Destructionator XIII wrote:Sikon, you'll have to forgive me for scraping your brain here...
Sikon wrote: Space warships orbiting at hundreds to thousands of miles altitude could destroy fighters in the atmosphere underneath.
What kind of energy would be needed for this? I ask because I know there have been problems with the atmosphere absorbing too much of the energy from a laser to make it economical to use as a ground to space energy transfer system for modern day spacecraft. If a significant amount is lost to the atmosphere, then the laser would need to be higher power on the ship, which also means more waste heat, and obviously, more power generation is required, which may be the limiting factor for these tactics.
Also, if the warships can shoot down, could a ground based system shoot back up (assuming, of course, any ground weapons survive the first wave of bombardment), or would that be too inefficient to be feasible on the ground (I know lasers generate a lot of waste heat, and on the ground that could be an environmental disaster)?
With a projectile or missile, how much mass would be required for it to not burn up before hitting the target? Also, how hard would it be for them to hit a moving target from that range?
I have no doubts that hitting a stationary thing on the ground with a kinetic kill mass would be easy, but fighters I always assumed would be much harder to hit.
It is an interesting topic. I was going to write some thoughts on this anyway, and this post will also be relevant to your questions.
Megajoule weapons could be enough, but attainable firepower in the gigajoule to terajoule range is likely.
Space warship power generation, waste heat, etc.
Usually the power generation and storage system is considered a major limiting factor for electrically-powered beam weapons. Future ultracapacitors could have an energy density higher than 60 Wh/kg along with a power density greater than 100 kW/kg. Such is from a MIT study on ultracapacitors for future cars: implied
here. That would be up to 0.2+ TJ of energy storage per 1000 metric-tons of ultracapacitors, able to be discharged at a rate of 0.1+ TW. For example, a hundred thousand ton warship with just five percent of its mass being ultracapacitor banks could store a terajoule, then discharge it at a rate of half a terawatt. Technology of the distant future is unknown, perhaps far superior, but the preceding is a reasonable expectation for a probable lower limit. For a large ship, let's figure the maximum output energy of each beam-weapon or mass-driver shot is most likely in the 0.01 TJ to 1 TJ range, depending upon assumptions like the size of the space warship, power handling per unit mass, inefficiencies, etc.
Another question is the recharge rate, depending on warship power generation. One nuclear-electric concept with a MHD generator was
estimated to obtain 0.37 kg/kWe, which would be 2.7 MW/metric-ton. For perspective, car engines of today are sometimes hundreds of kW of mechanical power per ton (i.e. 200 hp engine = 150 kW), with aircraft engines up to much higher power density. Even with need for electricity rather than mechanical power alone, the many thousands of tons involved in a space warship would allow it to have power generation at least in the gigawatt range or higher, likely terawatts for large ships. Even hard sci-fi technology could be well beyond today's concepts. Either fission or fusion reactors would work. There would also be inefficiencies.
What about waste heat? Deploying large radiator panels while firing weapons wouldn't be desirable. Internal phase-change-material (PCM) heat sink possibilities could include using ice/water to absorb some terajoules of energy. Actually, if the space warship has structure, armor, and individual weapons massing thousands of tons, such would be able to temporarily absorb some waste heat. But such could not sustain a high rate of fire for long without needing a "cooling off" period, so a different system would be needed, at least as a supplement. Interesting options include liquid droplet radiators, charged (solid) particle
radiators, etc.
Radiator mass for the weapons is going to depend much upon their acceptable operating temperature. If most parts of the weapons can operate at moderately high temperature, transferring away heat fast enough becomes plausible without excessively large radiator area and mass being needed even when a lot of power is involved. That is particularly plausible at the high technological level implied in this sci-fi scenario. One study of what is obtainable
for heat rejection in space with merely today's technology indicates that 30 MW of heat could be dealt with by a 45 metric-ton Curie point radiator (
CPR) or by a 29 metric-ton liquid droplet radiator, for an average temperature of 380 degrees Celsius or 650 K. The space warship would operate at least in the gigawatt range, with at least around a couple orders of magnitude greater heat rejection from its weapons, but it could afford to have orders of magnitude greater radiator system mass. And it would be much more advanced technology. The preferred radiator design for an armored warship would tend to be a droplet radiator or a particle radiator, not large flimsy panels.
Let's add an intuitive illustration of the overall picture. Consider 10% of the mass of a 100,000-ton warship being a beam weapon, with the maximum energy of each shot it could fire being somewhere between 0.01 TJ and 1 TJ. That proportionally corresponds to as much firepower per unit mass as a half-kilogram energy pistol firing shots between 500 J and 50 kJ of energy. Such is equivalent to the energy pistol being able to vaporize a volume of ice between 0.72-cm and 3.3-centimeters in diameter per shot. While the whole range is conservative by sci-fi standards, one could take the low end of the range if concerned about the reliability of it being plausible. The comparison is proportional since the sample space warship's weapon masses 20,000,000 times more than the energy pistol.
Yet the warship's shots each correspond to the equivalent of approximately between a 2500-kg bomb and a 0.25-kiloton tactical nuke in the energy delivered. Beam weapons of such energy can have "unlimited ammunition," powered by the discharge of the capacitors, which are recharged by the warship's nuclear reactors to fire thousands of shots in a period of a few hours. Or smaller shots could be used for an even higher firing rate.
For perspective, a 100-kJ vehicle-mounted laser concept is considered by the Department of Defense to be lethal against common rockets, aircraft, and light
ground vehicles. Yet, at the technological level implied by sci-fi interplanetary or interstellar space war, set in the distant future, average firepower of large space warships could be astronomically higher, either in the energy per shot, the number of shots fired per minute, or a combination of both. Every 0.01-TW of average weapons power corresponds to 100,000 times the energy per second: 400 million times it per hour.
Propulsion system power would likely be even much greater. For example, the MS Word document from researchers
here describes a magnetic compression pulsed fission concept with a magnetic nozzle, in which a vehicle of 1310 metric tons initial mass and 100 tons final mass could have 263 GW jet power. That is between 0.2 GW/ton and 2.6 GW/ton, with relatively straightforward technology. For this sci-fi scenario with advanced technology, the preceding is just a probable lower limit. A much larger 100,000-ton space warship could be more than 1 GW/ton, corresponding to an exhaust jet power above 100 TW. But one conservatively treats weapons power as orders of magnitude less than what is sure to be possible for propulsion, not counting on more than the 0.01-TW previously implied.
Beam weapons against atmospheric fighters and other planetary targets
Before considering other weapons types, let's first illustrate with a space warship firing a lethal radiation beam against planetary targets including aircraft. Against humans, on the order of 10 kJ per square meter of some types of radiation would be enough to cause enough exposure for relatively quick mortality, much above the level for slow death. The end result is a little like the effect of the radiation of a neutron bomb, for which 8000 rads or 0.08 kJ/kg-tissue are enough to immediately incapacitate enemy soldiers like tank crewmen according to an U.S. military
estimate, a couple orders of magnitude above the dosage usually lethal over a longer period of time (1% as many neutrons = 80 rads = 800-1600 rem in long-term). But the radiation would be like penetrating cosmic rays, not neutrons.
Since natural cosmic radiation experiences an attenuation factor of 600 going through earth's atmosphere from space to ground at sea level, assume the wide-beam radiation should have an intensity on the order of 6 MJ/m^2 before entering the atmosphere. (Penetrating natural cosmic rays = 16 rem/yr for
interplanetary space --> 0.027 rem/yr
sea level). The situation could be better with more optimal choice of particles and when firing against aircraft above sea level, but, to be conservative, don't assume better.
The result is that each shot of 0.01 TJ to 1 TJ energy can deliver a pulse of quickly lethal radiation to an area around 46 meters to 460 meters in diameter. If a given intensity level is insufficient, such as firing against a relatively hardened unmanned target, dropping the beam diameter by an order of magnitude would increase the intensity by a factor of 100, and so on. But wide beams can kill ordinary tanks, aircraft, infantry, etc. The beam is unaffected by weather and sufficiently penetrates the 10000 kg/m^2 mass shielding of the atmosphere. Unlike even neutron bombs, the beam would have no blast effect when set to sufficiently wide-beam mode, leaving structures unharmed aside from disruption to electronics, yet killing the occupants.
Some kinds of beam weapons could be more limited in propagation through the atmosphere. For example, as implied by what happens to sunlight, visible light from space doesn't always reach the ground well on cloudy days. So visible light lasers might be an unreliable weapon against low-altitude enemy aircraft, unless the basic principle of
this could be applied with ultra-intense pulses.
But microwaves can go through clouds. Against non-hardened targets, as little as a few joules per square meter or less can be enough, allowing gigantic "EMP pulse" microwave beams hitting up to multiple square kilometers per shot. Against ordinary civilian targets, such might be about the opposite of lethal radiation beams: At the wide-beam setting, such could devastate infrastructure without killing any people, aside from a few indirect deaths like crashing aircraft.
If necessary against hardened targets, the microwaves could be more focused, for physical overheating and destruction of targets, e.g. MASERs. At the 0.01 TJ to 1 TJ energies, a beam a few meters in diameter could be many megajoules per square meter, possibly gigajoules per square meter.
Nuclear projectiles and missiles
For another potential weapons system, consider space warships firing nuclear projectiles or missiles. For example, a cheap "brute force" method of dealing with atmospheric fighters trying to avoid shells or missiles might be to have them explode with sub-kiloton to single-kiloton yield. The equivalent isn't done by terrestrial militaries for reasons like political issues, but those don't necessarily apply so much in a sci-fi planetary assault scenario. Even in the real-world today, nukes don't have to cost more than
merely hundreds of thousands of dollars each or less in mass-production, compared to fighters costing orders of magnitude more: tens to hundreds of millions of dollars each.
Fallout from such nukes would tend to be harmful to the planetary defenders and localized regions without making the planet unusable by the invaders. Localized radiation levels shortly after a detonation can be lethal, but such drops over time, since the radioisotopes emitting the most initial radiation are those which decay most quickly. (The rate of radiation emission per unit time from a radioisotope is inversely proportional to half-life, to a degree such that stable elements can be thought of simply as those with infinitely long half-lives). Compared to residual radiation one hour after
the detonation, radiation levels are 1% as much after 2 days and 0.1% as much after 2 weeks. As implied, most is gone after the short-term timeframe. The fallout of a nuclear weapon detonation of low or moderate yield can much elevate radiation levels over a limited number of square kilometers, but it can do very little overall over the half-billion square kilometer total area of a planet like earth.
Historical above-ground nuclear weapon tests in the 20th century amounted to 440 megatons cumulatively, with 189 megatons fission yield ... 189000 kilotons (
large PDF file). Total collective dosage to the world's population from such past tests corresponds to 7E6 man-Sv, for the UNSCEAR estimate for total exposure in the past plus the result of currently remaining radioisotopes projected up through
the year 2200. The preceding total over the decades and centuries is less than what is received every year from natural
sources of radiation, which is in turn orders of magnitude less than what would make an eventual death from cancer probable. Of course, from a real-world civilian perspective, any potential increased risk of cancer is undesirable, but, from the perspective of the hypothetical space invaders, the bulk of the planetary surface is not harmed enough for them to necessarily be concerned.
For example, even with fission devices, if the orbiting warships are firing quarter-kiloton-yield nuclear shells or missiles against targets like enemy aircraft, it would take on the order of 800,000 warheads even just to exceed the limited radiological contamination from the 189-MT fission component of the preceding nuclear tests. Only some invaders would care about that level of fallout. And the preceding is for fission devices. A hard sci-fi scenario could alternatively have pure-fusion devices, which would be cleaner.
Non-nuclear projectiles and missiles
Yet another potential weapons system for space warships is firing non-nuclear mass driver projectiles and missiles to hit air, sea, and ground targets on the planet below, impacting at hypersonic velocities.
A 1977 NASA Ames study referenced
here determined that an earth-launched mass driver projectile going up vertically could pass through earth's atmosphere from ground level to space with a few percent of its mass being an ablative carbon shield, losing only 3% of its total mass in the transit. Such is for a telephone-pole-shaped projectile of a metric ton mass. That means the reverse is also possible for projectiles with the right mass, dimensions, ablative shield, and trajectory. For example, consider a similar projectile or missile fired from space, reaching the upper atmosphere at 12 km/s velocity and going nearly straight down. It could hit a ground target at about 11 km/s, a kinetic energy equivalent to about 15 tons of TNT explosive.
Projectiles and missiles fancier than the cheapest unguided shells could use small thrusters to adjust trajectory to home in on a target. Even with a proximity fuse for a large shrapnel pattern, hitting an enemy atmospheric fighter could be more complicated than with the nuclear missiles or wide-beam lightspeed weapons described earlier. However, advanced robotic missiles tracking by the right combination of infrared, visible, radar, and/or other sensors could help make the space-to-air missiles much harder to evade than today's air-to-air missiles.
Sci-fi technology might also mean other possible ordnance, such as biological weapons genetically engineered to have a non-lethal temporary incapacitating effect or infectious nanobots. Different attackers might use different techniques depending upon their psychology, ethics, objectives, etc. But the earlier parts of this post imply the general trend of space warships having vast firepower by the time of a sci-fi war like this.
Space warships versus planetary anti-space weapons
What about space warships fighting planetary anti-space weapons?
Fighters and missiles launched from a planet may tend to be smaller and more limited than space warships. For example, a mass driver sending even just 10 tons per hour to orbit could over a decade put almost a million tons up, enough to be potentially the seed of a society processing eventually billions of tons of extraterrestrial material into habitats and ships. But, in that scenario, billions of tons of spaceships might exist without the planet necessarily being able to launch more than a proportionally miniscule amount in a day. There is likely shipment offplanet of some valuable goods and also passenger traffic, but X million people per decade going offplanet only corresponds to just 20 * X * Y tons per day needed, where Y is the ratio of total launch mass to body mass.
A planet with some atmospheric fighters launching anti-space missiles would typically fail when fighting space warships. Warships can have point defenses. For example, a 100-kJ projectile can destroy an ordinary missile. (For perspective, 100-kJ is like the kinetic energy of a 200-gram projectile going 1 km/s, though the analogy shouldn't be taken too far since the momentum is different for a much higher velocity but smaller projectile). A 0.01 TJ to 1 TJ mass driver firing pellets like a shotgun could deliver on average a 100-kJ pellet per square meter within a 360-meter to 3.6-kilometer diameter pattern per shot, making it typically rather easy to hit an incoming missile from the planet.
Launch a missile from a planet with a regular rocket, and more than 90% of its mass is involved just getting off the planet. In principle, a planet could do better by instead launching nuke-pulse missiles, nuke-saltwater rockets, or other advanced propulsion concepts. But having such launched from a planet during a battle would make them relatively easy targets during their boost phase. The planet could do better by having missiles and warships in space long before the start of the battle, but such wouldn't be air/space craft or planetary weapons. The case of planetary assault presumes the attackers have already won the space battle.
What about planetary anti-space weapons other than launching missiles? While a planet could have gigawatt to terawatt range beam weapons (i.e. water-cooled, especially if by the ocean), the effective range of such against space warships would tend to be less than the effective range of space warships against planets. In a duel at up to light-minutes or greater range with lightspeed weapons, a properly utilized space warship fleet will win against an immobile planet.
A lot of examples earlier in this post have implied how enough firepower could make relatively wide beams effective against a lot of planetary targets, such as the lethal radiation beams killing any people not deep underground. That allows the ability to engage an immobile target like a planet initially at extreme distances if desired.
For example, if technology allows under 0.1-meter dispersion of a particular beam weapon at 100,000-km distance, the same weapons technology would tend to allow under 100-meters dispersion at 100-million kilometers distance. If thousands of GJ to TJ-level shots can be fired per hour with electricity from the nuclear reactors while only one has to hit, an immobile planetary target can be hit by a warship with under a hour of firing from several billion kilometers away. That corresponds to a few light-hours distance, giving the mobile warship plenty of time to evade any lightspeed weapons fire from the planet. Such would arrive hours later, long after the warship has moved to another location in the vastness of space.
Most likely, if a planet had anti-space beam weapons, warships would destroy those from long-range, then move closer to provide closer targeting, like initially engaging at millions of miles but then going into low orbit for the final fire support.
More on close fire support from warships and the use of recon drones
To clarify more on my last post, with good enough targeting information transmitted from recon drones through a computerized system, space warships could help kill even individual vehicles or even individual enemy soldiers from orbit when possible. Such wouldn't be their primary mission, and initially the warships would attack more valuable targets. But afterwards, a warship would still have practically unlimited ammo for its electrically-powered beam weapons running off nuclear reactors. Using a hundred-thousand-ton warship to kill a couple enemy soldiers riding around in a truck might superficially seem wasteful, but there is next to no marginal cost in the preceding scenario.
Consider a warship orbiting at a couple hundred kilometers low-orbit altitude for final fire support. A little like a terrestrial sniper can shoot an enemy from a half-kilometer away, some beam weapons on the warship could be designed to hit precise locations on the ground below, with potential accuracy of within a meter. If there was a single person or handful of people on the warship manually trying to search for targets, aim, and fire the weapons, it would be a slow process. Yet, if there were a large number of robotic recon drones searching for enemy vehicles and soldiers, transmitting their precise coordinates, a computerized fire control system on the warship could shoot thousands of designated targets per hour, continuing for hours or days if necessary.
Space warships would initially destroy all targets they could see from space, but, for foreseeable technology, one wouldn't expect orbital surveillance to find every last target. Robotic recon drones deployed on air and on the ground could help give further targeting information. For example, if a golfball-sized robotic drone with a miniature jet engine flies up to the window of a building and sees enemy soldiers inside, it can transmit a signal causing the warship's computers to fry the area within a 50-meter radius with a lethal radiation beam a fraction of a second later ... potentially very effective yet still with less collateral damage than just nuking the whole city.
Given the level of firepower and capabilities possible on one space warship, imagine what a fleet of thousands of such warships (or more) could do against a planet.
The preceding could be done before sending in regular armies or occupation forces in order to drastically reduce ground combat casualties, although use of non-sapient robots and/or telepresence whenever possible might make casualties beyond expendable robots be low anyway.
The unpredictability of future technology
Even in a hard sci-fi scenario, predicting the capabilities of technology that may be centuries or millennia beyond the 21st-century is highly uncertain. A little like a person from centuries ago couldn't very well predict the capabilities of modern combat, the preceding is mainly just a lower limit on what hard sci-fi technology could accomplish with the high technological level implied by a interstellar war scenario like this.
For example, perhaps technology would allow a million tons of raw materials to be quickly and cheaply converted to its mass-equivalence: a billion one-kilogram missiles to be dispersed at low altitude. Or there could be other weird military technologies.
But overall the advantage tends to be on the space side, not the planetary defenders. If technology is different like allowing even more warship firepower, such would probably make atmospheric fighters be even more outmatched by space fleets.
