Kyiv has claimed two hits on Russian navy surface ships following an innovative attack by seven unmanned surface vessels (USV) on Russia’s fleet base at Sevastopol. At present Moscow is admitting to a single hit, on a mine countermeasures vessel, but more hits may emerge.

Some footage of the attack has been released which is far from definitive as to its results. Before looking at the competing claims for the attack it is worth considering the mechanics of this new form of naval warfare – the first of its kind in actual combat.

The vessels used in the attack are around 8 metres in length, and powered by a jet-ski engine which drives a water-jet. A satellite transceiver aerial is fitted astern, and an infra-red/visual camera is mounted amidships about 1 metre above sea level. Forward, there appear to be a pair of impact detonators – a pair probably to avoid premature detonation caused by random impacts with waves as the USV powers through open sea. 

Small vessels, with a 100 kg warhead

The USV has a displacement of around 1 tonne. Deducting the weight of the hull, motor, fuel and command and control equipment leaves something in the order of 100-150 kg for the warhead, which is located amidships. A fuel load of 200 kg would give a twelve-hour endurance at full speed, which would be 60 km/h in flat water but probably half that in open sea and even less in moderate sea conditions. The effective range of the USV is therefore probably around 300 km. In this case it appears that the USVs were launched from the Ukrainian naval base of Ochakiv, about midway between Odesa and Mikolayev, and about 300 km by sea from Sevastopol.

In open water the USV cannot make anything approaching its full calm-water speed. In the Ukrainian video we see one operating in a moderately rough sea at a speed of somewhere below 30 km/h – not even on the plane. A manned vessel of the same size would be able to move at a higher speed than this, but most of the “feel” and anticipation needed to move fast is lost when the operator is not aboard the vessel.

Command and control needs a satellite link or a linking aircraft

The USV’s command link operates with a line-of-sight radio signal. With its aerial located only half a metre above water level, line of sight to the USV’s radio horizon falls to around 10 km. To operate at a distance of 300 km from its controller the USV must therefore be provided with a data link at great height, which can be achieved either by using a satellite link or a suitably fitted aircraft flying sufficiently high to maintain a line of sight to both the USV and to the controller. We don’t know which option was used here If the link was airborne then it was probably provided by Nato forces flying over the Black Sea. Moscow has accused the Royal Navy of taking part in the attack.

 

The attack will have achieved tactical surprise

The USV flotilla will have taken some ten hours to transit from Ochakiv to the sea west of Sevastopol. These vessels are small but have no apparent stealth capability, and will have been clearly visible on radar to any correctly operated emitter within their radar horizon. However, Crimea’s west coast is for the most part low-lying, and the USVs have a physical height of only half a metre. These circumstances combine to limit the likely radar horizon to a range of probably 20-30 km, and it is likely that they passed well outside that detection range until the final approach phase of the operation west of Sevastopol.

A Russian airborne radar would have been able to detect them on passage, but it is likely that none was on task (the perceived threat environment would have been “low”). Even if one was on task and did indeed detect the USV flotilla it would be unlikely to classify it correctly as an imminent threat, as it would have seen a flotilla of small boats heading south-west and south into the Black Sea.

In this attack it is probable that Sevastopol only became aware of the threat when the flotilla turned east to appear on radar at about 20 km from the entrance to Sevastopol’s roads.

At that range the flotilla would have had about 40 minutes to run to their targets inside the naval base (their speed would have increased as the drones found calmer water close inshore). The base is apparently protected by a floating boom, but a small light fast craft can jump a low boom, especially if it is propelled by a water-jet and not a propeller. Alternatively, one of the USVs could be detonated on the boom to open a path for the others.

 

Seven USVs reported in the attack, probably three penetrated Sevastopol

Some of the Ukrainian video is filmed in low daylight, and the rest in infra-red. This suggests that the attack took place towards the end of the day and continued into dusk. A short piece of video has emerged, shot from Crimea by what appear to be passing civilians, of a USV being attacked in daylight by a heavy helicopter outside the harbour, and appearing to explode. This may well be the same USV whose on-board camera footage records multiple attacks in daylight by a machine-gun, and whose footage cuts to footage from a quite different USV.

A helicopter is the ideal way to destroy a USV. Attacking from behind until course and speed are roughly matched gives the gunner a simple aiming solution and a low angular velocity (the rate of change of bearing and therefore aim). The USV camera footage shows the helicopter approaching from ahead (and missing), before passing astern. Shots land around the USV. A few seconds later the footage cuts to the view from another USV (suggesting that the first USV was hit and destroyed). Before the cut this USV’s footage clearly shows a Grigorovich-class frigate on its horizon, and also shows a single shell-splash, from a projectile which was probably fired by the frigate.

From the perspective of a frigate a USV attack presents something of a challenge. At range the flight time of a shell (10 seconds) makes accurate aim at a fast, jinking target impossible. A hit with a frigate’s 100mm main armament is therefore a matter of (improbable) luck. None of the frigate’s suite of missiles is optimised for, or even capable of, hitting a small, slow, cold surface target.

As the USV closes its target the range, and shell flight time, fall. However, a jinking target is too agile for a ship’s main gun to establish an accurate aim, so the kill must be left to smaller and quicker armaments. Russian warships are fitted with radar guided Gatling guns. The Grigorovich class has two, fitted aft.

These can open fire at 5 km, giving an eight-second flight time from gun to target, too far to get a reliable hit even when firing 50 rounds per second. As the USV closes flight time shrinks, along with angular velocity (the USV must, eventually, take an average course towards the ship), and the probability of a hit on the USV rises fast. As the USV closes its target and the probability of being hit rises it must increase speed both to reduce the time during which it is exposed to fire, and to counter the target’s own ability to turn away (warships sail faster than USVs in most conditions).

The critical time for both USV and target is the last 100 seconds of the attack (from about 1.5 km), when shell flight time for the defence falls to one second, and the USV must stop manoeuvring and go straight for its target to gain a hit. Once the range falls to a couple of hundred metres the defence’s guns will be unable to depress low enough to get a hit, and the USV’s chance of success is high.

 

Video footage is equivocal, but Moscow has admitted at least one hit

The Ukrainian footage shows a USV making its final approach to what clearly appears to be a Grigorovich frigate under way at some speed (we can see her wash clearly). The USV is also moving at speed and pitching violently while also jinking left and right as it approaches at around 40-50 km/h. However, in this critical minute we see no evidence of any defensive fire from the frigate, either from her starboard chain gun or from manned machine guns (muzzle flash would show up brightly in infra-red).

This may be for one of many reasons. One possibility is that Grigorovich’s on-watch crew failed to see the USV at all, due to inattention, surface clutter on the frigate’s radar (clutter increases as distance from the transmitter falls) or equipment failure. Another is that the USV was seen but the correct tactical reaction was obstructed by panic, or by distraction from another threat (airborne UAVs were part of the strike). A third might be equipment failure at the critical moment. A fourth may be that the USV’s track was chosen to bring it into the CIWS’s dead ground, low and ahead. The frigate’s calm measured pace suggests that she was not even aware that the USV was in its final attack phase.

The USV footage is continuous up to the last few metres of its approach. Frame-by-frame examination of the attack shows it veering wildly in track (perhaps to throw off counterfire).  However, there is no clear video record of impact and detonation (a sharp termination in the footage, possibly with added flash) what is actually shown is the USV cresting a steep wave (the camera aims upwards), followed by a plunge, and a featureless grey image for several seconds, which then cuts to footage from a different USV.

Critically, there is no visual evidence of either contact with the target or a detonation. This does not necessarily mean that the USV failed to detonate, and we will in due course see evidence of whether it did or not. If the USV failed to hit, or hit but failed to detonate, we would expect to have seen, by now, footage of the three Grigorovich frigates unharmed. The absence of that footage does provide some corroboration that the attack gained a hit.

 

USVs are too small to guarantee a kill

A USV hit on a 4,500-tonne frigate is unlikely to inflict fatal damage. The warhead is small, and because it detonates on the surface and outside the target’s hull less than 50% of its kinetic energy will reach the target (most of the blast will head away from point of impact into empty space and sea). Unlike a missile, whose fuse is designed to detonate inside the target, a USV warhead detonates a metre or two outside the hull, and will just punch a hole in the ship’s side slightly above the waterline. It is unlikely to start a large fire, it will injure and kill few men, and will inflict only slight shock damage on the target’s equipment and people.

However, a USV detonation does hole the target next to her waterline, so can sink a frigate if her damage control teams are not well-trained. That would not be a normal result, but the sinking of Moskva (corroborated by other non-public sources) suggests that damage control is not a strong suit in the Russian navy. For comparison we can look at the damage inflicted on USS Cole, a destroyer hit in 2000 by a 300 kg charge while at anchor in Aden. Cole, twice the size of a Grigorovich, neither burned nor sank, but was out of service for a year and lost 17 men to the blast.

The Ukrainian video shows three other sets of footage from USVs which seem to have penetrated Sevastopol harbour. One shows herself manoeuvring violently inside the harbour near to a large (but unidentifiable) target. Another shows a USV circling among ships tied up alongside apparently selecting a target. There is no other impact footage, but a shore-based camera appears to have caught a large detonation (whose time and location are not verified). The detonation looks considerably larger than one from a 150 kg warhead, and is definitely suspect.

Russia has admitted one hit, but there may well have been two, or even three. It looks very much as if the first hit may have been on the Grigorovich frigate outside Sevastopol.

 

Whatever the tactical success, the attack is a major strategic win for Kyiv

The attack may or may not be a tactical success (we have yet to see validated evidence of its results) but it is unarguably a considerable strategic success.

Carried out at trivial financial cost – a USV will come in at around $100,000 – and with no loss of Ukrainian lives, the USV threat will now place considerable sustained strain on the Russian fleet in and near its main Black Sea base. Ships alongside will have to maintain a much higher state of alert, ready to slip and sail at fifteen minutes’ notice, meaning at least one third of crews on duty at their working stations 24 hours a day. This will mean no leave, less sleep, less deep maintenance and therefore less effective and reliable warships.

The first line of defence must be in the air (a guardship placed up-threat simply becomes an easy target), which will mean allocation of scarce helicopter and fixed-wing resources to a 24/7 guard task outside Sevastopol. The ratio of aircraft on-task to total aircraft in a task group is at least 4:1 and more likely 5:1, so just the threat of repeated USV attacks will absorb a dozen aircraft to maintain two on station at all times.

A flotilla of small agile fast attack craft would probably have almost as much success in creating an anti-USV screen – the 140-tonne Ropuchas and 40-tonne Raptors would serve – but the task would place them in the target zone for Ukrainian drone strikes. Moving at one quarter of the speed of a helicopter, the screen would require four times as many ships as helicopters, and the same 4:1 ratio would overwhelm Russia’s Black Sea fleet fast attack craft numbers.

Another remedy will be to upgrade the floating boom to raise its height, but it is always going to be vulnerable to a hit from the lead USV, and cannot protect shipping transiting into and out of Sevastopol.

Thus with a threat costing a few million dollars, Ukraine will now tie down hundreds of millions of dollars of equipment and hundreds of men to stand permanent guard.

 

Repeat attacks are hard to prevent, and expensive to counter

There is no real prospect of pre-emptive prevention. A USV can be built in a shed, transported in a small commercial vehicle and launched across a beach or from a merchant ship at sea. As there is no need for a USV base there is nothing that Russia can do to prevent repeated attacks by striking their source – it must just wait for them and hope to react fast enough to counter them.

Best of all for Ukraine, the knowledge gained in each attack is preserved for the next one, since the operators are completely safe and have a full video record of their attacks. We should expect to see more, and more successful, USV attacks in the near future. Ultimately Russia may even be forced to abandon Sevastopol as an operating base altogether.