Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Large Mars rover engines
Date: Sun, 30 Jul 2000 05:16:44 GMT

In article <3981F7E5.3F5B@ix.netcom.com>,
Scott Lowther  <lexcorp@ix.netcom.com> wrote:
>...One idea that has cropped up is to use an IC engine burning
>methane and oxygen... more or less sorta a modified car engine. But a
>problem I see is that such engines on Earth swallow about 4-5 times as
>much nitrogen as oxygen; this provides cooler buring, plus extra
>"reaction mass." Are engines burning straight oxygen a doable system, or
>will the engine stand a good chance of melting without regenerative
>cooling?

Well, first, don't rule out regenerative cooling.  For that matter, don't
rule out normal liquid cooling, although you'd need a big heat exchanger
("radiator") in the thin air.

>Are there any gasses
>easily available on Mars that can serve the nitrogren role? Carbon
>dioxide is out, as I believe it will form carbolic (carbonic?) acid at
>the elevated temperatures, and chew up the engines (as well as possibly
>adding to coking problems).

Uh, if so, the engines are already in trouble, because about 1/3 of the
exhaust gas from oxygen/methane is CO2.  You don't get carbonic acid
without liquid water, and while hot CO2 is an oxidizing agent, I doubt
that there'll be a big problem in an IC engine.

Any LOX/methane system has the problem that hydrogen will be a scarce
resource for early Mars expeditions (until good sources of water are found
and equipment for using it is thoroughly tested), and recovering water
from the exhaust is a bit problematic.  You may be better off using
LOX/CO, which is not as energetic but is made entirely from local
materials.

>Alternatively, could a small turboshaft engine be a betterr choice?

Turbines unfortunately do not scale down well, and they have their own
problems dealing with very hot exhaust (gas generators for rocket pump
turbines generally either burn a very unbalanced mixture or have lavish
amounts of inert coolant, e.g. water, injected).  An external-combustion
engine -- steam engine or Stirling engine -- might be preferable.

My own favorite way of propelling Mars surface vehicles is to extract
magnesium from the soil.  It will burn merrily in CO2, ripping the oxygen
away from the carbon.  The Mars buggy chugs across the ochre landscape,
belching black soot from its smokestack, stopping occasionally to shovel
the ash out of the firebox...
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)