Claim
The Upward Acceleration Target of 17.3 G's for 6.8 Seconds is Reasonable
Evidence
The upward-acceleration target applies to the ramp that redirects the vehicle skyward after horizontal acceleration. Allowing up to ~17 g (normal to the trajectory) over a short, controlled interval (≈6.8 s) enables a smaller minimum curvature radius and therefore a shorter ramp, expanding siting options without altering the upstream acceleration profile. In practice, the vehicle's speed and the ramp's radius of curvature generate 160 m/s2 of centrifugal acceleration in addition to the 9.8 m/s2 of acceleration due to Earth's gravity, for a total of 169.8 m/s or 17.3 g.
The upward-acceleration target is a design input for sizing the ramp curvature and associated structures; it propagates through engineering and economic models into the cost estimate of the launch system. For baseline economics, we assume operations across 10 Mars transfer windows with crews of highly trained, physically fit astronauts adapted to Earth gravity.
In +Gx (“eyes-in”) orientation, the load vector is aligned front-to-back through the torso, which markedly increases human tolerance relative to +Gz. The crew rides in water-immersion acceleration couches that rotate to maintain +Gx during the pitch-up. Historical human-tolerance data with water immersion show sustained tolerance of 12 g for exposures of up to ~4 minutes; for much shorter exposures (seconds rather than minutes), higher peaks should be tolerable with appropriate countermeasures. (A representative chart from “Human Tolerance to Some of the Accelerations Anticipated in Space Flight” is shown in the ISDC 2025 presentation: Electromagnetic Launch — What is (and is not) Holding it Back.)
In a 1960s study titled "A new method of protection against the effects of acceleration on the cardiovascular system", three male subjects were exposed to peak accelerations—limited to the maximum each felt comfortable enduring—of 26, 28, and 31 g. The acceleration followed a 25-second versine profile, and the subjects were protected inside a “total water immersion G-capsule.”
An example of a possible g-force countermeasure is a respiration protocol wherein, just prior to the onset of high acceleration, an assisted-breathing system helps to deflate the lungs to promote a more homogeneous density distribution within the thoracic cavity, then immediately reinflates them at the end of the high-acceleration period.
The end of the acceleration section and the beginning of the ramp will be engineered so that over a time period of one second, the load will shift from ~8 g forward to ~17.3 g upward, and the crew couches will rotate ~83° to maintain +Gx (eyes-in) orientation. The bed rotation follows a smooth ease-in/ease-out profile.
The claim is not that every crewmember can tolerate 17.3 G's indefinitely, but that with state-of-the-art countermeasures, the short (≈6.8 s) upward-acceleration window lies within a reasonable, testable envelope for an astronaut-rated system. Further validation testing and certification would refine the exact limit used for crewed operations.
Reviews
The following reviews are limited in scope to the validity of the claim made above, and do not imply that the reviewer has taken a position regarding any other claim or the overall feasibility of a concept that is supported by this claim.
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Reputation: 0Verdict: ChallengesGraduate studies in physics and math at Michigan State University Honors College“16 G's is too high, and abrupt change from high horizontal to high vertical acceleration would be uncomfortable, dangerous, and probably unnecessary.”
An AI search on acceleration tolerance returned information that a passenger in an auto accident in a vehicle equipped with an airbag had a good chance of survival after 30 G deceleration. But that's for less than a second, and "a good chance of survival" isn't reassuring.
While it's critically important that the spacecraft exit the evacuated tube at as steep an angle as possible in order to minimize travel distance through the atmosphere, I see no reason that the vertical component of velocity has to be acquired in a short interval following the horizontal component. Couldn't the submerged section of the evacuated tube start at a surface station, head downward, and then gradually curve upward?
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“Countermeasures are essential and historical data show promise; comprehensive literature review and modern validation program needed”
From a medical perspective, the 17.3G upward acceleration proposal represents a significantly more challenging physiological environment than the horizontal phase, and the acknowledgment that state-of-the-art countermeasures are essential is appropriate and necessary. At this acceleration magnitude, even for 6.8 seconds, the human body experiences extreme stresses that absolutely require active protection beyond passive positioning alone. The cited 1960s water immersion study showing tolerance of 26-31G peaks is encouraging and demonstrates that the physiological limits can extend beyond the proposed 17.3G target when proper countermeasures are employed. However, it's important to note that these were experimental conditions with small subject pools, and the transition from tolerability to operational mission requirements represents a significant step requiring careful validation. The proposed countermeasures, particularly water immersion combined with innovative approaches like assisted respiration protocols to optimize thoracic density distribution, show promising mitigation strategies grounded in sound physiological principles. The concern about rapid cardiovascular load shifts, chest wall compression, and potential for g-induced loss of consciousness at these levels is real, but the historical literature suggests these risks can be managed with appropriate technology. What's critically needed now is a comprehensive systematic review of the existing acceleration tolerance literature, followed by a structured research program that incorporates modern medical monitoring capabilities, advanced materials for immersion systems, and contemporary understanding of cardiovascular physiology to validate and refine these countermeasures.
Submitted: - 1Reputation: 0Verdict: SupportsIndependent Researcher on the history of human acceleration tolerance in early space medicine.
“More research needed. There is enough to suggest that this may indeed be possible”
The study referenced, "A new method of protection against the effects of acceleration on the cardiovascular system" indicates that a test subject experienced 31g for 5 seconds with no adverse effects. The test subject experiencing health issues in the study was subjected to pressurized breathing which seems to be the likely cause of his adverse outcomes. The other two test subjects had no issues. The author recommended additional animal studies before resuming human studies because they recognized a host of potentially dangerous unknowns. I second this approach, but the 31g experienced with no adverse outcomes serves as a powerful proof of concept. More research is needed, however.
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