Table of Contents
Updates for 2/22/18
Bolt Calcs (formulae from Roark's Stress & Strain)
Force F = Chamber x-section * chamber pressure / # bolts
Bolt (effective axial) Area At = 0.7854(D-0.9743/n)^2 = 0.7854(0.25-0.9743/28)^2 = 0.03637in^2
Bolt (effective shear) Area As = pi*n*Le*Ds*(1/(2n)+0.57735(Ds,min - En,max))
Mcmaster ¼-20 cap screws: 150 KSI
Bolt Axial Failure
Inner Ring (10 bolts) (150000psi)/((830PSI*pi*(2.5in/2)^2)/(0.031in^2*10)) --> FOS = 11.4
Outer Ring (20 bolts) (150000psi)/((500PSI*pi*(5.0in/2)^2)/(0.031in^2*20)) --> FOS = 9.47
Inner Ring (10 bolts) (40000psi)/((830PSI*pi*(2.5in/2)^2)/(0.34818in^2*10)) --> FOS = 34.2
Outer Ring (20 bolts) (40000psi)/((500PSI*pi*(5.0in/2)^2)/(0.34818in^2*20)) --> FOS = 28.3
Vessel Calcs (formulae from Roark's Stress & Strain)
Sigma_axial = pr/2t = (500PSI * 2.5in)/(2*0.25in) = 2.5 ksi (16 margin)
Sigma_hoop = pr/t = (500PSI * 2.5in)/(0.25in) = 5.0 ksi (8 margin)
Structures Action Items
- Spec out O-rings
- Spec out lubricant
- Torque ratings for 0 separation condition
- This is probably unnecessary - we have concentric O-ring fits
- Get heating estimates, apply strength knockdown
Thus far, only a feasibility study has been completed on the combustion chamber to determine how difficult it would be to operate at an internal pressure of 500 psi. The following presentation shows this this pressure is very feasible.
A more in-depth analysis will need to be completed once a more finalized geometry is completed. Additionally, combined loadings and temperature considerations will be included into the analysis to ensure that our metallic factor of safety of 2 is met.
A quick hoop-stress calculator can be found here. The last tab shows some temperature considerations on stress and the resulting factor of safeties.
Prior to our Preliminary Detailed Design Review, we finalized our decision to make the combustion chamber of aluminum. This was primarily due to mass and cost concerns. Strength should not be an issue unless the hot gasses find a path to the casing. A huge goal of our testing will be to ensure that the casing is not heating up from lack of insulation or gaps between insulative components.
The two primary choices for combustion chamber material at this point are:
- Aluminum 2024-T4
- Aluminum 6061-T6
The grade is to be finalized based on availability and cost from chosen supplier. We are aiming for the following dimensions:
- 6” OD, 0.25” Wall Thickness, 3’ Long
- 14 lbm (6.350 kg)
As mentioned prior, the potential max temperature of the combustion chamber is an area of concern at this time. According to some hand calculations, we will be able to maintain a FoS of 2 up to:
- 500 psi internal pressure
- 472 °F (245 °C)
These hand-calcs are documented here.