Missile guidance systems

PMN1

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According to BSP4, 'Artemis used an early form of semi-active homing, whereby the target was to be illuminated by AI radar in the launch aircraft with the reflected signal picked up by Artemis'.

If semi-active radar was being looked at in 1943/44, why did missiles such as Sea Slug use beam riding homing?
 

Grzesio

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A guidance system choice depends of many factors.
For example semi-active or active guidance may not be appropriate for air-to-surface missiles with somewhat steeper trajectory, as the radar cannot 'see' the target against the background surface. For example this was the reason for equipping Roc bombs with a TV guidance instead of initially intended radar one.
In case of a surface-to-air missile as the Seaslug, the reason for not using active or semi-active radar guidance may be caused by a fairly complex way of guiding a missile towards its target. Generally it's easier to have such a missile remotely controlled (by signals or a beam) instead of self-homing (at least at the majority of its trajectory) - as in this case the missile's path can be calculated with appropriate advance, heading etc. by a fairly elaborate control system in such a way, that missile's trajectory is optimized according to given conditions of interception, allowing the missile to reach its target quickly and with minimum maneuvering.
An early self-homing missile would usually always point at its target, moving at the pursuit curve, what would usually result in relatively violent maneuvers at the final stage of its flight, what could lead to a miss. Such a missile needs also to cover a greater distance before hitting the target than a missile guided with e.g. constant bearing method etc., what means pursuit curve missile's kill range is actually smaller, even if its flight range is the same as of a constant bearing missile.

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Grzesio
 

RP1

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Further to Grzesio's comments, one should note that Artemis was more of a course corrected 5-inch rocket, rather than a missile in the modern sense.

In "British Destroyers and Frigates", Friedman mentions that the guidance range of a beam-riding missile is much greater than a semi-active homing weapon due to the limited power of the illuminator. I find this a little confusing, as the receiving SARH missile is closer to the target than the receiving director in a B-R system. Perhaps it relates to the size and sensitivity of the missiles receiver, and choice of frequency?

There is also the issue of changes in signal strength as the SARH missile closes on the target - IIRC this was a problem in the development of the BAT active homing anti-ship weapon.

RP1
 

Grzesio

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You're right. I've once read a very interesting book of 1947 about advanced weapons development by the NDRC - there were problems with the Bat indeed - as the Navy for some unexplainable reason chosen the active homing Bat instead of the semi-active Pelican (unexplainable, because rules of engagement always required visual identification of the target). This caused a lot of problems due to very rapid increase of reflected signal strength at the final stage of the trajectory (as both the transmitter and receiver were closing to the target) leading to a miss. The problem caused serious delay in development and eventually the Bat was put into service a couple of months later than the Pelican could have been.

Well, I understand the problem of the beam riding missile in such a way:
The illuminator size and power of an active homing missile is limited by the missile size, but there are no such limits when an external illuminator is concerned.
When the actively homing missile closes to its target both the illuminator and receiver are close to the target indeed - but at the moment when the missile is launched, the onboard illuminator is equally far from the target as the external, ship mounted illuminator would be, while the latter can be much more powerful. This obviously limits ARH missile's max. range as compared to to SARH missile.
Then, there's some unavoidable loss of signal strength during the reflection by the target - while the beam riding missile always receives a direct signal. And the BR signal covers the smallest distance, only the ship-missile one. SARH needs to cover ship-target-missile distance.

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Grzesio
 

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Real problem with beam riding guidance is that the beam spreads with distance so accuracy decreases with range whereas with SARH (or active RH) accuracy improves as distance to the target decreases (or range from the launcher increases - same thing)

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Talos, of course, had the best of both worlds - midcourse guidance on a beam with SARH terminal homing. IIRC this was used to advantage in Vietnam, bringing the missile down on the target under beam-riding guidance and then switching to SARH illumination too late for the enemy RWR to give meaningful notice before the end. The problem there of course is the size and complexity of the system (and not just the missile handling gear and the missiles), but it offered trajectory optimisation (with the bulky computers in the most optimal place - on the ship, right next to the guidance-beam projector) with reasonably precise terminal guidance, a combination you didn't otherwise get until AEGIS arrived (I can't remember to what extent SM-1 performance is optimised - if at all - through being directed by an AEGIS ship, but I'm of the understanding that the old Terrier ships which fired SM-2 ER did better to allow AEGIS to control the missiles so long as there were enough channels of fire available).

I remember reading something about Seaslug Mk2 being a SARH weapon; which might imply that it could ride the beam in the boost and approach phases, until it got close enough that returns off the homing head gave better control than ship direction, or it might not.
 

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Artemis was based on a 3" rocket, what diameter would you need to go to to have made it effective?
 

PMN1

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Was it planned to use Artemis with an existing AI or was a new system to be developed?
 

T. A. Gardner

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Further to Grzesio's comments, one should note that Artemis was more of a course corrected 5-inch rocket, rather than a missile in the modern sense.

In "British Destroyers and Frigates", Friedman mentions that the guidance range of a beam-riding missile is much greater than a semi-active homing weapon due to the limited power of the illuminator. I find this a little confusing, as the receiving SARH missile is closer to the target than the receiving director in a B-R system. Perhaps it relates to the size and sensitivity of the missiles receiver, and choice of frequency?

There is also the issue of changes in signal strength as the SARH missile closes on the target - IIRC this was a problem in the development of the BAT active homing anti-ship weapon.

RP1
It's not so much the power of the illumination radar but rather the dispersion of the beam. As the distance grows longer, the beam gets wider as does the reflected energy from the target. This means the missile has a harder time distinguishing where the target actually is within the return.
Many early SAM's like Nike and Talos got around this issue by incorporating both systems into the missile's guidance. It also permitted the missile to fly a very fuel efficient ballistic trajectory to the area of the target using beam riding. Once close to the target, it was illuminated and the missile could home accurately on that target.
The problem with both missiles was the size of the tracking and illumination radars had to be huge to get sufficient accuracy to guide the missile. The other issue was developing a guidance computer that could align the radars accurately and keep them centered on the target.
Beam riders that fly directly to the target are very energy inefficient in terms of flight to the target. Semi-active systems using a collision course intercept are better.

With Artemis, the guidance system was particularly crude and what the missile really needed was a control system that allowed for a reasonable level of precision. This would have greatly complicated the design however. First it would require the missile be gyro-stabilized to remain steady in flight and have a zero reference plain as it flew. Next, control surfaces would be necessary in addition to stabilizing ones to make the flight smooth and turns accurate.
My guess would be that you could do it in an 8" rocket, possibly a 5," but nothing smaller.

The original Artemis design tried to cram everything into a new head that fitted on the front of the existing missile, and that just wasn't going to work. You also really need a proximity fuze to give a good Pk, something that had yet to enter service.
 

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First it would require the missile be gyro-stabilized to remain steady in flight and have a zero reference plain as it flew.
I thought the whole point of Artemis was to let the body spin as much as it wanted; the homing head supplied control deflections in whatever direction the radar return was coming from.
With Artemis, the guidance system was particularly crude
This seems like an unfair accusation given when it was started. All the WW2 guided weapon projects were essentially zeroth-generation; mistakes were going to be universal and common, because there was no previous experience to learn from, and the learning curve was going to be a combined function of the expertise, resources and desperation you could throw at it.
 

T. A. Gardner

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First it would require the missile be gyro-stabilized to remain steady in flight and have a zero reference plain as it flew.
I thought the whole point of Artemis was to let the body spin as much as it wanted; the homing head supplied control deflections in whatever direction the radar return was coming from.
With Artemis, the guidance system was particularly crude
This seems like an unfair accusation given when it was started. All the WW2 guided weapon projects were essentially zeroth-generation; mistakes were going to be universal and common, because there was no previous experience to learn from, and the learning curve was going to be a combined function of the expertise, resources and desperation you could throw at it.
On the first, it was. But if there was any wobble in the homing head it would increase error in corrections. Also, there was likely to be some induced spin in the homing head if the missile spun. You really need a stable gyro of some sort to make AAM's work. I think the rolleron on the Sidewinder was probably the most elegant and simple solution ever devised.

On the second, it isn't. Artemis proposed a simple single spoiler that would pop open then close to alter course. Getting this to do so at the correct instant for the correct amount of course correction was going to be a real trick given the state of electronics at the time. Since no other early missile proposed or used such a method, it doesn't appear a workable solution. Other unworkable solutions that were proposed in early AAM missile designs included CLOS (human hand-eye coordination was simply too slow to make this work) and wire guidance (the X-4 used two 34 gage spring steel wires--about the thickness of a hair-- that would spool out for thousands of feet with no support).
 

Dilandu

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On the second, it isn't. Artemis proposed a simple single spoiler that would pop open then close to alter course. Getting this to do so at the correct instant for the correct amount of course correction was going to be a real trick given the state of electronics at the time. Since no other early missile proposed or used such a method, it doesn't appear a workable solution. Other unworkable solutions that were proposed in early AAM missile designs included CLOS (human hand-eye coordination was simply too slow to make this work) and wire guidance (the X-4 used two 34 gage spring steel wires--about the thickness of a hair-- that would spool out for thousands of feet with no support).
Well, I should point that the idea of Artemis was to use it in massive salvos - basically to saturate the target area with homing-a-bit missiles.
 

Dilandu

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P.S. Personally, I think that the only way to get viable AAM on WW2 tech was to use ACLOS (automatic close-to-line-of-sigh) with either conical-scan or lobe-switching radar. The beam is locked on target (maybe even manually), the position of the missile in the beam is tracked by automatically comparing the strength of the signal on opposite positions (left-right, up-down) and tone-modulated command are send to missile.
 

P-STICKNEY

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P.S. Personally, I think that the only way to get viable AAM on WW2 tech was to use ACLOS (automatic close-to-line-of-sigh) with either conical-scan or lobe-switching radar. The beam is locked on target (maybe even manually), the position of the missile in the beam is tracked by automatically comparing the strength of the signal on opposite positions (left-right, up-down) and tone-modulated command are send to missile.
That would be a Beam Rider. In order to keep the missile on the line of sight, you can't rely on signal strenGoth comparison - it's a glinty, wobbly part of the spectrum. The more practical way is to have each quadrant send a series of coded pulses that combine to a continuous tone when centered (well, in the center cone). That way, a simple comparison of the received pulse pattern in the missile will generate the appropriate steering commands.
 

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