Re: [amsat-bb] Fly to the moon question-2

[Bruce Bostwick <lihan161051@xxxxxxxxxxxxx>]

On Monday, Nov 24, 2003, at 12:55 US/Central, William Leijenaar wrote:

> Hi AMSATs,
>
> I see that with an ion-engine it is possible to go
> from a GTO into lunar orbit. This is very
> interresting, because then I only have one more
> question:
>
> What we need to get from this lunar orbit save to the
> lunar surface ??? How much Newton thrust for how much
> radio weight is needed ? I guess a ion-engine will
> have not enough thrust, or not ??

Not even close -- an ion engine is good for maybe a few pounds of 
thrust, and you're going to need a lot more.  You might be able to 
manage some sort of slow capture into lunar orbit (the quick and dirty 
integration in my head tells me it might be possible but I may be WAY 
off) but descent is going to need more thrust.  During the final phase 
of the descent you're going to need to balance the entire weight of the 
vehicle in lunar gravity for a hover landing -- there's no way around 
that because you can't cheat with a parachute the way you can on a Mars 
landing -- so you'll need a propellant that will give you at least a 
few hundred pounds of thrust (and a total vehicle weight less than that 
in lunar gravity) to make the final descent.

One thing I've been thinking of is what I call an "involute" descent, 
one where your descent engine is pointed exactly in the direction 
you're going for the entire descent.  If the thrust and burn timing are 
right, you'll end up in a gentle involute curve that ends up in a hover 
over the lunar surface, where your landing radar can take over and give 
you a controlled descent and soft landing.  Apollo didn't do it that 
way because of the requirements for a safe abort at any point during 
the descent and limitations of the AGS computer they used,  but I think 
if you do that you can save some fuel ..

"Oh yeah? Well, I speak LOOOOOOOUD, and I carry a BEEEEEEEger stick -- 
and I use it too!"  **whop!**   -- Yosemite Sam

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