New Mars Forums

Looking at the post that started this thread,  in the hybrid chemical-nuclear engine we are talking about adding oxidizer and burning chemically,  after heating the hydrogen through a NERVA-like scheme. 

In that kind of scenario,  the temperatures will go high enough to ionize the propulsion stream quite significantly.  The more chemical combustion you add,  the more ionization you get.  That's the same problem that limits plain ramjet combustion to Mach 6 airspeeds:  combustion temperatures getting into the big-time ionization range when they exceed about 4500 F (2480 C).  All chemical rocket engines already suffer a little from this effect,  since they burn with chamber temperatures in the 5000-6000 F range (2760-3320C).

Why ionization is a problem:  the nozzle converts internal energy to kinetic energy,  but it does not recover anything from that portion of the applied energy that went into ionization.  The recombination contributes nothing to exhaust velocity. 

Somewhere,  I think I saw a proposal to afterburn a NERVA-like device with injected oxygen.  But the injection point was somewhere in the nozzle bell,  to avoid the ionization problem. 

It's something to worry about,  anyway.

Sensible air is really a concept for airbreathers and folks calculating drag forces.  It's a fuzzy limit,  depends on how large the forces are,  in which you are interested,  compared to the other forces in your problem.  Ramjet,  scramjet,  and turbine thrust forces tend to get rather small compared to vehicle weights,  somewhere in the 60-80,000 ft range.  Vehicles propelled only by those kinds of devices tend toward 0.01 gee (or less) accelerations at around that altitude,  instead of the 0.1 gee+ we'd like to see.

GW

As best I understand,  injecting oxygen into an otherwise-designed supersonic inlet-type ramjet would not sensibly affect thrust,  but it would lower Isp.  What it really does is stave off the low pressure-induced high-altitude flameout problem,  simply because the oxygen concentration is higher.  It makes the chemistry a tad more vigorous,  when it otherwise doesn't want to be.

Whether that effect is actually worthwhile,  is more than a little problematical.  At the high altitudes where that becomes significant,  the air is so thin that frontal thrust density is already rather small compared to frontal weight density.  Thrust margin over drag divided by weight is your acceleration,  which at low-"density" aero forces and high-"density" weight force,  is small indeed.

If your acceleration is low at high altitude (and it will be,  oxygen injection or not),  it takes large numbers of range and time to accelerate to some desired speed.  Especially for a vehicle that must also fly back to launch point,  that adds greatly to weight,  for the extra fuel that the long burn time requires.  It's still a killer,  even for a one-shot missile.  You are way better off,  if you can accelerate abruptly,  rather than gradually.  That simply takes thicker air (i.e.,  lower altitude).  True for turbine,  too.

Nothing absolute about this,  it's a trade-off,  different for every design.  But,  forced to generalize,  I'd say stay under 60,000 feet (about 19 km) for "best" results with launch accelerators.  Although,  the design cruise altitude for ASALM-PTV was 80,000 feet (about 25 km).  That was a ramjet designed for a Mach 4 cruise up there. 

Like I said,  every design is different in detail. 

GW

Decimator:

Didn't you and I and Josh pal around at the recent convention?

GW

NERVA "did it" with limited-life and liquid hydrogen at a fully-ionized MW of 1.  If we were to reprise NERVA today,  I'd think Isp 900 s at engine T/W 4 would be quite feasible for a very first article,  again with LH2.  But,  it's LH2.  Problem!

I'd certainly do that again for a first article,  but I'd also start a parallel effort to solve the chemistry and materials problems with a "water NERVA",  as in the sci-fi of the 1940’s and 1950’s.  Performance might be somewhere near the 600 s Isp we are discussing here (not close to 1000 s),  again at engine T/W 4 in a first article. 

That's actually getting very close to what Josh started this thread about.  Follow-on articles would look more like Dumbo (metallic instead of ceramic reactor fuel containment) than Timberwind (a fluidized-bed particle-layer reactor),  at T/W approaching 10.  If not SSTO,  then that gets you a very practical TSTO with a minimal throwaway 2nd stage,  and perhaps a recoverable winged 1st stage working VTO and HL. 

The main attractive thing about a water NERVA is its rework into a space engine instead of a launch device to LEO.  Avoids all the safety-of-flight concerns regarding damage to the public.

Plain water is very easily transported as icebergs,  and is also almost impervious to meteoroid impact damage that way.  Water-as-ice is located on a variety of bodies,  including Mars,  and might well be fairly easily mined,  depending upon the exact nature of the deposits (which also points out how important prospecting/ground truth really is,  as we explore with men). 

Separation of solid contaminants from liquid water is SO very easy,  even if you have to use spin-derived artificial gravity.  My best guess is that any solution to the chemical/material challenge of a water NERVA could easily handle CH4,  NH3,  and similar liquid contaminants in the water,  so purity should not be much of an issue,  although salts might be.  Be nice if our NERVA could handle salt water,  wouldn’t it?  Think 2nd-generation device. 

How could one possibly "beat" a high-Isp/high-thrust engine (with VERY easily-obtained propellant throughout this solar system)?  It's a dream-come-true!

The follow-on is something that deserves very heavy-duty development,  too.  Consider an open-cycle gas-core water-propellant nuclear thermal engine as a deep space engine.  If I had to hazard a guess,  I'd say engine T/W could be near 30 at Isp near 2000 s.  The real limitation is that regenerative cooling won't be enough for the shell and bell.  But,  there is no core or core-containment to damage or melt,  it's already plasma.  (This might also mitigate some of the safety-of-flight issues for nuclear launch,  too.) 

All of that is fine for relatively-small vehicles.  But,  for colony-planting vehicles,  nothing (and I do mean nothing!!!) beats the nuclear explosion drive.  It works best in gigantic sizes (10^4+ tons),  so there's little point to using it in small vehicles,  as NASA "tried" to do in 1965,  looking at it for the manned Mars mission then planned for 1983.  They went with NERVA instead,  and chemical as its backup.  (Lost-water-under-the-bridge,  since all of that was cancelled in 1972,  in the middle of the Apollo landings.) 

Beyond that,  we're looking for Star Trek-style impulse engines and warp drive.  The physics ain't there yet for them,  much less the technology. 

As for “combined-cycle anything” in chemical propulsion,  the geometry changes have always been the bugaboo.  Anything we can practically do for changing geometry very badly (usually fatally) compromises the performance of the individual cycles. 

For example,  subsonic combustion ramjet usually features a constant area combustion chamber contracting toward a throat by a throat/chamber area ratio 0.65 max,  and not much less.  The exit bell falls in the 1.5-2 area ratio range.  Period.  That’s what works.  Anything far from that essentially loses all the performance potential.  I know,  I used to design ramjet missiles for a living. 

Scramjet (supersonic-combustion ramjet) on the other hand features (at worst) a constant area chamber,  and usually gently-expanding at around 5 degrees half angle.  There is no throat contraction at all,  but locating the final expansion bell axially can make-or-break obtaining a burn at all.  This technology is still very far from being ready-for-prime-time,  the recent X-51 flights notwithstanding.  There’s no good way to reconcile these conflicting geometries except by one-shot/throwaway ejected components,  and even that is very most certainly not a trivial exercise,  or a “sure thing”. 

And,  we have not addressed inlet geometry incompatibilities between ramjet and scramjet at all.  They are huge,  more especially in the internal ducting lines,  surprisingly enough.  The external compression features are actually quite similar,  which is terribly misleading.  Failure to get this right causes as many violent explosions in scramjet test articles as does too-low a scramjet takeover Mach.  It is quite catastrophic,  and (so far) quite incompatible. 

Integrating ramjet-or-scramjet with rocket is even worse.  Most rockets have a very large area ratio contraction from chamber to throat:  on the order of 10+.  And the exit bell area expansion ratios exceed 10,  often by a very,  very large margin.  I know of no variable-geometry techniques to accomplish this kind of geometry change,  except one-shot/throwaway ejectable insert items. 

You can go from rocket to ramjet that way,  but you absolutely cannot go back to rocket.  How are you going to change back to rocket in a combined-cycle engine,  if your trade studies say that you want to do that?  I don’t think anybody on Earth has a practical answer to that,  excepting maybe the Skylon folks with their SABRE engine,  and I am very definitely not even sure of that!  (I hope they do,  but I am most definitely not going to bet the farm on it.)

And that’s why I think parallel-burn options for the differing engine types are way-to-hell-and-gone far superior to any combined-cycle proposals I have ever heard of.  I have seen many of those for the last 4.5 decades.    None has ever led anywhere,  before.  Not a good track record. 

But parallel-burn works,  both ways.  Try that.  We can do it right now.  All-existing technologies.  Not trivial,  but very definitely do-able. 

GW

The real question is aeroheating damage to the airframe at speeds above Mach 6 (1.8 km/s in the stratosphere).  This was catastrophic shock-impingement damage to the aft fuselage of the X-15 at Mach 6.67 with a scramjet test article on the ventral fin stub.  The compression cone shock wave hit the underside of the X-15's tail.  Had the flight lasted seconds longer,  it would have cut the tail off the aircraft and caused a fatal crash.  Above M6 in sensible air,  the flight configuration has to be very "clean" so that shock impingement does not occur:  basically a simple dart with stub fins,  no nacelles.  There are enormous pressure gradients through the wave,  too.  You have to be structurally extremely tough,  as well as proof against ridiculously-high temperatures. 

On the other hand,  if you leave the sensible air before accelerating past M6,  then you can avoid this issue.  Shuttle's side-mount configuration avoided this by leaving the sensible air (about 80,000 feet or 25-ish km) at only about Mach 2.  There's no sharp boundary here,  the shock-impingement aeroheating problem can be significant at about M3,  depending upon the materials you are using.  X-15 had superalloy Inconel-X skins,  which pushed the critical speed to about M6.  They go into trouble by (1) going faster and (2) adding a nacelle. 

Maybe Terraformer has the right idea just above:  take off on rocket,  climb then pullover-and-accelerate on ramjet,  then relight the rockets and climb/accelerate to stage more-or-less in vacuum at 3 km/s.  The booster airplane is a bit bigger because of the high altitude burn of very significant delta-vee,  but the payoff is a second stage with a lower mass ratio requirement,  so that its payload fraction is much larger.  That reduces second stage size,  but probably not overall launch size. 

The portion of the first stage trajectory that can be powered by the ramjet shrinks in that scenario.  For a supersonic design,  takeover speed is about M 1.6-to-2 (0.5-to-0.6 km/s).  The max speed capability will depend more on vehicle drag than the engine design,  but with missile shapes falls in the Mach 4-to-6 range (1.2 to 1.8 km/s at stratospheric speeds of sound).  The numbers I have run indicate too little thrust margin over drag for the weight,  at altitudes over about 60,000 feet (20 km),  because the air is getting too thin.  Acceleration times become impractically long,  raising vehicle weight again for all the extra fuel.  Every design is different in detail,  of course. 

It's awfully hard to imagine a two-stage airplane design that (1) does not involve "effective nacelles",  and (2) does not have higher drag because it is a cluster.  Both are leading toward lower peak speeds from ramjet alone.  If you solve that dilemma,  pushing peak speeds toward Mach 5 or 6,  then you run up against the extremized form of the shock-impingement aeroheating problem.  This will be inherent,  because the staged aircraft configuration will almost certainly have a side-mounted second stage,  in effect a "nacelle" that sheds the offending shock wave,  and perhaps also gets hit by one from the first stage.  (THAT is also why I see no point to using scramjet for launch.) 

Spaceplane design is not easy.

GW

I like the electrolysis idea for Mars.  The combustion-based traditional methods are an artifact of our oxygen-bearing atmosphere.  Things are vastly different without that environment. 

Dunno if you can simply separate minerals by crushing,  they may still bind together until your particle size is of molecular dimensions.  That's impractically fine.  Even colloidal sizes are very expensive to do,  and that does not undo chemical binding. 

GW

Hmmmmm.  There is a concept for HTO TSTO (two-stage orbital craft with a winged airplane for a first stage) that I haven't looked at before.  In all the descriptions above,  The staging occurs at M6 (1.8 km/s) at 20 km altitude with a ramjet airplane.  It is beneficial to pull up to about 40 degrees at staging,  which requires rocket thrust be added to the ramjet thrust. 

New idea:  go back on rocket thrust at that point and thrust to a bit higher staging speeds in thinner-to-no air a bit higher up.  The penalty is a larger first stage airplane,  because it has to hold more propellants,  and more of its acceleration trajectory is powered at (lower) rocket Isp,  which compounds how big it grows to be.  The advantage is reducing the delta-vee required of the second stage below the 5.9 km/s in the prior discussions,  letting such craft be more potentially reusable while having higher payload fractions (for better economics).   

The ultimate limit on payload becomes more like that with an all-rocket TSTO airplane design (somewhat like the original concept for the shuttle,  except HTO,  not VTO).  There is a practical limit to the size of the airplane we can build,  because it starts to look like a water balloon supported on sharp nails,  when parked on its landing gear waiting to launch.  Square-cube scaling law at fixed material stress-strain limits.  And wing-loadings are limited by practical takeoff and landing speeds.  The bigger the wing the heavier,  by roughly the 0.58 power of the wing area. 

Unexplored trade study territory.  Interesting.  Hmmmmmmm,  again.

GW

ps --  I'd resist going VTO with the winged first stage,  because that way "traditional" rocket-launch huge logistical tail-type thinking gets into it,  making it hugely expensive.  Those government-type launch rocket guys never think like missile/weapons guys,  because the missile/weapons guys are all thinking one-shot by their tradition.  Spacex and ULA are the commercial exceptions that prove my point:  lower costs with reduced logistical support.  If it behaves more like a traditional airplane,  then traditional airplane-type thinking goes into it,  and that is aimed at extremely-low logistical support requirements.  That means far lower costs.

I really don't think kerosene is something we could practically manufacture on Mars,  but it might be somewhat representative of a hydrocarbon heavier than methane,  that we might dream up a process for.  It is a very well-known technology.  But,  I'd bet we can find a way to ignite or keep-unfrozen any of these choices,  though. 

I really don't think NTO or any of the hydrazines might actually be practically manufactured on Mars,  without a source of fixed nitrogen.  That's a huge obstacle there,  as far as I know.  But,  we already know those propellants can be easily stored,  and we have had engines that re-light multiple times in vacuum with them,  for decades now.  That's pretty much the technology of the shuttle OMS maneuver engine pods. 

I suspect LOX-LH2 would actually be the "easiest" to manufacture on Mars,  using mined ice and electrolysis as the basis.  LOX is not too much trouble to liquefy and store;  LH2 is much trickier to do,  with the ortho vs para form problem perhaps now the easiest problem of several to resolve. 

On Mars,  the truly fundamental problem is "where is the ice deposit big enough to be worth mining?"  We now know for sure that Mars has lots of water still (in the scientific sense),  the trouble is that it's just not "everywhere".  The kind of ice lenses Phoenix found near the pole is not the kind of deposit that supports practical mining and manufacture.  What we need is a buried glacier 10+ meters thick and many,  many km in lateral extent. 

BTW,  it'll be subliming as we dig it out.  Every mine hole has to be regolith-buried when not in use.  There will be one whale of a lot of regolith-moving operations involved in this activity.  The machines will look like heavy mining and road-building equipment.  That takes a big lander,  even if shipped in small pieces and assembled on site.  These things will not be carried by a series of Apollo-like dinky-little landing modules.  No way.  We need real "landing boats" of very significant size. 

They're not gonna fit existing payload shrouds for launch to LEO for this mission.  Something else to think about. 

We have orbital observations of where some such buried glaciers might be (emphasis on "might"),  but we have absolutely no ground truth about it.  I have never seen a robot probe design capable of determining that kind of ground truth,  either.  So,  if we are going to plan on making LOX-LH2 to return,  where do we land? 

Tough question.  We have to be close enough to walk to the ice,  or it ain't gonna work.  We're talking front end loaders,  bulldozers,  and large pressure-vessel process machinery here,  with maybe even some pick-and-shovel work by more than 2 men.  Long range transport is simply out of the question,  that first time up with propellant manufacture. 

As for making methane out of water,  and the CO2 in the "air",  the low inlet densities make all your machinery (whatever it is) look very large and heavy and energy-intensive,  compared to what we are used to here at home,  by about a factor of 14.  My guess is you can make 1's,  not 100's,  of kg per day.  You'll not accumulate enough to return a crew (tens of tons),  not even in a year's stay,  even if it doesn't break down or encounter unexpected problems. 

And you will encounter unexpected problems (lots of emphasis on "will").  Done robotically before the men arrive offers a potential way out,  except that robots-as-we-know-them-today are simply inept at solving unexpected problems.  Put the men there to solve those problems,  and you are right back to the inability to accumulate tons of propellants in time.  Plus,  with LOX-CH4 you still have to solve basically the same water problem as LOX-LH2,  to get the oxygen and the hydrogen. 

So I dunno which one to try.  And I don't yet see much of a path to resolving this in time for a mission in the 2030's,  much less the 2020's we'd all like to see.  NASA has no plans to send the right kind of probes that could locate the propellant-making resources.  I don't see anybody else sending the right kind of probes,  either.

That puts me back to the costly-but-sure-thing concept:  first mission relies on propellants-sent-from-Earth.  Which means it is an LMO-based mission,  sending down multiple ferries to multiple interesting sites,  and emplacing the machinery to experiment with propellant manufacture at the most promising ones after the men return home.  Leave the ferries in a higher Mars orbit,  with whatever propellant is left over,  for the next mission to use.  What's the point of going all that way with men,  and only making one landing?  That's really dumb!

Meanwhile,  we have to guess which propellants might actually be made on Mars most practically,  and build the first-mission ferries to use that.  That way subsequent missions (including planted bases) can refuel and re-use the same ferries.  Right now,  I'd guess LOX-LH2 from ice.  But with an engine compartment big enough to accommodate being refitted with different engines.  And with compartmentalized tankage to accommodate being re-plumbed for different propellants.  That's heavier,  and so is structural robustness necessary for long-term reusability.  My assumptions of inert structural weight 20% are quite likely too low. 

Any other ideas?  Opinions? 

GW

I don't pretend to understand quantum mechanics.  I understand special relativity,  except that I disagree that the infinite-mass result at V=c means there is a speed limit.  I think it means there is an observation limit,  speed itself is not limited.  So I think FTL flight is possible,  but navigation will be hell unless we find some new,  better theories for this. 

If I understood quantum mechanics,  then I might be able to offer an opinion about FTL communication-by-entanglement.  I do not understand that stuff.  I know those are English words when I read about it,  but I have no dictionary.  It's gibberish to me. 

That being said,  all I do know is what I've read in the journals.  The entanglement thing seems to be instantaneous,  or what Einstein termed "spooky action at a distance".  That by definition is FTL.  The disagreement seems to be whether useful info can be sent by this technique.  The consensus of what I read says no. 

But,  I don't believe it's settled.  Nothing ever is.  As we see experimental results that defy what we thought we knew,  we develop new theories (or new versions of existing theories) that encompass the new result.  That's been our history since we "invented" science. 

Josh is right:  by our current knowledge and theories,  the consensus is that FTL communication is not possible (although FTL quantum entanglement effects certainly are).  Once we have had time to play with this stuff and find out more about what can be done,  our theories and knowledge base will be adjusted.  That's the right way to do it. 

Whether that will lead to FTL travel and communications,  who knows?  But,  it is already clear that some quantum things really do happen FTL.  That gives me hope for the rest (the travel and the communications). 

GW

Has anybody considered hybridizing this reusability concept?  A combination of reusable and throwaway components?

How about a reusable first stage core and strap-ons?  Don't worry too much about recovering the second stage.  Entry is less challenging at 3 km/s than it is at 7 km/s.  That 3 km/s figure would be typical for the first stage of a TSTO vehicle,  as they are currently designed.  Use the strap-on concept to help make up for the extra inert weight in the reusable core,  since that weight hurts you worst near takeoff. 

Use a "pointy-ended" first stage core and a "Russian-style" truss interstage,  cheap enough to throw away.  The pointy end is the heat shield to protect the tankage,  and coming back streamline avoids broadside crush load breakup.  The engines at the rear are protected from airloads and entry heating better. 

Stream out a series of chutes/ballutes of increasing size out the rear to slow to subsonic,  then swap ends suddenly by clever chute redeployment,  for a rocket-braked touchdown on land.  Here on Earth with our dense atmosphere,  one can use chutes like that to reduce the amount of rocket-braking propellant. 

For launch sites like Canaveral,  first stage touchdown will be at sea.  I'd land by chute alone,  pointy-end first.  Better chance of not breaking up on ocean impact that way.  No rocket-braking in that scenario.  You can keep the truss interstage.  If it survives entry at all,  it can act as a sacrificial spike to help open the cavity into the water upon ocean impact. 

Either way,  the tankage is going to see some rather heavy airloads,  and some of them will be lateral,  as you swap ends.  Even subsonically,  that is tough to do.  But we have a history of heat-shields,  including hitting the sea successfully with them.  Ocean impact loads are really enormous,  too,  even with really big clusters of really big chutes.  Anything over about 30 mph sees loads not unlike those striking hard Earth.  Ask any waterskier about that. 

Just tossing out some ideas that seem feasible to me.

GW

Hi Josh:

I would think those hematite "blueberries" would be an easy source of fairly high-grade iron ore to utilize.  Just scoop 'em up with the equivalent of a front-end loader,  and process 'em in a reducing atmosphere.  Hematite is considered a pretty desirable ore here on Earth.  Mostly iron oxide mixed with other rock minerals,  I think it is. 

Based on some of the other discussions I have seen on these threads,  mined ice could be a source of hydrogen with which to reduce the hematite.  Products could be an equivalent to pig iron,  perhaps silica from the slag (as you suggested just above),  and probably some water from the gas exhaust (on Mars,  one should not think smokestack,  should think recovery instead). 

If such a process were emplaced and operated successfully on Mars,  it could teach us here at home a great deal about how to do heavy industry far more cleanly.  There's a really nice "spin-off" for you.

GW

This quantum physics stuff is way beyond me,  I'm an old aero/mechanical engineer. 

But here's thought regarding the "great silence" of ET's.  If they are capable of rapid interstellar communication and flight,  I would guess that capability depends upon physics we don't yet have,  and probably not upon anything detectable anywhere in the EM spectrum all the way past gamma ray frequencies.  There's no radio or laser to detect,  they do it differently. 

Here's another thought (not original with me,  see the 1951 film "The Day the Earth Stood Still").  We have ventured out into interplanetary space with probes,  and may soon find some means to send an interstellar probe.   We are also at constant war with ourselves.  If ET is out there,  why would he reveal himself to us?  Not until we-as-a-species outgrow being a threat to all bystanders.  Anybody with the capability of interstellar travel and communication could easily stay hidden from us. 

GW