German.

[Zinegata]

However, for passenger flights to farther planets (transneptunian missions, etc.), especially considering the asteroid belt passage, Orions still run. Solars are, however, the primary passenger transport inbetween Mercury and Mars, possibly reaching out as far as Ceres, and they also can work for Jovian missions, unless transit time is absolutely essential.

A bit of a theoretical technology here - but in High Frontier (the game, not the book) there are some proposed designs for a power satellite which essentially gathers solar power and then uses a beam to transfer it to a waiting space ship. This makes solar-powered ships viable for Jovian missions and possibly beyond.

Also, been wondering a lot about the possible Venusian Aerostat - there's been proposals for a research station that is essentially a hot air balloon high on the Venusian atmosphere, where the air pressure is relatively earth-like and thus not needing a pressure suit. Would large scale colonies here be more viable?

[HopDavid]

He's pessimistic, mostly pointing out the difficulties (but with some good points about the difficulties). I think the main problem is that his argument is essentially that space colonization is a question mark because of Peak Oil, which I don't find quite as convincing.

Tom Murphy, Chris Martenson et al fancy themselves Paul Revere. They're shouting an urgent message "Peak oil is coming! Peak oil is coming!"

I sympathize. I agree that we should conserve resources and live within our means.

However urgency doesn't give Murphy license to use wrong arguments and bad math. Which is what he's done with Why Not Space? as well as Stranded Resources.

Murphy's delta Vs are wrong. A 1st year aerospace student knows how to patch conics correctly, but not Murphy. He apparently hasn't heard of the Oberth effect. Nor aerobraking. Nor 3 body mechanics as practiced by Belbruno and others. I go into more detail at Murphy's Mangled Math.

Here is a comparison between the straw men in Murphy's "Grab That Asteroid!" scenario vs the asteroid retrieval described by Keck Institute of Space Studies:



Co-authors of the KISS paper include Chris Lewicki, chief engineer for Planetary Resources.

Murphy suggests 5 km/s to retrieve an asteroid. But there are asteroids much closer in terms of delta V. The KISS paper suggests less than .2 km/s.

Murphy suggest lox/methane as the propellant to retrieve an asteroid, this has an exhuast velocity of around 3.3 km/s. Lewicki et al suggest ion engines with an exhaust velocity of 30 km/s.

Murphy suggests a kilometer sized asteroid. Page 15 of the Kiss paper discusses safety. Murphy's kilometer sized asteroid is absurd not only because of the massive undertaking it'd require, but also the safety issues.

The propellant mass between the two scenarios differ by 9 orders of magnitude.

[Terralthra]

Why are people so obsessed with Mars to begin with? Space habitats (like an O'Neill Cylinder would be cheaper and - since they would be human-habitable by design - not require extensive terraforming, either. Put bundles of them at the L4 and L5 Sol/Terra LaGrange points and they're at closest approach, 20 million kilometers out. That's less than half the distance to Mars at the closest possible orbital approach, and orders of magnitude less when Mars is on the far side of Sol.

Mars' orbital radius is roughly 227,939,100 km, eccentricity 0.093; Terra's is 149,598,261 km, eccentricity 0.016; closest approach = 54,570,768 km; furthest approach = 498,418,600 km (though that's the straight-across-the-sun distance, not a useful trajectory). If "something went wrong" on Mars when Terra and Mars were at their actual furthest approach, the best we could do would be to shoot a rocket from Earth in a hyperbolic trajectory skirting how close we could send a spaceship to Sol and then back out to Mars' orbital radius. Without knowing how close we can send a manned ship to Sol, that trajectory is difficult to calculate.

Space habitats make way more economic, pragmatic, and even doomsday-scenario sense. If our concern is getting our eggs out of the Terra-gravity-well basket, getting some eggs into a second basket doesn't really help all that much. We're still bound to planetary gravity wells. Our population is still constrained by a finite surface area. Space habitats would offer insurance against anything short of a stellar catastrophe, and infinite (though bounded-growth over time) living space.

Hell, there's even an asteroid at L4 to get us started with some construction materials!

[PeZook]

Maybe people are obssessed with Mars because building a non-shitty habitat on Mars surface is possible (and more importantly, far less expensive despite the distances involved) with our current technology?

I mean, constructing a space station that will house a thousand people and be able to sustain itself with "local" (ie. orbital) resources is immensely more difficult. For one, the modules have to be pressurized like spacecraft, and we're only beginning to understand the issues with long-term habitation of deep space (did you know the ISS has problems with mutated space microbes eating their equipment?)

Mars on the other hand has gravity, an atmosphere (a thin one, but still) resources you can get to with surface rovers and can be settled by using what are essentially high-tech tents. It would still require massive amounts of technology compared to settling an actually habitable planet, but less than actual space stations.