Frontier Transportation Systems - Abstract
Frontier Transportation Systems an integrated Mars transportation system


Matthew Kaplan, et. all


Spring 1992

Executive Overview

Frontier Transportation Systems (FTS) has designed an integrated transportation network to support an advanced Martian base. The following paper represents the completion of the SIMPSONS project (Systems Integration for Mars Planetary Surface Operations Networks).

Our project focuses solely on the surface-to-surface transportation at an advanced Martian base. Several elements, such as interplanetary transfer vehicles, orbiting nodes, and ascent/descent vehicles will be necessary for the sustenance of such a base. Any one of these components would be a significant project in itself; thus, they do not fall within the scope of our project.

Assumptions and Goals

FTS defined the SIMPSONS project with the following assumptions.

In order to precisely determine the exact goals of our transportation system, the supported base needed a clear definition. To this end, FTS researched the most likely arrangements and locations of an advanced Martian base and selected a specific configuration. The base after which we modeled our system will be located at Utopia Planitia (30°N,240°W). Its favorable proximity to possible mining locations will facilitate the transport of raw materials to the base. Also, this latitude aids the ascent/descent vehicles by minimizing the plane change required to reach the orbiting transportation node which is at an inclination of 25°. Furthermore, this region is the largest flat area on Mars, which makes spacecraft landings, long distance travel, and communications easier. Finally, radiation shielding provided by the Martian atmosphere is increased at this location due to its low altitude (-1 km).

The following operations will be required of this type of advanced base.

The base will accommodate a crew of 12 to 18 persons with the possibility of expansion. The main components of the base shown in Figure 2.1 include the centrally located habitat area, a manufacturing facility, two nuclear power plants, two landing pads, and a garage/maintenance facility.

Surface transportation is needed for travel between some of the more distant elements of the base, as well as for mobility of crew and payload from one area of the base to another. Scientific expeditions require the use of both manned and unmanned transportation systems to reach distant sites of interest. Likewise, outposts such as scientific stations or mining sites need maintenance and replenishment of supplies. This can be accomplished by surface rovers or rocket hoppers. Closer to the base, raw materials must be delivered to the manufacturing facility for the production of necessities, such as oxygen and fuel, for base sustenance and maintainability. All of this requires a flexible transportation system, capable of transferring heavy cargo on a regular basis, and of transporting cargo over distances farther than the confines of the base.

Ultimately, the transportation system selected for the Mars base should be compatible with all payloads and should be adaptable to meet many tasks, including those unforeseen. Along with vehicles for the transfer of non-pressurized cargo, pressurized vehicles will also be needed for long range excursions. A transportation system composed of a set of modular vehicles which fulfills the needs of an advanced Martian base is presented in the following report. These vehicles include an aerial tram, a heavy lifter, a rocket hopper, Martian aircraft, and several different rover designs. This executive summary outlines the purpose and design of each vehicle, as well as recommendations for future analyses. Aerial Tram

To support the mining operations of this base, it will be necessary to refine 216 MT of regolith per day. Upon analyzing the important aspects of a fixed route transportation system, FTS selected an aerial tram as the most efficient and economical mode of cargo delivery. The aerial tram is easy to construct, as it merely involves the setting up of two stations and intermediate trestles for support. In addition, the tram is easily automated, inexpensive to build and operate, and it requires little maintenance. The other fixed route types of transportation that were evaluated included trains, elevated rails, and magnetically levitated trains.

The aerial tram requires little mass to construct. The carriers are a simple automated design made of light-weight aluminum. The main mass to be concerned with, other than the payload chambers, is the weight of the hauling/carrying rope which will be made of a zinc-coated steel. The trestle mass should not be of concern, as the use of in-situ materials to form Martian concrete from which to construct these structures will eliminate the need to deliver the heaviest materials from Earth.

The tram we have designed will transport 36 MT per hour for the duration of only a quarter of a day. This will increase the lifetime of the system because it reduces the likelihood of fatigue and the opportunity for failure in general.

This system was a good choice from the perspectives of both the present and the future. From the present point of view, it can be constructed with the technology of today, and has proven to be a safe and reliable system on Earth. From a futuristic point of view, the aerial tram is an advantageous choice with regards to its expandability. First, the tram was designed to be strong enough to carry four times the amount it will actually be carrying. Thus it will be possible to increase its transport capacity in the future to support a larger crew. Secondly, the expansion of the base can be facilitated by first expanding the tram itself since it is possible to construct an additional route that is powered from the same driving station of an existing route. Finally, the tram may be used to efficiently transport humans in either pressurized or non-pressurized passenger cabins on future routes.

Heavy Lifting Vehicle

The lifter is designed to perform the loading and unloading processes within the base vicinity. This vehicle has to operate off of many platforms, ranging from the descent vehicle and the rover flatbed to the Martian surface. FTS will require that there be at least three lifting vehicles. One would be located at the landing pad, one at the base, and one extra should be present at any given time in case of mechanical failure. A crane design was chosen after evaluating forklifts and other lifting vehicles.

The crane must meet the following requirements.

In our evaluations, we looked for the least massive crane which still satisfied the original requirements. Therefore, FTS selected trusses as the main lifting component to reduce weight. The selected crane, shown in Figure 4.1, is a composite of many Earth lifting vehicles. As shown, the crane has a horizontal truss and a vertical truss structure similar to the tower configuration of lifting cranes. The horizontal truss moves along the vertical truss in a forklift type movement. In the back of the crane is a large container which is filled with indigenous material to act as a counterweight. The horizontal truss has a maximum extension of about 10 m which provides flexibility in reaching the payload, and the vertical truss has a height of about 13 m. The grasping mechanism which hooks onto the payload can vary in position along the horizontal truss.

The crane is supported by a tracked wheel, which enables the crane to carry the cargo from one place to another. The empty weight of the crane was computed to be no more than 30 MT. By adding regolith as a counterweight, the total weight could go up to as much as 100 MT. The trusses and the grasping mechanism will be made mainly of aluminum alloy materials, which provide lightweight and high strength characteristics.

The grasping mechanism has three degrees of freedom that can accommodate a maximum cargo width of 6 m. The lifter will get its power from a closed-cycle, internal combustion engine, using CH4 and LOX for fuel and oxidizer, respectively. The lifter needs about 422 kW of power to travel 2 km/hr with a mass of 100 MT. The hoisting of the cargo using the grasping mechanism needs about 40 kW for a hoist rate of about 0.4 m/s. Also, the power required to translate the horizontal truss at a rate of 0.3 m/s along the vertical truss with 30 MT attached is about 50 kW. The crane will be controlled telerobotically by an operator from a command module located either in the habitation module or near the landing pad, where the majority of the loading and unloading processes will occur. To aide in telerobotics, the crane will need various sensors to accomplish the following tasks.

Ballistic Martian Hopper

A ballistic rocket hopper provides a shorter transit time and a greater operating range. With a given payload of 6.5 MT, this vehicle can complete two missions: 1) carrying one autonomous rover with various scientific payloads, or 2) carrying a rover and a crew of two, with supplies for seven days. Our hopper can transport either payload to a site up to 1000 km from the base, where the small rover would then enable exploration within a 10 km radius around the landing site. Due to the fuel selection for our overall system (methane/oxygen), on-site refueling away from the base would not be feasible; the hopper is, thus, limited to one hop from and one hop to the base.

As can be seen in Figure 5.1.1, the cargo bay was placed at the vehicle’s center. The hover engines were then balanced around the bay in two equal, self-contained, and coordinated sets. This arrangement provides stability in firing and reduces shifting of the center of mass as fuel is consumed.

The trajectory of our craft was modeled by three phases -- launch, ascent, and touchdown. The launch is basically a hovering maneuver until the reaction control system (RCS) jets are fired to attain the proper attitude for the ascent phase. The main engines located at the rear of the vehicle are then fired for the ascent phase, sending the vehicle into a ballistic trajectory. When the hopper descends to 100 m altitude, a parachute is deployed.

The end result of our analysis consists of a partial load-bearing, functionally gradient material (FGM) skin supported by a graphite/magnesium interior structure. The skin is limited in its ability to bear loads by two main considerations. First, the engines of the undercarriage are recessed, and the ascent and descent thrust cannot be carried by the exterior skin. Also, there are several panels in the skin (clamshell doors, payload door) which would have to be carefully supported so as not to provide weak points in the structure.

Since the hopper requires the ability to operate autonomously and possibly telerobotically, its command and data handling system will have to be provided with information such as attitude, altitude, velocity and position, and surface mapping. The hopper will require an IMU capable of measuring the changes in attitude and position in three axes to fully define the state of the vehicle.

Unmanned Martian Aircraft

With the low martian gravity and despite the thin atmosphere, studies performed at the Jet Propulsion Laboratory in the late 1970's underlined that there were no technical difficulties involved in designing and operating a remotely piloted Mars airplane. It also appeared that such a vehicle could be most useful in increasing the capability of a Mars surface crew by providing for long range exploration and mapping.

The Mars airplane is well suited for long range scientific exploration, especially over rough terrain, and it can fulfill a wide range of missions such as surface imaging, atmospheric sounding, high altitude meteorology, and radio science. In addition, an unmanned Mars airplane can perform other useful functions such as deployment of remote observing stations , servicing of manned outposts, and search and rescue missions.

In order to enlarge the scope of operation, FTS evaluated both a large and a small aircraft. Because of the thin atmosphere and the need to keep aircraft dimensions and power requirements reasonable, the payloads of the aircraft need to be restricted. The large and small aircraft are restricted to 300 and 100 kg, respectively. The large aircraft has a range of over 12,000 km, and the small one has a range of 8000 km.

For both aircraft, a classical configuration was adopted since this configuration allows high lift-to-drag ratios and high stability. Moreover, the high tail volume can tolerate large shifts in the center of gravity resulting from payload deployment. Other features include an inverted V-tail to reduce mass as well as drag , high aspect ratio wings (22) in order to minimize the induced drag and large propellers for efficient high altitude flight in the thin martian atmosphere.

Because of the composition of the martian atmosphere (95% CO2), only non air-breathing engines combined with propellers can be used. Because of its high power/mass ratio, a closed loop, internal combustion engine (CH4/O2) was chosen. In order to cut the total weight, an all composite structure was chosen, composed of high strength Thoronel 300 carbon-fiber and epoxy composites. This allows for a structural weight fraction between 15 and 20 %.

In order to minimize the take-off distance and thus the runway length, both aircraft are supposed to use a short-take-off device such as a catapult. The landing distance will also be shortened by using slow-down devices, such as nets. The landing gear for both aircraft will be a simple skid very similar to those used on gliders.

Like the rocket hopper, the small aircraft must have the capability to select a suitable site to land and to perform the landing autonomously. Nevertheless, the capacity to be remotely piloted should be available as an emergency back-up or for complex maneuvers. The computer will navigate mainly by a terrain-following procedure, using medium and high resolution images provided by previous or current remote-sensing satellites. The very high resolution images needed for high-precision procedures (vertical landings, for example) would have to be provided by previous aircraft missions. The command and data handling system could also rely on ground-based beacons for navigation. In addition, the avionics also need aircraft attitude, attitude rates, position, and position rates for navigation.

Rovers

The seven rover configurations which were designed for the SIMPSONS Project are: 1) fuel transport vehicle (FTV), 2) manned, short-range vehicle (MSRV), 3) materials transport vehicle (MTV), 4) Mars autonomous rover for ground exploration (MARGE), 5) human-operated Mars exploration rover (HOMER), 6) light cargo vehicle (LCV), and 7) heavy cargo vehicle (HCV). The following table shows the range and payloads for each vehicle.

The FTV will refuel the lifters, aircraft, and rovers, and serve as a backup to the pipelines that provide fuel for the hopper at the launch pad. The MSRV can be used for transportation in the base area, or it can serve as a short range exploration vehicle when included as payload on the hopper. The MTV is designed as a backup system to the tram. Since the transport of mined materials to the refining facilities is essential to life support, it is very important that we do not allow this operation to have a single point failure. MARGE will conduct autonomous long range unmanned exploration. HOMER will serve as a mobile lab for long range manned missions. The light cargo vehicle is an autonomous/telerobotic rover whose main purpose is the transportation of light cargo around the base area. The HCV, which will be operated telerobotically, will transport payloads of up to 10 MT within 20 km of the base to aid in such operations as base expansion by moving habitation modules from the descent vehicle to the base.

All of the components of the rovers should be modular. The advantages of this concept are that the modular blocks can be used as spare parts on almost any vehicle, and that new configurations can be made in-situ to meet unforeseen needs of the base. The astronauts will be able to construct (with robotic aid) any new vehicle configurations within the maintenance facility. The modular components were designed to fit on both a large and a small basic chassis design.

FTS selected hemispherical wheels as the mobility system for both the large and small chassis. The chassis was designed to be constructed of two-celled monocoque aluminum alloy beams. We selected a cell thickness of 5.0 mm for Al 2014-T6 beams after performing a static analysis of several different thicknesses and materials using NASTRAN.

For the purpose of commonality, the main power system for all vehicles with the exception of the rocket hopper and the tram were designed to run on a methane/oxygen internal combustion engines. This commonality in power source will facilitate maintenance of the vehicles and will also simplify the production of fuel since a common fuel is utilized. In addition, a modular concept (coined “legobility”) designed for the rovers which employs the interfacing of various subsystem modules (blackboxes) to configure a task-oriented rover (i.e. an unmanned autonomous rover or a manned mobile habitation module) is presented. This concept facilitates maintenance and also introduces redundancy into the system since spare parts are more readily available when needed. All these vehicles, when working together, will provide the support required for the sustenance of the advanced Martian base and indirectly, will lead the way to the settlement of Mars.

Recommendations

Due to the time frame and scope for which this project was undertaken, further analyses of each vehicle and its subsystems should be performed. Although this project gives an overall design for each of the vehicles which will be included in the integrated Martian transportation system, future studies will be required to develop these vehicles beyond the preliminary design stage. For further design of the tram, we recommend an analysis for reliability, and we recommend further research into the feasibility of using indigenous materials for the construction of the trestles. For future studies of a Martian lifting vehicle, we recommend a more detailed structural analysis of the grasping mechanism and the analysis of truss stability. For the hopper, the following areas must be studied further in order to achieve a complete vehicle.

The Martian aircraft needs further analysis in its thermal system, state estimation, takeoff and landing and artificial intelligence for surface terrain following. A dynamic analysis is required for further studies of the rover, as well as a more in depth analysis of the engine performance characteristics.

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