[ExI] De-Orbiting Gold

[spike]

... On Behalf Of Kelly Anderson
To: ExI chat list
Subject: Re: [ExI] De-Orbiting Gold

On Mon, May 21, 2012 at 4:48 PM, spike <spike66 at att.net> wrote:
>>...> Thanks Kelly, I didn't explain that very well.  If we go for 
> aerobraking, we are talking about very shallow reentry angles.  That 
> means high uncertainty on the reentry path.  Small uncertainties in 
> atmospheric density mean large uncertainties in landing spot.

>...Ok, that makes more sense. It seems to me that there are two ways to get
down... One, which is what everything I am aware of is to fire the rocket
straight ahead which slows your speed and you drop to earth...

Ja.

>...Another, which probably isn't used, is to fire the rocket straight up
into space pushing you down but not slowing you...

It isn't used because it doesn't work.  There are orbit sims available
online.  If you find a good one, post the URL.  Playing with those
demonstrates that if you fire your rocket straight up, you don't de-orbit,
you just waste your fuel and get stuck in orbit.

>... I figure that with monkeys in the can, you want to be going as slow as
possible, is that right?

Ja, but it is a tradeoff, as you can find if you fiddle with those online
orbit sims I mentioned above.  The shallow reentry angles require less delta
V and produce lower peak decelerations, but introduce greater risk of
skipping off the atmosphere and landing far from the recovery area.  This
was the biggest danger during the Mercury days.  Gemini and Apollo could be
steered to some extent by an offset CG, but the Mercury astronauts were spam
in a can.  They didn't like that.

 >...So would firing the rocket straight up give you more predictability,
but a hotter entry?...

No, it would probably result in no reentry at all.  It doesn't do what your
intuition tells you it does.



> >...upper atmosphere has waves and surges, like the ocean only thousands
of times bigger.

>...But would it not also be true that the bigger the variability, the less
it would actually impact the orbiting object? If it's thinner, it seems that
it would affect it less, and also be more exposed to solar wind effects,
etc...

What I mean is if you look at the atmospheric density in the critical areas
where the most aerobraking is taking place, the variability is a lot higher
than it is down here, where atmospheric density doesn't change all that
much.  If you find a good reentry sim, it might have atmospheric density
uncertainty programmed into it.

>> If the reentry angle is too shallow, the reentry body (RB) might 
> actually start to go back out, a skip-off event a little like a flat 
> stone skipping off the surface of a lake.

>...I've heard of that. I always figured you had to be going faster than
orbital speed to get this effect...

Orbital speed is quite sufficient to cause that effect if you come in at a
shallow angle.  You need a competent control system to prevent it, for if
you skip off just once, it introduces a great deal of uncertainty in the
landing spot.

>...And again, if you have a transponder in the object, does it matter, so
long as it lands SOMEWHERE in the Pacific ocean?

Sure but the Pacific Ocean is really big.  If your RB lands in international
waters, then whoever gets there first owns that payload.

...

>...Ok, so there is a balance that has to be reached. What I'm asking is
would that balance be significantly different if apes and fragile equipment
were not on the list of concerns?

I wouldn't think so.  The reentry path you would choose would likely be one
that is survivable by apes.  Reasoning: to get the steeper reentry paths
that result in higher peak Gs would require more delta V to start with,
heavier structure to tolerate the higher heating and higher loads, you would
need a sturdier heavier everything.

Now to your point of using the gold itself as an ablative sacrificial heat
shield, I don't see it.  You need to slow the thing down enough to not have
it vaporize on impact I would think.

That whole task is easy to underestimate.  Consider that once you get your
hunk of gold on board, you need to secure it such that the CG stays in one
spot, you need a thruster system, tanks to hold the fuel, you need actuators
to steer the nozzle, you need a control system to tell the actuators what to
do, you need a gyro of some sort to stabilize the thing, or failing that
some kind of really good thrusters to try to keep it all balanced, you need
an inertial reference system so you know where to point the reentry thrust
nozzle.  It would be possible, but it wouldn't be easy, and all that stuff
would be heavy and costly.

I will leave it at this: if it is feasible to reenter a hunk of gold from
LEO, it has only been recently that it is worth doing, as the price of gold
went crazy.

>> ... all I have seen so far is a number 
> of approaches which will not work for known reasons or are very high risk.

>...High risk? Really, what's the risk? I mean risking a big ball of gold is
clearly a risk, but you would practice with copper or something to get it
right, no?  Kelly

The risk is you would spend all that money to get your reentry system into
orbit, then any number of things might go wrong and you don't get your hunk
of gold back.  Space flight is still hard, even fifty years into it.  It is
still an unforgiving game, still expensive as all get out and still no
economies of scale.  You wouldn’t practice on a hunk of copper, you would
practice on a hunk of gold.  Reasoning: even if your copper reentry scheme
worked, you wouldn't get back a hunk of gold to pay for building and
launching your reentry system.

At this point I am still inclined to think if you had a hunk of gold in
orbit, you would make stuff out of it there, wire, reflectors and such.
Gold has some advantages over other metals for that sort of thing.

spike