ESA Document - Emits - ESA
ESA Document - Emits - ESA ESA Document - Emits - ESA
s 4.4.3.2 Assumptions and trade-offs HMM Assessment Study Report: CDF-20(A) February 2004 page 342 of 422 During descent four possible sceneries are possible. The cut-off of the engines at 2 m height with vertical velocity at this point between 0 and 2 m/s; cut-off of the engines at zero height, with velocity also between 0 and 2m/s. In all cases, it was assumed to have a maximum horizontal velocity of 1 m/s and a deceleration of 0.5 s; consequently a maximum horizontal force of 60 000 N is present. The vertical distance between the SHM and the Martian surface was assumed to be 1 m, due to the length of the retro rockets, and possible rocks in the landing site. The leg footprint was assumed to be 6 m. 4.4.3.3 Baseline design For landing stability, four legs with crushable shock-absorbing system and round footpads were selected. A three-leg design has the problem of stability in the presence of side-velocity if the spacecraft touches down moving away from one leg. A five-leg design does not improve much more since the leg structure is strongly driven by the one-leg- hits-first case. With five legs it would not be possible to make it as lighter as the number of legs increase. So the smallest number with reasonable stability is 4. One principal leg, and two secondary legs constitute each leg. The one-leg-hits-first case was applied to the principal leg, which means that this one has to be able to withstand all loads, during touch down. Figure 4-84: Landing-leg configuration Aluminium was selected for the landing legs material, due to its low density and high strength. For all the cases the horizontal and vertical forces involved were calculated, as well as the resultant force and it was concluded that the higher forces were involved when the cut-off of the engines occurred at 2 m height and with a vertical velocity of 2 m/s. Hvertical (m) Min.Vvertical (m/s) Fvertical due to Deceleration (N) Total Fvertical at motion extreme (N) Angle (degrees) Vector Force (N) 0 0 0 114 000 30.3 128 702.9
s 2 3.9 233 923.1 347 923 12.3 352 722.7 Hvertical (m) Max.Vvertica l (m/s) Table 4-39: Velocities and Forces with Vo=0 m/s Fvertical due to Deceleration (N) Total Fvertical at motion extreme (N) Angle (degrees) Vector Force (N) 0 2 120 000 234 000 16.9 337 875.9 2 4.4 262 906.8 376 906.8 11.5 533 805.2 Table 4-40: Velocities and Forces with Vo=2 m/s HMM Assessment Study Report: CDF-20(A) February 2004 page 343 of 422 The method used for designing the landing legs consisted of assuming that each leg is a truss, case and it must be able to support all the load. There are three possible cases for the angle of the legs with the SHM: it can be smaller, equal or higher than the angle of the resultant force. A brief analysis concluded that when these angles are equal, the force applied along the axial line of the leg is higher. Due to this the buckling and stress analysis were performed to this case. As the difference between these two angles increases, the lateral force increases, and the axial decreases. The maximum lateral force is 20% of the axial force. The case, which introduces higher stresses, is case 1; due to this the principal leg was designed to this one. A safety factor of 1.5 was applied to the resultant force, for the stress analysis, which results in a stress of 215 MPa for an axial force 1.5 higher than the expected. Through the buckling analysis a minimum radius of 2 cm was obtained for the leg, but to fulfil the strength requirements a higher radius was necessary. A radius of 15 cm and a thickness of 4 mm was selected after the strength and buckling analysis. 4.4.3.4 Budget Item Nr. Mass [kg] Margin [%] Mass with Margin [kg] Principal Leg 4 61.8 10 67.96 Secondary Leg 8 16 10 17.70 TOTAL 413.44 4.4.4 Communications 4.4.4.1 Requirements and design drivers Table 4-41: SHM Structures Mass composition • The vehicle shall support Tracking, Telemetry and Command (TT&C) communications during all mission phases and any attitude. • Communications availability should be maximized during all mission phases.
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s<br />
2 3.9 233 923.1 347 923 12.3 352 722.7<br />
Hvertical<br />
(m)<br />
Max.Vvertica<br />
l (m/s)<br />
Table 4-39: Velocities and Forces with Vo=0 m/s<br />
Fvertical due<br />
to<br />
Deceleration<br />
(N)<br />
Total<br />
Fvertical at<br />
motion<br />
extreme<br />
(N)<br />
Angle<br />
(degrees)<br />
Vector<br />
Force (N)<br />
0 2 120 000 234 000 16.9 337 875.9<br />
2 4.4 262 906.8 376 906.8 11.5 533 805.2<br />
Table 4-40: Velocities and Forces with Vo=2 m/s<br />
HMM<br />
Assessment Study<br />
Report: CDF-20(A)<br />
February 2004<br />
page 343 of 422<br />
The method used for designing the landing legs consisted of assuming that each leg is a truss,<br />
case and it must be able to support all the load.<br />
There are three possible cases for the angle of the legs with the SHM: it can be smaller, equal or<br />
higher than the angle of the resultant force. A brief analysis concluded that when these angles are<br />
equal, the force applied along the axial line of the leg is higher. Due to this the buckling and<br />
stress analysis were performed to this case. As the difference between these two angles<br />
increases, the lateral force increases, and the axial decreases. The maximum lateral force is 20%<br />
of the axial force. The case, which introduces higher stresses, is case 1; due to this the principal<br />
leg was designed to this one. A safety factor of 1.5 was applied to the resultant force, for the<br />
stress analysis, which results in a stress of 215 MPa for an axial force 1.5 higher than the<br />
expected.<br />
Through the buckling analysis a minimum radius of 2 cm was obtained for the leg, but to fulfil<br />
the strength requirements a higher radius was necessary. A radius of 15 cm and a thickness of 4<br />
mm was selected after the strength and buckling analysis.<br />
4.4.3.4 Budget<br />
Item Nr. Mass [kg] Margin [%] Mass with Margin [kg]<br />
Principal Leg 4 61.8 10 67.96<br />
Secondary Leg 8 16 10 17.70<br />
TOTAL 413.44<br />
4.4.4 Communications<br />
4.4.4.1 Requirements and design drivers<br />
Table 4-41: SHM Structures Mass composition<br />
• The vehicle shall support Tracking, Telemetry and Command (TT&C) communications<br />
during all mission phases and any attitude.<br />
• Communications availability should be maximized during all mission phases.
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