Fortification Friday: Star Forts and not-so Star Forts

Our discussion of Mahan’s simple “intrenchments” brings us to the star fort today.  And I feel the star fort is the most troublesome form to discuss in public.  Not that folks get upset about the “star.”  Rather that folks have some preconceptions as to what it should look like.  If one is not particular about the term, the definition might be widely applied.  But when one is particular about the term, a lot of works that we often refer to as “star forts” are not actually star forts!  With that, allow me to get particular about this particular form of entrenchment for a bit.

First, what did Mahan say about the Star Fort, and why we should be so particular:

The star fort takes its name from the form of the polygonal figure of the plan.  [Figures below]  It is an enclosed work, with salient and reentering angles; the object of this arrangement being to remedy the defects observed in redoubts.

Note, he offered no requirement for the number of points on a star, or even that the trace somehow match up to our conceptual notion of a star.  So we have four different basic plans offered, starting with a four point star:

Plate1Figure8

Then a five point:

Plate1Figure9

And a six point:

Plate1Figure10

Oh… and the favorite of those doing the digging… the eight point:

Plate1Figure11

This latter diagram offers a good point to discuss the evolution of the star fort. The implication is that an eight point might derive from a redoubt which has salients added to the sides.  (Recall the discussion at the end of last week’s post about “remedies” for the redoubt.)  All of this sort of stretches our notion of the shape of a star when categorizing what could be called a star fort.  Again, going back to Mahan’s definition, it is the presence of reentering angles which defined the star fort, not the shape that is the result.

But there is more to this evolution thread than simply tacking salients upon a redoubt.    The form actually derived as a modification of the standard bastioned fort.  Recall during the discussion of re-entering angles, curtains, and such we highlighted the “dead space” that existed in front of a curtain wall.  One remedy applied to that was a work known as a tenaille (referring to the French word for tongs, as these works were to look like the jaws of a set of pincers).   Consider a drawing of a basic tenaille:

tenaille1

This is from a larger set, on Wikipedia, which demonstrates more evolved tenailles (and demonstrating a truism about fortifications – the longer you have the more elaborate they become).  For permanent fortifications, a tenaille was a work placed in front of the curtain which allowed the defenders a secure area from which to defend the ditch.  And you can see how from the viewpoint of the attacker, it smoothed out the face of the defense into a simple reentering angle between two salients.

The usefulness of these tenailles lead to a broader application as a stand-alone enclosed work.  These featured alternating salients and reentering angles, none less than 60º (note the eight pointed above with 90º angles complementing).  Faces were usually between 30 and 60 yards. In order for all of that to work with some balance between the salients, the resultant polygon had a certain shape… and that’s how the star was born!

But you see what I just walked you through here – the star fort’s name was derived from the resultant shape, which was the result of a particular arrangement of the angles.  We might just as properly call these tenaille fort.  But in that case we’d have a long discussion of how that polygon gave us the jaws of a pincer.  If you ask me, “star” is the easier term to use here.

But there were down sides (as always) to this arrangement. Referring to the ability of the star fort to remedy the defects of the redoubt, Mahan wrote:

This, however, is only partially effected in the star fort; for, if the polygon is a regular figure, it will be found, that, except in the case of a fort with eight salients, the fire of the faces does not protect the salients; and that in all cases there are dead angles at all the re-enterings.

And there was more….

The star fort has, moreover, the essential defect, that occupying the same space as the redoubt, its interior capacity will be much less, and the length of its interior crest much greater, than in the redoubt;  it will, therefore, require more men than the redoubt for its defense, whilst the interior space required for their accommodation is diminished.

For those reasons, by the time of the Civil War the star fort had fallen into disfavor… or as Mahan put it, “led engineers to proscribe it.”  Yet, we know from so many Civil War maps and accounts that star forts were used.  Simply put, there were places where a star fort worked for the particular defensive arrangement desired.  Places where engineers might prescribe, instead of proscribe, a star fort.

On the other hand, there are a lot of places which are described as star forts which do not meet the definition… for those of us being particular… of a star fort.  An example is Fort McHenry:

FortMcHenry

Fort McHenry was not an “according to definition” star fort.  Rather it was a pentagon with five bastions… or in other words, a bastioned fort.  Mention of such brings us to the next form mentioned by Mahan, as we look towards next week.

But to summarize the discussion of star forts – these came in four, five, six, eight- pointed varieties (and where practicality was set aside, some with even more points).  It was the integration of salients and reentering angles which defined the star fort, not the overall shape itself.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 13.)

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