3 ms·

Let's do the math on that shall we. Aegis is built for defending against missiles going at most mach 4-5. Mach 5 is ~1700 m/s, so during a millisecond GC pause

by WJW 5y ago

Let's do the math on that shall we. Aegis is built for defending against missiles going at most mach 4-5. Mach 5 is ~1700 m/s, so during a millisecond GC pause it will travel slightly under 2 meters. Ship-based SAMs will have proximity-fused fragmentation warheads and even back in the 70s those had a kill radius of >100m (see for example https://en.wikipedia.org/wiki/S-75_Dvina https://en.wikipedia.org/wiki/S-75_Dvina). So even a GC pause at the worst possible moment would not significantly impact the probability to hit the target. Even really fast things in the real world are really slow compared to computers. (Of course, in practice the Aegis system will only provide midcourse corrections to the missile. The fuse and end phase guidance control software would not interface much with the rest of the combat management system anyway.)

lmilcin 5y ago
I think you don't appreciate how rockets kill other rockets. The rockets close head on (so the speeds add together) and then the rocket trying to kill the other one explodes at right distance to the side of the other rocket, exploding in a cone of debris. That cone of debris must intersect the other rocket at a right point. It can't just hit anywhere, to reliably kill it must hit a specific part of the other rocket. So, realistically, you have to time your explosion to something like tens of microseconds. Also getting there is a control loop problem and it requires very precise control. The more jitter in execution of the control loop the worse steering it will be and less chance it will get to the right place at the right time.
- WJW 5y ago
  I was a weapons engineering officer for the Dutch navy for 14 years; I'm well aware of the mechanisms involved. First of all, a SM-2 will not use the flight path you describe as they tend to "dive down" on a sea skimming missile. But even the shorter range Sea Sparrow missiles that do behave as you describe use multiple guidance phases. In the early and mid-phase guidance phases, the radar reflections from the target missile will not be strong enough for the relatively small receiver in the missile, so the system must depend on midcourse guidance updates (little bit to the right, little bit up, etc) from the combat management system on board the launching vessel to navigate to a "close enough" location from where the sensors on board the missile can acquire the target. The end-phase guidance and fuse timings are quite critical, as you say, but they are also not really under command of the CMS but rather done by embedded systems on board the missile. Generation of the mid-course guidance updates is not nearly as time critical and won't suffer much from a 1 ms pause. The error in location estimation of the target due to sensor imperfections (limited angular resolution, clock jitter, athmospheric effects, etc etc) will be way larger than the 2 meter from the GC pause.
pjmlp 5y ago
Impressive, but you need to factor in it is a real time JVM, with corresponding soft real time GC, and hard real time threads.

by WJW 5y ago

lmilcin 5y ago

I think you don't appreciate how rockets kill other rockets. The rockets close head on (so the speeds add together) and then the rocket trying to kill the other one explodes at right distance to the side of the other rocket, exploding in a cone of debris. That cone of debris must intersect the other rocket at a right point. It can't just hit anywhere, to reliably kill it must hit a specific part of the other rocket. So, realistically, you have to time your explosion to something like tens of microseconds. Also getting there is a control loop problem and it requires very precise control. The more jitter in execution of the control loop the worse steering it will be and less chance it will get to the right place at the right time.

WJW 5y ago
I was a weapons engineering officer for the Dutch navy for 14 years; I'm well aware of the mechanisms involved. First of all, a SM-2 will not use the flight path you describe as they tend to "dive down" on a sea skimming missile. But even the shorter range Sea Sparrow missiles that do behave as you describe use multiple guidance phases. In the early and mid-phase guidance phases, the radar reflections from the target missile will not be strong enough for the relatively small receiver in the missile, so the system must depend on midcourse guidance updates (little bit to the right, little bit up, etc) from the combat management system on board the launching vessel to navigate to a "close enough" location from where the sensors on board the missile can acquire the target. The end-phase guidance and fuse timings are quite critical, as you say, but they are also not really under command of the CMS but rather done by embedded systems on board the missile. Generation of the mid-course guidance updates is not nearly as time critical and won't suffer much from a 1 ms pause. The error in location estimation of the target due to sensor imperfections (limited angular resolution, clock jitter, athmospheric effects, etc etc) will be way larger than the 2 meter from the GC pause.

pjmlp 5y ago

Impressive, but you need to factor in it is a real time JVM, with corresponding soft real time GC, and hard real time threads.