Modsim Dev Diary #1 - Cannister Missile Launch

This is the first installment in a new series that unveils the intricacies of flight simulation development.

Ever since I was first introduced to the SA-12 (S-300V) surface-to-air missile, I have been captivated by vertical, cannister (tube)-launched missiles.

Missile launches come in two types: hot and cold. A hot launch occurs when the missile's rocket motor is ignited within the cannister. In contrast, a cold launch involves the missile being expelled from the cannister by gas before the rocket motor is ignited. The term "cold gas" is relative. In many missile launchers, this gas is actually generated by burning a propellant, which is why you might see flames emerging from the cannister during launch.

For more information on the physics of gas generators, refer to the paper "Research of the Burning Rate Coefficient on the Interior Ballistics of Gas Ejection Power System" by Renfeng Li, et al.

The subject of the next several diary entries will be the SA-12 missile. There is much to learn from it in terms of simulation, employing Dr. Carlo Kopp's extensive information available at Air Power Australia (https://ausairpower.net/APA-Giant-Gladiator.html). My ultimate objective is to develop a 6-degree-of-freedom (6DOF) fly-out model for this missile. Along the way, I will also create simulation components that will be useful for other missiles while using my entire Simulink flight simulation tool suite.

Since my current focus is on cannister ejection and I have not yet developed 6DOF quality models, I will use a 3DOF point mass simulation with Cartesian equations of motion. The key information required includes the missile's launch weight and the net force exerted on the missile by the ejection system. Based on the above reference, the pressure curve in the canister resembles that of a solid rocket motor. Therefore, I will utilize a solid rocket model block from my Propulsion Model Blockset to generate a "thrust" force on the missile. This thrust is calculated as the difference between the pressure force from the ejection system and the friction forces caused by any seals between the missile and the canister.

My best estimate is that the ejection lasts about one second. The main question is determining the maximum force. Fortunately, I can review some launch videos to estimate the timing and altitude of the booster rocket ignition after launch initiation.

However, there's a challenge: the SA-12 system includes two types of missiles—the smaller 3M83 (NATO: Gladiator) and the larger 3M82 (NATO: Giant). I'm particularly interested in the Giant, but video footage of this missile is less common than that of the Gladiator. Nonetheless, I can use both for my analysis because, while not positive, I believe they employ the same ejection system. It makes sense from a design perspective. Also, a drawing on Air Power Australia shows the ejection system's gas generator is nestled in the nozzle of the Giant's booster motor. It should also fit in a Gladiator's nozzle. If they do share this system, it is probably sized for the Giant, meaning the Gladiator ejection is going to be quite a bit more sporty.

The suspected Giant video (See on You Tube) appears to show the missile taking about four seconds from initiation to booster motor ignition. Altitude looks to be about 40-50 meters. The missile also appears to have a really low velocity at ignition.

The suspected Gladiator video (See on You Tube) appears to show a much shorter sequence, on the order of 1.5 seconds. The booster ignition appears to be 40-60 meters, but with a higher velocity this time. This backs up the same ejection system theory.

Moving back to the simulation, I adjusted the ejection system thrust in multiples of the launch weight for each missile, while ignoring aerodynamic drag, to recreate the observed behavior. The best answer was about 6x the weight of a Giant missile. The altitude and velocity profiles are provided below. Note, I'm allowing the missiles to coast to apogee before stopping the simulation. The key times to look at are 1.5 and 3.5 seconds.

Just a SWAG, but it appears the developers might have intended on the booster motor to ignite around 50 m for both missiles. Anyways, I now have a velocity and altitude estimate for the missile at booster ignition. I can choose to initialize later simulations at this flight condition and ignore the cannister or use the altitude to trigger booster motor ignition for a cannister launch. The great thing is I now have a decent surrogate model for the ejection system that can be moved to the 6DOF.

In the next installment I'm going to moving to a 3DOF pitch model to look how the Giant orients itself after ejection.