Le Mar/18/2018 Ã* 9:10 PM, Fred J. McCall a écritÂ*:
Alain Fournier wrote on Sun, 18 Mar 2018
19:35:57 -0400:

On Mar/18/2018 at 5:59 PM, Alain Fournier wrote :
On Mar/18/2018 at 4:58 PM, Jeff Findley wrote :
In article , says...

On Mar/18/2018 at 2:32 PM, Niklas Holsti wrote :
Hmm. In his "Making Life Multiplanetary" address in September 2017,
Musk
showed a simulation of a BFS Mars landing with text saying "over 99% of
energy removed aerodynamically". If less than 1% of the orbital energy
remains for rocket braking, is that really a significant difference
between Mars and Earth?


One percent of the energy of Mars' escape velocity (escape not
orbital energy, I think that is the relevant value to use here)
means one tenth its speed. That's about 503 m/s. According to
https://www.reddit.com/r/spacex/comm...tegy_analysed/

or https://tinyurl.com/hk252mj
the falcon 9 first stage inbound at 1.1 km travels at 105 m/s.
So 5 times the speed, 23 times the energy on Mars vs Earth.

I'd say it is significant but not a big problem. Of course, less
than 1% leaves plenty of room, 0.1% is less than 1%. But I would
think that it would be close to 1%, 5 times the speed seems
reasonable to me.

That looks like a pretty good first approximation.Â* But secondary
effects like gravity losses will be different because the two planets
have different gravity.

Yes but those secondary effects are very small to the point that
they can be ignored. Gravity loss in the last few meters are negligible.
Gravity loss before the last few meters are taken care of be air drag.

I should add that things would be different if you didn't have
sufficient thrust. If you fire your engines for two minutes to
to cancel that 503 m/s then the gravity loss isn't negligible.
But if you do like falcon 9 boosters do and decelerate rapidly
just before touching the ground then gravity loss is negligible.


This is intended to be a manned vehicle. As such, they won't want any
sustained g loads above about 3g (and lower is better). Unless you
restrict wetware to trained military folks in peak condition, that's
really the desired 'limit'. Soyuz peaks at around 4-4.5g for brief
periods of time during takeoff and landing. Shuttle deliberately
managed engine power to not exceed 3g. Falcon 9 hits around 3.5g
during launch (and throttles back in order to do that). Mercury and
Gemini were pretty ugly, with peak loads above 7g. Mercury took
almost 11g on reentry and jolt much higher. But they launched on
ICBMs which weren't optimized to give the wetware a tolerable ride and
were all military test pilots. Apollo hit something like 6g on the
return from the Moon.

Yes, especially after a few months of travel between Earth and Mars
you have to take that into consideration. But this would be only
for a few seconds. I think say 4g for 13.5 seconds should be OK,
it's enough to decelerate from 503 m/s and the gravity loss is small.


Alain Fournier