2 - Purdue University College of Engineering



2.0 Comparison to the Conceptual Design

2.1 Propulsion/Parachutes

In the Spring 2001 conceptual design the descent stage of the trajectory was controlled by a supersonic parachute, a subsonic parachute, and a retro rocket that was used to maneuver the vehicle to the desired landing spot. However this retro rocket added 3.5 tonnes of propellant to the system. To reduce the mass of the vehicle, the retro rocket was scrapped and instead a parafoil like that of the X-38 Parafoil Landing System was utilized. The parafoil added only 1.6 tonnes to the vehicle mass. This reduction in mass correlates with a reduction in cost of nearly 190 billion dollars (using the $100,000/kg correlation).

The propulsion system uses the same RCS engines and non-cryogenic hypergolic propellants but not the same retro engine. Project Marvin uses 1 Shuttle OMS engine to accomplish the en-route maneuvering and vehicle spin-up. Project Marvin also landed without doing any apogee burns to raise or lower the periapsis to control aerobraking. Since 20 m/s of (v was saved from not performing any apogee burns, 460 kg of propellant was not needed in our vehicle which again saved nearly 4.6 billion dollars.

Overall the parachute and propulsion system of Project Marvin greatly reduces the cost of the project. Further analysis should be done on the parafoil stage of the descent. How much of a horizontal maneuvering range is needed should be studied. If it turns out that the vehicle needs to be maneuvered over a larger range (greater than 5 km) so that the vehicle can reach its predetermined landing point, a different parachute configuration may be needed.

2.2 Aerobraking Trajectory

In the Spring 2001 trajectory scheme, the design was to use aerobraking to slow the vehicle down to land by slightly lowering the periapsis altitude through each pass. However, in comparison, the optimized design does not use aerobraking. It uses controllers in g-loading, bank angle, and angle of attack to cause the spacecraft to land on its first pass. Using these controllers the vehicle is able to land without exceeding the limit of 6 g’s and is well within the heating limits. If aerobraking were to be used, an additional propulsive mass of 223 kg would be added to the overall mass, costing more for the spacecraft to be launched. This vehicle uses the range of flight path angle designated by the Spring 2001 class and the initial velocity the spacecraft approaches the Martian planet. This design trajectory minimizes cost and the amount of time the astronauts spend in orbit and is safer.

2.3 Aerothermodynamics

The main difference between the analysis of this semester’s vehicle and last semester’s vehicle is that last semester they only analyzed the lift and drag at hypersonic conditions using Newtonian flow. This semester the vehicle was analyzed using both hypersonic and supersonic methods as well as viscous interaction effects. Also, the heating analysis last semester was only analyzed at the stagnation points. Last semester it was assumed that the stagnation points had the highest heating rates and the TPS was designed only using those rates. This semester more points on the vehicle were analyzed as well as using different equations for the different types of heating rates was used. Overall, this semester the vehicle was looked at in much more detail, with emphasis on the different flight regimes the vehicle would be flying in.

2.4 Thermal Protection System

The TPS for the previous iteration mars lander (the HAB) required a total mass of 17,069 kg. Marvin requires 825 kg of TPS to keep all materials within acceptable temperature limits. This decrease in mass is mainly attributed to an accurate method of computing temperatures as a function of wall depth and time (see section 4.3). The HAB TPS was modeled using a lumped heating analysis where temperature is considered uniform throughout the design space. The current method allows for high temperatures on the surface while keeping the structure materials cool with less TPS mass. Also, data for a greater selection of materials was gathered, allowing for lower density materials to be applied where possible. Provided accurate heating rates, the TPS mass estimation for Marvin is more accurate than the HAB’s because more points on the vehicle were analyzed. The HAB analysis only incorporates stagnation point heating, whereas Marvin’s TPS was designed based on heating at eight selected points on the vehicle. These additional modifications to the TPS design have led to a significant decrease in required mass while increasing the accuracy of the results.

2.5 Structures

The table below compares the HAB from last semester and this semester.

|Dia [m] |Length [m] |# floors |Mass [kg] |tface plates [mm] |tcore [cm] | |Last semester |8.75 |13.625 |5 |8,500 |2 |3 | |This semester |9 |14.5 |5 |8,200 |2 shell

others vary |3 shell

others vary | |Table 2.1: Values for HAB from last semester and this semester.

A larger HAB was used to increase the reference area to help the spacecraft capture.

Another difference between this semester and last semester was that the hoop stress equation used last semester was wrong. It takes into account the thickness of the core. This made the Hoop stress last semester much less than it should have been. A FEA was done on the vehicle for the launch loads. No FEA was done last semester. An analysis was done on the landing gear last semester, but not this semester. The thicknesses of the floors varied this semester also and a pillar and rings were added.

2.6 Systems

The HAB layout for Project PERForM was a very good initial design, which I borrowed from heavily. The main difference between our design and PERForM’s is the addition of a permanent hemisphere at the top of our vehicle. This differs from the cone of PERForM which broke away. The hemisphere added much needed storage volume and allowed greater center of gravity manipulation. The order of the levels was also changed. Floors one, two, and three are now the sleeping, kitchen, and ejection levels respectively. This was done for a relatively easy center of gravity manipulation. The inner diameter was also changed from 8.74 m to 8.8 m. However, this change is minor.

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