Interception against an actively manoeuvring target is a question of kinetic energy. If the intecepting missile is smaller all the hypersonic missile has to do is zig-zagging randomly and you're toast.
If you're going to wait for the hypersonic missile to be close enough that this isn't a problem, you have the issue that your missile won't be able to manoeuvre right and might still be building up velocity. Plus there won't only be one!
> the interceptor might also allow for active cooling of the IR window in that brief period.
The issue isn't the IR window, it's the air directly in front of the interceptor missile, which has to target from the frontal aspect.
>Also, even detection from a fixed ground base (or from space - see SpaceX's newest missile detection project) is still detection in the first place, which is what I was commenting on. Wasn't even going to veer into interceptor guidance, but I'm sure there's multiple ways to do it.
Detection is not enough. We can already detect missiles from satellites even. The issue is calculating their velocity accurately and locating them at +- 10m from 100km+ away is not feasible without really large apertures and pretty long focal lengths. You wouldn't be able to accurately track more missiles than you have optics and the optics would be mind-boggingly expensive. Besides that, the target acquisition radar to maintain a feasible or even physically possible relative aperture for target acquisition would be unable to resolve any detail about the missile, so it would have no way of telling the difference between a missile and a flare of the same brightness and color.
> Interception against an actively manoeuvring target is a question of kinetic energy. If the intecepting missile is smaller all the hypersonic missile has to do is zig-zagging randomly and you're toast.
What you're describing is not a matter of kinetic energy (clearly if it were about kinetic energy, this would be defeated by making the interceptor weigh 30 tonnes) but a matter of transverse acceleration. On that matter, as far as I understand it, current AAMs/SAMs are somewhere in the 50g region. I strongly suspect that hypersonic vehicles due to their lower L/D are nowhere near that -- hell, many of them seem to have a hard enough time flying straight, if US military's experience is of any relevance here.
Also, due to low L/D ratio in the high Mach region, if your missile is zig-zagging randomly, it won't stay fast for long. And if it's not zig-zagging randomly, then the question is how it gets the information how to zig-zag, since the nose of a hypersonic vehicle seems to be an extremely poor sensor platform -- which you admitted yourself. It's virtually certain that it won't be able to see the interceptor.
> The issue isn't the IR window, it's the air directly in front of the interceptor missile, which has to target from the frontal aspect.
It seems unlikely that the interceptor will have to fly at a comparable speed in a frontal aspect interception situation (considering that it doesn't need to catch up).
But as for terminal interception, I suspect that here the inverse-fourth-power-of-distance operation of an active radar homing solution might help you if all else fails -- at a small enough distance this will work even better than the inverse square applicable to passive optical/IR detection.
> would be unable to resolve any detail about the missile, so it would have no way of telling the difference between a missile and a flare of the same brightness and color.
It would seem that the easiest way to distinguish a flare would be to look if it's in a controlled flight? Unlike a flare, the hypersonic vehicle will continue flying, and the difference is readily apparent. Besides, you can't have that many flares on it, so you can't keep firing them continuously -- again, the only thing that's available to you if you can't see the incoming interceptor.
>> Also, due to low L/D ratio in the high Mach region, if your missile is zig-zagging randomly, it won't stay fast for long.
These are not airplanes with wings. These are missiles very high in the atmosphere flying complex flight paths more accurately describes as trajectories. The air is thin, the drag low, the energies extraordinarily high. A successful zig-zag maneuver could be as small as one degree left or right, up or down. And sacrificing speed to escape an interceptor is a valid tradeoff considering that, not being an airplane, the missile intends to slam into the ground/target in a minute or two no matter what its speed.
>What you're describing is not a matter of kinetic energy (clearly if it were about kinetic energy, this would be defeated by making the interceptor weigh 30 tonnes) but a matter of transverse acceleration. On that matter, as far as I understand it, current AAMs/SAMs are somewhere in the 50g region. I strongly suspect that hypersonic vehicles due to their lower L/D are nowhere near that -- hell, many of them seem to have a hard enough time flying straight, if US military's experience is of any relevance here.
Not so. AAMs/SAM can only pull anywhere close to 50g at gigantic cost in drag and can only do that once and briefly. It is a question of kinetic energy because the missile that can regenerate kinetic energy and that loses the least kinetic energy relative to it's mass is at a massive advantage.
>Also, due to low L/D ratio in the high Mach region, if your missile is zig-zagging randomly, it won't stay fast for long. And if it's not zig-zagging randomly, then the question is how it gets the information how to zig-zag, since the nose of a hypersonic vehicle seems to be an extremely poor sensor platform -- which you admitted yourself. It's virtually certain that it won't be able to see the interceptor.
Traditional aerodynamic designs suffer from poor L/D ratios, yes. Waverider designs probably have much fewer issues. Given the advent of HGVs, which are made to turn multiple times without any engine to back them up, we can be fairly confident that this is not the case.
As far as being a poor sensor, this is an oversimplification of a complex mechanism. Plasmas block radar up to a certain frequecy depending on their temperature. So you make it so that the radar is somewhere where the air is slightly colder and make its frequency higher. You don't even have to put the radar in the nose if the target you're trying to hit is sluggish and large enough.
It doesn't need to maneuver in response to the interceptor. The missile knows when it is in LOS to the target, and knows that the interceptor will be fired soon after. So it already knows when the missile will be fired and thus when the missile would hit. The window for interception is pretty short. It would have to do evasive maneuvers at maybe 4G for maybe 30 seconds, all the while gaining back Ke from reducing in altitude in combination with engines.
>It would seem that the easiest way to distinguish a flare would be to look if it's in a controlled flight? Unlike a flare, the hypersonic vehicle will continue flying, and the difference is readily apparent. Besides, you can't have that many flares on it, so you can't keep firing them continuously -- again, the only thing that's available to you if you can't see the incoming interceptor.
This is a REALLY hard problem, and it's the reason why imaging infrared guidance was invented. It takes a lot of time to be certain that a flare in a ballistic trajectory at high speed is different from a missile coming towards you! Those are precious seconds you can't afford. Remember that you have no way of estimating distance.
> Not so. AAMs/SAM can only pull anywhere close to 50g at gigantic cost in drag and can only do that once and briefly. It is a question of kinetic energy because the missile that can regenerate kinetic energy and that loses the least kinetic energy relative to it's mass is at a massive advantage.
Except since the interceptor has only one chance at interception, the situations of the interceptor and the missile are not symmetrical. "Doing that once and briefly" seems quite acceptable for the interceptor. The missile pretty much must regenerate kinetic energy, or its subsequent suboptimal flight regime might make it even more vulnerable to the second interceptor incoming, or the third one after that.
> The window for interception is pretty short.
With early detection (orbital, for example), the window seems to be as long as the range of the longest-range applicable interceptor missile.
> It would have to do evasive maneuvers at maybe 4G for maybe 30 seconds
That definitely seems to be a "too little, too late" kind of situation. Especially in a point defense scenario where maneuvering away from your line to target means not hitting the target in the first place (which is as good as a successful hit of the interceptor), and where being close to the target really favors the short-range defense systems.
> Remember that you have no way of estimating distance.
You may very well have that way in the end if SpaceX's/L3Harris's system gets implemented.
If you're going to wait for the hypersonic missile to be close enough that this isn't a problem, you have the issue that your missile won't be able to manoeuvre right and might still be building up velocity. Plus there won't only be one!
> the interceptor might also allow for active cooling of the IR window in that brief period.
The issue isn't the IR window, it's the air directly in front of the interceptor missile, which has to target from the frontal aspect.
>Also, even detection from a fixed ground base (or from space - see SpaceX's newest missile detection project) is still detection in the first place, which is what I was commenting on. Wasn't even going to veer into interceptor guidance, but I'm sure there's multiple ways to do it.
Detection is not enough. We can already detect missiles from satellites even. The issue is calculating their velocity accurately and locating them at +- 10m from 100km+ away is not feasible without really large apertures and pretty long focal lengths. You wouldn't be able to accurately track more missiles than you have optics and the optics would be mind-boggingly expensive. Besides that, the target acquisition radar to maintain a feasible or even physically possible relative aperture for target acquisition would be unable to resolve any detail about the missile, so it would have no way of telling the difference between a missile and a flare of the same brightness and color.