11.1 Overview

11.1.1 Introduction

EVAS is primarily responsible for providing the ability for individual crew members to move around and conduct useful tasks outside the pressurized habitat. It includes any activity performed by a crewmember wearing the EVA suit or operating the large pressurized rover in the low-pressure Martian environment.

EVAs fall into three classes: (1) Scheduled EVAs are planned and included in the mission operations timeline. These EVAs will consist of constructing and maintaining the habitat and scientific exploration; (2) Unscheduled EVAs are planned but not included in the nominal schedule of mission operations activities and may be required to achieve optimal habitat performance and safety. These EVAs will consist of unexpected repairs to the outside of the habitat, unscheduled repairs to the rovers, and unforeseen scientific exploration; and (3) Contingency EVAs are unplanned but required to ensure the safety of the crew. These EVAs include evacuating the habitat due to rapid depressurization or fire, external repairs of any life critical ECLSS subsystem, repairs of vital subsystems such as the power reactor, and the search and rescue of a large pressurized rover that is having difficulties in a distant excursion.

The following components are integrated into the design of the habitat to provide the primary operational elements which make EVA possible for the crewmembers: an EVA suit designed to be used on the surface of Mars and be compatible with other EVA equipment, tools, and transport aids; an umbilical system to provide connections from the habitat to the airlock and rover systems; a large pressurized rover that allows the crew to safely explore distant sites; and an airlock providing the suited crewmembers with the ability to safely transition from the habitat pressure to the Martian surface.

Due to the scope of this project, designing the EVA suit and large pressurized rover was not the responsibility of the EVAS team. However, for these two systems the EVAS team was responsible for establishing the requirements driven by habitat operations and designing the interfaces between the rover/suit and the habitat. The bulk of the EVAS design includes the airlock layout and the design of its interfaces (including the umbilical system) driven by requirements of habitat operations. Derived requirements for the EVA suit and large pressurized rover requirements are listed in later sections. The airlock layout and interface design is presented at the end of the EVAS section.

11.1.2 General Responsibilities and Considerations

The design of the airlock, rover, and suit interfaces with the habitat need to ensure maximum crewmember performance and safety while operating these systems. In order to accomplish this, we need to incorporate any EVA-relevant human performance capability into the design of EVAS.

11.1.2.1 EVA Workload Considerations

Although habitat systems should be designed to help keep EVA activities to a minimum, periodical EVA activities will still be needed to: (1) ensure proper deployment and maintenance of the habitat and (2) enhance the productivity of local and distant scientific explorations. In order to minimize crew workload and maximize crew efficiency, the following workload-related factors shall be considered in the design of the EVA suit and rovers:

-Endurance of the individuals performing EVAs.

-Decreased performance due to the overloading or under loading of the EVA crew.

-The effects of de-conditioning due to reduced Martian gravity.

-Effects of adapting to a new and unfamiliar environment.

-Effects of inappropriate systems design.

-The scheduling of complex tasks in accordance to the crew’s level of fatigue.

11.1.2.2 Crew Safety

EVA crew safety shall be the paramount consideration in all EVA designs. All subsystems designed for EVAS shall incorporate safety requirements that ensure the safety of the crew and habitat. The following potential hazards shall be considered for the design of all EVA subsystems:

-Temperature extremes in which the suit and rover will operate.

-Radiation exposure for the duration of the EVA.

-Micrometeorite hazards.

-Uneven or rocky surface characteristics.

-Chemical and dust contamination.

-Equipment and support tools with sharp edges and protrusions.

-Elevated radiation exposure during EVA approaches to the nuclear reactor or radioisotopic power generators.

-Electric voltage shocks from EVA approaches to underground electric lines.

11.1.2.3 Hypoxia and Decompression Hazards

In the low-pressure environment of the Martian surface, the EVA suit pressure garment must maintain a minimum pressure of 3.1 psia (21.7 kPa) to protect the EVA crewmember from hypoxia. All habitat and rover suits are predicted to operate at 4.3 psia (29.6 kPa) and the habitat will normally be maintained at 10.2 psia (71.4-kPa). Due to the fact that the crew is acclimated to the 10.2 psia atmosphere, there is a critical physiological transition to the 4.3-psia (29.6-kPa) oxygen environment of the suit. If the transition is not performed properly, nitrogen bubbles could form in the EVA crewmember’s blood and lead to an incidence of decompression sickness (DCS) or the ‘bends’. In order to prevent the bends, the airlock transition depends on a 40-minute pure oxygen pre-breathe protocol to remove excessive inert gases from the body prior to exposure to the pressure of the suit.

11.1.2.4 Carbon Dioxide (CO2) Toxicity

Excessive CO2in the bloodstream can ultimately lead to serious physical disorders and loss of consciousness. If the EVA suit stops scrubbing CO2, a high level of CO2will be detected and EVA termination must be initiated.

11.1.3 EVAS Input/Output Diagram

The input and output diagram shown in Figure 11.1 illustrates how each habitat sub-systems interfaces with EVAS. As can be seen, most EVAS interfaces occur with the CREW and ECLSS. EVAS to CREW interfaces exist mostly to supply EVA crewmembers with vital consumables such as water and oxygen. In return, crew interacts with EVAS to transfer mainly waste to the EVAS buffer. EVAS interfaces with ECLSS to supply the airlock (and therefore EVA suit) and rover with life-supporting consumables, which are then supplied to the astronaut. The EVAS unit works as a buffer to supply EVA components and crewmembers with consumables originating from ECLSS. This diagram also illustrates the expected EVAS interfaces with the habitat power, communications, and thermal subsystems. The EVAS interface with the robotics and automation system is recommended to preclude major intervention from C3 in rover-to-rover and rover–to-suit communications. The EVAS interface with the Martian environment is modulated by the airlock contaminant control system by containing dust particles from entering the habitat. This diagram does not show how the EVAS integrates with the habitat architecture.

Figure 11.1: Input/Output Diagram for EVAS

11.2 Design and Assumptions

11.2.1 EVA Suit

The EVA suit is a key design driver for any Martian habitat. The suit must provide the following functional requirements: pressure shell, atmosphere and thermal control, communications, monitor and display, nourishment, and hygiene. Although designing the suit was not within the scope of this project, it was necessary to determine certain requirements that are driven by the operations of the habitat. These include:

  • Minimal mass
  • Minimal storage volume
  • Maximize mobility and dexterity
  • Maintain 4.3 lbs/in2 internal pressure
  • Regenerable, non-venting heat sink
  • Durable, reliable, and easy to maintain

Minimizing mass and volume will be important in order to meet the overall habitat requirements. Minimal mass as well as a high level of mobility and dexterity will allow astronauts to travel further and longer around the Martian surface without growing tired. The suit needs an internal pressure of 4.3 lbs/in2 in order to meet established pre-breathe protocols. If the current pre-breathe time was increased, the internal pressure of the suit could be lowered even further (3.7 psi) to increase the mobility and dexterity of the suit. One of the most important features will be the regenerable, non-venting heat sink. If the water used to cool the suit was not able to be recycled, the required amount of water transported from Earth would more than double. A durable suit decreases the mass of spare parts needed. This mass will also decrease if the parts are easily interchangeable (especially those that acquire the most wear).

The suit will have several interfaces with the habitat. It will receive non-potable water (used for cooling), oxygen, and power from the habitat. Water will be carried into the airlock via an “ankle pack” and urine will be carried out via a collection bag. The urine will then be added to the collection tank and processed. Food will also be carried in and out of the airlock as well as the waste collection garments and LiOH canisters used during an EVA.

It was decided that there would be two EVAs per week each involving at least two astronauts (approximately 32 hours of activity). The astronauts will have access to a total of thirteen suits over the 600 days. This provides seven backup suits that can be rotated to minimize wear. Each suit would therefore have to be able to withstand a minimum of 106 hours of EVA.

11.2.2 Umbilical System

An umbilical system, along with the hatch, will serve as the interface between the habitat and the airlock/rover. Figure 11.2 shows the consumables, power, and data that will be transferred to/from the habitat to the airlocks. The astronauts will carry any consumable that is going to/from the two hatches. Everything else that is required, for both short and long term EVAs, will be transferred though the umbilical system, including: data, power, water (potable, non-potable, and waste), air, N2, and O2. The original plan was to have an identical system for the rover and the airlocks; hence, a rover or any of the airlocks would be able to be “plugged” into any habitat external port. Once the amounts of consumables had been calculated and it had been decided how each of the consumables was going to be transferred, it was apparent that identical systems would not work. The final design was a specific external port for the rovers and three identical external ports for the airlocks.

Figure 11.2: Input/Output Diagram for the Rover and the Airlocks
11.2.3 EVAS Consumables

Table 11.1 shows all of the considered consumables for the EVA system. The numbers for the airlock/suit are for one EVA and the numbers for the rover are based on a 20-day mission. The EVA suit values were calculated per crewmember. The rover consumables were calculated for four (contingency), with EVA supporting consumables for two. These numbers also include a 10% safety margin. The suit consumables are based on current technology and will be drastically affected by the final suit design.

Table 11.1: EVAS Consumables
Airlock / EVA Suit / Rover
Oxygen (kg) / 0.96 / 0.63 / 136.7
Nitrogen (kg) / 0.98 / n/a / 28.5
Power / 5.6 kW / 26 A*h at 16.8 V dc / self powered
Cooling/Hygiene Water (kg) / n/a / 5.5 / TBD
Potable Water (kg) / n/a / 0.58 - 1.17 / 220
Food (kg) / will vary / will vary / 202.4
LiOH (kg) / TBD / 2.9 / TBD

The amount of oxygen and nitrogen needed for the initial pressurization of the airlock and rover were calculated based on the volume (35 m3 and 23 m3, respectively), pressure (10.2 psi), and temperature (23 C). For the habitat it was found that 9.8 kg of N2 and 9.6 kg of O2 are initially needed. The values found in Table 11.1 reflect how much of each gas is lost due to leakage and cycling the airlock (10% of the total air). The value for the rover also accounts for oxygen used for EVAs that will be done during the 20-day mission (will be designed to accommodate 16 hours of EVA per day). The oxygen needed for the suit is based on current EVA suits (2). The power for the airlock is for the pump that is used to extract the air from the airlock prior to an EVA as well as for the dust removal system (3). It is assumed that one pump will be needed for five minutes per EVA to evacuate the airlock. The power needed for the suit will depend on how long the suit needs to be recharged and cannot be determined until a final design has been reached. The water needed to cool the suit is based on current EVA suit (2). It was assumed that the suit will have a regenerable, non-venting heat sink so no more than 0.5 kg of water will be lost per EVA. The cooling/hygiene water needed for the rover will be determined once a rover design has been chosen. The potable water needed for the suit is based on how much water a human requires in four to eight hours (need 2.5 kg per day). The potable water needed for the rover was based on the assumption that each crew member requires 2.5 kg of water for drinking and hydrating the food per day and that the rover would not be equipped with the capability to treat and recycle any of this water (2). The food for the airlock/suit will vary depending the length of the EVA and can be determined case by case. For the rover, the food mass was determined based on the assumption that each crewmember would consume 2.3 kg of food per day. The mass of LiOH canisters is based on current EVA suits (2). The CO2 removal system for the airlock and rover still needs to be determined.

11.2.4 Large Pressurized Rover (LPR)

Distant excursions will be a major part of the exploration and science done by the crew on Mars. The LPR will be used for such missions (see Figure 11.3). The following are the requirements for the LPR that are driven by habitat operations:

  • Shall support EVA and payload activities
  • Nominal crew of two but must be able to carry four
  • Airlock of rover must be capable of surface access and direct connection with the habitat
  • Rover must consist of: driver station, workstation, hygiene facility, galley, and sleep facilities
  • Rover must support 16 person hours of EVA per day
  • Rover must have facilities for recharging portable LSS and minor repairs to EVA suits
  • Workstation must operate 2 mechanical arms from pressurized environment
  • Arms must be able to be operated from inside and outside the rover
  • Power trailer must be designed to support for other operations when the rover is dormant
  • 500 km range
  • 20 day maximum excursion time
  • Transit speed so that half of excursion time is used for travel
Figure 11.3: Conceptual Drawing of a LPR

The first cargo carrier will transport the LPR to the Martian surface. When the crew arrives, they will be using the LPR to explore the local vicinity around the habitat. When that is accomplished, seven distant excursions will be conducted during the astronauts’ 600 day stay on Mars (about once every two months). Figure 11.4 shows the timeline and protocol for the LPR. After the seven distant excursions are completed, the astronauts will set up the LPRs to be ready for use by the next crew.

Figure 11.4: Protocol and timeline for extended excursions

Figure 11.4: Protocol and Timeline for Extended Excursions

11.2.5 Airlock


The airlock that has been designed by the EVAS team is an independent element capable of being relocated or ‘plugged’, if the mission requires. The airlock will also be a solid shell as opposed to the inflatable airlocks that have been used in the past (see Figure 11.5). This was decided upon for durability issues and for ease of relocation during the mission. One of the major drivers for the EVAS team was to ensure that in case of fire or rapid depress, it would always be possible to get all astronauts out of the habitat. With this in mind, it was decided that there would be three airlocks, two operational on the bottom floor and one emergency/back up airlock on the second floor. The third airlock being placed on the second floor came out of the lack of area on the ends of the habitat, but it also ensures that astronauts on the second floor (living quarters) could easily escape the habitat. In addition to the emergency airlock, each airlock is equipped with three EVA suits, two operational and one emergency/backup suit. The emergency suit will be sized so that any of the six crewmembers can fit into it.

Figure 11.5: Conceptual Isometric View of Airlock

The dimensions of the airlock are 4 m long, 3.5 m wide, and 2.5 m high, with a total volume of 35 m3. The choice of this layout was to ensure enough room for donning, doffing, storing, and servicing the EVA suits (see Figure 11.6). Other considerations included: providing sufficient storage space for EVA and scientific equipment, offering a space for a scientific workstation, and to have a majority of the plumbing and wiring in one wall (the wall behind the EVA suit lockers). A four-stage pump system, similar to the one used in the ISS airlock, was chosen because of the similar volumes. This was a coincidental similarity, the volume of the ISS airlock was not considered during the design process.

Figure 11.6: Conceptual Floor Plan of Airlock

11.2.6 Dust Removal System

11.2.6.1 System Responsibility and Problem Description

In order to ensure that the astronauts are living in a clean and healthy environment while inside the habitat, any interfaces of the habitat with the Martian environment must limit the amount of dust particles entering the pressurized volume.