ROCKET AERODYNAMICS FIN STABILITY - Part 2

1. Finertia

   1. maximum acceleration

2. Fdrag

   1. maximum angle-of-attack

3. Falpha

   1. fin restoring force

      1. non-zero angle-of-attack

4. Fhandling

   1. out-of-plane

FIN STRUCTURAL STRENGTH

Rocket fins and their mounting to the rocket body (or tube or other support structure, if fin can is utilized) must be capable of handling the following loads:

1) _________ : In-plane fin mass inertia load due to ________ of the rocket

2) _______ In-plane aerodynamic drag load at Max-Q or ___________

3)_________ : Out-of-plane loading due to_____________ due to ____________

4) _________: Handling load, typically ______

• force resultant.

• subsonic flow,

• 1⁄4 Chord line

• fin leading edge

• mean aerodynamic chord

FIN STRUCTURAL STRENGTH

• Location of the C.P. of an individual fin, as this is the location of the _______

• For _______< the C.P. of a fin is located at the intersection of the _______ relative to the ______ X̅ ) and the _________ (Y̅ )

• fin attachment:

  • FInertia × Ycg

  • Fdrag × Y̅

• fin bending.

  • FAlpha × Y̅

  • FHandling × YH

• shear loading,

• EX rocket

FIN STRUCTURAL STRENGTH

The maximum (fin root) bending moment (M) for the four cases are:

The first two cases are considered to be critical solely for the _________:

1) M = ________

2) M = _________

the third and fourth cases are critical for ______

3) M = ______

4) M = _______

The fin and its attachments are also subject to ______ however, this loading is not critical for most ________ and may be neglected.

• force-couple

• top fastener in tension

• linear distribution

FIN STRUCTURAL STRENGTH

• he applied moment (M) is reacted as a _______ (R) by the ________ (T) and the lowest part of the fin in bearing against the rocket body.

• It may be conservatively assumed that solely the top fastener reacts the applied moment (if additional fasteners are to be considered effective in reacting the moment,

• a __________ reactions over the remaining fasteners may be assumed).

• Bending stress

• 2 or higher.

__________ is then compared to the material strength and the resulting Safety Factor determined. Safety Factor should be _________

1. Gyroscopic stabilization

   1. thin atmospheres

   2. low speeds

2. Reduced trajectory dispersion

   1. wind disturbances or fin misalignment

• spin

• pitch–roll coupling,

FINS TO INDUCE ROLL

Inducing roll (spin) about the rocket’s longitudinal axis can improve flight performance in two main ways:

1. ____________ – Spin adds stability, especially for rockets flying in __________ (e.g., upper stages) or rockets with ______ where fin restoring forces are weak.

2. _____________ – A rolling rocket is less affected by ___________ or __________, because the spin distributes aerodynamic disturbances evenly around the rocket.

Disadvantages of roll:

● Parachute line tangling during recovery due to the ____.

● Dynamic instability, particularly __________ which can negatively affect flight stability.

1. Canted Fins

2. Fins with spinneron

3. Asymmetrically airfoiled fins

FINS TO INDUCE ROLL

The three most common methods of inducing roll are:

1)____________

2)___________(or tabs)

3) ____________ fins

Planar fins

• EX rocket

• maximum restoring (stabilizing) moment

ALTERNATIVE TO FINS

_________ are nearly always the best option as the stabilizing device for an _____, providing ____________ combined with minimum weight and minimum drag, and of course, are simple to fabricate and mount.

1. Ringtail fins

   1. restoring moment

      1. higher drag

      2. base drag

      3. ringtail and the exhaust plume

2. Conical flares

   1. hypersonic speeds

      1. tube-launched rockets,

      2. weathercocking and trajectory dispersion.

• less than 8°

• flow separation

ALTERNATIVE TO FINS

TWO Alternative Stabilizing Devices:

1. _________ produce about twice the _________ of planar fins with the same span and chord at both subsonic and supersonic speeds.

   1. Their disadvantage is _______ at subsonic speeds, but

   2. at supersonic speeds ______ can decrease due to favorable interaction between the ________ and the _________, especially for long-burn motors (e.g., end-burners).

2. _________ can generate more than twice the normal force of planar fins at_________, but they usually produce much higher drag.

   1. They may be useful for _________ where exhaust gases pressurize the tube and provide an initial boost in launch velocity, helping reduce _____________ and ____________

• For effective operation, the flare angle should be _________ to avoid _______, which would reduce stability effectiveness.