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Wednesday, 12 April 2017

Interstellar Trade Is Possible Part II: Travel

This is Part 2 of the series focusing on realistic Interstellar Trade. Here we will look at the various propulsion systems available and discuss which are appropriate in different settings and futures. Summary at the end.

In Part 1, we determined that the earliest attempts at interstellar trade would be done using contemporary technology and at the lowest costs possible. We also had to consider the financial aspect of the attempts, as shorter travel times are more interesting for investors. 
Project Daedalus
To be considered affordable, we had to send a micro-sized payload (about 1 gram) on a one-way trip to another star system at speeds between 0.15 and 0.8C. Higher velocities run into significant Lorentz factors that greatly increase the cost of travel without reducing travel times by much.

There are four steps in the interstellar trade cycle that involve travel: 

-the outbound journey where the seed is accelerate out of our system
-the braking burn, where a rocket is employed to stop at the destination
-the return journey, in which products are sent back to our system

Our main priority is reaching a balance between cost and speed. Travel velocity is not something very flexible, as the optimal fraction of lightspeed with be about 70 to 80% at most. Below that velocity, the relationship is nearly linear between velocity and cost, if we assume that cost is represented mostly by the price per megajoule added to the spaceship. Above that velocity, we reach more and more extreme Lorentz factors and quickly diminishing returns.

How can we greatly reduce costs?


The solution is to build the spaceship to use the most of the infrastructure that exists at the time of departure. Previous proposals for interstellar travel attempted to tie in the construction of massive solar stations or millions of nuclear bombs as a solution for providing the megajoules involved without any existing need for them. 

Yes, R. Forward thought in terms of thousands of terawatts.
The form this infrastructure takes is in the hands of the creator or writer of the setting, but the purpose is singular: make interplanetary travel easier. Due to the rocket equation, sourcing the energy for propulsion externally allows for smaller, faster ships. This favours large, immobile structures being built to handle the propulsion of small, mobile payloads between start and end points. The spaceships would be able to exploit the massive power of the structures individually, but the cost of the structure would be divided among all users. 

Modern examples of this sort of division of benefits and costs include large industrial ship docks so that the sea vessels don't have to carry their own landing boats, or national electricity grids so that not every home needs its own generator. 

Pushing thousand-ton payloads on journeys inside the Solar System actually involves similar amounts of energy as accelerating gram-sized payloads outside the Solar System. It can be delivered in many different ways to the spaceships. The methods used define the type of infrastructure in place, and the interstellar operation will design their spaceship towards making the most of it. 


Three propulsion types are available. The first is beamed propulsion. Proposed many times for interstellar travel, it involves using lasers either directly (light pressure) or indirectly to power a rocket. The second is a self-contained rocket. Usually, this involves nuclear propulsion. The third is external propulsion, where the propellant or fuel is send ahead of the probe to create a 'track'

Beamed propulsion 


Leave the power plant at home, but take the power with you! 




Beamed propulsion allows dozens of gigawatts of laser power to be produced by lasing stations of several thousands of tons, but can be used by laser sails massing only a few grams. This means extreme power-to-weight ratios on the spacecraft's end.

Light sails use the momentum of reflected photons to accelerate. The acceleration depends on reflectivity and photon pressure.

  • Acceleration = (Photon Pressure / Mass) * (1+Reflectivity)
Photon pressure depends on the flux of the laser beam, measured in watts per square meter. Mass is dominated by the laser sail's surface area. Reflectivity determines how many photons are bounced back for extra thrust, and how much of the laser beam is absorbed and turned to heat. It is a percentage.
  • Total Photon Thrust: Laser Power * 6.67 * 10^-9 * (1+Reflectivity)
Laser Power is in watts, and the Thrust in Newtons. We can calculate that a gigawatt laser beam, perfectly reflected, gives the sail 13.34 Newtons of thrust. 

However, no material is perfectly reflective. If even a fraction of that gigawatt beam is absorbed, it will vaporize the laser sail! The solution is to spread the beam over a large area. This increases mass, and lowers acceleration. Therefore, the laser sail will be built to the maximum thermal limits of its materials. It is not usually the melting point of the material, but the temperature at which the delicate reflective material starts degrading.


The choice of laser wavelength is another factor. Microwaves are easy to reflect with a lightweight metal mesh. Huge laser sails can be built to high reflectivity and lightweight masses. However, microwave beams suffer from terrible divergence. As the laser sails accelerates away from the laser generator, the 'spot size' of the beam expands greatly, which requires large sails to catch the full width of the beam, so might negate the advantages of the lightweight construction. 

Microwave and infrared sails are best suited to sharp accelerations by powerful lasers. The sail reaches several percent of lightspeed in a short distance. 


Visible wavelength lasers have better range and allow for smaller sails to catch the beam over longer distances. However, high reflectivity can only be achieved by dielectric mirrors. These are fragile and mass a lot per square meter. 


Even shorter wavelengths, such as ultraviolet or x-ray, can focus the beam tightly enough to continue accelerating a laser sail over millions of kilometers. The downside is that maintaining good reflectivity becomes more and more difficult. A significant fraction of x-rays might even pass right through their target!


A major factor for selecting this mode of propulsion is the existing infrastructure.


The cheapest interstellar operation is one that relies on systems already built and operated by someone else. In Laser Weapon Webs, I noted that lasers could be used to power interplanetary rockets. To move thousands of tons at rapid rates, gigawatts of laser power had to be available across the Solar System. It is best for a laser sail to be designed to use these very same beams to reach high velocities.


Therefore, the design objective would not be which combination of laser wavelength, sail area and heat tolerance would make for a perfect propulsion system, but to design a laser sail that best uses existing infrastructure.


First, we must ask what kind of lasers are best for interplanetary travel, as those would be funded and built long before any interstellar operation is conceived. If they are military in origin, they are likely to based on short-wavelength technology that can double as a destructive weapon. If it developed under civilian patrons, it will be built for the highest efficiency. This is because increasing laser energy is more expensive than building larger focusing mirrors. 


The interstellar operation would only have to 'rent' a laser beam instead of asking investors to build their own gigantic laser. A mature Laser Web that handles thousands of tons of traffic daily across interplanetary distance can reasonably track a laser sail over millions of kilometers. Such a web could extend over is 8.8AU, the distance between Mercury and Saturn. 


Let us suppose for an example that the web operates on microwaves. It is easy to achieve high reflectivity with a lightweight metallic mesh.


Let us target a trip to Alpha Centauri within 10 years. Travel velocity has to be 0.42C. Payload and braking engine are assumed to be 1kg. Acceleration must reach an average 6km/s^2.


These accelerations make it difficult to construct a probe that doesn't melt.


One option is to boost a section of the laser web out of the solar system. This would allow the laser to be focused over much longer distances, which reduces the stress of the sail itself. 

Laser sails might resemble white-hot disks shooting through space.
Another option is to do away with thermal limits of delicate reflective surfaces entirely and use melting points of materials instead. While the sail would lose the x2 factor in thrust, it could gain a much better resistance to high laser intensities. For example, an aluminium-based coating for a microwave mesh could achieve 99% reflectivity of 5 cm microwave light. It would melt at 933K, but would probably start losing reflectivity at much lower temperatures. A graphite microwave-absorbing mesh can survive 3800K, easily allowing five times higher laser intensities. A reflective laser sail would need a reflectivity of 99.6% or better to match the performance of an absorbing sail in this case. This is possible with dielectrics, but then it would become extremely difficult to get the low masses per square meter necessary. 

The best laser sail is one that manages high reflectivity at high temperatures. 



Carbon-carbon tip exposed to 3.9MW/m^2
The acceleration of a laser sail is limited by the photon pressure divided by the mass per square meter. If the photon pressure is temperature limited, the sail cannot accelerate faster by using a larger sail. The mass per square meter is therefore an important variable. Using larger wavelengths allows for lighter sails, as larger and larger empty holes can be built into the sail material (like the grid in a microwave door), but normally the beam divergence negates this advantage. In an Interplanetary Transport Web, re-focusing stations or mirrors are evenly spaced and make beam divergence a non-issue for our purposes. Advances in materials technology allow for the creation of paper-thin meshes that can survive extreme accelerations without bending. Carbon nanotubes, for example, have been tested to survive 6 tons of strain or more using strands on a millimetre thick. 

Particle beams can be used as alternative means of beamed propulsion. They provide better thrust and better thrust-to-weight ratios than lasers. The spaceship would need to extend a magnetic field around itself to catch the particle beams, which means having to install electromagnets on-board. Powering the magnets can be done by extracting a fraction of energy from the particle beam. They have similar ranges as infrared lasers.


An combination of particle beams and lasers can be used to boost a hybrid sail. Using electromagnetic fields to both deflect particles and hold in place conductive wires can allow the sail to extract the energy of a high-momentum beam and a long wavelength laser simultaneously. This concept will be explored in a future post.

One major downside to beamed propulsion for interstellar travel is that has to be a separate propulsion system for stopping at the destination. This imposes a minimum mass on the payload being carried by the laser sail, to be used as a second stage.


Laser-Powered Rockets

This propulsion system was described in Laser Launch to Orbit.


Instead of the momentum of photons, we use the energy contained in the beam to energize a propellant for propulsion. 

Laser ablative propulsion
The main advantage of this method is a much better thrust output than using laser sails. Instead of waiting for a solar system-wide laser web to be put into place, an LPR probe can be launched by much smaller set of lasers and mirrors. 

Of the designs described in the previous post, Pulsed Ablative-Plasma rockets have the highest exhaust velocities. Terawatt-level pulsed lasers depositing megajoule level pulses in microgram masses of propellant can create temperatures exceeding hundreds of billions of kelvin. Using the root mean square velocity of the gasses, we can estimate the exhaust velocity. 


For hydrogen (1g/mol, 22J/g/K), we can get 33672 km/s out of a 45.5 billion K gas, but it is hard to store. 

Lithium (6g/mol, 3.56J/g/K) can produce an exhaust velocity of 34173km/s out of a 281 billion K gas. 
Carbon (6g/mol, 0.52J/g/K) is the easiest to build a Pulsed Ablative-Plasma rocket out of, and reaches 63223km/s exhaust velocity from of a 1923 billion K gas.  


A carbon-based propellant ablative laser pulse rocket does not need much in terms of engine mass or structural components. To achieve a 0.15 to 0.5C deltaV (departure burn), a mass ratio between 2.03 and 10.7 is required. This is a rather reasonable condition. 

The departure mass is measured in a handful of tons, and about 32 newtons of thrust are produced per Gigawatt. A set of 1000 GW average power pulse lasers can propel a 1 ton spaceship with a mass ratio of 2.03 to 15% of the speed of light in about 12 days, and a mass ratio 10.7 spaceship to 50% of the speed of light in about 29 days. 

An alternative laser propulsion method?
The problem with this method is that it is difficult to construct large-scale (GW) high-pulsed power (PW) high-pulse energy (MJ) lasers that are useful for interplanetary propulsion compared to much lower-energy pulsed lasers (GW/kJ) or simply continuous lasers. A setting where these types of lasers exist would need a very specific path of development for laser-powered propulsion, starting with lightcraft and continuing with two-staged pulsed propulsion into the GW ranges. 

Another difficulty is the lack of economic sense of combining high-powered pulsed lasers with a laser web. The pulsed lasers are a valid method of accelerating spacecraft efficiently. Increasing the power output allows for quick acceleration over short distances, short enough to only require a single focusing mirror. Laser webs allow weak lasers to track and 'follow' a spaceship out to tens of millions of kilometres or more and use lower accelerations, so combining the two concepts defeats the advantages of both. For an interstellar operation, this means that it would be easy to exploit one or the other, but hard to justify the cost of putting the two together. 


A solution to this would be to create a situation where dual-laser webs exist: one continuous mode for regular travel, and a pulsed mode for military use. The interstellar spaceship exploits the latter. 


Nuclear Power


Lasers are expensive. Some settings might instead develop nuclear-powered rockets as an alternative to a costly laser web infrastructure. It would make sense for an interstellar operation to choose the nuclear option when it is available.

Hendrick fusion rocket. The blades are radiators.
The main focus of interstellar nuclear propulsion is high energy density and high exhaust velocity. Of the designs with the highest exhaust velocities, we have fission-fragment rockets, nuclear pulsed propulsion and pure fusion rockets.

Pure fusion rockets are currently outside of our technical reach. Although they allow exhaust velocities in the tens of millions of meters per seconds, it is not known whether they allow high thrust-to-weight ratios required to reach interstellar velocities quickly, or any form of miniaturisation. Smaller fusion reactions are harder to maintain than larger reactions in huge chambers as heat escapes more easily; this is a result of the square-cube law.


A common problem with all fusion rocket designs is that they require a large energy input to 'ignite' the fusion reaction. This is very unlikely to be available for a kilogram-to-ton sized spaceship. One method that could scale down to such sizes is antimatter-catalyzed fusion... but the availability of antimatter depends on the setting. It is dangerous, it is hard to use, and might be treated as nuclear weapons are today. It is likely that antimatter is considered too costly or be simply unavailable to civilians. Fusion ignition through an external laser might be an alternative option. The pulse energies required for high exhaust velocity ablative pulsed propulsion are sufficient for generating 100 million K temperatures or higher. However, accuracy requirements for directing and splitting a beam into a uniform pulse around a fuel pellet inside a tiny spaceship travelling at a large fraction of lightspeed might be an insurmountable challenge. 


An antimatter design is very well suited to an independently powered braking acceleration at the destination star system. Externally ignited fusion is only suitable for the departure burn. 



From the ICAN-II antimatter-fusion rocket design. 
Another option is nuclear pulsed propulsion. Orion drives using magnetic fields instead of physical pusher plates, powered by Teller-Ulam fusion/fission pulsed units can produce very high exhaust velocities. This could be an evolution of the Mini-Mag Orion engine. It is suited for both ends of the journey. The only downside is that there is a minimum mass to the pulse units in versions that use self-contained units. A z-pinch could be used to decrease the minimum mass by compressing the nuclear fuels to criticality. This would, of course, run into the same on-board power requirements as fusion propulsion, albeit at a much lower power level.  
Dusty Plasma rocket design
Fission-fragment rocket engines use the ejected fission products of a radioactive source for thrust. These fragments provide exhaust velocities of several percent of C. With no external power input and no minimum size, these can push a tiny payload to large fractions of lightspeed. A dusty plasma rocket can achieve an exhaust velocity of 10 million m/s. 0.3C of deltaV would mean a mass ratio of 8150. This is a 10 ton or so vehicle that enters the destination star system with only a few kg left of dry mass. For higher velocities, the mass ratio explodes into unfeasible values (million-to-one or worse). 

Overall, nuclear power means that an independent spaceship can depart and stop at the destination without relying on a multi-gigawatt laser infrastructure to exist. If lasers do exist, they can supplement nuclear rockets by powering the compression of pulse units or igniting fusion products. 


Different types of nuclear propulsion can be used for departure and braking. For example, departure can be performed by externally-compressed fission units, while braking can use antimatter-ignited fusion pellets. Using antimatter for the braking stage instead of departure is cheaper as less of it is required.


Nuclear propulsion can be used as the second stage of a spaceship that departs under beamed power.


'Track' propulsion


There is no need to send a single spaceship. The only objective is that a small payload reach the target solar system.


One way of delivering the incredibly useful laser power to the spaceship for the braking burn, without sending along a laser web spanning lightyears or delivering all the energy in a very short period, is to 'store' the laser energy in a physical object.

A seeded track was first conceived as a solution to the Bussard Ramjet problem. 
In practical terms, this means that before the seed is sent on its way, the interstellar project planners pay for a stream of objects to be sent on their way to the destination star.

These objects can be blobs of propellant, nuclear pulse units or solid impactors. They travel at a lower velocity than the seed ship intends to reach, such as a range of velocities from 0.01 to 0.5C. This way, the seed ship 'catches up' to a track unit, converts the stored energy into propulsion then accelerates to the unit in the stream. 


As the spaceship starts slowing down near its destination, the situation is reversed. The stream overtakes the spaceship instead. 


This concept has been described as a 'bomb track' for nuclear pulse propulsion or a 'pre-seeded trajectory' for fusion ramjets. Isaac Kuo has his own version relying on inert solid propellant to deliver kinetic energy through impacts.


The main advantage of this method is that the spaceship only needs a payload and an engine. Everything else is provided by the track. 


Propellant tracks allows for incredible effective mass ratios with only a small spaceship and onboard fuel. Kinetic impactors in streams of variable velocity (a faster stream of impactors send after the slower initial stream to intersect just as the spaceship flies by) don't even need fuel. They are likely the cheapest option, but their efficiency falls and rises with the relative velocity between spaceship and propellant. 


Nuclear pulse unit streams, where each object is a miniature Casaba Howitzer, are quicker to set up than multiple propellant streams, as the relative velocity between the spaceship and the units is of little importance (they can be sent as slowly or quickly as needed).



Collision of two 'gun fusion' bullets loaded with fusion fuel
The two concepts can be combined. Using the concept described in Gun Fusion, the impact of nuclear fuel with a target can create sufficient energy to ignite fusion. This produces a lot of energy with a only small relative velocity. In other words, a low-cost, low-velocity track can be shot ahead of the spaceship, and still provide a lot of energy.

The downside of track propulsion is that you'd still need a method of accelerating the track units, and that waiting for some forms of track propulsion to start getting close to the destination (and become the braking track) requires years of waiting. 


For example, a cheap kinetic impact-ignited fusion track sent on its way at 0.05C would take over eighty years to reach Alpha Centauri. This is far too long for an interstellar operation that wants to be ready in 10 years. It is therefore best to use track propulsion only to accelerate the payload, and use another form of propulsion to brake at the destination. One hidden benefit is that the fuel and/or propellant for braking can be collected from the acceleration track.  


Propulsion summary


The propulsion system chosen by the mission designers of an interstellar operation will depend strongly on the infrastructure and technologies in place at the moment of departure. If there is an interplanetary transport system, such as an Interplanetary Transport Web, then it will be exploited instead of more conventional alternatives. 


Having to brake at the destination favours nuclear propulsion, but there is no obligation for the same propulsion system to be used for both departure and braking. This allows the spaceship to best use the facilities available at home (externally laser-ignited fusion), while minimizing the usually more expensive braking propulsion systems (nuclear pulse units or antimatter-initiated fusion). 


'Track' propulsion is the most economical for acceleration, as it allows large deltaV capacities using small spaceships. Using a laser to accelerate track units over a long period of time allows for a much smaller laser to paid for by the interstellar operation. Attempting to extend the 'track' all the way to the destination might take longer than simply sending the spaceship alone, so it is best combined with other propulsion options! A
 hidden benefit is that the fuel and/or propellant for braking can be collected from the acceleration track; the spaceship only need to carry along empty tanks and an engine to the end of the first track.

28 comments:

  1. A couple of thoughts:
    1.There's a neat trick detailed in Atomic Rocket with reflective lightsails. You separate a large portion of the sail, back it off aways in the direction of travel, and use it to focus the beam on the back of the sail. This lets you use the same beam at home to decelerate the probe without all this mucking about with track propulsion or internal engines.

    2.Suppose you could use an LCD shutter on the rocket to convert a continuous beam into a pulsed beam before focusing it on the propellant?

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    1. Hi VIPM!

      1. The reflective sails do allow a laser-powered spaceship to use the same laser for acceleration and braking. However, this entails focusing a laser across interstellar distances... and tightly enough to fit its laser spot onto the sail.

      This means that a super-laser array that could focus onto the sail at a distance of 100AU (more than the entire width of Pluto's orbit) would have to be either seven million times more massive or 2656 times more powerful to reach the sail at 4.2 lightyears. The scaling is even more incredible for destinations further away.

      Unless laser propulsion becomes cheaper on the same scale, it is more efficient to add a small braking engine than to build thousands of identical lasers.

      2. An LCD shutter would be able to convert a continuous beam into a series of pulses. However, it would only be able to 'block out' the beam. It doesn't focus the energy.

      Pulsed lasers today reach dozens to thousands of gigawatts in power. This allows the propellant to heat up to the desired temperature so quickly that it does not have enough to time to expand and blow apart before it reaches the desired temperature.

      A continuous laser, turned into a pulsed laser by an LCD shutter, would still have to deliver the necessary energy at the necessary rate t heat up the propellant before it blows up. This means that the peak power of a pulsed laser would have to be equal to the continuous power of the laser+LCD combo.

      In other words, the concept is useful... if you have petawatt continuous lasers. Today, the average power of massive pulsed lasers is a few watts, because pushing so much heat through the system makes for very long cooling and recovery times...

      World record: http://www.laserfocusworld.com/articles/2017/02/llnl-hapls-petawatt-laser-reaches-highest-average-power-ready-for-delivery-to-eli-beamlines.html

      16 J pulses at 3.3HZ = 4.84W average power.

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    2. 1.Perhaps we could extend the trick another logical step. The light sail could have a series of transparent sheets. Individual sheets could be left behind and manipulated by rigging into Frensel lens.

      A continuous line of these, left behind periodically as the lightsail accelerates, could refocus the beam as it passes through, passing it from lens to lens like a miniature laser web until it reaches the lightsail.

      2.What I meant was, use an LCD shutter to pulse the laser as it arrives at the ship, in front of the optics that focus said laser onto the propellant. This would allow use of a continuous beam to drive a ablative laser pulse rocket without the inefficiencies of a pulsed laser, in theory.
      Is that feasible at all?

      As another thought entirely, a magsail or electric sail could be used instead of a rocket for the deceleration phase. Both would drag against the interstellar medium as well as solar wind from the destination.

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    3. Extending the laser web is one of the solutions I mentioned to the limitations of beamed propulsion.

      The LCD shutter would work, yes, but the requirements (a continuous beam as powerful as a pulsed one) would make its use extremely inefficient for the task.

      I looked into the interstellar medium drag sails concept, and it was interesting, but it has such a low 'speed limit', in a few percent of lightspeed, that applying it to fast interstellar travel would not be possible. Would you really build a massive magsail with heavy magnets just to shave of 5% off your 42% of lightspeed velocity over a period of decades?

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    4. "The problem with this method is that it is difficult to construct large-scale (GW) high-pulsed power (PW) high-pulse energy (MJ) lasers that are useful for interplanetary propulsion compared to much lower-energy pulsed lasers (GW/kJ) or simply continuous lasers."
      You seem to be implying here that a high energy continuous laser would be either more efficient or easier to build than a pulsed laser of the same output. Is this not the case?

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  2. In my universe, interstellar travel is done by antimatter propulsion for humans, military and important cargo; laser sails for less - important cargo and more civilian transport, because the antimatter is controlled by government; other "civilian" interstellar propulsion is the fusion pulse ship working on principle of Medusa that I found on Atomic Rockets; and also, there are some more prototype ships, powered by blackholes, again, controlled by government and biggest companies.
    Oh, and I forgot to mention that in my universe, there are some unobtanium materials that can't be syntetised, so they have to be shipped from mines to detonations.
    What do you think about it?

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    1. Its got the elements you need to justify interstellar travel, so here's some questions to put you on the right track:

      -What are the unobtanium materials used for? It has to be such an incredible benefit that we are willing to invest huge amounts to get them.

      -Black hole/antimatter = these are ways of storing energy. Black holes can't really be captured in the wild, because the smallest ones mass more than the Sun. Creating them artificially required a lot of energy and some strange matter. Antimatter is the same. It cannot be harvested realistically - its has to made. It cannot contain more energy than was consumed to make it, so it has to be considered as an energy storage method...

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    2. A micro black hole can be used for matter-energy conversion. If it is small enough, it produces non-negligible to massive energy in the form of Hawking radiation.
      The trick is to feed it with matter at the same speed as it emits it back. Too little and it goes runaway, emitting more and more, making it harder and harder to feed, until it disappears in an orgy of energy. Too much and it is quenched, and you have to wait a long time (hours, years, millennia) before it is emitting useful amounts of energy again.
      That and feeding something the size of an atom's pinhead.
      And keeping it in place - the safer it is, the heavier and the more you need of them for energy.
      And collecting the charming mix of gamma rays and high energy particles that is said Hawking radiation.
      And convince a star system that no, you won't let a runaway MBH punch their planet in the face.

      My point being, MBH can be more than energy storage, even through creating it in the first place is a massive, energy-hungry ordeal and they are inherently unstable in a way that make 1st generation Soviet fission powerplants run Party administrators with no knowledge in nuclear engineering a model of safety.

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    3. Ok, so, for now I have one material that is used to manipulate gravity in a very specific way, and for now can be used only to make artificial gravity on spacecrafts; second material is standard SFnal room temperature superconductor type of thing, that allows building drives many times better than before, and also makes coilguns easier to build; I don't know what to put next, maybe the strange matter or something? Also, the distances between stars are shorter in my universe, so that makes it more viable to transport things that normally wouldn't be economical to do so.
      Black holes and antimatter are probably the substances with highest energy density of all materials, so their usage in interstellar travel would be essential I think. But I am not sure which one is better for powering the drive, what do you think?

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    4. A-grav could also be useful in an industrial setting. Even if all it can do is produce a gravity like force towards itself.

      Imagine being able to heat and mix materials without a physical stirring device, or molding them into the desired shape while fluid and letting it freeze in place.
      Or using it to manipulate MBHs without an electric charge.

      These are all applications that can happen within a box, so surrounding the work area with A-grav generators can't be that hard.

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    5. @MrAnderson: focus on properties that cannot be replicated by conventional technology. Artificial gravity can be generated by spinning habitats, while super thermoconductors cannot.

      Something that can reliable generate or emit muons can allow for cold fusion. Room temperature gigawatts, anyone? (https://en.wikipedia.org/wiki/Muon-catalyzed_fusion)

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    6. You can make artificial gravity by spinning, but it makes vehicle less structurally hard, bigger, gives some technological problems, and still gives worse results. On space warship, artificial gravity without spinning would have many benefits, like smaller cross section of ship, more easy to armor habitat, and better accomodation for crew. On civilian vehicles it could mean small crafts with artificial gravity, and also it would be possible to make true "belly landers" - the gravity would be in the same direction in space and on ground. Also, gravity manipulation could give us antigravity using crafts - flying cars anyone?
      Muon generating material - I like this idea, totally forgot about muon catalyzed fusion.
      Also, what do you think about making a blogpost analysing interstellar drive that uses blackhole?

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    7. Given the mass required for spinning a hab module, and even pivoting it around to deal with milligees from the engine, I'd say that a small arti-grav mechanism would be very useful. My own setting has centrifuges for space manufacturing purposes and a few passenger craft, but otherwise minimal spin habs. Better medicine and exercise machines seem like a better hard sf solution.

      I do find it interesting though that people always assume that such a mechanism that allows flying cars.... what if it can only pull things down like a tractor beam? You might have aircraft with a 'tractor-beam' like jet engine, but a car might not be possible.

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  3. Hm. What about a ram-assisted rockets? I.e. rockets, which used interstellar medium (ionised and scooped by the magnetic fields) only as propellant, while having the fuel supply onboard (or, possibly, using the Sol System-based laser to provide them with power?)

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    1. Ram-assisted rockets would fall into the nuclear propulsion category. While they overcome the limitations of the original Bussard Ramjet design by providing the fusion power themselves, they would not be terribly useful in our stellar neighbourhood.

      For one, the density of the interstellar gas within a few lightyears of our Sun is terribly low. Second, the exhaust velocity of the fusion products (about 20% C maximum, usually only a few %) means however much drive power you have, you are not going to get a lot of thrust out of the micrograms of hydrogen you are collecting over propellant. Finally, the amount o hydrogen you collect is determined by how fast you are going. Starting from a very low velocity means very little propellant is collected, which means low thrust, which again prolongs the acceleration... this means it is a terrible method for either starting up or braking to interplanetary speeds.

      A laser-energized ramscoop however... that's original. Never heard of it before. I'll look into it.

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  4. HM. Well, of course the stellar ramscoope is useless on low velocity - so the initial propellant supply must be carried. But, only to accelerate to the ramscoop effective velocity. So, to reach 20% C on ramscoop-augmented ship we need less than a third of propellant that the usual fusion rocket with the same thrust would require.

    And, of course, ramscoops are excellent decelerators. We could use both the scoop as "magnetic parachute", and as collector of reaction mass to our decceleration. We could fill our propellant tanks while cruising with enough propellant to the whole braking.

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    1. I doubt that ramscoops are good braking devices - the drag is only significant at high velocities, because drag forces increases to the square of the velocity.

      To give you an idea of the forces involved:

      Imagine we have a 100km wide magnetic field that perfectly halts gas particles. The interstellar medium in the Local Bubble has "0.3 atoms per cm^3". That's 300000 atoms per m^3.

      Let's give out spaceship a cruising velocity of 0.5C. It encounters 353 zettaatoms per second. The mass per atom averages down to that of monoatomic hydrogen, which is 1.66*10^-27 kg.

      This spaceship encounters about 0.5 grams of matter per second.

      The drag force is 75kN. Is this enough to stop a spaceship capable of generating a 100km magnetic field quickly? It depends on the strength of the magnets.

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    2. Robert Zubrin preformed several simulations of so call "magsails" and found they were actually rather good deceleration systems, especially if it is made using super conductive coils as the field generators because it can make a sail field hundreds of miles wide.

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    3. Magsails are a great way to reduce the speed of a probe without adding a lot of mass.

      However, their effectiveness depends on the strength of the magnetic field of the system's star. It falls rapidly with distance, and has a maximum strength of a nanotesla in the heliosphere or 0.1 tesla in the middle of a sunspot (https://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)).

      The practically limits the usefulness of magsails to a few percent of the speed of light. The velocities required for fast interstellar travel are much higher.

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  5. But why we need rely only on drag? Our scoop, while providing drag, also could do its primary function: collect reaction mass. After our velocity would drop below the "effective drag" level, we would just fire up the engines, using the collected reaction mass.

    I.e. the ramship flight is something like this:

    * Ship launched from home system.

    * By using the onboard supply of reaction mass (or, perhaps, laser acceleration), ship accelerated till the ramscoop start to work.

    * Using the ramscoop, the ship accelerated to the 20% C.

    * Ship coast, while ramscoop still in action - but just collecting reaction mass, without using it.

    * Close to the destination, the ship maxed the ramscoop to upper limits, using it as ram parachute.

    * When ship decelerated over the minimal effective speed of ramscoop, the ship would fire the engine, and deccelerate by the collected reaction mass.

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  6. When thinking about cargo transport in the Solar system, I usually think of the equivalent of ISO containers (AKA Sea Cans) being shot to their destination by a mass driver. There is no requirement for a ship, life support or any of the other paraphernalia except for a corresponding mass driver at the receiving end which can decelerate the incoming cargo. The minimized energy and other costs, and I suspect that unlike SF novels where tramp cargo ships roam the space ways, this will be the main form of transportation of bulk cargo.

    Now, if you scale up this idea, you can effectively send your interstellar cargo as well, although remembering the calculations done in Marshal Savage's book "The Millennial Project" suggested the mass driver would be an artifact that spanned multiple AU. I don't have a copy of the book any more, but I'm thinking that this would extend from the inner Solar System to the orbit of Neptune...

    This would be great for sending the one way probes (they could have their own rocket engines or solar sails for deceleration), but not so much for getting home. The first order of business for the colonists or colonizing robots would be to begin assembling a corresponding was driver at the new star system, both to receive incoming pods (as the mass driver becomes bigger, it can take more and more of the job of deceleration), and to launch cargo back to Earth (being scooped up and decelerated by the Solar System mass driver).

    Routing and scheduling issues are left as exercises for the reader.

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    1. Investment projects are typically expected to fund their own growth. The cost of a multi-AU mass driver would have to come out of the profits from selling interstellar products...

      This creates a chicken-and-egg problem with the first interstellar shipment to arrive. You haven't generated the money yet to build the infrastructure needed to catch the payload.

      What will likely happen, at least for the first couple few shipments, is that the payloads will stop at the destination system under their own power. The problem is, for the mass driver idea, is that once you have the colony programmed to collect fission fuels and add nukes to the payloads, you can't update the programs for 4-10 years. That's 4-10 years of any infrastructure project being kinda pointless, then another 10+ years of 'already on their way' payloads arriving.

      Complacency and long-term planning are human truths - it will be difficult to motivate us to build such an immense mass driver for no monetary benefit. The 'cost' of using independent nuke drives is absorbed by the colony, so you're not saving any money by building the mass drives.

      Instead, we'd have to wait for the colony's size and output to become so massive that supplying the fission fuels becomes a restricting factor.

      But then again, by the time this happens, we might have programmed the colony to use fusion fuels, increased the power of an interplanetary laser transport web to levels that can handle interstellar payloads or simply have moved on to new star systems...

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    2. The mass driver pushes the pods out to the colonies, reducing their energy and reaction mass requirements by a considerable amount. It is taken as a given that they will need to decelerate on their own in the target system for the initial flights while things are getting set up.

      Interestingly enough, while the return cargos might have to be accelerated by on board power or one of the other means described upthread, the mass driver in the Solar System already exists to decelerate the incoming cargos. Once again, this reduces the propellant and energy fractions considerably, since the catch is done using the on board systems of the mass driver in deceleration mode.

      Not having to accelerate or carry the fuel needed to decelerate is probably the biggest advantage of the Solar System mass driver, and smaller and cheaper pods going out (and smaller and cheaper pods coming back) will considerably increase the profitability of the system as a whole.

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    3. Ah, but this runs head-on into the criticism I had for current interstellar exploration designs. Who will build the mass driver before the colonization attempt is successful?

      It will only be useful after the interstellar seeding attempts have returned with a bouquet of interesting products. This has been a truth tested again and again: people don't put big money into projects without a reasonable expectation of reward.

      Now, the mass driver idea is entirely reasonable if interstellar travel is a regular thing that stands to gain in efficiency and profitability from its use. But... the implication is that you've already established a system where you don't pay a penny for an automated colony to deliver tons of valuables to your doorstep.

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  7. Hello!
    The track propulsion is a awesome way to deal with the tyranny of the rocket equation. How do I calculate the max velocity for track propulsion? If the ship can accelerate 10m/s for 27.6 million seconds, will it reach 0.92c? Or do I have to convert the relativistic velocity to a newtonian equivalent of it? Also won't track propulsion allow for relatively cheap RKKV's?

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    1. It is indeed.
      The max velocity will depend on the mass of the track and how it provides propulsion.

      A kinetic impactor track is composed of inert impactors that collide with a small puff of propellant gas on the travelling spaceship. The resultant plasma explosion is caught by a magnetic field to generate thrust.

      With this track, you get more and more energy per impact the faster you travel. Conversely, its pretty poor at low relative velocity.

      If you have a track composed of 1kg impactors spaced by 60000km, launched out of the home system at 0.1C, you'll start out with 4.5TJ released per impact. At higher velocities, you'll have even greater relative speed and incredible effective power output. At 0.5C, the relative velocity is 0.4C and you'll meet an impactor every 0.5 second. Your output will be 14.4PW.

      However, as you might have noticed, there's a range of velocities where your relative speed with the impactors approaches zero. You cannot extract any energy from impacts and cannot break 0.1C.

      This can be solved by a two-way stream between you and your destination. You ride one stream up and the other stream down.

      Another propulsion type uses a stream composed of fusion fuel. Your relative velocity is used to compress and ignite a pellet of fuel to release a lot of energy. The energy extracted from the stream is mostly independent of your relative velocity, as even small differences (~100km/s) can be used to ignite fusion.

      You are limited, however, by the velocity of the fusion products. If the plasma explosion you are riding is slower than your own velocity, then you won't extract any energy.

      Yet another propulsion system uses simple bags of gas. You scoop up the gas and channel it into your rocket engine. This is the track type the RAIR depends on.

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    2. Some infos for the RAIR Track, please? Would it avoid the problem Kinetic Fusion has? The fuel isn't slowed down.

      Again wouldn't this technology cause RKKV's being feasible? With 27,6KT D-He³ I am able to accelerate an 2,67kT ship to 0.92c. with a yield of 89 teratons of TNT. The Chicxulub asteroid had a yield of 130 teratons for comparison.

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    3. Sorry, I'm not familiar with actual proposals for RAIR. If the 'problem' you mention about kinetic fusions is the reason I have for maximum velocity... well, it's a general problem fro all types of fusion, as maximum particle velocity is a function of maximum temperature, and that is a physical property of the fusion fuels you are using. Nothing can be done about that!

      This doesn't make RKKV's possible because accelerating your track will produce a lot of waste energy which can be detected directly as a laser pointed at your target system or indirectly as a spike of waste heat. You will produce relativistic projectiles out of spaceships riding the tracks, but you'll lose the largest advantage of RKKVs, which is that they give little to no warning before they arrive.

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