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Subsections
The OOM comprises three distinct, major structures: the engine
mounting, the propulsion support collar, and the lander adapter.
Minor structures, which are defined as structures that neither carry
nor transfer major loads, are the bipropellant and toroidal fuel
tanks.
The engine mounting acts as the foundation of the whole spacecraft.
Its primary function is to be the attachment locus for the two Kaiser
Marquardt R4-D bipropellant engines and the engine's peripherals such
as fuel lines, gimbal actuators, heaters, etc., needed for the
engine's proper functioning. Its second function is to support and
restrain the main fuel tank assembly throughout the mission and also
to transfer loads to the propulsion support assembly. The third and
final function of the engine mounting is to be the attachment point
for the launch vehicle adapter. The design, materials, and
construction of this structure are intended to be rugged yet
lightweight in order to support the large loads that it will be
subjected to while carrying out its three functions.
The second major structure on the OOM is the propulsion support
assembly (PSA). This structure is designed to be very rugged and
lightweight, but is actually reinforced to a lesser degree than the
engine mounting due to the fact that it is machined from one piece of
aluminum-titanium alloy and is composed of composite materials as
well. The PSA has five major functions that it must fulfill during
the mission. The first is to take the loads being transferred to it
from the engine mounting structure. It acts as the conduit for the
loads to the rest of the spacecraft. The second function is to serve
as the attachment point for the toroidal fuel tank and the rest of
the bipropellant fuel system peripherals. The third function is to
support the main fuel tank assembly and the pressure/flow regulation
systems. Its fourth function is to serve as the attachment site for
scientific and navigation instruments that are to be used in outer
space, such as the magnetometer and the star tracker. Its last
function is to transfer loads to and couple loads with the lander
adapter. This structure is very significant as a junction for many of
the loads and other equipment the spacecraft houses.
The last major structure that is part of the OOM is the lander
adapter. The adapter is an aluminum-titanium and graphite/epoxy
structure. Its main function is to join the SOM to the OOM during the
mission until their separation. With this in mind, it too will also
be used as a conduit for much of the thermal pipes, cabling, and
propulsion lines. The adapter is also the location for the
reaction wheels because the center of mass for the entire vehicle is
located within the area inside of this component. The adapter is
designed to be lengthy in order to accommodate these articles and to
give clearance for the lander's 4 large REA 20-4 engines. The adapter
is attached to the lander via explosive bolts that will self-destruct
when the modules are to separate.
The bipropellant tank assembly consists of three components: a fuel
tank for storing hydrazine, an oxidizer tank that will store nitrogen
tetroxide, and the bipropellant outer liner. Both internal tanks are
made out of 6Al-4V titanium with pressurant and liquid outlet lines
entering from the bottom of each tank. The tanks have passive vane
propellant management devices (PMD's) and operate at 330 psi nominal
pressure [2]. Both tanks are designed with a system of
baffles to prevent center of mass shifting due to slosh effects. Both
tanks and the liner are designed to have very high hoop strength due to
the largest loads being generated from within, due to pressurization
and liquid mass pressures resulting from any accelerations of any
kind. Both tanks are insulated and house various components of the
propulsion system inside: tank heaters, propulsion piping, and
other peripherals for the bipropellant system. The liner is
constructed by winding graphite/epoxy composite fibers around the two
inner titanium tanks when they are assembled together.
The launch vehicle adapter is an integral part of Asterius' mission.
The launch vehicle adapter's mission is twofold: first this unit must
impart a force to Asterius evenly during the launch period and also
during the subsequent staging operations until solar orbit is reached.
This goal requires distributing the immense forces from the Titan IV
launch vehicle perfectly through Asterius so that the vehicle is
firmly supported, as well as to ensure that local stress deformations
are not produced, resulting in structurally weakened modules. The
second goal of the landing adapter is to support the weight of
Asterius while on the launch pad and during the launch, and as a means
to attach Asterius to the Centaur upper stage. The adapter is
constructed of titanium-aluminum alloy reinforced with composite
graphite/epoxy. The composite is designed for most of its strength
along the axis of the launch vehicle, due to compressive forces being
the greatest, and also possesses torsional and lateral stiffnesses to
resist loads arising due to vibrations and oscillations generated by
the launch vehicle.
The launch vehicle is equipped with passive vibration control
equipment that is very similar to rubber grommets on the mounts of a
car engine, and is needed to keep vibrations in the lateral direction
out of the 6-10 Hz range. The vibrations due to the Titan IV launch
vehicle are expected to be in the range of 17-24 Hz in the axial
direction and roughly greater than 2.5 Hz in the lateral
direction [1]. The adapter appears as a truncated cone
with parallel top and bottom surfaces. It attaches to the Centaur
upper stage at its bottom surface, and to Asterius on its top surface.
Asterius is attached to the adapter with explosive bolts (similar to
those on the lander adapter) and will detach itself from the upper
stage during the destruction of these bolts.
The toroidal fuel tank houses the monopropellant fuel that will be
used during the deep space transit. It also houses the helium
pressurant that will be used to pressurize the monopropellant and
bipropellant systems jointly. Its outer skin is comprised of
graphite/epoxy resin composite. The skin contains two tanks of
titanium construction (similar to the bipropellant's arrangement).
Its unique geometry enables its center of mass to be located on the
z-axis of the spacecraft and yet store plenty of fuel and
pressurant. The toroidal tankage assembly also is designed with a
system of inner cell baffles to resist center of mass excursions due
to slosh affects.
The engine mounting is machined and constructed from aluminum-titanium
alloy. This rugged construction is due to the intense loads that this
structure will experience during its mission lifetime. The PSA is
also constructed from aluminum-titanium alloy (as previously
mentioned) along with composite reinforcement which will allow the
structure to be constructed lighter yet very strong. This structure
relies heavily on the composites being constructed to support
the compressive and extensional loads which will be
experienced. The aluminum-titanium alloy serves as the skeleton
structure that provides the most stiffness and resistance to all
loads; with the composites being used to reinforce these strengths and
enable the structure to be lighter.
The OOM will experience its greatest accelerations during launch;
therefore, any calculations regarding the maximum loads it will see is
adequately described by the calculations based on the following launch
accelerations. The Titan IV has been rated at a maximum
(non-impulsive) acceleration of 32.34 m/s
and at
-63.7 m/s
[1]. With these upper limits in mind,
the following calculations show the forces that the engine mounting,
propulsion support assembly, and lander adapter must endure without
failure.
- 1.
- Wet mass of SOM: 1690.61 kg
- 2.
- Mass of lander adapter: 42.83 kg
- 3.
- Mass of bipropellant: 987.43 kg
- 4.
- Force seen by engine mounting: +90,200 N to -177,000 N
- 5.
- Force seen by PSA: +88,000 N to -173,000 N
- 6.
- Force seen by SOM adapter: +55,000 N to -108,000 N
With these loads calculated, the engine mounting, PSA, and SOM adapter
were designed to support these loads with a 5% margin. The other
significant acceleration that the above structures will endure is the
braking and capture cycle at Jupiter. To demonstrate that these
forces are insignificant in comparison to the above calculated launch
forces, the maximum possible braking acceleration during this maneuver
is calculated to be 0.311 m/s2. It is therefore apparent that this
acceleration (and resulting forces) are considerably lower than the
launch acceleration of 32.34 m/s
and should not cause any
structural failures due to the design constraints previously
mentioned. The above braking acceleration was calculated based on
Asterius' full wet mass and both main engines firing simultaneously.
The greatest axial accelerations that Asterius and its launch vehicle
will have to endure are those encountered during the launch phase as
mentioned in the above section. Below is a calculation of the largest
sustained (non-impulsive) force that the adapter will see throughout
its use.
- 1.
- Maximum sustained (non-impulsive) launch acceleration:
32.34 m/s
or -63.7 m/s
.
- 2.
- Total wet mass of Asterius: 3574.24 kg
- 3.
- Maximum launch force (non-impulsive): 116,000 N to -228,000 N
The LV adapter is designed to support the above force with an
additional 5% margin. The adapter materials have been selected with the
above loads in mind and yet designed to be lightweight as
well.