Monday, December 31, 2012

On Long-Term Sustainable Interplanetary Travel

For long-term sustainable interplanetary travel,  I'd look first for the most widely-available,  highly-abundant volatile to use for propellant.  What we're seeing from all the planetary science probes is:  that answer is "water".  It may not be pure,  but ice is everywhere,  and solid contaminants are really easy to separate with gravity,  even if it is artificial (centrifugal force). 

So,  if we use the water,  we also find it is really easy (and safe) to ship anywhere,  because we can ship it frozen,  with minimal vapor containment to prevent sublimation.  An iceberg is not lost if you take a meteor hit;  a remarkable safety feature!  Just patch the hole in your vapor containment,  which is holding at most about 6 mbar pressure.  That containment might just be a big plastic bag.  We do have to learn how to patch holes in plastic bags in-vacuo and in zero-gee.  But that can be done.  One way is to use an aluminized thermoplastic,  and just heat-bond it with a hand-held “iron”. 

OK,  so water makes the most practical propellant material we have.  So,  now how exactly do we use it?  Solar or nuclear electrolysis is a known way to produce hydrogen and oxygen,  which can be both rocket propellants and fuel cell reactants.  We already know we're going to do this at some level,  and how to do it.  These are known tinkertoys in-hand.  But the energy cost to make the hydrogen and oxygen,  store them successfully,  etc,  will never be trivial,  no matter how the space commerce economy finally takes shape.  You'd really like to use the water directly,  as water,  and just use your propulsive waste heat to melt the stored ice as you go. 

That's a nuclear thermal rocket,  a water-variant of the old NERVA.  “Everybody” thinks the Isp will be far lower due to the molecular weight effect;  and at the same core temperatures,  it is.  But water-as-steam will transfer a whole lot more heat as a coolant than hydrogen.  You can run the reactor core at a higher temperature,  and a whopping lot more generated power.  I'd bet the as-finished Isp isn't as low as everybody fears,  once the development is done.  How would you like Isp 600+ seconds at engine T/W > 5?  I'd bet real money this could be done,  and within about 5 years of having resurrected the original LH2 NERVA,  an item that itself should take at most about 5 years,  if done by the right team.

The follow-on is a gas-core nuclear thermal rocket,  which takes away the core temperature limitation,  and eliminates the safety concerns associated with a "live" core inside an engine shell,  between burns.  Operated up to a modest power level,  regenerative cooling is possible.  How would you like 2000+ seconds of Isp at engine T/W > 30?  Beyond that power level,  you need a place to put all the waste heat,  so a heavy radiator is probably needed.  How would you like Isp 5000+ seconds at engine T/W around 0.1?  Even an engine T/W like that is still high enough for an impulsive burn,  for low-loss interplanetary travel.  I'd bet we could have one of these working within about a decade of having the water-NERVA working. 

High-Isp / high thrust nuclear propulsion,  with plain water as the propellant.  Water,  which you can get nearly everywhere you go.  Water,  which is easy to store and ship as minimally-enclosed icebergs.  To me,  this sounds like a series of very smart technology development programs we should already be undertaking.  Too bad no one is doing this stuff. 

To me,  it also sounds also like the government monopolies on nuclear power need to be broken,  so that somebody who is actually motivated,  can go do this stuff.  Political anathema to some,  I know.  But it must be done.  

I’d suggest testing these things on the moon,  so that a plume capture feature is not needed on your nuclear engine test facility.  The resulting facility savings might well exceed (in the long term) the costs of transporting all the necessary stuff to the moon. 

This nuclear thermal rocket stuff starts looking really attractive if you use a thorium reactor.  Thorium is very likely available "everywhere",  too.  It's very plentiful here,  and on the moon,  much more so than uranium.  The probes should be looking for it "out there",  shouldn't they?  Too bad they're not.  Not yet,  anyway. 

Just how sustainable and low-cost do you want to be? 


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. 


Wednesday, December 26, 2012

On the Looming “Fiscal Cliff”

Cassandra was a mythical seer whose doom was to make accurate predictions no one believed.  I often feel like her,  as few believe what I think about what is happening to our country.  Here are five points to ponder,  which taken together comprise my prediction.

Point 1:  the “fiscal cliff causing a return to recession” refers to Wall Street,  not Main Street.  We on Main Street have never recovered from the recession,  it is still going on (see the lousy unemployment figures for proof).  

By the way,  that recession started (for Main Street) long before the market crash of 2009.  Main Street’s recession was initially caused by a reduction in consumer spending,  in large part due to excessive fuel prices.  (See “Iran,  Oil,  and Economies” posted 3-8-12 on this site,  for proof.)  

Most of the corporations are still sitting on piles of cash,  as they have since 2009,  which has been publicly reported,  although not widely.  The giants among them (mostly the Wall Street investment firms who caused the 2009 market crash) got to add most of the “stimulus money” to their piles of cash.  

Point 2 : we have a bunch of greedy political hacks serving the special interests who bought their offices for them,  and themselves,  instead of we who elected them.  (We keep voting for malfeasants like that in our party primaries,  so this is partly our fault!)  These idiots will continue to posture and to scheme for party advantage,  instead of coming together on the budget deal our laws require.  They will take us over that “fiscal cliff”.  [verified 1-1-13:  Senate vote on tax side only,  no House vote and with evidence of foot-dragging,  no plan from Senate on amount or timing of spending cuts][Verified further 1-6-13: Senate and House passed tax cut bill without tax reform,  and no attention to spending,  nor to debt ceiling]{1-19-13: no change to this yet:  temporary partial tax cut,  but not payroll taxes ,  thus really effecting a partial and regressive tax increase,  and nothing done on spending,  just a looming political battle over the debt limit,  which is just posturing,  nothing serious}

They will thereby cause another market drop back into the recession where the rest of us still are.  They will thereby do further enormous damage to our 401K’s,  etc.  There is a reason that Congress’s popularity rating is effectively below that of Gaddafhi’s Libya.  Please remember that next time you vote!

Point 3: on the other side of the fiscal cliff lurks higher taxes and fewer government services for all,  but this burden is laid disproportionately (to income level) upon the poor and the middle class.  This is a back-door way of foisting right-wing economic ideology upon the rest of us as de facto public policy.  It is precisely why the Tea Party-dominated GOP will not deal with Obama and the Democrats;  they will get what they want anyway.  

That ideology says tax everybody but the rich,  and cut all government spending except that given to the rich.  In this case,  those “rich” are the giant corporations that actually run this country because they bought the offices of nearly all our Congressmen and Senators,  and also most of the heads of all the bureaucracies. 

And,  the Democrats also want to go over the cliff so they can blame the Republicans for the short-term troubles it will cause.  Neither side can think beyond the next election,  and they come every 2 years.

Welcome to the corporate welfare state,  in which government’s sole effective function is to pick your pocket to maintain the corporate welfare system.  What a gravy train for them!  And we are distracted from seeing it by the political bread-and-circuses nonsense. 

Point 4:  also lurking on the other side of that “fiscal cliff” is a further downgrading of the credit rating of US government bonds,  and a first-ever government default,  the start of many.  These outcomes will not be immediate,  but they will come,  and all within about a decade or so.  

That chaos will destroy us,  but the last to fall will be the corporate giants who have benefited most from the corporate welfare state.  Being international will help cushion their fall.  No one will cushion ours.  Welcome (eventually) to the Third World,  America!  

Last point:  Unless you screw up the courage and wisdom to throw these bums out and take back your country pretty damned quickly,  this dismal prediction is your future!  You have but two choices:  do it at the ballot box,  or do it in the streets with the guns they haven’t yet taken away from you.  

My personal preference is the ballot box,  as there is far less mess to clean up afterwards.  So,  please stop voting for ideological extremists and corrupt hacks!  It is almost too late!

Cassandra has spoken (yet again).