The first step; into orbit.

[chrispenycate]

The longest journey starts with a single step; and the first step for the journey into space is the one from the Earth's surface to orbit. Living at the bottom of a gravity well with a nice, thick atmospheric blanket protecting you from radiation, flash freezing, micro-meteorites and suffocation might be a good idea for an organism, but it sucks when you want to go for a really long walk.
I've prepared a list of possibilities for that first step: multistage chemical rockets, single stage to orbit chemical rockets, space-planes, laser launchers, Orion drive, MHD plasma drive, space elevator, orbital Ferris wheel, single pulse takeoff, explosive follower, or waiting for a new physical principle or technology, antigravity would be a good one. What did I miss?
1) Multistage rockets: pros - we know how to make them work. They're practically a mature technology, now, with the number of satellites, moon flights and the like they've done.
cons - incredibly inefficient; throw away technology, expensive at all levels, ninety plus percent of the energy goes into accelerating fuel in the wrong direction. The shuttle, despite its reusable bit, suffers from many of these faults.
2) SSTO: using ultra-light materials and high efficiency motors, it should be possible to get to orbit in one jump
pros - considerably cheaper than multistage systems
cons - close enough to the limits of materials science that we know that payloads will be minimal for some considerable time
3) Space planes. Instead of fighting the atmosphere, you use it, at least at the beginning of the journey, for both lift and reaction mass. A relatively conventional hypersonic jet aircraft flies into the upper stratosphere. There, it either: a)starts feeding oxidiser and reaction mass into its jet engines, effectively converting them into rocket motors, b)lights up a second set of motors designed to work in space or c)separates into two parts, one of which, rocket powered, continues to orbit, while the other flies back to refuel for the next flight
pros - most of the technology is a direct linear development from existing aerospace or military material. Payload is decent, fuel economy far better than existing systems. Probably possible to use existing facilities for take off and landing (though, for fuel economy and noise reasons I'd like to catapult it into the air using a giant linear accelerator)
cons - though the system is acceptably cheap to run when it is built, development charges are high. For thermal reasons, the plane is relatively heavy, so accidents would
4) Laser launchers: If you apply energy to a lump of air correctly, you can make a jet engine. This doesn't necessarily involve burning fuel; the energy could be applied from the launching ground, in a laser beam (no, lasers don't obey the inverse square "law". In fact, that only holds for spherical wavefronts, essentially omnidirectional, while a laser is practically a plane wave). When there is not enough atmosphere to produce thrust either you pour in reaction mass (not fuel, it could very easily be water) or vaporise the engine itself. This is not a reusable system; you use the capsules as building material or counterweight mass. I suppose you could parachute down in one, in an emergency; not that much worse than an Apollo capsule.
pros - if a source of cheap energy; maybe fusion power, maybe something I've not even thought of - is available, the capsules can be made cheap and fast, Not really suitable for putting humans into space, they could lift quantities of supplies, or the bits to build your space elevator.
cons - This is an energy hungry system. Some power will be lost in the atmosphere, some will be reflected back (it's probably an infra-red molecular laser, so this will only cause minor fires) and multi gigawatt lasers , finely controlled (precision control is vital) are an obvious target for terrorists; they could melt the wings off a conventional airliner.
5) Orion drive: The basic idea is a very solid heavy plate beneath which one explodes a nuclear bomb. Atom bombs in the original design, I proposed hydrogen bombs for launching the counterweight for the space elevator. When the thing is in motion you drop another bomb to give it another kick, and so on until it achieves orbit. Any living quarters had better be well sprung, with good shock absorbers
pros - we have the necessary power source on hand, and it's pretty well certain that the system would work. Payload could be enormous, although multiple flights are inadvisable.
cons - Plutonium is nasty stuff, and it would put a fair amount of pulverised plutonium into the atmosphere, to drift with the winds. An accident with one of these things could be the biggest man-made catastrophe in history. It would definitely be a bad idea to have a lot of these things taking off. The mass which finishes in space would be quite radioactive, so difficult to use for construction purposes.
6) MHD plasma drive; controllable fusion is practically a necessity for this to work. Lots of power in a portable format, If done right it can protect the vessel from atmospheric heating as well as propelling it. This is the only system considered which can achieve ground to orbit and in-system
pros - the best "S.F. story" drive.
cons - relies on a "portable", extremely high power energy source. For the time being, I can't see one on the horizon; fusion experiments are towards "huge, immobile" installations, antimatter in nanograms and beamed power (à la Waldo)? You might as well use the laser launcher.
7) Space elevator, orbital tower or beanstalk: We've talked about this one a lot; a piece of string dangling from geostationary orbit, up which you run rails and send vertical trains, handing spaceflight over from NASA to AMTRAC. A bit like an upside-down pyramid in some ways; the place where it needs the maximum tensile strength is where the weight considerations are the least stringent. It has to be built from the middle( geostationary point) inward, and outward,to keep the centre of gravity in place, and thus gives, free of charge, a slingshot for launching spacecraft further out.
pros - Obvious, when built. Efficient, cheap to run, the safety considerations reasonable (in case of accident or sabotage only the cable would wrap itself round the equator; devastating, to be sure, but not in the same league as dropping the weight) and easy to control who was going where, the ultimate bungee jump…
cons - Needs an enormous counterweight at the geostationary point, which has to come from somewhere, either in one big hunk (flown in from the asteroid belt or lifted by my orion project, from Earth or the moon) or stuck together out of smaller bits; linear accelerated from the moon, old laser-launched capsules, dredged junk from Earth orbit, captured comet fractions… All that's necessary is mass, but it would be convenient if a significant fraction of it were potential construction material for the tower itself. Would any government on Earth be happy with engineers juggling a multi-megatonne hunk of matter into a stable orbit, where every nudge is a nuclear explosion and a difference of sign could eliminate all higher lifeforms?
8) Orbital Ferris wheel: When your giant orbital habitat is rotating, to produce simulated gravity, you add long arms that radiate out from it. It doesn't matter much if they're cables or supposedly rigid; they're tens of kilometres long, and any girder will flex. The ends of these arms dip into the upper atmosphere, and a space-plane can fly up, match velocities, and hook itself on, to be yanked up to a higher orbit.
pros - Efficient, and once built, cheap to run. Elegant, too, and engineering-wise, a considerably lesser problem than the elevator.
cons - every space-plane lifted, even atmospheric drag slows both the rotation of the station and its orbital velocity by a tiny fraction, so it requires continual "recharging" (to be expected; you don't get anything for nothing) But it's not just energy we need to add - that could probably be obtained through solar panels - but momentum, and that means lifting mass up to it and squirting it out, pushing it up continuously, if only by small amounts. This is important; that close to the atmosphere, if the station itself ever got appreciable atmospheric drag, it'd heat up, like a shuttle re-entry ('cept that a shuttle's designed for it) before plunging to Earth in a simulation of "the night the dinosaurs died" Inconvenient, to say the least. But humans have a tradition of letting things slip, from time to time (wars, dark ages, other minor inconveniences) and a million ton Damoclean lump…
9) Single pulse takeoff: Shades of Verne and the Baltimore gun club; put in all necessary energy at the bottom of the well, not merely enough velocity to get it to orbit, but to do so despite losses due to atmospheric friction. Not proposing explosives, evidently, but electric energy, a Laithwaite linear accelerator; one proponent of this method suggested suspending the final coils from lighter than air craft, to economise some of the worst of the atmospheric friction, or building a mountain higher than Everest on the equator.
pros - All engineering in already studied environments. Can be run continuously, putting large (if molten) masses up for construction.
cons - Inefficient, noisy, requires enormous quantities of energy (only the multi-stage rocket is comparable) not elegant.
10) Explosive follower: We're good at guns, right? What if, instead of carrying the explosive fuel up in the rocket, we shot it up from a cannon (well, actually several cannons, with very sophisticated fire control and aiming) Removes all the first stage fuel from the equation, and all that would separate and fall to Earth would be a very battered bullet-proof plate
pros - Beating swords into, if not ploughshares, at least leaf springs. Easy enough to have backup capability in case of breakdown. All your servicing done in a known environment.
cons - It's silly - would you go up in a clay pigeon? It's not particularly energy efficient (each explosive shell needs to be accelerated individually); as the thing rises you're aiming at where it's not yet got to, because of the time the shells take to get there, and the slightest error in prediction, you can't compensate for because your feedback's too slow
11) Waiting for new technology: Well, something will doubtless come along; if not antigravity, a new energy source, or teleportation, or - I don't know. If Idid know, it would be on the list. But the question is, "when?"
pros - given time, enough time, a better technique is bound to appear
cons - that argument holds for never doing anything; if antigravity arrives, it'll have its problems, perhaps we should wait until…until… until a hunk of orbiting rock puts civilisation back into the hunter/gatherer phase, and requires thirty thousand years to get back to where we are now.

[SpaceShip]

Wow! Chris, well I certainly wouldn't know if you missed anything - where do you get it all from?

I liked the idea of the orbital Ferris wheel - that sounds great fun!

[chrispenycate]

Since "discussion" seems here to be me giving a lecture and saying "any questions" to a bemused public, I'll post this on the same thread as the previous part.

The second step; within the solar system

While our first step was mainly involved with energy, the second goes with time, and momentum. There's no easy way to climb the Earth's gravitational slope, but, once out, you can get to the planets without too much effort, if you're not in a hurry. The trouble with this is; decades to get to Mars, centuries to Jupiter and after millennia of travel, you're still inside the orbit of Neptune. This is not generally acceptable. The problem is, with action and reaction equal and opposite, if you're to go forwards, something has to go backwards, and, if this something is not to be as massive as you, considerably faster. Carrying enough of this something to accelerate you up to the requisite speed, then slow you down at the end of the trip, is a major problem, particularly as the non-expended part of this mass has also to be accelerated and slowed (assuming you want some for going home), while delivering supplies from outside is considerably more difficult than in our Earth orbiting exercise.
Various suggestions: gravity sling, solar sail, ion jet, plasma jet, photon drive, traditional rocket, magnetic sail, nuclear pulse, linear accelerator, atmospheric braking, eddy current braking, Bussard ramjet, waiting for new technology.

Gravity sling: doesn't in fact add any energy to a craft; what you gain going into the gravity field you lose coming back out. But, turning a corner is acceleration, too, and that's free, while any drive applied deep in the field ( like a quick bomb explosion) has its effectiveness multiplied. So, choosing your path so it has a planet where you want it, when you want it there, is likely to be a major consideration as long as fuel economy is important.

Solar sail; Energy has mass. Radiant energy, from the sun or a laser, is moving at light speed. Therefore it applies pressure; just not very much of it. If you could build a big enough, light enough membrane, and attach a very lightweight ship to it, it could accelerate out from the sun (yes, at maybe a thousandth of a g, but it keeps right on accelerating, and one centimetre per second per second adds up to 2,500 Km/hr in a day, for free, without using any reaction mass. The sails are big (huge, continent size), you have to plan your manoeuvres long in advance, "tacking" back into the system is hard enough that it's probably worth building up the maximum possible velocity outwards, then furling the sails and using a slingshot technique to bring you back. Slowing down (on the way out) can't be done wit this drive, landing is unthinkable. All in all, it seems better adapted to automatous mechanisms than human beings.

Ion jet: A stream of charged particles are accelerated up to relativistic speeds by something resembling a terrestrial particle accelerator, or the electron gun from a very large cathode-ray tube (television tube before the introduction of flat screens) The amount of mass actually used is small, and, despite the high ejection velocity, accelerating forces are low, but, like the light sail, it can be run continuously. Energy might be solar, or nuclear, or something else (no, not hamsters in a wheel; we still need a fair amount of energy) It uses mass extremely efficiently, and assuming a way to dispose of the steadily growing electric charge can be found, is a serious contender for slow interplanetary travel. I believe a small, low efficiency (uses heavy nuclei, and doesn't drive them very fast) ion drive is at present pottering round the sola system; this might well give us some ideas about the practicality of the system)

Plasma jet (torch ship): Much less efficient than the ion jet, it's still far more powerful. A fusion reactor vents its waste products, channelised by immense magnetic fields, back behind it. The velocity depends on the temperature, and should be as high as possible. It is extremely inconvenient for another ship, a space station or a research habitat to find itself in this exhaust stream. Astronomers complain that they might as well be on Earth with all the light pollution and radio noise generated. Even fully fuelled, they can probably pull half a g (although in that case, they won't be fully fuelled for long). Steering is a long, slow process, like oil-tankers; none of your right-angle turns here.

A photon drive can either be a "solar sail" equivalent, normally driven by fixed lasers or the ultimate version of the ion drive. If the efficiency of a reaction drive increases with exhaust velocity, then the highest efficiency is reached when the mass is emitted at light speed, ie. as radiant energy. We can't transform matter into energy at present, and it's probably just as well, giving our record: a beam of a gram or so of energy could be used for slicing planets. Still, it'd be a good trick if you could bring it off.

Traditional rocket: We know how this works; aim the craft at where you expect your destination to be when you get there, give it a big push, then freewheel for however many months are required. braking done by locally available methods, which (hopefully) leaves you enough fuel to get back (Unless you're a probe, in which case you don't plan on getting back)

Magnetic sail: a technique (which looks more like the rigging than a sail) of putting part of the inertia into a planetary, or stellar, magnetic field. Requires good superconductor technology, and only works close to the requisite celestial body (you thought inverse square dropped off fast; magnetic fields fold in much faster). so a lot of time freewheeling here, too.


A nuclear pulse rocket is the spacegoing equivalent of our Orion drive; you chuck an atom bomb out the back, and explode it. Not as effective as in proximity to a planet, since it hasn't got a hard surface to push against, but the pollution doesn't get anywhere important, either. You don't try and land it; if going to planets, you have to carry a lander. The bombs are heavy enough that, well into a journey, the individual jolts are much harder. Not many planetary inhabitants are going to be overjoyed at the idea of a couple of thousand tactical nukes coming to visit, but face it; any drive which can accelerate a decent weight ship by a decent amount, is going to have serious destructive capabilities, merely based on its energy requirements.

Linear accelerator: No, not throwing metal bricks out the back (though that would work, too, after a fashion), it's building a space station at your departure point, with a multi-kilometre railgun, and flying another to your destination, and the ultimate in freewheeling/precalculation games (yes, minor course corrections, with compressed air, available en route to its fortunately bug-free onboard computer) the incoming mass is so accurately placed that it comes in down the accelerator coils, which generate energy as they slow it down. This energy, generated from incoming fresh air tanks, swordfish steaks and caviar, is used to accelerate back purified platinum and crystals that would never grow in a gravity field; a game of catch bigger than a thousand planets, but, I suspect, freight only.

Atmospheric braking: Slowing down is acceleration, too (well, it is if you're a mathematician) And most of the major bodies in the solar system have atmospheres, which can be used instead of fuel for this purpose. Earth's has already seen frequent use, Mars' and (I believe) Venus' have both demonstrated the validity of the principle; it is only sad you can't yet reclaim any of the energy wasted in friction

Eddy current braking: As an alternative to the precedent; only for use wit planets possessing a strong magnetic field. Advantage; you can most certainly generate energy from your velocity.

Bussard ramjet: while developed for interstellar space, the ramjet should be able to work with a smaller magnetic collection funnel within the solar system, collecting solar wind and general junk for use as reaction mass. The funnels will have to be moving fast, accelerating forward, for this to work, and approaching a planet with a strong magnetic field will be - delicate,

Waiting for new technology: The same argument holds as for the previous section; certainly, new physics will bring new technologies, perhaps vacuum energy can be persuaded to act directionally, or gravity waves can be directionalised, rendering the heavy, expensive drives proposed here expensive white elephants; and if interstellar drives are ever to be considered, this needs to happen; but without the competition from the elephants I don't see why the new, esoteric opportunities will ever be investigated.

[Urien]

Really nice post. Thanks. So... no warp drive then? Bum.

If only we could fold space and step right through the curtain of continuum.

[chrispenycate]

Originally Posted by andrew.v.spencer

Really nice post. Thanks. So... no warp drive then? Bum.

If only we could fold space and step right through the curtain of continuum.

Warp drive, esoteric drives in general, not essential for insystem work; they're for part three, interstellar travel, which is only notes as yet. It might even get written, who can tell.

[mosaix]

Originally Posted by chrispenycate

Warp drive, esoteric drives in general, not essential for insystem work; they're for part three, interstellar travel, which is only notes as yet. It might even get written, who can tell.

Chris - just get on with it - how dare you keep us waiting!

Good thread by the way.

[chrispenycate]

Stage three: past the Oort cloud
Assume a ship with a perfect drive depending on traditional physics, which is accelerating at 10 M/sec/sec, one gravity, for comfort.
After one day, (rounding off figures everywhere) it is four million kilometres (thirteen light seconds) from home, travelling at nine hundred kilometres a second, and has ejected one three thousandth of its mass at light speed. After thirty days, we're at almost a tenth of the speed of light, three thousand million kilometres out (two and a half light hours) and one percent of our original mass is left behind. By this sort of calculation, expending twelve percent of our original mass, after 345 days we're into relativistic speeds, the drive is becoming less efficient, and we're less than an eighth the way to the nearest star. Now, we can choose between fuel economy (turning off the motor and cruising) or time contraction, which might relieve the boredom for those aboard, but not help the politicians who've just understood that they'll be long out of office before the budget they'd voted for comes to fruition (or a compromise, so you can go on eating soup)
And this is a superb drive, orders of magnitude better than anything we could even imagine today. It's almost a year before the universe's speed cop starts giving us any hassle, which is the first time any "space contraction" or "hyperdrive" becomes essential. If half of the ship's mass at the beginning was fuel, there is enough to slow down at the first star, potter around for a while, go on to one further star and come back without refuelling. (in fact, it's a bit better than this, because the mass to be accelerated is continuously reducing) Superb, but five years passed on the shortest journey, and two stars visited.
A generation ship, with a more reasonable hundredth of a g acceleration, even if sling shot out of the solar system with as much velocity as we can reasonably get in locally, needs to blast for fifty years and slow down for ninety; cruising time is roughly the same.
Not very practical for interstellar empires; oh, the natives are revolting (well, they were five years ago when you sent the message) we'll send you some troops right away; they should arrive in a hundred and forty years, max.

Still, not bad for interstellar colonisation; either as generation or hibership, if you can stand a two century voyage, then the time in orbit as the terraforming gets under weigh, as sealed domed cities are built on the planetary surface, and enough breathable air is generated to fill them; in fact, pioneering might turn out to appeal to a minority, giving a ship/station society, à la Cherryh, where planets serve as anchors and (rather poor) sources of raw materials


After the law of conservation of momentum, let us move on to the law of conservation of energy. It is obvious that the amount of energy required to accelerate a mass to near light speed is aproximately equivalent to the energy obtained by converting that mass into energy (mV squared not far from mc squared) Our plasma fusion drive, that was looking so dangerously powerful in Earth orbit is now looking a bit wimpy, the few nanograms of mass converted into energy at bikini start to look very minor compared with the tonnes required to accelerate a ship fast enough that time contraction becomes a serious aid, or, for that matter, slow it down at the destination. And, basically, the slower the ship is the bigger it has to be; an ecosystem designed to be stable over five centuries is far to big to land, and the extra surface to orbit craft it has to carry only add to the total. Perhaps a robot craft (with downloaded human personalities for making choices outside the computer's preprogramming?) would need fewer resources, thus less weight; but we're aiming at spreading our baskets out to prevent the disappearance of humanity, not merely human knowledge. Human eggs to be thawed out and educated up when conditions are suitable?
Hydrogen fusion will suffice for hiberships, or generation ships, but is not up to empire building, or trading in perishables. Please don't believe that these figures render conventional drives impossible for interstellar use, but they are not convenient. The only potential fuel known to modern physics giving the performance needed is antimatter, tonnes of it; and you'd better have a good contaiment system. And, until we can achieve these speeds, the universe's speed limit doesn't disturb us too much (except for messages home), any more than a mountain disturbs a fish. Squeezing space, universes with differing physical constants (and different speed limits) don't change this problem; nor will reactionless drives or gravity control - Weber's multi-megatonne "gravity-shear" ships doing 400g accelerations on fusion power just aren't practical on the energy front. For interstellar distances in reasonable time intervals your conventional spacecraft that fuel up, and do multiple star systems don't make the grade. Let us view a few conventionalish solutions before moving on to the "new, unknown physics"

Firstly the Bussard ramjet, which we glanced at in planetary drives, but comes into its own with a bit of a run-up; still, a framework of hair thin, superconducting wire, held rigid buy its own magnetic repulsion, doesn't lend itself to great accelerations, and the magnetic fields involved are going to make servicing a serious problem (you can't shut it down; it's a large percentage of your particle shielding, as well as your propulsion)

Light sails: running on direct starlight (sunlight) they drop in usefulness as the square of the distance from the source. Building a laser (oh, oodles of terawatts - removes the "free energy" part of the argument) puts the energy where you want it (how come nobody's proposed a lense system to collimate sunlight? It wouldn't be as parallel as a laser; but a laser only works at it's own chosen frquencies, and wastes the rest. A few fresnel lenses, perhaps as big as australia, hovering in roughly Mercury orbit, concentrating a beam aimed at where the ship is supposed to be when the light gets there - no?) And they have to be gigantic, and changing direction even a smigin requires days; but no fuel, and no energy problems (if it's not how to get rid of the excess heat generated in the actual ship part; the "cold of space" is fairly irrelevant if you've a star behind you, and a ruddy great mirror infront reflecting it all back at you, and you're generating heat that has to be disposed of all the time, too.

Launch it fast and let it cruise: despite the higher density of matter within the solar system, it would be possible to use up all the outbound fuel deep in the gravity well where it has most energy, then cruise at the attained velocity (much as at present we use minimum energy orbits to go to Mars). This means the difficult bit (well, the first difficult bit; slowing down's not that easy, either) is in "friendly territory", where one can deliver provisions in one-way containers or, in theory, bail out at the last minute. It's a very efficient way of doing the job, but not very romantic; the crew have about as much control of their flight as Laika going into orbit.

Where is all this going? With a conventional reaction drive, empires, general political groupings even, are not practical over interstellar distances; your colonies send back only one thing to the mother planet, albeit the most valuable thing in existence; information (and even that takes many years to make the trip). For a trading ensemble, some radically new physics is going to be required.