New Mars Forums

Actually should be about the same performance as direct plate ablation.  At least very crudely.  The difference is you don't lose the plate. 

The reaction mass could be just water.  Probably as ice,  with carbon dust in it to make it opaque and black.  It has to absorb the laser energy efficiently.  Dirty ice should be cheaper to make than plastics or steel. 

It'd be a bit smoky,  though. 

GW

Hi Josh & Void:

In the old nuclear Orion proposal,  the reaction mass was a huge block of polyethylene on the side of the nuke charge toward the vehicle.  The neutron reflector around the fission device was shaped to produce the gamma blast in two sharp spikes,  one pointed toward the vehicle through the reaction mass,  side/lateral gamma minimized.  That's the shaped charge I spoke of. 

That gamma spike toward the vehicle converts the reaction mass into super ionized plasma moving at super high velocities toward the vehicle.  The pusher plate diameter subtends the entire plasma spike at the blast distance.  Out in space,  they were talking 1-3 KT charges to push spaceships in the 10,000 to 20,000 ton class at near 2 gees,  and at Isp in the 10,000 to 20,000 sec range.  It works better the larger the vehicle;  their NASA-mandated small Mars design was several hundred tons and was under a miserable 5000 sec Isp. 

If one explodes inert mass with a laser blast,  one should be able to package the inert reaction mass in such a way as to produce the same double-spike shaped charge effect.  You suddenly and catastrophically blast it into oblivion with a death ray-strength laser.  I doubt it would go far into the plasma ionization range,  so the achievable Isp would be far less than the nuke.  WAG:  1000-2000 s Isp,  or 10 times less than the big nuke Orion. 

Still a pulsed explosion drive,  should still be better that lighting fires inside tin cans (chemical rocketry).  Ha ha.   

GW

(1) I cannot understand how any structure moving through the air does not displace air,  causing drag,  and my education and experience is very heavy in aerodynamics engineering.  You are talking about super-hypersonic orbital speeds,  too.  Drag will be enormous,  as will the aero-heating problem.  The space elevator centered at the geosynch point,  and non-rotating in any way,  does not move through the air.  What you described doesn't sound as if it is centered on any geosych point in any way at all.  Therefore it has to move through the air.  If it does,  there will be drag.

(2) Structures like this loop of yours have the same problem space elevators have:  they require some sort of "unobtainium" material,  even supposing you are right about no drag.  Carbon nanotubes offer a possibility of making progress toward materials like that,  but we cannot even successfully spin a long thread with them yet.  They may or may not ever be strong enough for applications like that. 

GW

I like what I read about the dried and "canned" foods,  just above.  Add in the frozen food capability that I suggested,  and you have almost the same menu capability we have down here.  The only thing missing is fresh foods,  which would requite some sort of garden.  We can do without that,  as proven by the 90+ day war patrols US navy diesel-electric subs undertook routinely in the Pacific during WW2.  A few of those went close to 150 days.  They had dried,  canned,  and frozen foods in those pigboats.  It worked.  Crews stayed healthy,  although "happy" is not something one associates with war.  We still do the same thing today in our nuke boats. 

I'd like to add the notion of thawing the occasional frozen brisket and doing barbecue (and other options).  My wife has an oven brisket recipe that will add pounds to you just from the smell while cooking!  You cannot do that sort of cooking in zero gee,  either:  convection ovens don't work without gravity to drive the convection.  Good cooking smells inside the vehicle (or home) add to the happiness of those inside.  We've known that since we tamed fire,  over a million years ago. 

With foods like that,  we are talking about cooking with free-surface water,  such as spaghetti.  You don't do that in zero-gee.  So,  we are inherently talking about implementing spin "gravity" here.  I like that,  because it makes all sorts of life support processes far easier,  including water and wastewater management.  Makes going to the bathroom far easier,  too.  And real bathing becomes possible again.  Not to mention avoiding all the microgravity diseases we are still uncovering. 

A spin gravity hab on a vehicle long enough for one gee is a rather large assembly,  right at 112 m long.  You simply cannot launch a thing like that in a single shot,  not even with an SLS,  or even a Saturn 5.  There,  I just went and shot down the SLS team arguments that you cannot go to Mars with just Falcon-9/-H and Atlas-H/Delta-H. 

But you cannot be a brain-dead bureaucrat and understand that argument,  when politics yells far louder.  Politics is government,  which is invariably run by committees,  whether elected or appointed.  A committee is a life form with six or more legs and no brain,  just as Heinlein said.  That's why I am not sure the first manned trip to Mars will be a government mission.  A motivated visionary private outfit like a Spacex might get there first. 

If you know exactly where you are going to land on Mars,  and you also have robot precision landing capability,  and also you have robot collision avoidance capability during your landings,  then landing unmanned stuff direct surface-to-surface makes sense.  But,  if you are missing any one of those three things,  then it does not make sense,  because you are guaranteeing that some kind of failure will occur.  Failures that will most likely be lethal for any crew sent there.  Or else you will spend a lot more than you planned on,  re-trying to land the replacements for the stuff you lost or landed too far away,  before you send the crew. 

Since I don't see all three things in existence and ready-to-go,  I'd prefer basing from orbit on the first mission,  if for no other reason than safety and limiting the potential for lost cargoes.  Even more important,  given a reusable lander,  that basing from orbit also enables landings at more than one interesting site (and there are many,  don't kid yourself). 

What's the point of going to the trouble of sending people all that way,  as difficult as it is,  and just land at one site?  That smacks of the Apollo "flags-and-footprints" nonsense.  You have to ask yourself "Why do we send people at all?"  Answer:  We send men to do the exploring that our machines cannot do. 

One has to have a very clear picture of why we go and what we intend to do while there,  before settling on mission architectures.  Doing that out of order ends up producing nonsensical designs and wasting a lot of effort.  Mission objectives must come first,  and will dictate some (not all) of the constraints within which your mission architectures must operate.  Then you select the "best" one of those. 

I gave you the fundamental mission objective just above ("We send men to do the exploring that our machines cannot do").  Now you have to correctly define what you mean by "explore".  We have millennia of experience to support that definition of "explore",  too.  It should mean to find out the answers to two very-deceptively-simple questions:  (1) "what all is there?",  and "where exactly is it?" 

That's not Texas drawl,  I meant them exactly as I wrote them,  word for word.  Think about it for a while.  For one thing,  you'll need a real drill rig,  because some of those answers could be on the order of km underground,  just like here at home.  Ask anyone in the mining or petroleum industries,  if you don't believe me.  Mining water there will be very much like mining coal here. 

Once you've thought about that,  then you understand exactly why "flags-and-footprints" missions are total nonsense.  It actually was nonsense long ago when we went to the moon with Apollo.  Most of the real discoveries about the moon have come from probes sent since then.  The Apollo astronauts couldn't do any real exploring,  until they took the car on the last 3 landings.  Even so,  they did not have a drill rig.  Sampling only up to half a meter down found very little of what we wanted to know. 

Guys,  I really recommend docked-module assembly in LEO,  and on-orbit basing at Mars,  for the first mission.  I like using the ISS as a base and home for the assembly workers,  that makes a lot of sense.  A docked assembly will require a lot of launched modules.  Better to bite the bullet and just do it that way,  even if you have to launch more thrown weight than some other notions.  That flexible orbit-based,  multiple-landings exploration trip will tell you where you really want to set up your first base.  It'll very likely be one of the sites you just explored. If and only if it is,  then you can start that base on the that first trip.  You might or might not get to do that,  and that outcome (either way) has to be part of your mission plan. 

GW

Hi Bob:

If you haven't seen it yet,  I posted some payload fraction comparisons on "exrocketman",  in one of two articles posted today (11-17-13). 

The DARPA spaceplane thing looks relatively undefined,  which may be a good thing,  as most federal government new-item development projects are way-over-specified.  It appears they want a hypersonic winged stage that can reach about 3 km/s just outside the atmosphere,  so it can land horizontally.  Vertical vs horizontal launch seems to be wide open. 

Based on the numbers I have been playing with,  that sort of thing is possible only with hydrogen-powered rocketry,  whether or not any airbreathing propulsion is used.  It's a nice wide-open solicitation,  often typical of DARPA.  I have very real doubts about getting anything flying in 5 years,  though. 

Looks to me like they are begging both XCOR and Spacex to respond.  I have good contacts at XCOR.  Do you know anybody at Spacex? 

GW

Hi Midoshi:

Good luck and Godspeed on your flight and the flight of your probe!

GW

You could use beamed-energy (probably super-high power laser) for launch to LEO,  if you solve the line of sight problem by using multiple laser installations.  There would be one near the launch site,  and one or two more downrange,  or even one already in orbit. 

The problem with this is that we're talking about lasers big enough to be death rays.  Death rays and atom bombs fall in the same sort of fear category as nuclear pulse propulsion.  The ignorant fear will kill the concept.

Technologically,  we're just about there.  The same kinds of tracking and pointing skills that enable ICBM intercept with anti-missiles can point the laser.  You build this beam launch system,  you've also built an anti-missile defense system,  no way around that.  Political nightmare.

The downside of laser for this launch or weapon application is poor damage-coupling into the target.  The beam heats the target,  which smokes,  and the smoke blocks the beam.  There's a different beam that gets around this limitation:  the electron or proton charged particle beam.  They have 100% unimpeded damage coupling,  but they wander all over the sky in the atmosphere,  like the lightning bolts they essentially are.

The way around this is to combine the two:  fire a laser bolt followed immediately by the particle shot.  It'll be a series of discrete hits,  no way around that.  The laser creates a straight line ionization trail,  which the particle shot follows as if it were a copper wire. 

There I went and told you how to do it.  And I also told you why you really don't want to.  Which is why the the anti-ICBM defense weapons are missiles,  not the energy weapons they first investigated in the 1980's (originally ca. 1950 in Project Seesaw). 

GW

Wow!  The air is mighty thin where this high-flying conversation has gone.  Allow me to bring it down nearer the surface for a moment. 

I'd rather there be a trained human pilot in any airliner I ride,  a pilot with the authority to override all automatic systems and take full manual control of the airplane.  The pilot's inability to override the computer in Airbus machines has already caused more than one fatal crash.  That's why I don't like riding Airbus products. 

On the other hand,  the pilots taking manual control need both training and experience at hand-flying airplanes,  as the recent Asiana crash at SFO so clearly demonstrated.  Same for the Colgin Air crash some years ago.  And many others. 

I do not want aircraft (or cars,  or trains,  or ships,  or anything else) operated solely by computers.  They can only do what they are programmed for,  unlike people.  I have seen nothing to suggest that will change even in our childrens's or grandchildren's times,  notwithstanding some of the stunts done with supercomputers in recent years.  Winning chess games is one hell of a long way from self-awareness and spontaneous creativity. 

Computers as we know them are like any other tool,  they exist to amplify and augment human abilities,  not replace them.  Robot manufacturing does require human supervision and intervention,  even today.  I see great dangers for any society that would ignore those facts of life. 

Not until the technology advances to the point where machine self-awareness and creativity can be verified (something we do not yet really know how to verify!!!),  should we allow any total job replacement.  Maybe we no longer need to weld the parts on the assembly line,  but we still need to supervise,  program,  intervene,  and repair the machines that do it for us.  Those are the proper roles for humans and machines.   

GW

Hi Bob:

One of the ramjet missiles I worked on was a low drag wingless finned "dart" that accidentally went M6 at 20 kft,  setting the record for airbreathing flight.  The ramjet worked fine,  and the bird was still slightly accelerating when it ran out of fuel.  It's skin was beginning to melt,  too,  especially since it was supposed to cruise at only M4,  with a terminal dive speed near M5. 

This was a throttle runaway incident on a first test that was supposed to fly sedately at its takeover M2.5.  Getting the M6 speed depends on the vehicle drag more than anything else.  That speed record stood from 1980 until NASA broke it with their X-43 in 2004. 

You do not need scramjet to fly between M4 and M6.  You need it to fly M7+.  But,  scramjet min takeover is M4!  Ramjet can be in the M1.5 to 2.5 range. 

Vehicle aero heating gets really,  really difficult above M6,  which is why M5 to 6 is what these pipe dream programs are all about.  You can tell a gravy train technology program from a real flight development,  simply because the gravy train guys want to fly scramjet at M5 to 6,  when it isn't necessary. 

Ramjet features relatively lower chamber pressures compared to rocket (50-100 psi typical vs 1000-2000 psi).  Ramjet nozzles have throat areas that need to be as large as possible,  right up to 65% of the combustor area (a flame stability limitation).  The max exit expansion area ratio is the pretty close to 1/0.65= 1.53. 

Those geometries are very,  very,  very far from what works with a rocket nozzle.  Compromising nozzle geometry "kills" (quite thoroughly) the performance potential of both rockets and ramjets.  That's why most designs feature an ejectable booster nozzle with suitable rocket proportions,  nested within a ramjet nozzle sized to work well.  It's a one-shot geometry change,  you cannot go back-and-forth.  Boost,  then sustain.  Period. 

There are ways to close off the inlets during rocket boost:  frangible or moveable or ejectable port covers.  Their best location is at the point where the inlet enters the combustor.

Most of the missile designs have you cast a solid propellant rocket inside the ramjet chamber.  As it burns out,  you open the inlets,  start the kerosene flow,  eject the booster nozzle assembly,  and light-off an ignition charge,  all at once.  The bird I spoke of transitioned from full rocket thrust to full ramjet thrust in 100 milliseconds.  You do need a very sharp booster tailoff to make that work,  which means a very careful interior ballistic design for almost no sliver. 

Some designs I worked on accepted a big loss in booster performance to avoid the ejectable nozzle.  There is a design called "nozzleless booster" for that.  The interior ballistics are very tricky,  there's more than one layer of propellant each with a different burn rate,  and the aft-end propellant shape is the nozzle.  It's a difficult art. 

There's no reason ejectable nozzles could not be used with liquid propellant rocketry,  using the ramjet chamber as the rocket chamber.  Nobody has ever done it,  but it could be done.  So could a hybrid booster. 

Solids came from missile work,  where they are preferred for a variety of very good,  practical reasons.  I even worked on some designs with fuel-rich solid propellant gas generators supplying combustible effluent as fuel,  instead of pumped liquid kerosene.  I actually know two really good ways to throttle a solid generator fuel supply,  too,  one of them with no moving parts at all. 

GW

For a different reason,  I had to rough out essentially the same plane circa 1985.  Mine was turbojet/ramjet parallel-burn propulsion,  a similar layout,  and designed for M5 at 100-150 kft.  I did it from all open sources. 

It was so close "to reality" the FBI confiscated all my design notes,  but not my sources or my slide rule,  because I did not possess the clearances "to know about such a craft".  I have often wondered if such a thing ever got built. 

It appears in hindsight apparently not.  Although it could have been.  The delta-wing pulse detonation experimental craft (seen above Groom Lake) of about 1995 apparently led nowhere. 

GW

I'm not sure what the terminology biamese,  triamese,  etc,  means.  I looked at the link for the OTRAG article,  and there is merit in that idea;  it's quite similar in fundamental concept to what I posted over at "exrocketman" for using simplified ramjet missile technology as a first stage.  Production savings often does overcome technological shortfalls,  even in rocket work (although logistical tail is the real cost driver).  Otherwise,  I'm not "up" on what y'all are really discussing here. 

But here's a thought anyway.  Why not think modular like OTRAG,  but not quite the same way he did.  More like what I proposed for the Mars manned mission vehicles. 

Come up with some sort of a common engine design,  with mix-and-match bell extensions.  Come up with a common engine "module" that is just a mounting frame for anywhere from 1 to maybe 10 of those common engines.  Then come up with a common propellant module (both sets of tanks),  that can be linked end-to-end or side-by-side (or both),  and plumbed together easily.  (BTW,  these will not turn out to be 5% inert.  More like 10+%.)

A bigger stack of propellant modules and more engines in the engine module,  is your first stage.  A smaller stack of propellant modules and maybe just 1 or 2 engines in the engine module,  that's your second stage.  Vertical launch on a typical rocket fast ascent trajectory leaves the sensible air at around Mach 2 at near 60-80,000 feet.  Cluster drag is really not much of an issue,  and neither is ascent heating.  You can use that "textured" lithium-aluminum panel that folks like so well.

Plus,  any dropped stage is easily split into individual modules for the best chances of successful recovery and reuse.  Smaller is better as far as chutes are concerned.  Same for deployed parasails or wings.  Just remember,  adding recoverability could double inert weight.  Or worse. 

Just an hunch on my part that something like that could be somewhat-tailored to any payload you want to launch,  up to the max TO thrust your design allows.  That kind of modular thing has been working for me up to know,  far better than any of the "classic" ideas. 

GW

If I had known,  I could have seen that last Grasshopper test at half a mile peak altitude,  6 miles from my house.  I certainly heard it.  Bravo,  Spacex!

GW