Preliminary Design Review: Loads, Structures, and Mechanisms Michael Cunningham, Shimon Gewirtz, Rajesh Yalamanchili, Thomas Noyes
Crew Cabin Structure Height of ~3.7m from heat shield to top of the cone Internal pressure of 60 kpa Power, Propulsion, and Thermal systems mass of 1853 kg o However, all following calculations use a gross mass of 4795 kg Chosen because it had the fewest external structures previous to the following design
Choice of Shell Material Considered aluminum, high strength steel, low strength steel, and titanium. Material Density (lb/in 3 ) σ u /ρ Aluminum 0.1 420 High strength steel Low strength steel 0.29 390 0.28 204 Titanium 0.16 906
Choice of Shell Material All materials would be able to withstand the stresses that they would undergo at a 0.1m wall thickness with a reasonable safety factor Chose aluminum because the only consideration left is mass, and aluminum is the least massive Specifically, chose aluminum alloy 7075 T6 because it is the strongest of all aluminum alloys
Load Analysis
Pressurization loads Cabin pressure of 60 kpa Max pressurization load occurs in a vaccum This max stress is 5.26 MPa Pressure is maximized along the edges and at the bottom of the capsule
Pressure Loads
Docking Loads Assume a Δv of 0.10 m/s, a damping coefficient of 2000 N-s/m and a Δt of 2.1131 sec based on research Use a damper to absorb the force This means there is a max force of 800 N acting on the craft.
Vibrational Loads Used SolidWorks to compute resonant frequencies Mode Frequency (Hz) 1 138.04 2 224.1 3 242.62 4 242.72 5 244.04
Vibrational Load Mode 2
Vibrational Load Mode 3
Vibrational Load Mode 4
Vibrational Load Mode 5
Vibrational Load Summary Mode 1 did not produce any meaningful displacement Modes 4 and 5 produced large displacements (27.78 mm and 33.10 mm respectively) at high frequency and would very likely result in complete structural failure
Earth Launch Acceleration Force Assumed max acceleration of 4.8 g's based on notes Assumed Pressure force and propulsive thrust act on the craft Max acceleration force is 87165 N Due to 25 half cone angle, this force breaks down into: o o Axial force of 78998 N Lateral force of 36838 N
Deformation Due to Launch Force
Earth EDL Deceleration Force Assumed max deceleration of 10 g's because research indicated that this is near the upper limit for safe re-entry Based on research, assumed a temperature of 176 Celsius reaches the capsule Max deceleration force is 181594 N Assumed Pressure load, thermal load, and frictional deceleration force all act on capsule
Earth EDL Deceleration Force
Velocity at Impact with Water Will deploy parachute at Mach 2 Deploying at Mach 2 will give the craft sufficient time to decelerate to terminal velocity. Assumptions: o Radius of parachute = 8.08 m o C d = 0.62 o γ = -30 degrees
Velocity at impact with water calculation Used following formulas: o β = m/(c d *A) o V = sqrt(-2*g*β*sin(γ)) Velocity at impact is 32.9698 m/s Assumed a max g load of 6.2 g's at splashdown based on loads during Apollo 11
Stress at Impact with Water
Stress at Impact with Water Max stress felt by craft during splashdown is 6.05 MPa Stresses are concentrated along the bottom of the craft Max displacement is under 0.3 mm
Basic Design of Crew Propulsion Stage
Engine and Nozzle Design We determined that if we were to generate a thrust of 1.5 MN we would have a nozzle diameter of 0.9322 m^2 with an area ratio of ~46.68. In addition the diameters of the two paired spherical fuel and oxidizer tanks are 1.57 m and 1.61 m respectively. The tanks are distributed around the nozzle with the nozzle protruding from the center.
Landing Structure
Landing Gear Key Designations (in mm)
Landing Gear The landing structure is a truss with a telescoping foot The foot is surrounded by honeycomb material to attenuate landing loads and bounce. It ensures that the maximum acceleration the astronauts feel never exceeds one-and-a-half earth gravities
Landing Gear For our cross section we chose a bending moment of inertia and generated a contour of the radius ratios
Landing Gear The landing structure was analyzed for the critical buckling load (P crit ) in both the foot and main compressive truss member. P crit for the foot = 1.6 MN P crit for the main truss member = 294 kn
Landing Gear Deployment The main leg member is stowed by having joint C initially unattached and members AD and BD bent along their lengths to retain contact with point D in the retracted configuration. To deploy member CD is pyrotechnically actuated downward to lock node C in, where node C is attached at point C with a ball socket joint, and locked in to the node by the tension in AD and BD.
Leg Cant Angle The main leg strut was canted at a 45 o angle based on the desire to keep the maximum truss member force, F cd = 63 kn (C), lower down and retain stability of the craft to withstand tipping. This graph varies force applied through cant angles to get F effective. Fig. A gives forces resultant from this.
Figure A
Member and Reaction Forces (Truss) (For the worst case of landing on one leg) React d = 2.5595e+004 React c = 5.3500e+004 React b = 1.3953e+004 React a = 1.3953e+004 F ab = 9.6002e+003 F ac = 3.6811e+004 F ad = 3.5896e+004 F bc = 3.6811e+004 F bd = 3.5896e+004 F cd = 8.3231e+004
Safety Factor The landing gear calculations were done with a 1.2 Factor of safety to ensure conservative estimates for safety, while not adding too much to the initial launch and lunar launch masses.
Impact Attenuation We used honeycomb cylinders designed to crush at a designated pressure,p crush = 800 psi, to control our rate of compression of the honeycomb.
Impact Attenuation The honeycomb acts like a crumple zone to extend the maximum time over which the total impulse (I total = 62113 N-s) of impact is integrated (t crush =~0.25 s ) This decreases the transmitted acceleration (to A transmit = 1.49*g earth ), which is calculated by dividing the transmitted force by the mass, M tot =16905 kg, of the craft.
References http://www.faa.gov/other_visit/aviation_industry/designees_ delegations/designee_types/ame/media/section%20iii.4. 1.7%20Returning%20from%20Space.pdf http://www.aerospaceweb.org/question/spacecraft/q0218.s html http://www.braeunig.us/space/comb-nm.htm http://www.hexcel.com/resources/datasheets/brochure- Data-Sheets/Honeycomb_Attributes_and_Properties.pdf
References http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720 018253_1972018253.pdf http://www.structsource.com/analysis/types/beam.htm http://www.astronautix.com/craft/lmlggear.htm http://www.hq.nasa.gov/alsj/alsj-lmdocs.html http://history.nasa.gov/ap11fj/26day9-reentry.htm