

Stress Overview



The airframe will have to withstand the compressive
loading due to drag and acceleration. 
 It will also have to withstand the bending stresses as the rocket corrects it's flight path. 


Compressive Strength


 Compressive strength will be the strength needed to withstand not
only the drag on the rocket, but also the acceleration of the
airframe. 
 For these calculations it was assumed the that fins would be secured
to the internal structure and would not be added to the overall compressive
forces. 

Figure 1
Figure 2

Drag

 The plot in figure 1 shows a graph of the total
drag on the airframe. 
 From this graph we see that the maximum drag on
the airframe occurs at approx. 3.75 seconds into the flight. 
 Maximum drag appears to be about 13,500 Newtons (3,033 pounds). 
 Using Rocksim it is determined that "Fin
Drag" represents 45% of the total drag. 
 Therefore a force of 7425 Newtons (1,668 pounds)
must be acting on the the nose cone, and body tube. 
 This value will be used as the maximum
compressive force on the airframe due to drag. This value is in error
as the forces are not distributed evenly along the airframe but will
give us a good worstcase figure. Stress at any point in the airframe
will be less than this value. 
 A flat thrust curve is assumed for the three
"P" motors giving us a force of 27,000 Newtons (6,067
pounds) 

Acceleration Component

 The acceleration component represent the additional compressive
loading on the airframe due to acceleration . 
 From the graphs on the right, about 9.8 Gees will be the maximum acceleration. 
 The Gforce will be acting on all masses supported by the
airframe. 
 From Rocksim we get the initial (empty) mass of
the rocket to be 369 pounds. 
 From this we subtract the engine mount and the
fins leaving us with 261 pounds of nose cone, airframe, and internally
supported structures. 
 By multiplying 9.8 Gees times the mass of 310
pounds we get and additional compressive loading of 2557 pounds. 
 Again this is a worstcase value as the stress at
any one point along the airframe will depend upon it's location. 

Compressive loading total

 Drag accounts for 1,668 pounds. 
 Thrust gives us and additional 6,067 pounds on the airframe. 
 With acceleration placing an additional 4,650 pounds of force on the
airframe. 
 The total combined forces equals 12,385 pounds. 
 Using a design safety factor of 1.5 the airframe must be designed to
withstand 18,577 pounds of compressive force. 
 Wow, that's a lot of force. 



The calculations to the right show the safety factor for
each strength value (Design, Yield, and Ultimate) of the airframe
material. 



Individual Component Stress


 Centering Rings: 
 Much more work is needed in this area, but initial results look very
promising. 




Bending Strength


 See this page for calculations on airframe
bending stress. 

