Monday, December 31, 2012

Mars Landing Options

In the paper I presented at the Dallas convention two years ago,  I posited a transport vehicle from Earth to Mars and back,  to be based in low orbit at each end of the voyage.   Some or all of the tankage has to be jettisoned by voyage end,  depending upon selected transfer propulsion,  but the engines and habitat module are recovered and reused on subsequent missions.  That reduces long-term costs. 

Based from orbit like that,  it is possible to send a single vehicle to multiple sites on Mars in the single mission,  depending upon how many landers you send,  and exactly how you send them there.   That increases “bang-for-the-buck”.  Sending a single landing vehicle to any given site presents the “standard” risks that we already well-understand from Apollo.  Given sufficiently powerful propulsion,  these landers can be single stage reusable,  even without refueling while on the surface.  Otherwise,  these are two-stage one-shot chemical vehicles,  unless you can refuel them on the surface.   Anyone can prove that,  by plugging in realistic numbers into the rocket equation. 

If you add refueling while on the surface,  so as to make single-stage reusable chemical propulsion feasible,  there are two choices:  (1) carry the fuel-making equipment with you,  or (2) send it down separately to the same site.  If you carry it with you,  there are two issues to address:  (1) your lander is necessarily much bigger and heavier,  and (2) the fuel-making devices must work very fast,  within the time frame of the surface stay,  which is limited by the men,  for any of a variety of very good reasons.  (Long surface stays are not very realistic for a first mission,  due to all the life support uncertainties.) 

Plus,  you are betting lives on the fuel-making gear working correctly,  at that particular site,  which might be quite different from “typical” Mars.  Although,  that last risk can be effectively eliminated by suitable development testing,  which of course takes calendar time. 

If you send it (the fuel-maker) down separately,  that opens a whole host of other safety issues that I have not yet seen discussed very well.  The most obvious one is the capability of actually landing multiple vehicles close together at the same site,  not too far out of range of each other.  This takes a radar transponder and a vehicle that is steerable during entry.  These are things we already have (even capsules have been steerable since Gemini),  but we have never actually carried out such a homed-in landing before.  That’s another issue that can be effectively eliminated by suitable development testing,  which again takes calendar time. 

The other issues involve the achieved range between landed vehicles.  If the return vehicle is too far from the fuel-maker,  how does one transport the fuel from one to the other,  when there is no fuel transportation infrastructure on Mars?  By truck?  By pipeline?  By strung hoses?  That last requires a very close range indeed,  between landed vehicles.  The other two require equipment that raises lander vehicle size considerably;  if you do that,  you might as well carry the ascent propellant down with you.

Landing really close together (so that strung hoses are feasible) brings into play another very serious risk:  rocket blast effects.  Even a chemical rocket produces a very high velocity stream whose stagnation temperature is very high,  until reduced by mixing with ambient atmosphere gases.  These are very destructive plumes,  and the forces they impose on impacted structures are very high (in effect the same size as the thrust force produced on the vehicle,  multiplied by the fraction of the entire plume that is intercepted by the structure,  and factored for effective perpendicularity).  You run the risks of puncturing the propellant tanks on your fuel-maker,  and/or cooking-off the propellants with the heating of the jet blast washing all over it (that plume spreads widely at low backpressures). 

By the way,  the supersonic expansion that reduces static gas temperatures is not “permanent”:  as soon as the gas flow shocks down subsonic,  its static temperature is once again very nearly stagnation,  and that’s essentially the rocket chamber temperature,  until reduced by mixing.  That’s what happens as soon as the plume strikes anything solid.  The source temperature for heat transfer across the boundary layer is the recovery temperature,  which is only a little lower than stagnation.   This is standard textbook stuff on heat transfer in very compressible (supersonic) flow.

The other risk with close-range landings is the obvious collision risk.  That can be handled by a human pilot taking manual control,  as it was on the moon with Apollo,  which in turn eliminates the possibility of landing multiple robot vehicles close together at the same site.  But for the manned vehicle that maneuvers,  you have to budget descent propellant to handle that contingency.  You cannot trim margins “to the bone” and still do that effectively.  Minimalist mission plans never address things like this.  

That’s one big reason why I rarely believe the claims of practicality regarding anybody’s minimalist mission design approaches. 

What I proposed in my Dallas convention paper was powering single-stage reusable landers with solid-core nuclear thermal engines (basically a resurrected NERVA),  and avoiding entirely the “making-return-propellant issue”,  by simply carrying it all with you in a bigger,  more capable vehicle.   These same nuclear landers could push the entire landing propellant supply to Mars,  separately from the manned ship.  That could be another cost savings. 

Resurrecting NERVA for this purpose might well be a faster engineering development time than any of the in-situ propellant technologies.  I think NERVA could be resurrected by the “right team” in 5 years.  I’d bet any of the in-situ propellant things will take longer,  because we have never done them before,  while we did do NERVA before,  back in the 1950-1973 time frame.  Fundamentally,  the choice of what technologies to attempt really all boils down to:  “how soon do we really want to go?” 

That brings up a very serious caveat:  I think the US government goal of “sometime in the 2030’s” is really code for “never”.   NASA is spending its resources on a rocket mandated by Congress instead of one we really need,  and on a capsule that competes with one developed commercially that is already further along in its development.  They are doing this instead of looking seriously (meaning substantial funding) at practical and safe transfer habitats for astronauts,  Mars lander vehicles,  or advanced propulsion that would cut overall program costs.  This is a recipe for failure,  not success. 

Long development times for the truly-necessary “tinkertoys” mean this landing will not happen in our lifetimes,  if it is to be done by NASA (or anybody else who does not understand the consequences of these choices).   If we humans are going to do this any time sooner than “2030’s = never”,  it has to be with tinkertoys we already have,  or that we can obtain very quickly (under 5 years),  because the final development checkouts will add 5 more years to that.  That already puts us about 2020 to 2025. 

And I think it needs to be done by somebody non-governmental like a Spacex.  Somebody actually motivated to go,  and free-enough of bureaucratic chains,  to go. 

Pessimistic,  I know.  Sorry,  but I’m a practical realist. 


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