Russian's Spacecrafts Soyuz TMA-03M & Progress M-13M docked at the International Space Station, ISS. Credit: NASA



Smoke and fire on board a module has been always a serious concern for long deep space missions. The International Space Station (ISS) has experienced small asteroid strike or collision with another spacecraft during docking or undocking operations since many years.
We can say that, in the ISS, many risks are mitigated by extensive procedures and experience of ground operations and astronaut crews, as well as the availability of multiple modules and return vehicles. 

For long duration missions beyond LEO, as well as Mars transit missions of about 1100 days, a quick return will not be possible. For that, the mass penalty for multiple volumes and operating has been always an important concern for mission concepts.

In 2016, a study has been done to investigate a variety of dual pressure vessel configurations for habitats that could protect the crew from these hazards. It was found that with a modest increase in total mass it should be possible to provide significant protection for the crew.
To help resolving these issues, the Safe Haven Concept determine the mass impact for providing a second pressure vessel.
Several configurations were considered that either had a small safe haven to provide 30-days to recover, or a full duration in using two equal size pressure vessel volumes.


EMC Mars Transit Habitat

The starting point in this study came from the Mars Transit Habitat baselined in the Advanced Exploration Systems, Evolvable Mars Campaign (EMC) as shown below, Configuration 1a (C1a).
The primary mass drivers are in structures, avionics, and the life support system, with about 3000 kg for a 30-day and 5000 kg to 6000 kg for a full duration.
The advanced life support system planned for the 2030s included in the EMCs C1a is about 1000 kg lower in mass, which would bring the total impact of the safe haven mass down to only 5,000 kg.
The C1a represents the standard single volume monolithic habitat planned for Mars missions in the 2030s, which includes an advanced closed loop life support system designed to support 4 crew for 1100 days.

The Configuration 1b (C1b) is the same concept, but uses current technologies required a little more volume for stowage. Also, they use structures and end domes based on current SLS manufacturing standards, and current life support systems from the ISS.

The risk of a collision during deep space missions could yield disastrous results.

All of these configurations appear to package in the standard shroud currently planned for SLS payload flights. It should be noted however, that growth in the habitat length will place limitations on growth of the hybrid propulsion stage.

With the use of the unique EUS manufacturing capabilities, the C4 overall length grew of about 6.3m beyond the C1a, which precluded packaging with the hybrid propulsion stage. In add, part of the habitat could be extended into the nose cone volume.

The C4 and C5 use a small built in propulsion system to complete the habitat’s transfer to cis-lunar space where it is then assembled to a chemical stage for Mars transfer missions.

Note that, a section of the habitat is the same diameter as the core stage and will be exposed to aero loads during launch. So, this section will require aero shell panels to protect it. But, it will be possible to overcome this problem if we use the 10m diameter payload fairing to completely encapsulate this habitat configuration.

Again, if the C5 is compared with C1a, we have about 5.7m longer, which also precluded use of the hybrid propulsion system. So, like C4, a separate propulsion element is planned.

The internal avionics, primarily communications and data handling equipment, is duplicated to provide a full system inside each pressure vessel. The highest risk for pressure loss would be from collision during docking operations when communications, data handling, and vehicle controls would be critical. In total, the additional avionics mass is to be around 742 kg.

The interior layout is arranged in a vertical orientation on two deck levels. The lower deck includes all of the crew systems for research, vehicle controls, galley, exercise, and waste management. The life support systems are located below the floor in the lower end dome.
Through a large opening in the center of the module, crew translate from the lower deck to the upper deck, which includes the four crew quarters with stowage packed in between and around the quarters. This disposition further enhances radiation protection for the crew.

The Common Bulkhead Habitat

The Configuration 2 (C2), showing below, has the same exterior appearance than the C1b. Between the upper and lower decks, we have an internal bulkhead and an Intra-Vehicular Activity (IVA) airlock. This latter link creates a nice safe haven with an duplication of a closed loop life support system that provides for full duration capability on each sides.

In fact, the crew systems are split between the two deck levels to minimize loss if evacuation from one side is required. Each level includes two crew quarters and about half the stowage, a split in the crew systems functions, and a complete life support system packaged in the decks over the dome and bulkhead for each level.
Because the end dome needs to support pressure loads from either side, given tension or compression load, the internal bulk is nearly three times more massive than the standard one.  This is why we see some difference between a pressure vessel with an internal bulkhead and two separate pressure vessels.

Vehicle Configurations 1 and 2.
The C1 is the simplest with a monolithic habitat. For C2,  the same habitat is used with an interior bulkhead integrated to a hybrid propulsion system for Mars transfer missions.
The bulkhead is a more compact solution but requires an additional airlock for Extra-Vehicular Activity (EVA) access outside the habitat. However, the dual pressure vessel solution offers simpler manufacturing, possible use of the IVA airlock for EVA access too, but is much longer and less compact. Then, the additional structures mass is around 1700 kg for both bulkhead and dual pressure vessel options.
In the event of pressure loss in one side, duplicate life support and avionics are provided on each side to sustain life for the full duration and provide communications and vehicle control systems on each side. In concept, the IVA airlock provides passage through the bulkhead for transfer of stowage and equipment needed for the duration of the flight. Conveniences like research equipment, hygiene, and exercise, might be lost or downgraded in one side or the other, but with access to the unpressurized volume it is believed that operational workarounds can be found to resolve life sustaining issues.
It is possible to make some repairs inside and outside the habitat. The interior uses a modular pallet system that can tilt up from the floors above the domes and away from the walls. An external inflatable airlock is also available at one of the radial hatches, to provide EVA crew access to the entire pressure shell for repair operations.
When C1b and 2 are compared, the increase in mass for a full duration safe haven can be found. It came to 5,969 kg, which included 1700 kg for the additional structure required for the internal bulkhead and airlock, 742 kg for the duplicate avionics required for each volume, and 3,527 kg for the duplicate life support system. So, for around 6,000 kg it seems reasonable to assume that a full duration can be provided in most Mars transit habitat designs that will protect the crew from smoke, fire, and pressure loss for the duration of their mission.

The Dual Pressure Vessel Habitat

Configuration 3 uses dual pressure vessels to provide a full duration safe haven integrated to a hybrid propulsion system for Mars transfer missions.

The Configuration 3 is the same than C2, but uses two pressure vessels of equal volume. When both are compared it was found that, C3 is about 300 kg more massive, all attributable to the additional structures mass.

The Exploration Upper Stage (EUS) Derived Habitat

The Configuration 4 (C4) shown below is a new concept utilizing the pressure vessel volumes planned for the Exploration Upper Stage (EUS). It yields a convenient large volume habitat with a closed loop system paired with a smaller volume using a 30-day open loop system.

The C4 uses a large pressure vessel (Propellant tank) as the primary volume and a second smaller pressure vessel as a 30-day safe haven. This configuration uses the EUS that is attached to chemical propulsion stages for Mars transfer missions.

The idea of this concept is the use of any attached pressure vessel for logistics could be utilized as a temporary safe haven. The condition is to have an airlock that permits Intra-Vehicular Activity (IVA) transfers between volumes.
Here, we have a much lower mass for the open life support system of about 629 kg, if we compare structures and avionics mass for the full duration habitat. For the 30-day safe haven, the additional mass is about 3,071 kg.
The mean risk for the full duration safe haven is that the crew would have only 30-days to make repairs to the primary volume.
If we use only the basic EUS design (without propellant loads), that is simpler for manufacturing and, optimized for habitat, the pressure vessel mass is reduced of about 2900 kg.
Also, the volume available between the two pressure vessels makes it possible to build in an airlock that can be utilized for both IVA transfers and EVA access.

The Dual Pressure Vessel Habitat

The Configuration 5 (C5) give the same volume than the C4, which includes two equal pressure vessels and a duplicate closed loop life support system for full duration in either volume.

The C5 uses dual pressure vessel to provide a full duration safe haven in each volume. It is attached to chemical propulsion stages for Mars transfer missions.
When C4 and C5 are compared, it shown that the C5’s mass is about 1500 kg more, indicating that further optimization of the EUS structural design in C4 may be possible.  Theoretically, the difference should only be the additional 3000 kg required for the duplicate life support system.

Two life support systems is proposed: one for the 30-day and, another for the full duration.

The 30-day open loop system has an overall mass of about 629 kg for hardware and consumables to support a crew of four.

The full duration included a duplicate closed loop life support systems that give  an overall mass of about 3,527 kg, based on current ISS technologies.

These technologies are used because they have a known reliability for long term use. So, planning for maintenance, repair, sparing of components, supplying of filters and consumables is well  understood.

Below image show the ACLS that will be installed on the American Destiny module on the ISS

Airbus has delivered the ACLS (Advanced Closed Loop System), an advanced life support system to purify air and produce oxygen for the International Space Station. The system also produces water, more or less as a by‑product of the technology. ACLS was developed by Airbus for the European Space Agency (ESA) and is set to be used as a technology demonstrator on the Station from summer 2018.

The ACLS extracts a portion of the carbon dioxide in the cabin atmosphere and, using hydrogen obtained from splitting water molecules, converts it to methane and water in what is known as the Sabatier process. Oxygen is then produced from this water using electrolysis. This increases overall system efficiency and reduces the need for supplies from Earth.

Credit: M. Pikelj / AIRBUS.

In the goal to prevent loss of mission and/or loss of life, a detailed concept of operations is needed to examine a variety of scenarios of possible uses.

Based on the ISS experience, windows as well as cameras provides external views of the habitat and approaching vehicles.

The EMC Mars transit habitats represented by C1 through 3 include view ports in each of the docking port hatches, or 5 windows when counting the forward hatch, three radial hatches, and a standard Window Observational Unit in the 4th radial port position. The remaining views were planned to be covered by high definition cameras and internal screens.

The C4 and 5 do not include radial ports. So, one Window Observational Unit is provided in a radial position and a forward and aft view port in each docking hatch were included.

ISS's Cupola

The Earth view from the cupola onboard the International Space Station. 

Credit: Scott Kelly / NASA / May 14, 2015

C1 uses a vertical layout on two deck levels with open circulation between levels similar to the way Skylab was designed. Large crew quarters are provided in a horizontal orientation for the bunk area with the intent of applying this design towards surface habitats and artificial-g configurations.

C2 and 3 are the same, but the open space is restricted by the addition of a bulkhead/dome and an IVA airlock.

C4 and 5 use a horizontal layout on three deck levels with vertical circulation through each end dome. The layout is intended for in-space use only so the crew quarters are in a vertical orientation with sleeping bags on the walls as done in the ISS crew quarters.

In general, the vertical layout appears to provide more open public space for the small volume provided. And, the horizontal layouts appear to provide more private spaces and improved packaging efficiency.

Habitat C1 through 3 were pre-integrated with a large hybrid propulsion system for the initial transfer to cis-lunar space. The hybrid system includes a large Solar Electric Propulsion (SEP) system, so some avionics and most of the power systems are on the propulsion element.

Once in cislunar space, the habitat is loaded with logistics and the hybrid propulsion system is re-fueled in preparation for the crew’s journey to Mars.

The greater length of C4 and 5 precluded the pre-integration of the hybrid system so an alternative chemical system was utilized. It includes a small integrated propulsion system for the initial transfer from Trans-Lunar Injection (TLI) to cis-lunar space where it is then loaded with logistics and berthed to chemical propulsion stages for the transfer to Mars.

In Mars orbit the habitat berths with another set of propulsion stages that have been pre-deployed for the return trip. All avionics and power systems are integrated as part of the habitat element.

Internal Layout of Skylab

The power requirements for all configurations were about the same since they all had similar requirements to support 4 crew for 1100 days.

Habitat C1 through 3 draw power from the solar arrays provided by the SEP system on the hybrid propulsion stage. The C4 and 5 utilize a set of deployable solar arrays sized for the habitat power requirements in Mars orbit.

ISS is Powered by Solar Arrays

The thermal requirements for all configurations were also about the same since they had similar thermal loads. Each utilized both passive and active systems with body mounted radiators.

ISS show its Heat Rejection System radiators (light color) Credit: NASA

A noted benefit for the full duration safe haven is that a second waste management compartment is also included.

Long-term stowage of food is a concern in the radiation environment of space and may require greater utilization of refrigeration and freezer units than currently planned.

All configurations were found to have limited packaging volume except C4. C1 through 3 and 5 required a higher packaging density than currently used on ISS.

Credit: David Smitherman* Tara Polsgrove†, and Justin Rowe‡ NASA Marshall Space Flight Center, Huntsville, AL, 35812 and, Matthew Simon, PhD§ NASA Langley Research Center, Hampton, VA, 23681