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The BBC News article The Rocket Scientists Mixing Up a Giant Firework begins with:

In a remote corner of tropical South American jungle, French scientists are mixing a ‘cake’ with a difference – a highly explosive rocket fuel for a new breed of space rockets.

and goes on to mostly talk about how to mix the ingredients and then bake them "like a cake" into a form ready to become an SRB.

Answers to If not constrained by underpasses, etc., would Falcon 9 have been less of a flying noodle? suggest that Falcon 9 rockets are driven down the road (on a truck of course) and this is confirmed by this answer, and photos of that happening are not uncommon:

Question: But what about SRBs and solid rocket motors? Are they ever transported on public highways? Or are they considered too dangerous for that? If not, does it mean these "cakes" are always "baked" in close proximity to their intended launch sites?

note: While even the segments of the largest SRBs are extremely heavy and that might be a practical limitation to highway transport, there are smaller ones, and solid rocket motors are even used as parts of payload spacecraft as well (example below). So let's not limit answers to only those that are too heavy for public highways, and instead keep the focus on the safety of transporting solid rockets on public highways.

below: Image of Orion 38 motor from Orbital ATK's Propulsion Products Catalog, borrowed from What inclination change could the 5th stage Orion 38 motor provide ORS-5 (SensorSat)?

Orion 38 motor from Orbital ATK's Propulsion Products Catalog

Above could probably fit in a 1-ton pickup truck, but below is probably strictly off-road.

below: "Transporting the rocket boosters is a slow and steady process" (Credit: Esa) Source

enter image description here

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    $\begingroup$ I wouldn't call the booster in the lower image "strictly off-road" - that transporter is pretty clearly on a road. $\endgroup$
    – Vikki
    Jun 20, 2019 at 0:28
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    $\begingroup$ If you're not going to restrict the size--hobby solid rocket boosters are shipped as ordinary hazmat. $\endgroup$
    – Loren Pechtel
    Jul 3, 2021 at 21:21
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    $\begingroup$ Most of military missiles are transported through public infrastructure - railways, roads. Russians have mobile ICBM launchers that travel normal roads and off-road with nuclear warheads on them (they are liquid fuel though). Plenty of SAM rocket launchers are road vehicles and SAM are often solid fuel powered. $\endgroup$
    – SF.
    Oct 24 at 21:37

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A few data points:

  • Shuttle SRB segments were transported by rail, from Utah to Cape Canaveral.
  • Ariane 5 SRB: the top segment is filled in Italy at the Avio factory, then transported by ship to Kourou. Transport from the factory to the ship is by truck over public roads, unless the factory is right next to the dock, which I doubt. The 2 large segments are filled on-site in Kourou.
  • Vega: empty stages are produced in Italy, and filled on-site in Kourou.
  • Atlas V SRB were transported by road.

I haven't found any documents on SRB transport safety.

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Not exactly public roads, but shuttle SRB segments were transported on roads inside of Kennedy Space Center (KSC).

New, loaded segments were delivered from Utah on trains directly to the Rotation, Processing, and Surge Facility (RPSF). After being removed from the railcars, they were processed and subsequently delivered to the Vertical Assembly Building (VAB) for stacking on a large wheeled vehicle called the Solid Rocket Motor (SRM) Transporter.

enter image description here

Spent SRB segments from the Shuttle program, after cleaning, were transported on trucks from the cleaning facility to the NASA KSC rail yard to be loaded onto railcars for the trip back to Utah.

enter image description here

All photos: NASA

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The British can be an inventive lot, the Skylark sounding rocket

components and payloads are made in the UK and then flown to Australia by transport planes. These include a special dedicated “explosives” transport plane that carries the rocket engines to Australia fully-loaded with their solid propellant.

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Decades ago I learned two interesting things from a friend involved with the US Navy Trident missile program, one relating to transport. The weapons used multi-stage, solid fueled rockets. The motors were cast in carbon-fiber cases and shipped to the prime contractor, in special containers, on railroad flat cars, across the Western United States. All rail freight is subject to occasional gunfire and at least once a bullet pierced the case of a motor.

When nobody wanted the damaged motor, anymore, the top of the case was cut open, the motor attached to a test stand, in a suitable location and the grain ignited. The propellent was flammable at STP but produced far less thrust, without the high temperature / high pressure environment inside a sealed case. Maybe it burned slower.

In a working weapon, the guidance system would reduce thrust by pressure release (blowing open vents in the case) when or perhaps before the missile reached the path for the first reentry vehicle. The RV would then be released on its selected path. Additional acceleration would change the flight path (direction and speed) of the 'bus' before the next release.

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  • $\begingroup$ While the rocket with a hole in it story probably cannot be sourced, it would be better if you could at least link confirmation of the rail transit (photo, news article, press release) and consider what the last paragraph is doing here in context of answering the question. $\endgroup$
    – GremlinWranger
    Oct 24 at 11:06
  • $\begingroup$ My father claimed to have thrown rocks at the 200 inch cast mirror blank destined for the Hale telescope on Mt. Palomar being shipped by train from Corning, NY. i.stack.imgur.com/pzkkb.jpg (from here) I guess times have changed :-) $\endgroup$
    – uhoh
    Oct 24 at 12:22
  • $\begingroup$ This answer is about weapons, not space exploration. $\endgroup$
    – Organic Marble
    Oct 25 at 3:24
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    $\begingroup$ With respect its about solid fuel rockets. The specific instance is a weapon but still, at its core, about solid fuel rockets. The fact that venting is used to modulate thrust is interesting, generally. SRBs for the STS were similarly pressure sensitive, I believe, and thus vastly safer to transport unstacked. $\endgroup$
    – Bill IV
    Oct 26 at 16:15

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