[extropy-chat] Pluto New Horizons launch -getting ready

[spike]

bounces at lists.extropy.org] On Behalf Of Robert Bradbury
Subject: Re: [extropy-chat] Pluto New Horizons launch -getting ready

Sounds like you need a combination orbiter + lander.  Let the lander just
worry about getting signals up to the orbiter and the orbiter worry about
getting them back to Earth.  But they you still have the deceleration
problem again.  I think an inflatable antenna on a hard-impact lander might
be the way to go.  The crater problem can be solved if you can bring it in
at a low angle and/or let it bounce/roll along until it came to a
stop...Robert


Robert and all space fans,

I thought this over and remembered a project I was on about fifteen
years ago.  When the kinetic energy of a projectile is large with
respect to the sum of the heat of fusion plus the heat of vaporization
of the projectile material, you are in what is termed the hydrodynamic
impact regime.  Upon impact, a compression wave is sufficiently violent
to break the chemical bonds in both the projectile and the target.  The
projectile and the target are vaporized in this regime, regardless
of the material of either the projectile or the target.

Above the hydrodynamic impact regime velocity, increasing velocity 
of the projectile does not punch a deeper hole in the target, it
only punches a wider hole.  This explains why the craters on the
moon are wide but not particularly deep.  It also explains why
super high speed armor piercing shells are long skinny things: the
length is all that matters above a certain speed.

On the project previously mentioned, the velocities we were working
were all in the 3 to 4 km/sec range.  This was sufficient to
enter the hydrodynamic impact regime for the second highest
energy of vaporization material we know of: tungsten.  Only
carbon is higher, and that not by much.

So if we were to try an impact landing on Pluto, 
we would still need to reduce the relative velocity down to
about that of a high-speed rifle bullet, which is in the 0.4
to 0.6 km/sec range.  You know what happens to a rifle bullet
when it hits a target; you can dig out the slug usually.  It's
messed up, but still mostly there.

So if we attempt to land on Pluto, the required deceleration
is not 14 km/sec but rather only about 13.5 km/sec.

If we suggest something like what one poster proposed, to
drop a cable and allow the drag across the surface to
decelerate the vehicle, keep in mind that it might take
a lot of cable.  If the relative velocity of spacecraft
to planet is 14 km/sec and it is strong enough to withstand
a tension of 100 times the mass of the vehicle, you need to 
reel out 14 km of cable in the first second after the craft
passes the planet, 13 km in the next second, and so on, for
a total of about 120 km.  This assumes you can somehow 
maintain a nice steady 100 G deceleration, and you 
somehow solve the problem of hydrodynamic impact 
vaporizing the cable upon impact.

If you decide to go the route of chemical rocket
deceleration, you have a problem of how to keep
the fuel from freezing if you use hydrazine or
how to keep it from boiling away if you use
cryogenic fuels.

Of all the possibilities, the one most attractive
to me is to send it out on a Hohmann orbit, which
obviates most of the fuel problem, since the relative
velocity of spacecraft to Pluto is low upon arrival.  The
main drawback to this approach is that it takes about
62 years to get there, assuming gravitational assist
from Jupiter.  That would make me 107 by the time it
gets there, assuming we launch right away.

spike