Understanding Rapid Depressurization in Hyperloop Systems

Research on the feasibility of the Hyperloop Transport System (HTS) has been ongoing since its conception. There are countless financial, political, and physical obstacles standing in the way of a reality where the “fifth mode of transportation” can safely and sustainably operate at scale. A central concern, at least for the focus of the depressurization team, revolves around a critical question encompassed by these obstacles: “In the event of rapid depressurization (i.e., if an orifice is found inside an HTS pod, exposing the pressurized station to the vacuum of the tube), how can we safely protect the passengers in the pod?

To even begin answering this question, we must start at square one. Although it seems morbid, we must ask ourselves if it is even necessary to delve deeper into this topic to avoid an unnecessary waste of time and resources. To initiate a motivation for this topic, we must ask: “What happens if we don’t explore it?”. Therefore, let’s explore the effects of rapid depressurization on humans themselves.

The paper of interest this week goes back many decades to the late 60s. “Rapid (Explosive) Decompression Emergencies In Pressure-Suited Subjects” by Emanuel M. Roth explores exactly what the title suggests: the effects of a rapid depressurization event on the human body. For the sake of brevity, we are limiting our summary of this paper to section III, which specifically looks at the lungs, with a couple of explanations from section I to help provide context.

Section III of Roth’s paper, titled Pathological Physiology of Explosive Decompression to a Vacuum goes into excruciating biological detail about the sequence of mechanical and physiological stresses inflicted on the lungs. The play-by-play process can be divided into three phases. In phase 1, defined by Roth as the isometric phase, there is no volume change in the lungs. This will last a little longer than 10 ms in humans as the effects of depressurization begin. During the second phase, or the attenuation phase, the chest expands and gas escapes. Disruptions of the tissues can occur as their tensile strength is exceeded. This could lead to pulmonary hemorrhage and edema, which describes the excess of fluid in the lungs due to bleeding. It could also lead to pneumomediastinum or pneumothorax, or the abnormal presence of air or gas in the peritoneal cavity (the space within the abdomen that contains most of the abdominal organs). During the third phase of maximal lung expansion, the lungs return to an isometric state as lung volume decreases. This is when the penetration of bubbles into the bloodstream takes place as gas emboli enter the bloodstream and pass to the arterial circulation.

   The next four subheadings go in-depth into the effects of the process described in the previous paragraph on the rest of the body. The first describes the contusion and disruption of the lung parenchyma. When the pressure gradient exceeds the tensile strength of alveolar tissue, the lung parenchyma can tear, leading to hemorrhage and edema that would damage gas exchange, as seen in phase 2 from before. In the section Pneumomediastinum and Pneumothorax, as described before, Roth describes air being forced out of ruptured lung tissue into the pleural cavity, causing chest pain. The third section explains aeromboli, which is when gas bubbles enter torn pulmonary veins and reach the arteries. This could obstruct blood flow and trigger ischemic (of lacking blood flow) injury to the brain or heart. This transitions into the final section: Ebullism. At very low pressures, especially at elevations above a limit known as Armstrong’s line, body fluids may boil at normal body temperature, causing skin swelling and mucosal blistering.

   Obviously, the consequences of a rapid decompression event are not to be taken lightly. The sheer horror and, for lack of a better word, gore that results from such a quick and violent event establishes a clear motivation for our work as a research team: to mitigate the effects of such an unforgiving monstrosity. Finding solutions to this problem would open up a huge discussion on the safety of the HTS for passengers. Ultimately, addressing these challenges is essential if the hyperloop is ever to move from a theoretical concept to a safe, practical reality.