This page discusses how separation events occur. This is crucial to most missions for recovery or payload reasons.

Body Section Retention

Prior to a separation event, the sections that will separate must be held together. Most of the sections below relate to tubes separating axially with respect to each other, but are not limited to this geometry.

Friction Fit

For smaller rockets, friction fit can be used. This essentially means that the sections slide together with enough friction to keep them together until separation. This is common for rockets through Level 1. The danger of this comes after motor burnout, when there is a large drag difference between the motor section and the nose cone of the rocket. If the rocket is going fast enough, this drag difference can be enough to cause premature separation.

Shear Pins

Shear pins are small nylon screws or inserts that go between two sections as follows. At separation, the sections move away from each other, shearing the nylon screw. It is important to have a relatively tight tolerance on the holes for the shear pins.

Usually, #2 screws will be used for rockets flying on H, I or J motors. Above that, #4's can be used. Usually, larger shear pins are not used because of the force required to shear them at separation. Ground testing is the only way to ensure you have the correct number and size of shear pins!

Initiation Methods

Separation events are typically released by pyrotechnic charges up to altitudes of about 20,000 feet. Above this altitude, BP combustion is less reliable. However, for relatively low altitudes, pyrotechnics are far and away the most widely used separation mechanism.

Pyrotechnic

See "Ejection Charge" for more details. Pyrotechnics are usually used for separation because of their reliability.

Pros:

• Reliable
• Simple
• Relatively easy to integrate
• Flight proven

Cons:

• Messy
  • Leaves thick, dusty, smelly residue inside the rocket
• Requires thermal protection for parachutes, webbing, payloads, avionics, and other internal components
  • adds weight and reduces available volume for internal components

Ejection Charge (electronically initiated)

Ejection charges can also be initiated by electronics. Typically a charge is set up as follows:

It is important to consider the safety of the altimeters when using this method. Ejection gases can damage altimeters from the heat, and the sudden pressure difference can damage the barometer. This is also a highly reliable method, but it is more complex because the altimeter, its connection to the charge, and the integration of the system must be checked meticulously prior to launching the rocket. This method is almost always used for dual deployment scenarios.

Motor Ejection

Most COTS solid rocket motors have an ejection charge in the top of the motor casing. It is ignited after a delay grain between the rocket motor and the ejection charge is burned through. See below:

This is the most reliable method of pyrotechnic separation, and is common for all rockets from 1/4A to L motors.

CO2 Ejection

Small, pressurized canisters of CO2 can be used to pressurize a tube section in the place of pyrotechnics. A COTS example is the Perigrine. These use a small, well-packed pyrogen to release compressed CO2, which in turn pressurizes the section to be separated. This system has the benefit of leaving less residue than a pyrotechnic-only system while still having the same initiation method already used for ejection charges. This is typically used for high-altitude flights (above 20k feet).

Non-pyrotechnic

Pyrotechnic methods of separation are highly reliable, but are more difficult to test, and are inherently variable unless the packing of the pyrotechnics is well regulated. All of them typically use electronic initiation.

Nichrome Wire - Shear Pin

Pin Release