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Subsections
The lander's skeletal structure and the landing gear are the only
major load carrying structures. Other structures that carry only
local loads include the antenna gimbaling beam, the WISP and DIPS booms,
and the teardrop-shaped lander fuel tank.
The SOM's major structure is the lander skeletal structure. This
particular structure is designed to fulfill many functions during the
mission even though it is just one unit. The skeletal structure is a
complex unit assembled and welded together from many smaller pieces.
This architecture is very integral for lightweight operations and the
strength the lander will need to have during the most stressful
accelerations of the mission: the launch cycle, the apogee kick
acceleration stage, the braking stage approach to Jupiter, and finally
the lunar landing on Europa. The lander has five main functions. The
first function is to protect the electronics during the mission. The
second function is to house and protect all of the scientific
instruments that reside in or on it. The third purpose is to house,
protect, and deploy the WISP upon reaching Europa. Fourth, the lander
structure will have the distinction of delivering all of its cargo
safely to the surface of Europa without structural failure during
descent or the landing cycle. The last function the lander serves is
to mount the maneuvering thrusters for the entire spacecraft.
The lander itself can be broken down further into areas: the top of
the lander is the bus which houses most of the electronics for the
mission and is where the antennas attach to. The sloping tubular area
is the scientific instrument attachment area. The midsection collar
is where the thrusters are mounted. Finally the base of the lander
skeletal structure is the area where the landing gear are located,
where the landing engines attach to, where the internal propellant
tank is supported from, and where adapter loads are distributed to the
lander.
The landing gear is solid aluminum with pneumatic damping and
actuation cells to take the shock of landing and adjust the height and
attitude of the craft on the surface if needed. The landing gear is
solid to support the shear loads that will result upon landing.
The teardrop-shaped tank's liner is of composite (graphite/epoxy)
construction and houses two internal 6Al-4V titanium tanks that hold
hydrazine and helium. The tank is located centrally
within the lander structure to ensure minimal center of mass shifting
due to slosh. The tanks have passive vane fuel management devices as
well as a system of baffles, heaters, liquid transfer lines, and
pressure regulators.
Both the low and high gain antennas (LGA and HGA) will be of
graphite/epoxy construction, the the HGA supported by an arched aluminum
variable direction, arched beam. The position of the antennas will be
changed via motors that are integrated with the support beam and
mounted to support structures that attach to the bus. The DIPS power
unit and the WISP surface probe will be mounted on booms from the
scientific instrument attachment area. The WISP's boom will be
equipped with actuation gear to move and deploy the probe, and the DIPS
power unit will also be actuated to move the unit if center of mass
issues need to be fine-tuned or altered after probe deployment.
The High gain and low gain antennas, the fuel tank, the equipment
booms, and finally the thruster booms are all minor structures because
the loads they will experience do not support the majority of the
craft's weight but instead only support specific, localized loads that
these structures are specifically designed for. The DIPS booms and
WISP booms will be tested for shear loads as well as the landing gear
and the main structure itself; these calculations and comments on them
can be seen below the current section in the section labeled `SOM
calculations.'
The SOM's skeletal structure is to be manufactured from aluminum and
will be composite reinforced. The many parts such as load bearing
rings, support tubing and the bus will be welded together, as
mentioned previously, and therefore must be manufactured from either
6000- or 8000-series tempered, heat-treated aluminum. Most of the
members of the lander structure will be hollow for weight
conservation, yet designed thick enough to support the intense loads.
The structure's center of mass is designed to be fairly low on the
craft for stability, and by the time it lands on the surface of
Europa, the fuel will have been consumed thus shifting the center of
mass lower still.
The SOM structure along with the DIPS boom and WISP boom will all, as
mentioned before, see maximum accelerations of 32.34 m/s
and
-63.7 m/s
during the whole mission. Therefore the following
calculations demonstrate to what criteria these devices were designed
to meet and exceed.
- 1.
- Wet mass of SOM: 1690.61 kg
- 2.
- Mass of DIPS: 60 kg
- 3.
- Mass of WISP: 98 kg
- 4.
- Max. force seen by SOM (approx.): +41,000 N to -81,000 N
- 5.
- Max. shear force seen by DIPS mount: +1940.4 N to -3822 N
- 6.
- Max. moment seen by DIPS mount (2 m moment arm)= +3880.8 N-m to -7644 N-m
- 7.
- Max. shear force seen by WISP mount: +3169.32 N to -6242.6 N
- 8.
- Max. moment seen by WISP mount (2 m moment arm): 6338.64 N-m to -12,500 N-m
Thus, from the above numbers, the SOM skeletal structure and both
mounts were designed to carry loads, with an additional a 5% margin.
It should be noted that even though the the magnetometer boom has a
length of 4 meters, this device will not be extended during launch and
therefore will be supported by imbeded structure. However, this
device will see some acceleration during braking.
The landing gear maximum loads will come from their support of the SOM
on the surface of Europa. The following calculations show the maximum
loads the magnetometer boom and the landing gear will see during their
mission lifetime.
- 1.
- Max. possible acceleration seen by magnetometer boom:
0.311 m/s
- 2.
- Acceleration on the surface of Europa 1.323 m/s
- 3.
- Mass of magnetometer w/boom: 18.97 kg
- 4.
- Dry mass of SOM: 892.81 kg
- 5.
- Max. shear force seen by magnetometer boom: 5.9 N
- 6.
- Max. moment seen by magnetometer boom (4 m moment arm): 23.6 N-m
- 7.
- Max. force seen by landing gear (4 legs): 1181.19 N
- 8.
- Max. moment seen by one landing gear leg (2.3 m moment arm):
675.06 N-m
The magnetometer truss and the landing gear were also designed to
carry 5% past the above requirements.