Oxygen Life Support Systems in a Spacecraft

Introduction

Looking into the very near future, tourists traveling into space no longer seems like some fantastic science fiction. The Blue Origin and the Mojave Aerospace Ventures companies are ready to operate their respective manned suborbital spacecrafts in the coming year[1]. While, The Boeing Company and the SpaceX are finishing the certification of their crewed spacecrafts to deliver people at the Low Earth Orbit. This is only the tip of the iceberg in the great competition.

The next ambitious goal of the space industry is to create space hotels (see Figure 1). For example, NASA already has announced opening the ISS for tourists. These objects are long term human habitations and as such have specific requirements for oxygen life support systems (OLSS). If these requirements are not met, people can die. Small variations in the chemical composition of a mixture of the gases all influenced by, pressure, temperature, a humidity and etc.[2]  can have disastrous effects. The work of some of these partial system can be analyzed and optimized using AxSTREAM NET™.

Figure 1 - The art image of the Aurora Space Hotel
Figure 1 – Art Image of the Aurora Space Hotel[3]
Types of life support system of a spacecrafts

The type and complexity of OLSS depends on the duration of the tourists staying in the artificial environment. For example, let’s consider the oxygen life support systems. A hypothetical manned spacecraft has an internal volume 15 m3 (530 ft3) and can carry six space tourists. The amount of the oxygen for the metabolism of one person is 0.830 kg/day[4] (Figure 2). The atmosphere should consists of 19.5 to 23.5 % of an oxygen[5]. Also, the amount of the reserve oxygen should be 0.035 kg (0.077 pounds) per human/hour. If our six space tourists start their journey with the environment gas in the craft at 23.5 % of the oxygen , it will take 3.5 hours to reach critical level. It’s enough time for a suborbital flight, and the oxygen life support system would only be needed as a reserve source.

For a flight any longer, an oxygen life support system would be necessary[7]. Currently, these circuit type systems can be opened, partially closed, or fully closed.

Open Type OLSS

The open circuit oxygen life support system consists of storage tanks onboard. In this type, the oxygen is delivered through a pipeline and a pump to a cabin to provide the necessary concentration. Spacecrafts with journeys taking less than one week usually used this type of life support system. The Apollo spacecrafts also used this type of the open circuit OLSS (see Figure 2). This type of OLSS can be analyzed easily with AxSTREAM NET™.

Figure 2 - Cryogenic Oxygen Storage System
Figure 2 – Cryogenic Oxygen Storage System[8]
Partially Closed Type OLSS

Needing to carry the store of the oxygen is the main disadvantage of this OLSS type. For example, one-way flight to the Mars one astronaut would need 100 kg of oxygen. A partially closed and fully closed circuit OLSS reduces the need to store large amounts of oxygen.

In a partial closed circuit, the required oxygen is generated from wastewater. In this case, circuit is closed to a direct water circuit, and the oxygen reservoir is only sized as a backup system. Only the water reserve is need in this partially closed system.

Fully Closed Type OLSS

A fully closed system is based on recovery of exhaled carbon dioxide. In the future, scientists and engineers aim to fully recycle the oxygen circuit on the spacecraft for the interpenetrate missions [10] (Figure 3). This type of future OLSS also can be analyzed with AxSTREAM NET™.

Figure 3 – Recycling Water and Air on the Spacecraft[9]
AxSTREAM NET™ Simulation

For the example an analysis of the closed loop OLSS of a spacecraft  was selected. In this example, there would be several separated branches with different fluids such as oxygen, water, carbon dioxide and methane. AxSTREAM NET™ can simulate and analyze all of these streams. For simplification, let consider only the oxygen and water branches.

To simulate the work of the system, the thermal-fluid model was designed in the AxSTREAM NET™ (see Figure 4). To simulate non-typical parts, the Local Fluid Resistance can be used. The hydraulic resistant for this element can be calculated inside the software with a user’s script. The unity of the separate branches is provided by additional user’s parameters and scripts. The example of a script which associates the mass flow rates in the oxygen and water branch is shown in the Figure 4.

Figure 4 - Thermal-Fluid Model of the Recycling Water and Air
Figure 4 – Thermal-Fluid Model of the Recycling Water and Air

Operation conditions of various oxygen life support systems for a spacecraft can be analyzed by a simulation in  AxSTREAM NET™. The results of the calculation can be used for selecting geometrical parameters of the pipelines or determining a mass flow rates at a given pressure difference at a pump. The thermal-fluid model can be used to design the pump at the requirement  conditions by a connecting AxSTREAM®, AxSTREAM NET™ and AxSTREAM ION™. Additionally, the thermal-fluid model can be upgraded to account for heat transfer between the walls of a hydraulic channels and the environment.

References
  1. Charlotte Kiang, The 2019 Guide To Space Travel,forbes.com, 25 Nov 2019
  2. NASA Opens International Space Station to New Commercial Opportunities, Private Astronauts, nasa.gov, 7 Jun 2019
  3. See plans for the first luxury hotel in space, cnn.com, 25 Nov 2019
  4. Advanced Technology for Human Support in Space, nap.edu
  5. Environmental Control and Life Support Systems for Flight Crew and Space Flight Participants in Suborbital Space Flight, FAA.
  6. Environmental Control and Life-support Subsystem (ECLSS), FAA.
  7. Soyuz breaks speed record to ISS, com
  8. Apollo 14 Mission Report,hq.nasa.gov, 2 Dec 2019
  9. Closing the Loop: Recycling Water and Air in Space, nasa.gov, 25 Nov 2019
  10. Kathryn Hurlbert, Draft Human Health, Life Support and Habitation Systems Technology Area 06, NASA