Date: Wed, 19 Jan 2000 14:37:36 -0800
From: Doug Jones <random@qnet.com>
Newsgroups: sci.space.policy
Subject: Re: Earth-Mars orbit?

Karel Jansens wrote:
>
> The reason why I asked: the (IIRC) March 1999 issue of Scientific
> American had an interesting article featuring a "slingshot-tether"
> launching system. I wonder if this system could be used to accelerate
> cargo/passengers to meet with a passing cycler-type station that would
> bring the cargo to Mars, where a similar system could be used to catch
> and slow down the cargo. I'd wager this would greatly reduce
> transportation costs to/from Mars.

Two words: hyperbolic rendezvous.

Three more words: dock or die.

Scares me and I'm fearless.

--
Doug Jones
Rocket Plumber, XCOR Aerospace
http://www.xcor-aerospace.com

Date: Wed, 19 Jan 2000 18:23:54 -0800
From: Doug Jones <random@qnet.com>
Newsgroups: sci.space.policy
Subject: Re: Earth-Mars orbit?

Karel Jansens wrote:
>
> On Thu, 1 Jan 1970 02:59:59 Doug Jones <random@qnet.com> wrote:
     ^^^  ^ ^^^ ^^^^
Uh, I think your system is having a bit of trouble with Y2K :)

> > Two words: hyperbolic rendezvous.
> >
> > Three more words: dock or die.
> >
> > Scares me and I'm fearless.
> >
> Real men don't scare from a bit of escape velocity <G>.

Hey, I used to jump out of airplanes for the hell of it, and this stunt is
the equivalent of jumping without a chute and having your buddy jump out of
another plane to hand you one. Eep.  Not me.  (I've been making this
analogy since Case For Mars II in 1984.)

> Seriously though. I was obviously not suggesting some kind of "Hail
> Mary pass" (how very ad rem the term suddenly has become <G>) for
> manned capsules. But I also don't think it would be impossible to come
> up with an acceptable back-up system (Heck, a rocket engine a some
> fuel could do the trick.). That's why all you smart people are here
> <G>.

The trouble is, by the time a rendezvous failure is discovered, the
vehicles are several hundred thousand kilometers from earth, and receding
at 4-5 kilometers per second.  The delta-v needed to return to earth under
those circumstances is difficult to provide; the transfer vehicle must have
(at least) two stages to get out of earth orbit, rendezvous, fail to dock &
return- and the total mass is multiplied by five to seven.  This wipes out
the mass savings due to the cycler.

> Besides, what's the worst that could happen if you miss (OK, provided
> the sling doesn't direct you back to Earth surface, of course)? You'll
> be on a different trajectory from the cycler, but with the right
> velocity already delivered at your rear end. I'm not saying it would
> be easy docking, but impossible it doesn't seem either.

Small errors in perigee burns turn into large errors in hyperbolic
velocity. The equation remains the same- dock or die, no viable abort
mode.  The absolute requirement for zero failures would drive up costs
dramatically.

Cycling spacecraft are Buzz Aldrin's brainchild- and they don't call him
Doctor Rendezvous for nothing.  I really feel that they are a solution in
search of a problem, and that Mars Direct (either by HLLV or EOR assembly)
would be a far better way to send people to Mars.

--
Doug Jones
Rocket Plumber, XCOR Aerospace
http://www.xcor-aerospace.com