ROCKET AERODYNAMICS FIN STABILITY - Part 1

• fin normal force

• straight flight path

• non-zero angle-of-attack.

• nosecone, conical shoulders and boattail

• destabilizing rotational moment.

• Centre of Pressure (C.P.).

• Stability Margin

• Fins

• aft

• ahead

Fins and Rocket Stability

• The stability of a fin-stabilized rocket is dependent upon the ___________ generated when a rocket tends to deviate from a ___________, in other words, flying at a _______________ This force, acting normal or perpendicular to the fin plane, imparts a rotational (corrective) moment to the rocket, about its Centre of Gravity (C.G.), which tends to counteract this deviation and to restore the desired straight flight path.

• At a small angle-of-attack, not only the fins, but other components of the rocket, such as ________, _______ and _________ also generate a normal force. In the case of the nosecone and boattail, the forces generated by these components cause a ___________

• The resultant of all of these normal forces acts at a point on the rocket body deemed the ____________This resultant force, which diminishes to zero as the angle-of-attack is corrected to zero, generates a stabilizing moment about the C.G.

• It is clear from this discussion that the _____ must generate enough force to overcome the destabilizing forces to the extent that the C.P. lies ___ of the C.G. by a certain distance. This distance (measured in body diameters, or calibres, for convenience) is deemed the _______ (SM).

• If the fins are too small, and as such generate a relatively small fin formal force, the resultant of the forces will lie _____ of the C.G. It is obvious, looking at the figure on the right, that the rocket will become unstable and tend to flip over.

• positive

• 1.5 to 2.5

• 3

Fins and Rocket Stability: Stability Margin

Fins Considered to be

• Last component to be designed

• Stability margin to be ________

• Good target value: ______

• High stability margin, approx. ___, may tend to veer severely into the wind

• Center of Gravity (CoG)

• mass distribution

  • 65% to 70%

  • 1.5

• forward shift

• apogee.

• liftoff.

Fin Sizing

• sizing the fins last is for the adjustment of the computed value of the exact location of the __________ of the fully-assembled rocket must be known (usually with a loaded motor).

• The C.G. depends on the ________ of all the components of the rocket. Once the C.G. location (xcg ) is known , the desired location of the C.P. (xcp) based on the desired Stability Margin.

  • For assumptions, location of the C.G. is about _______ of the rocket length.

    • Desired value of S.M. is at ____ when giving assumptions for Xcg

• The fins are then sized to attain such a Stability Margin.

  • Note that the location of the rocket's C.G. will change during its flight. As propellant is consumed, the rocket's mass will decrease.

• This will result in a ________ in the rocket C.G. which has a positive effect on the Stability Margin.

• After burnout, the mass of the rocket is constant and the stability margin will likewise remain constant for the remainder of the flight to ______

• As such, the minimum Stability Margin exists at ________ This is the design condition.

DELTA

• High speed rockets

• rounding tip

Rectangular

• Simple

• Ugly

Swept

Trapezoidal

• Less aerodynamic

FIN PLANFORM PROFILE

Delta

• Pro:

  • Flutter Resistant,

  • good choice for _________

• Con:

  • Sharp tip may be susceptible to damage and may be a handling hazard (mitigate by _____)

  • Force coefficient less than the other shapes

_____________

• Pro:

  • _______ to Make,

  • Structurally Robust

  • Flutter Resistant

• Con:

  • Rather ____

  • May be susceptible to damage upon landing

____________

• Pro:

  • Structurally Robust

  • Flutter Resistant

• Con:

  • May be susceptible to damage upon landing

______________

• Pro:

  • My go to shape

  • Structurally Robust

  • Can be clipped to shift CP Forward

  • Flutter Resistant

• Con:

  • _________ than swept flatforms

Tapered Swept

• high speed flights,

• flutter

Clipped Delta

Ellipsoidal

• aerodynamically

• Barrowman shape

Clipped Taper Swept

FIN PLANFORM PROFILE

Tapered Swept

• Pro:

  • Can be clipped to shift CP Forward

  • a good choice for ______ in particular for an upper stage

• Con:

  • May be susceptible to _____

  • Can be susceptible to damage upon landing

__________________

• Pro:

  • Structurally Robust

  • Can be clipped to shift CP Forward

• Con:

  • Potentially susceptible to damage upon landing

_________________

• Pro:

  • apparently __________ efficient

  • Hardest to make

  • Structurally robust and Flutter Resistant

• Con:

  • Hard to Fabricate

  • Not a ___________ so calculating fin CP is less straightforward

___________________

• Pro:

  • Can be clipped to shift CP Forward

  • an improvement over Tapered Swept

• Con:

  • Can be susceptible to damage upon landing

  • May be susceptible to Flutter

• finset

• airflow.

• non-zero angle-of-attack

• wind gust

FIN RESTORING FORCE

The ______ on a rocket serves to provide stability by generating a restoring force, that is, a force normal (perpendicular) to the fin surface when a fin is deflected relative to the _____

This occurs when the rocket is flying at a _______, denoted by the symbol alpha (a). This angle-of-attack may result from a flight disturbance such as _________.

The restoring force is a function of angle-of-attack, such that when a=0, no restoring force is created (nor needed).

• span width

• fin surface area

• dominant S/d squared

SPAN AND CHORD

Regarding fin design, there is one notable thing to bear in mind. As alluded to earlier, fin effectiveness, for a given shape, is more influenced by __________ than by chord length.

• In other words, fin effectiveness (in shifting C.P. aftward) is not solely a function of _________.

• The greater effectiveness of span width, compared to chord length, is a consequence of the _____________ term in the Barrowman equation for fin force coefficient (CN)F slope.

Along those same line, a particular fin shape influences its effectiveness in generating fin (restoring) force.

• Fin skin friction and fin pressure drag

• LoPER class,

• subsonic range

• 20-25%

FIN DRAG

• ____________ and _________ are the primary drag forces generated by a finset.

• For a __________ EX rocket that flies exclusively in the ___________, fin pressure drag force is generally not significant compared to the overall drag force acting on the rocket,

• Skin friction drag accounts for ______of the total drag.

FIN FLUTTER

• bending and twisting

• airflow energy

• natural resonance frequency.

• High-speed airflow

• oscillations.

• increased drag

• buzzing or whining

• Metal fins

• low shear modulus

_______________

• It is a rapid oscillation of rocket fins caused by a combination of _______ and _______ when _______ excites the fin at its __________

• This typically occurs at high speeds when the fin does not have sufficient stiffness.

Key points:

• Cause: _________ interacting with fins that are not stiff enough.

• Mechanism: Aerodynamic forces excite the fin’s natural vibration frequency, producing _______

• Result: Can lead to _________, structural damage, or catastrophic fin failure (fins snapping off).

• Sound: Flutter often produces a distinct _______ or _______sound.

Material effects:

• ________ rarely experience catastrophic flutter because of their high stiffness and strength.

• Plastic and wood fins are more vulnerable due to their _____________