I was watching a video about "The Big Stick" aka Project Pluto, a nuclear powered SLAM. In the video (I couldn't find the exact video) it said that after deploying its weapon payload a SLAM could fly over its target country as a "vengeance weapon", causing destruction with it's sonic boom for weeks and an altitude of just a few hundred feet.

What kind of damage would a mach 3 flyby at 200 feet cause?

I'm interested in both building damage and possible human injury.

  • $\begingroup$ Flying supersonic "for weeks" is far, far off into the realm of science fantasy. The linked article states that in testing the prototype engine never ran for more than 5 minutes before the project was cancelled, and nuclear powered aircraft have never proven to be viable. Leaving aside all of that, asking about low altitude sonic booms & the damage they cause is a perfectly fair question, so the upvote is based on that. But please let's set aside visions of "nuclear-powered aircraft flying for weeks..." $\endgroup$
    – Ralph J
    Oct 24 '18 at 5:38
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    $\begingroup$ @RalphJ the main reason for the short test duration was the inability to provide the Mach 3 airflow through the reactor for longer than that. $\endgroup$
    – jwenting
    Oct 24 '18 at 5:54
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    $\begingroup$ The Mythbusters proved that the sonic boom itself would do essentially no damage to glass. To be fair, though, the boom would become a psychological weapon against people after a while. Also, they were testing at Mach 1.0 give or take a bit, not Mach 3. $\endgroup$
    – FreeMan
    Oct 24 '18 at 16:43
  • $\begingroup$ "Causing destruction with its sonic boom for weeks" is what originally limited the Concorde to a few overseas routes. These issues will hopefully be resolved with more modern designs and materials. $\endgroup$ Oct 25 '18 at 11:39

The video is highly inaccurate in that area.

The actual damage the missile would have caused would have been largely radiological, from ejected radioactive material in its exhaust plume.

While windows and weak structures may receive damage from extremely low flying Mach 3 bodies, this would probably be limited to say some broken glass.

The plan was also never really considered. What was considered is having the weapon, now without its warheads, fly to a final target area and do a high speed dive into the ground, the kinetic energy alone combined with the spread of radioactive material from the rupturing nuclear reactor causing the damage there.

Mind that the damage (both physical and radiological) of the weapon flying overhead isn't clear. Different sources list different predictions. Some way the damage would be minor, others that it would be severe. But rest assured, the noise would shatter eardrums, the supersonic shockwave flatten crops and lightly constructed buildings, and the radioactive exhaust plume wouldn't have been healthy either.

I've always found this to be an excellent article on the project and the people surrounding it.

Edit: a very interesting article by the designer of the reactor about the problems inherent in the reactor design, and how he thought about the radiation hazard for people on the ground working with the missile: Merkle public paper

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    $\begingroup$ While the extra info is interesting, It would be good to focus more on the title question and the question in bold in the post. The stuff about radiation doesn't answer this and wasn't asked about. $\endgroup$ Oct 24 '18 at 10:21

Unrelated to the nuclear ramjet aspect, a close high-Mach flyby has many of the same effects as an explosion. The shock wave has a pressure profile very similar to the "N wave" of an explosion -- positive overpressure followed immediately by a vacuum underpressure -- and depending on the shape of the supersonic object, there may be multiple overpressure waves.

Historically, sonic booms were fairly common when I was a child, if you lived close to an Air Force Base; I grew up about 150 km from Fairchild AFB (Spokane, Washington) and used to hear the booms once or twice a week. This was in the 1960s, before the ban on non-emergency supersonic flight over populated areas.

Even with an aircraft flying at high altitude (FL200, at a guess), these booms could and would rattle windows; they were loud enough to startle people, sometimes leading to kitchen injuries (dropping hot items, for instance) or falls. Given that the intensity of the pressure wave from a linear source like this obeys an inverse law, such a boom (from aircraft that were only low supersonic, probably Mach 1.1 to 1.2) from a hundred times closer would be a hundred times stronger -- and go from rattling windows to breaking them, flying glass injuries, many falls (people knocked off their feet by the pressure, never mind off ladders), and so on.

Look at the damage at Chelyabinsk from their bolide explosion a few years ago -- while that object was, well, rock-shaped, at high altitude and much higher speed, that's the kind of damage you might expect with a fairly close supersonic flyby of a streamlined aircraft or missile.

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    $\begingroup$ I don't think a shockwave from supersonic object follows inverse square law, just inverse law. The shockwave is a cone, but it is also constantly fed energy, so energy from any point on the flightpath spreads as a ring on the cone and that only grows linearly with radius, not quadraticly, so the power density only drops with distance, not square of distance. $\endgroup$
    – Jan Hudec
    Jun 29 '21 at 15:07
  • $\begingroup$ @JanHudec You're probably correct -- like the light spread from a fluorescent tool of "infinite" length rather than one that looks like a point from a couple km away. $\endgroup$
    – Zeiss Ikon
    Jun 29 '21 at 16:16

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