

Landing on the Moon without a Rocket - gusgordon
http://underamp.com/?p=181

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lutusp
There's a problem with this idea -- as you decelerate out of orbit, in order
to reduce the craft's velocity so the inflated-bag method will work, the
craft's angle with respect to the surface naturally changes and becomes nearly
vertical. This is unavoidable.

Remember the early Mars missions that used the inflated-bag method? They had
to expend a lot of fuel to slow down enough to safely approach the surface, at
which point the air-bag method was used (and worked). But this only reduced
the amount of fuel required to get to the surface, it didn't significantly
change that cost and burden.

Just do some numerical orbital simulations -- see what happens when you slowly
reduce the velocity and altitude of a craft orbiting the moon so it is able to
graze the surface. See what happens when you impact the surface at shallow
angles. Hint: the craft's velocity is still extremely high and the air bags
have to absorb too much energy -- they will self-destruct.

In a hypothetical approach to a perfectly smooth plains area (the moon has
plenty of those), in which there are no altitude irregularities and the craft
can approach at nearly orbital speed and impact the surface at a very shallow
angle, the craft would begin to roll along the surface at such a high velocity
that the centripetal forces would tear the craft apart.

Speaking as a former NASA Space Shuttle engineer.

~~~
ColinWright
They do specifically say that they're intending to come in at 0.02 degrees or
so, which seems to indicate they've considered this. Personally, and not
speaking as a former NASA Space Shuttle engineer, it seems unlikely that
they'll find ground flat enough for this to work. They mention mountains, but
even just craters will provide far, far more variation than 0.02 degrees, so
they look like they'll skim the surface nicely, then slam into a wall.

But they're not stupid - I wonder what they're not telling us.

~~~
lutusp
> They do specifically say that they're intending to come in at 0.02 degrees
> or so, which seems to indicate they've considered this.

That won't prevent the craft from contacting the surface at very high speed
and beginning to roll. As soon as the roll starts (as soon as linear velocity
becomes rotational velocity), the velocity will decline further, which will
change the angle of attack and increase the conversion of orbital kinetic
energy into centripetal force. Believe me, the craft will disintegrate if it
touches the surface at near-orbital speeds, which is implied by the proposed
angle of attack.

> Personally, and not speaking as a former NASA Space Shuttle engineer, it
> seems unlikely that they'll find ground flat enough for this to work.

Yes, but my reason for stating it like that was to focus on one factor and
eliminate others for the moment.

> But they're not stupid - I wonder what they're not telling us.

I think the author is someone who thinks about these problems, but not with
the benefit of experience in orbital analysis or intuition about the kinds of
problems that come up.

