Mars Entry

By launching from Earth with a few kilometers per second more velocity, the velocity leaving the Earth-Moon system can be greatly increased, and the transit time from Earth to Mars reduced from a 240 day Hohmann to a 90 day oblique trajectory. This costs extra boost, but the reduced travel time reduces consumables, shielding, and zero-gee atrophy for crewed missions.

However, the entry velocity at Mars will increase from around 7 km/s to 12 km/s, faster than Apollo 16's return from the Moon. In order to remain within the narrow band of atmosphere where the radial acceleration is "just right", the lift must be adjusted by vehicle pitch angle, and the drag will vary proportionally, because the lift to drag ratio of the Apollo command module was about 0.3. The equatorial radius of the Earth is 6371 km, while the radius of Mars is 3396 km, so the radial acceleration to stay in a circular trajectory ( v²/r ) increases inversely proportional to radius, a factor of 1.88 .

That neglects gravity and density altitude; Apollo peak entry acceleration occured at 80 km (?) altitude (Earth's atmosphere is denser), Mars peak entry acceleration might occur at 40 km altitude (like Mars Pathfinder). A more accurate computation of radial acceleration at 12 km/s:

The maximum gee force on Apollo 16 was actually 7.19 gees, not 3.3 gees, so there is a scaling error factor of 2.2, perhaps due to buffeting, path corrections, and gremlins. If the 2.2 error factor was applied to FAST Mars entry, the gee forces would be 30 gees. The ratio of Mars FAST to Apollo acceleration in the table is 4.1 times.

The maximum heating rate for Apollo 16 was 346 BTU/ft²-sec, or 3.9 MW/m² (!), with a total heat load of 27939 BTU/ft² or 317 MJ/m². While the chemistry of ionization of CO₂ will be different from N₂+O₂, let's presume the heat load is similar. However, the shorter entry time for Mars corresponds to a higher heating rate, about 16 MW/m², assuming the same vehicle mass to area ratio.

Parachutes: Martian atmospheric pressure is 0.00628 times Earth's (less dense), Martian gravity is 0.376 gees (more dense), and Martian scale height is higher (11 km vs 7 km, less dense). The Earth's atmosphere is about 1.2 kg/m³, Mars is about 0.02 kg/m³, 60 times less dense. For the same terminal velocity, parachutes must be 0.376×60 or 22 times larger. Assume that we jettison the red-hot heat shield to reduce the weight on the parachutes and the heat load on the capsule, and that airbags can deploy on landing, to emulate the final ocean splash of the Apollo capsule.

Then there is the return journey. If that is also 90 days (for the same logistical and atrophy reduction reasons), then it will arrive at Earth with the same increase-over-escape-velocity as the launch towards Mars. If Earth reentry is at 14 km/s rather than 12 km/s, the gee forces and heating rate increase to 1.44 times Apollo 16. Not as rigorous as Mars entry was, but consider that the astronauts have endured a total of 6 months in zero gee, and the many more months on Mars at 0.376 gee, with bone loss throughout the mission.

Concerns:

• 1 The quadrupled gee loads at Mars (relative to Apollo) may injure astronauts who have suffered zero-gee bone loss for 3 months.
  • Total mission-time atrophy (almost two years to next opposition) may be fatal at Earth FAST reentry.
• 2 The higher Mars entry heating rate may push engineering limits.
• 3 The higher required structural strength and parachute mass for Mars entry will seriously reduce payload fraction.

Land animals are not fish in "air suits", spacelife will be different than landlife as well. We are adapted to our environment, not weightlessness, radiation, and long confinement. The "humans" who travel to Mars may be technologically adapted homo sapiens, perhaps with many small implants. More likely, genetically engineered to live permanently in space. One-way machines may be the closest approximation to spacelife for foreseeable future.

References:

• GDN personal email

• Apollo By The Numbers

  • p56 Apollo 16 Entry, Splashdown, and Recovery: velocity 36196.1fps → 11032.57 m/s (assumed relative to rotating surface)

• NASA Mars Fact Sheet

• Wikipedia Mars article

  • note: Mars equatorial rotation velocity is 255 m/s at 40 km altitude compared to 471 m/s for Earth at 80 km altitude. Rotation velocity is assumed to be already subtracted to produce the entry velocities given.

• Wikipedia Martian atmosphere article

• Wikipedia Mars atmospheric entry

• Sitting Kills, Moving Heals by Joan Vernikos. Optimal gravity for human beings may be 1 gee or higher.