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Sending unmanned stuff one-way to Mars is no longer particularly expensive.  For one thing,  it is far smaller than any two-way manned vehicle would have to be.  I say the cost is down,  because we are no longer using the shuttle at $1.5B to send max 25 metric tons to LEO.  At max payload,  I calculate about $27,000/pound for shuttle.  (Much higher at part load,  of course.)

The Atlas-5 -551/-552 configurations send 25 metric tons max at near $130M per launch.  That's about $2400/pound.  Delta -4 is comparable payload but about twice as expensive per launch.  Falcon-9 is 10.1 metric tons at about $56M per launch.  That's about $2500/pound at max payload.  Falcon-Heavy is projected to be 53 metric tons max at near $100M per launch.  That's around $900/pound.  These launchers,  used at or near their max payloads,  work out an order of magnitude cheaper,  or more,  compared to shuttle.  These per-pound figures are also much higher,  if the rocket is flown at part load,  too.  So it pays off to fly near max load. 

This is 20-20 hindsight to be sure,  but,  the ISS could have been about 10 times cheaper if we had had these rockets back then!!!  It was built out of 25-ton modules ferried up by shuttle.  Today,  Atlas-5 and Delta-4 could do that,  and Falcon-Heavy could soon do it 3 times cheaper still,  and with bigger modules. 

The necessarily-bigger two-way manned Mars missions could built by docking assembly in LEO the same way ISS was.  But with these newer rockets,  this could be a whole lot cheaper than anyone has ever dared to think in previous years.  By perhaps that same order of magnitude,  or more.  I don't think very many folks realize this yet. 

And,  any such Mars transit-capable vehicle is capable of transits to Venus,  Mercury,  and the NEO's.  You need landers for Mars and Mercury,  but that's a separate problem.  Reusability becomes very practical and financially inviting,  if one starts thinking about orbit-to-orbit transit vehicles (with separate landers as completely-separate transit vehicles),  instead of the more "traditional" Apollo-like approaches. 

GW

Just for the record,  the spinning ship I was proposing was not cable-connected.   It was a stiff structure of docked modules,  of moderately high overall L/D.  Typically,  these ship designs were between 100 and 300 meters long.  They would resemble the "2001" movie's "Discovery" that was spinning end-over-end when it was salvaged in the movie "2010". 

Parallel-stacks of modules could be docked and connected together axially and laterally,  if a single-module stack was too long to be dynamically stable and structurally sound.  No cables,  no fancy space trusses (which do not contain propellant or life support!!!),  no reconfigurations to do anything.  Just spin it up and coast a while (could be thrusters or a flywheel,  not sure which might be lighter,  but either could be made to work right now).  De-spin to make maneuvers,  so the dynamics and control are clean and uncomplicated.  Simple.  Clean.  Very do-able.  Right now. 

The spin rate criterion is a fuzzy boundary.  People with extensive military flight training in high-performance aircraft can tolerate spin rates above 10 rpm for quite a while,  usually.  Not always.  Getting sick from spin is both training-dependent and exposure time-dependent.  The astronaut who can take 12 rpm for an hour in a centrifuge would quite likely get sick,  even incapacitated,  if spun that fast for days,  much less months.  Less training,  lower short-term tolerance,  too,  much less the long-term tolerance.  For "civilians",  4 rpm seems to be tolerable for very long intervals,  typically.  That's why I picked it.  Maybe 2 or 3 is better.  That's an experiment needed to be done right here on Earth right now.  Any of those numbers (2 to 4 rpm) are tolerable in some very practical designs.

I had sizes and numbers worked out for 30 metric ton modules (before Spacex revised it to 53 tons),  with the human vessel powered by a gas core NTR,  as the baseline in my original paper at the Mars Society convention last year in Dallas.  The lander-plus-landing propellant vessels had solid core NTR's which pushed those vessels to Mars,  then got used again as single-stage reusable landing boats fueled from orbit.  These landing boats were 60 ton max weight,  with a 6 ton payload,  at 20% structure,  and 70% LH2 propellant.  These were good for a two-way trip,  fully loaded both ways,  rocket-braking-only descent (no benefit from aerobraking),  and 30-degree out-of-plane both ways. 

I did a revision of this mission design for using solid NTR instead of gas core on the manned ship,  which made it quite a bit larger.  I had to "stage" by leaving several of the empty fuel tanks in LMO,  but still was able to slowboat it two-ways Hohmann transfer without ditching anything at all into deep space.  (The gas core version was a fast-trip at 75 days one way.)  I did this revision because solid core NTR was ready-to-fly in 1973,  while gas core needed some development years.  The right team might get one working in 10-20 years;  the wrong team?  Never. 

I haven't checked it for all-chemical,  but you'd have to stage a lot more by ditching most of the empty tanks into deep space.  But you could still reuse the habitat,  engines,  and those tanks not staged off by the time of return-arrival in LEO.  Myself,  I think the NTR is just a better deal,  especially if maximum re-usability is a design requirement (and it should be from the get-go!!).

Gas core would take longer to develop,  and looks more like a later upgrade to me now.  At high payload fraction,  and simultaneously high structural fraction (for tough-as-an-old-boot reusability),  I rather think the solid core NTR makes more sense,  especially for a single stage lander item.  It is really hard to beat 900-1000 sec Isp to get you practical mass ratios.  I'd love to have a nuclear light bulb gas core at about 1300 sec for the lander,  but that would be some years' worth of development away.  NERVA could be resurrected by the right team in under 5 years.  The gas core in my original paper was a design projection for an open-cycle machine of 6000 sec,  but penalized by a waste heat radiator requirement. 

I've got some illustrations and numerical data for the all-solid core NTR machines worked out and posted over on "exrocketman".  See "Mars Mission Second Thoughts Illustrated" dated 9-6-11.  You'll have to scroll down or navigate down by date and title.  The site is http://exrocketman.blogspot.com

Not only are the performance and size numbers posted,  but the rationales for why I chose what I chose are there.  There's a version of the original paper posted there as well,  dated 7-25-11.  The discussions and rationales  for why maximum self-rescue capability is required,  and why we ought to make multiple landings in the one trip,  are there,  and I think those are very good rationales.  Those are all things we did not do during Apollo,  but we can now,  and we should. 

Too many are still focused on another Apollo-style flag-and-footprints stunt.  That's no reason to go to Mars.  It's just too far,  too dangerous,  and nobody is racing anybody else this time.  Exploration is the real reason to go to Mars.  And that word requires a proper definition,  one based on about 500 years of history. 

GW

What Impaler said in the prior post is very true.  Although,  if one has a smart strategy,  the "slow steady pace" that ensures success doesn't have to be so very slow.  I'm not so sure that most of the current government proposals so far are really that smart of a strategy.  We had the same problem before the Apollo design "gelled".  The key then was lunar orbit rendezvous (using a lander),  which let us do one launch/one mission,  without having to prove orbital assembly in LEO.  That outcome was an artifact of the easier numbers to reach the moon,  relative to the technology of that time.

Since then,  we have proven orbital assembly in LEO by docking:  it's called the ISS,  and we have proven we can build things like that out of 25 ton payloads (what the shuttle could carry).  Can we still do that?  Sure.  We have Atlas-5 -551/552 at 25 tons,  we have Delta -4 in the same class,  and very soon now we will have Spacex Falcon-Heavy at 53 tons.  We're already close with Falcon-9 at 10 tons.  All those are factor 10+/- cheaper than shuttle was.

The numbers for Mars preclude one-launch/one mission.  No one can build a rocket a mile or more high.  That's ridiculous.  The way around that is simple:  orbital assembly in LEO by docking,  as big as you want,  from already-practical payloads in the 20+ ton range.  It's just docked modules plus plumbing and wiring.  How big do you need to build?  We can now build it,  if we choose to,  in any size we want (something not possible in the 1960’s). 

The difference between a max 2-week moon mission and a 2+ year Mars mission with men is life support.  That topic divides into consumables and protection from lethal hazards.  We knew very little about any of that in 1959.

Consumables:  either you build a closed-cycle recycling ecology (which we cannot yet do),  or you provide enough packed supplies to support a botched-up,  stretched-out mission,  oh,  say,  50-100% longer than intended.  The current freeze-dried and/or wrapped-sterilized foods,  that we have been using on Apollo and shuttle and ISS,  only last a year,  maybe 15 months.  That's not long enough.  But,  there's a proven way out:  frozen foods.  They last for decades.  It's just bigger and heavier.  So,  we build a bigger ship in orbit.  No way around that.

Hazards:  zero-gee.  Microgravity illness sets in a bit beyond a year's exposure in forms that are not fully reversible,  and we don't even know the full extent of the scope of these illnesses yet.  The mission is 2+ years.  So,  we must provide artificial gravity,  no way around that.  That has spawned some ridiculously large,  complicated,  and expensive ideas.  We don't need all that.  We've already seen a glimpse of the real answer in the centrifuges we train in.  Just build the assembly in orbit as a long stack with the habitat at one end.  Spin it end-over-end in coast.  Humans can take up to roughly 4 rpm,  and that spin rate only needs a measly 56 m "radius" to provide 1 full gee.  We've already built far larger stuff in space.  We can design it to take all the spin and de-spin forces in the structures.  They’re not that large.

Hazards:  radiation.  Two kinds,  a slow drizzle of super-high energy "cosmic rays" at 24 to 60 REM per year,  depending on the solar cycle.  No shielding is as yet practical,  but we currently allow astronauts to absorb 50 REM a year.  There are career limits that will preclude second trips to Mars,  unless we learn how to shield this stuff effectively.  But,  there’s not much difference between the 60 REM max threat and the 50 REM max limit.  Minimal shielding effects for solar storm particles will make that up difference. 

The second type of radiation is sudden bursts of solar storm particles.  These are events lasting a few hours,  but at radiation exposures like standing in the initial fallout pattern of an atomic bomb: quite quickly lethal.  Shielding is required,  fortunately,  we can already do it,  as these are much lower energy particles.  About 20 cm of water is enough.  So,  provide one place on the ship where all persons can go to shelter,  and surround that space with the water and wastewater tanks you already know you have to have!  If you're really smart,  that shelter is also the flight deck,  so that maneuvers may be flown regardless of the solar weather.

A multi-module habitat and command deck,  some crew return capsules,  a bunch of propellant modules,  and some engines,  all assembled by docking,  could take men from LEO to LMO and back.  Or to Venus.  Or to Mercury.  Or to any of the NEO's.  Same ship.  Build one,  do all those missions with it.  Why launch all that hardware more than once?  Just launch propellant.  Re-use the ship.

OK,  Mars and Mercury need landers,  the other destinations do not.  We're going to need one,  and the Apollo LEM approach is way too inadequate for Mars.  Actually,  IMHO the lander is the true pacing item right now for men to Mars.  Send the landers and all their propellant as a separate ship or ships,  waiting in LMO for the manned ship.  If you're really smart,  you'll use nuclear engines in the lander to build one-stage reusable "landing boats".  That way one mission makes dozens of landings,  not just one.  (Those very nuclear rocket engines all but flew back about 1973,  then got cancelled.)

BTW,  those same nuclear engines can cut the mass of the ship or ships you assemble in Earth orbit,  by a factor near 4.  Hmmmm.  Seems to me like a nuclear rocket engine is a lot more important NASA project than a new giant Saturn-5 class rocket.  We’ve got launch rockets from ULA,  Spacex,  and maybe ATK/EADS big enough already for LEO assembly. 

Yeah we could do this,  and long before the 2030's.

GW

To answer Impaler's question,  yes,  Mark is correct.  There are several really big solid motor applications active.  These include the ICBM's I named,  the submarine-launched BM's,  and all the big strapons used on both Atlas-5 and Delta-4.  ATK and a couple of others build these.  My names might be obsolete,  but ATK used to be Hercules in Utah.  There is also Thiokol in Utah right across the Salt Lake valley from ATK.  Plus,  UTC-CSD had a big motor operation somewhere near the middle of the Mississippi River. 

GW

If somebody was working on a real Mars lander,  we could easily go that soon. 

But we do need that lander.  What's the point of sending men to Mars,  if we don't land?

GW

Any spaceplane will resemble an airliner in its safety aspects.  It's just too hard to get a lot of folks out.  The more on board,  the worse this problem is.  So,  like airliners,  we will have to pay very careful attention to making them as safe as is humanly possible.  That's the way to build a passenger spaceplane. 

I think the choice between vertical launch rocket and horizontal spaceplane (one stage or two) depends really on the intrinsic value vs size of the cargo.  Using Atlas-5 and Falcon-Heavy as examples,  launching 25+ metric tons to LEO can be done for something like $1000-2500 per pound (as crude as this is,  $2000-5000/kg),  but only at the max payload capability of the vehicle. 

That last phrase is the key.  Some payloads will fall below that max flyable value for any launcher,  and it would be "wasteful" in a financial sense to launch them on rockets meant to haul much more.  The various spaceplane ideas could possibly have lower launch costs,  but the payload is a much smaller fraction of launch weight in systems like that. 

I think the spaceplanes will be carriers of smaller items (including crews if we really can make them safe enough).  The great massive things will fly mainly on vertical launch rockets.  So says "Cassandra".

As for launching propellants to LEO,  pre-tanked stuff comes in many sizes.  Big ones ought to fly on vertical launch rockets,   little ones perhaps on spaceplanes. 

Really "hard" tanked items could be shot into space by a light gas gun,  with a small solid motor for circularization.  That could be done for well under $100/lb.  Maybe even $10-20.  We'll see.  But your tanks (or other items) must withstand a 1000's-of-gees launch,  like artillery shells. 

GW Johnson

The Area 1-X that flew was a 4-segment motor with a dummy mass on board for the 5th segment.  I know the solid business,  making that 5 segment motor work right without instability or erosive burning will be a challenge.  For example,  a bit of pressure oscillation can easily feed back into vehicle dynamics,  something not seen in static testing on the ground.  I  heard "thrust oscillations" mentioned multiple times during the Constellation effort. 

GW

Might it not be easier to just replenish atmospheric gases every few centuries-to-millennia with asteroid/comet impacts,  than to attempt planetary engineering on the scale of adding a massive moon?  Or building a planet-girdling conductor and energizing it?  Just the odd thought from an old guy.

GW

I think Void may be right.  This sort of pond or ice cave thing would work on the Moon,  or even the larger asteroids.  Anywhere there was ice. 

For gravitational induction of pressure by overburden weight,  you have to allow for the strength of the gravity.  Material depths here for a given pressure get divided by the relative gee there.  On Mars it is the Earth depth/0.37.  Depths are about 6 times larger on the moon,  and way larger on the asteroids. 

It would probably be easier and more effective to take at least partial advantage of the structural strength of the ice layer as a pressure shell.  But,  cracks are leak paths,  and ice cracks easily.  Failure would be catastrophic.  That's a problem yet to be solved. 

GW

See the "Gator" thread this same topic,  for info about "air breathing" engines for Mars.  I looked up a bunch of submarine and torpedo propulsion stuff.

GW

Gases into solution:  bubblers like in an aquarium,  I guess. 

GW

Very interesting stuff about ballutes.  I'm glad somebody's really working this problem,  there's great potential there. 

Radiative heating is the dominant heating effect by far for high-ballistic coefficient items like capsules,  warheads,  and the shuttle.  I dunno about the low-ballistic coefficient regime,  but I suspect it may still be dominant,  or at least very important.  However,  at 2 watts/sq.cm,  you might just actively-cool your way through,  by maintaining gas flow through the ballute somehow.  Just an odd thought. 

I never used them in hypersonics.  My experience was subsonic to about Mach 2.5.  In that regime,  ribbon chutes flew more stably than the conical towed ballute we tried.  But the real bugaboo for us was inflation.  We were trying ram air inflation,  and it failed about half the time for us.  Powered positive gas inflation eliminates that problem,  and allows inflation in vacuum,  too. 

GW