STRUCTURAL ANALYSIS

Airframe Strength

Compressive buckling analysis for airframe integrity under aerodynamic loading.

Loading Mechanism

During boost, drag acts on the nose while thrust pushes from the aft end. This puts the airframe in compression between the motor mount and nose.

[NOSE] ← Drag Force
   |
   | Body tube in COMPRESSION
   |
[MOTOR] → Thrust Force

For thin-walled cylinders with moderate L/D ratios:

Critical Stress

\sigma_{cr} = 0.3 \times E \times \frac{t}{r}

For very long, slender tubes (L/D > 15):

Critical Load

P_{cr} = \frac{\pi^2 EI}{(KL)^2}

E

Young's modulus of tube material

t

Wall thickness

r

Tube radius

I

Moment of inertia

K

Effective length factor

Material	E (GPa)	Strength (MPa)	Density
Kraft cardboard	2-4	10-20	0.6-0.8 g/cm³
Blue Tube	8-12	40-60	1.1-1.3 g/cm³
Phenolic	6-10	80-120	1.3-1.4 g/cm³
G10 Fiberglass	18-22	150-200	1.8-2.0 g/cm³
Carbon Fiber	70-150	300-600	1.5-1.6 g/cm³
6061-T6 Aluminum	69	240	2.7 g/cm³

The weakest section is typically:

Safety Factor

Apply a safety factor of 2.0-3.0× to account for material variability and construction quality.

SOURCES

Analysis and Design of Missile Structures — Bruhn, E.F.
Comprehensive buckling analysis for cylindrical structures

Rocket Body Structural Strength — Nakka, R.
Practical body tube strength analysis for amateur rocketry
Compression Tests of Tubing Used in Rocketry — Apogee Components
Peak of Flight #510 - Experimental data on tubing strength
Rocket vehicle loads and airframe design — Aspire Space
Structural design methodology for amateur rockets

Put this methodology into practice with the interactive calculator.