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, as fast as Apollo 16's return from the Moon. 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 . Assuming the same lift-to-drag ratio as the Apollo capsule (0.3), the maximum gee force on the Mars capsule would be 1.88 times Apollo (7.19 gees for Apollo 16), or 13.5 gees, and a correspondingly shorter entry time (814 seconds Apollo 16, 433 seconds Mars).

The maximum heating rate for Apollo 16 was 346 BTU/ft²-sec, or 3.93 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 7.4 MW/m², assuming the same vehicle mass to area ratio.

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 Mars-bound trip. 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.

Land animals are not fish in "air suits". 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 will be vastly cheaper for the foreseeable future.

Concerns:

• 1 The almost-doubled gee loads may injure astronauts who have suffered zero-gee bone loss for 3 months.
  • Total mission atrophy may be fatal.
• 2 The higher heating rate may push engineering limits.
• 3 The higher required structural strength and parachute mass will cut into payload fraction.

References:

• GDN personal email

• Apollo By The Numbers

• NASA Mars Fact Sheet

• Wikipedia Mars article

• Wikipedia Martian atmosphere article

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