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  1. @ItsJustLuciCould you please provide an estimate for how much RAM is needed to play with the 32k texture pack in RC1?
  2. I am very impressed than an 8 year old mod is still being maintained. Kudos to @NathanKell
  3. This is from the latest ToughSF blog post. Read here: http://toughsf.blogspot.com/2021/03/fusion-highways-in-space.html Fusion Highways in Space A transport system that can get spacecraft to Jupiter in 10 days, but without a massive onboard reactor, using antimatter fuel or riding a gigantic laser beam? What we need instead is a Fusion Highway to connect the Solar System in unprecedented ways. The art above is by GrahamTG. It depicts a Bussard Ramjet, which is relevant as all the same components (collection scoop, reaction chamber, magnetic nozzle) are necessary for the Fusion Highway to work, but are used in slightly different ways. The ideal rocket In Star Trek, propulsion is never a problem unless the plot demands it. If you had to imagine the perfect rocket, what features would it have? Solving the troubles we have with our existing chemical-fuelled engines can serve as a starting point. Limited specific impulse, limited thrust, great complexity and high cost are standard features of today’s rockets. Logically, a perfect rocket would have maximal propellant efficiency, incredible thrust, minimal complexity and cost… or how about no propellant at all? The perfect rocket takes us up to relativistic speeds, but is also lightweight and accelerates quickly. It is instantly available and safe to use. Only a few propulsion systems have approached this ‘ideal’ status. A Bussard Ramjet, as initially conceived, would need no propellant except what it could gather from the interstellar medium, and it could accelerate all the way up to the speed of light and back. Relativistic ramjet. As we know today, it didn’t really work as advertised. An antimatter beam rocket promises amazing performance with great thrust and efficiency, but fails with regards to cost and safety. Fission fragment propulsion attempts to provide similar efficiency and uses a much safer fuel, but it lacks thrust and no-one would call it a perfect rocket. There is another type of candidate for ‘perfect rocket’ status. Externally propelled ‘beamrider’ rockets leave the power and propellant at home and receive instead a beam that they only have to convert into thrust. Laser-driven sails are the most famous example of this approach. Powerful generators produce a laser beam that gets focused by a huge mirror so that it can concentrate its output onto very distant targets. That target, a spaceship, only has to reflect the laser beam to accelerate towards its desired direction of travel. A kinetic mass-beam rider and its magnetic nozzle. Kinetic mass-beam propulsion creates a stream of high velocity projectiles that the target can deflect magnetically. However, you would need a very expensive beaming installation or very long accelerator to make these beamrider concepts practical. We will be focusing on another external propulsion system that has many advantages over laser sails and kinetic streams. The ‘beam’ is a trail of fusion fuel pellets that is simply pre-positioned ahead of a spaceship so that it can ‘ride’ it with no additional power input of its own, up to relativistic speeds. Fusion Highways There are three elements to a Fusion Highway: -A ‘road-laying’ system that moves pellets into position -A series of fusion fuel pellets that align into a ‘road’ -A spaceship that ‘rides’ the ‘road’ by igniting the pellets as they pass into a reaction chamber. There are many ways to position pellets in space. There will be very many of them in number, so a positioning method that is very inexpensive would be preferred. The ‘pellets’ are not necessarily dumb masses of frozen fusion fuel. At the very least, they are coated in insulation and devices that report its position (like a corner reflector or low power transmitter). If they are not placed immediately ahead of the spaceship, they would need a method for correcting their position in the long term. The spaceship itself is very simple. It has an opening that guides the pellets into its reaction chamber, using magnets or laser pulses to make last-second adjustments. Specially shaped targets. The reaction chamber holds a specially shaped target mass. Ignition itself is the result of the high velocity impact between the fusion fuel pellet and the target mass. ‘Impact fusion’ can take place at velocities as low as 100 km/s, if we are able to convert the linear force from the impact into a more efficient 2D or even 3D compression. The result is an expanding volume of energetic plasma. It bounces off the fields generated inside a magnetic nozzle so that energy is converted into thrust, and so the spaceship accelerates. When the spaceship reaches the next pellet, the cycle starts again. The main advantages of this method is that the spaceship does not need to have a heavy reactor or a complex fusion ignition system. It just drops masses in front of the pellets and harnesses the plasma with a relatively lightweight magnetic nozzle. Unlike a remote laser beam, the energy that propels the spaceship is not the result of a massive beaming installation, but derived from the fusion fuel on-the-go. None of that energy needs to be transmitted by immense focusing optics either, and it does not get harder to operate as the spaceship gets farther from its starting point. The pellets themselves do not need to have a huge velocity, another major advantage over a concept like kinetic stream propulsion. This means you don’t need massive accelerators to bring the projectiles up to incredible velocities, with the expectation that the spaceship can achieve at least a fraction of that velocity. On the Fusion Highway, the spaceship’s velocity is mostly independent of the fuel pellets’ velocity. These factors mean that a Fusion Highway can be affordable and have open-ended performance. The actual performance of this propulsion system depends on several factors. They are: The mass ratio between pellet and target The impact velocity Fusion fuel energy content Average molar mass of the pellet/target mix Fusion burnup and use efficiency Nozzle thrust efficiency Let’s go through two worked examples to demonstrate how those factors are used. Imagine a spaceship of 1,500 tons travelling at a velocity of 300 km/s relative to a fuel pellet track. The track is composed of 1 kg pellets, composed of 500 grams of Deuterium and Tritium fusion fuel, surrounded by 500 grams of frozen hydrogen ice. It has the potential to release 170 TJ of energy. The mass ratio between pellet and target is 0.001; this means the spaceship is dropping a 1 gram target for the 1 kg pellets to hit. The impact velocity is 300 km/s. At this velocity, the impact of 1 gram releases 45 MJ of energy, enough to ignite the fusion fuel if the appropriate techniques are used. The frozen hydrogen ice can be shaped to help direct the kinetic energy of the impact into a compressive force that ignites the fuel. We know that the maximum potential for the fusion fuel is 170 TJ, but not all of this energy will be transferred to the spaceship. Firstly, not all the fuel will undergo fusion. The burnup percentage might be just 10%, so only 17 TJ is released. Of that energy, 20% will be in the form of X-rays and charged particles, which will be easily converted into heat by the frozen hydrogen layer. 80% will be in the form of neutrons, which escape more easily. However, hydrogen ice is an excellent neutron absorbing material, and it should be thick enough for half the neutrons to be captured and turned into heat, so the final amount of ‘usable’ fusion energy is closer to 10.2 TJ. The kinetic energy from impact adds a negligible amount. All this energy converts the target+fuel mix into a very high temperature plasma that expands (if timed right) inside the spaceship’s magnetic nozzle. The temperature is high enough that all particles involved become fully ionized, which simplifies our calculations as we can use perfect gas laws with reasonable accuracy. We also assume that all heating is done while the target+fuel mix is solid (so at constant volume) and that the contribution of phase changes and ionization is negligible. The heat capacity of a perfect monoatomic gas at constant volume is 12470/Molar Mass, in J/kg/K. The temperature of a gas is its energy density (Joules per kilogram) divided by its heat capacity. The rate at which the gas expands is the Root Mean Square gas velocity, which is (24942 * Temperature / Molar Mass)^0.5. If we put these equations together, we find that the molar mass cancels out and therefore: Plasma RMS velocity = (2 * Energy Density)^0.5 In this example, 10.2 TJ of energy is distributed in 1.001 kg of matter. This gives a value for the plasma expansion velocity of 4,511 km/s. A noteworthy consequence of molar mass and heat capacity cancelling out is that the nature of the gases expanding does not matter. In theory, we are free to use abundant propellants like water or silicate rocks instead of being restricted to bulky hydrogen, although lighter molecules absorb neutrons better and lead to greater overall efficiencies. We must make an adjustment to this expansion velocity. The fuel pellet is initially retreating from the spaceship at 300 km/s. After impact, it loses 0.1% of its relative velocity, becoming 299.7 km/s. This must be subtracted from the expansion velocity to find the actual velocity of the plasma relative to the magnetic nozzle. That value becomes 3,198 km/s. Fusion plasma within a magnetic nozzle. The performance of this propulsion system is quite spectacular. Nozzle thrust efficiency is realistically 80%, so the spaceship inputs 1 gram, and it gets 1.001 kg exiting the nozzle at 2,558 km/s. The effective exhaust velocity is multiplied by a thousand to 8.52 times the speed of light. If there is a 300 km gap between the fuel pellets, the net propulsive power the spaceship outputs is 3.27 TW and its average acceleration is 0.17g. Now let’s repeat these calculations for a much higher relative velocity. The same 1,500 ton spaceship rides a track of the same 1 kg pellets, but at 90,000 km/s. The mass ratio between pellet and target is increased to 2. The spaceship drops a 2 kg frozen hydrogen target to impact the same 1 kg fuel pellet. The total mass of the mix after impact is 3 kg, so the ‘retreating velocity’ is reduced to 30,000 km/s. This also allows us to extract two thirds of the potential kinetic energy from the impact; 2700 TJ. The fusion fuel is compressed by a large amount of target material at much higher velocities, so excellent burnup percentages are to be expected, up to 25%. We can hope for 42.5 TJ to be released, and all of it to be absorbed by the extra target mass we’re putting in. Total energy adds up to 2742.5 TJ. Energy density is 2742.5 TJ over 3 kg or 914.17 TJ/kg. We can expect a plasma expansion velocity of 42,759 km/s. You will notice that the margin between plasma expansion velocity and retreating velocity at 12,759 km/s is much slimmer than in the previous calculation. The spaceship puts in 2 kg of propellant and gets 3 kg of plasma, so its effective exhaust velocity is a bit higher at 19,138 km/s, or 15,310 km/s if we consider nozzle efficiency. That same 300 km gap between fuel pellets means that the spaceship encounters 300 pellets per second. Net propulsive output is 105,478 TW (if the spaceship’s nozzle can survive it!) and average acceleration is 3.12g. Velocity Bands The performance of the Fusion Highway depends on the velocity of the spaceship relative to the fuel pellets. There are four distinct ‘velocity bands’ that significantly affect performance: Logarithmic scale on y-axis, all units in C. Green is Fusion band, Yellow is Kinetic band, Red if Relativistic Band. -Sub-ignition band The sub-ignition band of velocities is where the relative velocity of the fuel pellets and the spacecraft is insufficient to ignite fusion reactions. With dumb pellets of fusion fuel and a simple target, this can be as high as 1000 km/s. With specially shaped sphere-section imploding targets and other features that improve compression upon impact, this can be brought down to below 100 km/s. Further into the future, a few tens of km/s might be all that is needed for impact fusion thanks to hotspot ignition or the assistance of external magnetic fields. A Fusion Highway would have multiple entry and exit ramps. A spaceship would have to reach this minimum velocity by some other means before it can start using the Fusion Highway. Think of it as a car accelerating along the entrance ramp to a highway. This could be accomplished by consuming the first few fuel pellets using an onboard ignition system. The frozen fusion fuel could be compressed by magnetic fields, blasted by plasma jets or compressed by ablative laser beams… ignition of the fusion reaction would produce energy that is converted into thrust, allowing acceleration up to the impact fusion threshold velocity. It would not be an ideal solution, as the heavy fusion ignition system would not be of much use for most of the spaceship’s journey, but it would allow for free entry and exit from the Fusion Highway at any point. A better solution could be the use of ‘boost tracks’ that have a high relative velocity to the spaceship, somewhat like a conveyor belt that the spaceship can ride until it reaches the Fusion Highway at the necessary velocity. The boost track is a series of fusion fuel pellets that are shot at the spaceship’s position at above the threshold velocity for impact fusion ignition, doing away with the need for heavy onboard propulsion or ignition systems. The spaceship can then ride this short boost track and then divert to the main Fusion Highway once it has built up enough speed. If the threshold velocity is very low, then some alternative options become available. For example, the boost track is composed of pellets put on a retrograde orbit that the spaceship only needs to intercept at the right time. A spaceship in Low Earth Orbit would be travelling at about 7.7 km/s relative to the surface. Pellets in a retrograde orbit would be travelling at 7.7 km/s in the opposite direction, adding up to a relative velocity upon impact of 15.4 km/s. Pellets on a retrograde near-escape trajectory, perhaps falling from the Moon, could reach a peak velocity of over 11 km/s and achieve 18.7 km/s upon impact. If these orbital velocities are too low, then interplanetary relative velocities can be used. An Earth-orbiting spaceship facing retrograde fuel pellets along the same orbital path would achieve a relative velocity of up to 7.7+29.8+29.8: 67.3 km/s. -Fusion band A fusion rocket at full blast, featuring liquid droplet radiators. Imagine it has a collection scoop for fuel pellets in front. The fusion band of velocities is where the spaceship’s velocity relative to the Fusion Highway is enough to ignite the fuel pellets by impact. There is a minimum and maximum velocity here. The minimum velocity, as described above, is the threshold for igniting fusion reactions upon impact. The maximum velocity is more complicated. In this band of velocities, the energy gained from each impact is dominated by the output of the fusion reaction. In the 300 km/s example that was calculated in the previous section, 99.9996% of the energy was derived from the fusion reaction. Because the same amount of energy comes from igniting the same amount of fuel, the expansion velocity of the resultant plasma is nearly constant. However, as the spaceship’s velocity on the highway increases, the retreating velocity increases. At very low relative velocities, the difference between expansion velocity and retreating velocity is huge. Effective exhaust velocity is at its highest. At increasing relative velocities, the difference between expansion velocity and retreating velocity becomes smaller and effective exhaust velocity falls quickly. At some point, the relative velocity is nearly equal to the expansion velocity and no thrust is generated; effective exhaust velocity becomes zero. This is the limit of the fusion band. The maximum velocity is therefore close to the expansion velocity of the ‘pure fusion’ plasma. This depends, as shown in the previous calculations, on how much energy can be extracted from the fusion fuel divided by the mass of the fuel pellet. For example, a fuel pellet that is 50% Deuterium-Tritium fuel, has a 10% burnup ratio and is able to convert 60% of the fusion energy into heat would manage an energy density of 10.2 TJ/kg, and create a plasma that expands at 4516 km/s. The maximum velocity in the fusion band using this pellet will be around 4516 km/s. A better pellet helps extend the fusion band of velocities. Deuterium and Helium 3 release nearly 95% of their output in a form that can be converted into heat. Advanced compression and confinement techniques can improve burnup to perhaps 25%. If the fuel pellets can be made entirely of DHe3 fuel, we could manage an energy density of 83 TJ/kg and therefore have a plasma that expands at 12,950 km/s. It is important to extend the fusion band of velocities to be as wide as possible as this is where the outrageous effective exhaust velocities are possible, multiple times the speed of light in many cases. The spaceship only needs to drop the smallest target masses to ignite the fusion reaction, and can then ramp its speed up and down easily. -Kinetic band A RAIR spaceship. After the fusion band’s maximum velocity is crossed, there comes a point where tiny target masses are no longer possible. The target/fuel mix must have a retreating velocity lower than the plasma expansion velocity. Calculations show that this requires a target to fuel pellet mass ratio of over 2, i.e. 2 kg of target mass to catch 1 kg fuel pellets. The kinetic energy added upon impact quickly becomes dominant. In the 90,000 km/s example above, the kinetic energy from the impact represents 98.45% of all the energy that the plasma gains. The fusion fuel in the pellets can actually be replaced with inert material and we won’t see a significant drop in performance (and this will really help keep the overall costs low!). There is an optimal mass ratio between the target and the fuel pellets that provides the best effective exhaust velocity at any impact velocity. Since the fusion output provides only a small fraction of the energy gained from impact, this optimal mass ratio depends mostly on the performance of the magnetic nozzle and less on the composition of the fuel pellets. Furthermore, as the impact velocities increase, retreating velocity increases linearly (it is a momentum transfer) but the kinetic energy added to the expanding plasma increases quadratically. Calculations show that effective exhaust velocity improves gradually at higher velocities. A spaceship can ride the Fusion Highway more efficiently the faster it goes. However, the great reduction in effective exhaust velocity and the extreme velocities involved make this unsuited for interplanetary travel. Also, in this band of velocities, a spaceship travelling along Fusion Highway acts very much like a Ram-Augmented Bussard Ramjet. -Relativistic band After a while, relativistic effects come into play. The equations we’ve used to estimate the performance of this propulsion system tell us that a spaceship can ride a Fusion Highway up to large fractions of the speed of light with only moderate amounts of target masses. However, some assumptions start to break down. For example, we assume that the collision between the target mass and the fuel pellet is elastic, that the kinetic energy is fully absorbed and converted into heat, and that the fusion reaction has time to ignite and spread its energy throughout the mix before it all expands outwards. Some of these things won’t hold up at relativistic velocities. The fuel pellet will start to act instead as penetrating radiation that digs through the target masses. The plasma might expand too quickly for the fusion reaction to transfer its energy efficiently, or it might reach temperatures so great that there is significant energy loss through blackbody radiation before it fully expands. When do these relativistic effects come into play? It is hard to say. 30% to 50% of the speed of light seems like a plausible limit. At 0.5C, the Lorentz factor is only 1.15, but hydrogen acts as 145 MeV radiation and the plasma temperature is supposedly in the hundreds of billions of Kelvin. This is not to say that a Fusion Highway can’t be used beyond 0.5C, but that a much more complicated analysis is required to determine how its performance is affected. What we can conclude for now is that attempting to extend a Fusion Highway beyond the Solar System, to enable interstellar voyages, is a topic that needs its own separate treatment. Interplanetary Design Let’s go through two complete Fusion Highway designs for use in interplanetary travel. One is modest and uses conservative assumptions, the other is more futuristic and fully illustrates the awesome potential of this propulsion method. You will note that we do not go beyond velocities within the fusion band. Entering the -Modest example For the modest example, we will use 0.5 kg fuel pellets that are 10% Deuterium, surrounded by 90% water ice. Deuterium is abundant throughout the Solar System and provides about 80 TJ/kg of fusion energy. Fusion burnup will be about 10% and the usable fraction of that energy is 70%; the expected energy density is about 560 GJ/kg. An interplanetary transport system will consume a lot of fusion fuel and propellant, so it would appreciate getting to use cheaper options. Deuterium is a relatively abundant fusion fuel and it can be extracted from water anywhere in the Solar System. This could be a solar sail carrying deuterium off a comet resupply station. Each fuel pellet is covered in multiple layers of very thin reflective aluminium sheets, which serve as thermal insulation from sunlight, as well as a ‘harness’ made of plastic wires. That harness allows for clusters of pellets held inside a payload bay, and attached to large solar sails. These sails depart from Earth and dive down towards the Sun. A close pass allows for great acceleration and a trajectory that shoots back up to Earth with a relative velocity of about 100 km/s. They then drop the fuel pellets in a line, forming a boost track. Each solar sail can position these boost tracks with only a few months’ notice. It is more practical to send off multitudes of these sails, to create regular opportunities for travel, perhaps every week. After dropping off their payload, the solar sails can adjust their outwards trajectory to encounter a gas giant planet for a gravity assist back into the Solar System, and as they are dozens of times lighter than before, they can very slowly cancel out their velocity and return to Earth. The Fusion Highway itself is a 35 million km long track of fuel extending away from Earth, consisting of around 12 thousand fuel pellets. They are positioned in sections of perhaps 100 pellets by solar statites, which are solar sails large enough and lightweight enough to counter the Sun’s gravity and hold a position in interplanetary space indefinitely. Another 35 million km long segment leads up to the destination. The spaceship is a 711 ton vessel. It carries a 100 ton payload and 11 tons of target masses. 500 tons are dedicated to the propulsion system, including a magnetic nozzle that is only 50% efficient at converting the expanding plasma into thrust. Small spacecraft performing rapid trips to the Outer Planets and back. We set the power density of the propulsion system to 2 MW/kg (totalling 1 TW), which might seem excessive, but note that this is only a magnetic nozzle and very high temperature radiators, nothing else. It must not be compared directly with typical fusion rockets, who have to use heavy ignition equipment, power recovery cycles and lower temperature radiators. The remaining 100 tons consists of shielding, electrical equipment and comfortable living spaces. As mentioned before, the performance of the Fusion Highway depends on the velocity you ride it at. In the table below, we can see that the deuterium releases 280 GJ of useful energy, allowing for a plasma expansion velocity that is a rather constant 1058 km/s. The initial effective exhaust velocity is an impressive 479,420 km/s, dropping to 79,800 km/s at a relative velocity of 900 km/s. Here’s the performance table: Repeating the calculations for every 50 km/s increase in relative velocity allows us to calculate the necessary mass ratio required to accelerate across each 50 km/s step. To accelerate from 100 km/s to 150 km/s, the spaceship needs to expend about 147 kg of target masses. For the final 850 km/s to 900 km/s, it expends about 890 kg. The cumulative mass ratio for accelerating all the way from 100 km/s to 900 km/s involves multiplying the mass ratios of each step, for a final value of 1.008, or about 5.6 tons on top of the spaceship’s 700 ton dry mass. Here’s a table of mass flow, acceleration and displacement parameters for a spaceship limited to 1 TW riding this modest Fusion Highway: Acceleration increases over time because the exhaust velocity of the plasma decreases the faster the spaceship goes. For the same propulsive power, lower exhaust velocity translates into higher thrust. Because each 0.5 gram of target mass is matched with 0.5 kg of fuel pellets, we can say that accelerating up from 100 km/s to 900 km/s requires 5600 tons of fuel pellets. That’s 560 tons of deuterium and 5040 tons of frozen water. To slow down back to 100 km/s and with an additional margin on top, we used a mass ratio of 1.016, or about 11 tons of target masses. What sort of performance do we get out of this set-up? The spaceship has an initial acceleration of 0.6g. It takes 23.2 hours to complete its acceleration, with a peak acceleration at the final pellet of about 3.6g. It will cruise at 900 km/s, enough to get it from Earth to Jupiter in 10 days, or from Venus to Neptune in 2 months. As an interplanetary transport system, it does not require very advanced technology or huge amounts of rare fuels. It is rather easy to replenish the bulk of the fuel pellet material, and while the 35 million kilometre long tracks might seem excessive, they are only constellations of a few hundred satellites holding positions in interplanetary space. Today’s mega-constellations are far more complex! Replenishing thousands of tons of water and hundreds of tons of deuterium would be the bigger challenge, but there are a few months to accomplish that task while the boost track solar sails make their trip around the Sun. -Futuristic version For this second example, we use more optimistic assumptions and have no care for costs. We will use 10 kg fuel pellets that are 0.5% Deuterium and 0.5% Helium-3, surrounded by 99% frozen hydrogen. Burnup will be 25%, and the usable energy fraction is 95%, so each pellet is expected to release 8.38 TJ. We are smothering the fusion fuel in inert mass so that average energy density and therefore exhaust velocity is reduced, in favor of increasing thrust and acceleration. A large SDI-era railgun meant to shoot masses at many km/s. In the future, waiting around for months so that a booster track is ready might be inacceptable, as there will be a need for trips to be completed upon short notice. So, instead of propellant-free and very cheap solar sails, we use coilguns to shoot out a boost track. The coilguns will only need to achieve velocities of only a few km/s, but this will be sufficient (and energy/infrastructure costs remain low). The boost track will consist of fusion fuel pellets encapsulated in fissile material, such as Plutonium 239. Fission reactions can be ignited by high velocity impacts, and at much lower velocities than fusion reactions. A fission-fusion hybrid booster track will be very expensive, but it would mean that a spaceship can start impact ignition from an initial velocity of less than 1 km/s! Each fission-fusion pellet consists of 2 kg of Plutonium surrounding the 10 kg fusion fuel/hydrogen mix described above. They are struck by 1 kg frozen hydrogen target masses at multiple km/s. Average energy density after impact is 12.95 TJ/kg, so the plasma expands at 7090 km/s. The penalty from the retreating velocity is negligible. Effective exhaust velocity (1kg in, 13kg out at 80% efficiency) is 942,170 km/s. A mass ratio of just 1.004 is needed to accelerate from 0 to 200 km/s. The boost track would be about 3.6 million km long. The spaceship would accelerate at about 0.67g along this track and exit after 8.9 hours. The spaceship then switches from the booster track to the main Fusion Highway. It is going faster than what is strictly necessary to ignite an advanced fusion fuel pellet upon impact, but it will help enable the following setup: The Fusion Highway will consist of several lanes. The ‘high speed lane’ is composed of many 10kg fusion fuel pellets, intended to be consumed by large spacecraft trying to get to places quickly. Parallel to this are ‘service lanes’ that propel smaller ‘tender’ craft that replenish the high speed lane. A tender craft has its own magnetic nozzle and is loaded with fuel pellets. It accelerates up a service lane to 100 km/s, and then drops off the fuel pellets to replenish the high speed lane. This causes the high speed lane’s pellets to move outwards at 100 km/s. Since the spaceship coming off the booster track is travelling at 200 km/s, it can catch up to the moving high speed lane pellets at 100 km/s. Why have a moving lane? The Fusion Highway will have to be millions of kilometres long. Having tender craft travelling at 100 km/s means that its entire length can be replenished quickly. For example, a 1000 km/s Fusion Highway is 69 million km long, and the tender craft can get it ready for the next trip in about 8 days. A 5000 km/s Fusion Highway will be 495 million km long and be ready every 57 days. Even faster tender craft, and a longer booster track to catch up with them, would be necessary for the longest Fusion Highways. The tender craft can also correct the positions of the pellets that they have dropped on their return journey back up the service lane. Frequent resupply flights can provide near continuous adjustments to pellet positions. The spaceship we’ll use carries a 100 ton payload. It has a 1000 ton propulsion system that is 80% efficient and can handle 10 TW in the exhaust plasma. 100 tons are dedicated to other equipment, and 7.6 tons to target masses, adding up to a total mass upon departure of 1207.6 tons. The spaceship can choose to exit the Fusion Highway once it has achieved its desired velocity. This can range from 100 km/s to 12,000 km/s (in addition to the 100 km/s granted by the boost track). It has enough target masses to reach the maximum velocity listed in the table below, and slow back down again. Acceleration on the Fusion Highway starts off at 2.7g, peaking at 23.5g at the highest velocities. It is likely that pellets start getting skipped to reduce acceleration if there is a human crew onboard. If a middling velocity of 2000 km/s is deemed sufficient, then the spaceship needs to ride the Fusion Highway for 27.2 hours. The acceleration length is 148 million km. Departing from Earth, the spaceship can reach Saturn in just 7.4 days. The spaceship expends 704 kg of target masses altogether, matched by 7040 tons from both acceleration and braking tracks. Closer destinations are limited by the length of the Fusion Highway. A continuous line of pellets from Earth to Mars, if both planets are on the same side of the Sun, may span as little as 55 million km. In this case, we can treat the spaceship as a classical Torchship that maintains a constant acceleration and perform a Brachistochrone trajectory: accelerate up the mid-point and then slow down to a stop. With 2.7g of acceleration, such a short trip can be completed in 25 hours. But what if we want to blaze a trail across the Solar System at 12,000 km/s? The spaceship would need to spend 3 days on either end of the track, and the acceleration length becomes 8.34 AU long, so the minimum trip distance is 16.68 AU. One possible use for such a velocity is crossing from Saturn to Neptune if they were on opposite sides of the Sun… a 39.6 AU trip which could be completed in a mere 9.4 days from stop to stop. A total of 7.6 tons of target masses would be expended on the system-spanning dash, matched by 76,000 tons of fuel pellets, of which 380 tons is rare Helium 3. That would make it a pretty expensive endeavour for delivering just 100 tons of payload. Comparison with alternatives These performance figures stand out even more if we try to recreate them using alternative propulsion systems. Let’s work out how large a fusion rocket we would need, starting with the modest Fusion Highway example. Normally, fusion propulsion can manage to produce the same exhaust velocity as the expanding plasma within a Fusion Highway rider's magnetic nozzle. A deuterium-burning rocket would have a maximum exhaust velocity of 12,900 km/s or 4.3% of the speed of light (it’s the average velocity of the reaction products from a ‘naked’ reaction), so normally accelerating up to 900 km/s and back down again is no problem. However, needing to have an electricity generating loop and fusion ignition equipment would bring down the average power density of a realistic fusion rocket down to 300 kW/kg at best. The maximum average acceleration of a 300 kW/kg fusion rocket that aims to achieve 1800 km/s of deltaV is about 0.02g. This limit exists even if we increase the power of the propulsion system to 10 TW or even 100 TW. At this acceleration, it would take 53 days to reach the desired 900 km/s transit velocity, which is clearly insufficient. If we want the same trip times, acceleration must average 1.1g, which means that power density must be increased massively. This becomes unfair to the assumption made for magnetic nozzle the Fusion Highway rider uses... The traditional fusion-propelled spaceship will struggle to match a Fusion Highway rider's performance, and start to look like Project Daedalus-inspired designs. The futuristic Fusion Highway is even harder to match. To perform a 12,000 km/s dash, a total deltaV of 24,000 km/s is required. A ‘naked’ Deuterium-Helium3 fusion reaction manages an exhaust velocity of 26,700 km/s or 8.9% of the speed of light, therefore we would need a mass ratio of 2.45. If we insist on recreating the 4.63g average acceleration while having the propulsion system representing 99.9% of overall dry mass, then the fusion rocket would need a minimum power density of 2.1 GW/kg. It would deliver 420,480 TW of fusion power. If we add 100 tons of payload and 100 tons of other equipment, we get a 200,000 ton dry mass and a 490,000 ton wet mass. About 145,000 tons of rare Helium 3 would be needed to deliver the 100 ton payload, despite the unfair power density advantage this super-advanced fusion rocket has over the already futuristic magnetic nozzle of the Fusion Highway rider. Now let’s compare the Fusion Highway to the Laser Beamrider. We start with the modest Fusion Highway. Achieving the 1800 km/s deltaV is no problem for a laser-propelled sailcraft. Acceleration is instead the main challenge. A laser perfectly reflected by a mirror delivers 1 Newton per 150 MW. Accelerating at 1.1 g means that each kilogram of onboard mass is matched by 1.62 GW of beam power. But how much beam power can a sail really handle? Zubrin's ultra-thin aluminium sail. A simple solid aluminium sail, even with 90% reflectivity, can only survive a beam intensity of 86.8 kW/m^2 and if reduced to 30 nanometres thickness, the minimum thickness needed to achieve such a reflectivity, it would have an area mass of 81 milligrams per square meter, giving us a propulsion system with a power density of 1.07 GW/kg. It would provide an acceleration of only 0.36g, without payload. To accelerate at 1.1g, a very advanced laser sail design will be required. Jordin Kare proposes dielectric laser sails that can survive much higher beam intensities, but require nanoscale engineering across kilometres-wide surfaces. In one example provided, a sapphire sail that is 57 nanometres thick and able to operate at 1563 K can handle 34 MW/m^2 but only masses 226 milligrams per square meter. Alone, it can accelerate at 100g. Or put another way, 1 kg of this sail material can accelerate 89.9 kg of payload at 1.1g. 100 tons of payload and 100 tons of other equipment could be attached to 2.22 tons of sapphire laser sail. The sail would have a diameter of 3.56 km and receives 327.3 TW of beam power. It will be difficult to keep such a gigantic structure from collapsing under 1.1g acceleration. Solid state lasers would have an efficiency of about 60%, so the electrical input required to generate such a beam is a whopping 545 TW. That’s over 250 times more than the world’s entire electrical output today. A beam generator station would be needed at both departure and arrival ends of the spaceship’s trip, or something like a Laser Web is needed to relay the beam across interplanetary distances. It might be expensive. We can now try to estimate the laser sail performance needed to match the futuristic version of Fusion Highway. Acceleration rises to 4.63g on average. The 200 tons of payload and other equipment must sit at the center of a 7.22 km wide sapphire sail that masses 9.26 tons. It receives 1,425 TW of beam power, requiring perhaps 2,375 TW of electrical power... Consequences A Fusion Highway has some clear advantages over other methods of rapid interplanetary travel. It might not be as flexible as a rocket engine or as versatile as a beamed propulsion system, but it allows small, lightweight spacecraft to reach very high velocities with minimal use of expensive fuels or complex equipment. What multiple Fusion Highways waiting to be used might look like. It does require time to set up and replenish, but as described in the multi-lane futuristic example, the Fusion Highway can be used to replenish itself. Multiple departures in quick succession might have to be served by multiple Fusion Highways aimed in the same direction, while multiple travel windows would require Fusion Highways spaced radially along a departure point’s orbital path. These requirements suggest that busy travel routes would end up having many interconnected Highways, making the ‘road network’ analogy valid. Furthermore, it creates the possibility that small waystations in interplanetary space would have a useful role. A comet full of water and deuterium could replenish routes bringing spaceship to it, and since so little target masses are needed for a spaceship to ride a Fusion Highway, the opportunity cost for using them to visit different destinations is low. This could result in chains of smaller bodies, from moons to asteroids, that can be visited one after the other at low cost to the spaceship. It might be entirely possible to have ‘road trips’ with many stops in space, which would be interesting to scifi authors. Another interesting consequence is that the consumption of large quantities of water and the deuterium it contains would favor the occupation of icy moons and outer Solar System bodies. Interplanetary colonization tends to neglect these sites for their poverty in terms of metals, minerals and solar power. With Fusion Highways, they instead become abundant sources of fusion fuel that are easier to keep connected to a wider interplanetary network of Highways than a dry inner Solar System body like a metallic asteroid. Different users will demand different types of Fusion Highways. The bulk of transportation would be done with the cheapest ices and fusion fuels, which is why we often mention water and deuterium, but there is a performance edge to be gained from using Helium 3 fuels. Some spaceships will have smaller magnetic nozzles that cannot handle as much fusion power, while others will want to maximize acceleration. This suggests that there might be Fusion Highways with small, infrequent pellets, other faster tracks with large, frequent pellets, and even military routes held in reserve that have the highest quality fuels. Uranus and its moons might become an attractive destination. Finally, it is important to consider that Fusion Highways won’t operate alone. They are best served in combination with other propulsion systems, whether it is solar sails that resupply the Highways or independent rockets that can complete the ‘last mile’ of a delivery. It creates the possibility that the typical interplanetary spaceship is actually a multi-modal craft, which uses many propulsion systems that complement each other. For example, a magnetic nozzle and a few target masses are not a major burden to a fusion-propelled spaceship that can also deploy lightweight sails to ride a laser beam.
  4. A most excellent development. I assume this will include a Raptor analogue?
  5. The trouble is power density. Linear generators, as I mentioned in the post, are easy to install inside the spring arms, but they are much heavier for the power they deliver than a rotating generator, by a factor 10+. The issue is where to get that nuclear fuel. There's plenty on Earth's surface, and little anywhere else.
  6. Thanks for the explanation. I'm loving your mods by the way!
  7. I was doing a few calculations and I got some funny numbers. Something like the 'NEXT Ion Thruster - 0.625m' engine. It has an Isp of 6380s and a thrust of 2.1 kN. Engine power in the real world is equal to Isp * 9.81 * Thrust /2, so I get 65.7 MW of power. However, the part mass is only 200 kg. This means the power density is a whopping 328.6 kW/kg. This continues for all the electric thrusters. The 'VASIMR - 1.25m' engine has an Isp of 6000s and a thrust of 24.9 kN, meaning that it outputs 732.8 MW of power. Power density is at 732.8 kW/kg. For comparison, real world electric thrusters have power densities on the order of 0.5 kW/kg, rising to 2 kW/kg with a large design like VASIMR. This means the in-game propulsion is 160 to 360 times more powerful than modern technology allows. Is this the intended balancing for NFT, @Nertea?
  8. This is from the latest ToughSF blogpost: http://toughsf.blogspot.com/2021/01/moto-orion-mechanized-nuclear-pulse.html Moto-Orion: Mechanized Nuclear Pulse Propulsion The Orion nuclear pulse propulsion concept has been around for over six decades now. It is powerful and robust, but lacks the flexibility and features we expect from many more modern designs. Can we give it those additional capabilities? That cutaway is one of Matthew Paul Cushman’s amazing pieces. Basic overview of Orion William Black has plenty of great Orion artwork. There is a lot of information on Project Orion, available mostly here and here. It is best to read through them to gain a complete understanding of how it works. We’ll only give a simple overview to start. Project Orion’s design for a nuclear pulsed propulsion system was pretty simple. A physical plate of steel, protected with a thin layer of oil, faced a plasma jet from a nuclear shaped charge. The force of that blast was translated into useful thrust for the Orion spaceship. In this manner, a propulsion system could tap into the immense power of a nuclear detonation while sidestepping the heat management issues that would normally come from handling such an output. Its thrust was huge, enough to lift thousands of tons into orbit, and so was its efficiency, with an effective Isp of 2,000 to 12,000s. That’s five to thirty times the specific impulse of a chemical rocket, with thrust and efficiency that only gets better as you scale it up. We call this combination of high thrust and high efficiency a ‘torch drive’; a term from ‘Golden Age’ science fiction where authors did not want to spend pages explaining things like deltaV limits and interplanetary trajectories to their readers. A torch drive lets you point at your destination and accelerate to get there. Even today, sci-fi loves this solution. It did have drawbacks though. The fissile fuel in each nuclear pulse charge is inefficiently used, with the majority being wasted. This was because each pulse had to be small, so as to not obliterate the pusher plate, and therefore could not produce the better burnup ratios of large nuclear charges. The rate at which these pulses were ignited could not be varied by much either. Timing the pulses with the motion of the pusher plate, so that the blast would meet the suspension system in the right position, was essential. There were three parts to the suspension system. The first is the pusher plate itself. When struck at a precise angle, it could be accelerated at 50,000g or more without being bent or twisted. It first slams into a gas bag, that acts similar to how a car’s airbags are used in a car crash, to turn a sharp shock into a more gradual shove. Momentum from the plate is then transferred to a set of pistons at a much slower rate. These pistons are connected to rigid springs that convert the series of pushes into a continuous acceleration. When the timing is right, the literally well-oiled machinery is very strong. When the timing is off, things break down. The suspension cycle, in short. If one charge ignited too early, then only a fraction of the suspension length can be used to absorb the blast’s momentum, so it gets translated into a hard jolt. Ignited too late, and it would further accelerate an already retreating pusher plate, with potentially devastating consequences. A complete misfire isn’t great either. The suspension arms would only be partially compressed, and so would not reach full extension on the rebound and it would become unsafe to receive another nuclear blast. The Orion spaceship would have to wait for the suspension to wobble to a full stop, and then use a half-powered charge to restart it from a fully compressed state. Waiting to restart the suspension cycle isn’t a nice position to be in when launching off a planet. Another drawback was the inability to convert any of the nuclear pulse drive’s immense output into electrical power. The two-step suspension system simply acts as a fancy spring to transfer momentum between the nuclear blasts and the spaceship. Most of the time, this is not an issue. Liftoff from a planet or moon’s surface does not take long, so stored power is sufficient. Cost-efficient interplanetary travel consists of short uses of the main propulsion system followed by long periods of coasting, during which solar panels can be deployed. An Orion warship accelerating, from the sadly incomplete sequence here. However, some of the more demanding applications require a lot of onboard power. Military spaceships especially want the ability to both accelerate out of harm’s way, while producing plenty of electrical power to feed lasers, RADARs and other energy-intensive equipment. Fulfilling this requirement means sacrificing payload capacity to mount an onboard nuclear reactor or some other heavy solution. It’s also a problem for very fast transports that want to use the Orion engine as much as possible; they can only extend tiny solar panels while accelerating as anything bigger would get burnt off by the nuclear blasts. Of course, there are many other problems too, that we won’t go into more detail this time. The fact that each nuclear charge is a fully functional nuclear warhead, for example, means that a crash-landing would spill out a full nuclear arsenal, worthy of arming a superpower. Or that the main propulsion system of an Orion ship cannot be used to turn, so huge Reaction Control thrusters would be needed for every single maneuver. We cannot ignore the existence of more modern and more refined nuclear pulse propulsion designs either. Orion was dreamt up in the 1960s and a lot has happened since then. Mini Mag-Orion. Most notably, Mag-Orion and variants thereof. Instead of a physical pusher plate, a magnetic nozzle is used to capture the momentum of nuclear-generated plasma. Fully self-contained bombs are replaced by subcritical masses of uranium. They have to be detonated by external compression devices, such as a Z-pinch or a magnetic pulse. The result; they are completely safe in storage and gain a not-bomb-like-at-all quality. Generating electrical power is a simple repurposing of coils in a magnetic nozzle into Magnetohydrodynamic generators, and turning is accomplished by unequally deflecting the plasma within the nozzle one way or another. However, these more advanced designs cut away at the awesome potential of an Orion drive. The need for large magnets, cooling systems for the nozzle, capacitor banks for the ignition system, all add a lot of weight. Designs of this type have much lower thrust than the original Orion design. They can’t take off from large planets or even operate inside an atmosphere. They move away from that brutal, simple and resilient character that a nuclear Orion engine has, to become something flimsier and more complicated. Perhaps that is an unacceptable compromise, especially for someone seeking specific capabilities, or a sci-fi author aiming for a special aesthetic. Ad Astra Game's RocketPunk, seeking that aesthetic. Could we solve some of the original Orion’s most glaring drawbacks without moving too far away from the image of an atomic piston engine from a bygone era? Moto-Orion We alter the 60 year old design by giving it a crankshaft. It won’t be directly connected to the pusher plate - it can be connected behind the main suspension arms, so that it doesn’t have to receive the shock from a nuclear blast directly and become unreasonably long and heavy as a result. The crankshaft is connected to a crank that turns a large wheel. Depending on the pulse rate of the Orion drive, this wheel will turn at 54 to 69 RPM. A gear train would be needed to increase the RPMs into the thousands, suitable for an electric generator. Also necessary is a counter-torque mechanism, such as a second wheel or even just a counterweight turning in the opposite direction. Please note that the depiction in the diagram above isn't perfect, as all these mechanisms have to find a place in between the springs, hydraulics and other machinery above the suspension arms. A different arrangement would take up less room, but be harder to read visually. The concept is similar to a wind turbine and its generator, except the blades are replaced by a nuclear pulse-driven crank. The power that can be extracted through the crankshaft will be a fraction of the mechanical energy delivered through the Orion drive’s suspension. This is already a small percentage of the nuclear energy released by the pulse charges. The USAF design for a 10m diameter nuclear spaceship has a fantastic 32.9 GW output, but this is only 0.78% of the energy released by 1 kiloton yield blasts every second. We’ll call this the Motorized Orion or Moto-Orion. In practice, the electrical power that can be derived from an Orion drive will depend on the mass of the electrical generator and the equipment needed to manage waste heat. A high performance generator would have an efficiency of over 95% and a power density in the tens of kW/kg. Waste heat will be the main obstacle to generating a lot of electrical power, especially as electrical generators tend to operate at lower temperatures. As discussed in a previous post, temperature is the biggest factor in allowing for lightweight heat management systems. A generator would typically want to operate at room temperature 300K, but this would mean huge (and heavy) radiators would be needed to handle their waste heat. We want the hottest generators possible. They are mainly limited by the decreased performance of their electrical insulators at higher temperatures. Commercially available motors are available at 570K, but applying research like this could create generators that operate at 770K. However, increased temperatures also increase electrical resistance and therefore cut into the efficiency of a generator. Based on some studies, high temperature efficiency can be held at above 90%. A generator is a motor in reverse, so we will use these same temperatures and efficiencies. Estimating the power density of an entire heat management system is quite difficult, but we can make some estimates. 1 m^2 of double-sided 2mm thick carbon fibre radiator fins would be 4 kg and radiate away 8.3 kW of heat at 520K. Note how this is a slightly lower number than the operating temperature of the generators, as we need a temperature gradient throughout the heat management system to actually move heat from where it is created to where it is radiated away. With reasonable figures for a silicone oil pump, a microchannel heat exchanger and a +20% margin for assorted pipes, valves and backups, it all averages out to 1.2 kW/kg. This seems like a low figure, but it only deals with the <10% of power that becomes waste heat. 1 MW of mechanical energy coming through the crankshaft would become 900 kW of electricity, handled by 45 kg of generators, and 100 kW of waste heat, requiring around 83 kg of cooling equipment. Altogether, this makes for an average power density of 7 kW/kg. This ignores the mass of the crankshaft, counterweight and other mechanisms, but they will be small compared to the rest. There is also the complication of radiator placement; they want to extend out from the hull, but also must stay within the shadow cone of the pusher plate to avoid being disintegrated by nuclear plasma. The original USAF 10m Orion had a payload capability of up to 225 tons (on certain missions). If a quarter of this was dedicated just to producing electricity, we could expect it to output 393 MW. That is a respectable amount! Here’s what a Moto-Orion derived from that design, with fully scaled radiators, would look like: Though, it is only 1.2% of the drive power. You could imagine an Orion drive spaceship that extracts more of its output as electricity, but it is fundamentally limited by the difference between the power density of the propulsion system (on the order of 330 kW/kg) and that of the power extracting equipment (<10 kW/kg). Furthermore, equipment that consumes that electrical output will take up an outsized portion of the spaceship’s payload capacity, due to their even lower power density (<1 kW/kg). There are other ways to generate electrical power. A linear alternator should be an ideal option. A magnet is simply pushed through a series of conductive coils, producing current as it travels up and down. It is just as efficient as a rotating electric generator, and depending on the exact design used, can operate at the same high temperatures. Even better, it does not produce any sideways torque, is easier to fit in between the suspension arms and is more resilient to vibrations. However, their power density is far lower than that of rotating generators, with 1.49 kW/kg being the best figure mentioned anywhere. Another option still is to use a high temperature superconducting generator. NASA has designs that aim for 60 kW/kg at the multi-megawatt scale. Efficiency is 99%, meaning that 1% of the power becomes waste heat. Thankfully, this heat is produced not in the superconducting magnet, but in the non-superconducting stator. It can reach 570K, so we can use similar heat-management equipment as described above. 1 MW of input power becomes 990 kW of electricity and 10 kW of heat, which are handled respectively by 16.5 kg of generator and 8.3 kg of cooling equipment, for an average power density of 40 kW/kg. The downside to using superconducting devices is having to mount the bulky and sensitive equipment needed to keep them in that state. A high-temperature superconductor needs to be kept in liquid nitrogen, which boils at 77K. About 0.01 to 0.1% of the power that a superconducting device handles is expected to become waste heat inside the cryogenic part through ‘AC losses’, where alternating currents create magnetic vortices within a conductor. Progress is being made into megawatt scale superconducting generators/motors. This Honeywell 1 MW design achieves 8 kW/kg. The passive solution to handling this heat load is to just let the liquid nitrogen boil. It can absorb 198 kJ/kg during vaporization, so for every kW a superconducting generator outputs, 5 milligrams per second of liquid nitrogen needs to be expended. Using the expendable liquid nitrogen solution, we can have the USAF 10m Orion dedicate 40 tons to electrical production, and 16.25 tons to liquid nitrogen reserves (adding up to a quarter of its 225 ton payload, as before). It would be able to output a whopping 1.6 GW of electricity, but only for 33.5 minutes before liquid nitrogen reserves run out. It’s not too bad; the spaceship would likely run out of pulse charges before it uses up all this coolant. The active solution is to use a cryocooler. It raises the temperature of the waste heat to a level where it can be disposed of using radiator panels of reasonable size. If the high temperature superconducting material operates at 100K, then it takes at least 4.7 Watt of cryocooler power to move 1 Watt of waste heat up the temperature gradient to 570K. A realistic cryocooler will achieve 30% of maximum Carnot efficiency, so we increase the power requirement to 15.7 Watts. We choose the 570K temperature target to keep using the cooling equipment from previous calculations (all the better to compare each solution). Cryocooler power density for aerospace applications is about 133 W/kg, but 300 W/kg is cited as an achievable goal. Putting these elements together, we have 1 MW of input power becoming up to 1 kW of cryogenic waste heat, which requires 15.7 kW of cryocoolers that mass 52 kg. The active solution brings down average power density to 12.9 kW/kg. It is a respectable figure, better than the non-cryogenic design’s 7 kW/kg, and especially interesting for missions with prolonged engine use with no opportunity to refill on liquid nitrogen.. A USAF 10m Orion that used an actively cooled superconducting generator massing 56.25 tons would produce 725.6 MW as long as the engine is running. There is a ‘catch’ to these cryogenic designs though. Superconducting magnets are not known to be resistant to radiation or damage of any kind. It is especially concerning when a nuclear pulse propulsion spaceship bathes itself with penetrating neutrons and high energy gamma rays repeatedly. The magnets cannot be placed too far away from the pusher plate and suspension system either, so they can’t hide in the relatively safe environment the crew enjoys at the other end of the spaceship. Flexibility There are two other major benefits to the Moto-Orion. The first is during start-up. The original Orion design relied on the suspension system being pre-compressed before the first full-strength nuclear charge could be used. It was the job of a half-strength bomb to get the suspension ready. While this use of fissile material is not too wasteful when compared to the hundreds of bombs that are regularly used, it is very inflexible. Start-up would only be possible a limited number of times, and only when the pusher plate is standing still… not at all comforting when space travel involves must-not-miss burns. It is even worse for a warship that needs multiple successive starts and stops to effect dodges from enemy fire. A Moto-Orion can use its electric generator in reverse, to produce torque while consuming energy from battery reserves. It can draw in the suspension arms to a compressed position, or time its pushes and pulls to bring a wobbling plate to standstill more quickly. The batteries can even be charged from another power source, such as solar panels, if battery reserves are depleted. This gives the spaceship an unlimited number of restarts. It gains the flexibility to halt and ready its drive at any time. The second benefit is recovery after the pulse sequence goes wrong, whether it is late, early or missed completely. Accurate suspension cycle for an Orion craft, by ElukkaJ. A Moto-Orion might be able to react quickly enough to adjust the position of the suspension system in case of a late pulse. Once the nuclear shaped charge moves past its designated ignition point, the spaceship’s motors would draw power to slow down the retreating pusher plate. This could prevent it from being accelerated into the suspension arms at an excessive velocity. When things go wrong, unpleasant, up to destructive, g-forces are generated. An early detonation is especially troublesome. Not only does it erode the pusher plate, it cannot be predicted. The Moto-Orion’s crankshaft and generator can be turned into an additional suspension arm to absorb the unexpected shock, but it would usually be weaker than the massive steel springs the engine habitually relies upon. Still, it can assist in bringing the pusher plate velocity back in line and ready to receive nuclear plasma blasts again. When it comes to misfires, Moto-Orion can potentially add velocity to the slower pusher plate (as it did not receive the momentum from the missed pulse) and bring the drive sequence back into correct timing. It can avoid a complete halt by drawing energy from battery reserves, and if it is powerful enough, do so without skipping a beat. There are other forms of flexibility, gained indirectly from having access to huge amounts of electrical power. They might not be as flexible in this regard as a nuclear-electric ship could be, as power generation is tied to the use of the engine and not an independent reactor, but many possibilities open up. Orion nuclear spacecraft could deploy drones and beam power to them, by means of microwave emitters or laser beams. They could receive nuclear charges ‘on the fly’ using magnetic scoops. Electrical Reaction Control thrusters can be used, so that the spaceship can turn more efficiently. There are many more possibilities. Consequences An Orion spaceship staging off a aerobraking lander at Mars. Moto-Orions are safer and more flexible than the original Orions. For a simple transport ship that only uses its engines briefly and wishes to maximize payload, the extra weight is unwelcome. Any craft that carries people might instead find that the additional capabilities and securities are a worthwhile trade-off. Warships would absolutely desire Moto-Orions. The huge amounts of electrical power turn them into terrifying attackers that can both unload with weapons energized by hundreds of megawatts of power while also performing multi-g evasive maneuvers. In a science fiction setting, Moto-Orions can deliver the retrofuturistic aesthetic of spacecraft riding on nuclear blasts while also making possible the use of exciting hardware like lasers and coilguns. One setting, RocketPunk, is in development by Ad Astra Games (and by Rick Robinson, who inspired ToughSF). It features Orion-propelled warships battling for Mars in an alternate Cold War future. More engaging action could be made possible with these motorized variants. The fact that a Moto-Orion connects electrical output with drive power by a single-digit percentage ratio is an interesting feature by itself. We discussed how this avoids troublesome issues such as The Laser Problem, where overpowered lasers have excessive ranges and render maneuvering during ship-to-ship combat useless. Low electrical power and high drive power give room for dynamic combat that is more exciting for readers or viewers. Other types of ‘torch ship’, like a rocket with an immensely powerful fusion reactor, could have better performance than Moto-Orion, but would have proportionally more electrical power - this pushes combat ranges so far out that maneuvering is rendered pointless again. The military potential of Orion was always at the forefront. Another bonus towards dynamic and interesting space combat is an Orion drive’s ability to continuously accelerate and outrun missiles that have less potent propulsion systems. Due to how poorly nuclear pulse propulsion performs when scaled down (burnup ratio and thrust efficiency drop dramatically), a missile would not be able to keep up with a full-sized Orion drive unless it had its own large and expensive pulse propulsion system. They would be excessively expensive, so only smaller and less powerful engines would be available to missiles. Consequently, Orion warships have a good chance of outpacing missiles. It creates a situation where one side having more missiles than the other does not automatically guarantee a win. Instead, careful use of maneuvers and relative positioning to set up a shot with short-legged missiles is necessary. All the better to read about or play through! The Project Orion battleship. We suggest going out and applying these calculations to bring motorized variants to other Orion designs. Huge spacecraft like the 4000 ton USAF 'battleship' could benefit immensely from this concept. You could also think about how Medusa could extract electrical power from its tether strokes, or even more outlandish ideas, such as a propulsion system where high velocity kinetic impactors strike a lump of propellant to create a jet of plasma that strikes a pusher plate, like a non-nuclear Orion.
  9. Technically, it is infinite propellant and not infinite Isp. Antimatter definitely beats ion propulsion, but anything with less energy density than that cannot do so... The photon rocket will likely look like a solar sail, except glowing hot at 3000K+ on one side.
  10. This is the most recent post on ToughSF: http://toughsf.blogspot.com/2020/11/nuclear-photon-rockets-flashlights-to.html Nuclear Photon Rockets: Flashlights to the Stars In this post, we will have a look at the concept of using a nuclear photon rocket for interstellar travel. They are an old concept that should theoretically be the ultimate form of relativistic propulsion. However, today they are unknown or unpopular. Why might that be the case? The image above is by David A. Hardy. The interstellar challenge The Daedalus starship. Interstellar travel is on a completely different level than interplanetary travel. The distances involved are orders of magnitudes greater. The shortest distance between stars is measured in trillions of kilometres. To face such distances, high velocities are required. The closest stars. A robotic probe might not mind spending several centuries to reach a destination. A human crew would want the trip done in their lifetime. Taking longer than that means running into technical and ethical trouble. The closest star to our Sun is Alpha Centauri A, currently sitting 40 trillion kilometres away, or 4.2 light-years. It would take 4.2 years to reach it when travelling at the speed of light. If we want to complete the trip within 20 years, we would have to travel at 21% of the speed of light. We also want to slow down at the destination. This means that we need a way to accelerate up to 21% of the speed of light, and then slow down back to zero - the deltaV sum is 42% of the speed of light. So how do we go that fast? The Falcon 9's Merlin rocket engines. Rockets are the space propulsion system we are most experienced with. There are many ways to measure a rocket’s performance, but only some are relevant to interstellar travel. Thrust, for example, is much less important when the trip will take many years; taking one month to accelerate instead of ten months is no longer a significant factor. Instead, let’s focus on exhaust velocity. Using the Tsiolkovsky rocket equation, we can work out the ratio between propellant and non-propellant masses of the rocket we are using. Mass Ratio = e^(DeltaV/Exhaust Velocity) DeltaV in m/s Exhaust Velocity in m/s A chemical rocket consuming oxygen and hydrogen propellants has an exhaust velocity of 4,500m/s. We find that for a chemical rocket to achieve a deltaV of 42% of the speed of light, we would need e^28000 kilograms of fuel for each kilogram of equipment, structure, engines and payload. That is a number that lies between 10^8428 and 10^13359. For comparison, the entire mass of the Universe is estimated to be 10^53 kg. Chemical rockets for relativistic travel are beyond impractical. The needle array of Enpulsion's IFM nano thruster. How about a rocket engine with a better exhaust velocity? Something like one of our most efficient ion thrusters? The Ultra-FEEP thruster that accelerates liquid indium to nearly 1,000 km/s is the best we can expect for now. It would still not be enough for relativistic velocities. To achieve a deltaV of 42% of the speed of light, we would need 6*10^55 kg of indium for each kilogram of dry mass. If you run the numbers yourself and lower the deltaV target, you would still find ridiculously high mass ratios being required. A deltaV target of just 2% of the speed of light, which would turn the trip to the nearest star an endeavour that spans about half a millennium, would still require a physics-breaking mass ratio of 10^579 from the chemical rocket, and a mass ratio of 453 from the Ultra-FEEP thruster. The lower value for the electric thruster seems much more reasonable, until you consider that indium is found at a concentration of 0.21 ppm in Earth’s crust. At our current output of 700 tons per year, a 1,000 ton dry mass craft would require at least seven centuries of indium production to fill its propellant tanks. To get away from these extreme figures, a logical decision would be to increase the exhaust velocity all the way to the maximum. The maximum is the speed of light. Photon Propulsion When your exhaust is light itself, the mass ratios required for relativistic velocities become decidedly modest. Light, more specifically photons, can be produced indefinitely ‘out of nothing’. In other words, if you heat up a surface, you can create a photon rocket that spontaneously produces and emits light without ‘running out’ of anything. All that is required is a power source. The more energetic the power source, the more photons that can be produced and the higher the photon rocket’s performance. The theory fits together neatly. The concept of using a nuclear reactor to heat up a surface so that it emits enough photons to produce appreciable thrust is at least 50 years old. Nuclear photon rockets could solve our problem of interstellar travel by harnessing the greatest sources of energy and utilizing the exhaust with the highest velocity. All the fuel they would ever need would be loaded up at departure, so they do not have to rely on the existence of any infrastructure at the destination or any assistance along the way. Perhaps they would have enough to return to us without having to refuel! However, ‘photon starships’ are not a popular idea today. They are not featured in NASA’s NIAC programs, nor are aerospace engineers dreaming up modern designs for them. What ‘catch’ has them relegated to relics of the past? Fission Photon Rocket A nuclear photon rocket from Boeing's PARSECS study. Let us start with the most familiar of nuclear energy sources: the fission reactor. A fission reaction produces about 80 TeraJoules for each kilogram of maximally enriched fuel. 95% of this energy is in the form of gamma rays or fission fragments; they can be blocked by a thick wall and converted into heat. About 5% leaks out in the form of neutrinos. This reduces the ‘useful’ energy density of fission fuel to 76 TJ/kg. In a typical reactor, the fuel is in solid form. Only a fraction of its potential 76 TJ/kg can be extracted in one fuel cycle. The products of fission, such as xenon-135 and samarium-149, remain trapped next to the fuel. These isotopes have a high neutron cross-section, which means that they trap and absorb the neutrons needed to sustain a fission reaction. Nuclear engineers consider these products to be ‘poisons’. If enough poisons accumulate in the fuel, the fission reaction cannot be sustained. The result is that a single fuel cycle achieves very low burnup of the fuel, which is the percentage of fissile fuel that has undergone fission. Typically, this is 1% to 5% of the total fuel load inside a reactor. On Earth, nuclear engineers deal with this problem by shutting down a reactor, extracting the slightly used fuel and sending it off for reprocessing. This involves removing the poisons, mixing in a small quantity of fresh fuel, and then returning it all to the reactor. A spaceship does not have the luxury of regularly halting its reactor while also lugging around a nuclear fuel reprocessing facility. Instead, we need to use a type of reactor that grants high burnup with no reprocessing necessary. The best option seems to be a gas-core nuclear reactor. In this high temperature design, the fuel and poisons are in a gas phase. It becomes easy to filter out the poisons as they are chemically very different from the fuel. We can have the fuel circulate within the core for as long as needed to achieve near 100% burnup. With the burnup problem solved, we can convert those 76 TJ/kg into heat. From a physics perspective, only about 0.77 grams of matter in a kilogram of fissile fuel becomes energy. This leaves us with 999.23 grams of waste after consuming the fuel. With no further use for it, we eject it to lighten the spacecraft. Imagine a nuclear starship designed specifically to make our next calculations easier. It consumes 1 kg of fuel per second. The average power output is 76 Terawatts. Thrust = 2 * Power/ Exhaust Velocity Thrust will be given in Newtons Power is in Watts Exhaust Velocity in m/s Those 76 Terawatts should result in 506.6 kiloNewtons of thrust. With a 95% efficient photon emitter, we gain a real thrust of 481.3 kN. After producing this thrust, we eject 999.25 grams of waste. Effective Exhaust Velocity = Thrust / Mass Rate Effective Exhaust Velocity will be given in m/s Thrust is in Newtons Mass Rate is in kg/s The ‘effective exhaust velocity’ based on this thrust and the amount of matter being ejected is actually 481.7 km/s. The critical point we make here is that while the thrust comes from photons travelling at the speed of light, exhaust velocity calculations must take into account all the masses being ejected. So what can a fission photon rocket do with an effective exhaust velocity of 481.7 km/s? It certainly cannot reach our desired deltaV. Achieving 42% of the speed of light would require a mass ratio of 10^113. Unless we have access to multiple Universes filled with highly enriched fissile fuel, this is impractical. Even with an extraordinary feat of engineering so that we could load a starship with 100 kg of nuclear fuel for each 1 kg of dry mass (and not have it immediately go critical), the achievable deltaV is only 2,218 km/s or 0.74% of the speed of light. Fusion Photon Rocket What if we used the better nuclear rocket: the fusion rocket? There are many different fusion reactions involving different fuels, but we are interested in those that provide the highest energy density. Proton-proton fusion provides a whopping 664 TJ/kg. However, it is very slow, taking thousands of years to complete, and it is not realistic to ever expect to take place outside of stellar cores. Next down the list is Deuterium-Helium3. About 353 TJ/kg is on tap. We won’t dive into the details of the various reactor designs that could be used, but suffice to say that near-complete burnup of fusion fuels is possible, and all the energy released can be converted into heat. If we compare the mass of the Deuterium and Helium 3 before fusing, with the mass of the helium and proton particles after fusion, we notice that 0.39% of the mass is missing. That is the percentage of mass converted into pure energy. It is a much greater percentage than nuclear fissions’ 0.077%. The list of particles involved in fusion reactions, with their exact masses. Let’s repeat the previous calculation for the effective exhaust velocity of a nuclear photon rocket. 1 kg/s of fusion fuels are consumed, for a power output of 353 TW. This produces 2,235.6 kN of thrust out of a 95% efficient emitter. We expel 996.1 grams per second of waste, so the effective exhaust velocity is 2,244.4 km/s. This is nearly five times than a fission photon rocket’s effective exhaust velocity. However, this is still not enough. Our desired deltaV of 42% of the speed of light comes at the cost of a mass ratio of 2.4*10^24. While we could gather enough galaxies together to fuel our fusion photon rocket, we want something more practical. The reality is that a plausible fusion photon rocket with a mass ratio of 100 would only have a deltaV of 10,335 km/s or 3.4% of the speed of light. Barely enough for a multi-century generation ship to cross the stars and certainly not enough for travel within a lifetime. Staging the fusion rocket will not help very much. Also notable is the fact that an effective exhaust velocity of 3.4% of the speed of light is actually lower than the exhaust velocity of direct drive fusion propulsion, where charged particles are directly released into space through a magnetic nozzle. DHe3 releases a 3.6 MeV helium ion and a 14.7 MeV proton. Their averaged velocity is 7% of the speed of light. A photon rocket is a very inefficient use of fusion energy. Antimatter Photon Rocket The ultimate fuel should give the ultimate performance. Nothing beats antimatter! There are many types of antimatter. There are antielectrons, antiprotons, antineutrons and their combined form, anti-atoms like antihydrogen. Antielectrons annihilate with regular electrons in a ‘clean’ annihilation reaction that produces high energy gamma rays and nothing else. They are however the hardest type to store. Antiprotons are much easier to store, especially in the form of frozen antihydrogen ice. The downside is that their annihilation is ‘messy’, as it releases a plethora of products. With solid shielding, enough of the energy of those multiple products can be absorbed and converted into heat. We set the efficiency at 85%. Each kilogram of antimatter contains a potential for 90,000 Terajoules of energy. It must be matched by another kilogram of regular matter, so the average energy density is halved to 45,000 TJ/kg. As we only capture 85% of that amount, the useful energy density is 38,250 TJ/kg. If we consume one kilogram of antimatter/matter mix per second, we would have a drive power of 38,250 TW. A realistic emitter would convert this into 242,250 kN of photon thrust. The effective exhaust velocity is 242,250 km/s or 81% of the speed of light. With such a high exhaust velocity, an antimatter photon rocket would be able to achieve the relativistic velocities we desire. A deltaV of 42% of the speed of light would only require a mass ratio of 1.68. That’s 0.68 kg of antimatter/matter mix for each 1 kg of rocket dry mass. We might even be able to go much faster with high mass ratios; travel times to the stars in single-digit years seems possible. However, antimatter is exceedingly difficult to collect or create. A mass ratio that seems acceptable for a conventional rocket would actually imply an unreasonable amount of antimatter. Existing accelerator facilities, if tasked with solely producing antimatter, would require about 3.6 ZettaJoules to produce 1 kilogram of antimatter. That’s 3,600,000,000 TeraJoules, equivalent to 286 times the total yield of all nuclear bombs today (1.25*10^19 J), or the total output of the United States’ electrical grid (1.5*10^19 J) for the next 240 years. If we were very serious about producing large quantities of antimatter, we could design a superbly optimized antimatter production facility, with very efficient antimatter capture mechanisms. Production efficiency can be increased to 0.025%. This means that 1 kg of antimatter would require ‘only’ 360,000 TJ to manufacture. An antimatter photon starship would ‘just’ need the combined output of all humanity (8*10^19 J/yr) for the next couple of millennia to fill it up. An antimatter production facility. In practice, the awesome performance of antimatter propulsion would be reserved for civilizations higher up the Kardashev Scale. Verdict and Consequences All the calculations so far have assumed nearly perfect use of the energy released by fission, fusion or antimatter reactions. We have also ignored the massive complications that arise from trying to handle the power of those reactions. Despite this best case scenario, nuclear photon rockets do not seem to be up to the task of rapid interstellar travel. Fission and fusion power are just not energy dense enough. Antimatter is far too difficult to produce in huge quantities. The ‘catch’ is that physics is not kind to photon propulsion. For this reason, this sort of starship will remain a bottom-drawer concept for the foreseeable future. What effect does that conclusion have? If we want to use rockets, we must accept that interstellar travel will be slow. Other techniques or technologies have to be employed to make crossings that last centuries. Cryogenic hibernation, life extension or digitizing the mind can enable the original crew to survive that long; generation ships or embryo seeding can allow another group of people to arrive at the destination. Robert L. Forward's Laser-propelled lightsails. If we instead want interstellar travel done quickly, we cannot rely on rockets. All the popular methods for interstellar travel depend on non-rocket propulsion, such as Robert L. Forward’s massive laser-propelled sails or the ‘bomb-tracks’ discussed in a previous post. The energy cost of relativistic travel is no longer derived from a fuel carried onboard a starship, but from an external source. This external source takes the form of large infrastructure projects and preparations that require many years to complete; we trade away the flexibility and autonomy of rockets to gain huge speed, efficiency and cost advantages. A consequence of non-rocket propulsion is that interstellar travel cannot be a whimsical affair. It has to be planned a long time in advance (which has implications for the stability of the civilization organizing it all) and it would be evident to all observers at the departure and destination what is going on. No ‘secret’ missions to other stars! Of course, a scifi writer might not like the sound of that. Their options lie in more exotic types of rockets, more advanced civilizations or speculative science. Examples of exotic rockets include a starship powered by a rotating black hole, where matter is converted into energy at 42% efficiency (an effective exhaust velocity of up to 252,000 km/s or 84% of the speed of light) or a Ram-Augmented Interstellar Ramjet, where the thin interstellar medium is added to the exhaust of a fusion reactor for a greatly improved effective exhaust velocity. More advanced civilizations handle enough energy to be able to produce large quantities of antimatter, overcoming the main difficulty with this fantastic fuel. Speculative science opens up the possibility of using ‘quark nuggets’ to rapidly and easily create antimatter, as well as wormholes and Alcubierre warp drives. Though, we must warn you, that these different options might be more troublesome than photon rockets!
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