New Mars Forums

A while back,  I tried to make a fair comparison of unit launch costs,  given that size of launcher affects it.  My results are the curves posted over at "exrocketman" in the 9-13-12 article titled "Revised Launch Cost Update".  (That site is http://exrocketman.blogspot.com,  for those not familiar with it.) 

I calculated a min unit cost as max payload capability to LEO divided by turn-key launch price.  Some of the cost data I found on the internet were older,  so I tried to inflation-correct everything to 2012 dollars for that article.  (Actual unit cost could be much higher if you don't fly "full".)  I then plotted these versus max payload capability to LEO. 

The curves show very high unit cost for smaller launchers,  but a very gently decreasing trend with size from roughly 10 tons on up.  That slope is quite gentle.  All the commercial vehicles seem to fall on it,  including projections for Falcon-Heavy. 

Titan-IV (no longer available) was never commercial,  government only.  It's about 4 times higher than the commercial trend.  So also are projections for SLS,  which at 100 ton sizes,  shouldn't cost more than around $500/pound,  if it followed the commercial trend line (it does not).  Of course,  SLS might actually fall even higher:  the government is infamous for radically-underestimating costs early on in a program.  (Any program one cares to name.) 

Spaceplane systems like shuttle are higher still,  because that's a different way to launch and operate.  I suspect vertical launch rockets with reusable stages will also be quite different,  and on a higher trend as well.  I have a hunch these might be attractive in the very small tonnages,  while expendable vertical launch rockets will be very hard to beat in the large tonnages (until we start building real "Star Trek"-like propulsion). 

I'm no fan of SLS.  The wrong people are trying to build it.  The right people could do a far more cost-effective job.  (Sorry Congress,  that's just the history as testified by the real data.) 

GW

The natural background in the US is not a "good figure" for estimating medical effects because it varies so much from place to place,  even discounting radon gas in basements.  There are lots of places with a little less than average,  and a few places with a lot more,  by factor 10 !!!,  such as the mountains in Colorado.  Even in those areas of far higher background,  there is no discernible difference in cancer rates that I ever heard of.  Very little is understood with certainty about acceptable doses of very low radiation,  except that very little effective harm seems to occur with present lifespans 70+ to 90-something years. 

As for shielding while on Mars,  think a meter or two of regolith piled on top of whatever habitation structures you build there.  Wet it down and let that freeze as you pile it,  and you have a hydrogen-rich shield of all-indigenous materials (permafrost),  with very little processing and no refinement.  Subsurface ice seems quite common on Mars.  Put it in a tank,  melt it with solar heat (self-pressurizing as the vapor equilibriates inside the closed tank),  and let the solid contaminates settle out.  Easy source of water for a variety of uses.

GCR threats vary by roughly a factor of 3 through the solar activity cycle,  a solar wind effect inside the heliopause.  I don't know about the variation radially away from the sun.  At min activity GCR around these parts (Earth's distance from the sun),  is around 60 REM annually,  compared to NASA's astronaut limit of 50 REM annually.  At peak solar activity,  GCR is somewhere around 22 to 24 REM annual. 

20 cm water cuts those exposure levels in half without incurring secondary particle shower dangers,  and that same 20 cm water will protect against the largest solar flare events.  So,  wrap the water and wastewater tanks,  that you already know you must have,  around a designated shelter on your ship.  Easy shielding making water do double duty in your design. 

The smart ship designer would make that designated shelter the flight control deck.  That way,  critical maneuvers can be made regardless of the solar weather. 

As for peak GCR at solar min being slightly above the limit,  I'd bet the proposed vehicle structures will cut the 60 REM in open space to pretty near the 50 REM limit,  generally speaking.  No water involved. 

On Mars,  I wouldn't count on any natural atmospheric or magnetic shielding effects as regards GCR exposures.  But even without being down in a crater,  the presence of the planet cuts the exposures in half to around 11 or 12 REM annually min,  max around 30 REM annual.  And,  one is not outside one's habitations 24-7,  either.  If your buildings are shielded the way I suggest,  your exposure is near 0 for all those portions of the day you are inside. 

I don't remember the conversion from REM to Sieverts.  It's all supposedly some kind of metric stuff,  though. 

GW

I would think that,  once on Mars,  you simply put a lot of regolith (multiple meters) on the roof over your head,  thus providing excellent shielding.  This would take care of both the slow drizzle of cosmic rays,  and the brief flashes of solar flares.  That lets you maximize time outside doing good things,  without overdosing on radiation so soon.

For the cruises to and from,  it appears that the capsule or habitat structures might provide a bit of shielding.  However,  whether you do a closed or open life support system,  water and wastewater will be involved.  The "killer" in cruise is the brief flash of a solar flare.  20 cm of water will shield those solar flare events just fine,  and cut the cosmic rays by about half. 

So,  pick a space to be the shelter,  and wrap those water and waste water tanks (that you must have,  anyway) about it.  If you were a smart ship designer,  the radiation shelter would be the flight control deck or space.  That way,  critical maneuvers could be flown regardless of the solar weather. 

Midoshi is right:  the biggest factor in this is the allowable dosage.  Not everybody agrees.  We have a standard that NASA uses for astronauts that is at least twice what is recommended for civilian adults.  For children,  the recommendations are even less.  But there is little agreement among the various countries and groups-within-countries. 

I think we could use the NASA astronaut standards with reasonable confidence for explorers sent to Mars and then returning.  The doses are gender and age dependent.  For a colony or one-way mission,  we'd have to come up with dose standards for children.  Don't have any now.

GW

Midoshi & Bob:

Since Kepler is in a trailing orbit,  I'm assuming one gets there by an ellipse outside Earth's orbit,  with a period that differences from our year by just enough that its perihelion hits Kepler's location.  The delta-vees for that ought to be fairly modest (once escape is achieved),  but I'd bet the transit time is a few months,  not days or even weeks. 

Not much different from going to Mars!

Any ship capable of hauling men on that mission could also be used to go to Mars.  You just don't need landers to visit Kepler. 

For months one-way,  you will need radiation protection (a shelter with 20 cm of water) and you will need artificial gravity (centrifugal force).  Life support can be stored supplies,  especially adding some frozen food,  since the round trip is likely over a year (about the limit for our "astronaut foods" as we know them).  It wouldn't hurt to consider meteroid protection and repair (foam/foil layers seem to work well and are also good insulators). 

I think a manned repair mission to Kepler would be a good rehearsal for all but one critical piece of hardware needed for going to Mars (that would be the lander). 

Many would argue that artificial gravity is the "killer",  but it is not.  Especially if your vehicle is the constellation of docked modules that it has to be,  if we use the rockets we already have to launch it!  You just dock your modules (mostly propellant tanks) in a "slender baton" shape,  with the habitat at one end,  and spin it end over end.  Very stable,  as demonstrated in Friday night football stadiums all over the country. 

As you expend and jettison tanks of propellant,  you reconfigure to maintain the same length "baton",  just skinnier.  Brief intervals of zero gee we know how to handle while maneuvering,  but for the long coasts,  artificial gravity makes both life,  and life support,  a lot easier.

Guys,  this could be done within 5 years by the right team,  and that's just a couple of design/checkout-test cycles.  We actually could have done this several years ago,  just like we already did building the ISS.  It's very little different.  Experience then makes it cheaper and quicker now,  too. 

There's an overall purpose for NASA-as-an-agency there,  if ever I saw one.  But I'd rather see someone not stultified into rigidity (meaning non-governmental) actually do this.  That's how one does this for less time and money. 

GW

Some time in the mid-to-late 1950's,  there was a series of high-temperature experiments done at one of the New Mexico air force bases.  I don't remember which base.  These experiments exposed volunteers to oven temperatures,  but at extreme low humidity (the New Mexico desert).  The final experiment was somewhere around 1/2 or 3/4 of an hour at 350 F.  The men were fine,  but a tray of raw cookies that went in with them burned. 

I myself have worked for up to an hour in-and-out of jet blast from various things,  in much higher humidity.  Temperatures were quite variable,  but peaking around 300 F.  Not a whole-body immersion,  either,  just blast on one side or the other.  Sniffing for unburned fuel traces to diagnose combustion problems.  In the case of a turboprop running on biodiesel blend,  I could identify the source of the biodiesel by its burnt smell.  It was actually quite funny and a lot of fun to do. 

Biodiesel from waste cooking grease smells like french fries cooking.  Biodiesel from animal tallow smells like barbecue cooking. 

GW

Grasshopper uses one Merlin-1D engine,  which has a higher engine thrust/weight than the Merlin-1C engines powering Falcon-9 right now.  Merlin-1D will be flown soon on Falcon-9,  upping its 28-degree LEO payload out of Canaveral to 13 tons from the current 10 tons.  The 53 ton capability of Falcon-Heavy is,  and always has been,  predicated up Merlin-1D's.  (Metric tons.)  That's based on their website. 

I dunno what good Helium-3 is right now,  or why it might be valuable right now,  since there are no fusion reactors or fusion rockets to use it.  Once there are,  I understand that it could be a good fusion fuel.  But those technologies still appear to be a good ways off,  controlled fusion being "just around the corner" since the mid-1950's.  Not a good trend,  that.  We're gonna need a breakthrough. 

GW

The real cause of finite life out of the ISS is related to "stardreamer's" cause #2:  infrastructure degradation.  The systems inside,  and the structure of the thing,  will wear out and cease to function.  It is inevitable. 

They have already had troubles with most of the life support systems inside,  only repeated repairs and replacements keep it functional.  The same will prove true of the solar PV panels outside.  There's already been one repair. 

Eventually,  the shells of the modules themselves will degrade,  as will the docking structures that hold them together.  They were fragile enough to begin with,  just to be light enough to fly at all.  Only a low-thrust thing like some sort of plasma device would be feasible,  to move a fragile thing like that to another orbit,  and it would take years to accomplish such a move.  Same for leaving LEO,  just longer still (bigger delta-vee).

My bet is that it will be a race to see whether the life support gear inside,  or the solar PV gear outside,  will become unrepairable first.  Once that happens,  the station becomes uninhabitable and therefore useless,  and also a falling-debris threat wherever it might be located.  The projections for that outcome vary,  from about 2020 to about 2030.  But it will occur,  and sooner than anybody wants. 

I would not bet any money at all on it surviving to 2030.  But that's just me.

Same thing was just beginning to happen to Mir,  when it was deliberately crashed in the Pacific,  just before ISS construction started.  It wasn't quite finished deteriorating,  but the Russians wanted to participate in ISS,  and could not afford to participate in both. 

GW

I said water,  not hydrogen,  was the most abundant volatile around. 

One would accept the lower Isp of a water NTR just so one could refuel anywhere there is ice,  without any propellant processing other than melt and filter out the dirt.

I will post some data from the book,  but not now.  Most of devices tested were developmental/research.  There was one baseline engine (the XE device) tested as an engine.  Lots of improvements to that engine were tested in the other devices.  Several of those paper designs showed large improvement in T/W.  Many of those were candidates for the first or subsequent flight test engines,  which were never built. 

When I said flight-ready,  I was referring to experimental flight test.  Not operational (routine) use.  If you knew anything about the engineering development process or the actual events of Project Rover,  you would have known that.  But OK,  from now on I will say ready-for-flight-test,  not flight-ready. 

At the start of Sorensen's first article he quotes 15,000 lb thrust at 5000 lb weight.  Sounds like T/W 3 to me.  Some of the Enabler designs  were thought capable of 10:1.  Never got the chance to be tested though.

GW

Well,  it's a little fragile to be enduring the forces of an interplanetary trajectory injection burn.  Other than that,  I like the idea.

There are some other shortfalls in its design,  relative to the mission you suggest. 

We have found out that there is a fuzzy,  ill-defined limit to how long humans can endure zero-gee without irreparable damage.  It seems to be somewhere over a year.  That station needs a huge centrifuge for astronauts to live in and stay healthy for its current mission.  It needs the same thing at Mars or Venus.

At either Mars or Venus,  there is no protection from either solar storm or galactic cosmic radiation.  That protection would have to be added before the station left LEO,  or you'll kill the crew in it next solar storm.  It's an ugly,  miserable death,  too.

GW

Impaler:

I think your anti-NTR rhetoric may be a bit overblown.  Most of this stuff you read about it are exaggerations feeding off the real safety concerns.  And they are real,  but not actually as bad as is often trumpeted. 

NTR as NERVA suffered from high engine weights for the thrust,  although not all future NTR need be the NERVA design.  They may not all suffer from that inert weight problem. 

It would be rather silly to use NERVA for launch to LEO,  although that was the first slated application back about 1973:  a replacement upper stage for Saturn-5.  It was ready-to-fly,  and was about a year from actually flying,  when the whole thing got cancelled along with Apollo itself.

As an orbit-to-orbit transport,  that engine weight objection is far less severe,  as long as the engine thrust/engine weight ratio remains near 1,  and the nuclear improvement in Isp can "shine",  as an indirect cost reduction. 

As an old NERVA-type NTR,  such vehicles usually size out 2 to 4 times smaller,  compared to the various chemical options,  including those who have the same hydrogen packaging and storage problems.    I don't know what numbers you might be looking at,  but these are the typical ranges I see when I work it out for myself. 

Having been flight-ready 4 decades ago,  the "right team" really could resurrect this NERVA technology quickly,  if we decide it is the prudent thing to do.  SEP will likely always suffer from a low thrust/weight,  far worse than NERVA,  as you indicated.  The other separate problem is that SEP is much further away from anything like "flight-ready" status.  That's simply not a near-term option for anything.

VASIMR is very little different,  in that its power supply problem,  identical to that of all the other electric thruster types,  leads to an extreme low thrust/weight problem.  Any of the electric schemes could "shine" if there were a flightweight electric power plant in the MW range.  There will not be such a thing anytime soon,  as near as I can tell.  Some of the fundamental science solutions are still missing,  much less the engineering technologies. 

So for a Mars expedition,  or anywhere else nearby (visiting an asteroid has about the same orbit-to-orbit delta-vee requirements as going to Mars),  we are stuck with the various chemical means or the old NERVA.  Chemical Isp could be 460 s with LOX-LH2,  but only if you can successfully store very-demanding cryo-propellants,  in vacuum and in tankage exposed to all the other risks of the space environment,  for about a 3 year voyage.  I'm not at all sure that is a flight-ready technology.  Not for missions of that duration.

All the other practical liquid combinations have performance closer to Isp 280-310 s,  although many are easily long-term storable.  You compare them to NERVA at 700-1000 s,  and that's where the size and cost reductions I mentioned start looking "real".

If you make landing and Earth-return propellants in-situ at Mars (a still-unproven but very promising idea),  you have water-as-ice and CO2 to work with.  You still have to launch the propellants off the surface to refuel the orbiting vehicle for its return.  That's not going to be a trivial operation:  you will need a reusable lander to act as the propellant ferry,  and you will have to refuel it for each trip up,  too. 

There will necessarily have to be a lot of trips.  (I don't think I would choose an ablative heat shield for this,  more weight to carry for replacement,  and all that.)  And,  the landings will require direct rocket braking in those sizes,  aero-decelerators simply won't work at ballistic coefficients that high,  in "air" that thin.  That's a lot of delta-vee required out of your lander on each trip.

Water-as-ice on Phobos or Deimos might help relieve that refueling difficulty somewhat,  although that rules out carbon:  you're pretty much stuck with LOX-LH2 at that point.  The difficulty with that refueling-on-the-low-gravity-moons scenario is that we don't know if there really is any water-as-ice in either of those two places,  and we're not really going to know,  until somebody or something visits them with a big drill rig.  Ain't really gonna happen before we send the big expedition,  if we ever do at all.

Water-as-ice requires energy to melt,  and more yet to electrolyze into LOX and LH2.  Both chemical and NERVA suffer from this need to produce hydrogen.  Neither would be a very effective refueling solution,  due to the infrastructure required to produce these cryo-propellants,  especially the LH2.  The leak problem alone is tremendously-difficult to solve handling hydrogen down here on Earth,  much less out in space encumbered by these idiotic space suit designs we are still using. 

Water-as-water is really easy to handle.  You only need to melt the ice.  It's on Mars,  some asteroids,  and probably most comets,  plus most of the larger outer-planet moons.  Chemical cannot use this directly as propellant,  but a water-variant of the old NERVA could.  So could some forms of solar propulsion,  at least in principle.  We don't have a water NERVA,  never did.  But we could,  probably about 5-10 years after resurrecting the LH2 NERVA.  It's not a big technology leap.  A solar water rocket is decades away from being flight-ready. 

Refueling-as-you-go makes a lot of sense in terms of vehicle size and launch costs.  The most abundant potential propellant resource that seems to be very widely available is water-as-ice.  Does it not make sense to choose a propulsion scheme that can use the water directly as propellant with little or no processing infrastructure?  That's how you reduce launch costs and vehicle sizes without getting someone killed. 

Can you think of any such schemes besides a water-NERVA?  If so,  the crucial question is how soon could we make it flight-ready?  Most of us want to see the Mars expedition mounted in about 10-15 years at most.  Certainly no more than 20.  I'd rather go sooner with what we have,  even if it's chemical. 

GW

As Terraformer said:  What if, say, we already that the propellent in orbit, off the shelf habitats and landers, and a launcher capable of putting 50 tonnes into orbit? I can see the price tag coming down to a few billion dollars in such a scenario...

So can I. 

I just want to see how propellants really can be made in quantity off-Earth. 

GW

From the site videos I have seen,  the Spacex grasshopper is simply a rocket stage with a sophisticated flight control and some landing legs.  It appears to set down upon nothing but a concrete apron.  The thing is a dual-purpose test bed;  (1) lots of throttling-burn experience with the new Merlin 1-D,  and (2) experience with vertically landing a rocket stage under its own power.

Their idea with purpose (2) is to sacrifice some (perhaps a lot) of payload capability in order to not crash spent stages.  I suspect the operational concept uses both chutes and a rocket-braked landing.  They'll have to cut the chute to land on thrust,  otherwise the straggling chute will topple the stage off its thrust vector. 

As near as I can tell from the videos and their website,  this is for future versions of Falcon-9 and Falcon-Heavy.  They hope that reusable stages may lower total launch prices.  No one is sure of that outcome,  though.  If it works here,  then maybe later,  they might consider versions of it for Mars.  But not yet,  not now. 

The one very serious problem they will face is aero-load-induced damage and breakup of the tankage of the stage they are trying to recover.  These things are tumbling at hypersonic speed when they they hit the air coming back,  even just first stages.  It tends to break the stages up and crush the tankage,  long before it ever gets anywhere near the sea. 

Those items that do survive the aero loads,  tend to break up upon impact with the water,  which is only a tad less abrupt than hitting concrete,  at anything over about 20 mph (ask any water skier).  Even the Shuttle SRB's did that all too often,  and they were built to be 900 psi pressure vessels;  these ordinary liquid rocket tanks are far more fragile. 

GW