Fast Interstellar Travel: External Propulsion

Project Daedalus, a actual design proposal 

The Problem
 
   'Space is big, mindbogglingly big.  You may thing it is a long way to the chemists, but that's peanuts to space...'  says the Hitchhikers Guide to the Galaxy, and it's true.  Not only is space big in size, but anything associated with interstellar space grows to enormous proportions; spacecraft, cost, political resistance, fuel, mass ratio.   This is, quite simply, the problem.  To reach even a nearby star in a reasonable amount of time requires a vast expenditure of effort and money.  We could have dispatched a probe to another star system by now, if humanity was prepared to work together and use the entire world supply of nuclear explosives for Orion pulse drive charges.  Proposed designs continue the theme of big and expensive.  Daedalus, 190 meters long, 54,000 tonnes initial mass, and fifty years to take a 450 tonne payload to a target six light years away; it also does not have enough fuel to stop at the destination.  The Enzmenn Starship, 600 meters long and needing three million tonnes of deuterium fuel to deliver a colonising expedition of 200 people to a nearby star.  The Valkyrie, only 2100 tonnes initial mass, but ten kilometres long, and needing a thousand tonnes of antimatter, the most expensive and dangerous substance in the world.  I could go on, but I think you get the point.
   Although much of the cost of the first starship built is likely to be research and development, successive starships will have budgets dominated by the sheer unyielding bulk of raw and processed materials, not to mention skilled and unskilled labour, needed to construct and supply such a vessel.  Normally there are two basic types of starship, big vessels like worldships, and smaller starships that take advantage of high speed to cut down on supply mass and crew size.  With fuel measured in the hundred thousand tonnes, an smaller vessel is obviously the preferred option; but with a smaller vessel the trip must be faster in order to be completed within the vessels endurance.  As performance is increased, mass ratios and needed exhaust velocities become prohibitive, pushing the boundaries of physical possibility.  It reaches the point where it becomes clear not conventional rocket propelled starship will ever provide the transport needed by a truley interstellar humanity.  But what can?  Enter the concept of External Propulsion, the main classes of which I explain below.

Beamriders

   There are two kinds of beamed propulsion, beamed power, and beam-riders.  The idea of the first is to rid a spacecraft of the mass of a nuclear power supply.  As most interplanetary spacecraft have a large amount of the mass budget allocated to this, it drastically increases performance.  The same technique can be used with either a thermal engine, using a laser beam, or a electric engine, using a microwave beam.  Although great for interplanetary applications, as well as powering missiles, it is not as good for interstellar work as the fuel mass for a starship would still be prohibitive, due to the engines performance being the same, just with a lighter spacecraft to push.  The second removes the remass as well as the fuel, using a beam to transfer momentum to the starship.  As the entire propulsion system is no longer on the spacecraft, this frees the starship from the tyranny of the Tsiolkovsky rocket equation.  The mass ratio becomes infinite, and the top speed the craft can reach is limited only by the velocity of the beam that is transferring momentum.  Thus, with the correct technology, even a small spacecraft can make rapid interstellar transits.  Well, relatively rapid.
   First we'll look at the pros and cons of beamed propulsion in general, then look at specific systems and their foibles.
Pros: the interstellar craft itself can be smaller, and more of it devoted to crew and life support or cargo, making the trip more endurable.  The velocity attained is likely to be higher than an independent starship, and so the voyage is easier as a result.  The fact that the propulsion stations remain in the departure system means that they are not throwaway like many starship designs, but can be used to launch other missions, or as part of an interplanetary transport network, paying back some of the construction cost, which increases the chance of colonisation ever occurring.  Although it ight take a whole system to build the launching network, anyone with the mont might be able to by a starship, leading to interesting scenarios.
Cons: initial cost of the propulsion installations is likely to be similar to that of a starship.  All the stations must be coordinated, requiring a good deal of organisation and logistics, but then, perhaps no more than a starship.  The beams, have to be aimed over possibly lightyears, and kept running for years.  The starship is at the mercy of the people controlling the beams, and sabotage may be a bigger danger.  Also, a set of beams capable of accelerating a starship will be awesome weapons, and it is unlikely any government will want another government to own them.  So either everyone is basically at ware anyway, or there is a single government with power in space.  The most annoying downside is that for an exploratory starship there is no network of beam-stations to help it slow down, and so either rockets or some kind of Solar Sail must be used.

MagBeam
   This uses a beam of plasma to push the spacecraft.  It can work at much longer ranges than expected because the spacecraft uses a MagSail to capture and focus the plasma beam, preventing it from dissipating as much as it would otherwise.  Current designs have a maximum mass of ten tonnes, but this might be possible to increase.  If not, it could be used to launch and accelerate smaller masses as part of a mass-beam station.

MassBeam
   In this design a beam of solid particles of conceivably any size are fired at the rear of the spacecraft.  There they are deflected by the spacecraft, either using magnetic or electric fields, or a solid bumper.  Many devices can be used to create the Mass-Beam, rail guns, coil guns, particle beams, ram accelerators, or even small laser propelled LightSails.  It is also possible to use Orion nuclear pulse charges as the masses in the beam, detonating them in the normal way.  This combines the power of Orion with the mass ratio and delta-Vee of a beamrider, so is a plausible choice.  However, a particle beam, perhaps combined with an orion booster, is most likely for an interstellar application, as it can have high velocity and energy content.
LightBeam
   The spacecraft for this is a solar sail, reflecting the light from a battery of lasers.  Although the lasers have the highest possible velocity(3E8 m/s), they also need the highest power per unit of acceleration.  This makes me think that a particle beam starship is much more likely, but like the MagBeam, a LightBeam could accelerate hundreds of A4 sized foil sails as the mass in a MassBeam spacecraft.  The advantage of a LightBeam is the range over which it can be focused, and the possible maximum velocity attainable is as close to c as the energy budget can afford, as the beam itself naturally travels at the speed of light.  The disadvantage is the very low acceleration per unit of power in the beam.  A mass beam of the same input energy will transfer a lot more momentum.  But because of the advantages and disadvantages of both it is likely a MassBeam will be used for initial acceleration, and a LightBeam for reaching high cruising speeds.

Solar Sails
   There are various kinds of solar sails - magnetic, plasma, photon - but they all share common traits. Each depends on the output of the Sun or another star to create a reaction force.  Like a beamrider they have an infinite mass ratio, and a very high delta Vee.  Unlike a beamed propulsion vessel they do not depend on a station and the difficulty of supplying it, running it, aiming it, and so on.  If a Sail with high enough performance could be built it would easily become a likely candidate for interstellar travel, quite aside from being far more romantic than several hundred thousand tonnes of complicated pluming.  The main disadvantage with the designs that have been considered or proposed in real life are low accelerations and huge engineering challenges in building a sail of potentially hundreds of kilometres.  If the spacecraft dips closer to the sun in order to accelerate faster, in a kind of Oberth boot, then it is difficult to make the spacecraft strong sough to survive that close to the sun.

Photon
   Photon, or LightSails, are the simplest of all spacecraft.  Face a large enough mirror toward the sun and the momentum transferred by the impinging photons will provide a infinitesimal amount of thrust, although more than an unboosted sail of the other varieties.  It is small, but with an infinite mass ratio and steady thrust very high performance is possible.  A LightSail might not be able to reach a fast interstellar cursing speed alone, but with laser stations throughout the solar system it should be possible.  Like other sails it is also a good option as a potentially lightweight deceleration system when travelling to an uninhabited system.
Magnetic
   A MagSail acts against the solar wind, the steady stream of charged and uncharged particles that stream out from the sun at 900 km/s.  The sail itself is a huge superconducting magnetic ring that acts against the particles to create thrust.  Although it has more less(Edited; sorry to anyone who this misled) thrust than a solar sail, it also requires advanced high temperature superconductors, and an onboard power supply.  Originally conceived as a 'parachute' of kinds to decelerate an interstellar probe, the MagSail is often consider in that role for starships, and also ties in nicely with a particle peak based MassBeam launching plan.
Plasma
   Known as the Mini-magnetospheric plasma sail(M2P2), this Solar Sail is basically a recreation of the Earth's magnetic field.  Instead of using a massive superconducting ring like a MagSail it uses very low density plasma to inflate a magnetic field, creating an enormous area for the solar wind to push against.  Although it needs an onboard supply of hydrogen for the plasma it is achievable with today's technology with some development, and advantage over the MagSail.  It should also prove to be more controllable, than the other forms of Solar Sail.


Space Train
   Somewhat whimsically named, the Space Train is a variant of a Beamrider.  However, the beam itself does not provide the impulse needed for DeltaV, it is merely the fuel and/or reaction-mass.  The 'track' is a path of Orion nuclear pulse units that have been laid over a expanse of relatively flat space.  As the spacecraft passes each charge it detonates, providing acceleration.  Alternately, the track can be a path of particles or plasma used to feed a Bussard Ramjet style craft.  Admittedly I cannot see the benefits of this system, and it is unusual to say the least.  Laying the 'track' could prove as difficult as building beam stations for a conventional BeamRider.  Note that this is quite obscure and hard to find on the internet compared to the other systems in this blogpost.

Interstellar Ramjets
   I'm not sure if the Bussard Ramjet qualifies as external propulsion, but it shares many of the same advantages.  However, the technical achievements needed to make one plausable probably mean that it has few advantages over more brute force methods, and the actual vessel may still be overly large.  However, if one were built, an eternal propulsion method would be ideal for getting it up to the speed where it begins to function effectively, especially the 'railway' method.


Far Future
   All of the methods proposed so far have been ones that lie within the realm of the hardest of 'hard SF'.  But what of a setting in which there is some magitech allowed, but the laws of Relativity still hold?  A force field device may, as I have already explained in another post, make fusion drives of great power mere child's play.  But even so the amount of infrastructure needed to supply a interstellar ship with fuel and remass is staggering, and likely to be prohibitively expensive, so external propulsion is likely to still be considered, especially if routine travel is the goal.  Along with superconductors that work without cryogenic cooling that would allow huge improvements in the performance of most of the above mentioned technologies, all of which rely on some magnetic-based technology.  Another option is a forcefield that can be used as a Solar Sail, reflecting perfectly both light and solar wind, able to closely approach the Sun, and easy to stow or deploy.  The applications in power generation also help to make BeamRiders more practicable.
   In my own fictional 'Verse the richest of the rich routinely travel between stars at relativistic speeds in order to take advantage of time dilation to prolong their effective life, much as Ender does in the Ender's Game universe.  They use ships that are a mix of BeamRiders and Solar Sails, the beams provided by interplanetary transport companies.  This 'effective life' is a measure of social standing, and is the difference in time between what they have experienced, and what time has passed in their home system.  As the 'Verse is nearing Post Scarcity, interstellar flight is one thing the rich can afford that other individuals cannot; a Ferrari supercar is not such a status symbol when anyone with a medium level 'fabber' (a kind of replicator using molecular 3D printing) and a few thousand dollars for raw materials can have one.


What does this mean?
   There are only a few simple constraints that these forms of interstellar transport are likely to impose.  Interstellar commerce will be virtually extinct, except for things which only have value because they cannot be created in the destination system.  Works of art, historical artefacts, information, and talented individuals might therefor be the main items of trade, and it would likely be intermittent and sporadic.  Interstellar war is almost impossible.  To retreat you would need to build quite considerable infrastructure, in the case of a BeamRider, and anyway, what is the point in the first place?  Thus even when known to be doable, interstellar travel is likely to be nigh non-existent in a hard SF universe; but you never know, humanity is never logical at the best of times.