The rocket assist for an artillery round is much larger than any tracer by an order of magnitude or more and yes has a proper venturi design. I’ll try to find a good picture if someone else doesn’t put one up.
Here is some US RAP info.
The new EXCALIBER projectiles are another thing.
Rocket-Assisted Projectiles (RAP)
Within the field of artillery techniques there has been a continual striving to increase the range and precision of field guns. Increased range is achieved either by gun improvements, which even include such modifications to propellant charges that a redesign of gun parts is required due to for example increased gas pressure in the barrel, or by improvements in the projectile performance. The turnover time for gun parts in the meantime is long, and therefore it is more attractive to attempt to improve the performance of the projectile itself without altering the gun as the ammunition has a continual turnover time of a totally different character to that of the gun.
Improved projectile performance can be achieved in several different ways which to a certain extent can be combined in one and the same projectile. At present work is proceeding along three different lines, of which the first involves attempting to produce a low-resistance projectile where the air resistance is reduced to a minimum. This work has resulted in longer and slimmer projectiles. The second line involves work that has been concentrated around reducing the base resistance of the projectile, caused by the stream of air round the projectile generating a lower pressure immediately behind the projectile base than in the surrounding air. The third line consists of equipping special projectiles with their own source of power in the form of a built-in rocket motor, so-called “reatiles” or Rocket Assisted Projectile (RAP).
Such a rocket motor is usually rigidly affixed to the projectile and gas evolved from an ignited propellant provide augmenting propulsion efficiency. Ignition of the rocket motor can be accomplished after leaving the gun barrel with a suitably designed ignition system contained within the casing, or ignition can be accomplished by means of the high temperature gases from the launching charge in the gun barrel.
Artisans in the field of rocket propelled missiles had for many years sought to combine the capability of the ordinary cannon, or artillery piece operating by means of an explosive charge for launching projectiles and the like, with the steady state propulsion efficiency derived from the burning of a solid propellant in a rocket motor. A primary concern is the very high acceleration forces experienced by such projectiles when launched from a gun since unwanted and difficult problems arise therefrom. With respect to the projectile and its warhead pay load, these problems are of relatively little concern since adequate solutions have long been available. However, with respect to the rocket motor, a different situation obtains. Motor casings of ample strength have long been available; however, many state-of-the-art propellants are not so durable, hence in providing a rocket motor for augmenting the propulsion of the projectile after launching by the gun, the artisan must insure that a propellant grain is included which is capable of withstanding the launching forces, so that upon ignition, smooth combustion, and therefore propulsion is obtained.
The high acceleration forces experienced by the projectile when it is launched often causes rupture in different components. This specially holds for the solid rocket propellant grain, due to the fact that every known type of solid rocket propellant has rather poor mechanical properties. A crack in the solid propellant can cause misfunction of the motor and also cause explosion of the whole projectile.
In spin-stabilized vehicles the solid rocket propellant grain will experience a high load due to the centrifugal forces. For a cylindrical grain configuration this means high tensile stresses, especially in the center of the grain. This makes it necessary to use hard types of solid rocket propellants with a high tensile strength. Such types of solid rocket propellants will necessarily become brittle. Consequently the risk for cracks during the launch will increase. Besides, these types of solid rocket propellant generally have a low specific impulse.
These rocket assisted projectiles, or gun boosted rockets, had not, prior to the 1980s, proved entirely satisfactory nor performed in accordance with expectations. Many reasons had been advanced for this, not the least of which is the failure to develop a propellant grain which can withstand the accelerations experienced during the difficult launching, or boost phase of the projectile flight, or to develop means to protect the grain. Success had been elusive most often in the past because of propellant grain cracking, or other, similar deleterious effects induced by the high launching acceleration forces to which the propellant is subjected. It is of course quite obvious that the rocket motor propellant would be subjected to very high linear, tangential and radial acceleration forces during the launch of the missile and due to the spinning thereof imparted by the lands or rifling grooves on the inside surfaces of the barrel of the gun which fires the projectile.
However, the RAP system has a number of known disadvantages, since even minor changes in ignition time and rocket power cause considerable changes in range. It is difficult to control the exact point of ignition of the rocket charge which concomitantly causes difficulty in controlling the trajectory of the projectile. RAP-projectiles, even when seemingly otherwise similar and fired from the same point, land in a relatively widely dispersed area.
Navy warships using guns to a maximum range of 15 miles have traditionally provided naval fire support for the Marine Corps and Army. The introduction of Tomahawk missiles increased the range of the fire support, but the size and expense of the missiles precludes meeting the volume of fire needed by the Marine Corps and Army. Navy warships are built to meet multipurpose warfighting requirements and therefore cannot carry sufficient gun ammunition or Tomahawk missiles to support a 30-day naval fire support mission for the Marine Corps or Army.
Rocket-assisted projectiles are available for the 105-mm and 155-mm howitzers. They are designed to extend the range of the howitzers. The basic rocket-assisted projectiles are filled with HE material. They produce blast and fragmentation in the target area. Computation procedures for the two basic HE RAPs are identical.
The 105-mm RAPs are the M548 and M913. The 155-mm projectiles are the M549 and M549A1. For the M109A2/A3 weapons, these projectiles are fired with charges 7 (M4A2), 8 (M119A1), and 7 (M119A2). The M198 howitzers may use charges 7 (M4A2), 8 (M119A1), 7 (M119A2), and 8S (M203 only for the M549A1 projectile).