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Added failure modes and stability info
Positive = not falling out the bottom.
Positive = not falling out the bottom.
== Stability ==
== Static Stability ==
[[File:L1_Guide_Stability.png|thumb|200px|right|CG and CP for stable flight]]
[[File:L1_Guide_Stability.png|thumb|200px|right|CG and CP for stable flight]]
=== Brief History ===
In 1958, G. Harry Stine published a simplified discussion of rocket stabilization geared towards model rockets using fixed fins on the rear of a rocket. The fins, if properly designed, provide a means of inducing a return to the desired flight path when a disturbance acts to rotate the rocket around its center of gravity (CG or c.g.). The air forces acting on a rocket can be thought of as all acting at a center of pressure (CP). He suggested a method of approximating the CP by constructing a cardboard cutout of the model and balancing the plan-form cutout.
The next advance in design came in 1966, when James Barrowman, then of NASA’s Sounding
Rocket Division, presented a closed form algebraic solution to equations based on potential flow theory. The approximations used to achieve the closed form solution rely on the assumptions that the rocket (a) is traveling at a speed below that at which shock waves are formed (somewhat below the speed of sound), and (b) has a small angle between its flight path and the relative wind (i.e. a small angle of attack, or AOA). The Barrowman Equations continue to be widely used, both in graphical form and as the basis for hobby rocket design analysis software. From the "Launching Safely in the 21st Century: Final Report of the Special Committee on Range Operation and Procedure to the National Association of Rocketry", 2005.
=== Center of Gravity ===
=== Center of Gravity ===
As a rule of thumb, having your CP 1.5-2 calibers in front of your CG is considered good, while numbers outside of that range tend to be either under-or-over-stable.
As a rule of thumb, having your CP 1.5-2 calibers in front of your CG is considered good, while numbers outside of that range tend to be either under-or-over-stable.
== Failure ==
There is a fantastic NAR case study, "Launching Safely in the 21st Century", written by the Special Committee on Range Operation and Procedure which goes into rigorous statistical analysis of rocket failures. The following categories are common failure modes recorded from the fight log database of the Minnesota Amateur Spacemodeler Association (MASA, NAR 576):
'''Unstable.''' The rocket flies with at least part of the boost phase in a nose-down attitude. For
the purposes of this study, based on the goal to characterize unsafe events, comments such
as "kind of unstable" were not counted, nor were "horizontal", "cruise missile", or "coning"
flights (unless they resulted in a crash; see below).
'''Lawn dart.''' The rocket descends in ballistic flight with the nose cone still on. This category
includes "No ejection" and a few "power prangs". Some rockets are designed to do this and
these flights were not counted. Boosters on two stage rockets also did this.
'''Separation.''' The rocket descends in multiple parts with at least one part not slowed by a
recovery device. For the purposes of this study, the few flights with a comment of "stripped
chute" were included here. The unplanned ejection of motor casings should also have been
collected here, but MASA LCOs almost never recorded them as outcomes.
'''Motor CATO.''' The motor failures catastrophically at ignition or during boost. The nature of
the CATO (spit nozzle, forward closure failure, blow by, etc.) was sometimes recorded, but
not consistently.
'''Core sample.''' The rocket descends in ballistic flight, but with the nose cone off the rocket
and acting as a (not very effective) streamer. These events typically have lower impact
speeds, and higher surface areas at impact, than do lawn darts.
'''Motor unrestrained.''' The motor exits the rocket at ignition or during boost (thankfully, this
was rare).
'''Shred.''' The rocket comes apart during ascent, other than by design.
'''No chute.''' The rocket descends without a recovery device deployed, but is not ballistic. This
category does not include chutes that were described as merely tangled, although it is likely
that some LCOs write "no chute" in those circumstances. The rationale here is that if the
LCO described the result as "No chute", it was potentially unsafe.
These would be called "recordable incidents" in the jargon of safety professionals.
When more than one failure occurred during a flight (e.g., unstable flight leading to lawn dart), the
most severe event was recorded.
== Motor Specs ==
== Motor Specs ==
One other program of note is called FinSim, which can model possible vibrations within fins. If unchecked, these vibrations can grow and shear the fins off, likely dooming the rocket. The program is required for transonic and supersonic flights, and can be found here[http://www.aerorocket.com/finsim.html]
One other program of note is called FinSim, which can model possible vibrations within fins. If unchecked, these vibrations can grow and shear the fins off, likely dooming the rocket. The program is required for transonic and supersonic flights, and can be found here[http://www.aerorocket.com/finsim.html]
{{reflist}}
[[Category: Rockets]]
[[Category: Rockets]]