Progress Report 5

Progress March 25-April 8, 1998

Jesús Jimeno, Systems Engineer

The Odysseus Program overall objective is to design a set of spacecraft that will permit a manned landing on Mars and a safe return of the crew. The current report summarizes the progress accomplished by the Manned Space Systems Division of Acme Aerospace on the Odysseus Program during the week from the 25^th of March to the 8^th of April 1998. This weeks goals to be accomplished were:

1.- Astrodynamics

• Determine Mars orbit according to Mars arrival orbit and an equatorial landing.

2.- Propulsion

• Estimate the amount of Methane needed for the injection into Low Mars Orbit from the surface.
• Study Propulsion system for Odysseus I
• Study multi-staging.

3.- Attitude Control

• Perform calculations on the required attitude maneuvering and specify an attitude event list which includes what is the appropriate spacecraft attitude to perform the interplanetary burn, Mars capture, satellite deployment and Mars orbiting.

4.- Configuration

• Accommodate the hydrogen fuel tanks so that they are easily changeable while in orbit around Mars.
• Perform a structural analysis on the ship to start designing the structure.

5.- Thermal Systems

• Heat exchange calculations on the available spacecraft data.

6.- Telecommunications

• Definite calculations on the telecommunication systems
• Specify satellite array composition and telecommunication characteristics.
• Find out possible complications if a spinning spacecraft is used.

7.- Power Systems

• Detailing of the power system for Odysseus I and II and the Mars surface arrangement.

8.- Mars Production Facility

• Find out about the methane production rates and hydrogen consumption and production rates of the plant. Specify number of units and total mass of the system.

The Astrodynamics Group has devoted its efforts to searching for alternating orbits for the trip to Mars. Good orbital trajectories had already been found. The outbound trajectory used a Venus swing-by in order to save time and propulsion needs. However, due to the extreme temperatures of Venus, a heat shield that protects the entire ship will be required for the short period of time that the swing-by takes place. Due to the complexity of a system which shields the whole ship and the limited amount of time available to finish the design, it will be desirable to find a new trajectory which will avoid Venus altogether. Those orbits do exist, but they require extensive trip times or unaffordable propulsion maneuvers. The Astrodynamics Group is still in search of a trajectory which will satisfy our needs, but a design process for the heat shield has also started in case that shielding the ship is the most satisfactory solution. In case that a Venus swing-by is required, it would be desirable to go around the planet behind the sun so that it is only required to shield the ship from one side.

The Astrodynamics Group has also been studying the LEO and Mars orbits. The construction and ejection into interplanetary trajectory of Odysseus I and II will take place from a 191 km circular orbit above the Earth. Capture at Mars will take place in an elliptical orbit with eccentricity of 0.818, periapse of 339.7 km. and apoapse of 33970 km. Transfer to an alternative circular orbit of lower altitude (approximately 100 km.) will be most likely performed in order to keep the propulsion requirements for the Mars Ascent Vehicle the lowest possible. The proposed elliptic capture orbit at Mars has the benefit of being an equatorial orbit what avoids the need for any plane change maneuvers since an equatorial landing site has been chosen..

The Propulsion Group has been working in optimizing the current propulsion system for Odysseus II. Currently, nuclear thermal propulsion has been chosen to propel the space ship into interplanetary trajectory and orbit capture at Mars. Multistaging has been proposed as a way of reducing the spaceships propellant mass and to increase the efficiency of the propulsion system. Technologically speaking there are no problems with a multistaged system. However, ethical concerns arise due to the fact that the stages of the nuclear propulsion system would be left out in space without proper care. It is the belief of the Acme Aerospace Company that the exploration of space should be performed in the way in which alteration to the space environment is minimized. It has been decided that the benefits that a multistaged system would provide would not justify leaving nuclear thermal rocket stages in unknown orbits in space. Accordingly, the initial propulsion design of a single stage system will be maintained.

The Propulsion Group has also started a study on the possible rocket systems to be used in Odysseus I. Liquid chemical rockets would provide the cheapest and simplest alternative. But due to the unexpected large payload that Odysseus I will be required to transport to Mars, nuclear thermal propulsion will be the only possible solution for Odysseus I. This will speed the design of the Odysseus I propulsion system since only a replication of the Odysseus II system which has already been designed will be required.

It is now important that the Propulsion Group focuses on the propulsion system required for the Mars Ascent Vehicle. This will be a methane propulsion system and it will inject the lander, crew, samples and necessary equipment into the spaceships parking orbit around Mars.

The Attitude and Control specialist has started a more refined calculation on the attitude and control parameters required for the design of the appropriate control system. The center of mass of Odysseus II was calculated. Based on current mass estimates and allocations, the center of mass of the entire configuration is at 15.83 meters from the docking area. Moments of inertia of the ship were also calculated. In order to hold a dynamically stable platform with artificial gravity it is necessary to have the largest moment of inertia about the rotating axis. However, for the current configuration, the rotating axis has the intermediate moment of inertia what makes the ship unstable. Possible solutions are currently being studied by the Attitude and Control Specialist, but they may include reducing the overall length of the ship and allocating more mass near the current center of mass.

The Attitude and Control Group has also been studying the possible methods to spin Odysseus II to provide the artificial gravity. Thrusters appear to be the most convenient system to spin the ship. Placing the thrusters near the center of mass will provide very stable rotation, but it will require either large thrusters or a long time to attain the necessary rotational speed. On the other hand, placing the thrusters further away from the center of mass (on the crew habitats) may cause some rotational instabilities, but on the other hand it will require small thrusters and small spin-up time. As an example, using 100 N thrusters it would take 15 hours to spin the ship up to 5 rpm if the thrusters are located near the center of mass, while it would take 5 minutes if they were located on the outside of the crew compartments.

Finally, for attitude control purposes and slight adjustments in attitude, small gas jets with an output of 5 N will be employed. These jets will be coupled about the center of mass for maximum performance.

The immediate goal of the Attitude and Control Group is to find a stable configuration which will permit the rotation of the ship to provide the artificial gravity. It is also desirable to spin-up the space ship as quickly as possible as well as to de-spin it as fast as possible for security reasons. On the other hand it is not desirable to have large instabilities when spinning up or down so that a compromise must be achieved for the locations of the thrusters.

The Configuration Specialist has modified the previous design in order to accommodate for a series of recommendations. The most important change is the housing of the fuel tanks next to the crew compartment (see attached drawings). This was done in order to provide a location where the tanks could be easily ejected and where the new tanks for the return trip could be placed. The new location has already been criticized by the Thermal Systems Group because the extreme low temperatures at which the liquid hydrogen must be maintained may affect the environment in the crew compartment. On the other hand, the placement of the tanks at this location helps in the stabilization of the spacecraft. New schemes would be sought in the near future in order to accommodate the desires of both, the Thermal Systems Specialist and the Attitude and Determination Specialist.

Following with the structural analysis of Odysseus II, this week, the Structures Specialist performed a preliminary stress analysis and material selection for the crew compartments and the major structural elements. All the crew related sections (the outer ring sections, the connecting tunnels and the center section) will be made of 7075T6 Aluminum which has proven application in aerospace applications. In order to procure enough stiffness to the structure, a series of trusses will be required (refer to the drawings). These will be made of 6Al 6V 2Sn Titanium alloy which was chosen for its strength and again, for its well-known usage in aerospace applications. Based on the choice of material and the known size of the structures and more refined weight estimate of the structure was possible. The total weight for the crew compartment sections amounts to 5700 kg.

Finally, a first sketch of the general shape of the Odysseus I and its components was provided. This can be seen in the accompanying drawings. It is important for stability and control purposes to keep the interplanetary trajectory configuration as an oblate object. A better effort must be done next week to achieve this goal. For the immediate future it is also necessary to keep working on the analysis of the rest of the structure, such as the titanium trusses.

The Power Systems Group has finalized the design of the power systems for both Odysseus I and II. Some changes have been implemented in the past week. The most important one is the removal of the nuclear reactors as power sources. Due to environmental and security reasons, the nuclear reactors have been dropped from the program. This has not affected the power supply very much since the power system had been overestimated before. Accordingly, the power system mix for Odysseus II is formed of a 30kW solar array, 10 RTGs providing 100kW, 70 batteries providing 100.8 kW and 7 fuel cells providing 84 kW for a total of 314.8 kW. A power conditioner system capable of 500 kW will also be included. All weights and necessary volumes have been calculated and will be furnished to the Structures and Propulsion Specialists so that a refined design can be achieved.

The Mars lander and the surface mission have also been allocated the necessary power. The lander will have available 66kW of power in the form of 3 RTGs and 3 fuel cells. On the other hand, the surface mission will have 122kW available when the lander is on the surface. This is achieved by way of the equipment in the lander plus a100 batteries and a solar panel which will be mounted on the top of the crews habitat.

A cost analysis of the entire power system for the entire mission has also been performed. The power system for the entire mission amounts to approximately $471 M which is divided in $361 M for Odysseus II, $108 M for the lander and $2 M for Odysseus I.

The Power system is thus complete and the only requirement is to adjust the amount of power available to the actual amount needed once the different groups finalize their actual power needs.

The Telecommunications Group has narrowed the selection of satellites to two options, the already known Astrid 2 and the new SCD-2. A total of six will be placed in a mars-synchronous orbit at 17033.27 km altitude. The placement of the satellites in such a high orbit will need to be analyzed more closely. As it was mentioned earlier, the parking orbit for the Odysseus II spacecraft will most likely be around 100 km. Therefore, an ejection system must accompany each satellite in order to place it at the corresponding altitude.

Data rates for the Odysseus II spacecraft have also been determined. A total of 256 kbps bandwidth has been provided for the spacecraft. 36 kbps will be used by the avionics, power, propulsion, telecommunications, and science systems and 10kbps will be allocated to frame, parity and other error systems. This leaves a total of 210 kbps for digital voice, video and other requirements.

The Thermal Systems Group has done a thermal analysis of the main inhabited sections of Odysseus II. The sections involved in the study are the ring habitats, the connecting booms, the central section and the lander. The three different flight regimes analyzed include orbit around Earth, orbit around Mars and outer space between Earth and Mars. In all of these cases a net heat input must be provided. The maximum heat input is while in orbit around Mars and it totals 340 kW. This heat input will be provided by the RTGs from the power system since those provide 130kW excess heat from the power generation process. Therefore, a total of 1300 kW is available at one given time from the RTGs. The rest of the ships sections: hydrogen tanks and cargo bay are still under investigation.

The biggest problem, as it was mentioned earlier, arises from the Venus swing-by. A major heat rejection will be required while the swing-by takes place. This is due to the high temperatures of Venus and the proximity to the sun at that point in the flight path. The heat rejection will be most likely achieved with a heat shield which will be deployed during the specific trip time that it is required. Currently, the entire division is working on the design for such a deployable heat shield. In order to aid in the rejection of heat it would be very helpful to make the fly-by behind the planet, so that Venus blocks the sun and the actual heat input to the spacecraft is reduced. This would only require one heat shield instead of the two needed if the spacecraft passed between the sun and Venus.

Based on the current status, next weeks goals are established as:

1.- Astrodynamics

• Search for alternate orbits which either avoid Venus altogether or at least have a Venus swing-by from behind.

2.- Propulsion

• Estimate the amount of Methane needed for the injection into Low Mars Orbit from the surface.
• Calculate the propulsion needs for putting the satellites in a mars-synchronous orbit from the 100 km. parking orbit.

3.- Attitude Control

• Perform calculations on the required attitude maneuvering and specify an attitude event list which includes what is the appropriate spacecraft attitude to perform the interplanetary burn, Mars capture, satellite deployment and Mars orbiting.

4.- Configuration

• Continue with the structural analysis of Odysseus II
• Redesign Odysseus I so that it is dynamically stable (oblate shape)

5.- Thermal Systems

• Calculate required heat rejection needed at Venus and the thermal calculations for the other spacecraft sections.
• Start an analysis on the thermal requirements for Odysseus I and the lander on the surface.

6.- Telecommunications

• Choose satellite for communications and advise of a launch method from the ship.
• Find out possible complications if a spinning spacecraft is used.

7.- Mars Production Facility

• Find out about the methane production rates and hydrogen consumption and production rates of the plant. Specify number of units and total mass of the system.