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Post by bigbombr on Mar 10, 2019 13:27:42 GMT
Not true, who said anything about wings? You don't need wings to fly. Skylon is a really bad example, because it's a surface to orbit aircraft. The biggest dead mass tradeoffs involved in Skylon are involved with it landing. An aero-orbital vehicle, on the other hand, would come in at high speed and stay there, using a nuclear ramjet (it'd be operating well in scramjet envelope, but without scramjet's problems because there's no combustion) and a lifting body shape. It would not resemble a plane, but a hypersonic cruise missile like the X-51A. Which, incidentally, happens to be very close to the optimal shape for bouncing projectiles off your nose, in space. The only dead mass it'd have would be the ducting required to make the ramjet work. F-35 is a supersonic VTOL that has to operate in subsonic and supersonic envelopes. Aero-orbital fighter concept, meanwhile, has to operate in orbital and hypersonic envelopes. Nothing more. Skylon is a beast because it needs to go through all of them. It's also big because it uses liquid hydrogen, and it burns it through the flight, as opposed to just during the last stage. It stands to reason, the more environments you requires your vehicle to operate in, the more of a compromise it becomes. If you shed your preconception that an aircraft has to land on the ground and take off from it, a number of designs open up that were previously ruled out. If we're not talking about a winged spaceplane, then are we talking about something like a lightcraft? That might work. But making some manned missile bus that dips into the atmosphere, launches munitions at hypersonic velocity (good luck with that) and climbs back out doesn't make any sense.
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Post by bigbombr on Mar 10, 2019 13:29:25 GMT
If you compare this idea to what is proposed for the skylon, that has to carry 15 metric tons and an 83m long spacecraft into low orbit. Assuming all your munitions have either been fired or jettisoned before reorbit this fighter has to carry a pilot, some cockpit systems and short term life support, maybe a radar if you think you need one (although it sounds like you are mainly aiming for a ground attacker) and whatever control surfaces and reinforcement you need for acceptable agility given the missions planned for this aircraft. To my untrained ear that sounds like it might actually be do-able with engine performance similar to that predicted for skylon in something not too much bigger than a regular fighter, the main obstacle seems to be that last one and what kind of compromises will be needed to make it viable for various altitudes and missions. Vulnerability during re-entry may also be a problem, not sure how much of a factor that might be. It will be handicapped compared to a regular fighter for sure, but you might just end up with something useful. Compare Skylon with a regular cargoplane that can carry 15 metric tons and you get an idea how a space capable fighter would compare to a more conventional one.
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Post by dragon on Mar 10, 2019 14:39:58 GMT
If we're not talking about a winged spaceplane, then are we talking about something like a lightcraft? That might work. But making some manned missile bus that dips into the atmosphere, launches munitions at hypersonic velocity (good luck with that) and climbs back out doesn't make any sense. We're talking about a reusable, powered and manned variant of a Hypersonic Glide Vehicle (think Avangard). Manned, because if you're attacking with a small fleet, you can't get global communications coverage, especially since plasma that forms at those speeds messes with radio waves. Reusable because you need to get your pilot back. Spaceships circle the planet at a rate of once per one and half an hour or so, not to mention capital ship weapons suck at hitting ground targets. Just hop into COADE and try to design a railgun that'd be both usable in space and fire rounds that wouldn't burn up on entry. A hypersonic fighter, armed with some sort of high-velocity conventional cannon, is, in fact, the best way you can provide reliable support from space to your landing troops. The others are lasers and missiles, the former need enormous power, huge apertures and are heavily attenuated by atmosphere, the latter are either of little use in space combat, if not nukes, or rather troublesome to use for close support, if they are nuclear-tipped. Launching munitions at hypersonic velocity is a problem, but not an unsolvable one. Especially since they would probably be cannon rounds, fired from as close as the fighter's maneuverability would allow (which will be pretty far, given that you can't really go below hypersonic with that design), relying on their short flight time to avoid too much mass loss from atmospheric ablation. This circumvents the biggest problems with firing this stuff directly from orbit. It could drop some kind of missile or bomb, too, but fitting anything externally (a bomb bay is a waste of mass and would be of no use in space) would be serious engineering challenge. It should be noted that what I'm proposing isn't really an attempt to cram two roles into a single vehicle. It's an attempt to provide any support at all to ground troops directly from the orbiting fleet. As it happens, requirements for that also make it a workable orbital combatant. Yes, it's a rather difficult thing to engineer and makes a lot of compromises, but all the other options for that specific mission are even worse.
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Post by bigbombr on Mar 10, 2019 15:42:28 GMT
We're talking about a reusable, powered and manned variant of a Hypersonic Glide Vehicle (think Avangard). Notice how none is developing something like this, despite the US, China and Russia all pouring plenty of funding into hypersonic missile research.Manned, because if you're attacking with a small fleet, you can't get global communications coverage, especially since plasma that forms at those speeds messes with radio waves. Positioning of an orbiting network of communication sattelites can happen quickly, and there are ways around plasma sheaths messing with radio communications: www.technologyreview.com/s/422292/russian-physicists-solve-radio-black-out-problem-for-re-entering-spacecraft/ Furthermore, you can lob the weapon (or drone) in the right direction, have it enter the atmosphere, and give it terminal guidance as soon as the plasma sheath drops. Reusable because you need to get your pilot back. Spaceships circle the planet at a rate of once per one and half an hour or so, not to mention capital ship weapons suck at hitting ground targets. That very much depends on the specific orbit. The ISS circles the Earth in about 90 minutes. Furthermore, whether or not orbiting spacecraft (whether or not they're manned is irrelevant) can effectively hit ground targets depends on their weapon loadout. This seems pretty feasible with lasers or guided munitions.Just hop into COADE and try to design a railgun that'd be both usable in space and fire rounds that wouldn't burn up on entry. That's just a matter of size, and you could make rounds big enough to survive reentry. Such rounds are just not the most efficient option for space-to-space combat, the same way space-to-surface missiles will be very different from space-to-space missiles, especially if a noticeable atmosphere is present.
A hypersonic fighter, armed with some sort of high-velocity conventional cannon, is, in fact, the best way you can provide reliable support from space to your landing troops. The others are lasers and missiles, the former need enormous power, huge apertures and are heavily attenuated by atmosphere, the latter are either of little use in space combat, if not nukes, or rather troublesome to use for close support, if they are nuclear-tipped. Are you seriously arguing that the reentry vehicle like that of an ICBM is less efficient than a manned spacecraft firing rounds out of a high velocity cannon? You were talking about an Earth-like context right, not something like Enceladus? And there are several frequencies a laser can use that don't suffer much from refraction, absorption and scattering while still being high enough diffraction isn't much of an issue. Also, have you actually played the game? Missiles with fragmentation warheads can do some serious damage.
Note that airborne fire support nowadays is mostly guided bombs and missiles, because guns kinda suck, you need to come way too close and need to properly run up your strafing run. A hypersonic aircraft will have a turning radius measured in dozens of km at least, if not more. It is the worst possible platform for guns.
Launching munitions at hypersonic velocity is a problem, but not an unsolvable one. I get the impression you've never taken a closer look at hypersonic aerodynamics. Do you know the layout of an SR-71? That doesn't work for hypersonic vehicles at all, as the bow shock from the engine pods will erode/cut into your hull. Hypersonics are hard, and you've chosen to make this more complicated than it has to be.Especially since they would probably be cannon rounds, fired from as close as the fighter's maneuverability would allow (which will be pretty far, given that you can't really go below hypersonic with that design), relying on their short flight time to avoid too much mass loss from atmospheric ablation. I'll repeat it, since it bears mentioning: guns suck at close fire support from a distance. And with a hypersonic aircraft, you can't go close. That's a fact of physics. You lack the accuracy, your turning radius is too large and I don't even want to think what happens to an exposed gun nozzle at hypersonic velocities.This circumvents the biggest problems with firing this stuff directly from orbit. It could drop some kind of missile or bomb, too, but fitting anything externally (a bomb bay is a waste of mass and would be of no use in space) would be serious engineering challenge. As mentioned above, fitting any large external shape will erode your spacecraft's hull during regular hypersonic flight, don't even think about reentry. If you absolutely want to have a manned hypersonic fighter/bomber, you need an internal bomb bay, as guns will have even more trouble.
It should be noted that what I'm proposing isn't really an attempt to cram two roles into a single vehicle. It's an attempt to provide any support at all to ground troops directly from the orbiting fleet. As it happens, requirements for that also make it a workable orbital combatant. Yes, it's a rather difficult thing to engineer and makes a lot of compromises, but all the other options for that specific mission are even worse. If you want orbital fire support, use lasers against anything time-sensitive and use missile against any hardened fixed positions. Please, before you make any more statements about putting guns on a hypersonic vehicle, take give at least a quick look at the various hypersonic plane, drone and missile designs and ask yourself the why's of the designs (or take a quick look at the challenges associated with hypersonic flight) and guns in modern air-to-surface combat.
You're proposing what amounts to almost the worst possible solution.
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Post by dragon on Mar 10, 2019 22:33:34 GMT
Notice how none is developing something like this, despite the US, China and Russia all pouring plenty of funding into hypersonic missile research.
Of course they aren't. This is a solution to a problem that doesn't exist on Earth. We have plenty of ground to launch airplanes from, and every big country has global communications coverage. This is for times where you only have the orbit, which is the starting point for every planetary invasion, but unlikely to be the case on Earth. Positioning of an orbiting network of communication sattelites can happen quickly, and there are ways around plasma sheaths messing with radio communications: www.technologyreview.com/s/422292/russian-physicists-solve-radio-black-out-problem-for-re-entering-spacecraft/ Furthermore, you can lob the weapon (or drone) in the right direction, have it enter the atmosphere, and give it terminal guidance as soon as the plasma sheath drops.
If it's constantly flying at hypersonic velocities, then it's constantly going to be in a plasma sheath. That's the problem. The article is interesting, but it's highly theoretical work. The to-go solution to the problem is to have a satellite in orbit behind the spacecraft. For that to work, you need a decent commsat network. It is probably a good idea to set this up in advance of a planetary invasion, but it's hardly quick work, and if there's ground-based ASAT, you will have trouble doing that (then again, ground-based ASAT will make the invasion itself difficult, too). Also, the people on the ground will be doing their best to jam your sats, and they'll have much more power to that end than you could ever put on a satellite. That very much depends on the specific orbit. The ISS circles the Earth in about 90 minutes. Furthermore, whether or not orbiting spacecraft (whether or not they're manned is irrelevant) can effectively hit ground targets depends on their weapon loadout. This seems pretty feasible with lasers or guided munitions.
The question is not if, the question is for how long. At 400km, lasers without huge apertures suck. Atmosphere makes them suck more. They need a long-ish exposure time, and you're at 400km very briefly, the distance grows quickly and so does your view angle, lengthening the path the beam can travel through atmosphere. If you go lower, you fly faster, and if you slow down, you drop, so there's really no good way of achieving good time on target with an orbital laser. That's just a matter of size, and you could make rounds big enough to survive reentry. Such rounds are just not the most efficient option for space-to-space combat, the same way space-to-surface missiles will be very different from space-to-space missiles, especially if a noticeable atmosphere is present.
"Just a matter of size" is just like "A small matter of programming". You up the mass, speed goes down. Quick. Try it and see, once you get into kilogram range (and what sort of artillery shell weighs a kilogram?), you're making GW-level weapons to keep speeds competitive, and your ROF takes a hit. A dedicated bombardment weapon would likely be a conventional gun or a low-speed coilgun, and would be dead mass in an orbital engagement because of low muzzle velocity. A lot of dead mass, I might add, I did try to make such weapons and they tend to end up heavy, bulky and/or very inaccurate. Are you seriously arguing that the reentry vehicle like that of an ICBM is less efficient than a manned spacecraft firing rounds out of a high velocity cannon? You were talking about an Earth-like context right, not something like Enceladus? And there are several frequencies a laser can use that don't suffer much from refraction, absorption and scattering while still being high enough diffraction isn't much of an issue. Also, have you actually played the game? Missiles with fragmentation warheads can do some serious damage.
Note that airborne fire support nowadays is mostly guided bombs and missiles, because guns kinda suck, you need to come way too close and need to properly run up your strafing run. A hypersonic aircraft will have a turning radius measured in dozens of km at least, if not more. It is the worst possible platform for guns.
Guns are the best answer for hypersonic munition deployment. Anything else is an aerodynamics nightmare ( your very next point). Also, in atmosphere you can easily have guided cannon rounds, the cost of which would probably be worth it. Sure, it's a bad gun platform, but when you can't mount anything else, gun it is. Plus, guns work in space without too much fuss. TBH, considering the speeds we're talking about, the line between "gun round" and "missile with a really beefy separation charge" would be somewhat blurred (as is the case with several community chemgun designs). A dual-purpose frag missile could be a practical alternative, but it's also a tradeoff. Nukes are much better in starship combat, hands down. Hypersonic aero-orbital fighters would undoubtedly be heavier overall. But for the record, flak missiles do work, but I found even the humble Stinger drone devastating in a high-speed intercept with enough of them flying. I get the impression you've never taken a closer look at hypersonic aerodynamics. Do you know the layout of an SR-71? That doesn't work for hypersonic vehicles at all, as the bow shock from the engine pods will erode/cut into your hull. Hypersonics are hard, and you've chosen to make this more complicated than it has to be.You're back to planes again. We're talking hypersonic lifting bodies, not airplanes. I'm not the one making this mistake, and I do know a few things about hypersonic aerodynamics. Hypersonic separation is probably possible if you really needed it, but as I said, it's best avoided. Besides, powered missiles at this speeds are pointless anyway, the launch aircraft is already going at least as fast as most rifle bullets, the gun is there mostly to make the round clear the launch platform. Same as with high-speed orbital engagements, really, only there you need to overcome enormous drag. I'll repeat it, since it bears mentioning: guns suck at close fire support from a distance. And with a hypersonic aircraft, you can't go close. That's a fact of physics. You lack the accuracy, your turning radius is too large and I don't even want to think what happens to an exposed gun nozzle at hypersonic velocities.Again, guided rounds, adjustable firing angle, or just firing a lot of rounds. A hypersonic gun muzzle is easier than a hypersonic missile separation. As mentioned above, fitting any large external shape will erode your spacecraft's hull during regular hypersonic flight, don't even think about reentry. If you absolutely want to have a manned hypersonic fighter/bomber, you need an internal bomb bay, as guns will have even more trouble.
See above. That's exactly the reason for the gun. Opening the bomb bay in hypersonic flight would be even harder than shooting a gun. Also, mach 27 (the number quoted for Avangard) is reentry speed. It is possible to make turns in such conditions. If you want orbital fire support, use lasers against anything time-sensitive and use missile against any hardened fixed positions.
Again, lasers have serious problems, the least of which is that you're either too fast to get a decent dwell time, or too high to have a decent power density (well, that, or you have to somehow engineer an aperture that dwarfs HST primary mirror...). Missiles can work, but again, you pretty much need an array of missiles which have orbital bombardment as a primary purpose. Also, even with missiles, a ship in orbit can't respond as quickly to the situation on the ground as something flying around in the atmosphere itself. Please, before you make any more statements about putting guns on a hypersonic vehicle, take give at least a quick look at the various hypersonic plane, drone and missile designs and ask yourself the why's of the designs (or take a quick look at the challenges associated with hypersonic flight) and guns in modern air-to-surface combat.
You're proposing what amounts to almost the worst possible solution. So no, not the worst possible solution. You underestimate the sheer difficulty the atmosphere presents to an orbiting vessel. You've got drag, heat and the fact that your fleet is zipping over the planet at orbital velocity. The thing with atmosphere is: the less of it in the way, the better. Also, I'm a flightsimmer, so I know well where guns fit into modern air to surface combat. I also know that this does not apply to a design I'm proposing, since it's completely outside the box. Thinking in terms of modern combat in an interplanetary war is fruitless.
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Post by gyratron on Mar 11, 2019 4:01:47 GMT
[/quote]Compare Skylon with a regular cargoplane that can carry 15 metric tons and you get an idea how a space capable fighter would compare to a more conventional one.[/quote]
I just explained why that's not a fair comparison. The skylon has to carry a payload to orbit, the fighter based on earth must carry a payload up to altitude and reach a cruise speed to be maintained over long distances, but the fighter operating from space only has to survive re-entry, glide a short distance to the target area and then boosts back up to space only after all excess weight including the payload has been dumped. Your comparison only works for a fighter operating from earth which has to fight in space, which would be much harder.
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Post by bigbombr on Mar 11, 2019 6:03:51 GMT
I just explained why that's not a fair comparison. The skylon has to carry a payload to orbit, the fighter based on earth must carry a payload up to altitude and reach a cruise speed to be maintained over long distances, but the fighter operating from space only has to survive re-entry, glide a short distance to the target area and then boosts back up to space only after all excess weight including the payload has been dumped. Your comparison only works for a fighter operating from earth which has to fight in space, which would be much harder. Going from standstill to 20 km altitude at mach 5 is a negligible amount of delta-v compared to the delta-v required to reach orbit. You're cutting out the low velocity section, which saves a lot of mass, but you'll still likely need massive propellant tanks. And a human pilot, the radar and structural elements are a pretty significant part of the mass. You're not lobbing an empty spacecraft into orbit.
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Post by bigbombr on Mar 11, 2019 6:08:46 GMT
So no, not the worst possible solution. You underestimate the sheer difficulty the atmosphere presents to an orbiting vessel. You've got drag, heat and the fact that your fleet is zipping over the planet at orbital velocity. The thing with atmosphere is: the less of it in the way, the better. Also, I'm a flightsimmer, so I know well where guns fit into modern air to surface combat. I also know that this does not apply to a design I'm proposing, since it's completely outside the box. Thinking in terms of modern combat in an interplanetary war is fruitless. I'm stating that having an orbital spacecraft with a large laser and reentry capable missiles makes for better fire support than a needlessly complicated vehicle that can carry less payload, puts a pilot at risk and can't hit the broad side of a barn because it deploys it munitions by cannon while having a turning radius measured in Belgiums.
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Post by AtomHeartDragon on Mar 11, 2019 21:01:01 GMT
So no, not the worst possible solution. You underestimate the sheer difficulty the atmosphere presents to an orbiting vessel. You've got drag, heat and the fact that your fleet is zipping over the planet at orbital velocity. The thing with atmosphere is: the less of it in the way, the better. Also, I'm a flightsimmer, so I know well where guns fit into modern air to surface combat. I also know that this does not apply to a design I'm proposing, since it's completely outside the box. Thinking in terms of modern combat in an interplanetary war is fruitless. I'm stating that having an orbital spacecraft with a large laser and reentry capable missiles makes for better fire support than a needlessly complicated vehicle that can carry less payload, puts a pilot at risk and can't hit the broad side of a barn because it deploys it munitions by cannon while having a turning radius measured in Belgiums. This. The main problem with this kind of hybrid craft, is that it doesn't offer much advantages above orbiting craft at the cost of tremendous penalties.
For it to be worthwhile it would have to be able to slow down to at least transsonic regime and loiter, which implies either dropping down conventional aircraft (but very likely nuclear powered) with reentry shields, or some much beefier propulsion than just a nuclear jet/rocket, to allow easy acceleration back to orbit after fulfilling mission objectives. If you are not loitering then you might as well be lobbing munitions and zapping with laser beams from orbit.
I wonder if another kind of orbital/atmospheric craft would be possible, though - briefly reentering shallowly and with as little velocity loss as possible to execute plane changes aerodynamically. This would be a primarily orbital fighter.
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Post by dragon on Mar 11, 2019 22:45:47 GMT
For it to be worthwhile it would have to be able to slow down to at least transsonic regime and loiter, which implies either dropping down conventional aircraft (but very likely nuclear powered) with reentry shields, or some much beefier propulsion than just a nuclear jet/rocket, to allow easy acceleration back to orbit after fulfilling mission objectives. If you are not loitering then you might as well be lobbing munitions and zapping with laser beams from orbit. A hypersonic fighter can definitely loiter. Let's assume loiter at 1700m/s (nearly twice the speed of a .50 BMG round), which would be around Mach 5 at SL. Centripetal acceleration is v^2/r. Assume 5 gravities as something a human pilot can hold. Now, this gives 578km, which is indeed the size of a small country, but let's also look at the period. It's 2Pi*r/v. That gives you about 35 minutes. That gives you three hypersonic passes per one orbital one (assuming an ISS-like orbit), without any issues with inclination. Note that you can turn much tighter if the situation warrants. Time on target will be longer (since you're not zipping over it at 9km/s), you can come much closer (meaning easier job for the guidance system and a lot less atmo to bother you) and are somewhat less predictable than a ship in orbit. Also note, the above is for a 5G loiter turn. If we assume short-term G-loads, we can go up to 20 gravities, and that gives us, at 1700m/s, a turn radius of 144.5km. Still a lot, but probably sufficient if you come in at a shallow angle. If you can make liquid breathing or other such fancy tricks work, you could get better turn rates. Or if you can slow this thing down below 1700m/s, for that matter, which you should be able to, since it's a very conservative value. Mach 5 is what the X-51 topped out at, it might be possible to go into supersonic regime without dropping out of the sky. Velocity is squared in the turn radius equation, so it'll go down quickly. Unfortunately, information on how slow something like Avangard can go is rather scarce... X-51 started out at Mach 4.5, but this was because of the scramjet, not for lift. Meanwhile, some lifting bodies (of very different design, though some Avangard visualizations resemble the X-24B) have been landed safely, presumably only slightly faster than a typical airplane (steep approach and high AoA is usually the key with such planes). Wings help with this flying business, but are by no means essential.
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Post by bigbombr on Mar 12, 2019 5:39:33 GMT
A hypersonic fighter can definitely loiter. Let's assume loiter at 1700m/s (nearly twice the speed of a .50 BMG round), which would be around Mach 5 at SL. Centripetal acceleration is v^2/r. Assume 5 gravities as something a human pilot can hold. Now, this gives 578km, which is indeed the size of a small country, but let's also look at the period. It's 2Pi*r/v. That gives you about 35 minutes. That gives you three hypersonic passes per one orbital one (assuming an ISS-like orbit), without any issues with inclination. Note that you can turn much tighter if the situation warrants. Time on target will be longer (since you're not zipping over it at 9km/s), you can come much closer (meaning easier job for the guidance system and a lot less atmo to bother you) and are somewhat less predictable than a ship in orbit. Also note, the above is for a 5G loiter turn. If we assume short-term G-loads, we can go up to 20 gravities, and that gives us, at 1700m/s, a turn radius of 144.5km. Still a lot, but probably sufficient if you come in at a shallow angle. If you can make liquid breathing or other such fancy tricks work, you could get better turn rates. Or if you can slow this thing down below 1700m/s, for that matter, which you should be able to, since it's a very conservative value. Mach 5 is what the X-51 topped out at, it might be possible to go into supersonic regime without dropping out of the sky. Velocity is squared in the turn radius equation, so it'll go down quickly. Unfortunately, information on how slow something like Avangard can go is rather scarce... X-51 started out at Mach 4.5, but this was because of the scramjet, not for lift. Meanwhile, some lifting bodies (of very different design, though some Avangard visualizations resemble the X-24B) have been landed safely, presumably only slightly faster than a typical airplane (steep approach and high AoA is usually the key with such planes). Wings help with this flying business, but are by no means essential. Unlike a spacecraft, you need to constantly expend energy to remain airborne, you have more trouble deploying munitions, you need to be aerodynamic and you're limited in size. And hypersonic vehicles are even less stealthy than spacecraft. You won't surprise anyone unless you've already suppressed their air/space observation systems. At that point, why not simply use the spacecraft you've already used to get here?
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Post by AtomHeartDragon on Mar 12, 2019 19:59:41 GMT
A hypersonic fighter can definitely loiter. Let's assume loiter at 1700m/s (nearly twice the speed of a .50 BMG round), which would be around Mach 5 at SL. Centripetal acceleration is v^2/r. Assume 5 gravities as something a human pilot can hold. Now, this gives 578km, which is indeed the size of a small country, but let's also look at the period. It's 2Pi*r/v. That gives you about 35 minutes. That gives you three hypersonic passes per one orbital one (assuming an ISS-like orbit), without any issues with inclination. Which means that, other things equal, three spacecraft (or one spacecraft and two drones) are going to be at least as good as your fighter, in addition to providing more or less global coverage. Unlike a spacecraft, you need to constantly expend energy to remain airborne, you have more trouble deploying munitions, you need to be aerodynamic and you're limited in size. And hypersonic vehicles are even less stealthy than spacecraft. You won't surprise anyone unless you've already suppressed their air/space observation systems. At that point, why not simply use the spacecraft you've already used to get here? Energy expenditure is not a major issue if you're running an air-breathing nuclear engine (even a solid core one), but there is one that has so far been overlooked: Survivability. Spacecraft, despite their typically flimsy construction, are generally surprisingly sturdy, courtesy of incredibly forgiving environment they operate in. An orbiting spacecraft typically cannot: - Crash
- Sink
- Get torn apart by aerodynamic forces
- Get pulverized by shockwaves
Barring getting pushed into lithobraking or escape trajectory, or getting literally vaporized, once up it will stay up. An aircraft, otoh, has to contend with aerodynamic forces and the fact any unpowered trajectory it will take is generally going to be a short one. Those problems get progressively worse the faster the aircraft in question, so by turning your spacecraft into an aircraft you automatically make it much less damage tolerant on top of any other engineering penalties it might incur. Really, unless your propulsion makes surface to orbit almost free, it is hard to make a case for an aeroorbital fighting craft.
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Post by gyratron on Mar 13, 2019 2:20:25 GMT
It might just be possible to use a sort of monocoque construction where the armor for space is also the reinforcement against aerodynamic forces in atmosphere. It will still be a compromise but perhaps not a disastrous one. The idea behind fighters is generally that they avoid damage by not getting hit anyway, and if surface to air missiles need their own nuclear engine just to catch up to you that's already a pretty good start. The main part I don't see is how holding a constant 4g turn just to zip past the target area once every half hour is a worthwhile improvement on the weaknesses of an ordinary orbital bombardment using lasers and missiles.
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Post by AtomHeartDragon on Mar 13, 2019 19:46:34 GMT
It might just be possible to use a sort of monocoque construction where the armor for space is also the reinforcement against aerodynamic forces in atmosphere. It will still be a compromise but perhaps not a disastrous one. The idea behind fighters is generally that they avoid damage by not getting hit anyway, and if surface to air missiles need their own nuclear engine just to catch up to you that's already a pretty good start. The main part I don't see is how holding a constant 4g turn just to zip past the target area once every half hour is a worthwhile improvement on the weaknesses of an ordinary orbital bombardment using lasers and missiles. The problem isn't different amounts of armour. The problem is the environment. An orbiting spacecraft may sustain severe structural damage without it affecting its function too much and can also continue to function after losing engines (though it's definitely A Bad Thing(TM)). OTOH hypersonic airframe will get turned into composite/metal confetti from much lighter damage simply because it has to contend with tremendous aerodynamic forces and losing propulsion will also not end well for it.
There was a RL aviation accident where SR-71 Blackbird disintegrated in flight at Ma3.18 due to inlet malfunction (curiously, the pilot survived, though not the other crewmember) without anyone shooting at it. There was also, of course, Columbia disaster where single cracked thermal shield tile was enough to break up and burn the entire reentering orbiter. Your hypersonic fighter will be somewhere between those two which is not a fun place to be, especially considering that some kinetics are going to be fired into its flight path and that pitting the hypersonic airframe with laser and seeing what happens will also be a thing.
You need a pretty strong rationale for swapping peace and quiet of orbit for air trying to murder you, especially considering all the extra penalties it confers, including the propulsion necessary to climb back up. Unless you have some marvellous air-breathing mass-injected fusion motor allowing you to pop in and out of the atmosphere freely as circumstances require, it seems that the winning move is not to play and only send down stuff you intend to stay there.
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Post by dragon on Mar 14, 2019 20:37:30 GMT
You're trying not to get hit anyway. SR-71's problem was that it was a twin engine aircraft with a rather quirky inlet design. Also, that failure wasn't in level flight (IIRC, it could generally survive an inlet unstart), but in a sharp turn. Shuttle heatshield, meanwhile, wasn't the best design, anyway. RCC is rather delicate, and on the Shuttle it was really thin, too, to save weight. If you have a nuclear engine you can probably afford to mount something rather tougher. Say, for example, tungeten alloy armor (like Shuttle was supposed to have at some point), or perhaps our good old friend amorphous carbon. As it happens, the properties you need to resist lasers and nukes are also the same you need to resist reentry heat. AC is rather hard to beat in either regard, and is light and fairly robust physically, as well. Also, it's not stealthy, but it's got one thing going for it: horizon. If it's zipping around at low enough altitude, any ground-based installation will have a very limited window of opportunity to fire at it. A laser will either be too weak to burn though the heatshield, or will need a mirror so large that it won't be able to traverse fast enough to track it. An orbiting spacecraft, while also affected by this, is incredibly predictable in its orbit, because any orbit change costs dV, not to mention its altitude makes it move much more slowly across the sky (that said, I don't think that a ground-based laser of a reasonable size will do much against spacecraft, either). The same can't be said of an atmospheric fighter, which can change direction effectively for free thanks to its using air as propellant, and thus can fly a rather complex flightpath at low altitudes. Yes, this means my fighter can replace a pair of spacecraft. Each of which would have to be carrying vastly bigger weapons to punch through the atmosphere and still pack enough punch to do damage on the ground. Well, the rounds actually reach the target, for one, and retain most of their mass while at it. Orbital bombardment is surprisingly troublesome to implement, and weapons capable of doing it are dead mass in orbital combat, unless you have a gigantic reactor to power either gigantic laser arrays or multi-kg class rail/coilguns that still fire at high speed. Orbital combat favors sandblasters due to advantages of short round flight time, but they're completely worthless when firing at anything with an atmosphere.
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