Why putting missiles on roofs in London isn’t as dumb as it sounds.

The UK government has recently announced that it will be placing Starstreak HVM surface-to-air missiles on some roofs in London as a security measure during the 2012 Olympics. This has caused a bit of a kerfuffle.

Below are some thoughts on the issue from a physicist’s point of view.

What are Starstreak missiles and how do they work?

The Starstreak is a short-range laser-guided surface-to-air missile. When launched it very quickly accelerates to Mach 3.5 (1200 metres per second) and is then guided onto its target by a pair of laser beams projected from its ground-based aiming unit. Being laser-guided means that unlike heat-seaking or radar-seeking missiles the Starstreak cannot be avoided through the use of chaff or flares; however unlike those missiles it does not have fire-and-forget capabilities.

The Starstreak Light Multiple Launcher showing three Starstreak missiles and the guidance unit.

Once the Starstreak approaches its target it releases three 900 gram tungsten-coated beam-riding submunitions. Once one of the submunitions (or all three) impacts the target a short delay fuse is activated and the 450g of explosive inside the submunition explodes inside the target, throwing out tungsten alloy shrapnel and tearing it to pieces.

What scenario is the deployment of Starstreak missiles designed to prevent?

My guess is that the government is trying to defend against suicide bombers using aircraft as weapons. A heavy aircraft moving at high speed has a large amount of kinetic energy and this, coupled with the chemical potential energy in the fuel, makes it a formidable weapon.* The Olympics will concentrate a large number of people in a small space which makes the Olympic sites attractive targets.

If a plane is shot down, won’t it kill people when the wreckage lands?

It depends on the size of the aircraft involved. A light aircraft at high altitude wouldn’t produce much dangerous wreckage, a low-flying jumbo jet would. But falling wreckage will kill far fewer people than an aeroplane striking one of the Olympic sites would.

The force of the missile’s explosion will tear any aircraft into pieces, and once the structural integrity of the aircraft is ruined the force of the wind will tear it into further smaller pieces. Each of those falling pieces will reach terminal velocity relatively quickly and will therefore strike the ground at a lower speed than if it were flown into the ground under power. The video that has been going around showing a helicopter shot down by a Starstreak missile crash into the ground in a fireball is of a guidance test – the missile in the video was not carrying an explosive payload.

What about burning jet fuel hitting the ground?

This is much less of a problem. An explosion inside an aircraft, combined with the high-speeds involved would aerosolise the fuel, causing it to burn up very quickly in mid air. Again, this is a much lower risk than if a plane full of jet fuel were to crash into one of the Olympic stadia.

Won’t the missile launches damage the buildings they’re launched from?

No. The Starstreak missile is ejected from its launch tube by a low power first stage rocket motor that is extinguished before the missile leaves the tube. The powerful second stage motor doesn’t kick in until the missile is safely away from the launcher, meaning that there is almost no recoil at all. The launch of a Starstreak missile produces no significant overpressure so there is no danger to windows or walls. The missiles have to be launched from roofs or open spaces because the rocket requires a certain amount of space to accelerate to attack velocity.

* It was the chemical energy in the tens of thousands of litres of fuel that were responsible for the collapse of the Twin Towers in the 9/11 attacks. Had the planes had no fuel aboard the Towers would have survived.

12 thoughts on “Why putting missiles on roofs in London isn’t as dumb as it sounds.

  1. I should point out that I don’t think that putting missiles on people’s roofs without their permission is a good idea. London has plenty of open spaces that could just as easily be used.

    P.S. I would love to have missiles on my roof.

  2. It’s interesting to see how hi-tech weaponry is these days, I bet the control systems are fascinating.
    I would also love to have missiles on my roof.

  3. Not having fire and forget is definitely a good thing as a human stays in the loop. ‘Fire & regret’ is the nickname of fully autonomous missiles!!

  4. As an absolute non-expert, I was thinking about Lockerbie. I can’t compare the explosive capabilities of the bomb on that plane against those of the proposed missiles, but certainly the wreckage fell from around 31,000ft and sizable pieces made it to earth, not having been torn into smaller parts. You also mention one or maybe all sub armaments arriving on target. If it is just the one – what happens to the other two? Can they be remotely destroyed?

  5. Unfortunately, I think you’re overstating the likely effect of a missile hit on an airliner. Small MANPADS like Starstreak are designed to mission-kill (ie render militarily inoperable) smallish military aircraft which are densely packed with vulnerable systems and aerodynamically very sensitive.

    Despite this, many aircraft have successfully returned to base following a hit by the small SAM systems (see, for example, past Israeli air force experience). A recent example of a MANPADS hit on a large civil aircraft was the 2003 attack on a DHL Airbus in Baghdad[1]. It landed messily but successfully. Note that the system used in this case had a warhead weighing over twice as much as that in a Starstreak dart. Clearly, there are no guarantees of breaking the aircraft up in mid air, much less burning off or vapourising the fuel. However, the experience would be offputting for the pilots, to say the least, and an airliner crashing on more or less anything other than a full stadium is likely to be a net gain in terms of casualties.

    [1] http://en.wikipedia.org/wiki/2003_Baghdad_DHL_attempted_shootdown_incident

  6. Incorrect. You simply aren’t comparing anything remotely like-for-like.

    The missile used in the Baghdad incident was a Strela-3 missile which entered service in 1974. It is more than 20 years older than the Starstreak missile and therefore does not have the benefit of the up-to-date technology used in the Starstreak missile (e.g. a Strela-3 missile only travels at only one-third of the speed of the Starstreak and therefore only has one-ninth of the kinetic energy). The Strela-3 is a heat-seeking missile with a proximity fuse, whereas the Starstreak is laser-guided and uses a contact fuse. This means that the Strela will explode if it gets close to its target, whereas the Starstreak will explode only if it actually hits its target. When a missile like the Strela-3 explodes in mid air, rather than inside an aircraft, the vast majority of the explosive force is directed outwards into the air, rather than towards the target. Older systems like Strela are forced to rely on proximity fuses because they are older and cannot guarantee a hit in the same way that the Starstreak can. Also, because the Strela-3 is heat-seeking it aims for the hot engines and is more likely to cause damage only to the periphery of the aircraft (you can see the damage to the Airbus A300 in your example is mild and centred around the wing); as opposed to the Starstreak which is aimed directly at the aircraft’s centre-of-mass.

  7. You seem to have read the Starstreak publicity material, but not not with a very critical eye. Every new missile system is advertised as a revolutionary one-shot-kill system, but in general each generation of MANPADS simply has a slightly higher likelihood of making a hit from a wider range of aspects: overall they still miss a lot.

    Starstreak can’t “guarantee” a hit any more than any previous system can, though it will have a higher percentage of hits (we think – it’s never been used in action as far as I know, and we got all the same “guaranteed one-shot-hit” publicity with Blowpipe and Javelin). Less still can it guarantee a hit with all three darts. My understanding is that the lack of proximity fusing is much more driven by space restrictions within the very small darts* – it would clearly be preferable for darts that didn’t make a direct hit to explode nearby. Also bear in mind that centre mass of an airliner as seen from beneath is the luggage and landing gear, and that critical systems are nothing like as densely packed, nor is the airframe as stressed.

    Given civil airliners’ repeatedly demonstrated ability to remain flyable when subject to severe damage, I remain to be convinced that Starstreak is capable of bringing about the best-case “break up and burn at altitude” scenario you describe. What it can do, and why I absolutely agree that its being deployed is far from as daft as some are making out, is prevent the aircraft being flown into a specific target.

    *which have to be very long, narrow and dense to maintain a ballistic coefficient high enough to allow them to coast to the target, since they’re not self-powered. That doesn’t leave much space for an ECM-resistant proximity fusing arrangement, actuators for guidance, a sensor for the command guidance system and a warhead, and what space there is is arranged in a long column which minimises internal volume. So the proximity fuse, being the only inessential item, is omitted. Engineering is all tradeoffs, as ever.

  8. The bomb that exploded inside Pan Am Flight 103 over Lockerbie was relatively small and placed in the hold, close to the skin of the aeroplane. The type of explosion created by the bomb was very different to that that could be expected from a Starstreak strike.

    Also, the explosive in the Pan Am bombing is estimated to have been between 350-450 grams of Semtex (probably Semtex-H). This is less powerful than the 1.35 kg of modern explosive (probably HMX) contained in the Starstreak missile, and this 1.35 kg of explosive would be distributed across three locations on the target, making break-up into smaller pieces far more likely. There is also the impact of the very dense (and very sharp) tungsten shrapnel created by the Starstreak’s submunitions to consider.

  9. Despite the Lockerbie bombing taking place at 31000ft and being somewhat fortunate* in causing control movements that helped aerodynamic forces to break up the airliner, the aircraft reached the ground in large pieces with most fuel remaining on board and burning on the ground. In military terms, a perfectly valid kill, but far from the ideal scenario for those on the ground. As I say, divert away from a target, absolutely. Neatly destroy in a fireball at altitude, no.

    *from the bomber’s point of view.

  10. What is the terminal velocity of the largest fragments of an air liner that would be left after a it has been reduced to fragments by a Starstreak HVM? (I tried to dig out my old OU Physics books but couldn’t find the one with the equation.)

  11. It’s impossible to say, really, without knowing the mass, size and shape and drag coefficient of the fragment. Aeroplane wings tend to be large and light (to generate the most lift and reduce the use of fuel), and this would lead to a low terminal velocity.

    A 1 metre squared piece of 3.5 mm aluminium, with a mass of 9.45 kg tumbling through the air with a drag coefficient of 1.0 would have a terminal velocity of 13 metres per second, or 28 mph.

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